I'm having trouble finding the right prop for my Zodiac!

Hi Guys,

I have randomly tried a few props on my setup and some are better than others.

The boat:
1982 Zodiac 15ft Inflateable.
Weight: 200lbs
Bottom: Firm (NOT the hard fiberglass, but with removeable aluminum tiles that cover the inflateable keel)

The engine:
50HP 4cyl 2-stroke Mercury (1985-ish) Long Shaft with very recent total rebuild - all new gaskets, bores, rings and bearings.
Weight: 180lbs.
Has a dolphin fin.

Gross weight
(Me, passanger, gear, boat, engine, & fuel tank): 600lbs

It was my understanding when I got this boat that with a 50HP it should fly. My most recent trip (salt water ~60 degrees) got me to 20mph with a bit of a tailwind. I was expecting closer to 30-40 and my buddie's nearly identical boat does 24 with a 30HP Tohatsu.

The shop is assuring me that the engine is tip-top and running properly - it certainly drinks enough gas!! It does sound OK to me and has strong compression... I think the key lies in the prop diameter and pitch...

Please help!

Re: I'm having trouble finding the right prop for my Zodiac!

I am having the same problem..

Re: I'm having trouble finding the right prop for my Zodiac!

We need prop size,present max rpm and speed.Without this info there is no base line to start from.Has this motor run on the boat before?Does the boat require a long shaft?

Re: I'm having trouble finding the right prop for my Zodiac!

I have a 50HP Yamaha. I took this engine off a 27' MacGregor Sailboat. I don't know the prop size but it says C13.5 on one of the blades, the boat weighs 550lbs (without gas), It max RPM is about 4500, does not need a long shaft. I have used it in the ocean once and it worked ok. I know this prop was not made for this boat and it can run faster with an efficient proper propeller.

Re: I'm having trouble finding the right prop for my Zodiac!

C13.5 is probably Clockwise 13.5" pitch.I would think that with a proper setup
it would be easily over reving. If it is indeed a 13.5 you would need to go to an 8.5 to reach 5,500,less pitch than what you would put on a big pontoon. so either it is not a 13.5 or something is screwy.You might lose the foil for now and see what happens.

Re: I'm having trouble finding the right prop for my Zodiac!

Thanks!

Re: I'm having trouble finding the right prop for my Zodiac!

I am the original poster of this question

My motor is a long shaft and I posted the weight earlier. My current prop measures about 7-8" in Diameter and says A40 16P on it. I have no idea what my max RPM should be...

Could I find this information in the motor's shop manual? My local West Marine had one...