Prop Size

[weephee]

I am replacing my Atomic 4 with a Yanmar 2QM15. I used the prop calculator and came up with 13 dia x 12 pitch for a 3 blade. I have a 3 blade but it is 12 x 12. Would the 1 inch reduction in diameter make a big difference. Would it be ok to go ahead and use what I have. Thanks

[westwinds]

The Propeller Handbook by Dave Gerr, Page 45, Rules of Thumb, #1: If you reduce the diameter of a propeller by one inch, you need to increase the pitch by 2 or 3; however, this rule should not be used to select the propeller, need further calculations. Also, Item #3: For every two inches of propeller pitch change, the RPM will vary by about 450 RPM.

Now the real question here is how much power can be transmitted through the 12X12 propeller when the diameter changes from 13 to 12 inches considering RPM. It is going to be roughly half the horsepower.

By the way, what is the maximum RPM, transmission speed reduction ratio (if any), diameter and pitch of the propeller with the A4? With those numbers, you can use the formulas in the Propeller Handbook to arrive at the correct size of propeller for the Yanmar. This is with a clean propeller and boat bottom.

[djmarchand]

Try it. If the new engine will reach wot with the new prop then it at least is the right pitch/diameter or maybe the pitch is too high. If it won't reach maximum rated rpm at wot then that isn't good for the engine and you should do something about it. You can't do anything about the diameter but any prop shop can add or subtract 2 inches of pitch.

And if it will cruise where you want to cruise leave it alone.

But as westwinds noted your new engine probably has a different transmission ratio and may even have a different wot rated rpm. But in any case try it first.

[weephee]

Ok, Thanks for the replies. I finally found the specs for my boat with a Yanmar engine 2qm15 and it calls for a 2 blade 15 x 9. I talked to a prop store and they told me that if I wanted a 3 blade then I should drop the diameter and pitch by 1 inch because it would be too much stress on a 3/4 inch drive shaft. That means I can use a 14 x 9. Price is about $600. Someone has offered me a 14 x 6. From the previous post it appears that I can have a prop shop change the pitch from a 6 to an 8. Is this correct or can I leave it as a 6 inch pitch. Sorry for all the questions, but I obviously don't know much about what I'm talking about. Thanks

[Kokanee]

Still need more info to make a call on the correct prop size.

The Yanmar 2QM15 is rated at max 3000 rpm. There are two possible reduction gear ratios. Either 2.14 or 2.83 resulting in a maximum prop rpm of 1400 or 1060. Make sure you are using the right ratio when you input the data into a prop calculator such as this.
Vicprop - Prop calculator for Displacement and semi-displacement hulls

[weephee]

The gear ratio is 2.14 if that helps and the LWL is 21' 3"

[weephee]

I put in the information and it comes up with 2 blade - 14.5 x 8.5 and the boat manual says 15 x 9 so that's real close and for a 3 blade it comes up with 13.8 x 8.4 which is close to 14 x 8 so I guess I'm ok I'll save this prop calculator. Its more detailed than the one I was using and came up with a better result for me LOL.

[westwinds]

Originally Posted by weephee

I put in the information and it comes up with 2 blade - 14.5 x 8.5 and the boat manual says 15 x 9 so that's real close and for a 3 blade it comes up with 13.8 x 8.4 which is close to 14 x 8 so I guess I'm ok I'll save this prop calculator. Its more detailed than the one I was using and came up with a better result for me LOL.

The following link is the service manual for the Yanmar. Look at page 1-4 for the propeller curves. 2QM15http://j30.us/files/2qm15_servicemanual.pdf

I would go with the 2 blade 15 x 9 because this was the propeller that fits the boat and at the end of any calculation, there is usually the art of selecting the correct propeller for the particular boat. This final step has been done for the 2 blade 15 x 9. You could try a 2 blade 14X11 or 14X12, but you would be up against the art part of propeller selection. The problem with propeller size calculators is that they are developed for the average boat. For instance if the beam is not average for the length, that throws the formula off right there.

The Propeller Handbook by Dave Gerr does have a formula for propeller shaft size on page 89. The diameter and pitch of the propeller are not part of the equation because horsepower and shaft speed are the determining factors for this calculation. You might say RPM and HP take the place of propeller specifications. Using a shaft speed of 1400 and a flywheel horsepower of 14 and stainless steel 304 as the shaft material, the diameter works out to 0.784 inches, a little more than 0.75 inches. If Aquamet 22, Aquamet 18 or Aquamet 17 is used, then the shaft size is a little over 0.5 inches because of the higher yield strength in torsional shear for these propeller shaft alloys. At any rate the 0.75 diameter was apparently what the marine architect specified and probably used a safety factor of a little less than 3. The original propeller shaft might have been for an A4 without reduction gearing However, if you look at propellers of the size of interest the propeller shaft size is one inch. Are you sure the propeller shaft on the sailboat is not one inch? The following is a link to a propeller source I found on the internet, which is where I found the propeller shaft size that usually goes with these size of propellers of interest.

