Kevin, I had recently asked this same question to the Hunter forum a couple of weeks ago and got some good answers, albeit they varied in responses to various ecperiences. I would appreciate your take on the problem as I haven't been able to remedy it yet. As I motor, I get a strong pull on the helm. As the engine RPM increases the worse the situation gets. Obviously an effect of prop wash to the rudder of some sorts. I have a new engine, Perkins M30 with a three bladed prop installed by a dealer a couple years ago just before I bought the boat so I don't know what the previous situation with the Yanmar was. Shafts and alignment have been checked by the boatyard, no excessive vibration is present and everything appears to be in proper proximity from the original configuration; strut to prop distance, prop to rudder diference. I originally thought that the new engine might be off centerline but checking visualy, everything seems to be on the same centerline as the old installation except for the fact that the Perkins is moved back a bit due to it's much smaller size. The wheel telegraphs the prop wash considerably above 2100 RPM's and the wheel pulls quite hard.If you release her she'll go to the rudder stop to port in an instant. One difference that I am aware of, the Perkins is a higher RPM engine than the old Yanmar. Typical cruise for the Yanmar I'm told was 2100 RPM's where the Perkins will run at 2600. Max RPM is 3100 with the prop that is installed on my boat. Some other owners have experienced some of the same problems to a lesser degree with most not having the problem. No specific anomoly presents itself to two or three blade prop, engine size ect.. in the folks that responded to me. Any help or advice would be greatly appreciated! Tom Hadoulias S/V Lite Chop
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Richard
Prop walk
Kevin, I believe what your experiencing is called, "prop walk". If your shaft revolves CW it will want to push or walk the stern to starboard and just the opposite for CCW. This is caused by the pitch of the prop. The greater the pitch the more severe the walk. Have the prop checked by a prop shop. Based on your engine and boat size they should be able to tell you excactly what size prop and pitch is optimal for your set up. They can also re-pitch your prop, which is fairly inexpensively. Good luck, RIchard
Kevin, I believe what your experiencing is called, "prop walk". If your shaft revolves CW it will want to push or walk the stern to starboard and just the opposite for CCW. This is caused by the pitch of the prop. The greater the pitch the more severe the walk. Have the prop checked by a prop shop. Based on your engine and boat size they should be able to tell you excactly what size prop and pitch is optimal for your set up. They can also re-pitch your prop, which is fairly inexpensively. Good luck, RIchard
Prop walk!
Tom: The RPM's on the old engine are NOT correct. The Yanmars where rated to run at 70-80% of the max rpm. If your old Yanmar was a 2QM15 I think they were rated at something like 3200 RPMs. So your cruise RPM would have been more in the 2200 - 2600 range. When they do their magic with props, they need several things to go into their formula. Weight of the vessel, HP, Max RPM, Transmission Ratio and probably the most important thing in your case is the distance between the hull and the tip of the prop. I think that the minimum clearance is suppose to be 15% of the prop diameter. You mentioned that they moved the prop closer to the strut, which would have caused MORE prop walk. My suggestion is to call a prop shop in your area and give them all of the information that they will need. Have them calculate the correct prop for your new setup. Then you can compare what you have to what they think you need. There was a guy in the SF bay area that you may want to talk to about your problem too. His name is Bob Kilian. He was with with Kilian Prop and is now with Bay Prop in Alameda, CA. 510/337-9122. They do some magic with props that several of the site members have had done. They put winglets on the prop and cup the blades. The charge about $130 for this on a two blade prop so you may estimate $200 for a three blade. It is worth the call. He can be contacted on Tues - Fri. I am sure that any prop shop can make your situation much better than it is now, regardless of who tackles your problem.
Tom: The RPM's on the old engine are NOT correct. The Yanmars where rated to run at 70-80% of the max rpm. If your old Yanmar was a 2QM15 I think they were rated at something like 3200 RPMs. So your cruise RPM would have been more in the 2200 - 2600 range. When they do their magic with props, they need several things to go into their formula. Weight of the vessel, HP, Max RPM, Transmission Ratio and probably the most important thing in your case is the distance between the hull and the tip of the prop. I think that the minimum clearance is suppose to be 15% of the prop diameter. You mentioned that they moved the prop closer to the strut, which would have caused MORE prop walk. My suggestion is to call a prop shop in your area and give them all of the information that they will need. Have them calculate the correct prop for your new setup. Then you can compare what you have to what they think you need. There was a guy in the SF bay area that you may want to talk to about your problem too. His name is Bob Kilian. He was with with Kilian Prop and is now with Bay Prop in Alameda, CA. 510/337-9122. They do some magic with props that several of the site members have had done. They put winglets on the prop and cup the blades. The charge about $130 for this on a two blade prop so you may estimate $200 for a three blade. It is worth the call. He can be contacted on Tues - Fri. I am sure that any prop shop can make your situation much better than it is now, regardless of who tackles your problem.
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Don Alexander
Another Possibility
Tom, If your tip to hull clearance is okay there could be another cause, though I would not expect this to be as severe as you report. Hunters have semi balanced rudders. This is achieved by having some of the blade area ahead of the rudder stock. They all have a sloping leading edge which leads to more area ahead of the stock at the top and virtually none at the bottom of the rudder. The intention is to obtain an average balance whilst sailing. However the water immediately behind the prop is moving much faster than that lower down so the top is over compensated and, added to prop walk, Could result in an oversteering tendency plus a desire to turn the boat, and this gets worse the faster you go. I have noticed this on my 376 but it is not a problem. Yours may be a worse case. May I suggest speaking with the previous owner as to how things were with the Yanmar. Good luck.
