Aluminum to stainless prop; lost WOT rpm

agrazela · Apr 26, 2004

I have a 78 70hp Mercury on 15' Starcraft trihull. WOT recommended range in manual is 5000-5500. Original propeller is Mercury 10-5/8" 12p aluminum (part# 48-73134A40). With this prop, I consistently got 5200-5300 rpm WOT, doing 30-32 mph by GPS. This prop has dinged-up blades, so I am considering either reconditioning or another prop. Friend has an excellent condition Michigan Match 10-5/8" 12p stainless steel (part# 033001; note: my measurement puts it at actually 10-1/2" diameter, though catalog says it it 10-5/8"). I tried this on boat without making any other changes. Now 4500-4600 WOT, but still doing 30-32 mph by GPS. Went back to old prop, WOT back to 5200-5300 WOT @ 30-32 rpm. Why the drop in WOT rpm's with the stainless? What can I try to improve WOT rpm's with the stainless prop?

KCLOST · Apr 26, 2004

Re: Aluminum to stainless prop; lost WOT rpm

Assuming the props are basically the same diameter and pitch.... I would think this is a prop weight issue... Have you weighed them to determine if there is any signifigant difference.... Just a guess.... Lower rpm motors don't normally handle SS props for that reason if I'm not mistaken...

gss036 · Apr 26, 2004

Re: Aluminum to stainless prop; lost WOT rpm

It might be that the stainless prop has a lot of "CUP" to it and that would definately affect the engine because it is not slipping as much. I just put a Mercury stainless prop on my 200 Merc and it has a lot of cup compared to the aluminum one. They both are technically the same, 14 1/2 X 17.I haven't had a chance to run it yet to see if will change things for my boat. KLOST mentioned weight, there definately would be a difference in weight. My stainless one weights at least twice as much, although I never actually weighed them, just judgement (hand balance)

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agrazela · Apr 27, 2004

Re: Aluminum to stainless prop; lost WOT rpm

Yes, the stainless prop certainly outweighs the aluminum prop. Also, I can see that it is more cupped. I also am sure that the stainless prop is less flexible. I can see where all these things can put more strain on the motor. But, I haven't gotten any suggestions on how to overcome this...Should I try to: Trim out a little? Raise "x-dimension"? Try trim tabs or a "fin"? Advance timing a few degrees (perhaps with higher octane fuel)? Has anyone had a similar problem and found a way to compensate?

LubeDude · Apr 27, 2004

Re: Aluminum to stainless prop; lost WOT rpm

You nearly always have to drop a pitch when you change to a ss prop, I think it is due to the lack of flex in the blades more than the weight.

KCLOST · Apr 27, 2004

Re: Aluminum to stainless prop; lost WOT rpm

LubeDude!!!!!! You know nothing about props!!!!! So don't mess up my perfectly good answer!!!!!

However, Lube may be right! I wouldn't mess with the motor to solve your problem. Your friend was a great help to you, in that you know what doesn't work. Try one lower in pitch and test if you want to stay with SS... But there is nothing wrong with aluminum either, especially on your HP range. You'll get it dialed in sooner or later... By the way, make sure you are using the corret thrust washer for whatever prop you use. You don't want anything to rub obviously. I'm sure you are ok, but worth mentioning. I have four props right now, and I will probably never use 2 of them. I learned from trial and error...

LubeDude · Apr 27, 2004

Re: Aluminum to stainless prop; lost WOT rpm

Originally posted by KCLOST: LubeDude!!!!!! You know nothing about props!!!!! So don't mess up my perfectly good answer!!!!!

Do too, Do too!! Its that thing on the back of the boat that costs a lot of money if ya screw it up!!

I just do not know what to put on my own boat, but I have lots of advice for others!! What are you running on your Ranger and how long is it? Your boat I mean.

KCLOST · Apr 28, 2004

Re: Aluminum to stainless prop; lost WOT rpm

Right now I have a 24P Trophy on it... I need to take it out and check speed and rpm, since I just recently raised the motor a few notches.... I've tried a 23P Trophy and it ran about 6000 @ 55mph... I'm thinking about getting my hands on a 25P, try it out, and then call it quits... The 24 is ok I'm sure, but I always think there is a little more in this boat!!!

Oh, It's basically 18' long...

Dhadley · Apr 28, 2004

Re: Aluminum to stainless prop; lost WOT rpm

agrazela -- the SS prop is doing what its designed to do. The SS has many advantages and among those is that it allows the prop to be designed to run a higher X dimension. It has more "bite" than the aluminum you had. Therefore your rpms are down. The speed is about the same. Therefore your efficiency has increased (your slip % is down). You should be able to raise the motor and gain rpm and therefore speed. There will be a point that this prop starts losing bite as you go up. That will be the limit of that prop. Youre on the right track. Good luck with your testing!

agrazela · Apr 28, 2004

Re: Aluminum to stainless prop; lost WOT rpm

Finally, someone with advice! It will probably be another week and a half before I get another chance to put her on the water, so I won't be able to check out x-dimension or trim angle until then. In the meantime, I haven't given up on trying to get more power out of this motor. I intend to check timing, throttle settings, and possibly perform a decarb (don't know when, or if, decarb was last done, as I have owned this motor only about 6 months now).

Dhadley · Apr 28, 2004

Re: Aluminum to stainless prop; lost WOT rpm

Sounds good. Try raising the motor a little now that you have a baseline. You should pick up some rpm and speed accordingly.

