Boat Speed

Kayaks do not seem to follow the old ship building rule for hull speed.  For simple displacement hulls the maximum speed in miles per hour is easily determined by taking the square root of the water line length and multiplying it by one and a half. This works pretty well for all ships and the speed is easily attained with about 1 horsepower per ton.

So a I had a kayak with a little more than 16 feet of length which would equal about 6 mph max speed.  The problem is it weighs about a ton with me and all the gear so I can only touch 6 mph on the gps ever now and then.

This simple equation does not explain how I was able to easily paddle a Hurricane Cat 5 kayak with about a 15 foot waterline length to over 6.5 mph.  Nor does it explain how friends with racing kayaks paddle miles and miles at about 7 or 8 mph.

I read hydrodynamic design books that discuss speed to length ratios and skin drag and other issues, but I've yet to see a good model of what is happening with surfski and racing kayaks.  Don't even get me started on those flat bottomed 12 foot SUP's!  There is no way I can explain how a wide flat 12 foot boat can go so fast in flat water.

Once boats are up to planing speed the equation changes and about 100 hp per ton is required. I can understand planing on a wave.  Planing hulls should actually be slower than non planing hulls when they are not on a wave, because of the extra skin friction.

And yet whitewater boats with a Displacement hull speed of less than 4.5 mph regularly go upriver against faster current than this.  How do they do it?  Between the eddies there is fast flowing water they must overcome.

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