High Thrust Prop for 81 Merc 7.5 OB?

[pvanv]

Im going through this 7.5 OB, s/n 5848475, stencilled "COMET" on the side of the block, to get ready for spring. I believe this is a 1981 model year.

The motor is used as the aux on a 23', heavy sailboat. The current prop is marked 7P A1 (I believe that means a 7" pitch) and visually, is a relatively shallow-pitched 3-blade aluminum unit.

The boat will not back up. Seems the blades get washed in exhaust, and won't bite the water. I have found this to be typical of the through-the-hub-exhaust arrangement on sailboats. Goes fine in forward, and the prop seems perfect in every way.

On an old OMC 9.9/15 used as a sail aux, I have seen the exhaust housing relieved (just above the ventilation plate) by drilling a 1" hole all the way through the leg. Worked pretty well, though it was a bit loud.

For the newer 4-stroke Tohatsu/Nissan/Merc 8, Solas makes a rather radical-looking 4-blade High Thrust prop, (both 5 pitch and 7 pitch) which is very effective for these low-speed, high load applications. It has an oversized, relieved hub, allowing the exhaust to slip inside the hub, rather than washing over the blades. But I don't see one for the 81 Merc 7.5 application.

Does anyone know of a similar HT prop for the Merc 7.5?

 

[pvanv]

Re: High Thrust Prop for 81 Merc 7.5 OB?

Problem solved !

I traded emails and a phone call with Marc Feinman, the US marketing rep for Solas Science and Engineering. He came up with the correct Solas part number (1113-100-07) which is a 4-blade, high-thrust, 7-pitch prop to fit the old 8-spline prop shaft on the Merc 7.5 OB. No special hub sleeve needed. Retail is about $92.00 US. Our store can source it through Coast Distributors.

Marc is a great guy, very knowledgeable and quite helpful. I can't say enough about how he handled my customer service request. He's in their Tampa, Florida office. Bottom line is that he knows the applications very well. The prop selector charts didn't help me much. Most OB and prop manufacturers concentrate on the faster, bigger and newer motors, and go-fast boats. They tend to ignore sail applications. The correct part is in fact available for most small OB's -- IF you get Marc to help find it.

I know from experience that these props are excellent for sailboats -- I have one on my boat. I was able to downsize from 15 hp to 9.8 hp, and still get better thrust, especially in reverse. Anyone who wants to improve the low-speed thrust on their outboard-aux sailboat may want to investigate these high-thrust Solas props.