Reference to "Proper vs Correct Prop Size" by Frank Walkker, Frank, have you decided what prop size to go with in your case? I have a similar situation with my Hunter 36, and am still going over the pros and cons. Bottom line is that it takes a given amount of hp to go a given speed. The more efficient the prop is, the less the hp required. So I am looking for a prop that give high efficiency. Haven't found one yet. However, if you choose to go overpropped, you will not be able to acheive the max hp of the engine, and if you are operating close to the limit, it does not leave much room for any execess load such as battery charging. I have been using a three blade prop that limits my engine sped to 2900-3000 rpm, with "black smoke" as an indicator. In general our cruising speed has been around 2200 to 2400 rpm, but at times may have pushed it up to make the next bridge opening..etc. Presently have less than three thousand hours on the engine (1982) but it is beginning to show the need for an overhaul. Recently I replaced the three injectors, and whereas that helps, the engine is hard starting, indicative of low compression. Faced with an engine overhaul or new engine, I am revisiting the prop size as well. Any further discussion on this would be appreciated.
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Props..a black art
Ernie, This prop stuff seems to be a black art. I have found on my airplane that two props of the same size (diameter) and pitch can yeild very different performence due to differences in blade shape. It would seem from reading most of the reviews, that the early Hunters (Cherubini) were all under propped and benefited from the replacement of a three bladed prop. The number of blades not being the deciding factor, but the increase in total "area" of all blades. The scientists tell us that the fewer the blades the more efficient. In the case of the older Hunters, this would mean larger blades (instead of more). But due to clearence limitations the only option was more blades so as not to increase the diameter. It would be interesting to see how a two blade prop with much wider blades might do? I believe the engine should be able to attain max RPM asper the manufacturers spec. If it can't you are "over propped" (and will probably smoke like your's). If you can easily exceed this spec you would be "under propped". If you're right on, your prop "load" is correct but it still might not be efficient. This is where the black art comes in. I'm sure the engineers could tell us more on determining pitch, number of blades, blade shape, and area. For us none engineer types, it's trial and error.
Ernie, This prop stuff seems to be a black art. I have found on my airplane that two props of the same size (diameter) and pitch can yeild very different performence due to differences in blade shape. It would seem from reading most of the reviews, that the early Hunters (Cherubini) were all under propped and benefited from the replacement of a three bladed prop. The number of blades not being the deciding factor, but the increase in total "area" of all blades. The scientists tell us that the fewer the blades the more efficient. In the case of the older Hunters, this would mean larger blades (instead of more). But due to clearence limitations the only option was more blades so as not to increase the diameter. It would be interesting to see how a two blade prop with much wider blades might do? I believe the engine should be able to attain max RPM asper the manufacturers spec. If it can't you are "over propped" (and will probably smoke like your's). If you can easily exceed this spec you would be "under propped". If you're right on, your prop "load" is correct but it still might not be efficient. This is where the black art comes in. I'm sure the engineers could tell us more on determining pitch, number of blades, blade shape, and area. For us none engineer types, it's trial and error.
Solution - Buy an adjustable prop
Kill two birds with one stone: Buy a Maxi prop. It feathers, but it is also adjustable. One model allows adjustment while the boat is in the water, though it takes a surface dive to do it. I've got one on my 340 and found that it was overpowered the first year I used it. I dropped the adjustment (pitch or angle, whichever you wish to use) and got the engine to rev higher. If you want a smooth ride, buy the three blade. If you're not motoring much, buy the two blade which provides slightly more vibration but saves about a boat unit ($). "Boat" unit is an acronym for Bring On Another Thousand. Jim
Kill two birds with one stone: Buy a Maxi prop. It feathers, but it is also adjustable. One model allows adjustment while the boat is in the water, though it takes a surface dive to do it. I've got one on my 340 and found that it was overpowered the first year I used it. I dropped the adjustment (pitch or angle, whichever you wish to use) and got the engine to rev higher. If you want a smooth ride, buy the three blade. If you're not motoring much, buy the two blade which provides slightly more vibration but saves about a boat unit ($). "Boat" unit is an acronym for Bring On Another Thousand. Jim
Engine might not need complete overhaul
I know your post is mainly concerning props. But I wanted to make a comment on the engine overhaul.. Under 3000 hours is not bad (got alot of miles & years for the original engine- that good). But you might not need a complete overhaul. Hard starting (where its getting a little bit harder every year) and possibly lower compression can be indicative of the valves being "burnt". You should look into just having an "upper engine" overhaul on your valves. You might find tha,t that is all you need to get your engine working great again and can be alot less expensive than a complete overhaul. I am referencing this directly out of Calder's book on Mechanical and Engine Boat Maintenance.
