Big thanks to the MTU support team

[Hurricane]

This is the last of three posts
This one on our summer cruise
http://www.ybw.com/forums/showthread.php?408220-Summer-2014-Mallorca-cruise
And his one on a problem we had with our shaft seals
http://www.ybw.com/forums/showthrea...ands-not-what-you-want-to-see-on-your-holiday!!!

We all know that boats create problems.
The trick is to have a relationship with the experts so that things can be fixed with as little fuss as possible.
Jennywren is a Princess 67 and fitted with "top of the range" CR2000 MTU engines.
This summer, our starboard engine started to present us with an alarm. I was sure that it wasn't a huge problem because these engines will drop to a slower speed if and then eventually shut down if there is any danger of damage.
So, we simply reset the alarm and carried on.

During our summer cruise though, the engine kept alarming at undetermined times.
The manual referred to the alarm as a "Combined Engine" alarm but I have since realised that description is meaningless.
This year we have been carrying a heavy Jetski on our bathing platform. It is slightly offset so that we can still use our passerelle so we thought that maybe the engines were sensing an off center load.
On return to our home berth we decided to do some further tests.
The first thing was to remove the Jetski and see if the alarm still happened.
Testing was the exciting part - the alarm only went off at high speed (25 knots) and only in rougher seas.
So with the Jetski removed we tested again.
After a short while we found that we could reproduce the problem in high speed turns to starboard. This is when the starboard engine is under the maximum load.
So it wasn't the Jetski - time to get hold of MTU.
We contacted an engineer in the UK and agreed to email him with our tests.
He pointed us to some wiring on the engine and after a couple of exchanges of email, he explained how to strip the wiring connections down.
We carefully cleaned the connections with switch cleaner and re-assembled the wires.

Since cleaning the connections, the engine has not alarmed so it seems that these connections were, indeed, the problem. It seems that excessive vibration at planing speed caused the wire connections to shake and loose their connection.
BTW - it was really fun doing the testing - high speed turns etc!!

We have a really personal relationship with MTU. They have always helped us and this latest example shows just how professional a supplier can be. To have technical help available to find a fault like this really makes boat ownership a joy.

So, a big thanks to the team at MTU's UK office for their patience and support.

 

[Robg71]

great to hear of quality support and backup....
and that everything is now working as it should be....

 

[MapisM]

Hurric, just as a FYI, this summer a Predator 82 with the bigger version of your engines (16 cyl) was stuck in my marina for some time due to cracks in the manifolds, which had to be completely replaced. And as you know, that's not a trivial job, with such big lumps of iron.
I was actually surprised to see that, 'cause MTU reputation is second to none, and they would definitely be my preferred choice in the power node we are talking about.
But sh!t happens, I suppose....

 

[Deleted User YDKXO]

This is the kind of thing that worries me about modern complex electronically controlled engines. We have MAN V12 engines in our boat now which seem to have dozens of sensors measuring every parameter that the designers could possibly think of and I dread the day that one of those sensors goes down. You're lucky because you have a pre-existing relationship with MTU but most owners are like me and have no such relationship with either the engine supplier or any of it's dealers. I just hope that I'm close to a good MAN dealer when eventually I do get problems but Sods Law says I won't be

 

[Hurricane]

This is the kind of thing that worries me about modern complex electronically controlled engines. We have MAN V12 engines in our boat now which seem to have dozens of sensors measuring every parameter that the designers could possibly think of and I dread the day that one of those sensors goes down.

Yep loads of sensors.
Apart from this latest issue, we had a few problems in the early days but we now know what to do.
Before this boat, we used to cruise at about 21/22 knots
This boat doesn't like that speed at all so we either run at significantly lower or higher speeds.
21/22 knots translates to about 1900 RPM
These engines incorporate twin staged turbos
The first stage starts with the engine but the second "comes in" at that magic 21/22 knots / 1900 RPM
So, running at exactly that speed, the second stage is "coming in" and "going out" all the time.
Now, do this for long periods and a sensor will eventually register that the turbo isn't up to speed.
Alarms then go off and that engine will drop into its "idiot mode" and usually it is synchronised with the other engine - so electronic warfare breaks out!!
It took a few months to discover what was happening so now we run at 25 knots or 10 knots and everything is sweet.
Incidentally, I believe that the engines are far more efficient running at 25 knots than with the turbos switching in and out.
I've always considered that I get those extra 3 or 4 knots from 21/22 knots to 25 knots for free.
Makes me feel better about burning 350 litres per hour anyway.

