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Author Topic:   stainless vs. aluminum
jokor3 posted 08-25-2003 11:05 PM ET (US)   Profile for jokor3   Send Email to jokor3  
If I am achieving 5800 to 6000 rpm's with a 13.25 x 17 aluminum prop on a 70 yamaha mounted to a 15 ft. striper and am considering changing to stainless... do I need to reduce pitch since the stainless props have more cup? Recommendations appreciated
Bigshot posted 08-25-2003 11:51 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
No.....nothing wrong with doing 56-5900 instead.
jokor3 posted 08-26-2003 06:44 PM ET (US)     Profile for jokor3  Send Email to jokor3     
Bigshot.......if this will reduce rpm just that small amount how will this effect of overall speed and holeshot performance?
Bigshot posted 08-26-2003 09:26 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Top speed by 5% or so improvement, holeshot may suffer 1 second or nothing at all.
Peter posted 08-26-2003 11:05 PM ET (US)     Profile for Peter  Send Email to Peter     
What is your top speed at WOT? Something doesn't make sense here. The Yamaha 70 on my 15 Super Sport turns to 5900-6000 RPM at WOT with a Yamaha 13 x 19 aluminum propeller. Top speed is 43 mph indicating about 6 percent slip at WOT. The Yamaha is mounted all the way down.

Is the Striper that much heavier to require a 17 inch pitch? Before you go any further in selecting a propeller, report back your average WOT speed as measured with a GPS over several runs.

Bigshot posted 08-27-2003 10:31 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Peter, your 19 alum is not as efficient or strong as SS. If you were to put a 19" SS prop on your boat it would drop a couple hundred RPM's but pick up speed.
Peter posted 08-27-2003 11:13 AM ET (US)     Profile for Peter  Send Email to Peter     
I would expect a drop in RPM switching to a stainless steel prop of the same pitch, not because of strength but more due to additional cupping that is usually found on a stainless steel propeller.

My question/concern with respect to Jokor3 is that Jokor is achieving the same WOT RPM with a 17 inch aluminum propeller that I achieve with a 19 inch aluminum propeller. I can't believe that an extra 1/4 inch of diameter would have that much of an effect on WOT. Based on Jokor's data with a 6 percent slip assumption, he should only be seeing about 39 mph which is about 4 mph lower than the estimated maximum and what I see. With no slip(impossible), he'd only see 41 mph.

If his top speed is 39 mph, then back calculating with the speed predicting formula suggests his loaded boat weighs 1500 lbs, about 300 lbs more than my loaded Super Sport. Need more data.

Bigshot posted 08-27-2003 11:22 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Aluminum blades at speed flatten out because they are weaker than SS, this killing performance. His boat is heavier but not much. His prop may be double cupped which makes it a 18". Your prop might have little or no cup, etc. Too many variables. He also did not say his prop was OEM or Michigan, etc.
jokor3 posted 08-28-2003 09:54 AM ET (US)     Profile for jokor3  Send Email to jokor3     
The prop I am running is OEM yamaha 13 1/4 x 17 K. At wot and fully trimmed (to the max) I read 6000 and comfortable trim probably 90% I am reading 5800. Do not have access to Gps only paddle wheel on fishfinder to measure speed. I realize this is not a perfect speed check. I was mainly interested in the assumed top speed and hole shot performance as opposed to exact figures. If I were to use
a 13x17 stainless would I need to change the pitch size because of increase cupping? or use the 17 with its cupping probably equalling that of standard 18-19 w/0 cupping?
jokor3 posted 08-28-2003 09:58 AM ET (US)     Profile for jokor3  Send Email to jokor3     
Also if it makes any difference the motor is mounted in the second hole from the top leavin g approx. 1 1/2 inches between transom and bracket.
Bigshot posted 08-28-2003 10:28 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Setup sounds good, go with a 17" Stainless. You can find them on E-Bay. Merc props fit Yamaha as well.
jokor3 posted 08-28-2003 11:21 AM ET (US)     Profile for jokor3  Send Email to jokor3     
Thanks Bigshot for the input and advice.

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