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Author Topic:   Another prop question !
jacko posted 09-04-2002 06:48 AM ET (US)   Profile for jacko   Send Email to jacko  
Last weekend I unfortunately ran over something very hard, thankfully at low speed, but it damaged the prop enough to require a lot of work on it. The damaged 17 inch prop is aluminium and came with the new Yamaha 175 engine in 2000.
Before the damage, it would push the Outrage 18 2000 along at about 45mph but with a max rpm of only 4900 (fully trimmed out). The quoted max range is 4500-5500. I feel that if I bought a stainless steel prop there may be an increase in the achievable max rpm and top speed. May be the prop size/pitch isn't suitable for the boat/engine combination. Unfortunately I am not experienced enough when it comes to engines/props to have any idea. Before I get the old prop sorted out I hope someone can help me.
BillD posted 09-04-2002 11:39 AM ET (US)     Profile for BillD  Send Email to BillD     
Jacko:

I am not an expert by any stretch of the imagination, but I did recently change the prop on my 2000 18' Outrage with a Optimax 150, and spent some time researching it.

First, to answer your question about switching to a SS prop. If you switch to a SS prop with the same pitch and diameter as your old Alum. prop you should reach the same RPMs at WOT but gain 2 - 4 MPH in speed. You pick up the speed because the SS prop does not distort or flex as much as the alum. prop does. The distortion creates slippage as the blades flatten out. (see the reference article on props). If you want to increase RPMs you will need to either reduce the pitch or reduce the diameter (or both). I reduced the diameter of my prop by about 1 inch and gained a couple hundred RPM at WOT and a little speed. I also gained a better hole shot, better time to plane, and the RPMs wind up faster. These were the results I was hoping for.

I should also note that I switched from a SS prop to a Alum. prop for a couple of reasons:
1. Max speed is not that important to me. (where I boat the waters usually have too much chop for me to comfortably run at WOT)
2. Aluminum usually shifts easier than SS. I would always get the loud clunk when putting the boat in gear, with the aluminum prop there is less of a clunk. This is simply a function of the weight of the prop.
3. Aluminum is cheaper.

Also remember that the 90 gal fuel capacity of the boat adds a good amount of weight. If you are running at or close to the max RPM with a full tank, you may exceed the max RPM with a quarter of a tank.

Hope this helps.

jimh posted 09-05-2002 09:30 PM ET (US)     Profile for jimh  Send Email to jimh     
Are you sure the Yamaha OEM propeller is aluminum? They often supply Stainless Steel propellers which are painted black. Thus they look like aluminum, but are really SS.
prxmid posted 09-06-2002 02:56 PM ET (US)     Profile for prxmid  Send Email to prxmid     
My OR 18 with a 150 Optimax and 17 SS Mirage prop will max out at just over 5000 rpm at WOT. This seems consistant with other peoples results
tbyrne posted 09-06-2002 04:55 PM ET (US)     Profile for tbyrne    
I'm going to disagree with the previous opinions. If you put a 17" pitch SS prop on your boat, I'll bet your max rpms will drop 200-400 rpms. SS props generally have more cup than aluminum props (which helps the blades "grab" the water better) so a SS prop in the the same pitch as an aluminum will generally run at lower rpms.

I went from a 17p aluminum Yamaha OEM prop on my 80 hp motor to a 17p Stiletto and my max rpm dropped from 6,000 to 5,600. The Stiletto has more cup than the Yamaha cleaver-type blade, which I understand has virtually no cup.

All this can be thrown out the window if you are looking to go from the white aluminum Yamaha to the black SS Yamaha, since I believe they have the same geometry.

If you are going to change prop styles, I recommend purchasing from a seller who will let you trade/test various props. If you are going to use a different brand of SS, I think you should start at 15" pitch. Your max rpm with the old prop was at the low end of the recommended range. A highly raked or cupped SS prop would likely have a hard time getting over 4,600-4,700 rpm. Good luck!

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