Motor mounting height, ventilation & performance issues

[Bigdaddy74]

We've had our new boat out a few times since we purchased it and I had to swap out props to get the right performance numbers and finally settled on a Saltwater Series II 15 3/4" x 15P. That pushes the boat right at 46 MPH at 6000 RPM with 3 adults, 5 batteries, our gear and a full tank of fuel. Before I was running a 17P SWS II prop and getting 45 MPH but my RPM was maxed out at 5600. Now that I've got the performance numbers where they need to be I think I may have an issue with the motor height. My 250 VMax SHO is mounted in the next to the top hole which I guess is referred to as the #2 mounting hole. It looks like the anti ventilation plate is about 0.5-1" below the water line at speed and I have read where it should be right at the waterline. The problem I am having is that when getting up on plane the motor seems to rev high at first but the speed doesn't increase (feels sluggish) quickly and I can't trim the motor more that about 30-35% without it beginning to ventilate. It feels kind of like a prop when it spins a hub but that is not what is happening here because once I get over about 4200 RPM it starts to feel like it should and eventually hits 45 MPH. Should I lower the motor one hole even though the anti-ventilation plate is already just below the water line? I also notice the motor revs higher and the boat looses considerable speed when making fairly tight turns. In the past I owned a 24' Aqua Patio tritoon with a 250 Vmax SHO and didn't notice this issue so something is different. I checked the Yamaha performance bulletins and on the few Bennington's that they tested that are equipped with the VF250XA motor they are all running the same prop that I have but they are all mounted in hole #1 and all are running right at 6000 RPMs and topping out at 44-45 MPH so I'm right there with them on the numbers. Should I move the motor down and mount it in hole #1? Will that eliminate the slip/sluggishness that I'm feeling on take off and will it allow me to trim the motor up a bit more? Is my prop just not biting in the water enough? Attached are a few photos of my current setup. Thanks in advance for any input.

2021 LS23NPAPG tritoon w/SPS+ package
2021 Yamaha 250 VMax SHO

 

[Jb75]

I’ve run mine (also vf250, but ESP hull) in both the lowest and the setting you are in. A bit faster when mounted higher but definitely sacrifice grip out of the hole and in aggressive cornering. I’m back in the lowest position currently for those reasons. Think you’ll be happier down a hole. And likely won’t lose more than a mph or two

 

[PartyBarge]

+1 The SWSII is a great design, but seems to not respond to ventilation as well as some other props. Keep in mind that the design/parameters of various transoms vary a lot, so mounting hole info may not cross reference well. Not sure putting the motor all the way down will cure all your issues, but it should help. Although your calculated slip at top speed is very good (typical for an SWSII), sounds like you are blowing out the prop at low speed...there's just not enough water around/past the prop to handle the power. Somewhere in that process, ventilation becomes cavitation...or vice versa...

 

[SLXGUY]

I have the same prop as you do and my 250 SHO is mounted in hole No. 1. I wrote this in another thread.......
"I have just received my 23 SLX (with the SPS+ hull) with a 250 SHO last Thursday. My dealer installed a 15.75 x 15 SWS II prop although I had previously indicated that I wanted a 15.5 x 16 SWS II prop. The dealer said Yamaha suggested the 15.75 x 15 and when I went on the Yamaha website it shows that this is the best all around prop (the website also shows the 15.5 x 16 as better for speed). I suspect Yamaha assumes you will load up a pontoon with people and need a better hole shot. The 15.75 x 15 SWS II easily spins to 6000 RPM (I only did this a couple of times after 4 hours and only for a short burst) with 5 adults and a full tank of gas. The hole shot is also very strong. Although my dealer said they would exchange the prop for a 15.5 x 16 if I wanted to, I think I'll keep the 15.75 x 15 because I rarely look to max out my speed as I normally cruise at around 4000 RPM and really I like the instant throttle response of the 15.75 x 15. The 250 SHO is a beast of an engine and with your boat your performance will be excellent regardless of the prop you might ultimately choose."

My GPS says I hit 47.5 at 6000 rpms.....so I think you would be well-advised to lower your engine.

 

[Bigdaddy74]

Thanks for all the advice. I just picked the boat up this afternoon after having the motor lowered one hole. Hopefully we will get to try it out this weekend.

 

[lakeliving]

My motor is in that same hole. Came from the dealer Installed on the top one so it was as low as possible. I gained some speed but lost cornering ability for sharp turns.