My 1988 Hunter 28.5 has a 20 HP Yanmar 2 cylnder diesel motor. It has always vibrated or knocked a little but no really more than a 2 cylnder diesel rock. I knew the prop shaft was bent and corroded so this year I planned to replace it and fit a folding prop. I did this using a Martek Mk III Eliptec Folding Prop. 16" Dia * 10" pitch X 1RH. I renewed the shaft, the stuffing box and the cutlass bearing and all appears to be perfectly aligned. This resulted in an exterme knock and vibration. As a result I hauled the boat and replaced the folding prop with my standard 2 blade prop. Now I have an extreme "knock" at about 800 RPM and at 2800 RPM. I did not have it anywhere near as bad as this before I replaced all the items above. I really can only now run at idle and 2,200 RPM The engine almost rotataes in a vertical circle on its mounts at these speeds. We can not see anything out of alignment or touching where it should not. Has anybody else had this experience on a Hunter 28.5? Are there any suggestions as to possible cause? Are there any solutions? Any information would be really helpful because right now we are stumped and I do not know whether to returnm the Matec or not. David Smith Smyrna Yacht Club
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unrelated cause?
sometimes our assumptions are wrong - in this case, it may not be anything you changed. Have you verified the engine mounts are solid?
sometimes our assumptions are wrong - in this case, it may not be anything you changed. Have you verified the engine mounts are solid?
one step a time
As Don suggested I bet you mounts are bad also, the engine should not rotate in a " vertical circle" as you described. You may have had several problems causing your knock,and if you had a bad shaft it may have caused a mount it go bad. My suggestion is disconnect the shaft and see how the engine does without the shaft next with the transmission engaged etc. etc. The engine should not move more then 1/4 inch either direction at any rpm. I would bet you new prop is OK and you will be able to use it after getting the problem fixed. When you replaced the shaft did you check it for being straight and true before you installed it ? Also 800 rpm is a little low rpm for that engine (if it is a gm series) it should be around 825 to 875 f.y.i. Good Luck keeps us updated on the fix when you discover it !
As Don suggested I bet you mounts are bad also, the engine should not rotate in a " vertical circle" as you described. You may have had several problems causing your knock,and if you had a bad shaft it may have caused a mount it go bad. My suggestion is disconnect the shaft and see how the engine does without the shaft next with the transmission engaged etc. etc. The engine should not move more then 1/4 inch either direction at any rpm. I would bet you new prop is OK and you will be able to use it after getting the problem fixed. When you replaced the shaft did you check it for being straight and true before you installed it ? Also 800 rpm is a little low rpm for that engine (if it is a gm series) it should be around 825 to 875 f.y.i. Good Luck keeps us updated on the fix when you discover it !
Motor mounts
I replaced mine last year and what a difference. I have a 1994 Hunter 29.5. If you look at the mounts real close you may be able to tell if one of them is bad. Try to compare the space between the upper part that's connected to the rubber to the base that's bolted to the hull. I could see a .25 inch difference in one of them so I replaced all four. Hope this helps Joe
I replaced mine last year and what a difference. I have a 1994 Hunter 29.5. If you look at the mounts real close you may be able to tell if one of them is bad. Try to compare the space between the upper part that's connected to the rubber to the base that's bolted to the hull. I could see a .25 inch difference in one of them so I replaced all four. Hope this helps Joe
28.5 Fraternity
The boat is old enough to need a close look at the motor mounts as well as the motor mount attachment to the boats stringers; asuming you have alredy tried to do an engine alignment and got the flange coupling alignment down to .003" or less. That said, I'd recommend getting a Seloc/Chilton Marine Manual for the Yanmar Inboards 1975-98 and look up replacing the motor mounts. From that manual, front and rear flexible motor mounts are not interchangeable; the rubber elastic modulus is different. As someone has already said, even if you did have a bent shaft and it has been straightened and/or trued up, disconnecting the engine coupling and running the engine should indicate the level of vibration and presumably something of the condition of the motor mounts which can also be visually inspected. Again, remember to check the fastening of the mounts to the stringers, which can also work loose. Also look at the cutlass bearing in the strut and check it for excessive 'play'. IMHO, and with 20 years with a folding prop on my 28.5, a properly installed folding prop contributes no more or no less vibration and I have been able to get my engine flange static allignment down to less than .002" when I replaced the original nylon stuffing box with bronze many years ago. FYI, there are lots of 28.5 sailors with more experience specific to the 28.5 and the Yanmar 2GM20F engine at: www.huntertwentyeightfive.com
The boat is old enough to need a close look at the motor mounts as well as the motor mount attachment to the boats stringers; asuming you have alredy tried to do an engine alignment and got the flange coupling alignment down to .003" or less. That said, I'd recommend getting a Seloc/Chilton Marine Manual for the Yanmar Inboards 1975-98 and look up replacing the motor mounts. From that manual, front and rear flexible motor mounts are not interchangeable; the rubber elastic modulus is different. As someone has already said, even if you did have a bent shaft and it has been straightened and/or trued up, disconnecting the engine coupling and running the engine should indicate the level of vibration and presumably something of the condition of the motor mounts which can also be visually inspected. Again, remember to check the fastening of the mounts to the stringers, which can also work loose. Also look at the cutlass bearing in the strut and check it for excessive 'play'. IMHO, and with 20 years with a folding prop on my 28.5, a properly installed folding prop contributes no more or no less vibration and I have been able to get my engine flange static allignment down to less than .002" when I replaced the original nylon stuffing box with bronze many years ago. FYI, there are lots of 28.5 sailors with more experience specific to the 28.5 and the Yanmar 2GM20F engine at: www.huntertwentyeightfive.com
I had bad knocking and heres how I solved it
Things started off fine on my 1988 28.5, until I changed to a drip-less stuffing box. Then the knocking began and wouldn't quit. While the engine was running I could push on the engine from side to side and up and down and most of the knocking would diminish, in certain positions. I adjusted the motor to the least knocking position, and the knock would return at another rpm. I kept this up for about a week and couldn't find a sweet spot...where everything was perfect. So frustrated I hauled the boat out. Pulled the shaft, removed the drip-less stuffing box. I bought and installed a slightly larger bronze stern tube and bronze stuffing box, installed drip-less packing, realigned the entire assembly and engine and things have been fine since. It seams the small 2-cylinder Yanmar needed the traditional stuffing box as a pilot bearing. And I learned to stop messing around with something that was fine to begin with. Hope this helps.
