Kayak design, how to determine handling in wind?

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Irish Sea
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Kayak design, how to determine handling in wind?

Post by Irish Sea » Thu Sep 21, 2017 3:19 pm

Hi folks!

So I've been playing around with a design program for woodstrip boats (kayak foundry) for a while now and have come up with a couple of designs that I think should be fun to paddle. I'm happy with drag numbers, stability curves, volume, freeboard etc.

The one thing that I haven't really been able to figure out is how to determine how a certain design will handle crosswinds?

It doesn't have to be super precise but is there a rule of thumb to determine how much a boat will weathercock / leecock when neutrally loaded?

I obviously want to avoid leecocking at all costs but an excessively weathercocking boat wouldn't be pretty either...

I think I understand how to change the weathercocking characteristics by adjusting the hullshape (e.g. more or less V in the hull in the ends or giving the boat more or less front/stern rocker, built in skegs etc.) but how do I determine how much is enough?

Any input welcome!

Cheers,

Bjorn
Last edited by Irish Sea on Thu Sep 21, 2017 3:35 pm, edited 1 time in total.

twopigs
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Re: Kayak design, how to determine handling in wind?

Post by twopigs » Thu Sep 21, 2017 3:32 pm

To a first approximation it must depend upon the projected area of the boat presented to wind ....... So you want the projected area in front of the axis of rotation to be equal to the projected area behind the axis of rotation.

Here is a thought ... could you mount a skeg at the bow as well as the stern?? And just deploy the one that prevents cocking!
Canoeing - bigger boat, broken paddle, more skill!

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nickcrowhurst
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Two skegs on my Shrike

Post by nickcrowhurst » Thu Sep 21, 2017 7:01 pm

For "twopigs", here's my Shrike with two skegs, used to balance the kayak, particularly when fitted with her sailing rig. The bow skeg is used alone when going to windward, and the stern skeg alone when sailing downwind.
Nick

http://imageupper.com/i/?S1200010050011 ... 6721863536

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Jim
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Re: Kayak design, how to determine handling in wind?

Post by Jim » Thu Sep 21, 2017 8:48 pm

Never used kayak foundry (I downloaded it once), in autohydro I can get a print of lateral plane status for current draft and trim which displays the underwater and above water lateral centres of area. If wind areas are also defined they are included in the above water lateral areas in addition to the portion of the hull above the waterline.

If you can't do that sort of thing in kayak foundry, build an upside down model and set it to the reciprocal draft and use the LCB value from each model as an approximation of the LCP for hull and deck.

If LCP deck is forward of LCP hull, the bow will swing away, if is is behind the stern will swing away. If you are adding a lifting skeg it will move LCP hull aft when deployed.

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Re: Two skegs on my Shrike

Post by twopigs » Thu Sep 21, 2017 8:57 pm

nickcrowhurst wrote:
Thu Sep 21, 2017 7:01 pm
For "twopigs", here's my Shrike with two skegs, used to balance the kayak, particularly when fitted with her sailing rig. The bow skeg is used alone when going to windward, and the stern skeg alone when sailing downwind.
Nick

http://imageupper.com/i/?S1200010050011 ... 6721863536
Genius! So it wasn't such a crazy idea!
Canoeing - bigger boat, broken paddle, more skill!

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nickcrowhurst
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Re: Two skegs on my Shrike

Post by nickcrowhurst » Thu Sep 21, 2017 9:24 pm

twopigs wrote:
Thu Sep 21, 2017 8:57 pm
"Genius! So it wasn't such a crazy idea!"
Uffa Fox did it one hundred years ago: http://www.intcanoe.org/iclife/hist/fox ... owned.html
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rockhopper
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Re: Kayak design, how to determine handling in wind?

Post by rockhopper » Fri Sep 22, 2017 12:20 am

Didn't one of the main manufacturers do a version of a kayak with an additional front skeg for sailing?....possibly P&H...

Rog.

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nickcrowhurst
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Re: Kayak design, how to determine handling in wind?

Post by nickcrowhurst » Fri Sep 22, 2017 6:08 am

rockhopper wrote:
Fri Sep 22, 2017 12:20 am
Didn't one of the main manufacturers do a version of a kayak with an additional front skeg for sailing?....possibly P&H...

Rog.
Yes, for example see Douglas's blog here: https://seakayakphoto.blogspot.co.uk/20 ... 5.html?m=1
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Re: Kayak design, how to determine handling in wind?

