Major diff. in wot rpm from alum. to SS prop

[anzomcik]

my 1986 90hp merc, came with a 19pitch SS on my 17 foot sportcraft adventure. The motor reved to 4500 with it and a speed of 36mph, i was having lowend power issues off idle, so i put on my alum. 17 pitch to see if it made a difference. i rev to 5200 and go 33mph at wot.

I did the math 17 pitch should be my fastest prop.

Does a SS prop take that much power to turn. I gained 350rpm per inch of pitch. If i were to buy a new SS prop should i go to a 15 pitch to try to match the difference in rpm drop?

 

[SimpleM]

Re: Major diff. in wot rpm from alum. to SS prop

Uhhh... You are seeing EXACTLY what my math says you should be seeing. More revs with the 17 pitch, more speed with the 19. The math must take slippage into account. The higher you rev, the more % of power you loose to slippage. Actually there is an optimal speed for prop efficiency but neither takeoff or wide open is that optimal speed, it is somewhere in between. Also, aluminum just has more slippage than SS. SS will not flex from water pressure, and are less prone to ventilation/cav.

 

[lots of boats]

Re: Major diff. in wot rpm from alum. to SS prop

Hi

You can add to the practical application of the pitch/rpm/speed math equation if you have SS and Aluminum pitch props with the same rated diameter and pitch. A simple off the shelf aluminum prop will usually spool up faster and turn at a higher RPM than a similar SS prop. So there are practical applications, such as whether your boat has enough power to pull a skier out of the hole, it may not have it with the SS, but it may be able to do it with the same pitch aluminum and give you a little more RPM and a little more speed than you would get if you dropped to a lower pitch SS prop. Also if your motor will not turn up to the rated rpm the prop pitch is too high.

Cheers.

 

[anzomcik]

Re: Major diff. in wot rpm from alum. to SS prop

I am trying to find the correct prop but there are numbers that are do not seem right. According to my repair manual my motor operating range is 4500-5000 rpm. That seems low to me, because the 115hp has the same basic motor as mine, but it revs to 5000-5500. My go to mechanic says when he worked on the tower of power when they were big in the 1980s mercury told him to let them run about 6000-6200rpm.

I was shocked to see such a big difference from the 19pitch SS and 17 pitch Alum. I am using this boat for fishing out on lake erie, so trying to get every last drop of performace out of it isnt a big deal. I want a good relialable motor that can get me out and back with not having to worrie about over loading or over loading.

Any thoughts on 15 pitch SS, or stick with 17pitch? Any thoughts would be great

 

[towtruck1]

Re: Major diff. in wot rpm from alum. to SS prop

First of all, please post the Diameter of the various props. Diameter has an effect on rpm and slip. I am assuming they are the same, but are they?

As for max rpm on your rig, engine companies usually specify an optimum power range (most efficient power, best torque) which is often near the max rpm. MAX rpm is also usually dictated by motor longevity, reliability, and vibration reduction. IOW, go too high and your engine does not run as well or last as long. Your engine may be a de-rated version of a more powerful engine, but there might still be some limitations which would make it bad to exceed an rpm that is ok on the bigger engine. BTW, older engines are more likely to rely more on torque and less on RPM to achieve power. I would try to get a more definitive answer on what your max RPM may be. Merc should be able to do this. Have you tried online?

Comparing props is difficult because each manufacturer has design differences that go beyond dia & pitch. You are comparing too many variables. Assuming 5000 is your real max allowable rpm, find a prop that gives you that at WOT. Since you had power problems at low rpm, I would use aluminum, as this will flex a bit more and let the engine rev better.

I did the math 17 pitch should be my fastest prop.
Except that since you are comparing an AL vs a SS prop, the effective difference in pitch is greater that 2", as the Al wil flex more.

Your best bet may simply be to use an Al prop of the same diameter and 19" pitch.