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  1. #1
    Seaman Apprentice
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    Default hull speed?

    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. #2
    Lieutenant Commander
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    Default 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.

  3. #3
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    Default 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

  4. #4
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    Default 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

  5. #5
    Seaman Apprentice
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    Default Re: hull speed?

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

  6. #6
    Petty Officer 2nd Class kellmike626's Avatar
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    Default 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

  7. #7
    Petty Officer 2nd Class
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    Default 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.

  8. #8
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    Default 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?

  9. #9
    Seaman Apprentice
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    Default Re: how fast should my boat go???????

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

  10. #10
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    Default Re: how fast should my boat go???????

    Quote 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.

  11. #11
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    Default Re: hull speed?

    Quote 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.

  12. #12
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    Default Re: hull speed?

    Quote 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.

  13. #13
    Petty Officer 1st Class Challenger84's Avatar
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    Default 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!

  14. #14
    Supreme Mariner Frank Acampora's Avatar
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    Default 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.

  15. #15
    Commander 45Auto's Avatar
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    Default 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!

  16. #16

    Default Re: hull speed?

    I have a Bravo 200 ES also. I thought it was one of a kind, I've never seen another. Mine has a 350, and it will run 55 mph at 4300 rpm with a 21 prop. I've thought about trying a 20 prop, and also about putting vortec heads on it to try and spin the 21 a little faster. Overall I really like the boat, the removable fiberglass cover lets me take it from family bowrider to "miami vice" in about 2 minutes!
    Josh

  17. #17
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    Default Re: hull speed?

    Quote Originally Posted by smokingcrater View Post
    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.
    i have been bracket racin for a while now and i know that once u get into a certain et bracket it gets harder and harder to lose more time off the et but this boat is bone stock..i dont know much bout boats so this is why i ask....thanx for anyhelp for the time to post

  18. #18
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    Default Re: how fast should my boat go???????

    Maybe you have your boat in the water too much and need to trim up to bring more boat out and bring rpm's up. Just an idea.

  19. #19
    Lieutenant rwidman's Avatar
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    Default Re: hull speed?

    Quote Originally Posted by scon View Post
    i have been bracket racin for a while now and i know that once u get into a certain et bracket it gets harder and harder to lose more time off the et but this boat is bone stock..i dont know much bout boats so this is why i ask....thanx for anyhelp for the time to post
    Huh?

    Translation please.
    Ron

    2000 Camano Troll
    North Charleston, SC

  20. #20
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    Default Re: hull speed?

    i was refering to racing ....i race 1/8th mile and for example we race a 6 second index and for what we run as far as motor ..to gain any more power it cost $$$$$$$ easier to go for 8 to 7 second than 6 to 5 second...and well this boat is bone stock and i was asking is there certain power adders that make a descent difference in adding power,,and what should i not mess with

  21. #21
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    Default Re: hull speed?

    the racing is on concrete...lol not in a boat........guess i should be more clear sorry

  22. #22
    Moderator QC's Avatar
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    Default Re: hull speed?

    Do some reading over in the I/O section regarding horsepower additions. MOst will say don't waste any money on a 5.0, but that mods to a 5.7 might be worth a little. Read up on Reversion which is sucking water into the intake due to valve overlap and severely limits cam choices and hence limits typical street mods you may be used to.

    In your case you need to focus on proper trim, weight balance, and then dialing in the correct prop. This assumes that the engine is running properly.

    FWIW, we try to keep this section free of engine stuff. Very difficult as proper oepration (trim, balance etc.) does belong here. So the way I see it is: General discussion, here. Resulting focused discussions on the prop, prop section. Resulting focused discussions on the engine, I/O section etc. Good luck!

  23. #23
    Petty Officer 1st Class
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    Default Re: how fast should my boat go???????

    Challenger84,what do you recommend for WOT?And remember,its an inboard not an outboard!

  24. #24
    Moderator QC's Avatar
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    Default Re: how fast should my boat go???????

    OK guys. I merged these two threads as we are all posting the same kind of info, to the same poster, about the same issue . . . how fast should I go.

    There were no 305's (5.0s) in 1996, but there were two in 1995. Very common to have an engine that is a year older model than the boat.

    The 5.0 was rated 190 bhp at the propshaft and had a 1.65 gear ratio. The 5.0LX was rated 205 at the prop and had a 1.5 ratio. Both were 4200 - 4600 recommended WOT RPM.

    Oh, and I deleted a couple of posts to try and make a little more sense out of the merge.

  25. #25
    Lieutenant rwidman's Avatar
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    Default Re: hull speed?

    Quote Originally Posted by scon View Post
    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?
    I hope that from reading the posts, you have figured out that this applies to "displacement" type boats. That is, boats that don't climb partially out of the water when run fast. A cargo ship would be a displacement hull. My boat (in the photo) is a "semi displacement" boat. Most of the boats you see running around on lakes and rivers are "planing" boats. The other end of the spectrum would by a hydroplane racing boat that is almost completely out of the water at top speed.

    Either the person you talked to doesn't know what he/she is talking about, or one of you misunderstood the other.

    As for getting more speed from a boat or getting more power from an engine, these questions get asked all the time and the bottom line ie - If you want a faster boat, buy a faster boat. There is no magic pill to make a boat faster or an engine more powerfull.

    Tune the engine up to specs, make sure you have the correct prop that gets you to wide open throttle (WOT), make sure the hull is clean and free from marine growth, and reduce weight in the boat. If you're on a river, you can go faster with the current than against it. Same goes for wind.
    Ron

    2000 Camano Troll
    North Charleston, SC

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