2.75's are they just bad designs?

[nasdaqsam]

Long time boater, especially pontoons. I have had 4 of them the last had a 496 HO Mag in it.

Anyway, we purchased the latest one (2016 Southbay 522 2.75 w/Merc 150 and lifting strakes) few years back pre-loved. We did not get a chance to water test it. For the most part it does the job for us, but I am really getting annoyed at a few of its little quirks.

All of the following was done with myself and my wife in the boat at the helm seats at various speeds. So not an uneven weight issue.
This pontoon has been the hardest to find the sweet spot as far as trim is concerned. Even when I do find that sweet spot (once I find it, I usually give it one bump down) you can be running along for quite a while at various speeds (but typically only happens above 17mph) it will just start to cavitate and over rev. Putting the engine all the way down when it does this does not help. It will not dig back in. You have to bring it back down to a very slow speed or stop and start again. I thought it was the possibly a bad prop or the hub. But the prop looks like brand new. There is just under 200 hours on it and not so much as a smug where it hit anything. Thinking the hub I pulled it and brought it to my boat guy. He swears nothing wrong with it. But for $30 I installed a new one. I thought this may have fixed the issue. To test it I left the trim all the way down and ran it full throttle. It did not cavitate or over rev. So not the prop or the hub. But the same issue still happens all though maybe a little less. (That could be my imagination). It seems to do it more into swells.

Only one thing makes any sense and that is; the 2.75 is just a poor design. There is a massive amount of turbulence between the toons right where the center toon ends, and the engine pod starts. This toon has more wash coming between the toons at the rear when running than any pontoon I have ever owned. I just can't imagine there is thousands in costs between a 2.75 and a 3.0 that it would be worth giving up all the performance. Had I known this issue prior to buying I would have passed on this one. I attribute some of it to the shorter length than I am used to also. My shortest next to this one was a 24'. This one especially if we have more people on it and they are sitting forward wants to pick the back of the boat out of the water so have to run with no trim. Just seems to inefficient running with no trim all the time when it can plane out.

Bad design or am I missing something?

Thanks in advance for input.

 

[ahicks]

My vote (pontoon owner since 1978): missing something
There's nothing very precise about the water flow to the prop of a pontoon. I believe sir, that your issue CAN be helped with a different prop. My first 2 tries/thoughts, would be to try a 4 blade prop. 2nd, would be to try some type of super cupped design prop.

 

[jlh3rd]

is the motor at a correct height?
lowering my then new motor in 2021 on my 24' pontoon just 11/2" fixed my ventilation issue...

 

[nasdaqsam]

My vote (pontoon owner since 1978): missing something
There's nothing very precise about the water flow to the prop of a pontoon. I believe sir, that your issue CAN be helped with a different prop. My first 2 tries/thoughts, would be to try a 4 blade prop. 2nd, would be to try some type of super cupped design prop.

Thanks for the reply. I can check with my boat gut to see if he has any in stock I can try. It will have to be next summer now as the year is wrapping up for us on the lake for the year.

 

[Scott Danforth]

First make sure the motor is at the right height. Too low and it plows, too high and it ventilates. A pontoon sure could use a powered jack plate as the prop height changes so much with load

 

[nasdaqsam]

is the motor at a correct height?
lowering my then new motor in 2021 on my 24' pontoon just 11/2" fixed my ventilation issue..

is the motor at a correct height?
lowering my then new motor in 2021 on my 24' pontoon just 11/2" fixed my ventilation issue...

Taking a quick look with the motor all the way down puts the cavitation plate approx. 1" above the bottom of the center toon. There is approx. 1/2" between the motor mount and the top of the transom. So it could only be lowered 1/2". I am not sure if this is a short or long shaft motor.

 

[Scott Danforth]

add two coolers of beer to the back of the boat if you cant lower the motor easily

 

[nasdaqsam]

Also just wanted to add. When it is just me in the boat running at the best trim, I can achieve the boat runs 34.8mph (GPS) at 5300 rpm's. If I trim it anymore it loses all bite any less and I am scrubbing speed.

 

[nasdaqsam]

add two coolers of beer to the back of the boat if you cant lower the motor easily

The problem with that is it wouldn't be very long and the coolers would be empty and I would have the same issue......on the other hand at that point I might not care as much.....lol.

In all seriousness I could move anything heavy to the rear storage areas. I don't have a lot of heavy items to begin with though.

 

[Scott Danforth]

If it was me, I would invest in a hydraulic jack plate, however a manual one would help you dial in the motor.

maybe one from bobs machine https://bobsmachine.com/

or TH Marine https://thmarinesupplies.com/collections/atlas-jack-plate-atlas-jack-plate-accessories

 

[nasdaqsam]

If it was me, I would invest in a hydraulic jack plate, however a manual one would help you dial in the motor.

maybe one from bobs machine https://bobsmachine.com/

or TH Marine https://thmarinesupplies.com/collections/atlas-jack-plate-atlas-jack-plate-accessories

I'm not sure I would need a hydraulic one as this is not really a performance boat that would benefit from adjusting the height on a regular basis. However, the manual one or I see they sell extension platers that would accomplish setting the motor a little lower.

