Understanding boat horse power

Discussion in 'Boat Tips' started by tanner, Mar 27, 2006.

  1. tanner

    tanner New Member

    Messages:
    321
    State:
    Somerset, Kentucky
    I'm pretty new to boating. And am in the process of looking at buying a boat. But am a littler concerned about horse power. I don't want to buy something that will take me forever to get someplace and yet I can't spend a fortune. I find some boats that have 10 horsepower. Some that have 50. And some that have 150. I would only be able to afford in the 10-50 range. So my question is how will a 10 horsepower handle in the water as far as speed? Is it not worth my time? Can it be translated to speed? As in "You can expect a top speed of *speed here* with a 10HP motor" Any help with this will be greatly appreciated!

    Rock on!
     
  2. Dreadnaught

    Dreadnaught New Member

    Messages:
    5,444
    State:
    Henderson,Ky
    Tanner, I think we will have to know a few more things about the boat you are trying to push with the engine in question. Mine is a 16ft deep V Fiberglass hull with a 60hp Johnson and it will carry three people at 24 mph. and it will run 30 with just me in it.
    There are alot of variables to consider when buying a motor like, boat weight, boat lenght, hull type, boat weight when loaded w/gear and so on.
     

  3. JimF

    JimF New Member

    Messages:
    151
    State:
    IL
    I had a 14' alluminium v-bottom it had a 9.9 merc, I took it to the IL river all the time. I will have to say that, if I could of got out and run on the bank I probably could have out run it! It never left me stranded, it always started, but it was sllllllooooow. Don't even ask about when I had two people in the boat.
     
  4. Chanellocked

    Chanellocked New Member

    Messages:
    108
    State:
    Lake St. Louis, MO
    I run a 16ft Bass Tracker III w/Johnson 50hp, by myself I can run 35mph w/a buddy 30mph, which is plenty fast enough for me. With a 16ft mod-v I wouldn't go with anything less than a 25hp. Good luck in your search:thumbsup:
     
  5. dademoss

    dademoss Member

    Messages:
    524
    State:
    Ohio
    Tanner,
    As stated, there are two components to boat speed, one related to the engine, and the other to the "drag" or "prop slip" component.

    Prop slip varies with load, hull design, motor trim adjustment, etc, so isn't a constant, but we can fix the value for comparision purposes.

    One would logically expect a bigger motor to move the boat faster, but that is not always the case.

    Let us look at 2 typical outboards, a Johnson 9.9 and a Tohatsu 18

    Johnson MAX RPM 5500, gear ratio 1.92, prop pitch 10, slip .2
    Tohatsu MAX RPM 6000, GR 2.15, Prop pitch 9.8, slip .2


    Knowing the maximum RPM. gear ratio and propeller pitch, we can calculate the theoretical top speed for these two motors, and assume a comparable prop slip of .2 (20%) Your prop slip WILL vary.

    There is a handy calculator, here: http://www.rbbi.com/folders/prop/propcalc.htm

    Using the calculator, the theoretical speed for a boat with the 9.9 is 21.7 MPH

    Same calculation on the 18 HP yields 20.7 MPH, 1 mph SLOWER than the 9.9

    Now we know, it's not all horsepower :(

    Saved myself around 2300 bucks doing the math too :)
     
  6. dademoss

    dademoss Member

    Messages:
    524
    State:
    Ohio
    Oh yeah, my 14 ft deep v with the johnson 9.9 runs around 17 mph with just me in it, down to about 14 with 2 people and 11-12 with 3 people.
     
  7. metalman

    metalman Well-Known Member

    Messages:
    3,463
    State:
    IN
    Name:
    Winston
    Wow Dave, reading that has left me exhausted!!!
    Tanner, as stated above, give us a bit of info:-
    1 What kind of boat do you want, V, flat bottom aluminum / glass etc.
    2 How long
    3 How many guys in it typically
    4 Where do you plan to use it most
    5 How much gear do you typically take

    ...W
     
  8. tanner

    tanner New Member

    Messages:
    321
    State:
    Somerset, Kentucky
    1: Flat bottom aluminum

    2: 14-16ft.

    3 2-3

    4 River

    5 6 rods. plus tackle. cooler.