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As a Classic Merc owner, I am familiar with your engine, since it is 4 of the 6 cylinders of the in-line 6's that I have. Gear ratio is 2.33.

Referencing Mercury's prop charts for your engine, with a strong engine, and with 5500 RPM, you should be running 38-42 MPH. Before buying a new prop, I would see if your boat, MORE LIGHTLY LOADED, will do close to the 5500 redline, and achieve the speeds mentioned. If not, either the engine needs work, the prop is a poor or outdated design (whether it be an old Merc design or aftermarket prop), or the boat is much heavier than it should be.

My guess is that your engine is mounted too low (all the way down on the transom), and with the transom bracket design of those days (thumb screw clamps), not easy to raise up. But if you can do it, I would bring the engine up about 3/4" to 1". With that old design transom bracket, the best way to raise the engine is install a 6" setback jackplate (the anodized aluminum Teleflex/Detwiler is highly recommended), and buy the special adaptor needed for these old Mercs. Then you can raise or lower it at will, easily, and boat/engine performance will be maximized.

If not, a Mercury aluminum prop is your best bet, since I am convinced they are the best perfroming and holding aluminums you can buy. Since Mercury is indicating a 19" prop is right for your boat, for a first choice, and for better pulling power at the same time, the brand new design 4 blade "Spitfire" is recommended. For a little less, you can buy the "Black Max" 3 blade. Often when boating is saltwater, I use a pair of the 19" Black Max/Black Diamond on my classic 21 Outrage with pair of 115 "Towers", and the boat will do 52 MPH @ 5800 RPM, very fast for aluminum props. (engines have a different 2.0 gear ratio)

If you can succeed is raising the engine an inch or so, a 19" Mercury Laser II will be your fastest, best riding prop, but they are pricey. For less, the Stiletto 17" (runs like a 19") 3 blade Advantage could be a good choice. But only if you get the engine up. If not, and running the prop fully submerged, it's not likely a performance SS will give any better performance than the Merc aluminums considring your speed range of the high 30's.

I run a Merc Laser II on my jackplate mounted classic 1979 Merc 115 powered 1979 Montauk, and the prop is a screamer to say the least. Lightning fast acceleration, great hull lift and ride, and strong top end (over 50). (see personal website link)

Here is really good mail order source for Mercury props:

http://www.jacosmarine.com/new_mercur...r_sale.cfm

For just an aluminum Merc 3-blade, this is a good deal. Quicksilver Black Diamond are the same as the Black Max.

http://www.dansdiscountprops.com/BuyN...icksilverA