Rudders...WHY????^

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Gages
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Re: Rudders...WHY????

Post by Gages » Sun Jul 25, 2010 10:01 am

Geko wrote:Gages
Very informative info.No bait intended for you or a boat comparison but how does a Mirage type rudder shape up in comparison to a trailing type?
I have never paddled a skegged kayak but have often paddled with trailing rudders up and noticed a positive difference but I cannot notice a difference on the Mirage even with the rudder taken off infact it may even be worse.
Geko

Its still a trailing rudder

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Re: Rudders...WHY????

Post by cswalker » Sun Jul 25, 2010 8:19 pm

Gages wrote:Having a skeg down is definately slower than a trailing rudder and surprisingly having a skeg up is also slower than a rudder down as all the water causing cavitation in the skeg box results in a slower boat as well--Test it out yourself if you dont think so
Definitely not cavitation, hydrodynamically impossible at that level of energy over the rudder surface. Aeration is the term your looking for!

In terms of drag and slowing a boat, hydrodynamically yes it will but the energy consumption will be in joules so fairly in significant unless you paddle with the rudder fully wheeled over all the time.

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Re: Rudders...WHY????

Post by Big Ade » Mon Jul 26, 2010 5:52 pm

I think at some point, someone with a motorboat and GPS will have to step in.
Said motoboat will have to tow the various kayaks and paddlers at a given speed, using the GPS to keep the speed constant.
A lugage weighing scale or similar will have to be put in the towline.
And someone will have to record the drag on the scales with skegs up, down and halfway.
Then they'll have to do it again with the rudders. Up down and sideways.

Then we'll have a defintive answer.

If it proves to be fun we can meet regullarly to measure new boats...

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Re: Rudders...WHY????

Post by ian johnston » Mon Jul 26, 2010 6:16 pm

All this obsessing over fractions of a knot of speed, whether hypothetical or real!

I think I'll stick to paddling slowly and enjoying the scenery & wildlife :o)

Ian

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Jim
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Re: Rudders...WHY????

Post by Jim » Mon Jul 26, 2010 7:04 pm

Big Ade wrote:I think at some point, someone with a motorboat and GPS will have to step in.
Said motoboat will have to tow the various kayaks and paddlers at a given speed, using the GPS to keep the speed constant.
A lugage weighing scale or similar will have to be put in the towline.
And someone will have to record the drag on the scales with skegs up, down and halfway.
Then they'll have to do it again with the rudders. Up down and sideways.

Then we'll have a defintive answer.

If it proves to be fun we can meet regullarly to measure new boats...
Nonsense, if certain projects go ahead and my company get towing tank access you will all have to send me your kayaks and I'll do the tests properly.

Now, what are the correction factors required for a 1:1 model test?

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Re: Rudders...WHY????

Post by Mark T E » Mon Jul 26, 2010 8:40 pm

Not sure if anyone has factored this in, but........ if a boat is fitted with a rudder, in a beam wind, and the rudder isn't deployed, ie: up, then surely the aerodynamic effect will make said boat weather cock? Am I being naive? Anyhow ,taking this to a logical conclusion, why not instead of a skeg underneath at the stern, a skeg on the deck on the bow. Gets rid of your hydrodynamic worries.
Anyhow, what do I know, this bloke seems to be the expert on skegs:http://cgi.ebay.co.uk/ocean-kayak-/2206 ... 335f553724

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Re: Rudders...WHY????

Post by TechnoEngineer » Mon Jul 26, 2010 8:59 pm

Mark T E wrote:boat on ebay
Blimey - for a split second I thought he had fitted a hydrofoil ;)
XL-Burn-3 / Monstar / Kodiak / My Videos

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Re: Rudders...WHY????

Post by Chris Bolton » Mon Jul 26, 2010 9:00 pm

Mark's link is here (since he hasn't made 5 posts the anti-spam stops it working in his post). Those stabilisers are amazing - it looks in good nick otherwise. Probably needs bigger waves?

The aero drag from a raised rudder can be a nuisance, exactly as you suggest, Mark. A friend of mine spent the first third of trip in a hired boat with the rudder down, the second third with it raised and the final third with it removed.

Chris

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Re: Rudders...WHY????

