TexasKayakFisherman.com est. 2000

Kayak fishing the Lone Star State...


User avatar
By DarrellS
#1279546
I went fishing on the Brazos yesterday (see FW Board). It was the first time that I had taken my GPS along with the Strip Boat. The boat really likes to cruise at 3.7-3.9 MPH , but tops out at 5.1 MPH. I am not complaining but why is the top speed only about 1.2 MPH faster than the cruising speed. I am just trying to learn the dynamics of friction and such.
User avatar
By gerald
#1279579
Good question--and there's a good answer. Just for a matter of reference we'll go with the formula for potential top speed as 1.34 times the square root of the water line length. This is in knots but I always say mph...and it is really more of a reference figure than a hard figure set in stone. So--for a 16' boat (we'll ignore the waterline length because 16' is so easy to work with) the top potential speed is 5.36 mph. None of us go around paddling at top speed. I like to say that the cruising speed is ONE times the square root of the waterline length. That would mean 4 mph for your boat (it is 16'--isn't it?). Now--you CAN go faster than the calculated potential speed IF you have enough horsepower. Most people don't. And the effort required increases exponentially as you attempt to go faster. Your boat is wide so it's going to start feeling a lot of drag at the higher speeds. That's where you're hitting the wall at 5.1.

The length of the boat generally determines the potential speed of a boat. Other factors determine how hard it is to get to that speed. But remember that the effort increases exponentially and that's why the top speed is really very little more than a brisk cruise. I can paddle at top speed for a minute or two--maybe. I can cruise briskly for hour after hour.
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By DarrellS
#1279588
Close enough. It's 15'10". Thanks for the reply, I think I understand. :idea:
User avatar
By CR
#1279591
Huh? :shock:

just kidding. Very good explanation.

gerald wrote:Good question--and there's a good answer. Just for a matter of reference we'll go with the formula for potential top speed as 1.34 times the square root of the water line length. This is in knots but I always say mph...and it is really more of a reference figure than a hard figure set in stone. So--for a 16' boat (we'll ignore the waterline length because 16' is so easy to work with) the top potential speed is 5.36 mph. None of us go around paddling at top speed. I like to say that the cruising speed is ONE times the square root of the waterline length. That would mean 4 mph for your boat (it is 16'--isn't it?). Now--you CAN go faster than the calculated potential speed IF you have enough horsepower. Most people don't. And the effort required increases exponentially as you attempt to go faster. Your boat is wide so it's going to start feeling a lot of drag at the higher speeds. That's where you're hitting the wall at 5.1.

The length of the boat generally determines the potential speed of a boat. Other factors determine how hard it is to get to that speed. But remember that the effort increases exponentially and that's why the top speed is really very little more than a brisk cruise. I can paddle at top speed for a minute or two--maybe. I can cruise briskly for hour after hour.
User avatar
By bowgarguide
#1279635
Gerald
What are to other factors that influence how efficient a boat hull is?
Ron
User avatar
By gerald
#1279762
bowgarguide wrote:Gerald
What are to other factors that influence how efficient a boat hull is?
Ron


size and shape. How smooth it is can also make a difference. This is not an easy question. Just as in aerodynamics we also have aquadynamics (is that a word?) but it's complicated by the transition of hull from water to air. If a boat is completely under water it's a lot easier. Just picture a big old cigar. A surface going displacement hull can't simply be half a cigar because of the interaction between the boat, air, and water. Efficient also means different things. Do you want all out speed, or do you want a great cruising speed? Most boats that are fast are harder to paddle at low speeds than a boat designed for lower speeds.
The best thing is to use common sense. Is that doohickey poking out from the hull going to cause drag? Damn right. Why have it? The general physics of how boats work have been known for a long time. There are no secrets to learn and nobody is going to make a great new discovery about boat design. We just try to blend subtle variations to get what we want in a particular boat for a particular task.
User avatar
By gerald
#1279826
Night Wing wrote:If I had a GPS, I'd like to know how fast my OI travels.


Your OI will get on up there. It's narrow with a good hull shape--though you have a lot of overhang on the ends that don't contribute to the waterline length. I'm sure I could hit 6.5 for a short time (maybe--old people have delusions of youth sometime). 6mph for sure.
User avatar
By Night Wing
#1279920
Gerald,

All I know is, at 18' long and 21" wide, my OU is very fast. Makes my plastic 14' 9" long, 26" wide SPTW seem like a slug. :lol:
By rodloos
#1279967
I'm just curious if the graphite epoxy coating some of y'all are adding improves the drag coefficient against the water, or does that just help with protection against the rocks etc?
User avatar
By gerald
#1280010
The graphite impregnated coating only helps rocks, stumps, stobs, logs, etc. to slide off the hull easier. It is a sacrificial coating and contributes nothing to speed.

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