can propeller shops cut down propellers?

[kwikrnu]

Can they also change out the hub to a different size?

I have an omc cobra and the gears have been changed from 2.0:1 to either 1.68: or 1.59:1. I can't find a 13.5x13 prop that fits the prop shaft which is 15 spline. I'm guessing a 13.5 diameter because in the list of props omc recommends for my engine diameters range from 12.75-14. 13.5 is middle ground. I think my only options are getting it regeared, but that is pretty expensive $1500 range, or get a prop cut down. The prop I have is a 15.5x13 aluminum and have no problems trying this to see if it works. I also have some stainless props 13.75x13 and a 13.75x15 but they have 13 spline hubs. Can the hub be changed out at a shop for a larger 15 spline?

WOT with the 2.0:1 stock gears my top theoretical speed should be +-40mph with a 15" pitch prop.

WOT with the 1.68:1 gears and a 13" pitch prop should be around +-41 mph.

WOT with the 1.59:1 gears and a 13" pitch prop should be around +-43 mph.

 

[kwikrnu]

Re: can propeller shops cut down propellers?

I found out they can cut down propellers. I also found this article about it on the net.

I went to a local prop shop today. The guy pieced together something that I hope will work.

This is what the current setup looks like.
Michigan wheel prop 011016 for a large gear set cobra 800 omc.

This is the shaft with 15 splines.

Here is the label on the upper unit.

This is the prop I got from the shop today.
Quicksilver part #qa2032x. It is a 14x13. It has an interchangeable hub system and the hub I think is part #101 from michigan. The individual parts are thrust washer#12-835467-1, hub with splines #991203, and there is an exhaust spacer part #878421. We used a mercury prop because the hub is larger and would be better for the exhaust combined with the spacer. It fits pretty close as seen in the pics.

I choose the 14x13 prop because of the math it should be equivalent to a 15" pitch and the small gear set with a ratio of 2.0:1.

If it runs too much rpm I will try a 13.5x15 prop.

I should have time to try tomorrow if the weather is good.

 

[kwikrnu]

Re: can propeller shops cut down propellers?

I went out for several hours today. I like the new prop. It gives me much better acceleration, speed, and rpm increase.

2 people in the boat previously was 29 mph @4500 rpms. Now it is 31 mph @5100 rpms. Not only have the rpms come up, but the boat planes out quicker and accelleration is much faster.

I think I got about a 100 rpm increase for each 1/4 inch decrease in diameter. Since I still think there is room for improvement so I plan on trying a smaller diameter/larger pitch such as a 13 3/4x15 (qa2034x). If the 100 rpm increase in revs for the 1/4th inch drop in diameter and 400 rpm drop for the pitch increase holds true I would only be at 4800 rpm wot. Instead maybe a 14x11 (qa2030x)to see if I can get the rpms up 400 which would get me to 5500 rpm wot maybe. The possible top speed drop thing concerns me though.

Now we are having a little more fun with the tube.

 

[kwikrnu]

Re: can propeller shops cut down propellers?

I didn't know the sting ray whale tail was an aftermarket part. It looks as if it has been on there a long time. I went out by myself yesterday afternoon with a topped off tank of gas and the removed hydrofoil. Top speed was 33 mph and 5500 rpm. The only problem is it did take longer to get on plane, but is it worth the 2 mph and 400 rpm drop? I've seen the smart tabs recomended and looked at their videos...

 

[Esox]

Re: can propeller shops cut down propellers?

From what I have read the foils aren't a good solution. Those that have installed the smart tabs are very happy with them. If you can spend the money on the tabs my guess is you will be much happier with them than the foil. I plan on getting smart tabs for my boat this year. You also need to watch your wot rpm's. I'm not sure what the max recommended rpm is on your engine/drive combo but 5500rpm seems very high for an I/O.​

 

[Texasmark]

Re: can propeller shops cut down propellers?

Now that just goes to show you how dumb you can be. I looked at the two props and assumed the latter just plain flat sucked. Your existing prop looked like it had much more performance built into it.

But it didn't as you proved by your test data. So back to what I have said for as long as I've been on here.

TRY B4 you buy. Only way to know.

Thanks for the sobriety check.

Mark

 

[kwikrnu]

Re: can propeller shops cut down propellers?

You also need to watch your wot rpm's. I'm not sure what the max recommended rpm is on your engine/drive combo but 5500rpm seems very high for an I/O.​

The factory service manual states 5200 to 5600 rpms.