Post by Robert Craig » Mon Jul 26, 2010 9:37 pm

Jim wrote:
Nonsense, if certain projects go ahead and my company get towing tank access you will all have to send me your kayaks and I'll do the tests properly.

Now, what are the correction factors required for a 1:1 model test?
Jim
Did (some time ago) approach the Denny Tank people in Dunbarton. Got as far as a contact in (I think) NEL East KIlbride, but never got a reply. So gave up. If you get anywhere, give me a shout - I'd love to be involved.

For non-locals, the Denny Tank is a historic model testing tank, still used by a university. It's open to the public - well worth a visit.

[My son was disappointed to find, on our visit, that the Denny Tank didn't go "rumble, rumble".]

On the drag issue, consider a beam wind. Assume for the moment that the skeg and the rudder are identically shaped - say flat plates - and identically placed, the only difference beining the angle of the rudder.

Both are being used to counteract the torque generated by the centre of pressure of the wind being astern of the centre of pressure of the hull. So both generate the same force. So both are at the same angle to the flow (a plate aligned withn the flow doesn't produce any sideways force aka lift). In one case the angle is the rudder angle; in the other the whole boat is crabbing slightly through the water.

So the drag of the rudder equals the drag of the skeg (has to be, as the flow conditions are identical). Only difference is the crabbing of the skeg boat, which presumably gives more drag.

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Re: Rudders...WHY????

Post by Jim » Mon Jul 26, 2010 11:19 pm

Robert - yaw angle is definitely an important factor, but here is a conundrum for you:

If you use a skeg and therefore the entire hull is yawed, do you get away with a lesser yaw angle, than the rudder angle you need to keep the hull without yaw in the same conditions? If you need more rudder angle, is it enough to stall the flow over the rudder and create even more turbulence?

I would suggest, that there are going to be different regimes depending on vessel forward velocity, and the wind/current velocity (both magnitude and direction), where the optimum solution changes from being a skeg to rudder or vice versa as the leeway angle, yaw angle and rudder angle vary as a result of the regimes.

Conclusion:
Paddler A will report that rudders are faster and Paddler B that skegs are faster, but they won't have been in a position to compare apples with apples, or to collect enough data to really get to the bottom of it.

Postscript - I haven't read back through the whole old thread, but my main reason for not using a skeg or a rudder, is that I would almost certainly damage them. Feel free to find a contradictory statement from me, I often change my mind, such is the persuasiveness of this group! :-)

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Re: Rudders...WHY????

Post by Chris Bolton » Mon Jul 26, 2010 11:31 pm

Robert Craig wrote:Both are being used to counteract the torque generated by the centre of pressure of the wind being astern of the centre of pressure of the hull. So both generate the same force. So both are at the same angle to the flow (a plate aligned withn the flow doesn't produce any sideways force aka lift). In one case the angle is the rudder angle; in the other the whole boat is crabbing slightly through the water.
I would say they were each (in that situation) being used bring the centre of pressure on the hull aft so that it was aligned with the centre of wind pressure. The rudder would be in line with hull (otherwise the boat would behave differently to the way it would with the skeg) so there would be no effective difference. The boat would make some leeway, creating turbulence around the skeg / rudder - I'm not expert enough to know whether that would be worse in terms of overall drag than having the hull trimmed to balance it.

The idea of needing a rudder to trim for wind direction just doesn't seem right to me. Then again, I have a boat which (unless loaded wrongly) has neutral trim in all wind directions, so I don't feel the need for a skeg or a rudder.

Chris

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Re: Rudders...WHY????

Post by geoffm » Tue Jul 27, 2010 12:10 am

Now add some reasonable sized waves so the rudder spends half of its time out of the water and see which method works best. I would think the rudder would need to be turned a fair amount to correct the yaw that occurs whilst the rudder is "airing".

Geoff

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Re: Rudders...WHY????

Post by Robert Craig » Thu Jul 29, 2010 10:33 am

Chris
I think you're saying (and I entirely agree) that a rudder the same shape, size, and position as a skeg and used with no angle has the same effect as a skeg.

My arguement is that in this situation the boat has to crab slightly through the water so's to get the skeg/rudder producing the sideways force required. I was trying to explore the difference beween this situation, and the situation where there is a slight angle on the rudder and the boat doesn't need to be crabbing. I agree that on a boat with a rudder, one can choose how much rudder angle to use.

