TexasKayakFisherman.com est. 2000

Kayak fishing the Lone Star State...


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By bowgarguide
#1349092
Ok I have some questions
Why 300 lbs at 4 inch water line
Is stability increased with a deeper sitting hull say 4 inch compared to 1 inch
Ron
User avatar
By gerald
#1349115
You have to have a starting point from which to design certain small craft. 300 pounds consists of the boat, paddler, and anticipated cargo. As mentioned, for a skinny racing kayak I'd consider something like 200 pounds @ the 4" waterline. A 4" waterline is generally accepted as a starting point for small craft such as canoes and kayaks--considering the widths and lengths we are working with. I could design a 16' boat with a 1" waterline that would support 300 pounds but it would be 4' or so wide--maybe more. Not even close to what we want--unless you're wanting to stand up and slowly pole the flats.
Just one more factor is that most rules for competitive events involving paddled or rowed craft call for the measurements to be made at the 4" waterline.
There are even more reasons but these should suffice.
User avatar
By Light Keeper's Kid
#1349131
Wouldn't you want a racing hull to be sitting out of the water do to less drag :?: If I'm understanding this your saying that in order to get this is a trade off because of hull width :?:

Mike
User avatar
By gerald
#1349180
For a race boat you want the most speed with the least drag possible. Thus you have a long skinny hull with a shape that cleaves the water in front with narry a flutter, allows the water to gently caress the hull like the hand of a satiated lover in the after glow of the moment, to linger with the image as the water departs the stern with nary a ripple...

uuuhhhh....where was I?

Ah yes. Because of the skinny little hull with a heavy paddler the boat is going to rest in the water--tadah!--nearly 4". All designers fudge on that 4" waterline. So, now we have this image of a long, skinny hull that allows flow around it (actually the hull mostly displaces the water, though there is some flow--laminar and other wise). Hopefully we can understand that, but we haven't discussed the cross section shape--until now. To lessen drag you want the most efficient shape possible (color that the least surface area in the water). That shape is round. Unfortunately round is very unstable. A good compromise is elliptical in shape--which I generally use in most of my boats. The compromise for most race boats--and in this case we're mostly talking about sprint type or K1 kayaks--is a parabolic shape. The parabolic shape is generally round on the bottom where the water is but gives you a little flare for secondary stability. But don't be fooled. A race boat takes some time to learn how to paddle. Some people say you have to be born in a k1 to be able to paddle one....

All of this is generally beyond what most of us need to know about boats, but it's nice to have a little background....so we can understand what motivates us...
User avatar
By gerald
#1349982
Well....it appears that I've covered this little question to everyone's satisfaction...


...or something....
User avatar
By bowgarguide
#1350069
Ok now the stability same boat at 1 inch 2 inches on up to four inches.
Ron
User avatar
By TexasZeke
#1350093
feel free to correct me if I'm wrong.....

Using either of my boats for the test, both of which have a water line of between 3 and 4 inches. My understanding of water line is the depth of hull below the water. So if the boats would run with a water line of one inch both would be faster, but at the same time both would be harder to control wouldn't they. Seems like with one inch below the water and nine above it wind and outside forces would play a much greater roll. Without the added hull surface below/in the water tracking would be adversely affected because without the friction of the water to help hold the boat straight it would wander around or be blown around by the wind. If you could build a boat with a 1" water line 4" sides would be best because of the lower profile for wind.

or have I misunderstood all of this

kevin
User avatar
By gerald
#1350131
bowgarguide wrote:Ok now the stability same boat at 1 inch 2 inches on up to four inches.
Ron



Whew!.... Ok. If you know of any 10" tall men who weigh about 10 pounds you can have the same boat that you or I would paddle at the 4" waterline. Regardless of the size of the boat it HAS to displace an amount of water equal to the boat, paddler, and all cargo. That little 10" man would have great stability, but as TexasZeke alludes the boat would be subject to wind and other side forces. All boats handle and paddle better at their design capacity. The design capacity is NOT the same as maximum capacity. Maximum capacity is what most commercial boat builders put on their site. Bigger number are more impressive to the masses. A boat designed for 300 pound design capacity at 1" waterline would be HUGE. Surface area would be immense. Friction would be massive. It would be a SLOW barge....compared to a long, skinny boat designed to run at the 4" waterline. An empty canoe has very little waterline, but the wind can easily blow it around. True--it can ride above the majority of the swift water BUT there is no paddler in it. I'ts not going anywhere without a paddler--except downriver.

Lower profiles are better in the wind but this can be alleviated in great part simply by paddling a boat at it's design capacity. Those in canoes who say they are getting blown around a lot usually aren't even close to design capacity. PLUS many of those people don't really know how to paddle or trim their boat....

Ron: What detail or answer are you looking for? If you want to know the shape of the fastest boats look at ocean racing surf skis and/or K1 kayaks. Long and skinny measured at the 4" waterline. Stability is not a factor in these boats. Speed is the primary factor. Heck...in some race boats you can't even sit in them while they are sitting still in the water. They are only stable while moving. A C1 canoe is 31/32" wide (somewhere around there), but due to the shape and intended use it takes about 3 years to become proficient with one. A C1 is a good example of a boat that is "measured" at the 4" waterline but is usually much less.

Stability and speed are opposite ends of the spectrum when it comes to small displacement hulls. You can have a stable hull with a 300 pound load, or you can have a fast hull with a 300 pound load--but you can't have both. Anything in between is a compromise....

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