 

[Deleted User YDKXO]

Incidentally, I believe that the engines are far more efficient running at 25 knots than with the turbos switching in and out.
I've always considered that I get those extra 3 or 4 knots from 21/22 knots to 25 knots for free.
Makes me feel better about burning 350 litres per hour anyway.

Thats interesting because I drew up a fuel consumption curve for my boat based on the fuel consumption figures given by the engine instrumentation and the fuel consumption in terms of nautical miles per gallon is almost dead flat between 15 and 25kts so as you say, there is no financial penalty for doing 25kts rather than 15kts. Maybe it's because the hull designers for this size and type of planing boat optimise the efficiency of their hulls for the 20-25kts speed range that most owners will use and maybe they also work with the engine manufacturers to adjust the torque/power characteristics of the engines to produce max efficiency in this speed range?

 

[jfm]

almost dead flat between 15 and 25kts so as you say, there is no financial penalty for doing 25kts rather than 15kts. Maybe it's because the hull designers for this size and type of planing boat optimise the efficiency of their hulls for the 20-25kts speed range that most owners will use and maybe they also work with the engine manufacturers to adjust the torque/power characteristics of the engines to produce max efficiency in this speed range?

Mike, I get the same. As you may remember I have instantaneous mpg readout. Once fully planing the mpg stays flat ( within 10%) so there is no significant penalty in my boat for doing 26 knots as opposed to 18. The reason is simple: there is no material drag increase, which contrasts with a car where the drag increases with the cube of the speed or whatever. Broadly speaking, in boats like ours the engines do the same amount of work in moving the boat 1 mile whether they do it at 18 knots or 26. The way to save fuel is to pootle at D speed, and then there is a significant gradient in speed- drag curve. You use way less fuel per mile at say 4 knots that you do at say 8 knots

 

[Deleted User YDKXO]

there is no material drag increase,

You'll have to educate me, John! Surely the drag in the form of both hydrodynamic and aerodynamic drag increases with speed? Or is it because as the hull lifts out of the water, the extra drag is offset by the reduction in wetted area?

You use way less fuel per mile at say 4 knots that you do at say 8 knots

Oh yes I do know that!

 

[jfm]

Sorry I didn't explain we'll or accurately. The drag does indeed increase as speed increases. But the gradient is a straight line. Ie there is linear proportionality of drag vs speed. Ie at 2x the speed there is 2x the drag and your fuel flow rate is 2x. So if you apply double the power the boat goes twice as fast hence mpg stays the same within 10% or so. In a car when you do 100km/h instead of 50 the drag follows an increasingly steep curve so you burn 8x fuel rate into the engine but only 2x speed increase, due to aero drag which is pretty insignificant on a boat

 

[Deleted User YDKXO]

Sorry I didn't explain we'll or accurately. The drag does indeed increase as speed increases. But the gradient is a straight line. Ie there is linear proportionality of drag vs speed. Ie at 2x the speed there is 2x the drag and your fuel flow rate is 2x. So if you apply double the power the boat goes twice as fast hence mpg stays the same within 10% or so. In a car when you do 100km/h instead of 50 the drag follows an increasingly steep curve so you burn 8x fuel rate into the engine but only 2x speed increase, due to aero drag which is pretty insignificant on a boat

OK, yes understood

 

[Gentlemanracer]

. Ie at 2x the speed there is 2x the drag and your fuel flow rate is 2x. So if you apply double the power the boat goes twice as fast hence mpg stays the same within 10% or so.

The good, or better, bit is that burning more fuel but consequentially for less time for a given journey translates to more speed and more economy!

 

[MapisM]

Maybe it's because the hull designers for this size and type of planing boat optimise the efficiency of their hulls for the 20-25kts speed range that most owners will use and maybe they also work with the engine manufacturers to adjust the torque/power characteristics of the engines to produce max efficiency in this speed range?

While I don't think there's any boat builder big/strong enough to get Cat, MAN, MTU etc. tune the specs of their engines for specific hulls, it's certainly true that engine manufacturers are aware of what the typical application for their products demand.
In fact, the torque/power curves are fairly flat in the whole "planing" range.
But it's even more true that most hulls (and their propulsion!) designed for f/b pleasure boats are optimised for their typical usage, which is from the mid/high teens to the mid/high twenties.

