want to be faster

[bazboat]

I have a 10 feet seafire whith a 25 hp mercury im doing 32 mph an i want to reach 40 to 45 mph ,do i have to change the motor for a 40 hp jonhson vro whith power trim or theres something to do whith my 25 mercury .

 

[galamb]

If your boat will still be stable at the speed you want you will definitely have to upgrade the motor - you may need a 50 to reach that speed.

General rule of thumb - for 15% increase in speed, increase the horsepower by 50%.

So 40 horses would be about a 60% increase in horses = roughly 18% speed increase - 32 x 1.18 = almost 38 mph, but now the 40 horse is heavier than the 25 so the boat is pushing more weight (means more drag = loss of some speed).

That's why I say, for the speed you want you may be looking at a 50 horse if your boat can handle it...

 

[fastjeff]

Graham has a point. On my little boat, the Merc triple (about 75 hp) goes as fast as the Merc 850, which weigh 30 pounds more. (Both motors use the same lower unit setup.) It rockets onto plane with the bigger motor, however.

Jeff