First, full disclosure: My knowledge of motors is minimal. Now, my situation. I'm in my second year of running a 2001 80 hp, 4-stroke Yamaha tiller. (Love it.) Up until the past month or so, my top RPMs/speed (on plane without porpoising) was about 5400-5600 and 34-36 mph. Lately, I've been able to push 5900-6000 RPMs and top speed of 37-38. (Honestly, the 6000 scares me, so I usually back off.) I'm wondering why the change. The only thing I've done to the prop was remove it to remove fishing line back in July. (And this "newfound" power just began the past month.) So why might this be happening? Is it good? Bad? (And, yes, this is with the same basic weight in the boat - me and 1 passenger.) Thanks for any insights!

I can't explain where the added rpm's came from, but I don't think I'd be too concerned about that or running it at 6k - that's the upper end of the rpm band, and where it's designed to run.

marine_man

My motor is running a lot better in the cool temps lately too.

That would be my guess also. Cooler air temps.

Colder air = better running and a bit higher RPM's.

Like the others have said.. Outboards always perform better at cooler temps.. I also would not be too scared of 6k rpm, its on the high end, but your mot probably has a rev limiter, and if your not hitting that,you should be good.

IF you want to ower your rpms, mabye get the next higher pitched prop...

check your prop for damage as well.

Thanks for the info. I learn something new here all the time...This lesson starts with cooler weather (bit of a "duh" moment, even for me) and includes "rev limiter." (How do I know for sure I have the latter?) Regardless, thanks!

While I agree that cooler weather will increase horsepower, I don't know that it will get your 400 more RPM.

Regardless, with respect to a revlimiter - you have one. Almost all outboard motors manufactured today have them.

marine_man

Ya, 400 does seem like alot. I know any of the boats I run don't turn that much faster in cooler weather.

Might not be the same for boats (although I don't know why it shouldn't) but it's not uncommon for snowmobiles to pick up quite a few r.p.m's when run in a temp swing like that. I've experienced it plenty of times for eg. taking off on a ride around mid day with temps around 20 degrees and riding late into the evening when the temp has fallen to below zero, I wouldn't be suprised to see a change in r.p.m's of at least 300 r.p.m.'s. Now taking in to consideration a two stroke outboard which is set up to run during summer temps 60 to 90 degrees and put it out on the water on a 40 degree morning and you have a similar temp. variance. I would say it's more common than people realize. Cold air= more power and the ability to better pull the prop your running. Might be a fun little research project for ya, take note of where your boat runs on a nice warm summer day then check it against the numbers you get on a cool fall day.