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Old 25-03-2020, 06:13   #1
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Propeller sizing - Yanmar 3YM30AE

Hello everyone

I am trying to troubleshoot why my 2018 Leopard 40 with two 3YM30AE Yanmars will not reach "rated rpm plus 100 or 200" as specified by Yanmar, which means 3300-3400. It has Volvo 21351265 propellers (2-blade 17x16).

With those propellers both engines reach 3100-3150 rpm at WOT (measured by photo tach in flat water without wind).

I believe the root cause is that they were confused as to rated rpm of the new AE version and they thought it was 3000 instead of 3200, when they relaized that they downsized the spec to 17x15 but they could not find that in the Volvo prop catalog, hence they stuck with 17x16 until I complained.

Now the acid test for the theory. Has anyone been able to get a Yanmar 3YM30AE engine with SD20 or 25 saildrive (1:2.64 ratio) to turn Volvo 21351265 propellers (2-blade 17x16) faster than 3200 rpm?
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Old 25-03-2020, 06:49   #2
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Re: Propeller sizing - Yanmar 3YM30AE

"Yanmars will not reach "rated rpm plus 100 or 200" as specified by Yanmar, which means 3300-3400."

No big deal , the key to long diesel life is not underloading or overloading the engine long term.

IF the cruise speed you enjoy is 10% , in your case 300rpm , below the RPM you see at wide open (WOT), you are fine.

IF at WOT there is black smoke in the exhaust , you are overloaded , pull back till the exhaust clears , deduct 10% and away you go.

If the turbo has at least one pound of boost at your low cruise setting, you are fine.

IF you long term use the propulsion engine at idle for air cond , refrigeration or battery charging , every few hours, go for a long near WOT 30min run, and change the oil at 1/2 the hours the eng mfg sez.
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Old 25-03-2020, 07:22   #3
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Re: Propeller sizing - Yanmar 3YM30AE

Calculating with your numbers, those engines should be able to swing 17 x 16, 2 blade wheels. If you are overpropped its not by much. Those engines are not turbo so not as susceptible to damage from overpropping. Under propping will not hurt the engine. I would check throttle linkage to assure you are hitting the stops on the engines and also fuel flow to make sure the engine is not starving at high rpms.
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Old 25-03-2020, 08:02   #4
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Re: Propeller sizing - Yanmar 3YM30AE

A non turbo engine can easily be overproped it’s not just blown motors.
But a couple hundred RPM is very small overprop, you don’t cruise near wide open do you? If so then prop so you can get to full RPM.
In a perfect world boats are perfectly propped at light weight, so when loaded for cruising, we are all over propped a little.

On edit, any Cat that is properly propped for running both engines is certainly over propped when cruising on one, and many do that with no harm, slight over propped isn’t a bad thing, too much is of course.
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Old 25-03-2020, 09:16   #5
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Re: Propeller sizing - Yanmar 3YM30AE

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Originally Posted by a64pilot View Post
A non turbo engine can easily be overproped itís not just blown motors.
But a couple hundred RPM is very small overprop, you donít cruise near wide open do you? If so then prop so you can get to full RPM.
In a perfect world boats are perfectly propped at light weight, so when loaded for cruising, we are all over propped a little.

On edit, any Cat that is properly propped for running both engines is certainly over propped when cruising on one, and many do that with no harm, slight over propped isnít a bad thing, too much is of course.
Thanks for the thoughtful response; I failed to clarify that the measurement is with both engines at the same time and without any real load on the boat.. I also failed to clarify that its is a bareboat charter boat and by the end of its 5 year contract it will have lost a bit of compression, making the overpropping more significant. Been there already!

These boats have lots of windage and are a bit underpowered to start, hence it does not really help to be overpropped because that prevents you from using peak power (which can only be achieved around 3200 rpm) when you need it against the tradewinds. 3100 WOT in flat water with two engines easily turns into 2200 rom against wind and see with one engine when you are in trouble! I have gone down to 17x14 for the time being; just want to check if there is anybody else could get higher rpm on the 17x16s, whcih could be a sign of exhaust or fuel issue on my boat...
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Old 25-03-2020, 09:20   #6
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Re: Propeller sizing - Yanmar 3YM30AE

