RPM? 5000 or 5800

[zibzer]

Hello,

I have a 1979 100hp Evinrude with a 15 Pitch prop

At WOT and properly trimmed I get 27MPH (GPS) - 33mph (pitot)

At WOT i get 5000RPM Exactly

Myt manual says 5000RPM is WOT RPM, but some people on the forum are saying others with a similar engine should be at 5800RPM.

Now im very satisfied with the motors speed and acceleration and general performance, but is running at 5000 going to hurt it?

I just dont want to be swapping engines in a few years, but if its just carbon buildup and such that will happen then I dont mind cleaning the carbs occasionally.

Or if theres a better prop out there for me, let me know and i will price it out.

thanks for your help!

its on a 1987 Peterborough 17'5 Foot Runabout

 

[iwombat]

Re: RPM? 5000 or 5800

If your WOT is only 5000 you are lugging the motor. Shoot for 5.6k+ WOT. 5000 was ok for the previous generation of gas. With the new ethanol blends you need to shoot for a highter RPM range.

This is not to say you need to run it there. You need to set it up so its _capable_ of turning that high or you're lugging it throughout the rpm range.

 

[Theoutdoorsman]

Re: RPM? 5000 or 5800

I just dont want to be swapping engines in a few years, but if its just carbon buildup and such that will happen then I dont mind cleaning the carbs occasionally.

Cleaning the carbs has very little to do with extracting carbon deposits. I rebuilt a 1979 E'rude 140 two years ago that was horribly built up inside the exhaust housing. Needless to say, it was the demise of the powerhead. You definitly NEED to decarb those older bubblebacks annualy. Check the frequently asked questions (FAQ) forum for the thread on EXACTLY how this should be done. Follow it to the letter. If the outboard is new to you, I'd do it immediately without hesitation. Iwombat is a senior member here who has once again offered some great advice on your rpm's. Mine is currently propped to turn 5600-5800 rpm's depending on load and live well weights. The closer to 5800, the better off you'll be, in regards to the build up of carbon deposits. I agree, 5000 rpm's is lugging your outboard. Dhadley is one of the many setup guru's here. You might consider starting a new thread on suggestions for the setup of your rig before throwing money at it via a new prop. You may be surprised at the advice he can offer you!!!!! And it won't cost you a penny either!!! You have everyting to gain and nothing to loose......... ......... Good luck!!!............... ALAN


EDIT: I'll save you some searching on the decarb process. Happy reading!!!

http://forums.iboats.com/showthread.php?t=158076

 

[iwombat]

Re: RPM? 5000 or 5800

You can generally pick up a few hundred rpms just by raising it up on the transom a bit. 800rpms is probably not going to happen unless you're way off of an ideal set-up. It's not unheard of though.

 

[Theoutdoorsman]

Re: RPM? 5000 or 5800

You can generally pick up a few hundred rpms just by raising it up on the transom a bit. 800rpms is probably not going to happen unless you're way off of an ideal set-up. It's not unheard of though.

I'm sure you'll agree, I see some folks doing some pretty strange things these days. Some of them will do just about anything to make a sale....... LOL

 

[ezeke]

Re: RPM? 5000 or 5800

You can also pick up a few hundred RPM by having a clean smooth bottom on your boat. The real test is to do the math; Take your prop's pitch, the RPM, and the gear ratio of your lower unit and see how far from the theoretical maximum speed you are.

 

[zibzer]

Re: RPM? 5000 or 5800

Thanks for the tips guys. Sounds like i better do some research and gain a few RPMS. I had the motor put on the transom by a profesional, so i would imagine its in a good position, but ill take pics and get you guys to check it out. What do you mean exactly by setup of the rig?

 

[Theoutdoorsman]

Re: RPM? 5000 or 5800

Thanks for the tips guys. Sounds like i better do some research and gain a few RPMS. I had the motor put on the transom by a profesional, so i would imagine its in a good position, but ill take pics and get you guys to check it out. What do you mean exactly by setup of the rig?

Layout of the floor plan (weight distribution), height of motor on transom, and generally things of this nature. You should really consider posting a seperate thread in regards to the setup. This would likely catch the eye of some of the guru's on board. I'm definitely NOT the one to offer many suggestions on this. There are more quallified persons for that. They can offer much more knowledgable advice than I, as they've probably forgotten more on this subject that I'll ever know.......... LOL ......... Definitely post the pics in the topic thread!!! You will be surprised at what you'll learn. These guys are good!!!! Best of luck................ ALAN