I know your post is mainly concerning props. But I wanted to make a comment on the engine overhaul.. Under 3000 hours is not bad (got alot of miles & years for the original engine- that good). But you might not need a complete overhaul. Hard starting (where its getting a little bit harder every year) and possibly lower compression can be indicative of the valves being "burnt". You should look into just having an "upper engine" overhaul on your valves. You might find tha,t that is all you need to get your engine working great again and can be alot less expensive than a complete overhaul. I am referencing this directly out of Calder's book on Mechanical and Engine Boat Maintenance.
pays your money and takes your chances
Ernie, I ordered a Campbell Sailor 3 blade. I let them size the prop and installed it this weekend. I went from a 16 X 11( unknown mfg) two blade to a 14 x 8 three blade. From a little preliminary motoring it would appear to load the engine almost identically. Max speed is still 3400 RPM. I probably picked up .25 knots at 2400 RPM and the Campbell is noticeably smoother. Full boar is still 6.7 knots. I compared the blade area of the two props and they look about equal so I expect the sailing drag to be the same. I think the reason for the speed increase at 2400 RPM is that the Campbell probably loads up faster. This was the point of my original inquiry. The power curve of props and engines do not match up very well. If you match them at max RPM then the engine is loafing at cruise. I still believe that a little overproping is desirable but to paraphrase Clint Eastwood " you got to know your limitations" Your concern about the high output alternator is well founded as a 70 amp unit will suck up almost 2 hp at max output. Frank
Ernie, I ordered a Campbell Sailor 3 blade. I let them size the prop and installed it this weekend. I went from a 16 X 11( unknown mfg) two blade to a 14 x 8 three blade. From a little preliminary motoring it would appear to load the engine almost identically. Max speed is still 3400 RPM. I probably picked up .25 knots at 2400 RPM and the Campbell is noticeably smoother. Full boar is still 6.7 knots. I compared the blade area of the two props and they look about equal so I expect the sailing drag to be the same. I think the reason for the speed increase at 2400 RPM is that the Campbell probably loads up faster. This was the point of my original inquiry. The power curve of props and engines do not match up very well. If you match them at max RPM then the engine is loafing at cruise. I still believe that a little overproping is desirable but to paraphrase Clint Eastwood " you got to know your limitations" Your concern about the high output alternator is well founded as a 70 amp unit will suck up almost 2 hp at max output. Frank
Good choice, Frank
Frank: Thanks for the response. I have also been leaning towards the Campbell Sailor and am glad to hear about your results. By any chance did you check into the Kiwi Prop, a new one that feathers? Tom, hope you are right. Just finished checking the compression on the engine...150psi on #1, 290 on #2 and 280 on #3. Repeated the test with capfull of oil in each cylinder,....320 on #1, 340 on # 2 and 290 on #3. Left engine sit for a while, repeated then measured again, 220 on #1, 340 on #2 and 300 on # 3.. Noticed in one of Gordon's response to compression testing, he mentioned that below 350 psi indicated some kind of overhaul. Ernie
Frank: Thanks for the response. I have also been leaning towards the Campbell Sailor and am glad to hear about your results. By any chance did you check into the Kiwi Prop, a new one that feathers? Tom, hope you are right. Just finished checking the compression on the engine...150psi on #1, 290 on #2 and 280 on #3. Repeated the test with capfull of oil in each cylinder,....320 on #1, 340 on # 2 and 290 on #3. Left engine sit for a while, repeated then measured again, 220 on #1, 340 on #2 and 300 on # 3.. Noticed in one of Gordon's response to compression testing, he mentioned that below 350 psi indicated some kind of overhaul. Ernie
"I couldn't sleep at all last night"
Ernie, I looked at about all of the feathering props but I just could not sleep at night knowing that I was swinging 2 grand worth of bronze and probably pretty delicate at that. I could just imagine what a floating day marker piling would do to those stub shafts and bearings. Those compression numbers sounds like #1 cylinder might need a little work. Frank
Ernie, I looked at about all of the feathering props but I just could not sleep at night knowing that I was swinging 2 grand worth of bronze and probably pretty delicate at that. I could just imagine what a floating day marker piling would do to those stub shafts and bearings. Those compression numbers sounds like #1 cylinder might need a little work. Frank
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