How much H.P.do you need to get the M.P.H you want

[GCB069]

I am new to boats and would like to know how H.P = M.P.H. With me at 225 lbs motor 6gal. of gas and aal the gear i get 25 MPH on my GPS fishfinder. I have a Lowe Jon L1436 with my 2006 Mercury 9.9 hp 2S Outboard converted to 15HP on it.

You can see my boat in Members Boats page 6.

[triehl27]

I have a 14ft Duroboat with a 15hp mercury on it. I max out at 31mph with nothing but me, and optimal trim (no fin), but it tends to start walking around on the water at that speed. With 6 gals gas, gear and me I get about about 25mph. I went with the Doel-Fin on it and it helps quite a bit in stabalising it at high speeds in chop. With the fin and no gear I max about 28mph. Even in the heaviest winds and tides I have always had plenty of power in saratoga straights, and on lakes.

[A9]

How many MPH you are going to be seeing is dependent on a lot of things:
-Weight of boat (glass vs aluminum)
-Type of hull
-Weight displacement
-Overall weight
-Surface of bottom (bottom paint, rivets, waxed fiberglass, etc)
-HP
-Water conditions
-Temperatures
-Tilt of motor
-HP of motor + age of motor

I'm sure there are more, but I'm forgetting em....

I run a 12' Lund semi-V hull (riveted) with a '07 15hp Yamaha 2 stroke motor.
boat: 180lbs motor: 86 lbs me: 180 lbs gas: 20 lbs
So no gear I'm running at 466lbs. And I haul @$$ when I'm solo with no gear in flat conditions. It's darn slippery going full throttle, the lightest chop will pop my boat up and all that is in the water is the prop...I'd guess it goes over 30mph, and if I put a buddy and gear in it, it's going to be a bit slower because adding weight to smaller boats is going to have a big effect on overall speed. I'd say I still go well over 20mph with a load of gear and a buddy (700lbs) but you never know....

[Gisteppo]

GCB, give me your TOTAL weight (hull, everything in it, motor, the whole shooting match), your total HP, what pitch and diameter your prop is, and what speed you want. Ill give you an answer.

E

[ChrisB]

Call the dealer No offense, but not many people on this site are going to be able to give you an accurate answer to your specific boat.
But I'll answer your question, Mercury 175 Pro XS, that should give me the MPH I want.

[Gisteppo]

Chris, there is a normograph put together by Dave Gerr that makes speed estimation as a function of weight and horsepower very, very simple.

E

[GCB069]

Lowe Jon L 1436 Boat 145 lbs, motor 85 lbs, me 225 lbs, 6 gal. gas and 150 lbs gear, prop is a 9"-7" pitch..
just would like to knoe what top speed should be for this motor 2006 Mercury 9.9 hp 2S Outboard converted to 15HP.

Glenn

[ChrisB]

Chris, there is a normograph put together by Dave Gerr that makes speed estimation as a function of weight and horsepower very, very simple.

E

Where can I find it?? Learn something new everyday.

Thanks

[Gisteppo]

Its in the Dave Gerr book "The Nature of Boats", as well as his other book "The Propeller Handbook".

Speed prediction can also come in the form of a Crouch number, which is a calculation with weight and horsepower, with a hull shape coefficient. Its not terribly complex, but it isn't John Q 6Pack math either.

E

[Anglinarcher]

Call the dealer No offense, but not many people on this site are going to be able to give you an accurate answer to your specific boat.
But I'll answer your question, Mercury 175 Pro XS, that should give me the MPH I want.

At the risk of sounding wrong, "Trust ME", no dealer will give you the truth. I am on boat # 3 and they all said I'd get more than what I got.

[Anglinarcher]

Its in the Dave Gerr book "The Nature of Boats", as well as his other book "The Propeller Handbook".

Speed prediction can also come in the form of a Crouch number, which is a calculation with weight and horsepower, with a hull shape coefficient. Its not terribly complex, but it isn't John Q 6Pack math either.

E

Sounds like another book, or is that books, I need to look into.:bounce: