waterline and speed

I read that the speed of a boat is proportional to the boat length at waterline. Does anyone know if this is only for flotation length. Background, thinking out of the box alittle: I want to keep the boat short as possible for the reasons of weight, tranportation and storage in an appartment. What if I add an attachment to the front that could be put in the trunk. Does it have to have a closed bottom to provide flotation or could it have just two walls and no bottom?


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RE: waterline and speed

Jim,

The speed vs waterline length relationship is for displacement boats. The theory behind it is that since the boat is pushing through the water, it can only go as fast as the water can be pushed out of the way (since boat and water can't occupy the same space at the same time). The displaced water forms a wave. The water can only move as fast as the wave moves. The speed of wave movement depends on the square root of the wavelength - the longer the wavelength the faster the wave can move. And finally, the max wavelength is determined by the length of the boat.

So putting it all together, the longer the part of the boat that produces the wave, the faster the boat can move - in theory. In real life it's nowhere near that simple. The drag produced by the boat subtracts from the speed. The shape of the hull influences how efficiently the wave is produced. The angle the boat floats at determines drag. And on and on and on. So waterline length is only one part of the equation. But to answer your questions directly:

1. The "attachment to the front" is actually not that far out of the box. If you look at large ships you'll see that they have bulbous prows below the waterline to influence the way they displace the water. It's not exactly the same, but definitely in the same family as what you're suggesting.

2. Since you're trying to influence the displacement, the attachment has to actually displace water. While an open bottom and top could do that, you may run into water flow issues with an open bottom which could produce suction and actually slow your boat down. The most sure bet is a closed bottom.

Finally, if you're looking for improved storage, handling and transportation, the better solution would be a take-apart/nesting boat. This is because a small boat with a long nose would still be a small boat. The extension would cost materials ($$$), time and weight but provide only a tiny increase in speed, if any. (Remember, to double the speed you have to make the waterline length 4 times longer.) For typical small boats that's just a bit over 1 mph and in the meantime you're still sitting in a small cockpit.

Compare this to a larger boat that's designed to be taken apart. Out on the water you have the space, storage capacity and buoyancy of a larger boat, along with any speed increase. In storage, transport and handling you have 2 (or more) smaller pieces that are easier to lift, move and store. If it's a nesting boat, all you need for storage space is something the size of the largest piece.

So while your idea does have some merit, for a small boat there's other factors that come into play and make it less useful. That's why I think a take-apart nesting boat would serve your purposes better.

Laszlo

 

RE: waterline and speed

Amen, Laszlo!

RE: waterline and speed

Hi jimqr, 

everything that Laszlo said and let me add a summary of another way to think about the consequence of Laszlo's comments.

in summmary:  you cannot determine the fastest hull shape/length without first determining the power that will be applied.

so a boat that is fastest for a strong/athletic paddler of x weight applying power of "a" is not the same hull that would be fastest for a weaker paddler of x weight  applying power less than "a".

as a practical matter, for most recreational paddlers paddling a standard style sea kayak who are cruising at a relaxed power output, the 'fastest' boat at that power setting is going to be around 13 feet waterline.    for a fit paddler who wants to move along you get to around 15 feet typically....and fit racers often push 18 feet waterline length.

there is a really good thread on this topic from a couple years back....

https://clcboats.com/forum/clcforum/thread/50725.html

 

h

 

RE: waterline and speed

   thanks for all the replies

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