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how fast should my boat go???????

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  • how fast should my boat go???????

    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?


  • #2
    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.

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    • #3
      Re: hull speed?

      well that explains y it seems like it has good power till it plains out then.... what can i do to pick up 5-10 mph ive got a 21p prop now n it plains out pretty quickly

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      • #4
        how fast should my boat go???????

        boat is a 96 20ft gw invader bravo es i/o 305 inboard alphaone outdrive.21p prop,did some research on boat and its supposed to weigh w not people or gas or anything in it 3,225lbs i believe

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        • #5
          Re: hull speed?

          rpm is 3800-4000 @wot not sure on gps...how acurate is gps?

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          • #6
            Re: how fast should my boat go???????

            I'd say 40-50mph depending on load and prop. What's your current WOT rpm and top speed?
            2005 Bayliner 175 w/ 3.0L Mercruiser & Alpha One Gen II Outdrive
            2003 GMC Sierra w/ 5.3L Vortec

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            • #7
              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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              • #8
                Re: hull speed?

                ok 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?

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                • #9
                  Re: how fast should my boat go???????

                  rite now 4000 sometimes...3800 -4000 on avg n speed by dash says 40ish

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                  • #10
                    Re: how fast should my boat go???????

                    Originally posted by scon View Post
                    rite now 4000 sometimes...3800 -4000 on avg n speed by dash says 40ish
                    boat speedometers can be somewhat accurate, but they also can be wildly inaccurate. The ONLY way to start comparing speed is to use a GPS.

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                    • #11
                      Re: hull speed?

                      Originally posted by scon View Post
                      well that explains y it seems like it has good power till it plains out then.... what can i do to pick up 5-10 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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                      • #12
                        Re: hull speed?

                        Originally posted by scon View Post
                        ok 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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                        • #13
                          Re: how fast should my boat go???????

                          WOT @ 3800-4000 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.
                          GL
                          Compression, Spark, and Fuel is all you need!

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                          • #14
                            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 speed---and 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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                            • #15
                              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.
                              A displacement hull will not try to climb on top of the water. That's called planing. Powering a displacement hull past it's hull speed will push the bow through the standing wave and create another standing wave.
                              Any opinions expressed above are worth exactly what you paid for them!

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