Pontoon Proping, Why the 400-600rpm rule per 2" don't work!

[Ryan]

Hey guys, I just picked up a 2012 23 foot xcursion with 2 pontoons, I got the 115 yammy on her and I have now tried 3 different props from the dealer and can't figure out whats wrong. I use the gps for speed and I am getting a top speed of about 20 mph at 5500 rpm, the dealership said I should be closer to 30. However with the 15 prop I went to go full throttle and it just wound out the motor and pinned the tach at 7000. I pulled that back real quick... then with the 17 I went full throttle and still hit about 6600 rpm, so then with the 19 i hit like 6200 rpm. and still top speed about 20 mph. didn't matter if there was just 2 of us on there or 3 adults 2 kids a full live well of water and a pug chihuaha cross. It was 20 mph right away out of the hole and then cavatation over that... The dealership has my boat right now trying to figure out the issues but do you guys have any ideas? I don't know the length of the leg on the boat but could it need a longer leg if it is available? I really don't know much about boats so any help when I have to go talk to the dealership will be greatly appreciated.

You've got more problems than cavation If your able to hit 7k with that engine. It's suppose to have a rev limiter at ~6400-6450. I can tickle 6400 with just me in the boat with a 13.5x15 but Im 31+mph and cavitating while doing it. Look at you model number on the engine It will have some numbers after F115. Mine is F115 TLR. I was told that the 'L' was for Long. Supposely a 25" long shaft rather than the shorter 20" piece in other yamaha models.

[Littlehoe]

Well still haven't gotten the boat back from the dealership but I have been in contact. They still are unsure of the problem. they were thinking a tach issue but on idle the tach seems fine when I ran it and trust me when it pinned that tach gauge at 7000 it scared the heck outta me. But I never hit a rev limiter that I know of... That being said is the limiter similar to a sled where it will retard the engine and sound like it is bouncing off the limiter as a description?

I told the dealership I needed the boat back by Thursday evening and they said they would make sure it was at least useable by then. I hope its something simple and they can have it fixed by then. If not I will run it how it is and record some video of it to post.

[BobG]

The part of the formula you are missing (and most people will never have to think about this) is elevation. At about 5,000 feet, where I often boat, my Merc 115 is only putting out 95 HP. When I take it to Turquoise Lake, in Leadville, CO (Elevation 10,200 feet) that probably drops to 85 HP. Re-propping may be necessary.

[Shameful1]

I am brand new to owning a pontoon, have had several go fasts over the years (Eliminator, Hydrostream etc.) but getting the most out of a pontoon I know nothing about. i put a jack plate on last week (more for shallow running than anything) but it did not work out, it hung to low and plowed water and sent water up the plate and over the motor so that's off again. So now prop selection, I have a 2005 sweetwater 2019sc with a F90 yamaha turning a 13 1/2 X 15. WOT RPM is between 5000 and 6000 according to the manual. With two or three on board I am turning about 5300/5400, that seems low to me and want to be closer to the 6000 mark. So my question, will a 13 1/2 X 14 do the trick on a pontoon? I don't want to over do it and end up at 6500.

Thanks, Steve