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Originally Posted by FPNC View Post
Calculating with your numbers, those engines should be able to swing 17 x 16, 2 blade wheels. If you are overpropped its not by much. Those engines are not turbo so not as susceptible to damage from overpropping. Under propping will not hurt the engine. I would check throttle linkage to assure you are hitting the stops on the engines and also fuel flow to make sure the engine is not starving at high rpms.
Thanks; yeap, for the test I removed the end of the thorttle handles to make sure I hit the stops. I moved down to 17x14, which is slightly underpropped (3500 with both running, light load in flat water without wind) but gives me a cushion. I have not been able to run a full diagnostic on the fuel pressure side (boat is managed by Moorings and I canīt get all my toys to the boat), hence the call for empirical data to see in any similar boat coudl get better rpm. So far all sisterships get similar numbers as mine...
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Old 25-03-2020, 09:23   #7
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Re: Propeller sizing - Yanmar 3YM30AE

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Originally Posted by FAST FRED View Post
"Yanmars will not reach "rated rpm plus 100 or 200" as specified by Yanmar, which means 3300-3400."

No big deal , the key to long diesel life is not underloading or overloading the engine long term.

IF the cruise speed you enjoy is 10% , in your case 300rpm , below the RPM you see at wide open (WOT), you are fine.

IF at WOT there is black smoke in the exhaust , you are overloaded , pull back till the exhaust clears , deduct 10% and away you go.

If the turbo has at least one pound of boost at your low cruise setting, you are fine.

IF you long term use the propulsion engine at idle for air cond , refrigeration or battery charging , every few hours, go for a long near WOT 30min run, and change the oil at 1/2 the hours the eng mfg sez.
Thanks. I am engineer and understand overpropping. This boat is not overpowered to start (2x30HP for a 40-ft cat with lots of windage), hence I cannot bear a setup that will not let the engine(s) hit max power rpm when the **** hits the fan (one engine only, wind and wave on the nose, lee shore on the stern).
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Old 25-03-2020, 10:49   #8
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Re: Propeller sizing - Yanmar 3YM30AE

The best propeller calculator I have found is at http://www.vicprop.com/displacement_...tion=calculate
I had a 2 blade 17x10 on a mono hull with a 3GM30 and it was perfect. I am now replacing it with a 3 blade, feathering, 16x11.

I have also found the book, Propeller Handbook by Gerr very good.

I am guessing you have too much pitch and perhaps too big a diameter if your prop is 3 blade. You probably need more pitch than me since your cruising speed is higher than my mono hull.
If the engine isn't producing black smoke or overheating then live with it.
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Old 25-03-2020, 11:09   #9
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Re: Propeller sizing - Yanmar 3YM30AE

Quote:
Originally Posted by svlamorocha View Post
Hello everyone

I am trying to troubleshoot why my 2018 Leopard 40 with two 3YM30AE Yanmars will not reach "rated rpm plus 100 or 200" as specified by Yanmar, which means 3300-3400. It has Volvo 21351265 propellers (2-blade 17x16).

With those propellers both engines reach 3100-3150 rpm at WOT (measured by photo tach in flat water without wind).

I believe the root cause is that they were confused as to rated rpm of the new AE version and they thought it was 3000 instead of 3200, when they relaized that they downsized the spec to 17x15 but they could not find that in the Volvo prop catalog, hence they stuck with 17x16 until I complained.

Now the acid test for the theory. Has anyone been able to get a Yanmar 3YM30AE engine with SD20 or 25 saildrive (1:2.64 ratio) to turn Volvo 21351265 propellers (2-blade 17x16) faster than 3200 rpm?
A two blade prop is often noted as all a boat needs in normal conditions if powered correctly but a three blade prop is desired when facing significant wind and waves. That's the trade-off (three blade prop) many single engine cruisers make planning for the worst conditions. A hybrid solutions would be a feathering or folding three blade prop to minimize the drag.

~ ~ _/) ~ ~ MJH
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Old 25-03-2020, 11:49   #10
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Re: Propeller sizing - Yanmar 3YM30AE

Our 11.6m 38 ft FP Athena has 2 off 3 ym30's and the props are 16" dia x 13". If you have SD20 saildrives like us, I would say ours is fractionally underpropped so would suggest yours is massively over propped given your much heavier displacement and associated fatter hulls. That sounds like a non standard prop has been fitted, perhaps one for a SD50 or similar.
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Old 25-03-2020, 12:15   #11
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Re: Propeller sizing - Yanmar 3YM30AE

If I had a boat in Charter, I’d be sure it wasn’t overpropped cause your Charters are likely going to push the throttles to the stop and won’t know not to.
But once I got it correctly propped, I’d adjust the throttle stops so that they couldn’t get full throttle, cause it’s my belief that continuous full throttle operation just isn’t good for a motor, and Yanmar often restricts their engines to one hour at full throttle.

