YANMAR 2GM RPM / OVERPROPPED QUESTION

[Northeaster]

If a boat has been (and still is!) overpropped for years, and therefore has never been run past approx 2000rpm (yanmar 2gm), will it hurt now to slowly try to bring it up to max rpms (WOT)in neutral?

I cannot bring it anywhere near max rpms while in gear, as white smoke will come out, and I don't want to overwork the engine to the point of causing damage!

Explanation - just installed a new magnetic pickup tach, as the boat never had one, and i wanted to know my rpms. The specs say 97 teeth on my engine. I set the teleflex guage is for the range 90-110 teeth - it may need calibration, but I think is somewhere in the ballpark, as idle shows about 800rpm.
I can rev to about 3000 in neutral - may go more, but I didn't want to damage it!
Will put out white smoke above 2200 rpms (again guage may be off some).
On my 30' med displacement (9000-10,000lb) sailboat, I can do 5 - 5 1/2 kts around 1800-2000rpms, but will smoke it I try to go above 5 1/2 or 6 kts. I know my hull speed would not be much greater thna 6kts, but I am nowhere near cruising rpms (approx 3000rpms) at this speed, and white smoke starts coming out.

PS the engine works / sounds very good until it revs to the point of white smoke - does not overheat, etc!

I thought it was overpropped anyway as it has a 15" x10 3 blade prop - the prop caclulator showed I should have something like a 15" x 7.4 or a 14" x 8.6.

I will likely have some pitch taken off, once the boat is hauled in the fall, but want to know that it will not damage it more as is.

Advice appreciated!!

[Inlet Marine Services]

A 2gm or 2gm20f is a 3600 rpm motor, running at 2000 rpm will cause carbon build up in the head and exhaust to the point it will only smoke and not reach full rpm. Fix the prop and see what it will do, the cont rating is 3450.

[Northeaster]

Inlet - Thanks for the advice! I will fix the prop / pitch trouble next haulout - But is it worth the expense of hauling out with only a couple months of sailing left - East coast of Canada, since that prop has been on there for years and years?

Also, is it OK to try and see how high it will go -WOT, IN NEUTRAL, to see how it sounds, and what my tach reads - it is a 25 year old engine? Will it hurt to open it right up - I am thinking of gas engines where you don't want to redline them!!

[never monday]

i'd have to say the injection elbow is clogged. if not it was loved to near death.

[Northeaster]

Actually, i had the elbow off last year, and it was very clean. It certainly wasn't the original 25 year-old elbow! Probably replaced just a few years ago.

I still would like to know - will it cause any damage to rev it up to WOT in neutral, since it likely hasn't seen high rpms in quite a while??

[never monday]

Originally Posted by Northeaster

I still would like to know - will it cause any damage to rev it up to WOT in neutral, since it likely hasn't seen high rpms in quite a while??

it shouldn't cause any damage. But doing this isn't benefiting the engine. It will clear the exhaust. But no load on the rings won't gain you any thing.

[Northeaster]

NeverM - thanks for the reply! I realize it will not benefit the engine. I would just like to know approx waht rpms I can reach at WOT, and then I will look at the rpms under load while increasing the speed slowly until I start getting white smoke and / or no increase in speed with the throttle increase. Just trying to get a feel for how it is supposed to work, and how overpropped it is!!

I will try to see if the local mechanic has a portable tach, in order to calibrate mine. It shows about 800 rpm at idle, and if I can achieve near 3600 at WOT, in neutral, at least I will know the tach is close!

Thanks again!

I don't think there is a local diver, but if there is, I will see how much it would cost to remove the prop to be de-pitched!

[George Elliott]

Hi I've a 3gm and I was underpropped and wound up going to an excellent prop shop in Fl. He had me intentionally overprop the eng by giving me too much diameter and pitch. All he wanted to know was what RPM I pulled in dead calm conditions with a clean bottom. He then cut down the prop (it was his prop, he already knew the dia and pitch) It was like I'd repowered 3400, rpm only when the boat was in motion (she's a small 38') 2800 if I was stuck on the bottom like in the ICW. Sounds to me you're 1/2 way there, find a good prop shop and take the prop in. The GMs don't start putting out until 23-2400 anyway. Good luck George

[Northeaster]

George - thanks for the input. I will check with the yard at the marina, to see about the prop / depitching, now, or at the end of the season!


FYI - I did rev it up to full throttle., in neutral - soounded good and rpm read 3500 (bear in mind new tach hasn't been calibrated with portanle tach, but idle shows 800rpm, and top end 3500, so it must be close!!)

In gear, and motoring with the wind on the stern, it reaches approx 4.5kts at 1800, 5 or so at 2000 and tops out at 5.8-6kts at 2300rpm. It starts with alot of white smoke above that - I went as high as 2500, lots of smoke - didn't want to push it any further.

Does it sound right to want to take a couple of inches off the pitch, from current 15x10 to approx 15 x 7.5??

Thanks again for all of the advice!! I should mention taht the prop has 15x10 stamped on it from manufacturer. There is a chance the pitch has been altered already, but its seems consistent with the inability to go above 2500rpm!

[George Elliott]

Hi Again Yeah, you're definitely overpropped. Find out what the absolute max rpm she'll pull in calm conditions with a clean bottom and take it to the best prop shop in your area. They'll ignore the markings on the prop and scan it then cut it back so you;ll hit 3400. i think I used Miami Prop in Hialeah, Fl, it's a simple process and pulling a prop with a 3 jaw puller isn't hard without dive gear. Good luck. George