Is there a formula to calculate speed from hp and weight?

[Mark42]

Seems to me that there was a graph that used to be posted often that would give speed based on hull configuration, weight and hp.

Lets say I have a 18ft deep v weighing around 2800 lbs, and I want top speed to be 35mph from an outboard motor, how much hp will it take?

 

[JB]

Re: Is there a formula to calculate speed from hp and weight?

Well, there is a basic formula that has worked well for me.

WOT= (square root of HP/weight) x BF (bottom factor)

Bottom factors range from about 150 for a deep vee to about 225 for a jon flat bottom. You just have to guess where yours falls. The example below uses 150 for BF.

Do some high school algebra to solve for HP for a selected WOT.

Divide both sides by BF and cancel.
WOT/BF= square root of hp/weight

Square both sides and cancel.
WOT/BF squared= hp/weight

Multiplyboth sides by weight and cancel.
weight x (WOT/BF squared)=HP

Insert knowns:

HP=2800 x (35/150 squared)
=2800 x (.2333 squared)
=2800 x .0544 = 152HP

If your BF is more like 200 the solution is 85.75HP.

Now you need to determine the max HP recommendation for your boat.

Good luck.

 

[Tail_Gunner]

Re: Is there a formula to calculate speed from hp and weight?

http://www.go-fast.com/boat_speed_predictions.htm

for the less gifted........

 

[Bondo]

Re: Is there a formula to calculate speed from hp and weight?

Ayuh,...... I was never a very good Mathamagician,.......
JB Lost me at "me."

Good Link,..... Deserves a save......

 

[Rudy Brown]

Re: Is there a formula to calculate speed from hp and weight?

http://www.go-fast.com/boat_speed_predictions.htm

for the less gifted........

This one is dead on!! It said my top speed should be 46mph, and thats what my boat does.