Tom, If your tip to hull clearance is okay there could be another cause, though I would not expect this to be as severe as you report. Hunters have semi balanced rudders. This is achieved by having some of the blade area ahead of the rudder stock. They all have a sloping leading edge which leads to more area ahead of the stock at the top and virtually none at the bottom of the rudder. The intention is to obtain an average balance whilst sailing. However the water immediately behind the prop is moving much faster than that lower down so the top is over compensated and, added to prop walk, Could result in an oversteering tendency plus a desire to turn the boat, and this gets worse the faster you go. I have noticed this on my 376 but it is not a problem. Yours may be a worse case. May I suggest speaking with the previous owner as to how things were with the Yanmar. Good luck.
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Kevin
move prop aft
Dear Tom, PYI has seen this kind of problem in the past. What we determined is that a cone/helix of disrupted water leaves the propeller and expands as it flows aft until it either terminates or dissipates. If the disturbed water terminates on a fixed surface, like the rudder, it will cause additional loading to that surface. If the flow/cone/helix of water impacts the rudder on its starboard side one could assume that you would have a heavy helm to starboard. What we did to help eliminate the problem was to place a spacer between the two coupling halves, which placed the propeller closer to the rudder by apx. 1.75”. This altered the flow/cone/helix of water reducing the pressure to the affected surface. If you look at the pitch of your propeller and calculate the efficiency at a given propeller speed, you may only see that the helix distance might be 4.5”. (10” pitch X .45 prop. efficiency = 4.5”) Changing the location of the propeller will have an impact to the location of the helix/cone/flow of the water as it makes contact to the rudder. As propeller speed increases so does the amount of flow or movement, which you describe in your note. I would like to add that I do not believe Hunter, or any other mfg, screwed up if you are experiencing this. Numerous things like displacement, Hp, propeller styles, can alter water flow off the propeller… As any topic like this, it can be very complicated to pen, so if you would like to give us a call we can discuss it. Hope this helps. Kevin
Dear Tom, PYI has seen this kind of problem in the past. What we determined is that a cone/helix of disrupted water leaves the propeller and expands as it flows aft until it either terminates or dissipates. If the disturbed water terminates on a fixed surface, like the rudder, it will cause additional loading to that surface. If the flow/cone/helix of water impacts the rudder on its starboard side one could assume that you would have a heavy helm to starboard. What we did to help eliminate the problem was to place a spacer between the two coupling halves, which placed the propeller closer to the rudder by apx. 1.75”. This altered the flow/cone/helix of water reducing the pressure to the affected surface. If you look at the pitch of your propeller and calculate the efficiency at a given propeller speed, you may only see that the helix distance might be 4.5”. (10” pitch X .45 prop. efficiency = 4.5”) Changing the location of the propeller will have an impact to the location of the helix/cone/flow of the water as it makes contact to the rudder. As propeller speed increases so does the amount of flow or movement, which you describe in your note. I would like to add that I do not believe Hunter, or any other mfg, screwed up if you are experiencing this. Numerous things like displacement, Hp, propeller styles, can alter water flow off the propeller… As any topic like this, it can be very complicated to pen, so if you would like to give us a call we can discuss it. Hope this helps. Kevin
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Les Blackwell
Rudder could be the problem
While all your replies on the prop are excellent, please don't overlook the rudder as a possible problem. We have a Huner 380 and when the boat arrived, it had a terrible turn to starboard when under power. It would increase when the engine was increased in speed. We measure the rudder and it was not symetrical--indeed, it had a curve in it. Hunter replaced the rudder and our problem was drastically reduced. (Hunter sent a vice president to check it out and he agreed that something was wrong) However, I also at that time replaced the standard prop with a Max prop and that was the best thing we have done to the boat. The boat now tracks very well under sail and power. Good luck.
While all your replies on the prop are excellent, please don't overlook the rudder as a possible problem. We have a Huner 380 and when the boat arrived, it had a terrible turn to starboard when under power. It would increase when the engine was increased in speed. We measure the rudder and it was not symetrical--indeed, it had a curve in it. Hunter replaced the rudder and our problem was drastically reduced. (Hunter sent a vice president to check it out and he agreed that something was wrong) However, I also at that time replaced the standard prop with a Max prop and that was the best thing we have done to the boat. The boat now tracks very well under sail and power. Good luck.
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Tom Hadoulias
Thank You for your input!!
I believe that I have some good info to go on and I also believe that this is an obscure problem that may encompass a couple of points that you all bring out. Namely, a new engine and a new rudder. Although I have not measured the exact symetry around the centerline of the boat vs. shaft vs. rudder shape I agree it's got to be one of the factors mentioned. Thank you all for yor inputs and yes Kevin, I would like to call and discuss some options with you. Thanks, Tom Hadoulias S/V Lite Chop Tom
I believe that I have some good info to go on and I also believe that this is an obscure problem that may encompass a couple of points that you all bring out. Namely, a new engine and a new rudder. Although I have not measured the exact symetry around the centerline of the boat vs. shaft vs. rudder shape I agree it's got to be one of the factors mentioned. Thank you all for yor inputs and yes Kevin, I would like to call and discuss some options with you. Thanks, Tom Hadoulias S/V Lite Chop Tom
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