Can I improve Yanmar YM 13 (20GM) performance?

[SvenH]

Can I improve Yanmar YM 13 (20GM) performance? (fluctuating rpms)

I just did a weeks long upwind trip requiring about 24 hours of motoring.
Engine has been as reliable as ever since owning it for about 10 years now.

But I wasn't too happy with the speeds I could manage.

When I bought the boat it had a higher pitch folding prop which was showing its age.
At some point I asked for a quote for a new one, and they suggested one with smaller pitch.
I was then able to source a barely used second hand one of the same size as suggested, but a cheaper model/brand.

I found that with this prop I required more revs to get the same speed and the top speed was lower.
The top speed was about 6 knots, now it is 5.5. But with strong head winds, it was around 4.2 today.

At this point the revs max out at around 2800.

Can I get a better speed with a better prop? Or with a fixed one?
Can I fit any prop that still allows rpms to max out?

I am unsure of the size I have now, something like 15x12?
Boat is 9.36 x 3.15 x 1.80 meters with saildrive.

Second question:
At the top end of revs the rpm seems to get in a state of fluctuation. rpm climbs and then goes down again, then repeat.
With the old prop this happend too and more often than with the new prop.
Is that a regulator issue?
Is that fixable?

 

[RichardS]

I believe that the maximum revs for your engine are 3600, the same as my Yanmar, so there is something not right.

It could be that you are over-propped or have a partially blocked exhaust elbow or something else but you could to do some rev tests in neutral to try and pin down what's going on.

Richard

 

[SvenH]

That's a good thought, thanks.

I thought that the 2800 rpm was because it is under load but you say it should go up to the 3600's?
That is what the manual suggests.

I'll run in idle next time the engine is hot again and see what happens.

Oh, I did check, the power cable does reach maximum stroke on the regulator when the power lever in the cockpit is at maximum.

 

[scottie]

That's a good thought, thanks.

I thought that the 2800 rpm was because it is under load but you say it should go up to the 3600's?
That is what the manual suggests.

Speed in idle does not prove a lot


I'll run in idle next time the engine is hot again and see what happens.

Oh, I did check, the power cable does reach maximum stroke on the regulator when the power lever in the cockpit is at maximum.

 

[Tranona]

Yes, the pitch is too steep (or the diameter too large) by about 2". You should be able to achieve close to 3600 rpm in flat water and a speed of close to 7 knots and cruise at 2400 with a speed of about 5.5 knots.

What actual engine do you have, hp and reduction ratio and what is the design displacement? If you feed this information plus the LWL into a programme such as Propcalc on www.castlemarine.co.uk you will get a good estimate of the optimum prop. You may not be able to use the programme as I think it only runs on older PCs, but if you post the information here I can do it for you. Then you can check against the prop you have. If you then decide to go for a new prop you have a base to work from. However if you are buying new get your supplier to recommend the size, particularly if you are buying an expensive folder as sizes can vary according to the design of the prop.

 

[SailSouth]

two things come to mind....

Was/is the prop and hull relatively clean - fouling can make a big difference and I would check the max rpm possible in-gear under load with a clean bum before going any further.

When at max possible RPM under load were you getting black smoke from the exhaust (which would indicate it being over propped - or heavily fouled).

 

[lw395]

Sounds like the regulator might not be working properly.
Which might explain everything.
Will it rev more in neutral?
Or it could be fuel starvation.
Possibly worth checking the fuel pressure at the inlet to the injector pump?
Also is there a tiny filter at that point? a little gauze in the inlet?

It may be worth getting injectors and pump serviced.
Exhaust elbow- check it's not partly blocked.
While you are at it, check the air filter is not fouled.
Look at all the cheap things!


Did it use a lot of fuel or create a lot of smoke?

Check the hull is reasonably clean, nothing around the prop shaft.
Check the shaft turns freely. Sorry it's a saildrive, so you'll need to dive on it?
Is there oil in the gearbox? Not too little, not too much!

Do you have the old prop? That might give a reference as you know the new prop has a bit less pitch.

If your engine is down on power, which could have many causes, you do not want a new prop, you want the engine sorted.

You can use the prop calculator websites or Dave Gerr's book to work out how much power your engine is putting out at the revs it will do.
Is that consistent with what Yanmar say for a healthy engine?

