Kayak stability

[Andy B]

Hi there, Do I need to specify location of CoG to get accurate cross curves? How does Freeship calculate CoG? I am assuming it is the CoG of the ship only, but if I require CoG higher up, ie for a racing kayak design, so how would that be achieved? Also I am using Michlet and Godzilla for the same project with the default CoG at waterline and using Beam and GMT to control stability but would it be better to change the Ship loading formula parameters to raise CoG? I have tried to do so using the formula KGOA = a*(TOA + b* LOA^c , but if my a value is over 1.0 the program does not like it. I tried to get CoG at 0.3 from keel line with draft of 0.065 using a = 3.615, b = 0, c = 0.Thanks in advance, and many thanks to those who have made these programs available to us mortals.

[Rick Willoughby]

The cross curve is only part of the equation. You need to know the height of the CofG (KG) and then do the geometry through the range angle.

No need to fiddle with ship loading formula. Freeship gives the metacentric height above the keel. If you leave Michlet as is then the height is above the waterline. Difference between Michlet and Freeship is simply the draft.

A racing kayak is marginally stable. The paddle is used to maintain balance. For an ocean kayak the KMT is around 450mm.

Rick W

[Andy B]

Thanks Rick, it is probably the stability curve I am most interested in since only one displacement is needed (by the way correct stability is rather critical in this particular project). I know what height I want CofG but Freeship sets a different height. I don't know how to calculate stability manually and maybe laborious with design evolving so often. I was hoping to use the Freeship KN Sin curve. What are the other parts of the eqaution?

[Petros]

I would not waste time trying to reinvent or alter large boat design software to work for a kayak.

At the link below is free kayak design software that is used by Mariner kayaks to develop their own designs, and it already gives you the information you need for a kayak.

There are two programs, one for hull design to minimize drag, locate CG position, etc, and the other to determine roll stablity (with curves and examples). With instructions too. Matt at Mariner kayaks is also very helpful to answer questions as well, I have known him for many years and is helpful if you are not too abusive of his time.

http://www.marinerkayaks.com/mkhtml/downloads.htm

Good luck.

[Rick Willoughby]

Freeship provides a value for KMT in one of the reports it produces.

If you CofG is lower than the transverse centre of roll i.e. KG>KMT then the boat will be inherently stanble when on even keel. You need to use the cross curve for different angles of roll to see if it remains stable once heeled.

You can set the minium GMT as a constraint for Godzilla and it will give you the best underwater shape for that value. You can then play with the topsides to improve stability through the range of control.

Both Michlet and Freeship produce useful results for determining initial stability. Answers are identical providing you adjust for the difference in reference point.

Rick W.

[Andy B]

Thats what I'll do. And thanks for the link - I have Bearboat Pro and Matt Broze's spreadsheet, great program. I don't find Kaper as accurate as Michlett, 1% less drag can make a difference after 2 hours of racing. Cheers - Andy