What speeds are you gettin in your Zodiac? I need help with prop selection.

[Tyson0317]

Hi Guys,

This is a two-part question.

#1
I am curious what kind of speeds people are getting in 15ft Zodiacs with ~50HP motors.

#2
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 sport 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...

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...

 

[nobrainsd]

Re: What speeds are you gettin in your Zodiac? I need help with prop selection.

I have a zodiac futura mk11 (13'9" and has speed tubes) with a 40 hp nissan. I carry a full load (including a ten gallon gas tank). My boat hits 28-29 mph on smooth water (off the gps). Your prop should be selected based on the top rpm at wide open throttle. This spec should be in your motor's operating manual. I removed the original prop and went with a propulse composite four blade prop that is adjustable. Tried a few settings until I hit the right rpm. Didn't hugely change my top speed, but I didn't change the pitch much. For my nissan 40d a 16 pitch prop would only be appropriate for a very light load on the motor. At speed a zodiac has a pretty high coefficient of drag. The weight of the boat isn't as much of a limitation. The normal prop is a 12.5. My adjustable prop is set lower. By changing the pitch lower I bumped my wot rpm up about 700 to reach the rated range for the motor. I used to have a dolphin on the outboard. Setting the engine tilt so that the dolphin worked well at lower speeds killed my top end speed. Changing the tilt at speed helped. Got tired of always adjusting tilt at different speeds and installed self adjusting trim tabs. That definitely helped! The only other thing I can suggest is that you look at how much gear case is in the water. Does your boat really require a long shaft?

 

[Tyson0317]

Re: What speeds are you gettin in your Zodiac? I need help with prop selection.

My transom says that I need a long shaft... I will look up the RPM range in a book and probably get a cheapy tach. Thanks for the advice!