Starboard engine low revs underload

[Jobs_a_ good_ un]

My starboard engine only achieves 3400 rpm in gear it normally does 3800rpm,its ok in neutral
The port engine is fine it does 3800rpm in and out of gear
I went a bit low on depth comming in to lymington at low tide the ferry pushed me into low water heard the prop rattling on something could I have damaged the prop
Although after this episode it achieved 3800 rpm then I moored up for 2 weeks and went away
Ist noticed the problem last weekend on the way too Poole harbour
but Im sure it was achieving full revs on the way back from chichester the last time i went out and the boats been in the marina since although I did have a engine service and holding tank fitted while I was away
Any body got any Ideas how I can find out

 

[Freebee]

Tell us more about what you have , outdrives or shafts. engines diesel petrol? you have not filled out your profile so we cant tell!

 

[Jobs_a_ good_ un]

Ive got 250 yanmars on shafts

 

[Freebee]

Have you got rope cutters ...you may have picked up something?

 

[Jobs_a_ good_ un]

Yes rope cutters are fitted

 

[hlb]

Most likely [censored] or water in filters. Deal with this first.

 

[Jobs_a_ good_ un]

Jusr had a full service done all filters changed

 

[Freebee]

were the filters changed before or after the rev change was noted? If before as Hadyn says could be dirt or water stirred from tank bottom into fuel lines, could be the fuel system has not been bled of all air or even loose diesel pipe fitting allowing fuel pump to pull air instead of fuel at max`suction.

But lets go back a bit you said you may have dinged prop, praps bent blade or blades to artificially increase pitch but I would suggest you would get excess vibration from this

 

[Roy]

Actually seems like prop or props are well dinged hence drop in revs. Vibration not always an issue. I have gone several hundred miles with one blade bent and two blades half severed with no vibration but a drop off in speed. As HLB says, check out everything else before lifting the boat out.

 

[Jobs_a_ good_ un]

Service was done while we were away then took the boat out upon return and then noticed the lack of power as soon as we took it out after the service
Prior to the service the engine was fine

 

[Jobs_a_ good_ un]

When we last left the boat the revs were fine we travelled from chichester to lymington at 3400 rpm plus and moored up and then had the service done.
Ive been on to the guy who did the service hes had a look and says theres nothing wrong with the engines because in neutral they rev o.k

 

[Freebee]

well thats where you start then my guess air in the fuel lines! first bleed the system then trial at max revs under load, but while you are at it drain filter bowls of water.

How much fuel in your tanks at the moment? If they are not full you may have condesation happening inside the tank hence water bottom.

 

[Freebee]

I would say acheiving full revs in neutral is meaningless, its performance under load that counts. Couldn't the mechanic try max revs in gear moored up-securley moored up ofcourse! Are you sure this guy knows what he's doing, can you trust him did he change the fuel filters or just give em a wipe with an oily rag?

 

[hlb]

Dirt, water, Air are the first way to go. Turbo?? but there should be signs. Oil?? If no other noticable faults, it's a shortige of fuel. For what ever reason.

 

[Jobs_a_ good_ un]

The guy that done the service is the yanmar main agent for my area he also took off both the water pumps and put new shafts in both of those but he left all the water in the bottom of the boat from the leaking water pump.we only noticed the drop of revs after the service hence the thought the props wernt dinged

 

[rich]

If you had a cctv cam like mine, you could have a look at the props with out getting wet /forums/images/graemlins/tongue.gif

 

[Nauti Fox]

Personally I would look at what the mechanic done, check the invoice to see what he's actually "adjusted". Must agree spinning them up out of gear proves nothing and isn't particuarly good for them.
If you are SURE that it pulled full revs before the service and does'nt afterwards it sort of points to what he's done, as I said, if your sure.

 

[KevB]

All is not always as it seems

Had a friend last season whilst coming back from the CI complain that one of his engines would only pull 3000 revs whilst the other was fine (duo prop outdrives). We stopped and lifted the leg with the problem to see if anything was amiss, all was OK. He continued the journey home at reduced speed.
The following weekend he went to check out the problem. The engine that was pulling full revs had lost one of it's duo props causing the other engine to be doing all the work and not being able to pull full revs. A bit like trying to get on the plane with one engine.

 

[gcwhite]

Partially blocked fuel filter

 

[Its_Only_Money]

New filter not sealing properly....

... and allowing air to be drawn in???