low revs update

[Jobs_a_ good_ un]

Just got back from the boat the yanmar agent at lymington says he trailed the boat and got revs of 3600 rpm on one engine and 3800 on the other (twin yanmar 250s shafts)So I jump in expecting to take starboard engine up to 3600 rpm but it wont move past 3350.
However when I put the port engine on full throttle 3800 rpm the starboard engine starts to rise and ends up at 3580 rpm when before i put the revs up on the port engine the starboard would only do 3350
why should the revs rise when the other engine increases in revs ???????
Does anybody understand ?????help

 

[No1_Moose]

because when you increase your speed through the water i.e. increase speed on the port engine you are decreasing the load on the starboard engine. Another way to look at it is that the Port engine is having some of its work done by the starboard engine.

 

[oldgit]

If one engine is not doing much it will make the other do all the work preventing it from reaching full RPM.

 

[Talbot]

Have you checked to ensure that both props have the same dimensions, If the engines are OK, then the symptoms you describe could be due to the prors being slightly different, and the starboard is pushing more than the port, thus allowing port to achieve max revs earlier, and stbd only able to get t here when port is on max.

 

[omega2]

Have you got boost pressure gauges fitted? if yes then try to get them as even as possible, if not I would suggest you get some fitted as they will allow to balance the power output of the engines far more accurately than the rev counters, on most twin engine set ups neither engine will hit max revs when running alone as the props are built to be running together and both engines producing equal power. Talk to Paul Williams at Clements Marine 01234 378814. or sales@clementsmarine.co.uk. I have no connection with this firm but I have complete trust in them

 

[Jobs_a_ good_ un]

The only difference we have made is that we have put a 5 hp outboard on the back there used to be a 2hp could this affect the revs.
Its on the starboard above the bathing platform but its the port engine thats low.
The next thing I will have to do according to Dave Crawford marine is take all the injectors out have them serviced etc.

 

[david_bagshaw]

what was the max rpm before[ QUOTE ]
....Just got back from the boat the yanmar agent

[/ QUOTE ]?

for what it is worth, my view,

1 As mentioned above fit boost gauges to determine engine output

2 even if your bigger outboard was dragging in the water it would be unlikely to affect the load to that extent, even if in the prop stream.

3 I would look for :-
a small air leaks in fuel system preventing full power (incorrectly refitted filters perhaps)
b air leaks in the boost system ditto
c exhaust blockages ditto
d if the engine has electronic management...... sensor probs etc


4 also yard says ...trailed

I was told 30 years ago by mercedes benz marine divisions engineer Sandy Buccan when doing full power tests on twin installations, the trailed engine should be at least in gear and just above idle rpm, other wise the other engine would not reach max rpm, due to water flow disturbance near the props.


of course all tests should be done in plenty of depth of water. with our displacement boat the max rpm is not reached until the depth is at least 3 x draft

 

[hlb]

Just to be sure which engine is the problem, see what happens if you just run on one engine, then the other. Which one is best. Then get yer mechanic to relook at fuel system on which ever engine. is there any smoke?? Sack the mechanic and get a better one!!

 

[bob_tyler]

3580/3350. How about first checking that the rev counters are accurate. Try swapping them over and see how they read. You may find that suddenly your port engine appears slower than the starboard - 3350/3580.

PS Don't forget to swap them back again after this test.

 

[Jobs_a_ good_ un]

Thats a good idea but the port engine which is reving low is a lot quiter than the starboard at full throttle hence I know the port engine is not acheiving its full capacity does that make sense ?

 

[hlb]

Only sence I know here is some one dont know arse from elbo. Get a mechanic to sort it out. The engine needs some fuel...

 

[bob_tyler]

Sorry, I got your engines the wrong way round. Please substitute Port for Starboard and Starboard for Port in my posting.
Might be the simplest test to do first without getting your hands dirtier than necessary and the smell of diesel throughout the boat from redoing connections in fuel lines, etc.

 

[Wiggo]

Don't bother swapping rev counters. We had exactly what you describe a couple of years ago, and it was bug and blocked filters on one engine. I can't think why the crap would have got past the filters and clogged the injectors, though, as that's the whole point of filters.

Injectors may not need servicing (£££££) but could be worth checking (£). I am told there are some unscrupulous types out there who will charge £££££ to have them checked, only to find there's nothing wrong with them. If they are checked, ask to see the test results...

Lik ethe others, my money's on air leaks from the turbo to the manifold, or fuel leaks from the injector pump.

Of course, you could just have been unlucky with the timing and had a couple of injectors fail just at the time of the service, in which case it's just a coincidence. You could always have a word with Col...