RPM's on 60hp 4 stroke extremely low at altitude….

Just recently bought a 2010 Lowe 165 FM with Merc 60hp 4 stroke. 1.83:1 gear ratio. Prop was what it came with, guessing it was stock: 10-3/8″ dia. with 13 pitch. Operating rpm should be 5500-6000. In the last 2 weeks I took it to 2 lakes. One at 4800′ and one at 8600′.
Results…
4800′ lake-Max RPM 5200 Speed:@24mph
8600′ lake-Max RPM 4200 Speed:@18mph
I think I have to reprop for these lakes just not sure to what. Monday I’ll call Mercury but till then just wondering what others experience with this situation has been. Don’t want stainless right now so going with aluminum. Did the Mercury prop selector and it came up with the following:
11.25×10
11.625×10.5
10.875×11
10.625×12
10.75×12
Lot of variables there for me to make a call. I like my current trolling speed but am willing to be slower after a prop change but not faster if I can help it. Any advice would be appreciated….
Thanks.

Normally a good rule of thumb is one pitch will change you 200 rpm. So if you went down to a 11 pitch and ran it at 4800′ you should get around 5600 rpm. But a lot of the smaller pitch props don’t have much cupping in them which will actually give you more rpm’s than just 200 a pitch. The bigger diameter props will probably be flatter more like a pontoon prop. If you do go smaller your trolling speed will slow down as well.

Thanks for the input Bryan…Glad you mentioned the cupping because I was thinking of buying one “with” cupping but under my situation looks like I might want to stay away from that to get the most rpm I can. If I want to try to max out to about 6000 rpm I guess I should get a prop that will max out out near 6000 rpm at the 4800 foot elevation and just have to accept my top rpm at the 8600 elevation may be about 5000 rpm’s. I do a lot of trolling and some jigging so I typically don’t run long distance but I like to be able to get up to 25-30 when the weather comes up. Granted, the lower pitch in theory will make me go slower but I hope the increased rpm will over come that. I agree looks like I’ll most likely go with an 11 pitch, just not sure about the diameter. I’m currently at 10-3/8 dia 13 pitch the one 11 pitch the calculator put out was 10-7/8 dia. so that may be the ticket. I’m really curious what the Merc folks will recommend when I call them tomorrow.

Is that an EFI or carb motor?
Ive never dealt with marine performance at altitude but rather snowmobiles. I know at 8k you need to be looking into jetting on carbureted engines, not sure if prop would change that much if the engine can not make the power?

It’s an EFI. I expected some performance loss but not as much as I’ve gotten. When it’s all said and done I won’t have a lot of top end but I need to get the rpm’s up and try to minimize my speed loss. I’d really like to see a solid 25 MPH and 5600-5800 RPM.

Well, my local marine store recommended trying a Hustler 12×10-1/2. Tried it out at 8600′ and much, much improved. Was able to just reach 5500 and 26 mph but I had to trim to just about cavitating to hit that. When I made slight turns it would briefly cavitate so I had to trim down in turns. Also, riding into the wind and chop I couldn’t quite hit that 5500 RPM. Talked to my local dealer and Turning Point (Hustler manufacturer) and they both recommended going to the 12.5 x 8. My local shop told me they would change it out for me. WOT on the Merc 60 is 5500-6000. I’m hoping I can hit about 5700-5900, be able to trim the leg down a little and maintain at least 25 mph, which is fast enough for my needs. I’ll post again after I try it out early next week….

I put a 12-1/2×8 Hustler prop on and it’s definitely better, in some ways. It comes on plane faster, less ventilation in turns and chop, gets me into the proper RPM range (5800 rpm) but i did notice a drop in speed by just a couple of
MPH. I got 5800 RPM at 23 MPH MAX. This is good because now I’m operating at the proper WOT rpm but I wish I could get a little more speed. My last and final try will be a QuickSilver 12-1/4×9. That may just be the sweet spot prop for my boat. I assuming the prop design is somewhat different and the only to find out on any given boat is to try it….I get back with results.