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Old 19-10-2020, 06:27   #1
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Best Process to find Right Propeller for Diesel Auxiliary

What's the best process for finding the right propeller for a diesel auxiliary engine? I now know the boatyard's guess (years ago) was wrong.


Engine: A new Yanmar 2-cylinder 2YM15. 14hp at 3600rpm max. Gearbox is 2.62:1.

Boat: Bristol 27 full keel, 6 knot hull speed (5.96kt using 1.34 √LWL formula).
Current Prop: 12" x 8" 3-blade bronze on 1" shaft (RH rotation).


Situation: Engine was installed in 2014 by a boatyard 500 miles away. I restored the boat, splashed her last week, and started breaking-in the engine following the below Yanmar recommendations. 4.1 hours so far. I am very particular about breaking-in this great running diesel properly.



Problem: I pushed the throttle to WOT just for a moment yesterday. This nice new 3600rpm max engine hit 4000rpm for about one second before I backed off - and would have gone higher had I kept pushing.


What's the best process for getting the right propeller? It doesn't sound like this can be done without hauling her, but I'd like to hear recommendations from folks who have been here and done that.

Yamar's recommendations to properly wear-in piston rings and prevent cylinder walls to become glazed.



From the Operating Manual: During the first 10 hours of operation,

+ Engine should be run at 3100-3200 rpm most of the time.

+ Do not operate at WOT for more than 1 minute.
+ Do not operate at low speed or light load for more than 30 minutes at a time.


From a Yanmar Diesel America Service Advisory: During the first 50 hours of operation,
+ Operate at varying speeds - 2000, 2400, 2800 rpm.
+ Occasional 10-15 minute periods at 3000 rpm.
+ Limit operation under 2000 rpm.
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Old 19-10-2020, 07:09   #2
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Re: Best Process to find Right Propeller for Diesel Auxiliary

Thought these engines had a governor to stop over reving.
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Old 19-10-2020, 07:24   #3
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Re: Best Process to find Right Propeller for Diesel Auxiliary

The 12"x8" is a wrong prop for that engine and boat, definitely too small. We would recommend a 3 blades 14" if you have room enough under the boat.
If you get a prop with pitch adjustment you will have the chance to fine tune it to your specific boat
www.ewol-propellers.com


Quote:
Originally Posted by dbcreed View Post
What's the best process for finding the right propeller for a diesel auxiliary engine? I now know the boatyard's guess (years ago) was wrong.


Engine: A new Yanmar 2-cylinder 2YM15. 14hp at 3600rpm max. Gearbox is 2.62:1.

Boat: Bristol 27 full keel, 6 knot hull speed (5.96kt using 1.34 √LWL formula).
Current Prop: 12" x 8" 3-blade bronze on 1" shaft (RH rotation).


Situation: Engine was installed in 2014 by a boatyard 500 miles away. I restored the boat, splashed her last week, and started breaking-in the engine following the below Yanmar recommendations. 4.1 hours so far. I am very particular about breaking-in this great running diesel properly.



Problem: I pushed the throttle to WOT just for a moment yesterday. This nice new 3600rpm max engine hit 4000rpm for about one second before I backed off - and would have gone higher had I kept pushing.


What's the best process for getting the right propeller? It doesn't sound like this can be done without hauling her, but I'd like to hear recommendations from folks who have been here and done that.

Yamar's recommendations to properly wear-in piston rings and prevent cylinder walls to become glazed.



From the Operating Manual: During the first 10 hours of operation,

+ Engine should be run at 3100-3200 rpm most of the time.

+ Do not operate at WOT for more than 1 minute.
+ Do not operate at low speed or light load for more than 30 minutes at a time.


From a Yanmar Diesel America Service Advisory: During the first 50 hours of operation,
+ Operate at varying speeds - 2000, 2400, 2800 rpm.
+ Occasional 10-15 minute periods at 3000 rpm.
+ Limit operation under 2000 rpm.
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Old 19-10-2020, 12:25   #4
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Re: Best Process to find Right Propeller for Diesel Auxiliary

The normal procedure to find the correct prop goes something like this:
Take the physical characteristics of the vessel ( LWL, beam, displ., draft, power, etc.) and calculate the maximum speed obtainable with the available power. Then given the maximum obtainable speed, you enter the propeller tables and find the prop that will absorb that much power at that speed and at that shaft RPM. Then you add a good measure of experience with this type of hull and you select what you hope will be the correct prop. At least thatís what we did 40 years ago before we had computer prop programs.

