Cruising inertia

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Conor Buckboy
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Cruising inertia

Post by Conor Buckboy » Sun Jan 29, 2017 10:15 pm

Hello, I'm having somewhat of an argument with one of my paddler friends. The subject is whether a lighter boat or heavier boat will be more efficient in a simple long distance paddle. He thinks the lighter boat while I think a slightly heavier boat will be better. I think this due to newtons law of inertia, basically the more force on an object, the more force is needed to change the state of said object. In kayaking terms this could come about where a heavier kayak will hold its momentum and carry its speed more (due to inertia) because it's harder to stop, as such the paddler could use a slower stroke rate/ save energy with less powerful strokes.
So basically to narrow it down my friend is arguing a lighter boat because it requires less force to move, while I argue a heavier boat because it requires more force to stop. Forget about portaged or sprints, we are simply talking about cruising speed.
Surely the lighter boat will require a higher constant power from the stroke to stay at the same pace otherwise it would lose speed quicker, whereas a heavier boat can slow down the rate to let the boat run and cruise.
Originally I posted this in completion but I'd thought I'd post it here too as a lot of sea kayakers do a lot of long distance stuff and we all have the moment where we can stop paddling at cruising speed, get out a bottle and sip a drink, put it back and still be moving at a fair pace.
Thoughts?

Dyllon
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Re: Cruising inertia

Post by Dyllon » Sun Jan 29, 2017 10:42 pm

Makes no material difference in terms of energy expanded. Heavier boat will take a wee bit more to accelerate and carry more speed. So, ultimately, feel will be different.

One can make exactly the same arguments with bikes. But as they need to get the bikes uphill, lightweight is more important.

Shape, surface area and windage is what makes the difference.

R.

Conor Buckboy
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Re: Cruising inertia

Post by Conor Buckboy » Sun Jan 29, 2017 11:03 pm

While yes I understand the obvious differences with shape it's still open to debate.
Also while what you have said about bikes is correct in not sure it's truly applicable to kayaks due to a bikes propulsion being virtually constant with the chain on the cogs and clipped in pedals, a lot more constant that the amount of time a blade is in the water for a kayak, and also with bikes there are gears which can mess up this analogy for marathon kayaks and bikes.

Chris Bolton
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Re: Cruising inertia

Post by Chris Bolton » Sun Jan 29, 2017 11:16 pm

The key difference is that more weight means more hydrodynamic drag, wetted area and form. Competitive marathon paddlers will try to keep their boats as light as the rules allow, which should answer the question.

I do think the glide of the hull has an effect on the ideal stroke, but whether a light low drag hull glides better than the same shape more deeply immersed but with more mass, I don't know.

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Re: Cruising inertia

Post by Mark Gawler » Mon Jan 30, 2017 6:45 am

All things being equal, same hull shape paddling on flat water the increased weight will result in greater displacement therefore more drag, but there are so many other factors you won't notice the difference.
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Re: Cruising inertia

Post by Allan Olesen » Mon Jan 30, 2017 6:57 am

When you reach constant speed, inertia no longer comes into play.

So a heavier boat will be slower to accelerate and decelerate, but the cruising speed will be unaffected by inertia.

Of course, other factors not related to inertia will come into play: You will have to displace more water volume which will create more drag. But your hull will also be lower in the water so you get a longer water line, which will increase the hull speed slightly - if you cruise at a speed where hull speed is relevant.

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Re: Cruising inertia

Post by M-J-B » Mon Jan 30, 2017 9:07 am

During your long distance paddle you probably can not paddle precisely in a straight line from A to B. More energy is needed to alter the course of a heavier boat. That's fine as long as wind & waves are trying (and failing) to get you off course but any reason you might have to alter the course means wasted energy. Even with no wind or waves you probably end up making small adjustments to your course anyway so a heavier boat ends up wasting your energy. It's more efficient to reduce the windage than to add weight if wind is causing problems.

Also inertia related, some have also argued that slightly heavier paddles are more efficient than ultra lightweight paddles which are easier affected by wind gusts. Personally I believe lightweight is always better if all other factors are unchanged. Additional weight might make it easier to handle momentarily but energy is wasted in the long run.

Ken_T
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Re: Cruising inertia

Post by Ken_T » Mon Jan 30, 2017 1:13 pm

Hi Connor,
First comment the weight of the paddler will be far more significant. A lighter boat/paddler combination will accelerate harder with the same force (a=f/m) so the amount the speed rises & falls while cruising will be slightly higher & as the resistance increases as the square of the speed (or faster) the average resistance will be marginally higher. This effect will be negligible when compared to the extra drag created by displacing more water.
Ken

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Jim
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Re: Cruising inertia

Post by Jim » Mon Jan 30, 2017 4:03 pm

You are talking about glide, how well a boat maintains its speed between strokes.
For a kayak which has a naturally high stroke rate and opposing paddle strokes, glide has relatively little relevance other than how it makes you feel.
For a canoe, C1 or OC1 where the stroke rate is roughly halved and there is no opposite stroke to cancel the yaw effect, glide becomes a significant issue - the more glide a boat has, the more time you have to affect a steering component to the stroke before the speed drops. Once the speed drops, the effort to build it back up again causes additional yaw, which then requires a heavier steering effort, which takes the speed back off. Therefore a boat with more momentum can be beneficial when solo canoeing, but it is still better if the glide can be achieved by hull shape rather than weight.

