motor size and speed

[PTBadboy]

I have a 15ft fiberform with a 73 40hp jhonson on it top speed is only about 18 mph should it go faster?and if I put a 70hp on it would that be to big for my boat?

[MacEFL]

Look at the plate for max Hp. C
How does it come up on plane? If it pops up real quick, you may be ale to get a little more speed with a prop change but make sure you stay under max RPM's at WOT. (wide open throttle)

[PTBadboy]

Ive tried the prop thing but no results Im going to have it tuned up.where is the plate with the hp ratting on the boat?
Thanks

[Tacklebuster]

What is your RPM's at WOT? I "think" you should be turning around 5500. If you are, that is all you should be getting. If your not up in the 5000's, your motor is either propped wrong or you have some other issues going on :twocents:

[Draggin' Bait]

Another simple item to check is the height the engine is mounted on the transom. If you are too shallow you are likely cavitating and losing power. If you are too low you have too much drag from the lower unit. Larry

[Starfish]

The motor rating is usually just forward of the throttle control (on the small plate that shows the weight/persons capacity for the boat), but I've also seen the plate mounted on or near the transom on at least one boat.

Please don't take offense at the following suggestions since I don't know how long you've had this boat or how familiar you are with rigging. You may have already checked all these out.

Have you checked to make sure you don't have water between the double hull? Many glass boats have dual drain plugs.

When you're running, take a look at where the water spray is coming from and how the boat handles. If the spray line is coming from near the bow of the boat, you may be "plowing". I presume this motor does not have power tilt? If not, there should be a pin that lets you adjust the trim setting manually. (It's mounted in a row of holes in the bracket that is hidden when the motor is all the way down and visible when tilted up). The holes closest to the transom are "lower" and the ones farther away are "higher". Move it so the motor is a couple of notches higher (more tilted upward) and see if that raises the bow and moves the spray line toward the rear and increases speed. You can keep raising it until the boat starts porpoising then lower it back one notch. If you DO have power tilt, then raise the trim while running until it starts to porpoise and then lower just enough to stop it.

You indicated that you tried "the prop thing"... some more specifics about what prop pitches you tried and what your WOT RPM's were would help a lot.

No matter what you do the 40 horse won't make your boat a speed demon, but I would expect a lot more than 18 mph out of that combo.