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Old 14 September 2009, 12:04   #1
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Country: UK - N Ireland
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Boat name: JanJack /
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Yanmar Diesel acceleration problem.

I've been having this problem for ages now and thought I'd describe it here and see if anyone can help.

The boat is a Redbay Stormforce 8.4m wrap-around and the engine a Yanmar 6lp-stze 300hp with a Bravo 3 Leg.

The problem I'm having with it is the acceleration is painfully slow and the engine seems to get bogged down within the mid rev range.

What seems to happen is when you first push the throttle forward there's a rapid increase in speed up till about 1500 Revs. At this point the Revs increase very slowly as the boat comes onto the plane and up until about 2500 Revs and then again it accelerates rapidly from 2500 Revs up till about 3700 at WOT.

I'm fairly sure the problem isn't the boats weighting, and I assume the prop's are fine as it is within the recommended rev range at WOT.

I'm thinking the problem is something to do with the engine itself.
Does anyone have any ideas/suggestions etc of what to try or what to check?

Cheers

Conor
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Old 14 September 2009, 14:14   #2
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On two occasions I've come across a similar problem, although not on a boat; sorry. One time it was the fuel pump that needed re-timing and the second was the turbo (ouch). I'm afraid that probably wasn't a lot of use but as most marine diesels either are versions of HGV engines or are based on them, they may be places to start.

Nearly forgot. The diesel pump fault was exacerbated by injectors that needed a damn good clean.
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Old 14 September 2009, 14:15   #3
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Is it the same when you adjust the trim? It is a bit strange because you would expect it to bog down at lower revs. I know Goldfish RIBs inject air/exhaust into the prop to cause it to slip to keep the turbo on boost.

Could even be too much prop grip?
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Old 14 September 2009, 14:21   #4
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Quote:
Originally Posted by The Beard View Post
On two occasions I've come across a similar problem, although not on a boat; sorry. One time it was the fuel pump that needed re-timing and the second was the turbo (ouch). I'm afraid that probably wasn't a lot of use but as most marine diesels either are versions of HGV engines or are based on them, they may be places to start.

Nearly forgot. The diesel pump fault was exacerbated by injectors that needed a damn good clean.
I was also thinking it could be fuel related issues. Is there any way to isolate an issue in the fuel system or is it a matter of paying a mechanic to take a look?
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Old 14 September 2009, 14:25   #5
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Quote:
Originally Posted by codprawn View Post
Is it the same when you adjust the trim? It is a bit strange because you would expect it to bog down at lower revs. I know Goldfish RIBs inject air/exhaust into the prop to cause it to slip to keep the turbo on boost.

Could even be too much prop grip?
Almost exactly the same when trim is adjusted. Slight difference when the leg is trimmed right in but still painfully slow.

Too much prop grip? I've never heard of this before.

It's running 2 x 4 blade 26 pitch bravo 3 props, counter-rotating. I'd imagine that would be pretty grippy.

Again, anyway to isolate this as the problem?
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Old 14 September 2009, 15:32   #6
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What do Redbay say?

Hi there
Worth a discussion with Redbay as you are clearly on their doorstep.
I wonder if the initial set-up was too cose to ideal- in that a 5 % deteriation on any one aspect, drag, turbo loss of boost, engine efficency etc could all cause the symptoms.
I would think 24" props were more normal. Again ask Redbay.
Was the boat like this when you got it?

Paul
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Old 14 September 2009, 16:48   #7
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think i would have a look at air /fuel filters first ,,,even if its not smoking ect,,,also what type of diesel fuel are you using ,white ,red ,heating oil ,
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Old 14 September 2009, 17:00   #8
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wastegate on turbo sticking open possibly?
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Old 14 September 2009, 22:44   #9
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Quote:
Originally Posted by chorscroft View Post
Almost exactly the same when trim is adjusted. Slight difference when the leg is trimmed right in but still painfully slow.

Too much prop grip? I've never heard of this before.

It's running 2 x 4 blade 26 pitch bravo 3 props, counter-rotating. I'd imagine that would be pretty grippy.

Again, anyway to isolate this as the problem?
I may be barking up the wrong tree but basically too much grip causes the engine to bog down off boost - much the same as a quick 4wd turbo car - to get it off the line quickly you have to slip the clutch to keep the engine spinning at high enough revs for the turbo.

As I said Goldfish RIBs have done this mod to their diesels to make the prop slip at slow speeds to get the turbo to kick in earlier.

As your problem is after 1500 rpm it prob is not the problem but you never know - changing props could help if so.
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Old 15 September 2009, 02:45   #10
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Quote:
Originally Posted by chorscroft View Post
Almost exactly the same when trim is adjusted. Slight difference when the leg is trimmed right in but still painfully slow.

Too much prop grip? I've never heard of this before.

It's running 2 x 4 blade 26 pitch bravo 3 props, counter-rotating. I'd imagine that would be pretty grippy.

Again, anyway to isolate this as the problem?
Some props have holes (PSV) it allows exhust gas to exit into the water flow at slow speed,as an idea it came from peaky race engines in the states.

I'd be looking at filters first,although your engine picking up towards the top end sort of discounts that,my thoughts lead me to your turbo.....sorry mate.
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