Yamaha VMAX 90 on a 20ft pontoon

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Current prop I'm running is a 13.5X15-K. Any thing above 5000 RPM feels as the boat is slowing down and sounds like the prop is cavitating. Tried at different trim settings, no difference. Any ideas? Also, lost my rear nav light cover due to low bridge. Light still works, just need a cover.
 
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Didn't see any damage to the prop and the toons are clean. When I bought the boat, mine was the first VMAX 90 they installed. Put in the water 3 times to run it before they let me have it. I'm thinking they got frustrated with trying different props and gave up.
 

2fast4u

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wonder if your motor is mounted too high on the transom, usually motor cavation plate should be flush to 1 inch below the water flow feeding the prop at full speed. You might want to try it with all your people load in the rear of the boat. You have never stated what type of payload you had on the boat when this happens.
 

mowdoc

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Check the prop hub - could be spinning under heavy load. You will prolly have to take it somewhere that has a shaft to hold it with. If you try spinning it on your outboard you will just turn motor over.

The hub can be easily replaced in some props.
 

mowdoc

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Oh yeah, best bet on your stern light would be to order a new light complete online and just use the lens from the new one. They don't cost much and you may find an LED that you want to use.
 

basshawk84

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Current prop I'm running is a 13.5X15-K. Any thing above 5000 RPM feels as the boat is slowing down and sounds like the prop is cavitating. Tried at different trim settings, no difference. Any ideas? Also, lost my rear nav light cover due to low bridge. Light still works, just need a cover.
!5 pitch sounds like too much for a 90 hp engine. I think you would be better in the 11-13 Pitch range. When I had my 115 HP Yamaha, I went with 13 Pitch 4 blade Nemesis aluminum prop made for pontoons. It performed great.
 

PartyBarge

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The comment just above is spot on, except it isn't that the 15p is too much, it's that the k-series prop is a poor choice for a pontoon boat! Way too much slippage, just as you are getting big-time at high power. Same is true of the 13 through 15p K's that I tried (except one of the 15's, not sure what it's deal was...could only get about 4200 RPM). Anyway, switched to the pontoon specific 4-blade Nemesis 13p almost two years ago. I doubt there is a better prop for boats/motors like ours, although certain other 4-blade props may be just as good! We pull the Nemesis at a nominal 5500 RPM, so you would have some headroom if your VMAX has a bit more highend HP.
 

2fast4u

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The comment just above is spot on, except it isn't that the 15p is too much, it's that the k-series prop is a poor choice for a pontoon boat! Way too much slippage, just as you are getting big-time at high power. Same is true of the 13 through 15p K's that I tried (except one of the 15's, not sure what it's deal was...could only get about 4200 RPM). Anyway, switched to the pontoon specific 4-blade Nemesis 13p almost two years ago. I doubt there is a better prop for boats/motors like ours, although certain other 4-blade props may be just as good! We pull the Nemesis at a nominal 5500 RPM, so you would have some headroom if your VMAX has a bit more highend HP.
please tell me what " K " prop is, thanks
 

PartyBarge

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The K-series is Yamaha's standard small-case (4 1/4") 3-blade aluminum prop. No telling how many of those are out there, but they are not a great match for the high drag/low speed of a modestly powered pontoon boat. I expect they work well enough in other applications....

BTW: here is a general rundown of the various Nemesis props (they are typically just a lower cost twin of the Merc Spitfire). Note the pontoon recommended version is the 10-13p small case. I liked the performance so well that I bought a large case version (4 3/4" 23p) for our 190 Hp Stingray (although it has been tweaked a little). Works very well there too as an all-around low-cost aluminum prop. To be sure, nothing is going to dethrone the Merc Hi5 as the best pulling prop! I'll be putting that one back on for the 4th.

 
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DaveyJ

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The K-series is Yamaha's standard small-case (4 1/4") 3-blade aluminum prop. No telling how many of those are out there, but they are not a great match for the high drag/low speed of a modestly powered pontoon boat. I expect they work well enough in other applications....

BTW: here is a general rundown of the various Nemesis props (they are typically just a lower cost twin of the Merc Spitfire). Note the pontoon recommended version is the 10-13p small case. I liked the performance so well that I bought a large case version (4 3/4" 23p) for our 190 Hp Stingray (although it has been tweaked a little). Works very well there too as an all-around low-cost aluminum prop. To be sure, nothing is going to dethrone the Merc Hi5 as the best pulling prop! I'll be putting that one back on for the 4th.

