pvanv
Admiral
- Joined
- Apr 20, 2008
- Messages
- 6,567
I posted to the Prop and Merc forums, but got no replies. Maybe the Non-Repair experts have an idea...
I'm going through a 1981 Merc 7.5 OB, s/n 5848475, 8-tooth prop spline.
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. I may need to do this on the Merc.
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 reverse exhaust to slip inside the hub, rather than washing over the blades. But I don't see one for the 81 Merc 7.5 (8-spline) application.
Does anyone know of a similar HT or "Bigfoot" prop for the Merc 7.5? Will my only option be to bore a hole through the side of the exhaust housing? Maybe there is a conversion sleeve to use a modern (Tohatsu/Nissan/Merc) HT Solas prop on this motor?
I'm going through a 1981 Merc 7.5 OB, s/n 5848475, 8-tooth prop spline.
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. I may need to do this on the Merc.
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 reverse exhaust to slip inside the hub, rather than washing over the blades. But I don't see one for the 81 Merc 7.5 (8-spline) application.
Does anyone know of a similar HT or "Bigfoot" prop for the Merc 7.5? Will my only option be to bore a hole through the side of the exhaust housing? Maybe there is a conversion sleeve to use a modern (Tohatsu/Nissan/Merc) HT Solas prop on this motor?
