Recently we upgraded from a two bladed (15 X 12) to a three bladed (14 X 10) prop on our Hunter 310. The engine is a Yanmar 2GM20F. The new prop design was done by a professional shop. The original prop had realtively narrow blades while the new prop has blades of a larger area. The main reason for the change was to eliminate vibration and this it has done admirably. There has been no change in speed at the usual 2900 RPM which is 5.5 knots. The question which arises is this: is there some rule of thumb to determine just what the loading is on your engine at, let's say 80% of MAX RPM = 2900 RPM. Is the engine actually loaded to 80% or is it just loafing along due to insufficient pitch ? How can one tell ? The big problem here is that most sailboat engines (and this one) don't have a throttle but actually have a governor adjust which we refer to as the throttle ........ so the enigne turns over at the same speed for the same "throttle" position if you are in gear or out of gear. Any ideas would really be appreciated. Regards, s/v Island Hunter
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I think I see your point!
Ralph: I think I see your point. It seems to me that your boat may be slightly under propped. You should be getting hull speed at this RPM setting. I think you will find that they reduced the diameter of the prop to help eliminiate cavitation and prop walk. I am not an expert at this but in general when you drop your diameter by 1 inch and increase the number of blades, you would not need to reduce pitch. They reduced your pitch by 2 degrees and recduced your diameter. My guess is that you have lost about 500 rpms in total thrust. Now having said this, it may have something to do with the TYPE of prop. You may not have a sailor prop. I would ask them and get another opinion from another shop too. You should rev your engine in neutral and see what your maximum RPM's are (should get close to 3600). Then you should try running your engine at WOT under way. See how high your engine will rev. If you can get 3600 your probably under propped. PS: Have you noticed much difference under sail?
Ralph: I think I see your point. It seems to me that your boat may be slightly under propped. You should be getting hull speed at this RPM setting. I think you will find that they reduced the diameter of the prop to help eliminiate cavitation and prop walk. I am not an expert at this but in general when you drop your diameter by 1 inch and increase the number of blades, you would not need to reduce pitch. They reduced your pitch by 2 degrees and recduced your diameter. My guess is that you have lost about 500 rpms in total thrust. Now having said this, it may have something to do with the TYPE of prop. You may not have a sailor prop. I would ask them and get another opinion from another shop too. You should rev your engine in neutral and see what your maximum RPM's are (should get close to 3600). Then you should try running your engine at WOT under way. See how high your engine will rev. If you can get 3600 your probably under propped. PS: Have you noticed much difference under sail?
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Dakota Jim Russell
Smoke
I had some similar questions when I was setting my maxi-prop which is adjustable. I came to the conclusion that it was set pretty close to what I wanted as after the engine was really warm, I could get 3300 rpm (3GM in myHunter 340)with just a touch of smoke. If there had been no smoke, then I would have wondered. Have you had yours really warmed and run to its max speed?
I had some similar questions when I was setting my maxi-prop which is adjustable. I came to the conclusion that it was set pretty close to what I wanted as after the engine was really warm, I could get 3300 rpm (3GM in myHunter 340)with just a touch of smoke. If there had been no smoke, then I would have wondered. Have you had yours really warmed and run to its max speed?
Probably a good esitmate
JR: That is probably a good gauge. If you are not blowing black smoke at 3200 but you are at 3300 I would think that this is perfect. As long as there is also not an overheating problem either. Of course when you have an adjustable prop, you have a lot more flexibility.
JR: That is probably a good gauge. If you are not blowing black smoke at 3200 but you are at 3300 I would think that this is perfect. As long as there is also not an overheating problem either. Of course when you have an adjustable prop, you have a lot more flexibility.
