stability with speed

[ancient kayaker]

You may be right. I know it doesn't take a lot to stiffen up a canoe or kayak considerably. When I take a rest from paddling during a long (for me) trip my usual way is to stick the paddle across the gunnels and hook a leg on top. Then I can relax or even sleep, the boat feels rock solid even though the buoyancy of the blade is only a pound or so. Of course I can sleep anywhere in almost any position, genes from a naval family I suppose.

[DBM]

It is pure dynamic stability. At anchor I almost fall overboard walking from side to side in a 19 foot V bottom. At speed it is rock solid. I noticed years ago when I was in a flat stern round bottom canoe with a 9.9 outboard (fun). At rest it will tip over if you stand up wrong. Opened up you can't make it tip over.

[philSweet]

The reduction of yaw with increasing speed is a design factor. It is not difficult to design a boat that trips over itself as speed increases. It has to do with how the center of resistance is affected by speed and yaw angle as well as the magnitude of the damping force and paddle cadence. The latter two generally do help reduce yaw excursions; but I believe it is primarily a design feature.

[DBM]

I believe that the design is of no consequence so long as it is symetric. I think that this subject is more related to fluid dynamics that hydro dynamics and I have no background in fluid dynamics.

[ancient kayaker]

I have no idea what design factor Phil is referring to, but my canoe which I designed and built shows very similar behavior in this regard to some of my other boats, although they are quite different in other respects.

The canoe has a skeg and it takes a lot of work to turn at any speed, or even at rest. It hardens up further under way.

My recently completed sailboat can also be rowed, and that also stiffens noticeably under way. It is the opposite of the canoe, short and fat, no skeg, and at rest it can literally be spun end to end with 2 pulls on an oar. When starting I have to be careful to pull evenly to keep her straight. However, once under way holding a straight course becomes much easier, and it needs several pulls on one oar, at least a half-dozen, to execute a full turn around.

The only thing I can think of which makes any sense at all is that the blade's force on the water decreases at speed, but I don't believe that is the whole story. Petros' vortex theory is the most likely explanation I have heard. The rowable sailboat has a single 72 deg chine and demonstrates the effect most strongly, the shortest of the kayaks has a rounded bilge but it has 2 sharp-cornered grooves along the bottom and it also shows the effect clearly. The canoe has 2 chines at 25 and 46 deg and shows the effect somewhat less.

My longest boat is another kayak which also has a rounded bilge but a flat bottom and I don't recall really noticing the effect on that boat, however in the interest of full disclosure I don't have a lot of time in that boat as the homebuilt canoe is a much nicer boat.