2-Blade to 3-Blade Conversion Table?

[rardiH36]

I can't seem to find with Google or Yahoo search engines any table or formula for determining the 3-blade equivalent to my existing 2-blade prop.

I have read that actually calculating the right prop from scratch is quite detailed. (Inputs include rated engine HP, rated max engine RPM, gear reduction ratio, beam, length on waterline, displacement, and more). My current 2-Blade prop, 16(D)x15(P) RH, as specified by Hunter, I think is matched about as right as can be for the engine/boat. The engine's rpm's max out right at its rating and max speed motoring at about 7kts would seem to be near the boats hull speed. But I want to try a 3-blade to see if I can reduce the severe prop walk I have in reverse. Is there a simple conversion table for 2 vs. 3 blade equivalents?

regards,
rardi

 

[Steve Dion]

You can try contact Kilian Props in Alameda. These guys are very good and can probably help you out.

Kilian Propeller


510-614-0596
2900 Main
Alameda, CA 94501

Another option is to check out the Campbell Fixed 3 blade. A friend put one on his Jeanneau and it appeared to be a different motor. There was very little vibration and he claimed much better motoring performance.

Normally when you go to a 3 blade from a 2 blade they either reduce the diameter by 1" or take about 1 deg. out of the pitch. It is really better to speak with a pro about this.

http://www.westbynorth.com/CampbellSailerTests.aspx

 

[sailaway78]

Total agreement

Talk to a knowledgable prop shop. You can calculate till your green, and still be off a bunch. Are just too many variables. One thing I would suggest, is with the boat empty, you would want it to possibly over rev one or two hundred RPM, to put you right on with a normally loaded boat. A really good prop guy can get you right on the money first try. And a prop, and prop change is just too expensive a proposition to gamble on.

 

[Roger Long]

Is there a simple conversion table for 2 vs. 3 blade equivalents?

If the pitch is the same, you want to end up with about the same blade area. For the new prop you will be buying, there should be full information. Look for D.A.R., Disk Area Ratio. Multiply this times the area of a circle the diameter of the prop. D.A.R. for typical three blade powerboat props is .5, and four blade props, .67. I don't recall seeing the D.A.R. for a two blade prop since they never came up in my professional practice but they are probably about .33.

My 16" two blade would therefore have a blade area of 66.35 square inches. Divide by .5 (same as multiplying by 2) and the diameter of the 3 blade prop circle should be 132.7.
Divide by pi, take the square root and multiply by 2 for diameter and the three blade wheel should be 13" in diameter. 80% of the diameter would be a very rough rule of thumb.

For what you will pay for a new prop though, you should get better information, real numbers, and confirm that your present prop is a good match for the boat. Most prop sellers now have computer programs that do this quickly.

If you are interested in sailing performance, the greatest benifit of a full analysis will be to get the smallest prop that won't cavitate and will give you the bucking power in adverse conditions. Your prop may be a good match but a smaller prop with more pitch might be as good (aside from slightly lower effeciency and resulting higher fuel consumption) and have less drag under sail.

If you just make a direct (equal area and pitch) 2 blade to 3 blade swap, you will probably notice the increased drag. The blades on the three blade wheel are closer together so interact more when water flows around them while sailing. You want a 3 blade wheel to be as small as possible.

If your boat does well under power with the 3 blade wheel as mine does, my advice would be to learn how to use and work with the prop walk. You probably aren't going to eliminate it, just reduce it a bit and not by enough to significantly change the way your boat handles. Getting a slip where you dock on the opposite side is a lot cheaper than a new prop, for instance.

 

[Ross]

Re: Total agreement

There is diameter and pitch. Blade area ratio and blade shape. Too many variables and after reading Dave Geer for an entire winter I have come to realize how much I don't know about props.

