I need more power. I know it's a sailboat, but the 16 hp yanmar just won't do it in heavy winds with my 2 blade prop. I spent 5 hours in the intercoastal waterway in an 18 knot headwind traveling approx. 12 miles. I overheated the engine trying to make any headway at all. Sails were not an option. I have asked around and the general opinion seems to be that a 3 blade prop will help in these conditions. It can get a little spooky when you notice a barge coming up behind you just a little later than you would have liked to. What is your recommendation for my boat. 1987 Hunter legend 31 with the 16 hp 2 cylinder Yanmar. Thanks On a slow boat to China!!
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Pitch?
Capt. Jim: Do you know how your prop is pitched. I found that our boat (same model 1985 H'31) had a 15 x 12. We actually changed to a CDI composite prop that is 15 x 10. These boats should have about a 15 x 9. What RPM's are you getting out of the engine at WOT (wide open throttle). Should be able to get around 3400-3600. If you can then you are probably not over propped. If you can't you should think about re-pitching. 1" decrease pitch will give out around 200-250 additional rpms. Do you have any idea what type of hull speed you are really getting? Is your bottom clean? Is your prop clean? Are you blowing smoke? You really need to make sure everything is right before you invest your money.
Capt. Jim: Do you know how your prop is pitched. I found that our boat (same model 1985 H'31) had a 15 x 12. We actually changed to a CDI composite prop that is 15 x 10. These boats should have about a 15 x 9. What RPM's are you getting out of the engine at WOT (wide open throttle). Should be able to get around 3400-3600. If you can then you are probably not over propped. If you can't you should think about re-pitching. 1" decrease pitch will give out around 200-250 additional rpms. Do you have any idea what type of hull speed you are really getting? Is your bottom clean? Is your prop clean? Are you blowing smoke? You really need to make sure everything is right before you invest your money.
C
Capt. Jim
Reply to Capt.Steve Dion
Thanks for the advice. It's nice to communicate with someone with the same boat. I will check the pitch before replacing the prop. My engine overheated at 2700 rpms. This was after 4 hous of 2500 rpms against heavy winds. I generally try not to run over 2500 if I can help it. I can get higher rpms, but I am a little afraid to strain the engine. Usually 2700 would not be a real problem but I would not do it for a long period. Should I be able to run at higher engine speeds w/o a problem. May I just have an engine cooling problem? What is the recommended rpm range for everyday operation? Thanks, Capt. Jim
Thanks for the advice. It's nice to communicate with someone with the same boat. I will check the pitch before replacing the prop. My engine overheated at 2700 rpms. This was after 4 hous of 2500 rpms against heavy winds. I generally try not to run over 2500 if I can help it. I can get higher rpms, but I am a little afraid to strain the engine. Usually 2700 would not be a real problem but I would not do it for a long period. Should I be able to run at higher engine speeds w/o a problem. May I just have an engine cooling problem? What is the recommended rpm range for everyday operation? Thanks, Capt. Jim
D
Don
Steve, I thought you said before
That you were still running the stock prop that came with the boat? Given your discussion on this thread, I suspect that I might have the larger prop (15/12) and should consider the composite. I can run fine up to 3200 rpm, but then again I have a dirty bottom right now. No overheating issues to date.
That you were still running the stock prop that came with the boat? Given your discussion on this thread, I suspect that I might have the larger prop (15/12) and should consider the composite. I can run fine up to 3200 rpm, but then again I have a dirty bottom right now. No overheating issues to date.
M
Mark Johnson
Cruising RPM
My rule of thumb is run the engine at about 20 percent less RPM than you get at full throttle. My maximum RPM is about 3600..my cruising RPM is 3000. You should be able to run all day at that RPM. If you can't, I would check out your cooling system. The Yanmars like to run at higher RPM's.
My rule of thumb is run the engine at about 20 percent less RPM than you get at full throttle. My maximum RPM is about 3600..my cruising RPM is 3000. You should be able to run all day at that RPM. If you can't, I would check out your cooling system. The Yanmars like to run at higher RPM's.
J
Justin Wolfe/PYI
Overpropped?