Post by PeteW » Fri Sep 22, 2017 8:11 am

When considering wind impact - don't forget yourself if you are not sitting exactly at the axis of rotation - you may have a short lever but you are a lot bigger and more importantly higher into the wind than the deck is likly to be. Thinking back to some of the 'not so golden' oldies as the Seahawk etc. where you sat well back in the boat to see the impact - not so common now.

Irish Sea
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Re: Kayak design, how to determine handling in wind?

Post by Irish Sea » Fri Sep 22, 2017 9:28 am

PeteW wrote:
Fri Sep 22, 2017 8:11 am
When considering wind impact - don't forget yourself if you are not sitting exactly at the axis of rotation - you may have a short lever but you are a lot bigger and more importantly higher into the wind than the deck is likly to be. Thinking back to some of the 'not so golden' oldies as the Seahawk etc. where you sat well back in the boat to see the impact - not so common now.
Yes, Pete

that's actually one of the things I've been thinking about. How to account for the "sail" effect of the Paddler's body?

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nickcrowhurst
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Re: Kayak design, how to determine handling in wind?

Post by nickcrowhurst » Fri Sep 22, 2017 10:28 am

Bjorn, as you well know, CAD systems do not design the kayak. They are reliant on the software engineer and her choice of algorithms to give you some analysis of your design choices. I only use CAD with my designs after I have produced, tweaked and measured a full-size paddling prototype, and the CAD then enables me to distribute the plans to the world as internet downloads. That way I keep total control of the design. (BTW I have 40 years of computing experience, including creating a suite of programs for a major UK sail-maker to design yacht sails, including spinnakers and genoas. However, I based the software on interviews with craftsmen who previously did the task manually and laboriously. I had no input to the creative task.)
To answer your question, I would build the kayak you have so far designed, but leave the position of the paddler's body in the cockpit to be determined by sea trials. I would consider a temporary deck arrangement to permit this. Adjusting the position of the comparatively heavy weight and windage of the paddler will help compensate for any less-than-ideal choices you make. Shifting the cockpit and seat forward or back will change the trim of the kayak, and hence the relative areas of the bow and stern both above and below the waterline, which are your current concern.
The alternative, as I see it, is to spend years gaining the hands-on knowledge and experience of great designers like Aled Williams and Nick Schade.
I'm not recommending that you follow my procedure, but I thought it might be of interest.
Nick, of www.cnckayaks.com

Irish Sea
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Re: Kayak design, how to determine handling in wind?

Post by Irish Sea » Fri Sep 22, 2017 11:29 am

Thanks, Nick!
I was actually starting to think along those lines as well.
I am planning for a recessed cockpit so need a rather big cutout in the deck anyway to acchieve that recessed area. I could figure out a way for a temporary cover and seating arrangement and go for sea trials. That's probably the most sensible option. With my current hull shape I'm pretty sure its going to weathercock somewhat. The "how much" I can adjust by boat trim (moving seat) or potentially adding a smallish fixed skeg. I'll probably be adding an adjustable skeg anyway so I suppose I shouldn't worry too much about weathercocking.

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Re: Kayak design, how to determine handling in wind?

Post by Aled » Sun Sep 24, 2017 11:55 pm

For a skeg boat, place the CLA between 6cm and 8cm behind the LCB, making sure that Design and Target Displacements are close to equal

Irish Sea
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Re: Kayak design, how to determine handling in wind?

Post by Irish Sea » Mon Sep 25, 2017 10:14 am

Thanks, Aled!

That's a number I can work with :)

I'm pretty close as it is. Need to move the CLA slightly further backwards and I'm sorted. Fine adjustment will probably still be done by sea trials as suggested by Nick.

Cheers,

Bjorn

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Robert Craig
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Re: Kayak design, how to determine handling in wind?

Post by Robert Craig » Fri Sep 29, 2017 1:08 pm

Remember that the centre of lateral resistance varies with speed through the water - the faster you go, the further forward it shifts.

Irish Sea
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Re: Kayak design, how to determine handling in wind?

Post by Irish Sea » Mon Oct 02, 2017 12:36 am

Robert Craig wrote:
Fri Sep 29, 2017 1:08 pm
Remember that the centre of lateral resistance varies with speed through the water - the faster you go, the further forward it shifts.
True, but as the boat will likely be equiped with an adjustable skeg I should be able to compensate for that effect.

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Robert Craig
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Re: Kayak design, how to determine handling in wind?

Post by Robert Craig » Mon Oct 02, 2017 9:37 am

Exacrtly! That's why it has to be "adjustable!"

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