I would bring back the stops to knock off at least 200 RPM from max, at least.
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Old 25-03-2020, 12:58   #12
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Re: Propeller sizing - Yanmar 3YM30AE

I am furiously in agreement. In fact I adjusted the cables so that the "balls" at the end of the throttle handles hit the deck before the levers down below hit the proper stop. When I run WOT trial I need to get the balls out.
Quote:
Originally Posted by a64pilot View Post
If I had a boat in Charter, Iíd be sure it wasnít overpropped cause your Charters are likely going to push the throttles to the stop and wonít know not to.
But once I got it correctly propped, Iíd adjust the throttle stops so that they couldnít get full throttle, cause itís my belief that continuous full throttle operation just isnít good for a motor, and Yanmar often restricts their engines to one hour at full throttle.

I would bring back the stops to knock off at least 200 RPM from max, at least.
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Old 25-03-2020, 13:01   #13
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Re: Propeller sizing - Yanmar 3YM30AE

Do I suspect correctly thay your engines are 3Y30's without AE at the end, rated/max power at about 3600 rpm? What is your WOT rpm?
I have 3YM30AE's, which are very different and rated/max power 3200 rpm.
Quote:
Originally Posted by Rapanui View Post
Our 11.6m 38 ft FP Athena has 2 off 3 ym30's and the props are 16" dia x 13". If you have SD20 saildrives like us, I would say ours is fractionally underpropped so would suggest yours is massively over propped given your much heavier displacement and associated fatter hulls. That sounds like a non standard prop has been fitted, perhaps one for a SD50 or similar.
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Old 25-03-2020, 14:48   #14
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Re: Propeller sizing - Yanmar 3YM30AE

That sounds as if they are overloading the engine a little. I ran a three bladed prop with the 3YM30 engine with no problems at all--so the pitch may be wrong.

The maximum torque for such an engine under load occurs at less than the maximum revolutions of an unloaded or lightly loaded engine. As long as your loaded engine can reach the maximum torque number of revolutions plus a margin of few more revs when at full speed under load--then your prop, assuming it is not dirty or damaged (use Prop Speed) is not too bad.

The NO LOAD maximum RPM for your engine should be 3850 plus or minus 850 RPM, your no load idle speed should be 850 plus or minus 25.

The theoretical maximum torque RPM for your engine is 3489 rpm. My engine reached 3500 easily when under load, drove a 42 foot trimaran heavily laden at hull speed, and was thrifty on fuel.

I think using Prop Speed on your propellers makes a significant improvement. If your RPM reaches the designed maximum torque RPM of your engine, that may be all you need.
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Old 25-03-2020, 15:03   #15
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Re: Propeller sizing - Yanmar 3YM30AE

The fastest way to overload a diesel is to run it "over propped". Yes you engines should be able to reach at least 100 RPM over their rated top RPM when fully loaded.
What is required to know is the specified rated engine RPM, an accurate tachometer, and the size in pitch and diameter of your props.

Doesn't matter who made it and it doesn't matter what your size is now - it is just a starting point t work from. If you do your trial runs and measure your WOT RPM and it is lower than your target RPM then simply have the prop removed and have it re-pitched by an estimated amount. This amount may vary with the number of blades and such but the prop shop can give you a pretty good guess. This really a seat of the pants type thing with a bunch of estimates and rules of thumb that people offer from experience. Things like 1" pitch equals 200 RPM, 1" diameter = XXX RPM, etc.

The main thing is you do not want the engine overpropped so better to be underpropped a little.

Because all of these initial measurements should ideally be made with a clean prop and clean bottom with the normal full load of fuel, water, gear, etc. You can be assured that as the boat stays in the water and the bottom and props are no longer "Pristine" the load on the engines goes WAY up. Being slightly Underpropped allows for this normal dirty load use up to a point.

For longevity this is not something that should be done by the seat of the pants. Or by saying -" I don't ever run WOT so why be worried with if I can get the engine to run there". The engine load occurs throughout its power curve.

All that being said - if you really don't use your engines to travel long distances or cruise under power alone much and all you use the engines for is entering harbor and docking - it is probably no big deal.
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