Prop question about a 75hp Force by Merc

[JakeJef]

I have a 1998 75hp Force on a 17.5' Fisher. The boat planes out fast but will only run 33mph. It has a 10x15 prop on it that looks tiny but after researching it's the right size. Will a 10x13 increase the speed at all?

 

[flyingscott]

What are your rpms much more important than speed. You should have the engine propped so it reaches maximum rpms with a light load.

 

[SkiDad]

13 pitch will slow it down. Like the person above me said. What are you hitting for rpm ?

Oh and 33 is about normal. Are you measuring by a GPS app ?

 

[steelespike]

Pretty sure hp is developed at 4750 probably could run up to 5000.

 

[JakeJef]

Thanks for the info. I'm running around 4800 rpms so maybe that's all it'll do. I have a buddy with same size boat with a 50hp Johnson that smokes me. It drives me crazy when he beats me to my crappie holes ha

 

[Silvertip]

If the gear ratio is 2:34 then that prop is way wrong and should be 13-3/4 X 17 or 19. 10 x 13 goes on the small 1.83 Merc gearcase.

 

[Frank Acampora]

That engine is a very nice revision of a Chrysler 3 cylinder engine and runs remarkably well for its design. The problem is that it still is dirt common crossflow design. Yes, the OMC 50 loop charged engine will be competitive.

The gearcase is the small Mercury gearcase used in the 30, 40, and 50. You can use some Mercury props and yes, the correct prop is a 10 inch diameter prop.

NOW--As to whether or not a 13 pitch will be faster, MAYBE. Right now you are running 4800 RPM. The 13 pitch should raise the RPM up to about 5200 BUT--the decrease in pitch may still result in a slower top speed. Great hole shot though.

You really need to check compression ratio to be certain the engine is healthy. Then you need to experiment with props--and yes, this can get expensive. You might also consider a stock Mercury 15 pitch stainless prop which SHOULD (not will) give about 200 RPM more than your aluminum

You need to also maximise the engine height on the transom and weight distribution in the boat. If the engine does not have power tilt/trim you need to find the best position for the tilt bar.

Again, comparing your friend's 50 to your 75 is comparing apples to oranges. His hull may be lighter with less wetted area and his set-up may be optimised for maximum speed. Also, he may be a small 150 pounder and you may be a 300--makes a difference.

 

[flyingscott]

Sounds like your engine is running well if you want more speed could go to a stainless prop if you can find one. I am pretty sure that when mercury acquired force they put the smaller lower unit from the 40 hp on that motor. So you might be able to find a stainless prop. That's why you take the 10" diameter prop not the bigger ones.

 

[steelespike]

A little calculation indicates 1.83 gear ratio? and the numbers look reasonable.

 

[JakeJef]

Alright thank you guys for the info

 

[Frank Acampora]

Just for the record: Mercury did NOT acquire Force. Brunswick, which owns Bayliner, Maxxum and Mercury among other brands, acquired Force from Chrysler when they were forced to divest during the 1980s. At first they only changed the decals from Chrysler to Force. Later on they started to improve the brand and save costs by incorporating Mercury parts into Force engines.
Unfortunately, due to competition from their alternate low end entry level engine (Mariner, a rebadged Mercury) and environmental pressures, the Force line was discontinued in late 1990s.

 

[flyingscott]

Actually U.S. Marine bought the Chrysler outboard division in the early 80s. Brunswick bought U.S Marine in the late 80s early 90s. As far as mercury not acquiring force they sure wrote the name big enough on the side of the merc factory in Fon du lac WI.

 

[Frank Acampora]

Brunswick OWNS Mercury. As far as Force by Mercury, Mercury Force, whatever---Marketing hype. A play on words to get people to buy 'em. Fact is Mercury and Force engines were made by the same manufacturer and later engines have parts interchangeability. Some parts, not all.