Tachos fiddled

Rocksteadee

Rocksteadee

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Boat is a Princess 40 with TAMD63P
Just fitted new tachos to fly. Not yet driven boat due to lockdown.
Previous hand held digital tacho measured on engines read about 100 rpm lower than dash tachos at WOT.
Now checked setting on removed tacho revealed an offset of +3.5 %
Checked lower helm tachos and have same offset.
This gives same results as hand held
Meaning that each engine is not reaching full revs
I am assuming that it is over propped and manufacturer offset the tachos rather than fit correct size props.
Anyone else come across this and what would the implication of being over propped?
 
Not come across it .
Over propping leads to excess loads and premature engine failure ....it’s a marine diesel killer .
Having said that age, added Kgs ( fluids etc ) and seasonal fouling has the same effect .
Ideally it should be about 3% under propped rpm wise .
So for me reach 70 rpm over manufacturers WOT .....momentarily.

But I have load gauges and EGT in fact it’s theses I base my throttle position......just bomb about at 80 % load and accept what ever knots the boats runs at .Thus takes into account, fouling , fuel weight , extra folks etc .
 
Over propped is worse than under propped as the engine can’t reach its designed WOT. Not that you ever run a diesel continuously at WOT, you always throttle down a bit to be kind to the engine. But it has a knock on effect of your cruising revs being lower than they should so the engine labours. Under propped is easier to deal with as you don’t have to use all the revs but your top speed will be down. Ideally it should reach rated WOT on an accurate tacho, a little bit over is OK as when you load the boat up and the hull is dirty you will lose some of the top end revs and it will be nearer to where it should be.
Bottom line its easier to take some revs off if underpropped, but impossible to add them if its over propped.
 
Rocksteadee said:
Boat is a Princess 40 with TAMD63P
Just fitted new tachos to fly. Not yet driven boat due to lockdown.
Previous hand held digital tacho measured on engines read about 100 rpm lower than dash tachos at WOT.
Now checked setting on removed tacho revealed an offset of +3.5 %
Checked lower helm tachos and have same offset.
This gives same results as hand held
Meaning that each engine is not reaching full revs
I am assuming that it is over propped and manufacturer offset the tachos rather than fit correct size props.
Anyone else come across this and what would the implication of being over propped?
Click to expand...
If they are Volvo tachos you can calibrate them up or down to correct figure through the tiny hole back of gauge , use a small drill bit .
 
volvopaul said:
If they are Volvo tachos you can calibrate them up or down to correct figure through the tiny hole back of gauge , use a small drill bit .
Click to expand...
I think that what the OP means someone s turned them up “3.5%” to disguise the over propping .
As opposed to have adjusted the prop pitch .
So I assume when tested it indicated WOT rpm had been reached .
Since found this out .
 
volvopaul said:
If they are Volvo tachos you can calibrate them up or down to correct figure through the tiny hole back of gauge , use a small drill bit .
Click to expand...
That was how I found the offset to read 3.5 % high
engines are 2750 and 2700so allowing for offset are actually 100 less so not achieving the rated 2800. that was with half tanks no water or kit and clean hull
my concern is overloading engines and increased fuel.
When we spoke last year you said that these should go like the proverbial off a shovel, it did seem a bit sluggish on acceleration
 
Volvo won't sign off an engine installation without carefully checking it all out themselves - Volvo are in for the engine warranty bill not Princess. The condition of the boat at design sign off would have been brand new with realistic dummy/actual loads etc.
The tachos are not always spot on, as others have said they can be adjusted. They also will vary with age.
If you have fitted new ones and re-calibrated and find you can't get max RPM - I would investigate that rather than assuming Princess and Volvo somehow colluded to sign off an overloaded design.
How old is the boat? How clean is the bottom/props -it doesn't take much to loose the top few rpms. Turbos don't last forever on 63Ps irrespective of hours- that will make it slow to accelerate. Do both engines accelerate at the same speed. If not something is wrong with either fuel supply or air (turbo) supply. Also have the props been replaced at any point they may not be the originals.
 
kashurst said:
Volvo won't sign off an engine installation without carefully checking it all out themselves - Volvo are in for the engine warranty bill not Princess. The condition of the boat at design sign off would have been brand new with realistic dummy/actual loads etc.
The tachos are not always spot on, as others have said they can be adjusted. They also will vary with age.
If you have fitted new ones and re-calibrated and find you can't get max RPM - I would investigate that rather than assuming Princess and Volvo somehow colluded to sign off an overloaded design.
How old is the boat? How clean is the bottom/props -it doesn't take much to loose the top few rpms. Turbos don't last forever on 63Ps irrespective of hours- that will make it slow to accelerate. Do both engines accelerate at the same speed. If not something is wrong with either fuel supply or air (turbo) supply. Also have the props been replaced at any point they may not be the originals.
Click to expand...

Boat is 98.
Stbd prop replaced
Port requiring replacement as very badly pitted.
Test were done with an optical hand held tacho (same reading a 2 new tachos.
was clean and half fuel no water
Engine rev equally but seem a bit sluggish at 1500 waiting for turbo delay.
Every tacho reading the same at all revs. Have checked 3 and all have same 3.5 offset.
Tacho is measured from crank induction sensor.
 
All sounds pretty good. Tricky thing now is what to do next. New props will be expensive as will new turbos.
If it was me, I would have the turbo boost pressures checked under load, at sea by a suitably experienced volvo tech with the right kit. Probably cost a few hundred quids but well worth it for the confirmation of condition. If you start messing with props before veryifying the engines that will get expensive and very time consuming.

If the turbos and fuel supply are all signed off as OK or good enough bearing in mind they are 22 years old, it is possible to have the props tweaked either by pitch or diameter. If you are thinking of replacing the port prop might be worth waiting until you are 100% satisfied the engines are right. If it turns out you are slightly over propped it's not by much. So unless you drive around at max throttle fully loaded all the time no reason to panic. Any fuel savings will be very small and lost in the variations caused by sea state.
 
kashurst said:
If it was me, I would have the turbo boost pressures checked under load, at sea by a suitably experienced volvo tech with the right kit.
Click to expand...
Lots of aftermarket boost gauges available for the automotive market. These any good for boat use? What sort of boost pressures are we looking at?
 
Paulg25 said:
Lots of aftermarket boost gauges available for the automotive market. These any good for boat use? What sort of boost pressures are we looking at?
Click to expand...
I did this on a targa 43 with a pair of 1998 vintage 63Ps (similar issues to OP). I used a car boost pressure gauge and a selection of fittings. I think I was trying to measure @ 2.5 bar. In the end it got more expensive than I first imagined and took ages to find all the bits and bobs to connect to the manifold. You really need two set ups as well as boost pressure depends on load. If the engines are behaving differently, it helps a lot to see what they are doing simultaneously. I was never convinced about my measurements. It turned out both turbos were low on boost, I decided afterwards it would have been better to bite the bullet and get a professional kit on board and a more experienced second pair of eyes to observe the engines behaviour.
 
pretty sure the 63P has a smoke suppression device on the fuel injection pump. An airline from the inlet manifold goes to the injection pump to prevent over fueling if there is not enough air coming in.
 
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