I think that as the wind increase a skegged boat crabs move, and a ruddered boat (where the rudder angle is chosen to avaoid crabbing) needs more rudder angle.

Jim
I suggest that the boat crabbing doesn't produce any significant torque to counteract the wind torque. So the crab angle with skeg = rudder angle with no crabbing, and so the flows over the rudder/skeg are identical.

Veering off subject, I suspect that the crabbing does produce a bit of upwind lift - but no torque, just a reduction of leeway.

You got access to a CAD tool which can model this?

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Re: Rudders...WHY????

Post by Jim » Thu Jul 29, 2010 10:56 pm

Robert Craig wrote:You got access to a CAD tool which can model this?
No, but for my final year project I re-wrote a VPP to use investigating if a sailing boat could benefit from offsetting the keel with or without optimising the section for the required direction of lift. I think what you need is something like that. The VPP was in Fortran 77, I'm sure if you can work out the equations for balancing the kayak you can knock something up in VBA for excel with input from a spreadsheet or form which would be much easier to use than the cryptic text files I needed :-)

Right now, I really don't have time!

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Re: Rudders...WHY????

Post by Chris Bolton » Thu Jul 29, 2010 11:00 pm

Robert wrote:My arguement is that in this situation the boat has to crab slightly through the water so's to get the skeg/rudder producing the sideways force required.
Thanks, Robert - I understood that, but I don't think any crabbing is necessary. I'm suggesting that the skeg adds lateral area to the stern of the boat, so it becomes balanced; centre of force and centre of resistance line up. In that situation, it's exactly the same as if the boat was balanced without needing a skeg - it doesn't need any turning force, so it doesn't need to crab to generate it.

Chris

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Re: Rudders...WHY????

Post by Robert Craig » Fri Jul 30, 2010 10:27 am

Chris
I see your point. I'm taking the line or discussion that "balance" is like a balanced see-saw.

A = Wind on the boat plus water on hull = one side of see-saw

B = Water on skeg = other side of see-saw

A and B are torques

If it's all working, giving no leecocking or weathercocking, A = B.

A isn't zero (the kayak weathercocks without the skeg)

So B isn't zero.

A skeg with no angle to the flow produces no sideways force. As B isn't zero, there must be an angle. The only way the skeg can have an angle to the flow is through crabbing.

An analogy is an airplane in level flight, nicely balanced - but both the wing and the tailplane produce forces (and therefore torques). Trick is to get these the same to keep the balance.

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Re: Rudders...WHY????

Post by maryinoxford » Fri Jul 30, 2010 2:33 pm

Gages wrote:The secret is getting the rudder where the skeg goes and so far no one has made one that is workable and usable for sea kayaking.
I thought that was what Kari-Tek's Skeg Rudder system does. Has anyone tried it?
http://www.kari-tek.co.uk/SkegSystemsSRS.html

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Re: Rudders...WHY????

Post by Jim » Fri Jul 30, 2010 2:45 pm

Robert,
Whilst 'torque' is not wrong it's not the normal term, we would normally talk about moments and moment balance.

The basic problem is that the yaw centre of the vessel which is the centre of lateral area is not coincident with the centre of effort for the windage area, so you get an overall yaw moment.

For a quai-static case, you need to draw the above water shape and below water shape and find the centres of the projected areas.

Forget forward speed,the water flow is equal and opposite to the wind flow, the boat moving sideways through the water is mathematically identical to the water moving onto the side of the boat.

Forget any lift coefficients, they will be close to one in this case anyway, the wind force is half the air density times the projected area x square of the wind speed. The water force is the same equation using the water density and sideways speed of the boat. Since these forces must be equal and opposite you can calculate the boats drift from this.

Because the action of these forces is through the CLR and CE you have a force couple which creates the yaw. To solve this you have to take moments about one of centres of action, we tend to take them about the CLR since we think of the boat yawing in the water rather than in the wind - mathematically it is identical. The force times the horizontal distance between the centres of area is your yawing moment.

