Hi all

About a year ago I revved my Yanmar 2ym15 too high for too long and ended up cracking the exhaust elbow connection. I managed to sail home with almost no wind, and the mechanic who fixed it said the higher rev range was only to be used in short bursts. Makes sense.

More recently a couple of sailors I know have made the following comments: 1) Yanmar engines are very 'soft footed' and designed to shake/vibrate a fair bit and 2) they will vibrate excessively if matched with the wrong sized propellor.

The previous owner swapped out a 20hp engine (not sure which make) and put in the 15hp engine for the simple reason that the engine-brackets fitted the existing fibreglass mounts. I suspect he did not change the prop at the same time, so I'm wondering if this is making my engine vibrate more than usual? There are a couple of rev ranges where things are quite smooth so I tend to stick to them, but the top one is only 1500rpm, so I feel I should be able to get more power from this engine without shaking things to bits.

Any advice on this would be appreciated as I'm due to pull the boat out for anti-fouling in about two months' time and can inspect/change the prop then. The only thing I know about it now is that it's a fixed prop - see attached pic - and it's driving a 3 tonne, 30ft Ted Brewer designed cruising yacht (Nimble 30).

I am not an expert, and am pursuing my own issues in respect of a wrongly pitched propeller behind a Yanmar 3HM.

If you can over-rev your engine for a sustained period that suggests to me your prop may be too small or of insufficient pitch for the engine. As I understand it (see disclaimer at the beginning), ideally the prop should be such that the engine can reach its max continuous RPM and for short periods its max RPM and the boat should reach close to hull speed in the latter case (although your fuel consumption will be high) - unless the engine is underpowered for the boat. I'm not sure what the water line length and beam of your boat is.

As I understand it, to some extent, and within limits, propeller pitch and diameter can trade-off - more diameter less pitch and vice versa (because less slip (more efficiency) with larger prop - all else being equal it shifts more water per revolution (and so is also harder to turn at a given rpm)). Clearance from prop to hull can be a determining factor in choosing diameter. If you're lucky the pitch may be stamped on the prop. If you need to measure your prop to work out its pitch, this site may help: goneoutdoors.com/measure-boat-propeller-6101713.html

I think max continuous power for the 2ym15 is 9.1 kW (12.4 hp metric) at 3489 rpm. Max power (for limited periods) is 10.0 kW (13.6 hp metric) at 3600 rpm. All at the crank shaft. At the prop the power is about 3% less due to transmission losses. These taken from p84 of the Yanmar Operation Manual for Marine Engines 2YM15, 3YM20, 3YM30E, 3YM30. Also the rpm of the shaft will be less due to transmission ratio. If you have the KM2P-1 (S) then the forward ratio is 2.21 and the reverse ratio is 3.06 (p88 of same). This matters for calculating prop rpm at a given engine rpm. If you have another variant of the KM2P-1 or a different transmission, then ratios will likely be different.

Of itself, unless you have a bent shaft or prop (even slightly so), I wouldn't have thought the prop would, of itself, cause vibration. The 2ym15 is a two cylinder engine of 570cc displacement. As I understand it, two cylinder engines are not inherently balanced (has to be a six for that, if cylinders are in-line), and so would vibrate at all RPM, and that would be more or less apparent at different revs depending on the damping characteristics of the engine mounts (and the state of the engine mounts). I'd expect another possibility is the alignment of the engine to the shaft. If that is slightly out of whack that may also cause vibration, but I have no specific knowledge about that.

A useful site for calculating what prop diameter and pitch should be is vicprop.com/displacement_size_new.php/?m=1
Another is www.seahawk.com.au/propcalc.html Both require some assumptions about slip (to what extent the propeller moves less far forward through the water per revolution than its pitch.)

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BluewaterBound said..
Hi all

About a year ago I revved my Yanmar 2ym15 too high for too long and ended up cracking the exhaust elbow connection. I managed to sail home with almost no wind, and the mechanic who fixed it said the higher rev range was only to be used in short bursts. Makes sense.

More recently a couple of sailors I know have made the following comments: 1) Yanmar engines are very 'soft footed' and designed to shake/vibrate a fair bit and 2) they will vibrate excessively if matched with the wrong sized propellor.

