Anyone removed cup from prop ?

[SkiDad]

I have a 15 pitch stainless prop for my outboard... picked it up used for cheap. runs faster out of the hole than my 15 pitch aluminum but it is a little slower top end. I'm not too concerned about top end but the thing I have noticed it that it is harder to steer with it on. Stainless rides high in the stern, aluminum rides high in the bow.

15 pitch stainless 13.75x15 - 4800 RPM (37 mph)
15 pitch aluminum 13.125x15 - 5100 RPM (39 mph)

the stainless has some cupping - not a ton. Do you think the cupping is causing the steering issue ? I'm thinking of having it removed and polished up and one blade is just slightly off from the other too, maybe 1/8 inch if i measure - but not noticeable to the eye. trying to eliminate the steering issue and pick up 2-300 rpm

thoughts ?

 

[dan02gt]

Re: Anyone removed cup from prop ?

A good prop shop can fix you right up. Hard steering is usually a sign of incorrect motor height (mounted too high). The motor height has to be set for the prop you're running to get it to perform the best.

BTW what prop did you buy? It may be totally incorrect for your setup.

 

[SkiDad]

Re: Anyone removed cup from prop ?

here is a chart with the 2 props i have circled and also a picture. Both are michigan. The stainless one I have is off an 1988 force, but they fit anything from 84-94 force I believe. They don't make stainless anymore for my engine.



as for my engine height, the anti-cavitation plate is about 1 inch below the bottom of the boat - I would raise it but it's already in the lower hole and I don't want to mess re-drilling. It steers very easy with the aluminum prop.

 

[alldodge]

Re: Anyone removed cup from prop ?

A note to add - cupping should not effect FWD steering but it will effect REV steering, doesn't have the bite which is normal. If your getting FWD steerng issues there is probably other details as dan02gt mentioned.

The outer .5 inch diameter is probably causing the reduction in RPM

 

[SkiDad]

Re: Anyone removed cup from prop ?

Maybe I'm plowing and causing the steering issue ? I didn't have too much time with the prop the other day b/c lake was closing

 

[mpsyamaha]

Re: Anyone removed cup from prop ?

Was your trim angle the same with both props? I ask because you said the ss prop rides higher in the rear, to me that means more grip. If you were running with the bow pretty low(not enough trim) it would affect the steering like you describe. The cupped ss prop will likely allow more grip and get more boat out of the water at a higher trim angle. I have had that result, and it ended up with slightly lower rpms than a non cupped equivalent, but slightly more top end speed. And the increase the boats response to trim is nice when dealing with choppy seas, or even boat wakes. Too bad you cant raise the motor, I bet the ss could really pick up some speed.

 

[Frank Acampora]

Re: Anyone removed cup from prop ?



Depending upon the hull, early Force engines may benefit by raising the height on the transom. You have an early white Force-- I believe you said 1985. This was simply a re-badged Chrysler. Nothing was changed except the decals. You have screw-down engine transom clamps with two 3/8 bolts at the bottom holding them down.

You can simply unbolt the bottom two bolts and raise the engine on the transom. Temporarily fill the holes in the transom with GE Silicone RTV. This will be fine for testing purposes. If you find a "sweet spot, then simply drill 2 more 3/8 holes in the transom, seal them, and rebolt--no big thing. I have hulls with 6 or so holes in them. It does not compromise the transom integrity.

While you are at it, you really should consider putting a thin (.080-.125) aluminum plate under the outside transom clamps. They tend to bite into the transom glass. Eventually the power trim/tilt will also have problems. The plate avoids this.

Your engine has a 1.76 to 1 ratio lower unit. It can use any 3 or 4 cylinder lower unit up to about 1993 and if you are serious about more speed, find a 2-1 ratio lower off an 85, or 90. You can also use the 1,95 to 1 ratio lower units off late model Force engines. With existing props you will gain about 400-600 RPM and at least 3 MPH more--some engine/hull combinations show up to 8 MPH more.

In these photos you can see various examples of props I use as well as various aluminum plates on the transom and raised engines.

Small gearcase Mercury props are a direct fit. Notice the Ballistic prop in the last photo. Look at the aggressive rake and cup. I needed to modify the thrust washer so the blades would clear the exhaust snout. It loads down my engine to about 4500 RPM (which is still within specs) but speed stays the same as other props turning a higher RPM. --Excellent prop! Well worth the money.

 

[SkiDad]

Re: Anyone removed cup from prop ?

took the boat out this weekend and ran the stainless prop all day. If I trim up alot the engine the steering is very sloppy (so weird feeling) - like it has a few inches of play but I know it doesn't have any. It seems unstable like that. I'm guessing it's just b/c the prop is lifting the stern a lot higher than the AL prop. Props are both michigan but it seems the boat is happier with the design of the aluminum.



I skied behind it and it threw a totally different wake and prop wash. So I think I'm going to keep it for my spare.

Anything else out there beside the ballistic in stainless that I can run ?