Originally Posted by chorscroft
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?
When you say the problem has been "for ages" it suggests that it wasn't like that earlier so my comments are based on it was right once, but now it's gone pear shaped.
If that's the case, it's difficult to think it's anything to do with the prop unless you've replaced it and the problem is coincidental with that.
If the engine produces the same-old maximum power it's difficult to think it's an engine mechanical problem.
If, after getting out of the hole, you've got the same max speed - and therefore maximum power - then the turbo is clearly working at least in part.
If you can achieve maximum power and maximum speed that suggests you're achieving maximum fuel flow so why would it be 'restricted' at mid-range? Blockages and restrictions generally restrict flow at all flow rates, not just at mid-range flows.
This brings me back to the turbo or its control mechanism...do you know at what rpm the turbo did or should start to deliver? Typically, this should be around 1200rpm(ish). If the onset of boost is delayed, for whatever reason, then this might explain the smart get-away (by sheer engine cubic inches), lethargy in the mid-range where boost is not forthcoming, followed by restored performance when the turbo belatedly kicks in.
I wonder if there a control system fault in the turbo mechanism? Is it electronic or mechanical control? If the latter, try and figure out what the mechanism is and does all appear connected and correct? If it's electronic control I fear it's a dealer visit.