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Go Back   SailNet Community > On Board > Gear & Maintenance > overpropped vs underpropped
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Thread: overpropped vs underpropped Reply to Thread
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Topic Review (Newest First)
12-19-2012 11:20 PM
Lake Superior Sailor
Re: overpropped vs underpropped

I have a diesel YSM8 it's governed, if the prop is too high a pitch She blows black,if the prop is too low the governer kicks in and the boat is slow.So the prop that gets the speed without the black must be right.K.I.S.S.Then you just pick the throttle speed!...Dale
12-19-2012 11:39 AM
MedSailor
Re: overpropped vs underpropped

Quote:
Originally Posted by srah1953 View Post
The only data that I can find in the manual is the maximum output rating of 39hp at the crankshaft at 3,000rpm. It gives a propeller speed of 1,293 rpm and a reduction ratio of 2.32. Can I multiply one by the other to get max rpm (which would make 3,000 rpm)?
No you shouldn't be multiplying anything by your gearbox ratio. Your tachometer, if correctly calibrated, reads your CRANKSHAFT rpm, not your propeller shaft rpm. It's max RPM at the crankshaft at WOT that you're after.

My engine has at least 3 variants as well. One governed at 3000rpm, one at 36000rpm and one at 4000rpm. Mine is the 4K version.

The way my mechanical surveyor found this out for sure, was he used a sight gauge to verify that my tachometer was reading correctly as they often are not. Mine was off by 1000rpm EDIT: this should read "of by 100 rpm at 3000rpm"at 3000rpm. Once the engine was WELL WARMED UP to operating temperature after running under load for a while, he had me put it in neutral and advance the throttle to full throttle. He took a reading on my tachometer, and another from his strobe sight tach thingy, and I put the throttle back to idle.

In short, if you want to know for sure where your engine is governed, then floor it in neutral once it's warmed up. Where the govener stops you is the RPM that you should be able to reach (or get near) at WOT.

MedSailor
12-19-2012 11:33 AM
jrd22
Re: overpropped vs underpropped

From what I can find 3000 is WOT.

http://us.yanmar.com/products/marine-engines/
12-19-2012 11:15 AM
srah1953
Re: overpropped vs underpropped

Quote:
Originally Posted by Minnewaska View Post
Max rpm should be in the engine manual. You raise a very good point. My TAMD31 has about a half dozen variants, signified by a following letter. All but mine have max rpm at 3800, but mine is only 3000. I had one engine shop hear the 31 and assumed the higher rating and went down a rabbit hole trying to resolve an issue.
The only data that I can find in the manual is the maximum output rating of 39hp at the crankshaft at 3,000rpm. It gives a propeller speed of 1,293 rpm and a reduction ratio of 2.32. Can I multiply one by the other to get max rpm (which would make 3,000 rpm)?
12-19-2012 10:42 AM
jrd22
Re: overpropped vs underpropped

Yep, every variation can have different ratings, our MD31A is 3500.
12-19-2012 10:36 AM
Minnewaska
Re: overpropped vs underpropped

Max rpm should be in the engine manual. You raise a very good point. My TAMD31 has about a half dozen variants, signified by a following letter. All but mine have max rpm at 3800, but mine is only 3000. I had one engine shop hear the 31 and assumed the higher rating and went down a rabbit hole trying to resolve an issue.
12-19-2012 10:31 AM
srah1953
Re: overpropped vs underpropped

Quote:
Originally Posted by kellysails View Post
FYI

Yanmar 4JH5E naturally aspirated.
Hi
I note that you say your engine is a Yanmar 4JH5E and that it has a governed rpm rate of 3,400rpm. May I ask where you got this data from? The only data I can find for it from Yanmar is a maximum output rating of 53mhp at the crankshaft at 3,000rpm I'm asking because I'm trying to find a source to find out what is the maximum WOT rpm of my 3JH5E engine.
Thanks
12-18-2012 09:58 PM
MedSailor
Re: overpropped vs underpropped

Quote:
Originally Posted by jrd22 View Post
I agree with Minne. It's not just that you can't reach WOT, it's that at 2500 rpm you are asking the engine to do what it normally would at 2800 rpm (approx) with less hp so it puts more strain on everything. That's not a huge difference and your engine might do OK, but if you get into a situation where you are motoring into a stiff wind and large waves and you need to run near full throttle that is when you might blow a headgasket or overheat, etc.
Well put. From the mechanics I've talked to, being ABLE to reach max RPM at WOT is the only real way you can tell if you have the correct amount of stress on your engine at any given speed.

How many hundreds of RPM can you fudge it and get away with? That depends on a lot of things and many unknowns. If you adjust your new prop so that you can reach max RPM at WOT and you can still hit your cruising speed at a reasonably efficient RPM then there is no downside.

At what RPM do you currently reach hull speed/cruising speed? What engine do you have?

MedSailor
12-18-2012 08:59 PM
jrd22
Re: overpropped vs underpropped

I agree with Minne. It's not just that you can't reach WOT, it's that at 2500 rpm you are asking the engine to do what it normally would at 2800 rpm (approx) with less hp so it puts more strain on everything. That's not a huge difference and your engine might do OK, but if you get into a situation where you are motoring into a stiff wind and large waves and you need to run near full throttle that is when you might blow a headgasket or overheat, etc.
12-18-2012 08:12 PM
Minnewaska
Re: overpropped vs underpropped

Quote:
Originally Posted by Brezzin View Post
....Having just replaced my three blade fixed with a four blade feathering. I find that I can only get to 2900rpm with the max WOT at 3200. ....
This is confusing. If your new feathering prop won't allow you to get to max rpm under load, then it needs to be adjusted. If you're uncomfortable at any given speed, just pull the throttle back. But it should be able to get to max rpm.
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