What size prop for 2QM15

[Island Time O25]

A question to those in the know.

The PO had installed a funky plastic (!) 2-blade prop and since it looked brand new and more or less sturdy I figured it'll be OK. Low and behold on the last day of this season it somehow was chipped - a triangle out of one blade the size of a quarter. As whatever wind there was was on the nose we motored about 4-5 hours to where the boat is hauled out. And about 45min-1hr before arrival we felt that the prop started being and sounding funny. And as the boat was hauled out the next day I saw the reason why.

On the boat there was what seems to be a previous prop (not sure if original or later replacement) and it says 13X11. Yet the 2qm15 manual says the factory prop (if I calculated from the metric data correctly) is 15X9. Any advice if my manual info calculations are correct and also if 13X11 would do the same job as 15X9?

Thanks in advance for your input.

[DougR]

Need to know:

vessel size, weight, waterline length
engine h.p. , max engine RPM, transmission reduction ratio

Can a 15 inch prop actually fit in the space available and still have about a 2 inch clearance between the blade tip and the hull?

DougR

[westwinds]

Propeller Handbook by Dave Gerr on page 45 has some rules of thumb. Rule 1, one inch more diameter absorbs the torque of two or three inches of pitch. So no, 13X11 would not do the same job as 15X9. Also, you need enough tip clearance, figure 10% of the propeller diameter from tips of propeller blades to the boat hull.

[fryewe]

13x9 is a good prop for the 2QM15. That was the pitch of the 2 blade I used on that engine for 32 years, and of the 3 blade I replaced it with. Good luck with it. I think you'll be happy with the prop you found if you put it on.

[Island Time O25]

Fryewe, it's not 13X9 it is 13X11.

Thanks everyone for the input.

I stumbled upon this https://www.vicprop.com/displacement...tion=calculate

which tells me that to replace 15X9 with 13" diameter it would need to have a pitch of between 12 and 13. I am close with 11 (just as I thought) but not exactly. It would be OK to experiment if I just be using the boat for days sails etc. But since I am planning to take her to FL I'd rather be exact with this. So I will update any new info or calculations.
---------------------------------
Data Input

Waterline length in feet: 22.75 feet
Beam at the waterline in feet: 8 feet
Hull draft in feet (excluding keel): 1.5 feet
Vessel weight in pounds: 7450 lbs
Engine Horsepower: 14 HP
Number of engines: 1
Total Engine Horsepower: 14 HP

Engine R.P.M. (max): 3000 RPM
Gear Ratio: 2.14:1
Shaft R.P.M. (max): 1402 RPM

Number of shaft bearings (per shaft): 2
Desired speed in Knots: 6.5 knots
Horsepower Calculations

This will calculate the maximum horsepower and torque available at the prop(s).

Total available horsepower at the engine(s): 14 HP
Total available torque ft/lbs at the engine(s): 25 ft/lbs
Horsepower loss of 3% per gearbox: - 0.4 HP
Horsepower loss of 1.5% per shaft bearing: - 0.4 HP

Total horsepower available at the propeller(s): 13.2 HP
Total torque ft/lbs available at the propeller(s): 49 ft/lbs
Speed & Power Calculations

Basic displacement speed and horsepower required
Displacement hull speed (1.34 X sqrt of waterline length): 6.39 Knots
Minimum horsepower required at propeller(s) for Hull speed: 16.3 HP

Calculations based on desired speed and available HP
HP required at propeller(s) for desired 6.5 knots speed: 16 HP
Estimated maximum speed with existing 14 horsepower:
This is the speed we will use for the propeller size. 6.12 Knots

At this point it is important to note that all of the calculations above are based on full RPM and HP. Most engines are rated to run at a percentage of thier full RPM. This is what will determine your maximum cruising speed. The propeller sizing calculations below are based on 90% of full RPM. This gives the engine some reserve power to allow for variable loading in the vessel.

Propeller Size

Number of blades Diameter (inches) Pitch (inches)
2 Blade 14.5 X 8.7
3 Blade 13.8 X 8.6
4 Blade 13.0 X 8.4

The propeller sizes shown above do not contain calculations for cavitation or blade loading.
If you find that the recommended propeller is too large to fit your vessel, you can try increasing the shaft speed. Failing this, you can reduce the diameter and increase the pitch at the expense of your propeller efficiency. The rule of thumb is 1 inch of diameter is equal to 1 1/2 to 2 inches of pitch.

[Island Time O25]

Just talked to my marine pro buddy re: prop size calculator, etc. He advised me to input a smaller HP for my 35 year old engine, saying that no way it'll give me same power as a brand new. So he suggested inputting 11-12hp and seeing what happens. This:

Number of blades Diameter (inches) Pitch (inches)
at 12hp 14.0 X 8.3
at 11.5 hp 13.9 X 8.2
at 11hp 13.8 X 8.1

So since this:
If you find that the recommended propeller is too large to fit your vessel, you can try increasing the shaft speed. Failing this, you can reduce the diameter and increase the pitch at the expense of your propeller efficiency. The rule of thumb is 1 inch of diameter is equal to 1 1/2 to 2 inches of pitch.

my 13X11 should work fine, especially considering that the prop itself in "A" condition.