How far down should the motor be? Put the cavitation plate 1" to 1 1/2" below the bottom of the center toon?

 

[Scott Danforth]

Each boat is different

 

[nasdaqsam]

Each boat is different

True. Thank you for the help. I'm going to have my boat guy look at it also when I bring it in for winterizing. If we can grab that last 1/2" between the transom and the top of the mount for just the cost of drilling some new holes and then maybe try out some props with more bite like a 4 blade as mentioned above.....maybe that will fix it enough without having to add the expense of the jack plate. If it doesn't, I think I will probably sell it and go to a 24' true tritoon. I have it in my head now the 2.75 toon has something to do with this issue also.

 

[Scott Danforth]

Get 2 people to sit in the back to effectively lower the back a bit to see if the extra half inch is worth it before drilling holes

 

[nasdaqsam]

I'

Get 2 people to sit in the back to effectively lower the back a bit to see if the extra half inch is worth it before drilling holes

I will give that a try.

 

[jlh3rd]

yep..I wouldn't drill holes yet. If it's one thing I've learned about pontoons in the last seven years it is they are more susceptible to ventilation and prop height compared to a standard hull.
I just have a twin tube, the motor is a long shaft, and before I lowered it, I couldn't trim up at speed at all...
My understanding is that a ventilation plate should be level to the bottom of a boat's hull, as a starting point, which means your toon....
the other test is a visual of where the plate actually is while running at speed. It should be right at the water height....
1/2 inch?..new holes?...I just don't know....
a prop might work........a jack plate, yep....
I do believe the motor is too high...
Is that the original motor and factory setup?..

 

[nasdaqsam]

yep..I wouldn't drill holes yet. If it's one thing I've learned about pontoons in the last seven years it is they are more susceptible to ventilation and prop height compared to a standard hull.
I just have a twin tube, the motor is a long shaft, and before I lowered it, I couldn't trim up at speed at all...
My understanding is that a ventilation plate should be level to the bottom of a boat's hull, as a starting point, which means your toon....
the other test is a visual of where the plate actually is while running at speed. It should be right at the water height....
1/2 inch?..new holes?...I just don't know....
a prop might work........a jack plate, yep....
I do believe the motor is too high...
Is that the original motor and factory setup?..

Thanks for the reply. Yes the setup is all original.
For now, I am going to move the heavier items, again I don't have a lot to the back of the boat. And next time I have people over try putting them in the back only to see if motor height is the issue. I suspect it is.
I like the jack plate idea but that's a major expense for a hydraulic unit. I have found a few manual ones fairly cheap so that may be an option. Once the motor is at the right height, I wouldn't think moving it up and down is going to be something I would have to do each trip out.
I am also going to look into 4 blade props.

On that subject. It is currently set up with the stock aluminum 3 blade 15.25X15P prop. And as stated previously it runs 34.8 at 5300 RPM's with a full tank of gas and just me or me and my wife in it.
First, is that about right for current set up for this configuration?
Second, is there a chart that determines which 4 blade would be a close replacement? Or is it a try them out type of thing?

Thanks

 

[jlh3rd]

ah, propping a boat......I cannot, nor will, tell you what prop as propping a boat as I found out can be.....interesting. And I knew basically what that involved.
I'll say this: I put a new 115 merc. CT motor on my 24', twin tube (25") pontoon. Normal operating rpm range is 5000-6000 rpm.
They put a 4 blade, 14x14 on it. Max rpm is around 5700 with a 26 mph top end with me on board. But pulling two tubes with teenagers and people resulted in 5200- 5300 max rpm and speed below 20...too low for me..There was no 4blade, 14x13 so I went with a 3blade, 16x12 which gave me 6000+ by myself, but 5600-5700 pulling tubers and people....enough to keep teenagers from getting bored....this is the short story to my propping saga...
So propping a boat depends on what you want.
Short answer is I think you're 700 rpm's low....

 

[jlh3rd]

And I learned that reducing my prop pitch, or blade #, or diameter ( they ALL effect max rpm) my top end speed would drop but my rpm and "torque" would be better....
generally speaking.....

 

[nasdaqsam]

Update: taking a closer look at the height. I was incorrect. The boat is on a hoist so I couldn't get a good look from a level perspective. In doing so the cavitation plate is actually an inch below the bottom of the motor pod. So appears to be correct.
So thinking possibly prop issue. I called several prop sales companies and gave all the info, current performance and cavitation issues. All three independently recommended the same prop. The Blackmax XC 3blade aluminum 16x13P. The current prop is an aluminum 3 blade 15.25x15P.
This prop is apparently built specifically for pontoon boats. The wider 16" with deeper cupping helps with the cavitation pontoons experience. The 13p "should" get the RPM's into the correct operating range.

Thoughts?