Hi Jim, I'll be filling in for Kevin while he is road tripping... Something does sound amiss with your current prop engine combination. If you can't get 3500/3600 rpm out of your Yanmar 2GM20 then either you are overpropped or your engine is not performing correctly. As others stated you should be able to hit 3600 at full throttle AND you should be able to run all day in the 2800-3000 rpm range without overheating. Since we don't know what you current prop is all we can do is assume. As for a 3-Blade. Yes it will help you maintain boat speed against waves or a headwind. So yes in general a 3-blade would have helped in your situation. ASSUMING your engine is performing correctly. If not don't expect anything better than what you're getting now. A 14" x 11" 3-Blade feathering prop works nicely with your engine and boat combination. Justin Wolfe PYI, Inc. 1-800-523-7558
Hi Jim, I'll be filling in for Kevin while he is road tripping... Something does sound amiss with your current prop engine combination. If you can't get 3500/3600 rpm out of your Yanmar 2GM20 then either you are overpropped or your engine is not performing correctly. As others stated you should be able to hit 3600 at full throttle AND you should be able to run all day in the 2800-3000 rpm range without overheating. Since we don't know what you current prop is all we can do is assume. As for a 3-Blade. Yes it will help you maintain boat speed against waves or a headwind. So yes in general a 3-blade would have helped in your situation. ASSUMING your engine is performing correctly. If not don't expect anything better than what you're getting now. A 14" x 11" 3-Blade feathering prop works nicely with your engine and boat combination. Justin Wolfe PYI, Inc. 1-800-523-7558
Overheating is not good.
Capt. Jim: You definetly have some type of problem. Let's go over this before you spend any money on props and mechanics. 1. Put you engine in neutral and rev it to see how many RPM's you get. They can be rev'd up to 4000 + rpm's in neutral without any engine damage. The Yanmar guy did this to my boat while I was there and told me NOT to worry. This is not the same as reving to 4000 under a load. If you cannot get to 3600 in neutral then you may have a couple of problems. The first thing to check then is the mixing elbow. I will post some pictures on the Photo Forum so you can see the elbow. If this is clogged then you MUST replace it as soon as you can this is plugging up your engine and will cause overheating. If that is not the problem then you may need to have your govenor re-calbriated. I had this done. If you do not know what you are doing you should leave this one to the trained mech. 2. If you get the rev's out of the engine that you should then we need to see what you can get when it is in gear. This should also be somewhere near the max RPM's of 3600. This is the MAX rating of the engine. Now if you expect to get these rpms we must assume that the bottom is clean and the prop is not fouled. If both of these conditions are positive then we can move on to the prop. 3. If we are clean but cannot get the RPM's then your pitch is probably the problem. Once again, I think that a 15x9 is the best on a two bladed prop. 4. Just some other things to check. Be sure that the strainer on the raw water intake is clean. Then you may check the heat exchanger. I am not sure how to clean this thing because it has not been a problem. I assume that the Yanmar manual has instructions. Justin mentioned that you should have a 14x11 prop. We need to ask him how this relates to a 15x9 two blade. My feeling is that it should be a 14x10. Each inch of diameter or pitch is good for about 200-250 rpm's. But we need to keep in mind the additional blade. Yanmars recommendation on these engines is to run them at 70-80% of the max rating. You do not need to worry about straining these engines. Anything under 3000 rpms is fine and you can go higher for short periods of time. You will know when you are not doing any good because they start belching black smoke (too much fuel).
Capt. Jim: You definetly have some type of problem. Let's go over this before you spend any money on props and mechanics. 1. Put you engine in neutral and rev it to see how many RPM's you get. They can be rev'd up to 4000 + rpm's in neutral without any engine damage. The Yanmar guy did this to my boat while I was there and told me NOT to worry. This is not the same as reving to 4000 under a load. If you cannot get to 3600 in neutral then you may have a couple of problems. The first thing to check then is the mixing elbow. I will post some pictures on the Photo Forum so you can see the elbow. If this is clogged then you MUST replace it as soon as you can this is plugging up your engine and will cause overheating. If that is not the problem then you may need to have your govenor re-calbriated. I had this done. If you do not know what you are doing you should leave this one to the trained mech. 2. If you get the rev's out of the engine that you should then we need to see what you can get when it is in gear. This should also be somewhere near the max RPM's of 3600. This is the MAX rating of the engine. Now if you expect to get these rpms we must assume that the bottom is clean and the prop is not fouled. If both of these conditions are positive then we can move on to the prop. 3. If we are clean but cannot get the RPM's then your pitch is probably the problem. Once again, I think that a 15x9 is the best on a two bladed prop. 4. Just some other things to check. Be sure that the strainer on the raw water intake is clean. Then you may check the heat exchanger. I am not sure how to clean this thing because it has not been a problem. I assume that the Yanmar manual has instructions. Justin mentioned that you should have a 14x11 prop. We need to ask him how this relates to a 15x9 two blade. My feeling is that it should be a 14x10. Each inch of diameter or pitch is good for about 200-250 rpm's. But we need to keep in mind the additional blade. Yanmars recommendation on these engines is to run them at 70-80% of the max rating. You do not need to worry about straining these engines. Anything under 3000 rpms is fine and you can go higher for short periods of time. You will know when you are not doing any good because they start belching black smoke (too much fuel).
