Re: prop info for aquapatio with115 yamaha
Okay, a few thoughts.
Sounds like what you're experiencing upon turning is ventilation, which happens when air pockets develop around the prop, thereby denying it any water to "bite" into in order to propel you. When you throttle back the water settles enough for the prop to bite again. If it's convenient, you might consider asking a marine shop or your marina to take a peek at your motor height and ask if they think you need to lower it a little.
15-18mph at WOT might be a tad slow for that rig, so let's look at a few things. What condition are your toons in? Are they clean and straight? Gunked up toons will cost you several mph. Are they watertight? Water in the toons will slow you down as well and is more common than many people realize. If you trailer your barge or have some way to get it out of the water, grab the front of it and shake it back and forth while listening for sloshing.
If your toons are clean and watertight, the next thing to look at is RPMs. I think you said you're turning 5200 RPMs at WOT. Are you sure your tachometer is accurate? If so, is that the right number? Your manual will tell you, but if you don't have a manual ask a dealer, and while you're at it ask which prop is spec. If it turns out you aren't getting enough RPMs you need to find out why. Are you confident your motor is performing up to snuff? If not, you might consider checking compression, which is cheap and easy to do. Only after you have satisfactorily addressed all these factors will it be time for prop experimentation.
If and when that time comes, keep these things in mind. 1)Barges like props with big blades and some cupping. 2)For every 1" drop in pitch you'll pick up around 200 more RPMs. 3)More RPMs don't necessarily equal more speed. In fact, some times they equal less.
Hope that helps. I wish I could provide a simple answer to your question, but unfortunately there isn't one. If you haven't already, do some searching over in the pontoon section. There's a lot of discussion on this very subject and you may find some of it helpful. Good luck.