By adding a skeg (or rudder held straight) at one end you add a little bit of area at a much greater distance from the yaw centre. This will create an additional water moment, when this is equal to the wind moment your boat will balance perfectly and be blown sideways without yawing - this is why it's best to test for the optimum amount of skeg to deploy. If you don't have a skeg, fine adjustment of trim can be used to make the stern deeper and bow shallower, thus moving the CLR aft - once the CLR and CE are in line you go sideways without yaw.

Complications.
If the boat moves forwards the water flow is the vector sum of the forward speed and lateral drift. If there is a current or tide stream, you have to find the vector sum of all of the above. Now the water goes around the boat a bit so it is acting more like a wing and the resulting force vector will be at angle aft of the beam, but can be resolved into 'lift' (laterally) and 'drag' (longitudinally). The centre shold be of the projected area in the direction of the flow vector, this may not be in the same place as when whe just looked at it sideways on.....
Similarly the boat is moving through the air at an angle to the wind speed and all the same things apply - air and water are just fluids with different density (although one is also compressible).

Now if you have resolved the vectors to a transverse and a longitudinal force for each, you should find that both of the 'drag' components act on the centreline of a symmetric vessel (you were going to edge to control it? that will muck up this maths!) so you can forget them - they are both going the same way against your propulsive effort.
Transversely, although the centres have moved arouns and the projected areas we have a very similar situation, this time lift coefficients may be significant, especially when you add a skeg or rudder but we would need to do model or full scale) testing to work them out. Quasi-statically you still have a yaw moment which you can correct by deploying some skeg (or straight ahead rudder) using it's projected area and centre in the calculation of the moments. But now if you have a rudder, you can also alter the rudder angle to change the projected area and give you more or less lift - so by factoring in the change in projected area and centre due to rudder angle you will find that you have another option for fine tuning your dynamic CLR. However whilst the drag for the hull and skeg remains the same, you can increase or decrease it a fraction with the rudder depending on what angle is required to hold the balance and whether you are aligning it to the vector flow or swinging it more accross that flow.....

So you see it's all just relatively simple maths, if you know the lift/drag coefficients of the components involved and that includes the hull and all the stuff (paddler) above the water........ And of course know how to integrate to find the underwater and above water areas and centres of areas....

As I say, you could do it in VBA if you wanted. :-)

Jim

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Re: Rudders...WHY????

Post by Douglas Wilcox » Fri Jul 30, 2010 6:32 pm

Mary mentions the Kari-Tek Hydro skeg/rudder. I have used these on a Menai 18, Quest and SKUK Explorer. As a rudder they are really, really good when surfing fast downwind (especially when using a sailing rig). However, in strong winds from about 45 degrees to 135 degrees to the wind you need to use it partially raised in skeg mode. If you put it fully down in rudder mode the wind thinks it is a fully deployed skeg and the kayak will lee cock and you cannot paddle fast enough for the rudder to generate enough force to turn you back into the wind. (I am relying on Jim to correct any misuse of technical terms). I think it is a great product but it is accurately described as a "skeg rudder", it is not a rudder. Mind you we paddle an Aleut Sea II with a rudder in some pretty high winds and even though it has a huge rudder blade, if you put it down in a force 5/6 and try and paddle at 45degrees to the wind, the Aleut will lee cock and the rudder will not correct it so you need to lift it completely and paddle without it.

With respect to drag caused by a skeg box. I once had two Alaw Bachs and fitted a Kari-Tek hydroskeg to one. On a millpond day I paddled each as fast as I could (skeg up) and measured the maximum burst speed using a GPS with battery save mode off and EGNOS enabled to give the most accurate results. There was no difference! Maybe this was because a limiting factor to top speed in an Alaw Bach is that it squats on its stern and creates more drag at high speed and maybe this is a bigger factor than a little drag from a skeg box. Of course burst speed is quite different from a "little" drag adding up over 40km at normal cruising speed.

As it happens, I much prefer paddling the Alaw Bach with a skeg than without and when I decided to buy a Nordkapp LV, I sold the Alaw Bach without the skeg.

My own conclusion is that if you paddle in lightish winds (or downwind only, in stronger winds) rudders are great. If you paddle a kayak in high winds with varying loads, skegs are absolutely brilliant things!

Douglas

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Re: Rudders...WHY????

