The Boating Forum - yamaha f115 power band

How do I find the most efficient cruising rpm for my yamaha f-115. (I would like to buy a new prop that matches the the best rpm to my most common speed which is usually 17-22 knots.) Hope I phrased the question right. I currently have a prop which fits the basic criteria when propping (wot 6k rpm @ 31 knots), but was wondering if I can manipulate it somewhat to get better economy.
thanks,
Louis

What hull is it on? Did you look up Yamaha performance specks for the engine and hull?

yeah, would like to know what hull its matched with...

my f115 powered my19ft pathfinder.

WOT at 5400-5600 i would get 41 mph

cruise comfortably at 26-31, getting about 3-4 miles per gallon, 4200-4400rpm

that was a 4blade stainless prop

It's a 22ft panga built in mexico. the hull is much beamier than typical pangas (8ft). I currently have a 3 bladed alum prop: Pitch = 15, dia. = 13.25". Like I said, I usually run between 16 & 22 knts depending on seas. that speed translates to approx 4-4,500 rpms. I get approx 4 nm/g (again depending on seas). Rpm numbers are not exact...but pretty close.

Sandy, I took your advice and looked up the performance specs on the yamaha site. Obviously my boat is not on the list, but looking at many boats, I found that 3500 rpms or 4000 rpms were the two listed as most efficient. I assume that means the engine is most efficient between 3500 & 4000 rpm?

thank you for your time
Louis

My boat is a 20' Sea Hunt with a 8' 1" beam. But what is more important is the hull weight. Mine is 1850 lbs dry. Mine also runs, with a SS prop at 16 & 22 knots at 4 - 4500 rpms. The prop is 13 x 17. For my rig Yam. suggests a 13 1/4 x 18 alum. The dealer proped me down when I went to SS. You could look for a dealer who would let you try different prop sizes but can you measure flow? For the amount of gas I burn it is not worth much effort.
There are a lot of guys on this site who really know props. I'm not one. Poke around in search and see what you can come up with. Maybe someone will be kind enough to link you up.
But first I think you've got to find out the boat weight.

I have a yammie 115 on a 22ft pangamarine...1800lb hull..best, easy cruise is 4000 rpm, 25 mph....4.5 mpg

Thanks Sandy,
I really don't know the weight, but my guess is that it is somewhere close to 1800 lbs. (incl. engine & 1/2 tank fuel). my numbers are for 2-3 people aboard w/gear.
Thank you for your numbers.

Slickster, what prop do you use on your panga?
thanks,
Louis