To some people it feels better if the boat moves at a constant speed (good glide and/or lots of momentum), to others it feels better if there is a noticeable surge forwards during every paddle stroke - personally I like to feel the boat surge, particularly if I am trying to travel quickly because it gives me something to time my strokes against.

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PeterG
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Re: Cruising inertia

Post by PeterG » Wed Feb 01, 2017 10:25 pm

My anas acuta definitely has more 'carry' when heavily laden so glides along between strokes. The waterline is also longer so it ought to be faster. Yet as soon as I camp in the same place for a day or two and have an unladen day paddle the boat shoots ahead effortlessly. So I would say the lighter the better.

Kris_D
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Re: Cruising inertia

Post by Kris_D » Fri Feb 03, 2017 4:32 pm

Interesting question!
In the very simplified (impossible) case where the only difference between the two conditions is the total mass I think it should not make a difference. In real life, a greater mass for the same hull will increase friction drag because of more wetted area, and thus require more energy to propel. It will also have a slightly longer water line, so the theoretical hull speed will be slightly (very slightly, goes by square root of length) higher but only if the paddler has enough power to get to that speed anyway. It will cost a lot of juice.
Paddling involves alternated accelerations and decelerations and thus the added inertia which make the boat slow down less during the unpowered phase will equally make it be harder to accelerate during the powered phase. Should cancel out.
I'm sure in real life the light boat will always be faster. Except maybe in nervous chop where a heavy boat gets trashed around less, not sure.
I don't think we have the equivalent of downhill cycling, where extra mass might indeed help because more gravity = more forward force and with a similar air resistance (= more backward force) these two should come into an equilibrium at a higher speed.
Just my ideas! Kris

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Re: Cruising inertia

Post by twopigs » Fri Feb 03, 2017 5:34 pm

Kris_D wrote:I don't think we have the equivalent of downhill cycling, where extra mass might indeed help because more gravity = more forward force and with a similar air resistance (= more backward force) these two should come into an equilibrium at a higher speed.
Just my ideas! Kris
Please don't repeat this to anybody teaching Physics ...

Obviously when teaching Physics we start by ignoring air resistance .....

Gravitational potential energy = mgh
which is converted to kinetic energy as we descend the hill = 1/2 mv squared.

Notice m - the mass - appears on both sides of the equation - so extra mass does not result in extra speed.

When we factor in air resistance I'd suggest a cyclist with less mass might be subject to lower air resistance - think slim man in Lycra versus Mr Blobby.

So it might just be that slim man in Lycra ends up at the bottom of the hill with more speed than Mr Blobby (on the same bikes).
Canoeing - bigger boat, broken paddle, more skill!

Kris_D
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Re: Cruising inertia

Post by Kris_D » Fri Feb 03, 2017 7:35 pm

Why do you think I'm not teaching physics? ;-)
Obviously air resistance is the crucial thing in my reasoning, only a lazy physics teacher would exclude this as it is the predominant force in cycling over 20 km/h.
Your exchange of potential and kinetic energy is correct and it is why, in a vacuum, a feather and a bullet fall at the same acceleration (9.81m/s2) and will reach the same speed (up until infinity, if the fall goes on). I think you will agree that in air, a bullet will reach a greater final speed than a feather.
I consider the cyclist+bike as the "free body" upon external forces act, in this case resistance (mostly aerodynamic drag and a bit of friction of wheels etc.) and a propulsive force of equal magnitude. The latter is generated through gravity acting upon the bike+body's mass and will increase linearly with mass. Aerodynamic drag will also increase, as you say, but not as much since frontal area scales with mass to the power 2/3 only (simplifying a bit). And drag scales with this frontal area linearly. So I'd still say, all else being equal, heavier is faster downhill :-). But please tell me if I'm overlooking something, I'll be happy to stand corrected!
But anyway, methinks this was supposed to be about kayaking :-).

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Re: Cruising inertia

Post by pathbrae » Fri Feb 03, 2017 11:21 pm

Without peddling,
The physics sez - the skinny bloke should be quicker downhill.
The reality sez - the fat bloke pics up speed faster and attains a higher descent velocity.
I don't understand it - but it invariably seems to be true (and I don't think it has anything to do with kayak speed - but who knows?)
So much sea - so little time to see it.