The previous owner swapped out a 20hp engine (not sure which make) and put in the 15hp engine for the simple reason that the engine-brackets fitted the existing fibreglass mounts. I suspect he did not change the prop at the same time, so I'm wondering if this is making my engine vibrate more than usual? There are a couple of rev ranges where things are quite smooth so I tend to stick to them, but the top one is only 1500rpm, so I feel I should be able to get more power from this engine without shaking things to bits.

Any advice on this would be appreciated as I'm due to pull the boat out for anti-fouling in about two months' time and can inspect/change the prop then. The only thing I know about it now is that it's a fixed prop - see attached pic - and it's driving a 3 tonne, 30ft Ted Brewer designed cruising yacht (Nimble 30).

The 1st place I would look would be engine alignment, as it's done in water, there's heaps of online how to. Prop balance would be next and check the shaft is not bent and the cutlass bearing might be worn allowing the shaft to wobble.there's a lot of variables. Is it solid mount or flexible ? And does it shake the same when revved hard out of gear ?

Bluewater bound,
What rpm was 'reving too high for too long'?
In the higher rpm range below 3480 or prolonged periods above 3600?

It's important as your engine should be able to sit at 3480 all day long without ill effect, assuming it's propped right.

Your mechanics statement, 'said the higher rev range was only to be used in short bursts' is wrong.

A cut and paste from the 2YM15 manual
www.yanmar.com/marine/wp-content/uploads/2024/03/0AYMM-EN002D_YMseries_OPM_20240117.pdf

page 10
During the first 10 hours of use the engine should be run at maximum rpm minus 400-500rpm for most of the time at 70% load

Between 10-50hrs the engine should be run at high rpm most of the time.

There is also a description of the 'burn out procedure' on the same page and p44.
When in operation periodically use the engine near maximum load.

Many engines are bought to an end of an early life by low speed running continuously. As a general rule of thumb for every four hours at low load operations requires an hour at high load to burn out and restore the diesel engine.

Also check your polyflex connector if you have one.

They do work harden and need to be replaced from time to time.

Alignment out of whack, worn cutless, bent shaft, dodgy engine mounts are all much more likely to cause vibration than an under or oversized prop. So you've got a bit of checking to do as well, as trying to ascertain whether the prop is correctly sized.

Your engine is about right for that boat, maybe slightly under, but still should get you to hull speed in calm conditions with correct prop, and clean prop and hull.

I'm a bit intrigued about sustained high revs supposedly cracking the elbow - that's a new one to me.

Cheers, Graeme

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wongaga said..
Alignment out of whack, worn cutless, bent shaft, dodgy engine mounts are all much more likely to cause vibration than an under or oversized prop. So you've got a bit of checking to do as well, as trying to ascertain whether the prop is correctly sized.

Your engine is about right for that boat, maybe slightly under, but still should get you to hull speed in calm conditions with correct prop, and clean prop and hull.

I'm a bit intrigued about sustained high revs supposedly cracking the elbow - that's a new one to me.

Cheers, Graeme

I agree with the sustained high revs trip, rather odd I would assume that the engine will be governed & even if it's been hot rodded it ain't gunna crack the exhaust. By the sounds of things it cracked due to excessive vibrations. The torque curve of those little engines is reasonable flat 2600 rpm looks like the sweet spot ideally you would be propped to achieve hull speed at those revs. That's got nothing to do with vibrations though

Have you had the prop balanced
also looks a 1'' shaft so if poor quality it will whip quite a lot
last thing to check is distance from tip of blade to underside of hull as lack of clearance has big impact

Thanks for all the advice - quite a list of things for me to check. I can't remember exactly what revs I was using, but over 3000, so might have exceeded the safe continuous operation rating.
I might get an expert eye to have a look while its in the water, and then do any other checks when it's on the hard.
At low revs (1500) it's very quiet and pleasant - I took some colleagues down the Yarra from Docklands last week and it was easy to chat over the sound of the engine even in the cabin. But just a bit higher than than it gets into shaky territory, so as I say, I've got a list of things to check ahead of the haul-out. If it turns out to be a single factor I'll post the results ... though it could be several combined.
By the sounds of it I won't need to waste money on a slightly different sized prop