Got my new motor, Need a different prop? here are the results ??..

[chrisg]

I had downsized from a ?87 I6 115 Merc. To a ?99 60 bigfoot.

The Bigfoot has a WOT range of 5000-5500.

The marina put a used 14/13 Aluminum prop on it. My old merc had a 14/15? SS prop.

This is on a ?87 parti-barge, 24-foot pontoon, just me and 10gal of fuel. And only having the gauges on the dash to go by.

At WOT, the RPM was 5800 when trimmed out. The max speed appeared to say 15MPH. I don?t think the motor liked that, it was screaming. It appeared to be happy around 4000 RPM, which moved it at 10MPH.

Should I put my old prop on it, that would be a 2? increase, and if I recall that would result in a 2*200RPM decrease, or 400, resulting in 5400 RPM, within the range.



The Mechanic said don?t use SS, they are gear breakers. What is wrong with using SS, I know they are heaver, are they only ment for bigger motors?. Should I get something else? (Someone talked about elephant blades, did not find any). Will increasing the pitch increase my speed?

Other then that, it?s a heck of a lot quieter then the old motor. ( and it started with no issues!)

 

[crb478]

Re: Got my new motor, here are the results ??..

Re: Got my new motor, here are the results ??..

I would try it with your old prop since you don't have anything to loose. See if you likeit better or not. Your mechanic is steering you away from stainless steel props because they do not give when they strike something like aluminum ones will. When they hit something hard the weakest link will often break. That could be an aluminum prop or if using a stainless prop it coould be your gearcase.

 

[Dhadley]

Re: Got my new motor, here are the results ??..

Re: Got my new motor, here are the results ??..

The weakest link should be the hub. That's what they're for.

 

[chrisg]

Re: Got my new motor, here are the results ??..

Re: Got my new motor, here are the results ??..

Thanks, I was worried that it had to do with something else. I just read on another post that a SS prop actually may give better performance.

So, considering the RPM and speed I'm now getting, does it sound like changing to my old SS prop will do the trick? (I have to get the marina to locate and give me back my old prop.) I could try it out, but is it worth doing this, or do the numbers suggest a different pitch?

 

[rndn]

Re: Got my new motor, Need a different prop? here are the results ??..

Try your old prop as it seems it may be the right size for you. SS props do not give as the aluminum props do upon impact, however the hubs are supposed to give way prior to gear case damage.

 

[chrisg]

Re: Got my new motor, Need a different prop? here are the results ??..

Well, I tried putting my old SS prop ( they ARE heavier!) now, the speed appeared to improve, and the engine sounds fine at WOT ( not screaming, relatively quiet). If it was not for the fact that I have a tach, I would think everything was good.

But the tach now reads 4800RPM at WOT, and its just me and 6Gal of gas. So, while the speedo said 18MPH ( Ya, I don't trust it). It did seem faster - Perhaps because the water was smooth and I had a tail wind.(stern wind?)

Now The Prop the Marina put on was a 14/13 - it gave 5800RPM, the SS prop was a 14/15 ( That's what the mechanic said, I did not see the numbers on it ( it is dirty - perhaps cleaning it would make a difference). I expected to drop the RPM by 400-500 RPM not 1000!.

So either:
A) the SS prop is pitched higher then I was told ( it was on this same boat, but on a 115 MERC.) OR
b) the 200rpm/pitch change does not apply going between an aluminum and SS prop. ( I recall reading somewhere that SS props will result in a lower RPM then the equivalent aluminum prop).


So, should I:
1) leave alone: 4800RPM is not that far off of the 5000-5500 RPM range ( although that's with just one person- I still have to test it with a full boat.) Will this really hurt the motor, or am I just losing performance?

2) Buy a new 14/15 Aluminum prop, and hope that will put the RPMs in the correct range.


Thanks

 

[freddyray21]

Re: Got my new motor, Need a different prop? here are the results ??..

I would suspect the prop maybe mismarked. from one pitch to another the rpms will decrease (going up in pitch) about 200 or so. Get the right prop for your motor without over taching it or under taching it.

 

[Randybeall]

Re: Got my new motor, Need a different prop? here are the results ??..

Most designs of SS props are more efficent than the same pitch/diameter in alumnium. That means when a prop change of 1" in the same metal results in a 200 rpm change you can not say the same when you change metals. SS in the same pitch may slow the engine 100 to 200 rpm. I have to suspect your big change is either misidentified prop or damaged and modified prop. Perhaps the best next step is to take both props to a prop shop for identification before you spend a bunch of money.

 

[chrisg]

Re: Got my new motor, Need a different prop? here are the results ??..

Thanks all:

The new prop was a 14/13 ( I read it on the prop)- it gave me 5800 RPM.

I suspect that the old Stainless Steel Prop was different. ( as I could not read any numbers off of it - perhaps if I clean it up I will find out what it is)

Either way, I'm pulling the boat out for the winter, so I'll get a new aluminum prop next spring. Since I know the new Al. prop gave me 5800RPM, I'll get a 14/15 Al prop and that will put me in the correct range. (The marina took back the prop and gave me my mony back.).
Once I clean up the old SS prop, and determine the actual pitch, I'll see if I can sell it.

 

[chrisg]

Re: Got my new motor, Need a different prop? here are the results ??..

I was doing some looking around, and noticed there are no 14/15 props, they are 13 3/4 /15" (called a 13/15). But I also noticed that most of these 'wizards' indicate a 14/13 is the correct prop.

This again is for a 24-foot pontoon, 60hp bigfoot. does this pitch ( 13/15) sound right?

 

[chrisg]

Re: Got my new motor, Need a different prop? here are the results ??..

I found a prop wizard that had my parti-barge as a selection, so I entered my info for my boat, and it came up with 14/13 for water sports, and 13/15 for speed. Since this is a pontoon, I wont be doing watersports, so it sounds like a 13d ( actually 13.75) 14p is right.

My SS prop I found out is a 13/17, which explains the 1k decrease in RPM.

The marina that first sold me the 14/13 prop has a 13/15 Al. prop on sale, thinking of getting that, putting it on before taking the boat out of the water.

This should put the RPMs in the range of 5400 RPM, which is in the WOT range. That's with only one person. What reduction in RPM should I expect when the pontoon is loaded? (perhaps I should ask this in the pontoon forum)

Chris

 

[steelespike]

Re: Got my new motor, Need a different prop? here are the results ??..

Loaded speed could vary a lot depending on amount and placement of the weight.Also the pontoon length and diameter of the logs.

 

[chrisg]

Re: Got my new motor, Need a different prop? here are the results ??..

thanks, I'll ask in the pontoon forum