I can’t go larger than 25 HP on my 1986 Starcraft 16′ deep V aluminum boat.
Any suggestions on a prop for a boat which is no doubt under powered with a 2015 Mercury 25HP four stroke.
The motor didn’t come with a prop.
I don’t have a specific prop to suggest but if you feel like you are starting out being under powered, look at low end torque or hole shot over top end as your priority. Getting on plane with a loaded boat in a reasonable amount of time would be my goal.
Each make and model number of outboard has an optimum prop pitch for the prop for which the motors RPM can be optimized for optimum speed when combined with the boat it’s being used on.
In general the speed difference isn’t great. But a new prop with the correct pitch when combined with a motor tuned for the right RPM’s in the boat it’s used on can add a slight bit of speed.
Hope that helps. Had it done to mine this summer added a little over 1 mph
Can you contact an area dealer who specializes in Mercury outboards in about the prop and possible tuning?
Went through this with my motor. 14′ with a 25hp 2 stroke. As I recall from a past thread of mine maybe last spring?, anything under 75hp really doesn’t benefit much from different props. I’d have to double check, but I believe the stock prop is optimal for these motors. Again, mine I think is a 12 pitch, and was a beast to get on plane even solo, so I got a 13 pitch which did help, but on sharper corners kept blowing out the motor.
Went back to the original, and shimmed up the motor an 1″ to get the cavitation plate even with the underside of boat and that helped immensely. Kept it well tuned, high octane, with a good synthetic oil and it gained a couple mph top end. I’m happy with where its at now, a 25hp is only going to be a 25hp no matter what.
Couple years back on our Canada trip we tested out our 2 stroke 25Johnson against another parties 25 HP Mercury 4stroke. His motor was newer by about 15yrs, and I think had a 4 prop propeller, but he had probably at least 100# extra in body weights, and boat was heavier as well. We beat him out the gate but after about 200 yds they caught up to us and didn’t look back. Again, some may be faster out the gate or top end, but either way they are still similar motors when it comes to overall performance.
I would start with an 11″ pitch.Even better if you can find a good 4-blade prop.
Thanks to all for your help!!
I got so confused I decided to call Mercury Customer Support. Here is what I was told to buy.
9.3 Dia. – 9.5 pitch, four blade aluminum Mercury Spitfire. I was also told that optimum performance could be had if the cavitation plate was flush with the bottom of the boat, no more than 2″ longer than the bottom of the boat.
I’m glad I waited to make the purchase since Santa brought BPS Gift Cards.
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