OTOH there are boats out there massively over propped, because people like quiet fast cruising when there's no wind, pottering along at low rpm.

 

[Tranona]

OTOH there are boats out there massively over propped, because people like quiet fast cruising when there's no wind, pottering along at low rpm.

While you may be able to tolerate some over propping where the engine is too powerful for the boat and can achieve hull speed at less than maximum revs and power. it is not a good idea on a low powered boat like this that needs every bit of the rated power to achieve hull speed. At 2800 rpm it is only producing around 75% of its maximum which as his figures show gets nowhere near hull speed.

The figures I quoted in post#5 is what he should be aiming at. BTW the engine/prop combination he has would not be acceptable for commissioning a new installation.

 

[Ammonite]

Before you go thinking about changing the prop what revs were you getting with the old prop? Id also want to know whether you have always experienced the fluctuation in revs as that doesn't sound at all right unless the taco is faulty. Can you hear the rpm changing for example?

It is also worth checking whether the taco is actually reading correctly. This is easily checked with a £10 laser style taco off eBay where you stick a reflective bit of tape on the crankshaft pulley. My old taco was 500rpm out at the top end but fine at idle

 

[RichardS]

Speed in idle does not prove a lot

Sometimes it does, sometimes it doesn't.

In this case it may well prove everything.

Richard

 

[SvenH]

Thanks for the help guys, really appreciated.

Tranona, if you run the calculations, that would be great.

displacement is 3700 kilo's.
Length of the waterline is about 7.45 (gets to about 8.20 meters at speed)
Loa 9.36
Width 3.15
Width@ waterline (is that beam?) 2.37
Finn keel


I was unsure about the rpms, I thought I had read there were two options for the rpms, 3000 and 3600. I have now read the engine tag, it says 2GMC 3400 with 13 HP.

"Before you go thinking about changing the prop what revs were you getting with the old prop?"
I do not know, I don't think I actually watched for it at that time.
But I won't run out and get a new prop without trying out things you guys come up with

I found a mention of a prop in my diary, 15x12 12 dec 2009 and I found the date I picked up the new one, 24 feb 2010, but I am unsure if this size is the old or the new one. I have a vague recollection the other one could have been 15x19...

Boat has an SD20 saildrive, oil at the right level, good color.

Smoke at full power (or any power) is pretty much non existent, just a hint of greyness.
I do run GTL diesel which burns cleaner.

No fouling at the hull.
I can't be sure of fouling at the prop but last weeks performance was not significantly less than I am used to. I mean, a fouled prop would feel different in any situation, would it not?
Slow speed response and manoeuvring feel normal.

The rpm changes at top revs were quite slow when increasing (unnoticeable) and would then drop down a bit in a second or so. This would be quite usual with the old prop but rarely happened with the new one.
With last weeks trip it seemed to happen after getting back up to speed after having slowed down after running at top revs for a long time. Changes would be quicker than with the old prop, but there is quite a few years between now and the prop switch.
I rarely feel the need to go this fast but I needed it this week.

 

[Tranona]

Not good news I am afraid. The basic problem is your boat is underpowered for its displacement. The calculations more or less confirm what you are achieving. Maximum speed of 5.9 knots with a 15" prop and a pitch between 11 and 12 - so your current prop is close. However, dropping an inch in pitch will increase revs by approx 250-300. The boat really needs the next size up (YM 20), but of course this is a much bigger engine physically, but would increase your speed potential up to hull speed of 7 knots with a corresponding improvement in adverse conditions.

It is important to understand the power curves of these little engines. Have a look at the data sheet - at 2800 your engine will only produce about 6hp compared with over 13 at 3600. This is more than enough to cruise in flat water at around 5 knots, but you need to get further up the rev range to get the power to deal with adverse conditions - as you have discovered.

Given that you have near enough the right prop (if your memory is correct) then there is probably not a lot you can do. Suggest you do a bit more invetigative work. Go out on a nice calm day and run at 200 rpm intervals from 2000 rpm up to how fast the engine runs and plot against speed. If there is no tide then use GPS so make the runs long enough to get a good average. Check your rev counter is reading correctly - you can buy or borrow hand held electronic rev counters. You will then have a base to work from. Makes sense also to check the actual size of the prop.

If the prop is 15*12 and you cannot reach near full revs, say 3400, then you need to check the engine - fuel flow and blocked exhaust elbow are obvious things to look at.

