Declining Speed throughout the Year : a Turbo Issue?

[nveater]

I have 210hp Cummins 6BT-5.9M engine driving a 1999 Mainship Pilot 30. At the beginning of the season I was getting an average speed of about 12.7 kts at cruise (2400 rpm) and about 15.6 WOT (just shy of 2600). More recently the average speed has dropped to about 10.2 and 12.5 respectively. I have dived on the boat and there is nothing around the prop and the bottom is "normal" for this time of year . The engine sounds fine and all the other indicators (oil, temp etc) are where they always are. I have taken tides and currents etc into consideration . It shouldn't be the prop as the existing prop worked fine in the beginning of the year. I do have vacuum gauges on my fuel filters and the readings have not changed. I do not have a boost instrument, but I was wondering if it could be the turbo air filter ? I have never serviced or changed the filter in the 3 years of owning the boat ? Would welcome any ideas as I ponder this conundrum. Thanks

 

[SteveK]

IMO fuel flow is reduced, which puts filters as suspect. You have vac gauges on primary, but what about engine main filter, those vac gauges will not show anything.

 

[nveater]

Okay I appreciate that thought- I have a fairly robust fuel filtration system and I didn't initially consider that because when my sailboat's filters were clogged the engine would struggle to reach cruising RPMs- I have no trouble reaching cruising rpms however this boat's filters haven't been changed in about 2 years so perhaps time. I'll change them and see what happens- Thanks so much

 

[tiltrider1]

If your boat is going slower at the same RPM then it is not an engine issue. Primary culprit would be dirty bottom. Second most likely cause is weight. Third possibility is head wind vs tail wind and fourth possibility is your measuring device is inconsistent.

 

[Cigatoo]

What Tiltrider 1 said. I could lose a knot or two by end of summer if I had not cleaned the bottom well on my 26 foot Fortier. If you are making the same turns its a drag issue or a prop. The water has been abnormally warm this year making for lots of growth.
You will be surprised what an effect the black slime on the bottom has. Never mind a few barnacles.

 

[nveater]

Unfortunately that makes sense- I was hoping for a mechanical fix

 

[rgano]

Your "measuring device" for speed? GPS? How about RPM? Analog tachs, if you have them, in my experience are not accurate enough for me. especially in their long term repeatability.

 

[jleonard]

A turbo issue would likely result in black smoke at mid to higher rpm’s. And you wouldn’t reach those rpm’s under load.
When is the last time you checked you tachometer for accuracy?

 

[kapnd]

The 210 boost numbers are very low, as the turbo output is small.
Ordinary turbo boost gauges don’t measure in small enough increments to make them particularly useful for this motor.
Is the air filter possibly dirty or wet?
Does the turbo spin up freely from the air side?
Have you looked at the turbo from the exhaust side?
I worked on a Mainship with a Yanmar that was taking on seawater through the exhaust, interior of turbo was corroded beyond belief, definitely not spinning.

 

[Jeff F]

Weight makes a big difference. I had the same engine in my old 34 MY and there was close to 2 knots difference at top speed between all tanks empty vs full. How much stuff have you brought aboard? Is the boat sitting properly on its lines?

 

[Simi 60]

Ours is weird
Dirty bottom and tonnes of weight has negligible effect
But get some crap on the prop and nozzle and lose as much as a knot.

I don't bother with the bottom between 2 year lifts now
But do dive the prop every month or three.

 

[Comodave]

If your boat is going slower at the same RPM then it is not an engine issue. Primary culprit would be dirty bottom. Second most likely cause is weight. Third possibility is head wind vs tail wind and fourth possibility is your measuring device is inconsistent.

+1. If the engines turn to the cruising RPMs and the speed is slower then it shouldn’t be an engine issue. Check the bottom and props, etc.

 

[ksanders]

I have 210hp Cummins 6BT-5.9M engine driving a 1999 Mainship Pilot 30. At the beginning of the season I was getting an average speed of about 12.7 kts at cruise (2400 rpm) and about 15.6 WOT (just shy of 2600). More recently the average speed has dropped to about 10.2 and 12.5 respectively. I have dived on the boat and there is nothing around the prop and the bottom is "normal" for this time of year . The engine sounds fine and all the other indicators (oil, temp etc) are where they always are. I have taken tides and currents etc into consideration . It shouldn't be the prop as the existing prop worked fine in the beginning of the year. I do have vacuum gauges on my fuel filters and the readings have not changed. I do not have a boost instrument, but I was wondering if it could be the turbo air filter ? I have never serviced or changed the filter in the 3 years of owning the boat ? Would welcome any ideas as I ponder this conundrum. Thanks

If your RPM is the same and your speed is reduced your motor is not the problem.

I have noted on yeasr 3 of bottom paint my speed is reduced and my fuel consumption is increased at any given RPM setting.

Thats why I went to a 2 year bottom paint schedule

 

[Eagle 59]

Do you have a self polishing or a hard antifouling?

If it is a self polishing type, you can try to go for a few miles with almost full throttle with constant RPM. If you will recognize any increase of speed, (during constant wind and flow conditions), you will not have a technical, but a "biological" problem.

 

[nveater]

Thanks everyone for the replies- I think i have to agree that its probably a bottom issue. I did take apart the turbo filter and look at the blades and system and all seems well with spinning blades and a relatively clean filter. I have not added any significant weight to the boat in terms of "stuff" it's fairly lightly equipped. This is my second season with the boat and I have not painted the bottom which appeared in remarkably great shape when the boat was hauled last year. It is a hard paint and the boat will be painted this year.