 

[Hermit Scott]

I seem to be collecting props for my boat off ebay. My boat came with a 12LH10. I bought a 12LH12 and a 12LH11. I am going to try to have the 12LH12 repitched to 12LH10, but I think I will try out the different props to find the one that works best and sell the rest. The last prop I bought was under $50, to have it repitched it $75.
From what I heard, if you go with the same diameter prop going from a 2 blade to a 3 blade, they told me to subtract one inch from the pitch. I think there may be more to it than that, because llike Ross said there is blade area to condsider, and really I think some props are designed differently and are probably more efficient.
By the way what diameter and pitch 2 blade do you have?

 

[Don12364]

Calculations??

I have an DOS program which calculates the diameter and pitch from basic principles for any type of prop/hull combination. However it must be realised that the calculated prop for a small vessel should only be seen as a starting point and a prop shop will try this and then go either up or down to optimise the performance.
This is a luxury we who want to just buy one prop cannot afford but it does argue in favour of buying a feathering type as these can have their pitch adjusted - but they cost double a fixed prop in the first place!

 

[Ross]

Re: Calculations??

Wandering through a marina in the wintertime and studying the props on sail and power boats can be quite an education. Some blades look like mickey mouse ears and some look more like rabbit ears. Some carry a lot of metal and some seem rather skinny. I suggest that you could find a couple of dozen 12x12 props and they would all be different LH and RH not being a factor.

 

[Steve Dion]

Hermit:

If you just take your prop to a Prop Shop, they can calculate what you need and tune up one of your props. Why waste money guessing when there is a science to accomplish the end to a mean?

 

[Hermit Scott]

I bought my first prop that was close but not exact because I thought I would be launching my boat 4 months ago. Then I caught a good deal on a prop only one pitch away $46 with shipping. So the short answer is cheapness. Also I have little trust in anyone in my area to be able to calculate this properly. I do want to try out each prop to see the effect before I sell them. I should be able to break even by the time I am through and I will know for a fact what each pitch difference does for my boat.

 

[Chuckbear]

rardi, Michigan Wheel has a prop calculator on their web site and yes you should calculate what you need rather than do some kind of conversion. That is why you can't find a chart. Once you have the general information, contact Michigan or your local prop shop to nail down exactly what you need. Making the wrong choice would be expensive. Chuck

 

[rardiH36]

Thanks all for the good info and suggestion where to go for prop help. I've submitted querys with some of the firms. Once the prop selection is made, might be a while before actually installed. Will need to access whether to ask a diver, or wait until the next haulout.

 

[Roger Long]

Will need to access whether to ask a diver, or wait until the next haulout.

I can tell you've never been a party to taking one off on land. That diver could be down there a long time and they charge by the hour. He might not get it off at all.

Wait for the haul out. For a proper installation, the fit between the prop and the shaft should be checked.

 

[Ross]

My boat has a propeller aperture and the clearances are so narrow as to require shifting the engine forward to be able to provide enough room for the hub to clear the end of the shaft. When everything is in place it works very well but pulling the prop is a major job.

 

[rardiH36]

Ross:

Yes I have heard rumblings that changing a prop in the water can be a challenge. And also sometimes the shaft and prop might not match right which means the diver needs to come out a second time. You warning supports my instinct to wait until the next haulout.

rardi

 

[Maine Sail]

Rardi

I would highly recommend the three blade Campbell Sailor prop. It is the most efficient, both in motoring and in reduced drag, of just about any fixed three blade prop.

The Campbell prop, unlike the Michigan MP Sailor props is designed like an airplanes wing and is also cupped. I have not yet installed mine so I can't fully comment but after speaking with numerous customers each and every one was very enthusiastic about it.

Compared to a Michigan 3 blade prop at 7 knots the Campbell has about 26 pounds less drag. At five knots the drag of the three blade Campbell is nearly the same as a Michigan 2 blade and only about three pounds more than a Martec 2 blade folder..

Mine is sitting on my work bench and really is a beautiful piece of gear..