However, I think you have to accept that you will always be limited by the lack of power in adverse conditions.

 

[SvenH]

That is interesting and a little perplexing...
Thanks for doing the sums!

I do not think I'd invest in a different engine in this boat. I'm not sure how long we'd be keeping her. I love how she sails but she is a bit too small for the family.

But do you think I'd get better performance with a better prop?
Like a fixed one?
While browsing I found a 15x11 one that could be worth a try next season.

 

[Tranona]

Depends on the folding prop you have. Mostly there is little difference between fixed and folding and in some cases a folder can be marginally better. However there are some folders that have very narrow blades, supposedly for less drag for racing, but really achieve just less thrust under motor.

You don't say what your boat is but from your description so far suspect it is designed at the performance end of the spectrum and performance under power was not a priority, hence the use of a small engine. In historic terms it is nothing out of the ordinary but we have become used to more powerful engines in sailing boats. My 10m modern short ended (9.8m waterline) displacement 5500kg has a 30hp engine and deals with adverse conditions well. 5.5 knots into wind and waves is no problem.

 

[SvenH]

Indeed, the boat may be at the performance end of cruising.
But still, the boat did great yesterday.
I had the strongest headwinds (something like 20 knots) of this trip and was able to achieve speeds up to 6 knots with the engine running at 3100 rpm. I've never had the boat run this fast with this prop, regardless of head winds...

Which brings me back to the regulator (or is that the governor) question I posted earlier.
After doing the very speedy run, I managed to catch some weeds on the keel and after trying to get rid of them in reverse it felt I could not get the revs back up.
I also touched the bottom so my senses may well have been confused.

But I'd like to check if there is something wrong with how the regulator functions.
And another thing, I am also unsure if the power lever sticks in the maximum setting good enough, or if it can vibrate itself slightly looser.
When I reversed out of the channel the lever would definitely not stay at maximum.
Can these levers have their friction increased?

 

[Tranona]

Yes, there is usually a screw in the lever housing for adjusting the drag. The actual location will vary according to the specific make of control, but if you remove the handle and cover you should find it.

 

[lw395]

Sounds like the prop has to be OK.
The regulator itself is not a trivial thing to work on.
But it might be worth checking that the throttle cable is correctly adjusted?
The Yanmar workshop manual is probably online if you don't have one.
If the regulator is not able to demand 'max fuel' from the injector pump, that is possibly caused by the injection pump needing a service. Probably more likely than the governor components of the regulator being at fault.

A fuel issue is still quite likely, a partial blockage, dodgy lift pump or air leak?

 

[SvenH]

On the way back home I did a short run @ the highest revs I could get, about 3150 rpm.
That gave me about 5.7 knots on the gps.
Not a hint of the rpms changing.

Because of the loudness I throttled down a bit.

A few seconds after the revs settled they dropped significantly, giving me a bit of a scare.

I should have tried a longer run at top speed to check but it seems that fuel issues should not allow top speed at first and then seconds later at lower rpm give an even lower rpm, should it?

Oh, I did look at the power lever in the cockpit and found the friction screw. But the whole thing looks a bit tired and could do with removal and clean&polish.
But as often is the case on this boat, it has been gooed in with sikaflex...

 

[SvenH]

Revisiting a year later.
One of the things recommended to me was a fuel filter change. Ahum, It had not been done in 10 years...

But no real dirt to speak off. Today, again in strong head winds, rpms started fluctuating again, seemingly more often and further down.

But a part of the filter change got me thinking.

I was unable to use the lift pump to bleed the air.
I used a seperate one to finish and then studied the lift pump a bit.

I did not know it is a driven pump, thought it was only for manually bleeding air.

How about this for theory:
In the fuel line there is an electric pump. Maybe that was placed there because of a broken lift pump?

Maybe it has insufficient capacity so the engine needs to take a break to wait for fuel?

Replace lift pump and remove electric one?

 

[QBhoy]

I would perhaps be trying to open the throttle from the engine itself to see if it’s the remote throttle mechanism or cable that’s to blame. Easy things first.
Outwith the fuel filters being clean, injectors operating well and strong compression...perhaps look at the fuel pump and govenor.
If all is well you are Defo over propped but would be strange to be so far out. Most engines have a max hp figure between a certain rpm band. You really need to be in that band.