Rule of thumb for speed increase with increase in HP?

[peter1708]

Hello, I've seen a rule of thumb posted somewhere (think it was here) but I can't find it. Does anyone know of a general rule of thumb for percent increase in speed with a given percent increase in HP? This would be for mid-range 2 strokes (80-100Hp).

Assume a 25% increase in HP, any idea what speed increase would be?

Thank you,

Pete

 

[walleyehed]

Let's say you have a 100hp engine and go to a 125. You'll see about 1.47mph increase with all other parameters remaining the same.

 

[tpenfield]

As a general rule, to double the speed takes 8 times the HP. (2 X 2 X 2 . . or . . "2 to the third power"), but there are other factors that have an effect as well.

So, a 25% increase in horsepower would get you somewhere in the area of 7-8% increase in speed. . . (1.077 x 1.077 x 1.077=1.25)

 

[Frank Acampora]

Of course it really depends upon the boat and use. However, with my little 14 foot flat bottom go-fast, increasing horsepower from 90 to 125 increased speed from 50 to 55.

 

[tpenfield]

Of course it really depends upon the boat and use. However, with my little 14 foot flat bottom go-fast, increasing horsepower from 90 to 125 increased speed from 50 to 55.

Yea, sounds about right. Mathematically you would be around 11% increase using the "cube" formula.

 

[anzomcik]

I always ball park speed increase or decrease by saying it takes 5hp to increase speed 1mph (very general terms). That is of coarse based on a boat that can already plane out and stay on plane when loaded. So it doesnt apply to low hp applications.

 

[peter1708]

Thanks guys, I appreciate all the responses - very helpful!

Pete

 

[tpenfield]

I just got about 5 mph more out of my boat. . . 47 -> 52 mph . . . It figured to be about a 150 HP improvement across 2 engines

 

[Fed]

Real world GPS results, 70HP to 90HP = 27Knots to 35Knots.

My gut feeling is that if I added another 20HP it wouldn't make as much difference.

 

[peter1708]

Thanks for the info, some decisions need to be made!

Thanks,

Pete