new prop for new motor

[slo-toon]

I have a 95 Sun Tracker pontoon 24' I replaced the original 60 hp Mariner last summer with a 60 hp Mercury
4 stroke Big Foot. This motor came with a 14 x 11p Quicksilver Black Diamond prop. This motor turns four thousand RPM @ WOT. The owners manual says this motors max RPM is 5500 to 6000 RPM. I know this setup isn't going to be anywhere close to fast but, I don't feel that I am getting all this motor has to offer, so I am asking for help selecting a different prop so I don't have to run the motor wide open just to stay out of everybodies way. This is a lake cruiser only, no pulling tubes. Thanks for your help!

 

[Maclin]

Welcome aboard!

Some questions first...

About previous engine you replaced...:

• Was the Mariner a 2stroke or 4stroke.
• What prop was on it.
• Did that prop work ok, did the engine get into proper RPM range.

About new replacement engine...:

• What is the MPH at the 4k rpm.
• Is the Big Foot new.
• If not, what was it on before.
• Is it in good state of tune.
• Do you know the drive ratio.
• Are the controls working properly, and the engine throttle is getting opened all the way at WOT position.

Big Foots are supposed to be lower in ratio and allow for a bigger diameter prop if I remember their "reason for being", so 11p seems that it should be getting into the proper RPM range. But with the RPM's you are seeing, Conventional wisdom would say to go down in pitch. I have not looked up props for one, so do not know what all is available, but a 10p would be closer to what you need, assuming engine is good and throttle getting to actual WOT ok.

 

[slo-toon]

Thanks for the reply! Yes the Mairner was a 2 stroke, don't know what prop was on it but actual top end speed was a little more than what I have now. I do know that the prop was not near the size of the 14" big foot. I belive the motor was what came on the boat when purchased new. I checked the speed last summer, don't recall exactly what it was, something like 12-13 mph. I do recall that I lost some top end speed though .The Big Foot is new, installed by a local Merc dealer. At this point I can only assume that the throttle is actually adjusted properly. Wouldn't I want more cup in the prop instead of less?

 

[Maclin]

Right now the problem is the engine is lugging and will have internal problems later on if run like this. You need to get the RPMs' to at least the 5500 to get proper performance and engine longevity. Have you talked to the dealer about props?

Cupping is usually the last thing looked at, and mostly for getting the last 1-2mph out of a sport boat.

 

[slo-toon]

I just checked the throttle cable at wide open position, pulled it off of the linkage and had at least another 1/4 inch yet to go. I adjusted the cable and it appears to opening up more. Should I adjust it so the linkage on the motor is at max? This could be my whole problem (i hope).

 

[Maclin]

See if a 10p is available.

With assumptions of 2.3 ratio and 30% slip at 15mph and engine at 5500rpm a basic prop calculator came up with a 10p.

 

[slo-toon]

Yes a 10P and a 9P are available

 

[ahicks]

You just gave somebody a lot of money for that engine. Is there a reason you aren't discussing this with them? Generally the correct prop is part of the deal. From what you say, it's barely even close!

Regarding the extra 1/4" of travel, though it doesn't sound like it's been set up correctly, I doubt seriously that it will get you the rpm you're after.

 

[Maclin]

I just checked the throttle cable at wide open position, pulled it off of the linkage and had at least another 1/4 inch yet to go. I adjusted the cable and it appears to opening up more. Should I adjust it so the linkage on the motor is at max? This could be my whole problem (i hope).

Looks like we posted at the same time, I did not see this until now. Throttle travel seems to be a factor. In my opinion the Dealer still owes you a little bit of work.

If the engine still can't get to the 5500 rpm minimum, you definitely need a different prop. You do not want to run the engine with it lugging like that. If it cannot get into that top RPM range at WOT, even if you do not usually run there, internal damage will eventually occur. The Dealer should know this and want to finish off their sale for a happy customer, and not see you next year with a bum motor

 

[slo-toon]

The dealer I bought the motor from is nothing to brag about,I'll leave it at that. It will be 2 -3 weeks before I will be able to get my boat in the water. I'll see what I get out of this throttle adjustment and then possibly look at another prop.