2nd opinions on prop selection

[johnyrude200]

Hello All,

I’ve finally gotten my mini tritoon to a usable state and am trying to dial in the prop selection. I’ve run into a new situation and could use some opinions/recommendations.

Motor is a ’93 50hp Evinrude longshaft. Operating range is 4500-5500 RPM. The motor struggles to maintain 4500 RPM with a 12.5 x 15P prop running about 16-17MPH under light load. I tried a 11.25 x 13 pitch, and it was over-taching at 6200 RPM and the boat lost about 8-9 MPH. I expected it to just jump up about 500RPM, but I didn’t even run it at WOT for very long because it was clearly too small a prop and the boat was limping at 8-10mph.

For an experiment, I threw a 11.75 x 17P prop, and I GAINED 500RPM, hitting 4900-5000RPM, and 19MPH, but it was working hard. So it seems that diameter of prop is a factor with this boat, and pitch isn’t the usual +/- 200-300RPM swing per pitch up or down.

Now I don’t have any other sized 15P props, or 14P props, so now I’m left to buying something to see if I can get it to the 5200-5500 range sweet spot. I am considering buying a 11.75 x 15P prop to see if that gets me to where I need to be. Smaller pitch seems to be a big waste of time, diameter seems to be the larger influence.

I will say in moderate chop (say, 2-3 foot waves), at WOT, I’m having a bad ventilation problem. The motor is about as low as it should be in the water, with the anti ventilation plate sitting parallel to the bottom of the center tube. I have considered possibly adding a 1-piece hydrofoil to help cup water down in these types of waters, as of course the motor revs up by as much as +1000RPM when the prop slips out of the water (guessing this isn’t good to happen regularly, which it was in the big lakes).

There are no 16P props I can find for sale by anyone or listed anywhere.

Moving up to a 70hp might be a little two taxing on the mechanical tolerances to this boat, and I’d be happy just doing 20MPH anyway if I can get that with the right RPM range and prop size.

What is disappointing here is I had a ’78 35hp motor on this thing last year with a 11 x 9 prop, and it basically did the same speed (17MPH). The 50hp with powertrim was only a net gain of about 35-40lbs in weight as I had a panther powertrim unit linked up to the old 35hp, which I would say is really only the equivalent of a modern day 30hp motor? So +20hp only yields +1-2MPH? No significant weight changes to the vessel since last fall.

[jerry-ahrens]

This is a fairly common problem on pontoon boats, especially with smaller hp. engines. What is the length of your boat? From my experience, you need a LARGE diameter prop. The larger the better. These props are sometimes referred to as ”elephant ear props” that are made for pontoon boats. Some have extra cupping. If you have an unusually short boat with a larger engine, this will complicate the problem. I would suggest checking out Mercury Marine’s prop offerings. They have a very wide selection of aluminum props for different makes of engines. There is also the chance that your boat/motor combo may just not be compatible, due to the gear ratio/weight of your 50 hp engine.

[wiscoboater]

If you have a good prop repair shop in your area you could try having them add some cupping to your original 12.5 X 15 and reducing the diameter by 1/4 inch. You were not that far off from your goal with that prop. The diameter reduction would pick up the RPM’s and the cupping and RPM increase would pick up your speed. It would not cost much to do that work and might get into the range your looking for.

[david-bartlett]

Another thing is that most pontoons are more of a displacement hull, than a planing hull so your speed is going to be regulated somewhat by your waterline length.

[fleetwin]

Yeah, most pontoon boats have a certain "hull speed", trying to push past this barrier usually results in engine lugging, ventilation, and just about most other unwanted performance headaches. So, it seems like your rig is comfortable at about 16mph, I guess I wouldn’t try to push it much faster.
I remember my old 24′ pontoon, more fun than any other boat I’ve had, especially when playing Rodney Dangerfield pulling into the snooty sailing clubs asking about becoming a member! In any event, the old 73/74/75 65/70hp was comfortable at about 3/4 throttle with a big ol 13" prop, the engine wasn’t struggling and I’m guessing we were at about 15mph. Pushing the throttle down farther only resulted in ventilation, engine lugging, and erratic ride.
Most normal propping theory goes out the window when trying to deal with a pontoon boat, so keep experimenting.

[johnyrude200]

here is what the boat looks like, all aluminum contruction and SS components, including furniture, so I won’t have to replace anything down the road. It’s 7’wide x 17′ long, middle tube is a 22", outer tubes are 13". Middle tube sits lower. I have underlayment installed but I don’t know how much of a difference it makes because the only spot that gets hit with rip-tide splashing is the back 1/4 of the boat.

[fleetwin]

Cool looking rig, sure "looks fast", hoping it will live up to your expectations!

[fisherman6]

That is a cool looking boat. It looks like fun. If you don’t mind my asking, why the larger diameter middle tube? I’m wondering if that larger tube is causing part of the problem. Does the middle tube sit lower in the water? I’m no expert for sure, but if the larger middle tube sits lower in the water it is forcing it to be a displacement hull and slowing it down. All three toons really need to be level across the bottom.
-Ben

OldJohnnyRude on YouTube

[Tom]

Doesn’t the middle pontoon mess up water flow to the motor?

T

[johnyrude200]

My reason behind the middle pontoon being lower to simulate the hull design of a traditional boat. I looked at some of the big pontoons being made, as in, the $70,000 ones, and saw they were doing this, so I said "all right I’m not totally insane." It follows the same port/starboard to keel taper as a normal hull.

When I gun the boat, it does in fact get up on plane (you feel it rise up), but the center tube ‘splays’ the water laterally, the outer tubes cup it, and then it deflects back toward the middle tube around the last 1/4 of the boat. I added underlayment because this was effectively causing a straight shot of water at the engine (plus deflectors at the back). Without the deflectors, it was a like a fire hose of water being shot into the boat from the rear.

The motor sits with the ventilation plate at the same height as the bottom of the center tube, copying the design of a traditional hull. At WOT and on plane, it behaves the same. However if I trim it, due to the amount of turbulence, ventilation/slippage starts to become an issue. The boat handles fine in moderate chop (say, 2-3 foot waves), and bounces similar to a regular boat.

Granted, 99% of my experience as a boater is with a 14′ starcraft aluminum tiller, so I’m still cutting my teeth with other style hulls.

My goal is/was 20-23MPH, I’m not far from that right now (19mph), and I don’t think the 50hp has the right prop on it yet because again, it’s working hard to hit 4900-5000 RPM. I think 5500 would give me that little extra I’m looking for.

[Author]

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