prop thoughts for my boat

[thefox1051]

Good day all im tring to figure out what prop i should go with.
1979 Bomber bass boat 16'5" fiberglass heavy boat (heard outher marines call it a speed boat)
1978 55hp evinrude 2 cyl 2 stroke mod#55875S small gearcase
current prop 12.25X15 aluminam
engine rated at 5200 to 5600 rpms
1 person planes good 4950 rpms around 30 mph
2 people planes ok better if person up front to get it going 4900 rpms 27 mph
3 people plows water 4000 rpms about 12 mph

 

[usskydvr]

Re: prop thoughts for my boat

You provided good information. You need to, at a minimum, drop down to a 13" pitch prop. That will get your RPM up by another 300-400RPM so you would then be running at 5250-5350 with one person (at the lower end of the max for your engine)
A better choice may be to go to a 11" pitch which would raise that to 5550-5650 (right at your max) , but would give you more power when you have 2-3 additional folks onboard.

 

[SRC8778]

Re: prop thoughts for my boat

I think a 13 would be a good place to start. Changing the pitch is effectively changing the gearing so...

4900 x 15/13 = 5654

While it doesn't exactly work that way in the real world, especially since the engine should, despite making more hp, have slightly less torque at the higher rpm, that should put you toward the upper end of your rpm range when lightly loaded. That is what I would try first if it were me. 15 to 11 would be a huge change, especially if you are already running 4000 rpms without planing.

 

[thefox1051]

Re: prop thoughts for my boat

I got the 11 pitch prop very noticeable fixed every problem i had works GREAT with three people. But I do believe I should get a 13 for two people. Just cant believe that much of a pitch would what such a difference.

 

[steelespike]

Re: prop thoughts for my boat

The motor came with a 12 1/4 X 15 prop. And it is intended to push boats in the 15-16 ft range.and imo should do well and produce speeds in the 29-34 range.
While your prop change has solved your performance problems I wonder about your speed results. A gps would be better for speed.An 11" prop is close to what you would run for a pontoon boat.
You may have setup problems or perhaps a motor issue.Does the boat respond to trim?Does the carb open all the way? Is the motor in excellent tune? bottom clean and smooth? Prop in good condition? The anti vent plate, just above the prop, should be visable on plane.In the extreme raising the motor could net 2 to 3 mph.
I figure the dry weight of your boat is about 900 well within a 55s range.
I wouldn't be surprised to see a 13 on your motor but 11 seems extreme.
The motor is rated for a max rpm of 6,000.