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  1. #1
    Petty Officer 1st Class JJB's Avatar
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    Default Boat weight to HP ratio ?

    Is there a way to figure horsepower needed for a boat by it's hp to weight ratio ?

  2. #2
    Senior Chief Petty Officer
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    Default Re: Boat weight to HP ratio ?

    Don't think there is. Hull shape (length, draft, beam, deadrise, chines) will influence as well, making one hull faster than another even though they may nominally weigh the same.

    Typically, in boat speed calculators (like this one: http://www.go-fast.com/boat_speed_predictions.htm ), they operate with different constants for different types of hulls.

    In other words, you probably can't determine the optimal HP for a boat by its' weight alone.

    Now, if you know the weight and the hull type (i.e. constant), you could use that same calculator in reverse to give you an indication of the power needed to move the boat at a given speed.

  3. #3
    Honorary Moderator Emeritus tashasdaddy's Avatar
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    Default Re: Boat weight to HP ratio ?

    all boats since 1972 should have a plate on it with the maximum hp listed on it. you want atleat 70% of that rating or higher. if the boat doesn't have the plate, or it is homemade, you have to go by the USCG guidelines in this post,
    http://forums.iboats.com/boat-restoration-building-hull-repair/coast-guard-rules-191283.html
    FLORIDA GATORS
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    Please, NO PM's (Private Messages) regarding boat/engine problems. they will not be answered.
    That is what these forums are for. Post your questions, in the appropriate Forum.

  4. #4
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    Default Re: Boat weight to HP ratio ?

    i have an old popular mechanics book (from 1983) that has a slide rule that allows you to pick 3 of the following 4 options and it gives you the last option.

    -Planning Hull / Deep Vee
    -HP
    -Boat Weight
    -Speed

    The book states most boats with proper prop and properly tuned will receive 90-100 percent of the rated HP. Occasionally the speed may be higher than the slide rule dictates.

    heres an example

    Planning Hull
    100hp
    2000lbs
    ---------
    36.5MPH


    anybody post here 3 of the 4 options and Ill give you the 4th.

  5. #5
    Petty Officer 2nd Class txmntman's Avatar
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    Default Re: Boat weight to HP ratio ?

    the Coast Guard guidelines is the best way to go.

    That being said, another way to get the ESTIMATED MINIMUM HP required to plane a boat. This is assuming your weight is accurate, and you are dealing with a flat-bottomed boat in still water. It is very simple. Take your boat weight and divide by 50. So, a 1000 lb boat (boat, motor, gear, passengers), flat bottom or still water would take 20HP to plane.
    If your boat with all gear is heavier, it will take more HP to plane.
    If your boat has a v-hull, it will take more HP to plane.
    If your boat is in rough water, it will take more HP to plane.
    Note: this is only to plane, not run around at 30 MPH.

    So, let's look at this. You have an aluminum boat that weighs 450 lbs.
    You weigh 250 lbs. You carry 6 gal of gas (total weight approx 48 lbs). This is now a total of 750 lbs. You will need a MINIMUM of 15 hp to plane. So, you stick a 15 hp engine on it and you are happy.
    You then invite your friend to fish with you on your new boat. He weighs 200 lbs. You now have 950 lbs, and your boat will not plane, and you are not happy.....

    Use the Coast Guard guidelines and you find what is the MAXIMUM HP you can put on your boat......

    Just something else to think about....

  6. #6
    Petty Officer 2nd Class woodee148's Avatar
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    Default Re: Boat weight to HP ratio ?

    Modified Vee
    4000lbs (empty)
    260 hp

    thanks !

    2007 Seaswirl 217 Deckboat, 5.0 Merc MPI

  7. #7
    Petty Officer 1st Class JJB's Avatar
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    Default Re: Boat weight to HP ratio ?

    my two factors are 3700 lbs loaded and a modify deep v hull what would be mim plane hp ?

  8. #8
    Petty Officer 1st Class
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    Default Re: Boat weight to HP ratio ?

    This is a great post...I'm currently trying to find the best option for a boat that will suit my fishing/boating needs. I just fully rebuilt and overhauled a 1991 Johnson 60HP (bored over .030). Im thinking this motor will now last a long time especially since I will take care of it, so I'm looking for a boat that this motor can push and plane out nicely. Im not to concerned with super speeds but I do want the boat to plane out nicely.

    Right now the motor is on a 15' boat that is pretty light and only 5 feet wide...and honestly its probably to much power for this boat. Im hoping to find a nice "bay" style V shape up front to fish inshore and some offshore on calmer days as well. Hoping to find something 16-17 feet and atleast 7 feet wide or so.