Today with computer programs, you are really doing the same thing....you input the boat and power data, and the program calculates the vessel speed. Then you select the numbers of blades, the gear ratio and engine rpm, etc. and the program looks at its data base and finds a prop that will absorb the correct amount of h.p. without being overloaded, cavitating, etc.

So in your case you have (as you know) a prop that is too small to absorb the available engine power, so it overrevs. Probably the prop that is on the boat is a left over from the original engine, which most likely was an atomic 4 with a 1:1 transmission. But today you have a 2.62:1 reduction and the shaft RPM is much slower, so you need a larger prop to absorb the h.p. and keep the engine from over revving.

According to my program, the ideal prop for your engine is a 15 x 9 x 3 or slightly less efficient would be a 14x 10 x3 blade. Hopefully you have enough room in the aperture to fit a prop this large and still retain adequate tip clearance. If not, you will have to add pitch to your existing prop to hold the rpm down, though this isnít the most efficient way to go.

Another thing that could be done is to swap the existing transmission for one with a ratio of approx. 1.9:1. This will allow the use of your current 12 x 8 x 3 prop, although it will be somewhat less efficient than the 2.6 ratio and 14Ē prop.

Ultimately these numbers will probably need to be fine tuned a bit one way or the other to get where you need to be.
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Old 19-10-2020, 18:53   #5
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Re: Best Process to find Right Propeller for Diesel Auxiliary

DougR answered this far better than I could.
Just confirming that your 12x8 prop is too small as that is whats recommended for our 8hp Yanmar on a 26ft fin keeled boat. Wish I was closer, could do with your old prop.

google the vicprop calc but I bet it wont be better than DougR's programme
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Old 20-10-2020, 05:03   #6
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Re: Best Process to find Right Propeller for Diesel Auxiliary

I certainly appreciate DougR's response. My prop is in like-new shape, just coated it with the 2-step Velox product. Believe it's a Michigan Wheel. Fits a 1" shaft. Feel free to reach-out if you're interested in it. - Dean
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Old 20-10-2020, 09:13   #7
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Re: Best Process to find Right Propeller for Diesel Auxiliary

Quote:
Originally Posted by dbcreed View Post
What's the best process for finding the right propeller for a diesel auxiliary engine? I now know the boatyard's guess (years ago) was wrong.

Engine: A new Yanmar 2-cylinder 2YM15. 14hp at 3600rpm max. Gearbox is 2.62:1.

Boat: Bristol 27 full keel, 6 knot hull speed (5.96kt using 1.34 √LWL formula).
Current Prop: 12" x 8" 3-blade bronze on 1" shaft (RH rotation).

Situation: Engine was installed in 2014 by a boatyard 500 miles away. I restored the boat, splashed her last week, and started breaking-in the engine following the below Yanmar recommendations. 4.1 hours so far. I am very particular about breaking-in this great running diesel properly.

Problem: I pushed the throttle to WOT just for a moment yesterday. This nice new 3600rpm max engine hit 4000rpm for about one second before I backed off - and would have gone higher had I kept pushing.

What's the best process for getting the right propeller? It doesn't sound like this can be done without hauling her, but I'd like to hear recommendations from folks who have been here and done that.

Yamar's recommendations to properly wear-in piston rings and prevent cylinder walls to become glazed.

From the Operating Manual: During the first 10 hours of operation,

+ Engine should be run at 3100-3200 rpm most of the time.

+ Do not operate at WOT for more than 1 minute.
+ Do not operate at low speed or light load for more than 30 minutes at a time.

From a Yanmar Diesel America Service Advisory: During the first 50 hours of operation,
+ Operate at varying speeds - 2000, 2400, 2800 rpm.
+ Occasional 10-15 minute periods at 3000 rpm.
+ Limit operation under 2000 rpm.
I had a similar situation when I bought my boat; I wasn't sure the prop was correct. I called Michigan Propeller and used their propeller computer then called back to speak to one of their tech reps...the prop was the correct size.

But that is not the end to the process. With that information in hand take the boat out for a test run. The boat should be able to go to wide-open-throttle (WOT) without going over the engine max RPM and without any excessive vibration or prop cavitation (give or take a few hundred RPM). If it doesn't consult with them again for what prop you should try next.

Good Luck.

