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Post by wolferich on Aug 8, 2020 10:23:12 GMT
Last week I was travelling for some days on my boat and experienced a strange behaviour of the engine (Yanmar 4JH5E).
I came to the boat after approx. 9 month. When I first started the engine it ran immediately and smooth. When we started our trip I noticed that the speed was maybe 0.5 - 1 kn less than usual at 2200 rpm. I assumed that was due to the fact that I didn‘t renew the antifouling this spring. Later I increased the rpm and noticed that 1) the ship didn‘t accelerate althout rpm were increased and 2) when I increased the throttle to rpm above a certain level (maybe above 2500 rpm) the throttle came back by itself and rpm went down to the previous level of rpm.
In the next harbor we changed the oil filter which was a bit dirty. The engine was running better afterwards. The next day at sea the engine was still running better but the aforementioned rpm/throttle effect was still there, although less obvious. Later, after having the engine running for some hours, everything seemed to be normal again and the strange effect seemed to be gone. Full rpm and resulting speed were achievable again.
Does anyone have an theory what might have happened ?
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Post by rene460 on Aug 9, 2020 2:53:51 GMT
Hi Wolferich,
I have experienced similar behaviour. It turned out to be a single barnacle that had attached itself to one blade of the propellor. This does two things, it increases the torque necessary to turn the propellor, so the engine seems to lack power, and it reduces the efficiency of the propellor in terms of its ability to move water to produce thrust.
The key is in your comment about “after nine months and not recently antifouled”, a problem we all have at the moment. In my case, I was so worried that perhaps I had a rope around the prop, that I had a diver inspect the prop. He reported the barnacle, brushed it off, and said I would now be right. I was sceptical. Only one small barnacle? He assured me that he had seen it before, and sure enough, normal performance restored.
In your case I suspect it was just the normal weed and stuff that grows when you are not using the boat regularly, and after a bit of running, the speed of water over the prop cleaned it off. The hull would also benefit from that first running, and that would improve your boat speed.
These days, before I drop the ropes to leave the pen, I run the engine revs up in forward while looking over the transom. It is surprising how much weed and stuff appears in the wake. Only takes a few seconds then, when the wash is clear, the same in reverse. Perhaps I am paranoid, but I have a feathering prop, and I am conscious of the possibility that the blades might not rotate, leaving me without reverse. As soon as the boat moves back enough to pull on the lines, I know the prop is free, we drop the lines and back out of the pen and along the fairway. Once we are in the clear channel, we can go forwards again, and when we reach the end of the five knot zone, I try a short burst at wide open throttle to check, rpm and boat speed. Some times the boat speed is still a bit slow, but I ease back to normal cruising speed for a while and the hull is soon basically clean again. Then we put up the sails and enjoy the quiet.
The issue is that the prop blades do act as hydrofoils as they move through the water. Anything that destroys the flow around the low pressure side in particular, greatly reduces the water that is propelled back, and so reduces the thrust you need to drive the boat. Think of a problem on the upper surface of an aeroplane wing, when the flow is disturbed, it destroys the lift.
I suspect that had you run the engine speed up a bit in neutral, it would have behaved in the normal manner, and only when in gear, driving the fouled propellor did it quite accurately tell you it was overloaded. I think that the needs have now gone (until next time,) and all is well. I hope so anyway.
rene460
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Post by wolferich on Aug 9, 2020 11:02:28 GMT
Thanks for that valuable comment. That could indeed explain the things I experienced and I hope very much that I don’t have to worry about any deeper hidden fault somewhere in the system.
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Post by NZL50505 on Aug 10, 2020 1:03:03 GMT
I agree with Renee's habit before leaving the dock.
In fact the last checks I do before dropping the lines are probably a hark back to pilot training:
(1) Confirm full & free rudder movement (I turn the wheel from lock-to-lock to ensure nothing has got caught between top of rudder and hull restricting movement)
(2) Engage both fwd and then reverse for a few seconds to confiorm prop also spinning freely
I am surprised by how many other skippers don't do this and also by how many ask me "What are you doing?" when they see me go through my final pre-departure checks!
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Post by wolferich on Sept 3, 2020 18:08:48 GMT
Today we wanted to start our next trip but had to return to the harbor soon. Our local technician assume a propeller defect leading to a slip when stronger force is applied to the propeller. Tomorrow a diver will dismount the prop and then I will know more.
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Post by wolferich on Sept 26, 2020 13:57:28 GMT
So, finally it was a defect on the fixed blade propeller. There seems to be some plastic part close to the axis that can loose functionallity. Can someone explain a little bit?
Anyhow, the old propeller was replaced with a flexofold propeller and since then everything is fine. Speed at the usual rpm improved a bit and speed under sails gain by the folding od the propeller. First of all I am happy that there was a proven cause of the symptoms and I don't have to worry anymore.
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Post by johannes on Sept 26, 2020 18:02:53 GMT
Folding propellers usually have a piece of semi-elastic plastic that acts as a bump stop when the blades unfold.
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Post by vasko on Sept 27, 2020 5:08:07 GMT
The problem that I had with the speed some years ago :
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Post by wolferich on Sept 27, 2020 8:06:58 GMT
The problems I had were with a fixed propeller and I wonder about the technical background. Due to language problems the technicians could not really explain this to me..
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Post by sailbleu on Sept 28, 2020 20:14:05 GMT
Are you , they , talking about the cutless bearing ? This a rubber part ( in a bronze , brass or plastic housing) and keeps the shaft aligned . I can not imagine another “ plastic “ part in or around a fixed blade propellor . Did they change the propellor for that reason ? If so you were ripped off big time . Although I’m not sure a worn cutless can cause these symptoms . Unless something ( ropr , fishline) came in between the shaft and the cutless slowing the engine down. Not very plausible though .
Would never go for a flexfold type, too much that can go wrong . But that’s my opinion & choice of course .
Regards
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Post by wolferich on Oct 3, 2020 7:42:50 GMT
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