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JB-NL
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Re: Cruising inertia

Post by JB-NL » Mon Feb 06, 2017 9:51 pm

twopigs wrote:
Kris_D wrote:I don't think we have the equivalent of downhill cycling, where extra mass might indeed help because more gravity = more forward force and with a similar air resistance (= more backward force) these two should come into an equilibrium at a higher speed.
Just my ideas! Kris
Please don't repeat this to anybody teaching Physics ...

Obviously when teaching Physics we start by ignoring air resistance .....

Gravitational potential energy = mgh
which is converted to kinetic energy as we descend the hill = 1/2 mv squared.

Notice m - the mass - appears on both sides of the equation - so extra mass does not result in extra speed.

When we factor in air resistance I'd suggest a cyclist with less mass might be subject to lower air resistance - think slim man in Lycra versus Mr Blobby.

So it might just be that slim man in Lycra ends up at the bottom of the hill with more speed than Mr Blobby (on the same bikes).
Where do you get the Mass is on both sides of the equation?
Kinetic energy is what it is about: Ek = 1/2 · m · v2

So the more mass we have, the more Joules we need to reach/maintain a constant speed, given similar boats and circumstances.

If you have any doubt, fill up your kayak compartments with water and try to maintain your normal cruising speed.
For what other reason are racing machines ( incl K1 racing kayaks) built as light as possible?
JB
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Re: Cruising inertia

Post by Hengle » Tue Feb 07, 2017 1:03 pm

Hi All

Racing boats are not built as light as possible, e.g. they are usually built to the rules 12kg for a sprint K1 and 8Kg (ICF rule) for a Marathon K1.

Several years ago one of the London Clubs did several tests using K1's of the same design but varying weights with the same paddlers. the results were if there was no portage, that the heavy boat was faster possibly due to it being less lively and therefore the paddler can work harder. Acceleration was greater in the light boat but after 6 or so strokes the heavier boat comes through, therefore over the course of a 500m circa 200 strokes then 12kg is faster than 8kg.

Obviously as soon as you portage, or you increase the weight a lot, or the boat is scratched, or the seat is uncomfortable, or the conditions vary, or, or, or, the above would cease to be untrue.

Boat stiffness is also a large factor both for how stable they feel, and their speed. Stiffness will vary due to construction, quality, age etc, for example you can build both a Glass or carbon boat to 8Kg but the carbon one should be stiffer, also choice of resin will have an effect on stiffness.

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norb
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Re: Cruising inertia

Post by norb » Tue Feb 07, 2017 5:57 pm

I have opportunity paddling the same model of kayak in two different version.
One is a standard weight and another is super light.
Lighter boat is faster .I have no doubt about it.Is faster on sprint and distance.
Im not so sure if I want empty super light boat in strong wind condition but that is different story.
In calm good weather personally I will go for light boat.
Light is faster and more efficient
Norbert

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Re: Cruising inertia

Post by PSK » Tue Feb 07, 2017 10:41 pm

A bit rough and ready but added weights to the list of my short TT results:

http://www.performanceseakayak.co.uk/Pa ... alBBJW.php PROBS - THIS ONE DOESN'T WORK AT THE MO

All weight listed entries are for a Taran

For a longer distance, but with more sketchy records:
http://www.performanceseakayak.co.uk/Pa ... Puffin.php

Need to root through some older records to fill the gaps with this one.

Read into it as you see fit.

JW

PSK
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Re: Cruising inertia

Post by PSK » Tue Feb 07, 2017 11:19 pm

Apologies, can no longer edit the previous post - this should do it, perhaps, sort of, well hopefully...


A bit rough and ready but added weights to the list of my short TT results:

http://www.performanceseakayak.co.uk/Pa ... alBBJW.php
All weight listed entries are for a Taran

For a longer distance, but with more sketchy records:
http://www.performanceseakayak.co.uk/Pa ... Puffin.php

Need to root through some older records to fill the gaps with this one.

Read into it as you see fit.

JW

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Re: Cruising inertia

Post by Jim » Wed Feb 08, 2017 1:51 am

Best not put my old long course time in - my boat was 19.5kg at the time, but I clearly wasn't (and still am not) capable of driving it at the same speed as you and Aled, with or without the F6 headwind.

I should probably weigh it - the only addition is the electric pump so probably a bit over 20kg now.
Since I decided to chase the leaks on my WWR (long story) I have been doing FW sprints in my Taran instead for the last few weeks. In my WWR K1 I was sometimes able to stay level with Rachel in WWR C1 by the end of 200m. In the Taran I can sometimes finish ahead, or sometimes I hit the wall after about 150m and almost stop.
It is not the extra weight/glide that allows me to go a bit faster some of the time, it is the extra 4 feet of length, rudder so I'm not edging to steer, and stability so I am not unintentionally edging the boat trying to put the power down. I suspect the extra weight is the reason that on some runs I run out of steam before the end.

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