Post by Chris Bolton » Fri Jul 30, 2010 8:14 pm

Robert wrote:A skeg with no angle to the flow produces no sideways force.
Indeed not, but if (to simplify Jim's maths, which I agree with, even if nobody else has read them) you look at the boat without forward motion, the wind is blowing it sideways, so the flow is at right angles to the skeg and the skeg doesn't need to produce lift.

There is crabbing, because as the boat is paddled forward, it is blown sideways (leeway). The net effect is that it goes maybe 5º (depending on the wind strength) downwind of where it's pointing. So in that sense it's crabbing, but I don't think that's what makes the skeg effective.

Chris

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Re: Rudders...WHY????

Post by Robert Craig » Fri Jul 30, 2010 9:14 pm

Jim/Chris
Right, ok, in the static case I agree. I'd forgotten about leeway. The skeg is producing a drag force 'cos it's moving sideways through the water, and it's working 'cos this drag force produces a moment which balances the moment of (wind on hull + water on hull) from leeway.

However, the sums get hard for the non-static case, as the centre of pressure for the water (and to a lesser extent the wind) moves dramatically when the boat is moving. It moves towards the leading edge. That's why almost every kayak lies beam-on the the wind when stationary, but weathercocks when moving forward. It's also why beginners find river kayaks spinning out of control on flat water.

Aircraft wings are the same - the faster the plane goes, the more the cente of lift of the wing moves forward towards the leading edge. That's why there's a tailplane as well - the tailplane centre of lift moves as well, so the plane stays level as the speed varies.

These sums for the non-static case are way beyond me. It may well be that the drag is still the dominant factor, and the lift is unimportant .... Jim, comments? I'm happy about the aircraft stuff, but out of my depth in the nautical stuff.

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Re: Rudders...WHY????

Post by Jim » Fri Jul 30, 2010 11:40 pm

Robert Craig wrote:These sums for the non-static case are way beyond me. It may well be that the drag is still the dominant factor, and the lift is unimportant .... Jim, comments? I'm happy about the aircraft stuff, but out of my depth in the nautical stuff.
It is exactly the same, you just have 2 fluids involved with different speeds and densities, and you turn everything through 90 degrees and ignore gravity because buoyancy deals with it.

Pretend the water forces are the gravity forces, and the wind forces are the apparent wind forces on the wing. The main difference is that gravity is constant but you have to calculate the water forces in the same way as the wind forces.

Which component is dominant will depend on the speed and angle, much like the stall point of an aircraft wing. You would need to do the maths for each boat and skeg/rudder to find out where the point is.

Jim

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Re: Rudders...WHY????

Post by geoffm » Sat Jul 31, 2010 6:17 am

My own conclusion is that if you paddle in lightish winds (or downwind only, in stronger winds) rudders are great. If you paddle a kayak in high winds with varying loads, skegs are absolutely brilliant things!
A good summary in layman's terms Douglas. Although I am absolutely fascinated by Jim's comments and really enjoy trying to unravel just a tiny bit of info from them what you wrote about skegs and rudders above makes a lot of sense. There is a video clip on one of Justine's This Is The Sea videos where Andrew McCauley, Lawrence Geoghegan and Stu Trueman are caught in a vicious katabatic wind which nearly finishes them off. Stu Trueman was the only paddler with a skegged boat and he is heard to say that the skeg was a real advantage compared to the ruddered boats in the extreme winds.
Incidentally, we have an unused skeg/rudder for sale. It is a brand new and complete kit no longer needed. (We bought a skegged boat for my wife a couple of years ago and part of the deal was she could have a rudder fitted to it if she needed it. She didn't :-) Anyway, it is for sale. Postage from Australia would be a factor, nevertheless it is available at a substantial discount.

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Re: Rudders...WHY????

Post by Skua » Thu Aug 05, 2010 12:20 am

This seems to be over complicated by mathematics. It is much easier to think of the hull moving through the water, the bow meets a resistence, which is translated as the bow wave. The stern is trailing in displaced water and is more free to move, so it gets blown downwind. That is the effect of weathercocking, it is what is actually happening. However, paddlers look towards the bow of their boat and see the relative movement of the bow only and think the bow is swinging into the wind (not quite the same thing, but certainly the same visual effect).

What is the skeg used for? To counter the leeward slip of the stern in the wind. That is what it is for.