~ ~ _/) ~ ~ MJH
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Old 20-10-2020, 09:29   #8
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Re: Best Process to find Right Propeller for Diesel Auxiliary

Let professional propeller people do the science :-)!

Go here:

https://www.vicprop.com/calculator.htm

Input your boat's parameters as requested. Out will come the correct specifications for a propeller for your boat. No guess work involved - well nearly none :-).

The diameter will be correct. It will be the diameter that gives a "swept area" that will absorb the power your specified horsepower will feed the prop in a normal tranny/shaft setup.

The given pitch is not quite as accurate but its "close enuff". Since pitch is a function of speed through the water, the match of the pitch to the anticipated speed will be fine, but your boat may be a tad slow due to growth which throws of the pitch calculation a tad.

But as you probably know, spurious accuracy is a PITA, so what this calculator tells you will be the specs for a perfectly serviceable prop.

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Old 20-10-2020, 09:42   #9
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Re: Best Process to find Right Propeller for Diesel Auxiliary

A larger three blade prop may create noticeably more drag. I suppose it depends on how much time you typically spend motoring vs. sailing, but personally I would get the largest 2 blade folding prop that will fit, or if you have the budget get a feathering 3 blade. There will be a sacrifice of motoring speed but you'll notice improvement in sailing speed.

We have a heavy slow boat, but noticed a big improvement in sailing when we switched from 18" three blade to 19" feathering 3 blader. Did take a little hit in the maximum speed under power, but fuel efficiency at our 5 knot cruising speed seems the same.

On the other hand if you plan to motor for days against headwinds and sacrifice about 1/2 knot in sailing speed, then go for a bigger fixed 3 blade prop.

BTW props can be changed in the water by a good diver. Cheaper and more convenient that a haulout.
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Old 20-10-2020, 09:43   #10
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Re: Best Process to find Right Propeller for Diesel Auxiliary

Different prop calculator and I used 8000 lbs dispalcement for your boat (half laden). I get 14x10x3 or less efficiently 13x12x3. Interestingly my calculator says 14hp is not enough to reach 6 knots half laden. Predicts a HP load of 16hp which is 493 pounds per hp to reach 6 knots. 14hp is predicted to give you 5.7 knots at half laden.

Whats the largest diameter prop you can fit in that aperture and only fill 85% of the opening? Probably not 14" since the boat was designed for a high reving Atomic 4. Sometime you have to design backwards from the prop and in your case a 12" prop may be your diameter limit. If so, I'd guess a 12x13x3 would be a good place to start but you may have to modify the pitch to nail it.

On edit, 12x14x3 might be a better starting point.

Good luck.
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Old 20-10-2020, 09:44   #11
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Re: Best Process to find Right Propeller for Diesel Auxiliary

I once got 8 quotes for a propeller and got 8 radically different recommendations. Then I bought THE PROPELLER HANDBOOK by Gerr and followed his formulas. This worked out well. I have since found a web calculator which I believe is good:https://www.vicprop.com/displacement_size.php
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Old 20-10-2020, 14:22   #12
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Re: Best Process to find Right Propeller for Diesel Auxiliary

When I bought (new) my Alerion 28, it came with an undersized folding prop. It was probably sized for the then one cylinder diesel, not the two cylinder Yanmar I have. I called the prop company, gave them the specs and they confirmed that I needed a larger prop and recommended an appropriate one. I suggest you do the same. The boat runs much better now though Alerion wouldn't pay for the new prop.
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Old 21-10-2020, 01:19   #13
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Re: Best Process to find Right Propeller for Diesel Auxiliary

Never ever govern a Diesel engine by the prop ,the engine must be able to reach governed revs at hull speed ,all engine manuals will tell you this ,cruise revs is usually 200 rpm below obtained revs in gear clean prop clean hull .⚓️⛵️
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Old 23-10-2020, 18:43   #14
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Re: Best Process to find Right Propeller for Diesel Auxiliary

Have a pro do the math. I did and I am very happy with the prop. I have plenty of power and bite in the water. I am a bit overpowered too.
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Old 23-10-2020, 20:21   #15
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Re: Best Process to find Right Propeller for Diesel Auxiliary

Since your prop is in an opening, you are limited in diameter. Minimum clearance at the prop tips is needed. Take measurements of the opening. Prop is definitely under pitched. I would look at both 2 and 3 bladed props. A 2 bladed would reduce drag when sailing compared to a 3 blade.
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