Pontoon Boat & Deck Boat Forum

Anyone out there a prop expert?

I took my toon out a while ago and at full throttle, I was barely getting up to 3k rpm... My boat has an aftermarket ss prop (14 pitch) and under one of the seats is what I believe to be the original aluminum prop which is a 13 pitch... Do you guys think that change would allow my motor to get up to full rpm? Is there any reason that someone would use a prop that would limit full rpm? I am stumped...

Also, the original prop has some nicks and gouges (minor) on the edges, should I take it in and have it fixed up or not worry about it?

It's my understanding that 1" of pitch equals about 400 rpm.

Mike, I am not an expert but be sure you read these Mercury articles on props..
I guess you are thrusting too much with this pitch hence the RPM will drop.. (but I am not an expert though....)

all I know about props is stay out of the shallow water....and rocks

My first ( and i mean first ) outing I found some shallow rocks and i wasn't pretty....

I have a two piece Hustler prop.. and i like it

Thats great advice... I will probably keep the other prop onboard as a spare!

Two words prop guard, but I don't think they work on anything bigger than 90 horse the last time I checked.

Our rocks float, there call gators and their head is hard and will take out a prop, I hit a 10 footer that was slow at getting out of the way , I hit with the keel of the toon and then he hit the prop-guard, that saved him and my prop.

I got a cut on the inside of the guard, but I didn't lose my SS prop

Mike, I don’t mean to sound discouraging, but it sounds like you may have other problems. Even if the props you have are not "dialed in" you should be getting more than 3000 rpm. I'm not sure of the exact specs on your Merc, but WOT should be around 5500 to 6000 rpm, and 13 or 14 pitch prop should have you a lot closer that that. I would have it checked out before you do much experimenting. You don’t want to risk expensive damages.

There's something not right if you can only get 3000 rpm's. You did unload it off the trailer first, LOL. Just a joke. No way the props you mentioned would limit your rpm's to that degree. Get your motor checked out before you play with props.

Mike also make sure your tach is set up properly and you are getting an accurate reading. BTW I have a 90hp Yamaha with a 13 5/8" X 13 pitch prop and it turns 5,400 rpm at WOT. What is your top end speed now?

Mike a prop change is not going to get you 2000 more rpm's. You didn’t go into detail on how it ran at other RPM’s. I would start checking the Motor. Go in this order, if the first thing checks out move to the next.
1)Do a compression check. They should be within 10% of each other. Most motors new run 140-150 PSI. You will still run OK down to about 110 but motor will not perform as well.
2)Do a spark test to make sure you have spark at all cylinders. And check spark plug condition.
3)Check your fuel system. Filter, Pump, Carbs
It could be something simple like not getting enough fuel or major: bad cylinder or blown head gasket.
Good luck.
Here is a good site for DYI information
http://www.themarinedoctor.com

Very good advise... I just spent about two hours over at the other forum, man that guy is good with outboards!

Judging from the way the motor ran, my first guess is the tach is not setup right... I will go out and look when it stops raining... I will also do a compression check and replace any fuel filters...

She was running real smooth with no trouble accelerating or idle...

I was running at about 4900 RPM with my 14 pitch prop. Switched to a 13 pitch and my RPM went up to about 5400 or so...right where I wanted it. I think my engine is rated for 5550 RPM. I did not want the posibility of letting someone else drive and over rev my engine or if I was distracted, I did not want anyway to over rev it so

Ok, I just went out and did a few checks..

I removed the tach and the arrow in the back was pointing to 5, since the range was 1 thru 6 I assumed it was based on the number of cylinders and switched it back to 4... This may be the reason my rpm were not right??

I did a compression check (dont know if I did it right) but after 5 cranks with the other plugs out I was getting up to around 110 psi on each cylinder.. Seems low but they were all the same so that seems like a good thing?

The spark plugs were pretty fouled looking so I will replace them prior to the next outing...

Then it started raining on my so I had to put the cover back on!

Anyone have any comments on my action?

As long as there close to each other, you should be alright, and yes tach set to 5 would put your tach off a little, rain what's rain, my grass turned to dust a month ago.

But if I still had a grass cutting service like I had a few years back, it would have been raining to beat the band.

Ahhhh, good advice... I went to the Faria website and downloaded some tach instructions earlier but they must have been for an outboard motor! I just went back and looked again and for the inboard motor your are right, 12 pole and electrical pulses are 1/2 the number of poles so six is right!

Thanks!! This one answer has made building this forum worth it...