Things started off fine on my 1988 28.5, until I changed to a drip-less stuffing box. Then the knocking began and wouldn't quit. While the engine was running I could push on the engine from side to side and up and down and most of the knocking would diminish, in certain positions. I adjusted the motor to the least knocking position, and the knock would return at another rpm. I kept this up for about a week and couldn't find a sweet spot...where everything was perfect. So frustrated I hauled the boat out. Pulled the shaft, removed the drip-less stuffing box. I bought and installed a slightly larger bronze stern tube and bronze stuffing box, installed drip-less packing, realigned the entire assembly and engine and things have been fine since. It seams the small 2-cylinder Yanmar needed the traditional stuffing box as a pilot bearing. And I learned to stop messing around with something that was fine to begin with. Hope this helps.
KNOCKING YANMAR
I have a 1981 H27 that someone repowered with a Yanmar 2GM. This boat was laid up for a couple of years and I have not seen it in the water yet. After I took possesion in the spring I did start and run the engine on land. At low idle 800 RPM, there was a loud knock and I thought the boat motor wasa going to shke the boat off its stands. At no time was the engine ever in gear so this had nothing to do with prop , shaft or stuffing box. The sound and shaking went away as rpm increased, but I was concerned about this vibration. This is an engine that supposedly only has 500 hours on it. I am wondering if during repower they might have used old engine mounts, now twenty six years old and originally for a Yanmar 1GM. The engine has been winterized now so I will not be playing with it till the spring. Faulty engine mounts sounds like the first thing I am goig to look at in the spring.
I have a 1981 H27 that someone repowered with a Yanmar 2GM. This boat was laid up for a couple of years and I have not seen it in the water yet. After I took possesion in the spring I did start and run the engine on land. At low idle 800 RPM, there was a loud knock and I thought the boat motor wasa going to shke the boat off its stands. At no time was the engine ever in gear so this had nothing to do with prop , shaft or stuffing box. The sound and shaking went away as rpm increased, but I was concerned about this vibration. This is an engine that supposedly only has 500 hours on it. I am wondering if during repower they might have used old engine mounts, now twenty six years old and originally for a Yanmar 1GM. The engine has been winterized now so I will not be playing with it till the spring. Faulty engine mounts sounds like the first thing I am goig to look at in the spring.
KNOCKING YANMAR
I have a 1981 H27 that someone repowered with a Yanmar 2GM. This boat was laid up for a couple of years and I have not seen it in the water yet. After I took possesion in the spring I did start and run the engine on land. At low idle 800 RPM, there was a loud knock and I thought the boat motor wasa going to shke the boat off its stands. At no time was the engine ever in gear so this had nothing to do with prop , shaft or stuffing box. The sound and shaking went away as rpm increased, but I was concerned about this vibration. This is an engine that supposedly only has 500 hours on it. I am wondering if during repower they might have used old engine mounts, now twenty six years old and originally for a Yanmar 1GM. The engine has been winterized now so I will not be playing with it till the spring. Faulty engine mounts sounds like the first thing I am goig to look at in the spring.
I have a 1981 H27 that someone repowered with a Yanmar 2GM. This boat was laid up for a couple of years and I have not seen it in the water yet. After I took possesion in the spring I did start and run the engine on land. At low idle 800 RPM, there was a loud knock and I thought the boat motor wasa going to shke the boat off its stands. At no time was the engine ever in gear so this had nothing to do with prop , shaft or stuffing box. The sound and shaking went away as rpm increased, but I was concerned about this vibration. This is an engine that supposedly only has 500 hours on it. I am wondering if during repower they might have used old engine mounts, now twenty six years old and originally for a Yanmar 1GM. The engine has been winterized now so I will not be playing with it till the spring. Faulty engine mounts sounds like the first thing I am goig to look at in the spring.
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