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Kevin
Overload
Dear Ralph, Thanks for your note. Your engine manual most likely has a propeller performance curve in it. This curve gives you the amount of thrust that propeller is distributing at a given range of Rpm’s. This is of coarse only valid if the propeller is loading or governing the achievable Rpm’s by the engine. You should not have identical Rpm range’s in gear and out of gear. I would guess that you either have a faulty tachometer or a governor out of adjustment. Hope this helps. Kevin
Dear Ralph, Thanks for your note. Your engine manual most likely has a propeller performance curve in it. This curve gives you the amount of thrust that propeller is distributing at a given range of Rpm’s. This is of coarse only valid if the propeller is loading or governing the achievable Rpm’s by the engine. You should not have identical Rpm range’s in gear and out of gear. I would guess that you either have a faulty tachometer or a governor out of adjustment. Hope this helps. Kevin
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Ralph Johnstone
Steve .......................
....... although the blades are quite a bit larger, I cannot see any loss of speed under sail. Mind you though, the prop REALLY winds up under sail with the tranny in neutral. At this RPM, it probably is knifing its way through the water and greatly reducing the drag. Regards, s/v Island Hunter
....... although the blades are quite a bit larger, I cannot see any loss of speed under sail. Mind you though, the prop REALLY winds up under sail with the tranny in neutral. At this RPM, it probably is knifing its way through the water and greatly reducing the drag. Regards, s/v Island Hunter
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Daktoa Jim Russell
For thought: freewheeling props drag
You may be right about knifing through the water. However the concept reminds me that helicopters, and oak seeds rotate their "props" to slow their descent . . . . Hence I have always stopped the rotation on my prop and drive shaft to increase speed, and reduce the wear on the transmission. Any other thoughts out there?
You may be right about knifing through the water. However the concept reminds me that helicopters, and oak seeds rotate their "props" to slow their descent . . . . Hence I have always stopped the rotation on my prop and drive shaft to increase speed, and reduce the wear on the transmission. Any other thoughts out there?
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David
Prop drag
Dakota Jim I think you are correct. Dave Geer for one has written an article that supports the theory that a freewheeling prop produces more drag than a prop that is fixed with no rotation.
Dakota Jim I think you are correct. Dave Geer for one has written an article that supports the theory that a freewheeling prop produces more drag than a prop that is fixed with no rotation.
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Terry Arnold
freewheeling or not
In the July 15, 2000 issue of Practical sailor PS says "In 1993 we worked with graduate students and professors at MIT to test a nuber of propellers. One of the tests checked to see if allowing a prop to freewheel created less drag than locking the prop in place. The answer was that "substantial" savings in drag can be had freewheeling "across the whole range of speeds." This test assumed that both props were located in the open behind shaft struts not in apertures of full keels." In another PS I remember PS opining that transmission wear was negligible in freewheeling but that in blue water cruising, still a good idea to run the engine several times a day to lubricate the transmission.
In the July 15, 2000 issue of Practical sailor PS says "In 1993 we worked with graduate students and professors at MIT to test a nuber of propellers. One of the tests checked to see if allowing a prop to freewheel created less drag than locking the prop in place. The answer was that "substantial" savings in drag can be had freewheeling "across the whole range of speeds." This test assumed that both props were located in the open behind shaft struts not in apertures of full keels." In another PS I remember PS opining that transmission wear was negligible in freewheeling but that in blue water cruising, still a good idea to run the engine several times a day to lubricate the transmission.
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Colin
freewheeling
Just an idle thought..where drag is not of paramount importance..has anyone designed a power generator driven by the freewheeling prop rather than a separate water driven generator?
Just an idle thought..where drag is not of paramount importance..has anyone designed a power generator driven by the freewheeling prop rather than a separate water driven generator?
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David
Colin
I once met a fellow who sailed a Shamrock 38' from Cape Cod MA to Europe and back with an alternator driven off the freewheeling shaft. He stated it worked well for the 10,000 mile voyage. He had a hydraulic drive transmission and claimed no damage to it.
I once met a fellow who sailed a Shamrock 38' from Cape Cod MA to Europe and back with an alternator driven off the freewheeling shaft. He stated it worked well for the 10,000 mile voyage. He had a hydraulic drive transmission and claimed no damage to it.
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