someone told me today that my hull cant exceed a certain speed is this true and how do i find out what that speed is?
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how fast should my boat go???????
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Re: hull speed?
Hull speed is generally described as the square root of the length at the waterline in feet multiplied by 1.34 and the answer is given in knots. It's only meant for displacement hulls.
So if you have a boat that's 25 feet at the waterline, multiply the square root of 25 which is 5, times 1.34 and you get 6.70 knots. That's the fastest the boat should be able to go in displacement mode. Once you exceed that speed, the boat is starting climb on top of the water.
Once on top of the water, the top speed is limited by drag and HP. Drag increases with the square of speed, so it takes 4 times the HP to double speed, on plane.

Re: hull speed?
GPS is way more accurate than the one in your dash!
RPM sounds way low. Like you need to drop a prop pitch or two. before you do that, you'll need to make sure the thing is running good.
What is the gear ratio of your outdrive? And, you'll definitely need to post an accurate GPS speed for good advice.
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Re: hull speed?
Originally posted by scon View Postwell that explains y it seems like it has good power till it plains out then.... what can i do to pick up 510 mph ive got a 21p prop now n it plains out pretty quickly
What do you need to do in that boat to pick up 10 mph? I'd say doubling your horsepower would get you 10 mph. (assuming your engine is healthy and running the correct prop)
It takes A LOT of horsepower to gain a very small increase in speed. Someone else used this analogy, I'm just stealing it. In high performance cars going down a quarter mile, you need to increase HP a huge amount to gain a couple tens of a second. Increasing speed in boats is a similar concept.
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Re: hull speed?
Originally posted by scon View Postok will do i have gps in my truck that i can take with me ill repost this again next saturday night...if i drop pitch on the prop will my rpm pick up so i dont lose top speed?
You need your engine to put out as much HP as you can. That generally means running at the max rated RPM. If you going for top speed, you can adjust diameter and pitch but still maintain the rated RPM.
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Re: how fast should my boat go???????
WOT @ 38004000 is way too low!
You need to reprop with maybe a 19 pitch or even a 17 to get your RPM's up to ideal speed!
Make a post in the prop section of the forums and people there can help determine the rite prop for you.
I think 21 is a little to much for your setup.
Hope that helps.
GLCompression, Spark, and Fuel is all you need!
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Re: hull speed?
Too many unanswered variables to answer your question about gaining more speed but I can give you a concrete example: On my 14 foot speedboat, 90 horsepower gave me 55 MPH. Increasing horsepower to 125 gave me 60 MPH. So a 35 % increase in horsepower yeilded a 10% increase in speedand I was happy with it.
However, before you go the horsepower route, you must maximise performance with what you have: in general, make sure the engine is running at peak efficiency, experiment with propping, and trim out the hull so it acheives its maximum speed. Then play around with adding horsepower.
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Re: hull speed?
So if you have a boat that's 25 feet at the waterline, multiply the square root of 25 which is 5, times 1.34 and you get 6.70 knots. That's the fastest the boat should be able to go in displacement mode. Once you exceed that speed, the boat is starting climb on top of the water.Any opinions expressed above are worth exactly what you paid for them!
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