Older Merc 9.8 Prop Problem

I have an older Merc 9.8 (s/n 7117222), I think it's early 80's vintage. A few years back I stupidly damaged the tiller handle and transom mount. An acquaintance had the bottom half of a 7.5 of the same vintage (s/n 7036508) which appeared absolutely identical, so we just replaced everything from the motor down.

It worked perfectly. Except that I now have a lot less speed.

The prop on the motor now (p/n 48-47670-7) is a 3 blade, 9" diameter, 8 spline, apparently 7" pitch prop. I don't know what the original prop was as I gave the rest of the engine back to my friend and he has disposed of it. But I suspect it was a higher pitch. I've visited the Merc prop selection page, and although it doesn't go back far enough to figure out what prop I should be using, it recommends a 10.5" pitch for a current 9.9 on my boat.

My problem is I can't find any cross reference tables or catalogs that will give me a part number for a replacement prop. Any ideas where I could get such information would be greatly appreciated.

Or if there is any other likely cause for the significant loss of speed, I'm all ears.

Is the carb opening all the way?

Jeff

Yep. The engine runs beautifully. No hesitation, no coughing, no starting issues. Obviously there's no tach, but it seems to be hitting normal RPM's, just very slow speed.

On one occasion when I had the boat pretty heavily loaded, it repeatedly acted as if a clutch was slipping somewhere - rpm would suddenly rise, while the speed dropped off. I limped around all that day at less than full throttle, because that would happen every time I tried to run up the throttle. (I thought it might be cavitating). That only happened the one day, but it never gets decent speed any more.

It's on a twelve foot Bluefin rated for a 25. With two passengers I'm lucky to get 8 mph. With friend's 15 on it the boat gets 18 mph. I never checked with a gps before we changed out the bottom end, but I was getting noticeably more speed back then.

Do a websearch for "Michigan Wheel" and download their PDF propeller catalog. It will give you the OEM and their own part numbers for all available props.

I do agree the 7 pitch is "not enough" with that particular combo, a 9 pitch (factory standard) or the 10.5 should give you a little more top end speed...

Was your old prop a 2 blade or a 3 blade? A 9"x10 pitch 2 blade was standard on the old 9.8's. Could also be a spun hub in the prop.

I noticed that when I downloaded the Michigan Wheel catalog as suggested by galamb. To be honest with you, I don't recall whether it was two or three bladed. I guess I'm just not that observant.

How can I tell if the hub is spun in the prop? It seems solid enough.

And I have to thank everyone who's responded for all the help. It's really appreciated.

Scribe both prop and hub, then take her for a spin and check again. The lines should still be together.

Jeff