Prop question, 2005 180cc w/ yamaha 85hp

[fuzzymaster]

Currently have a aluminum 133/4 x 15 prop and am getting about 25kts at 5500rpm, this is with half a tank and two people. 4 people probably only 20kts. Seems like I should be able to do better, what do you all think. Thanks

[freezerfiller]

I agree that seems a little sluggish. is 5500 WOT?

[fuzzymaster]

Maybe 5600 but thats about it. What do you think about a 17" pitch?

[fuzzymaster]

Also I think I am going to use some local grain scales and figure exactly what the boat weighs.

[gnrphil]

A 17 pitch will bring your rpm's down, probably to about 5100 so I don't think that will help. do you happen to know the gear ratio for that motor ?

[Juan]

As gnrphil said, a 17" prop is only going to reduce your rpms... you might go the other direction and try a 13" to get you in the rpm range that you should be turning... 5800 to 6000 at WOT should be about right for that motor. But before you start changing props, be sure your motor is set at the optimum height and you're trimmed out at WOT..

[fuzzymaster]

I just watched a couple videos for proper motor height setting, I'll go out tomorrow, with someone, and see what the cavitation plate looks like at wot. Maybe I can get our local mech to go with me.

[freezerfiller]

X2 on the height if you are buried and raise the motor, your rpms will go up as will the speed. Before you start messing with props. You can use a slip calculator once you figure out the gear ratio as Phil mentioned to determine if better results are possible..

[fuzzymaster]

The gear ratio is 2.o. I will get some numbers today and try the slip calculator. Commander, I gotcha. I just liked to try to get the best performance and fuel efficiency out of it. Thanks

[acdaddy0]

Fuzzy, I have a 13.25x17 aluminum prop, I don't know what brand as it came with the boat two years ago. 115 hp ocean runner omc. It's pushes 5600 rpm at 41 or so mph two adults and them two little kids in avatar and full tank 42 gallons. Gear ratio is 2.0. I did raise my motor one hole after purchase to get more revs. I like it set up to just start to blow out If trimmed to far then backed down while at speed. A fifteen should be good for your 85 hp if it's the right prop. I hate that I don't know the make of mine as it works great and I'll be back to the drawing board when I bust it. It does have cupping and some rake. Maybe try a smaller diameter in the same pitch . Just a reference if it helps?
i bought a 13x19 solas prop because I have always kept a backup prop in boat and just to see if it would get me more speed when I'm by myself with less load. It resulted in one mile and hour less with a lot less weight and 300 less RPM's and more engine strain. It is under the helm for emergency use only now. 

[fuzzymaster]

I finally got back out and got some numbers and info concerning plate position in water. Max rpm is 5200, and right around 27knts, that is two people and 25gals fuel. I checked the plate when boat was on plane at about 4000 rpm, 18 knts and it was at least 5" under water. The boat gets on plane pretty quickly and the max rpms is about right, I seriously think I need to move the outboard up a hole, maybe not so muck for speed but economy. There was alot of water waking? around the lower unit above the cavitation plate.

[gtightline]

5" below the plate in my humble opinion is way too low, your right, it sure sounds like it needs to come up a hole, at least....I have a 180CC with a 115 Yamaha two stroke on it, and the plate is just below the water at plane, not even an inch, just barely below. As acdaddyO says, I too have mine set up so it blows out if trimmed too far and I can trim it down a tad to where it "catches" it runs well at 5600 to 5800 RPM at 46-49 MPH depending on which way the wind is blowing..hahaha...85 horses might not be enough to get the kind of performance you are looking for..just saying....Good luck and keep us posted.

[gnrphil]

Yes raise it for economy and you might get a little speed increase as well but your mpg will improve somewhat.