I use a 12 pitch Nemesis as the back up to the 14 Reliance on our Yamaha F150. It’s a good performing prop, especially for the cost!
 

PartyBarge

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LOL Without a rev limiter, your motor would likely take that Nemesis 12p beyond 7k RPM!
 

DaveyJ

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LOL Without a rev limiter, your motor would likely take that Nemesis 12p beyond 7k RPM!
Actually my boat is quite a load for the F150 so, with just me on board, it’ll just barely get to 6k and, with my typical load, will be around 5,500 WOT.
 

PartyBarge

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Actually my boat is quite a load for the F150 so, with just me on board, it’ll just barely get to 6k and, with my typical load, will be around 5,500 WOT.
Oops, forgot you would have the large-case Nemesis and the 150 Hp motor would have a different gear ratio too. What is the speed difference between the Reliance and the Nemesis?

Just finished swapping out a Nemesis 23p for the ole standby Hi5 23p on our Stingray followed by a short shakedown...all is well!
 

DaveyJ

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Oops, forgot you would have the large-case Nemesis and the 150 Hp motor would have a different gear ratio too. What is the speed difference between the Reliance and the Nemesis?

Just finished swapping out a Nemesis 23p for the ole standby Hi5 23p on our Stingray followed by a short shakedown...all is well!

29 with the Nemesis vs 33 with the Reliance
 

PartyBarge

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4 MPH is about the difference between our best Stingray speed of 56 MPH (with an SS Turbo three-blade 24p) and the 23p Nemesis at 52.

BTW: Your pic shows a well-distributed reasonable load. In the past couple weeks, I've seen a pontoon that did a nose dive with every application of power putting water up on the deck and two that were riding so low no tube was visible! The decks on both were pushing water like you'd expect from a barge! Thankfully, I didn't see what would happen if they took a wave from a wake boat operating a max-plow. Seems to me someone is going to discover what a watercraft with all flotation down low and most weight up high can do... Over the years I've seen people making their bid for a Darwin Award and it seems to be getting more common. Ya All be careful out there.
 

DaveyJ

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4 MPH is about the difference between our best Stingray speed of 56 MPH (with an SS Turbo three-blade 24p) and the 23p Nemesis at 52.

BTW: Your pic shows a well-distributed reasonable load. In the past couple weeks, I've seen a pontoon that did a nose dive with every application of power putting water up on the deck and two that were riding so low no tube was visible! The decks on both were pushing water like you'd expect from a barge! Thankfully, I didn't see what would happen if they took a wave from a wake boat operating a max-plow. Seems to me someone is going to discover what a watercraft with all flotation down low and most weight up high can do... Over the years I've seen people making their bid for a Darwin Award and it seems to be getting more common. Ya All be careful out there.
That was a 4th of July parade a few years back. We were at rated person capacity with 15 on board. Have done it a couple of time at a slow cruise. Normally run 7 to 10 people but everyone likes to sit in front which is the downfall of the lounger layout. Just have to pay attention....
 
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kaydano

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I've seen some pontoons riding extremely low too. I always thought that once the pontoons are just below the surface, adding one more pound of weight would sink and flip it over. One side of a boat will have more weight than the other (it's inevitable), and while that is fine if the toons are above water, as soon as they get submerged, that weight difference will be enough to flip it. And turn into a Darwin Award.
 
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Update to my prop issue. Thanks for all the replies, but I have a little more info. Benny 20sv, Yamaha 90hp SHO. Current prop 13.5X15-K.
40004pm, 1/2 trim 14mph
5000rpm, 1/2 trim 19mph
5800rpm, 1/2 trim 22mph
WOT, 6200rpm after hitting rev limiter (7000) 23mph
Smooth water, no load
with 8 people:
5000rpm - 17mph
WOT 5800rpm after hitting rev limiter 18mph
 

PartyBarge

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That's similar to what we had with the original 13p K-series. Both the 14p and the 15p (still have that one) ran calculated slippage of more than 30%. That all changed with the 13p Nemesis Pontoon prop. Yesterday, with 7 aboard and a bunch of gear we ran ~5200 and ~21 MPH. With two on a medium inflatable (one adult and one 6-year old) and in a hard turn the WOT RPM would drag down just under 5000 (could get about 18 MPH straight-line towing).

I don't use trim as you do... Likely our transoms are a bit different and I rigged an extra 2 degrees of tuck under (negative trim). I use no trim unless the bow is heavy or above ~15 MPH.

P.S. Never hit the rev limit even with light load unless I (purposely) trim out way too far. Max RPM is ~5500 with light load.
 
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