What is a rudder for? Funnily enough, to counter the leeward slip of the stern in the wind.

How do they do this? Quite simply by adding a resistence at the stern, a lateral resistence to neutralise the effect of the resistence of the water at the bow. Not so complicated after all.

Now the relative merits of an over stern rudder (a well designed one, not a self tapping screw on afterthought):
The hull integrity is not compromised by grinding a great slot below the waterline, then laminating a stiff box in the flexible hull, so you end up with flexible sides and stiff centreline...below the waterline!
There is no intrusion into the aft hatch space to reduce your cargo carrying capacity.
There is no possibility of grit preventing the skeg from dropping - Sod's law dictates this will only ever happen when you need it the most.
The activation lines are the same, in that you need a means of activation, so we'll call that evens.

An overstern rudder requires no holes to be cut below the waterline. The hull remains as designed with no stiff/hard spots introduced.
The foot braces do not need to be compromised. Proper, firm footbraces can be utilised with toe control of the rudder angle.
The rudder out of the water syndrome can be overcome with the use of a properly designed blade - which trails, yes, but the angle of trail self adjusts according to the amount of rudder immersed and the forward speed. As your stern lifts over a wave the rudder blade drops, as the water pressure on the forward edge increases (through immersion) it trails to its natural position again according to your forward speed.
Rudder up in the wind - if it is a light wind so that you don't need the rudder, then there is insufficient wind to cause a noticeable effect on the balde area. If it is windy enough or the seas are so steep that you feel the need to deploy the rudder, then it won't be in the air, it will be in the water.
Going upwind, keep the rudder up - it is presenting so little surface area that it creates no hinderance whatsoever. - Antagonists of the use of a rudder because of the windage argument are quite happy to put kit bags on their decks....and thos kit bags don't create any windage? With a rudder, there is room in the hatch to put your kit below decks!

The use of a rudder is so misunderstood. People who do not use them think it is for turning the kayak, it isn't it is for PREVENTING THE KAYAK TURNING - we worked this out in the early part of my post. It adds a lateral resistence that stops the stern from slewing downwind. If you want to manoevre, you lift the thing up, put the kayak on edge just the same, and let the stern slew. You wouldn't try to turn a kayak hard with the skeg down - for the same reason, it is there to stop the stern slewing, so you lift the skeg up.

Paddling long downwind runs the rudder will win every time. The only way to test it is to put them on the same kayak with an open minded, independent paddler and try one, then the other. The rudder is more versatile and puts the blade out astern of the waterflow around the actual hull - in use and underway the rudder trails clear of the waterflow around the transom of the kayak, reducing the turbulence at the stern and allowing better waterlflow.

The question about drag - yes, you can feel the difference when you put the rudder down unless you spend a significant amount of money on profiled rudder blades. A flat blade works perfectly well, but the 1.5mm of frontal edge is enough to be able to feel it when it is down (plus the bit of flow along the blade sides). However, in calm or upwind situations there is no drag because it is up,out of the way and your perfectly smooth hull can glide through the water as intended. Coming downwind, again, if it is light airs, you don't need it so there is no drag (but there is with a raised skeg....all around the slot the skeg fits in, unless fitted with a flexible membrane either side like a draught excluder for skegs.)

Coming thrugh weed or rocks the rudder will just kick up over them....your skeg will drag in the weed and catch on the rocks.If you get long strands, like sargasso weed, around the rudder, simply lifting it allows the strands to fall off if they haven't slid off by themselves.

So, in effect the only difference between a skeg and a rudder, is how it is fitted. The reason for using it is the same. To add a resistence at the stern to counter the resistence of the bow in the water. That's all. The overstern rudder is self levelling according to the forward speed and other conditions. A skeg has to be manually fiddled with to get the required amount down.

geoffm
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Re: Rudders...WHY????

