RPM and Speed when cornering towing tube?

TexasMustang

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Got to take the new (to me) boat out today (Easter Sunday) with all the kids. It was warm enough that the kids wanted to get in the water. They had a great time tubing. Boat is a 2008 2275rl tritoon with a Yamaha 150. Boat was loaded with 4 teenage girls, 4 younger boys, and 3 adult men. I was towing a large tub (Super Mable) with 3 people on it.

Going in a straight line the boat had plenty of power/speed. I backed off at 25mph with the teenagers on it but it would have gone faster. When I turn to wip the tube out the RPM's would shoot to redline (showing just under 7k on tachometer before I'd pull throttle back) and I'd loose all speed. It felt like the prop was coming out of the water. The trim was all the way down. Any thoughts on what is going on. Could it be the prop? I don't know exactly what prop is on it; it is a 4 blade stainless steel. Going in a straight line and not towing the boat tops out at about 35mph at 5900rpm.

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BulldogsCadillac

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Couple questions that may help figure this out, first, what hole is the engine mounted in. Second, is that top speed with light load (just one or two people), toons clean? Third, when underway and on plane, get someone, or you to SAFELY look at the motor , is the flat horizontal plate that is just above the prop , riding right at waterline, below, or above?

Totally possible that it is just the wrong prop, could also be motor mounted in wrong hole. Are there any markings on the prop for pitch and diameter?
 

BigKahuna

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Good advice by Bulldog. Could be a combination of all factors. With the trim down you're losing speed in the turns and your RPMs shoot up losing bite. Cavitating. Sounds like the motor could be mounted a bit high. Follow Bulldogs advice and check that cavitation plate position at speed. Don't know much about 4 blade props but 35mph at 5900 rpms sounds right. You might be able to get a few mph more with a 3 blade properly pitched prop. Also keep in mind that 2275RL is a big heavy boat (I had a 2004) plus you have 11 people onboard and towing a Super Mabel with 3 more people on it. Of course you're going to lose speed in the turns! You're making that 150 work!!!
 

PartyBarge

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Yup, the above includes good things to check, you are ventilating the prop in turns (feeding air to it). Another consideration is how does the pontoon ride with that load while towing and in turns? If it is pretty flat or carrying too much weight forward, the prop could be getting too much aerated water and just unloads. Again while towing with that load, how does it corner? Flat? Or leaned into or out of the corner?

Usually prop ventilation in turns means the trim is too much, in your case it could mean the motor is too high, but weight distribution makes a difference too. Could be the previous owner had the pontoon set up for speed not aggressive towing... IF the motor needs to be down more to cure the ventilation in speedy hard turns, expect to lose some top end.
 
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TexasMustang

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Thank you for the feedback and suggestions. I will check on these things - it will probably be next weekend before I get the boat back in the water.
 

Vikingstaff

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They’ve nailed it above. To me it totally sounds like the motor is not mounted well for towing and turns, but just straight line spread. That weight distribution with so many people is also likely exacerbating the issue. Tweaking prop may help performance slightly, but I think really think it‘s the engine mount height and weight distribution.

Just work your way through the advice above: first check engine mount (cavitation plate while at WOT will help you determine this issue). Next up, being smart about weight distribution while tubing with a full load of people.

Article to help a bit

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CLDave

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Did you try moving people further back or keeping the heavier folks as far back as possible? Sounds like too much weight in the bow to me.
 

Jb75

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excellent advice by all the above but I’m with cldave. Move the big boys to the back of the boat when towing and see if it hooks up better. That’s cheaper than any other option.
 

TexasMustang

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Got out on the boat again this weekend with much better results. I had fewer people on the boat. was towing a smaller tub, and kept people more to the back. That combination of changes fixed the cavitation issue.

The motor is mounted in the lowest hole so remounting it lower won't be an option.
When on plane the cavitation plate is about flush with the water - like 2nd picture from left above. (I think - hard to tell. Had a teenager take a picture of it from the boat leaning over the back as much as safe - not a clear picture)
 

Vikingstaff

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Sounds like its mounted in a good spot then TexasMustang. Looks like simply being attentive to weight distribution when you have people on board will solve your problem. That’s nice. Enjoy your boating.
 
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