Pontoon Boat & Deck Boat Forum

Hi all I have a 24ft Harris pontoon and have removed the mercruiser 135hp and fitted a mercruiser 5.0 MPI and with an Alpha 1 drive with a ratio of 2.00 I have tried several props from a 15 right up to a 24 four blade but I am blowing the props out in a straight line and even Turing left to right .. I have been told the drive ratio is to high and should be around 1.50 will this bring the revs down ...do I need to install rakes on the sides of the pontoons to help lift and turning...paul

A 2.0:1 is the low gear ratio and your prop is turning slower than it would with the 1.5 higher ratio. Technically, you'd need to be running a higher pitch prop than you would with the 1.5:1. How are you doing vs. the engine's posted redline after the motor's trimmed up?

You'd think that with the additional weight of the V-8 your boat would be riding lower in the water, and that you be less likely to be getting ventilation.

I would suggest you contact Harris' Customer Service Department. They may can get you to someone in engineering that's well versed in I/O's in tritoons, and could give you suggestions.

Hi thanks for the reply pontoon does not respond at all to trim up as soon as I try to trim the revs raise quickly I have tried 15p 3 blade right up to 24p four blade and it did not bring the revs down at all had an increase with the 24 of about 3mph and that's it. If I try to accelerate fast it will just blow it out and rev it's brain out .... There must be a prop that out there that will fix this.... ... paul

First of all, Welcome to ya Paul!

I have several questions in order to get a better idea of what you are doing. There are several of us here running I/Os. Hopefully one of us can help you.

Was this a twin tube that you added a third tube with an I/O bay, or did you modify a center tube that originally had an outboard?

I ask that question in order to determine if you have a factory outdrive mount or not. If you cut your own mount into an outboard tube, I could see a couple of potential problems that you might be encountering. 1. Transom angle may be different than design or 2. Height might be off.

Do you happen to have a picture of it (on the trailer)? That would tell us much.
Secondly, how does the boat sit in the water? Fairly flat with a small tilt sternward?
A running shot (taken from shore or another boat) would also be helpful.
At risk of alarming you, I am slightly suspicious that it might be mounted 'high' somehow.

Once we know that you are properly mounted, then propping is next. I can't tell you much about gear ratios but there are lots of guys here with prop experience. I can't think that gear ratio stuff would make you 'blow out'. Rather you would just be off on your rpms.
I agree with Bamaman in that you may need to pitch up even further, but let's make sure you are rigged right first.

Hi thanks for the help my pontoon is a two pontoon boat that was fitted at the factory with the 135 mercruiser in front of the engine well it had a arrow shaped aluminum structure that I think did nothing but divert water away from engine well I removed this alloy frame welded atop on it and turned it into a fuel tank ..I will post some pictures as boat is on my holiday cabin.. It does sit slightly raked to the transom but does ride a lot better than it did with the 135 but but can't stop the prop from slipping it seams to be worse when I turn to he right then the left but even in a straight line it slips

Very strange. Sounds like maybe a larger diameter prop will give it more grip on the water. Also, might want to check how you mounted the props. I would mark the prop hub and the cavitation plate to check if the hubs are slipping.
The engine in my barge sits in a fiberglass pod between the pontoons. If the original engine worked fine, having the rear sitting lower in the water may be disrupting the water flow to the lower unit. Trimming UP is not helping; try trimming in or down.