PROPELLERS, Used, New, inboard and outboard
Sea Chest Marine Propellers.

[weephee]

Thanks Westwings for your 2 detailed responses. I thought getting the yanmar engine and installing it was going to be the hard part but this seems to be beating it but at least I'm learning something, I think. Now, the Columbia 8.3 originally came from the factory with an atomic 4 as standard equipment. It has a 12 inch diameter x 6 inch pitch, two blade prop on a 3/4 inch stainless steel shaft. The atomic 4 is approx 30 hp with 1800 rpm at WOT direct drive. Optional was the yanmar 2qm15 which is approx 15 hp with 3000 rpm at WOT with 2.14 gear ratio. The shaft is still 3/4 inch and it came with a 2 blade 15 x 9 prop so larger diameter but more pitch. Would that not put more load on the shaft. A gentleman at the Millers Island Propeller in Baltimore told me that you need to reduce the diameter and pitch by one inch when going from a 2 blade to a 3 blade so that is why I was trying to get a 14 x 8 3 blade for this Yanmar. I have no problem sticking with a 2 blade but most people tell me a 3 blade performs much better than a 2 blade. Your comments on all of this would be much appreciated. The shaft is definitely 3/4 inch.

[westwinds]

Originally Posted by weephee

... Now, the Columbia 8.3 originally came from the factory with an atomic 4 as standard equipment. It has a 12 inch diameter x 6 inch pitch, two blade prop on a 3/4 inch stainless steel shaft. The atomic 4 is approx 30 hp with 1800 rpm at WOT direct drive. Optional was the yanmar 2qm15 which is approx 15 hp with 3000 rpm at WOT with 2.14 gear ratio. The shaft is still 3/4 inch and it came with a 2 blade 15 x 9 prop so larger diameter but more pitch. Would that not put more load on the shaft. A gentleman at the Millers Island Propeller in Baltimore told me that you need to reduce the diameter and pitch by one inch when going from a 2 blade to a 3 blade so that is why I was trying to get a 14 x 8 3 blade for this Yanmar. I have no problem sticking with a 2 blade but most people tell me a 3 blade performs much better than a 2 blade. Your comments on all of this would be much appreciated. The shaft is definitely 3/4 inch.

The propeller for the A4 is smaller in diameter with less pitch because of the direct drive so the propeller turns faster; the diesel runs at a higher engine speed because of the reduction gears. These two things put the A4 at a disadvantage. The larger propeller moves through a lot more water at a slower speed, which makes it more efficient, less turbulence. The A4 also does not reach the maximum horsepower because the engine speed is held to a lower RPM, as it is a direct drive. In fact the horsepower is about halved. Also, the A4 will go half the distance the diesel will on a gallon of fuel because of its obsolete design and the fact that is less efficiency because it is gasoline, but just getting in and out of harbor, well not much is needed. There is also the explosion hazard with gasoline, but a lot of safeguards exist with a gasoline engine in a boat so you do not hear of fires and flying boat parts much at all. The easy thing here is that you already have these several engine setups that have been designed by a marine architect and these fellows know what they are doing. Just go with what exists. The two vs. three blade arguments require careful design consideration. It’s complicated because of something called cavitation that has to do with how much power you can put to a propeller before the forces become so great that the water actually pulls away from the curved surface of the turning propeller. What happens is the water actually turns to vapor, like boiling water is a vapor, but this is at the cool water temperature the boat is in. It takes a awful lot of energy to do this and when the water moves to a different part of the propeller, it comes back with a force that actually can destroy the metal the propeller is made from. It’s somewhat like a hammer blow. It makes a noise you can hear. Quite a few pages of the Propeller Handbook concern the design to prevent this problem. If the propeller is made too small in diameter and the pitch too great, then this cavitation can happen. All a three blade propeller does is give more area to spread the forces around. It is actually a little less efficient, and when the engine is not running, it also creates more drag if it is not a folding propeller. A two blade is also cheaper than three or more blades.

[weephee]

I'll go with the two blade. Likely have to buy a new one as used props for 3/4 shaft are few and far between. Thanks for all the information. I've learned a lot and appreciate your help

[Opie91]

Maybe try a Campbell Sailor West by North Enterprises - Campbell Sailer

Pretty sure they are around $500 for a 3 blade and have a lot of good reviews.

[Chasing Summer]

Proper Pitch Marine Propeller, marine sales and service, marine boat propellers, shafting and drive train sales and reconditioning services, Selbyville, Delaware, Mid Atlantic USA ... and it's not a harmonica shop ...
Props is Scotts business. If he doesn't have it he can have it made. Also, he can change the pitch up and down a couple inches.
He's working with me for a 3 blade prop for my Spencer 42.

[roverhi]

FWIW, my 3GM30 with the 2.0 gear box has a 13x12 prop which seems just about perfect.

[Cheechako]

I'd try that prop you have. Calculators are not perfect any way. Get some real world info by doing so too.