KCLOST · Apr 28, 2004

Re: Aluminum to stainless prop; lost WOT rpm

agrazela, Make sure you don't lose water pressure after raising the motor... You should invest in a water pressure guage. If you don't it may be too late until you realize that no water or lack thereof is getting up to the block... $40 is a lot less than several thousand.... I did it, and it was an easy install...

robalofish · Apr 29, 2004

Re: Aluminum to stainless prop; lost WOT rpm

Greetings, If your not sure about the de-carbing I would do it first. I speak from experience here. I had a SS prop on my 200hp reconditioned and put it back on my boat still wasnt at top RPM. I then had the prop shop remove 1 of pitch and RPM was good. Read some of the info here on de-carbing, de-carbed and had to take the prop back to have the inch put back in! And as you can see by my post most of the prop shops can adjust the pitch for you so you don't need to purchase a new prop to test. Just some more information, for your files.

agrazela · Apr 29, 2004

Re: Aluminum to stainless prop; lost WOT rpm

Update: Checked timing; it was retarded from factory spec about 6 degrees. I suspected it might be overly retarded due to slight wet fouling I had been experiencing on plugs. I set timing to factory specification. Motor absolutely WOULD NOT START. I took a good look in the compression chambers with a small dental mirror, and they were incredibly crusted up with blackish deposits. So I did a 24 hr-soak decarb with 3 parts mineral spirits : 2 parts toluene : 2 parts acetone : 1 part xylene : 1 part propanol. During tank-testing (55 gal drum) the next day, the motor spit out an incredible amount of oily, greasy, foamy gunk, and smoked heavily for about 30 minutes. After all this, compared to how it was before, the motor seems a bit harder to start cold, but much easier to start warm. It idles more smoothly, and throttle response up to about 3000 rpm (max possible without blasting out water from tank) is INCREDIBLY smoother. Moreover, the motor is much quieter AND there is now far less smoke in the exhaust. I hope all this will ultimately give me better high-end power. We shall see!

Hooty · Apr 29, 2004

Re: Aluminum to stainless prop; lost WOT rpm

It sounds like you're on the good foot but exactly which timing was retarded. Was it full advance timing or pick-up timing? c/6 Hooty

agrazela · Apr 30, 2004

Re: Aluminum to stainless prop; lost WOT rpm

Hooty, If I understand your question correctly, it was pickup timing I was able to test and set in my test tank. I will not get to check timing at WOT until I get her to the water. Specifically: The SELOC manual states to set timing at cranking rpm, with the spark arm stop just touching, to 28 degrees BTDC...this is supposed to result in 23 deg BTDC at WOT. The motor WAS set at 22-23 deg BTDC at cranking rpm with spark arm stop just touching. My theory is that the prior owner either: -retarded timing to compensate for gunk in the cylinders and/or to make the motor easier to start cold; OR -mistakenly set the timing at cranking rpm's--rather than WOT rpm's--to 23 degrees BTDC.

agrazela · May 4, 2004

Re: Aluminum to stainless prop; lost WOT rpm

Another update: Still haven't gotten boat to water to see if I can now turn the 12 pitch s.s. prop at 5000+ rpm; however, I took a really beat up aluminum prop, cut off most of the blade areas with a hacksaw, and made it into a test prop. With this test prop, I can go to full WOT rpm's (5500 rpm...with some throttle to spare), in my test tank (55 gal drum), in gear, without losing water out of the tank. The timing is now dead on at 23 degrees BTDC at 5500 rpm by this method. I will say that throttle response is now as smooth as silk, and there is no bogging or "drop out" anywhere on throttle up/throttle down. I attribute much of this improvement to the decarb procedure...I am now sold on decarb (I come from a strong "gunk is good" upbringing). Dhadley, I did remember that I have already boosted x-dimension up by 1 inch by wedging thick-walled aluminum flat rod stock between the transom top and the mount bottom (and redrilling the mounting holes/filling the old holes)...I needed to do that shortly after buying the motor just to make the 12p aluminum prop get to the proper WOT range! I am absolutely determined to make a s.s. prop work out on this motor...and unfortunately no one makes an 11p s.s. prop for this application. I have only owned this motor 6 months and have already ruined two aluminum props on all the kelp floating in San Diego Bay. I guess if none of my "tricks" to boost power from the motor work out, I could always have a prop shop remove some of the cup from the s.s. prop. What should I expect to be charged for such an operation?

Hooty · May 4, 2004

Re: Aluminum to stainless prop; lost WOT rpm

Do you think you could raise the engine another hole without venting? If ya can, that's where I'd go next. c/6 Hooty

KCLOST · May 4, 2004

Re: Aluminum to stainless prop; lost WOT rpm

Install the water pressure guage, if you don't have one... Raising motors, and kelp can kill water flow real quick!!!! You need to be sure that you are not going too high... Good luck...

agrazela · May 5, 2004

Re: Aluminum to stainless prop; lost WOT rpm

Hooty, Raising the motor further is the last thing I want to do...I would have to drill new mounting holes and fill old ones (again)...what a pain! Besides, the anti-cavitation plate is already about 3/4" higher than the bottom plane of the boat...I'm afraid to take it any higher. If my motor tune-ups do not get this s.s. prop up to 5000+ rpm, I figure I have three options: 1. Remove cup from prop. 2. Try trim tabs (probably smart tabs) 3. Go back to an aluminum prop and try to dodge the kelp! This boat is stern-heavy (being an open-bow with all the batteries/fuel/etc. in the rear), and does have a tendency to porpoise a bit, so I was considering tabs anyway.

Aluminum to stainless prop; lost WOT rpm

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