Reducing prop blade area

On my 30' trawler I have an 18" Michigan "MP" prop driven w a 40hp engine w 2.57-1 reduction. Of the 40hp I usually use about 18 to cruise at 6.15 knots. I'm slightly over propped but one inch pitch reduction would make me under propped.
Ideally I like to be just a tad under propped like about 50rpm. The prop is under loaded at 18hp so I'm think'in that reducing parasitic drag by taking off a bit of the prop edges or edge will gain the rpm I need, reduce the P drag and avoid using too much power to turn the blades through the water.
One could make an accurate line on the blades on each of the 3 blades, grind off to the lines and dress the edges appropriately.
Could take it all off on one or the other side or the same amount off both. I'm leaning toward all off the trailing edge.
What think all?

 

If you don't know props, don't mess with it. Go to a prop person who knows what they are doing.

 

Do yourself a favor and give Acmemarine.com a call. They can fit you with a prop that will do a much better job than that stock Michigan wheel. Of course it's means a different wheel, but it'll work better. On the other hand, you can take that wheel to a shop and have it cut down a bit.

 

What is the engine RPM? What is the propeller pitch? It doesn't seem likely that you will be able to reduce blade area enough to increase RPM by 50. The pitch could be tweaked a bit or as PAR stated a better choice might be a more refined propeller design made for the desired operating point.

 

Those "dog ear" props are obsolete and not very efficient. Michigan Propeller can also fit you with a new style prop.

 

Thanks all,
OK I'll take it to a shop. Now that I'm no longer in Alaska I have lots more options.
As to design I'm curious about the Acme Propellers PAR recommended. They look and sound good .. very good. Wonder how much more money they are.
I don't really need great efficiency as the boat burns about 1gph. And I have plenty of power. The engine (37hp) is rated at 3000rpm. Vetus and Westerbeke run the same Mitsubishi 107 cu in engine and rate the same engine at 42 and 44hp respectively. My redrive is a BW 2.57-1. I suspect my blade area is too much for my little engine. I'd rather reduce blade area than reduce pitch just on my perceived principals.
Re the old design element it dosn't seem any different performance wise than the skewed and more modern prop that was on the boat when I got it. It worked fine but was getting "thin" and slightly pink.
So I tried to buy the MP prop through Blue Sea and I thought I was getting an MP three blade when I ordered an "MP-3" prop. Wrong. They wouldn't take the prop back (the MP-3).
Had the MP-3 on the boat (same P&D) but seemed a little overpropped. Sold it to someone in Alaska and bought the "MP" from Tacoma Propeller. The MP has lots of reverse thrust and otherwise has good performance. The only thing I'd be interested in improving significantly is smoothness. But all the props I've had seem plenty smooth enough. But if an Acme prop could make my boat twice as smooth I'd be interested. Personally I don't think I'd gain much in smoothness or efficiency. I'm guessing 5 to 7%.
The only real prop problem I have is the poor mismatch in the bore or shaft taper. The prop obviously seats only on the fwd 1/4 of it's bore. Haha but that work's fine too. But again I want to make it right.

 

You are exactly right about the efficiency. The one you have probably is pretty good and there won't be much to gain. You may see an improvement in fuel burn if you run the engine at a better HP/RPM point. You could get a smoother running prop by increasing blade number and going to a skewed design. However, both of those will also reduce backing.
As I said earlier, reducing blade area alone probably won't get you much of an RPM increase. So making a slight pitch adjustment is probably necessary.

 

johneck,
Thanks for the input.
Re the more blades idea I did try this old Michigan Star. One inch larger dia at 19". Found it in a 2nd hand store. Took pitch out twice but still too much. Would need to reduce dia. Now that I don't have access to a tidal grid I'll probably not experiment w it more. Interestingly it was the least smooth and noisiest of all the props I've had on the boat. I think it was the water coming off the blade tips and hitting the bottom. Plenty of clearance as you can see though.

 

Interesting, generally with more blades each one has less loading and thus the pressure pulses are lower. Your description of water coming off the blade tips and hitting the hull is not really a reasonable explanation. The flow of water contracts as it goes thru the propeller disk and exits at about 85% of the propeller diameter. What is making the boat vibrate is pressure pulses created by the blades. These pulses are caused by the prop creating thrust, but more so by cavitation as it grows and collapses. if the 5 bladed prop had greater vibrations it was almost certainly due to increased levels of cavitation.
The skeg in front of the propeller is quite thick and blunt and appears to be almost as wide as the blade. This leads to a bad inflow situation for an unskewed propeller. There is a significant axial wake deficit that the entire blade enters and then leaves. This will make large thrust fluctuations and show up as unsteady shaft forces. So in my opinion this propeller is a bad choice for the boat. Is there any way to round the aft edges of the skeg? This might improve the inflow and make any propeller work better.