[westwinds]

If you are not getting any crankcase blow by, then I would go with the rated horsepower of the engine which is 14 horsepower continuous operation. The engine manual is available online and it's 224 pages. Download it you might need it. The propeller specification can vary with the dimensions of the sailboat. A sailboat with a narrow beam will take less horsepower for the same speed so there can be a little bit more pitch to the propeller. Since you have both 13X11 and 15X9 I would try both. If your RPM comes in at 2700 with the 15X9, from what I have read a 10% lower RPM than what is needed for maximum horsepower has very little power penalty and will not hurt the engine. The maximum RPM of 3000 gives 14 horsepower. At the same RPM this engine can be pushed to 15 horsepower for one hour maximum. There are propeller curves for this engine in the manual.

[uncle stinkybob]

My 30' sloop running the Yanmar 2QM15 came with a 14x9 3 blade michigan wheel.
I replaced it with a 3 blade Campbell Sailor prop. Kelsey at West by North recommended the 13x7. A much smaller prop, but a much different design prop. The results were amazing, prop walk is gone, astern thrust is very powerful, I picked up 1.4 knt at my max cruise rpm, 2800 rpm. Much lower drag when sailing also. The prop is made to order for your boat. Do not buy a used Campbell Sailor prop and expect it to work for you, it may or may not.

[groundtackle]

Our Lecomte 35 16,000 lbs 2QM15. Have 13x9 now and it's no good for me.I never actually measured it so possible someone flattened it, thats what it was stamped ( fallen into that trap before) Boat won't go and no brakes at all.
I bought a used 2 blade Vari 15x10, havn't tried it yet. It may be too much for the that little thing and will adjust some pitch out if I can't get to the RPM. Rule of thumb is also to reach no load vs full load WOT with no greater than 10% drop. That is a good place to start at least.
ce

[Island Time O25]

Originally Posted by westwinds

If you are not getting any crankcase blow by, then I would go with the rated horsepower of the engine which is 14 horsepower continuous operation. The engine manual is available online and it's 224 pages. Download it you might need it. The propeller specification can vary with the dimensions of the sailboat. A sailboat with a narrow beam will take less horsepower for the same speed so there can be a little bit more pitch to the propeller. Since you have both 13X11 and 15X9 I would try both. If your RPM comes in at 2700 with the 15X9, from what I have read a 10% lower RPM than what is needed for maximum horsepower has very little power penalty and will not hurt the engine. The maximum RPM of 3000 gives 14 horsepower. At the same RPM this engine can be pushed to 15 horsepower for one hour maximum. There are propeller curves for this engine in the manual.

I do have that 224 page manual and some others. And that''where I got the factory recommended specs for the prop (don't remember from which manual). But the specs were metric so I had to convert to inches.

No, I don't have the 15X9 prop and that's why I was going to use the 13X11 which came uninstalled with the boat. The prop which was on the shaft was a smallish 2 hard blade plastic(!) prop which although looked good enough had chipped a blade on the last day of the season, about 45 mins away from the haul out dock.

[Island Time O25]

Originally Posted by uncle stinkybob

My 30' sloop running the Yanmar 2QM15 came with a 14x9 3 blade michigan wheel.
I replaced it with a 3 blade Campbell Sailor prop. Kelsey at West by North recommended the 13x7. A much smaller prop, but a much different design prop. The results were amazing, prop walk is gone, astern thrust is very powerful, I picked up 1.4 knt at my max cruise rpm, 2800 rpm. Much lower drag when sailing also. The prop is made to order for your boat. Do not buy a used Campbell Sailor prop and expect it to work for you, it may or may not.

Thanks for the info. This is something to consider over the winter lay up. From various boats I have owned I have accumulated a few spare props in good shape which I could probably now sell and use the $$ toward purchasing a new custom one. Otherwise I'm not in the play for a brand new custom prop as I don't believe in 35 year old boats getting brand new expensive parts unless absolutely necessary.

[Island Time O25]

Originally Posted by groundtackle

Our Lecomte 35 16,000 lbs 2QM15. Have 13x9 now and it's no good for me.I never actually measured it so possible someone flattened it, thats what it was stamped ( fallen into that trap before) Boat won't go and no brakes at all.
I bought a used 2 blade Vari 15x10, havn't tried it yet. It may be too much for the that little thing and will adjust some pitch out if I can't get to the RPM. Rule of thumb is also to reach no load vs full load WOT with no greater than 10% drop. That is a good place to start at least.
ce

Wouldn't 14HP be too little power for a 16,000lbs boat, never mind the prop specs? My other boat a 36' is about 18,000lbs and has 3-blade prop on a 32HP engine which IMO is barely enough.

[groundtackle]

Island Time- Yes, HP is light but thats what she came with. I just assumed they had matched it with the correct prop to maximize that lack of hp. I'm also going to throw a 100A alt with a derate switch, poor little thing.
Happier the PO spent money on sails and she performs well. Hopefully even better with the folding prop.
ce

[uncle stinkybob]

Originally Posted by groundtackle

Island Time- Yes, HP is light but thats what she came with. I just assumed they had matched it with the correct prop to maximize that lack of hp. I'm also going to throw a 100A alt with a derate switch, poor little thing.
Happier the PO spent money on sails and she performs well. Hopefully even better with the folding prop.
ce

Hi GT, would your Le Comte's name happen to be DIVA?
from what I have found, they came from factory with a Vetus engine.
I would love to see pics of your's. Very beautiful boat imo.

[groundtackle]

Stinky, we boat her in SF 5yrs ago off craigslist. She now TRIP ex Aelous. Very little history came with her, owner was always in such a hurry to get his cash and get home.
I have some pics on the flush deck thread page thingy just before we went to mexico.
ce