Prop size for 20' pontoon w/ Mercury 4-stroke 115hp

[BamaPat]

I?m looking for some advice on prop sizes. I have a 2014 20? pontoon with a 115hp 4-stroke Mercury outboard. Typically driving with a few people in boat and often pulling a tube. Original prop was Aluminum 3 blade 13.75 x 15. RPMs at FOT were about 6,000. I wanted a little more top-end speed so I changed to Solas Aluminum 3 blade 13.25 x 17. I went from 21-22 to 29+/- MPH, an FOT RPMs have dropped to about 5,000. Engine specs suggest RPMS to be 5,000-6,000. Is being on the lower end of suggested RPMs making my engine work too hard, or is this still considered efficient? Last week I hit an underwater log and dinged the prop pretty good and need to replace prop. I?m wondering if it?s wise to get another prop with 17? pitch at 5,000 RPMs or go with original size of 15? at 6,000 RPMs. ? Or does anyone have any other suggestions on what would be a good prop choice to keep MPH close to 30 and RPMs in good range?

Thanks in advance!

 

[ahicks]

For me, it would depend on my priorities. If you're tubing or pulling anything, you'll want max pulling power, which will be best provided by an engine turning at max rated rpm. If you're just cruising, you might enjoy the lower rpms, and possibly better fuel mileage offered by an engine running on the lower end of it's max rated rpm range. The extra speed would just be a bonus....

5000 or 6000 your call. Neither will damage the engine.

 

[dussbake]

first of all, ROLL TIDE?...or war chicken? lol

I have a 24' pontoon with a Mercury 115 CT 4-stroke and have learned a lot lately about props thanks to people like the gentleman above me. My opinion based on your description above is to probably go down in pitch to get your rpm's back up. It will give you more torque for pulling tubes. Be careful on your diameter though. I would suggest going back up to your original prop diameter if not a full 14".

 

[rriggle2]

Have 2016 Suncatcher V20FC with F90LA, original prop was 13 5/8x14. Was getting 24 MPH at 5800 wot. Switched to 13 1/5x15 and getting 25 MPH at 5700 wot. Now going to go with a 17 pitch to see what happens, may also go back to 13 5/8 but the 1/8 difference I don't think matters nuch. With original prop, 1000lbs people, 27 gal gas and 400 lbs gear pulling 2 kids on tube I was 19.2MPH 5800 wot but make a sharp turn and down to 13MPH. The 24 and 25 NPH speeds are with 440 lbs people, 27 gals fuel and 100 lbs gear. Yamaha Performance sheet states the boat went 29MPH 6000wot with same fuel load and two adults? Would be happy if I could ever get it to that speed. Also 30% prop slip seems really high to me.
Now out with a 13 1/4X17 and not much differance, 26.2Mph at 5575 WOT. I guess no matter the pitch you put on a water plow it will still be a water plow. Not sure if a 13 1/4 X 19 would make any differance now.

 

[BamaPat]

Thanks for everyone's input! I typically do two types of driving with this boat, either tubing close to home at WOT or longer cruise 25 miles to friend's house on complete opposite side of our lake. I think I'll keep original 15 pitch prop on for tubing to keep RPMs up and better torque, as ahicks and dussbake suggested. I went ahead and bought the same 17 pitch prop as what I dinged up, and will spend the 5 minutes to change to faster/better gas mileage prop for the longer cruises.
Thanks again for the advice!

 

[ahicks]

Just a thought, maybe you could shoot for a compromise prop by sending in the damaged prop with the intention of having it repitched as part of the repair process. A good shop should be able to handle that pretty easily at a cost not much more than a regular repair. Tell them what you told us and ask them for suggestions. You could end up with a prop both you and your engine really enjoy in the process