    I guess the real question is...what might be the maximum weight that I could probably push with my motor and get up on plane nicely? My guess is probably around 1600lbs? maybe a little more or less?

    Cory

  9. #9
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    Default Re: Boat weight to HP ratio ?

    Quote Originally Posted by woodee148 View Post
    Modified Vee
    4000lbs (empty)
    260 hp

    thanks !
    41mph empty

    add 500lbs (batteries, you, and misc)

    38mph

  10. #10
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    Default Re: Boat weight to HP ratio ?

    Quote Originally Posted by BigNiner19 View Post
    This is a great post...I'm currently trying to find the best option for a boat that will suit my fishing/boating needs. I just fully rebuilt and overhauled a 1991 Johnson 60HP (bored over .030). Im thinking this motor will now last a long time especially since I will take care of it, so I'm looking for a boat that this motor can push and plane out nicely. Im not to concerned with super speeds but I do want the boat to plane out nicely.

    Right now the motor is on a 15' boat that is pretty light and only 5 feet wide...and honestly its probably to much power for this boat. Im hoping to find a nice "bay" style V shape up front to fish inshore and some offshore on calmer days as well. Hoping to find something 16-17 feet and atleast 7 feet wide or so.

    I guess the real question is...what might be the maximum weight that I could probably push with my motor and get up on plane nicely? My guess is probably around 1600lbs? maybe a little more or less?

    Cory
    according to Popular Mecahnics:

    1600+200lbs Load+60HP= 30mph.

    add another 200lb passenger and
    =28mph

    1600 sounds about right to me!

  11. #11
    Petty Officer 2nd Class woodee148's Avatar
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    Default Re: Boat weight to HP ratio ?

    Quote Originally Posted by zibzer View Post
    41mph
    Thanks for the info although I must say I had expected a little more out of this new boat. not complaining just sayin..

    2007 Seaswirl 217 Deckboat, 5.0 Merc MPI

  12. #12
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    Default Re: Boat weight to HP ratio ?

    Any gurus want to tell me if the P.M. book is telling the truth or not?

  13. #13
    Chief Petty Officer Mkos1980's Avatar
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    Default Re: Boat weight to HP ratio ?

    3550lbs
    270hp
    ________
    55.1 Gps
    90 Chaparral 2150SX 350 MAG Alpha 1
    Vortec Heads, LT4 HotCam Upgrade
    GM BowTie 1.6 Roller Rockers
    Edelbrock RPM Air-Gap Intake
    Edelbrock 600CFM Marine Carb * New MPH* 65.2
    Laser II 21 Prop 5400 RPM's Corsa Exhaust

  14. #14
    Supreme Mariner oops!'s Avatar
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    Default Re: Boat weight to HP ratio ?

    im sorry.....i dont know what your trying to calculate here.....

    do you want to know how much horespower your allowed?...hit td's coast gaurd link.....the answer is there

    do you want to know if the pop mech's book will tell you how fast you can go?.......
    sorry...it can only guess.....the hull dictates friction.....a hook will rob you 3 mph....(thats a lot).....a rock can gain you a few......so there is a ton of variables...it might be a perfect world guideline...but

    you can also have bigger torque and hp...but low rpms...(diesel)...

    so.... you might have 400 hp...but 2500 rpms......and rpms are what make it go.......ie....a 19 inch prop travels 19 inch thru the water with one full revolution......so there is a big difference between a 200 hp 2500 rpm diesel and a 200 hp out board that will rev at 5500 rpm

    *****disclaimer...(to all the tech guys,....this is just a very basic laymans explanation,)

    now,..... if you want to know how fast you might go with a certain hp....hit this link....

    http://forums.iboats.com/showthread.php?t=225803h

    that was posted by 45 auto......it cannot take into consideration hull designs...but it will give you a good base line......(btw...45 again..thanks...great work)

    i hope that helps...if it doesnt..please re-phrase the question

    cheers
    oops
    Last edited by oops!; March 27th, 2008 at 03:09 PM. Reason: sp

  15. #15
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    Default Re: Boat weight to HP ratio ?

    OOPS beat me to it... the info in that thread is dynamite... definately required reading....

  16. #16
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    Default Re: Boat weight to HP ratio ?

    Quote Originally Posted by Mkos1980 View Post
    3550lbs
    270hp
    ________
    55.1 Gps
    sorry, your off the charts. P.M. says after 40mph its hard to dictate what a given boat and weight will get. so on that note. your boat will get something over 40mph

  17. #17
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    Default Re: Boat weight to HP ratio ?

    Oh... and 2500 ponds, 220 horsepower, 58-60.

    That calculator based on weight has me doin like 70... maybe behind the truck headed to the water... but not with this engine!

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