Primary stability considerations in kayaks

Good grief ! You didn't believe the argument before ?

Get two kayaks or canoes with different lengths and actually test them !

Cchippo check out post #11.

 

Just to get back to something which might be relevant to what the OP actually wants, I did play around with checking out differences in stability when scaling length for a kayak design.

The only change in the two examples pictured is in length. Sections are identical. CG height above base also wont change noticeably, since the person weights so much more than the kayak. The longer boat floats higher in the water, and also has more stability.



Now obviously you can't go too long or the thing will get silly, but you can get some stability advantage by increasing length.

 

Even though length does add more righting moment, the correct way would be to add width. If I remember right the righting moment goes up log with width while it would linear with length, so you gain a lot more by width for the same material.

There is something else to consider. A coke tin has been calculated for the optimim diameter and tin height to use the minimum material, but have the most volume ! Any one tried this with small canoes and kayaks ? Lightest with the most buoyancy... ?

 

That would be half an sphere. Low wetted surface, but a lousy boat.

 

Ah, but you can change it so it would still be optimal. No use going around in half a tin

I think everyone has been looking for the optimal in everything... amazingly we haven't found the ideal yet, or every one would have that one

 

When I was making canoes what I read talked about primary and secondary stability and that all boats were a compromise between the two. The flat bottom on the right would have a lot of primary stability, in that it would be more stable in static conditions ie flat water. Secondary stability comes into play with waves, the round bottom on the left would have a lot of secondary stability, as it wouldn't be tossed around by waves so much as the flatter bottom. So beam itself adds stability, but so does hull shape. As far as being easy to paddle, entry and exit shapes have a lot to do with that, as well as minimal rocker and length.

 

You're assuming the KG remains the same, which is doesn't. It is the main variable which affects the stability and cannot be assumed to remain constant. That was the purpose of the GZ curves I posted. The lightship KG is assumed no change, but it will; how much is unknown. The KG (double length hull) in the full load is much lower but the stability is almost unchanged. If the KG remained the same for both hulls, the stability would be worse in the longer hull.

The moment is wholly dependent upon the location of the KG, regardless of hull form.

 

Going back to the specifics. I've just dug out an old paper on Kayak design (I personally don't design them). One option they used to increase stability was the addition of a small double bottom. In the boat used called "Duet" the DB measured 1.5inches in height and extended for 10.0 feet It added 110lb of salt water ballast (ok a bit extra drag/paddle power required). The improvement on the stability curve is impressive; for a kayak. (No surprises really, just adding weight low down). They increased the range of stability from 68 degree to 77 degrees, and the righting moment from about 93 ft-lbs to 120 ft-lbs.

Hope you understand the archaic imperial system!

 

I once built a 10 lb folding kayak that had 24 in beam and a 10 ft long. It was kind of tippy for the average kayaker because it was so short. I put two stuff sacks full of sand in about where your feet would go and it make it very manageable for most paddlers. Without it most would flip right over.

The surface area (and the weight) of the skin and frame goes up with a square of the length, so a short kayak will weight a lot more than a longer one. but the short was was far move tippy than a similar design that was longer. putting weight down low made a large difference.

 

The stability of a Kayak is in the back of the operator and his ability to paddle and predict tipping motion and how and when to make alterations.

Like a bicycle--it has no stability without a rider.

 

A8' long, 4' beam dinghy will capsize if a person steps on the gunwale to board. A 30' long, 4'beam canoe can't be capsized by one person, even hiking out. This is how a real hull behaves.

 

Sorry that's too simple

If you make a 8ft x 4ft dinghy out of steel, so its heavy and deep, you can safely stand anywhere on it. Even made in 3mm ply a 7ft 10in dinghy is more stable than a 30ft canoe, see photo

If you make a light semicircular canoe 30ft long it will have difficulty staying upright even with on one on board

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

You are standing inside and on the bottom. I am talking about hiking out or standing on the gunwhale.

 

I built a 20', 28" beam 2 chine "pirogue" with no rocker and little flair as I wanted a boat I could cut into 5 pieces to nest and throw in the trunk of my car. The middle section is the largest at 5' and it only adds ~1 inch of beam. The boat can also be configured with 4 pieces, i.e. a ~15' hull. This means I can easily test the configuration at 15' versus 20'.

I can tell you that in practice adding the middle section creating a 20' hull makes a world of difference in increasing "stability" all abstract considerations aside.

 

Cthippo has inadvertently caused an international incident. Respondents from Japan, Thailand, South Africa, UK, Canada, US, etc. dived right into the flap. Great fun! Congrats to you all for maintaining gentlemanly decorum.

SInce I do not know much and wish to learn, I will ask a simple question. Is it not true that a disturbing force will be resisted by the sum of all the individual section moments? If that is true, is it not also true that a longer hull, with a geometricly similar planform, will have a larger sum of moments? Assume that displacement and BmWL, are the same and that draft will be the variable.

What am I missing here?