J
Justin Wolfe/PYI
Answer for Steve
Steve you're very close on your assumptions. The key is when you go from two blades to 3 blades a good rule of thumb is to reduce the pitch 2". So a 15x12 2-blade becomes a 15x10 3-blade. Cool with that? If so, then with the 3 blade if we reduce the diameter by 1" we need to increase the pitch by 1" as well so a 15x10 = a 14x11. Since a 14x11 is comfortably within the range of angles why spend the extra money on a 15". Had we started with more pitch originally I would have stuck with the 15" on the recommendation because it would have been too much pitch for the 14", but in this case that isn't true. Our rule of thumb is that reducing pitch by 1" gives roughly 12% increase in max rpms. The other thing to consider is the clearance between the blades and the bottom of the boat. Should have at least 15% of the diameter of the prop in clearance on a 3-blade. I don't know if this is an issue with the 31. If so then the 14" becomes even more attractive. If not then a 15"x10" 3-blade feathering would work just fine as well.
Steve you're very close on your assumptions. The key is when you go from two blades to 3 blades a good rule of thumb is to reduce the pitch 2". So a 15x12 2-blade becomes a 15x10 3-blade. Cool with that? If so, then with the 3 blade if we reduce the diameter by 1" we need to increase the pitch by 1" as well so a 15x10 = a 14x11. Since a 14x11 is comfortably within the range of angles why spend the extra money on a 15". Had we started with more pitch originally I would have stuck with the 15" on the recommendation because it would have been too much pitch for the 14", but in this case that isn't true. Our rule of thumb is that reducing pitch by 1" gives roughly 12% increase in max rpms. The other thing to consider is the clearance between the blades and the bottom of the boat. Should have at least 15% of the diameter of the prop in clearance on a 3-blade. I don't know if this is an issue with the 31. If so then the 14" becomes even more attractive. If not then a 15"x10" 3-blade feathering would work just fine as well.
Tip clearance - JW
Justin: The tip clearance on the H'31 is just a little over 2 in. My understanding on this subject is that there must be enough clearance so the prop does not cavitate. Otherwise we end up pushing air instead of water? Is this correct!
Justin: The tip clearance on the H'31 is just a little over 2 in. My understanding on this subject is that there must be enough clearance so the prop does not cavitate. Otherwise we end up pushing air instead of water? Is this correct!
J
Justin Wolfe/PYI
Aha! Is that with the 15"
Ideally you would have at least 2.25" Yes, any closer and the 3-blade prop starts to ventilate. The biggest problem with that is noise. You get the infamous "whop-whop-whop" as the blade pushes water against the hull. A 3-blade 14" is sounding more appropriate. A 15" might work just fine (barely), but again why spend the extra $$$. The 14" will work just dandy.
Ideally you would have at least 2.25" Yes, any closer and the 3-blade prop starts to ventilate. The biggest problem with that is noise. You get the infamous "whop-whop-whop" as the blade pushes water against the hull. A 3-blade 14" is sounding more appropriate. A 15" might work just fine (barely), but again why spend the extra $$$. The 14" will work just dandy.
E
Ernie Rodriguez
What about efficiency?
In the discussion with respect to the 15 inch diameter and the 14 inch diameter, is there any consideration for the efficiency of either? Ie, would the 15 inch prop if pitched correctly for the rpms of the engine yield the same efficiency as the 14 inch dia prop if also pitched correctly? I would expect that the efficiency of the propeller would determine how much power ends as useful thrust, but I have seen very little discussion on that and would like to hear some opinions..
In the discussion with respect to the 15 inch diameter and the 14 inch diameter, is there any consideration for the efficiency of either? Ie, would the 15 inch prop if pitched correctly for the rpms of the engine yield the same efficiency as the 14 inch dia prop if also pitched correctly? I would expect that the efficiency of the propeller would determine how much power ends as useful thrust, but I have seen very little discussion on that and would like to hear some opinions..
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