Post by geoffm » Thu Aug 05, 2010 1:25 am

Skua,

You say that rudders are just as reliable as skegs. From what I have seen that is fine as long as the paddler maintains their rudder, many don't, and many rudders fail.Most skegged kayaks will paddle perfectly well, albeit with a bit more effort, if the skeg fails.Some ruddered kayaks have a design that necessitates the rudder (it is not able to be lifted up) and if a line fails on these in bad weather well offshore there could be a serious problem. Many other ruddered kayaks have poorly designed hulls that need a rudder to be controllable.
Your own design doesn't seem particularly perfect. To quote a previous poster:
"The skua I paddled had a rather poor rudder control system for a sea kayak, a bit like an aircraft foot control with a central pivot that broke when pushing with both feet."
I think I will stick with my skeg kayaks :-)

Geoff

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Re: Rudders...WHY????

Post by Ken_T » Thu Aug 05, 2010 3:49 am

Hi,
I have paddled boats with rudders, skegs & neither, My current solo has a skeg & I own 2 doubles fitted with rudders. I appreciate that which is fitted is dependant on the design of the boat. I agree in a lot of conditions that a ruddered boat is faster, that is why most racing boats are designed to have a rudder (also so that a Swede form hull can be used to allow the paddle to be planted closer to the centre line of the boat, but at the cost of reduced directional stability). In very windy conditions when paddling across the wind a skeg can be adjusted to make the boat naturally to run at the desired angle to the wind (although the setting will be dependent on speed, but this can be used to fine tune the angle of the boat). In the same conditions if the rudder is deployed it has to be hard over otherwise the boat will try to point downwind, if it is not deployed the boat will try to point upwind.
The second problem with an overstern rudder is that in rough conditions it is out of the water when it is needed most, when the stern is above the crest of a wave in the highest wind (not sheltered by the waves themselves), an understern rudder would sort this out, but unless it retracts is in a very vulnerable position. Maybe something like Karitechs skeg/rudder is the best solution, but I have no experience of this.
On the downside for skegs they do reduce the volume of the rear hatch & make it significantly more difficult to pack.
Ken

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Re: Rudders...WHY????

Post by MikeB » Thu Aug 05, 2010 8:15 am

Skua wrote: What is the skeg used for? To counter the leeward slip of the stern in the wind. That is what it is for.

What is a rudder for? Funnily enough, to counter the leeward slip of the stern in the wind.
Rudders turn. The boat turns. Skegs don't turn, they stop the boat turning.

Mike.

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Re: Rudders...WHY????

Post by Davec » Thu Aug 05, 2010 9:41 pm

I have an Nigel Dennis explorer with hydro skeg and have just changed it for hydro rudder.
I found Kayaks turn sharper without a rudder.In windy conditions i found the rudder to be more useful than the skeg as i do not have to alter paddling stroke to use slider.
My rudder is also underneath my kayak and not on the back deck so stays in the water in a following sea if needed.
Didn`t Freya use a rudder when she paddled around Australia??
Good enough excuse fo rme then.

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Re: Rudders...WHY????

Post by chrism » Thu Aug 05, 2010 10:41 pm

Ken_T wrote:The second problem with an overstern rudder is that in rough conditions it is out of the water when it is needed most, when the stern is above the crest of a wave in the highest wind (not sheltered by the waves themselves), an understern rudder would sort this out, but unless it retracts is in a very vulnerable position.
That's why surfskis have understerns - and as a consequence handle a lot better (and are more fun) in big downwind conditions. I'd still far rather have an overstern than a skeg though - it can at least be used to control your direction when on a wave most of the time, unlike a skeg.

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Re: Rudders...WHY????

Post by Chris Bolton » Thu Aug 05, 2010 10:46 pm

Skua wrote:The stern is trailing in displaced water and is more free to move, so it gets blown downwind. That is the effect of weathercocking, it is what is actually happening.
Skua, I agree with some of what you write, but not that. I have experienced leecocking, where (in my "mathematical" world) the bow is blown downwind - would you say the stern is being blown upwind?
Skua wrote:Antagonists of the use of a rudder because of the windage argument are quite happy to put kit bags on their decks
That's a ridiculous generalisation, for which there is no evidence.
skua wrote:The use of a rudder is so misunderstood. People who do not use them think it is for turning the kayak, it isn't it is for PREVENTING THE KAYAK TURNING
If that was the case, there would be no need to make the rudder turn - you could get rid of the cables and pedals and just have an overstern skeg. The ability of a rudder to turn the boat is why they are used on boats which are otherwise hard to turn, like racing kayaks and double sea kayaks.

Chris

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