Re: Make my boat go in reverse
If you want to stay with the somewhat underpowered 7.5, you will be looking for a 4-blade high-thrust prop. The HT prop will be shallow pitch, say 7", and will have "elephant ear" blades that are larger in swept area at the periphery than near the hub. Also, it will have a large-diameter center hub to relieve the exhaust in reverse, rather than washing the bubbles over the blades. This prop will NOT give you high speeds, but will give excellent control, especially in reverse. I do a lot of these for sail applications, and they work very well. I usually specify Solas Amita-4 HT props, as they seem to have the widest selection.
The search may get involved, and may end up being more expensive than you originally thought. Here's one similar story... I have a customer with a 1981 Merc 7.5 on a 23' O'Day sailboat. She complained that she couldn't back up. I searched high and low, could not find a high-thrust prop for her, and eventually contacted Solas directly. They recommended a 7" pitch x 10" diameter 4-blade HT prop with the old-style 8-spline Merc drive ($92.00 retail price). I ordered it, and after it arrived, I had to cut the old aluminum -- yes, aluminum, hub off of the prop shaft. It had been installed, probably without any grease, in 1981, and had never been off, so it was corroded onto the shaft. Sheesh. An hour of labor later (retail cost $75.00), I verified that while the new prop fit the shaft fine, it was too big in diameter to clear the ventilation plate. Damn. And there wasn't any prop with the correct dimensions that I could find. So I had the local prop shop re-cut the diameter and balance the prop ($75.00 retail price). But the story does not end there...
The re-cut prop fit on fine, and cleared the ventilation plate perfectly, but the hub was in contact with the skeg. So, to make it work, I had to file a semicircular relief in the skeg, just below the propshaft housing, about 3/8" deep, in order to provide clearance between the skeg and the prop. Another half-hour of labor (retail cost $37.50). The modification went well, and I'm sure she will be pleased with the performance. BUT... In hindsight, it would have been cheaper and easier to go with a different motor, such as a Nissan/Tohatsu 8, (as I can get off-the-shelf HT props for that application), and then just sell her old 7.5 Merc. She now has an expensive, 1-off, custom prop, and if she ever needs to replace it (though doubtful on a sail application), it will cost a lot of money. Again.
Bottom line: Sometimes it's better to change motors.
