I have a 1986 Mercury 50hp tiller on a aluminum Duracraft 1860 mod-v. The motor sounds great on the muffs. But under a load, its a dog. It's barely doing 10mph.

We have checked the compression with all cylinders hitting at 125. Spark test came back all sparking. Took the carbs off and did a cleaning, very little varnish if any at all. Blew out all the jets, and put back together. Fuel sample did not show in water, or trash. So, we started looking at the prop. The motor had 10 x 15pitch 3 blade on it when I bought the motor and boat. So, they had me put on a 10 x 13 pitch. The boat doubled its speed, and almost planed out. Had a little bit of hesitation at WOT, but it was finally stretching its legs.

So, they ordered me a 12.1 x 9 pitch prop, said it would make this rig sing! Got it put on Friday, ran it saturday morning, and its worse than the original!!!! At WOT, it is bogging down. I am at my breaking point now. Went back to the dealer/mechanic, and he said its in the motor, just bring it in. He said the 9 pitch should have jumped it out of the water, and humming. May even have to back off the throttle even. I asked about the diameter being larger, and they said it didn't matter.

After some research, this 9 pitch prop is for pontoon style displacement watercraft. I have been told to go with a 10.5 or 11" x 11 pitch prop, then another place told me to go with a 10.25" x 14 pitch.

Where do I go from here??????

Propellers are an uncertainty. Usually the right propeller is found through trial and error. You should also check your timing and carburetor adjustments, an issue with either can result in a similar problem.

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