 

johneck wrote;
"Interesting, generally with more blades each one has less loading and thus the pressure pulses are lower. Your description of water coming off the blade tips and hitting the hull is not really a reasonable explanation. The flow of water contracts as it goes thru the propeller disk and exits at about 85% of the propeller diameter. What is making the boat vibrate is pressure pulses created by the blades."

Yes but the blades on the "Star" are very high aspect ratio so the tips are very narrow and throw out very strong pulses.

"These pulses are caused by the prop creating thrust"

Actually loosing thrust. Those "pulses" are the product of tip losses ... the water not staying on the pressure side of the blade but boiling around the blade tips trying to get away from the advancing blade.

The wider the blade tip the greater opportunity the water has to escape around the blade tip and thereby cause loss of thrust.

The MP type (I refuse to call my favorite prop a "dog ear") gives the water lots of opportunity to get around the blade tips hence it's lower efficiency. I have another theory that the long graceful gentle curve of the MPs blade edge is probably better at cutting through the turbulence of the flat backed deadwood and perhaps regaining back some or all the efficiency lost over the wide tips.

If I was to continue experimenting w the "Star" I'd probably cut the blade tips off straight along the arc of the OD. Cutting them off almost square but w the trailing edge a little longer than the LE to reduce tip losses a little.

What think?

 

I would advise against cutting the tips off the other propeller. Once again the issue is not tip losses but the blade abruptly entering the slow moving water behind the skeg. The pressure differential at the blade tip induces a flow from the high pressure on the aft side to the low pressure on the forward side and this generates a spiraling tip vortex. the tip vortex varies in intensity depending on the instantaneous lift of the blade and therefore is not completely uniform. The exact shape of the tip and the leading edge into the tip does have an effect on the strength of this vortex, but the radial circulation distribution is a much greater factor. The three bladed propeller is producing the same thrust (basically) as he five bladed one, so each blade is generating 60% more thrust. So each of those blades has a stronger tip vortex.

The pressure pulses on the hull are caused by the rotating pressure field created by the blades. The pressure field is there due to the thrust (or vice versa) but there is also a varying pressure field due to the varying volume of the cavitation on the blades. This in most cases is far greater than the pressure due to the blade lift.

The basic efficiency of the propeller is determined by diameter, RPM, power. Some small variation can be achieved with a proper circulation distribution and blade shape, but it really can't be changed very much. Unsteady forces are affected by all the factors we have been discussing and can be greatly varied base on varying the propeller geometry (cord, skew, pitch, etc.) That is what makes propeller design such an interesting field. It has kept me interested for almost 30 years (and hopefully many more).

 

johneck,
Thanks but the only way I could work on that prop is to be able to pull it off several times and cut more off the tips of the blades until the WOT rpm came up to my engine's rating. It costs 2 or $300 for each lift out so would be a waste of money.

" Once again the issue is not tip losses but the blade abruptly entering the slow moving water behind the skeg." Yes and this is one of the reasons I think this MP prop is excellent on my Willard. The leading edge goes into the turbulence like a scythe .. not like a meat cleaver. Besides the rounded propeller blades go nicely w the rounded bilges of the Willard hull.

Yes propellers are interesting ... kinda like anchors. I can experiment w the latter much more easily.

 

Right, I am suggesting that the non-skewed 5 bladed prop is not a good option for your boat. I would sell it and look for something else. In the meantime, repitch your existing prop to get the RPM you want.

 

A question on semantics: with "overpropped" do you mean that the engine does not reach its nominal full power rpms (in your case 3000 rpm)? If so, what revs do you get then?

 

baeckmo,
Yes.
Almost though as in 2900 plus a tad. I'd rather be underpropped 50rpm.
I think I'd gain about 200 by reducing pitch 1".
This is just a fine tune issue.
Have the boat out of the water and blocked up. Don't plan to relaunch till next spring. Have some small blisters and a few other things like replacing the Kingston valve, refinishing cap rail, R&R prop shaft and rudder to replace the dripless seal bellows and match the prop bore to shaft taper ect ect.
So the boat's getting pickled for the winter.

I see your'e from Sweden. My previous boat was an Albin 25. WONDERFUL boat.
The Albin had an MP prop the whole time I had it.