In fact, I disagree with jfm when he says that the gradient of the drag increase is a straight line.
I mean, yes, it is sort of straight, but only within a rather narrow range of speeds, which is the one we are talking about.
You said that in your boat the MPG is constant from 15 to 25 kts, but I bet that it does go down at 30 kts, let alone at 35.
In fact, as you said in your other post, from 15 kts upward there is still a component of efficiency increase due to better lifting as the speed goes up, and that partially offsets the speed-related drag. But once you run out of such "improvement", the engines are let alone in fighting the drag increase, which is indeed exponential.
To the point that it could be impossible (or surely wouldn't make sense, anyway) to put enough power in a hull/transmission like yours for reaching 50 knots.

Otoh, I agree that aerodynamic drag is negligible with these boats/speeds.
It does matter - a lot! - with air entrapment catamarans for instance, but that's a totally different kettle of fish.

 

[MapisM]

The good, or better, bit is that burning more fuel but consequentially for less time for a given journey translates to more speed and more economy!

Not sure to see what you mean.
I think we all agree that within the typical speed range of a P boat the MPG (but NOT the LPH!) is just about constant, but that means more speed with the SAME, not MORE economy, in my books.
Unless you consider as an economy also the lower engine hours, but that has nothing to see with fuel burn.

 

[Deleted User YDKXO]

While I don't think there's any boat builder big/strong enough to get Cat, MAN, MTU etc. tune the specs of their engines for specific hulls, it's certainly true that engine manufacturers are aware of what the typical application for their products demand.

Obviously I don't know about the boat building industry but I do know that in the construction equipment industry, engine manufacturers do tailor the torque/power characteristics of their engines to suit particular applications which I guess is fairly easy to do with electronically managed engines. In fact, as we know, there are plenty of companies who will adjust the engine management protocol of your car engine to change the torque/power characteristics so it can't be that difficult. But yes I also do agree that the torque curve for most marine turbo diesel engines is pretty flat in their normal operating range so I guess any adjustments are small

You said that in your boat the MPG is constant from 15 to 25 kts, but I bet that it does go down at 30 kts, let alone at 35.
In fact, as you said in your other post, from 15 kts upward there is still a component of efficiency increase due to better lifting as the speed goes up, and that partially offsets the speed-related drag. But once you run out of such "improvement", the engines are let alone in fighting the drag increase, which is indeed exponential.

You may have a point here because you're right at least for my boat. The fuel efficiency does drop off again after 25kts which tends to suggest that the hull/engine design is deliberately optimised for the 15-25kt range rather than just being a function of drag

 

[Gentlemanracer]

Not sure to see what you mean.
I think we all agree that within the typical speed range of a P boat the MPG (but NOT the LPH!) is just about constant, but that means more speed with the SAME, not MORE economy, in my books.
Unless you consider as an economy also the lower engine hours, but that has nothing to see with fuel burn.

Not engine or drive hours (the latter are more expensive so far); more speed is more litres/time but with less time burning it, can deliver a "faster is better" outcome. I'm overseas at present and separated from my spreadsheet, but providing we have pretty flat water (tend to stay at home otherwise) this is what we find! Our beam is only 3.3m though, with sterndrives.

 

[Hurricane]

I think I agree with all of you - but maybe that could mean I disagree with all of you

Anyway, in my experience there is a difference in MPG at different speeds - and of course there must be.
But when you consider the big picture, the only appreciable difference is the huge saving made between displacement speed and planing speed.

At displacement speed, yes, you DO gain proportionally between (say) 10 knots vs 5 knots but is it really worth it - thats double the passage time.
If you are going to save fuel, the big one is between displacement and planing.
Once you have made that decision, it isn't worth fiddling round
So, as I said above - for us it is either 25 knots or 10 knots.

JW is still able to do speeds in excess of 31 knots (even with all the stuff we now have on board!!)
During the summer, the bimini is usually up all the time and any more than 25 knots, the bimini starts to rattle round a bit.

 

[jfm]

In fact, I disagree with jfm when he says that the gradient of the drag increase is a straight line.
I mean, yes, it is sort of straight, but only within a rather narrow range of speeds, which is the one we are talking about.

Yes, I agree that 100%. The effect I was describing applies only in that narrow range of speeds, eg for my boat 18-26 knots approx.

Also I agree, ref gentleman racer discussion, that except when the boat is on the edge of the hole/ on the "hump" you don't get more economy in terms of litres per mile by going faster

 

[MapisM]

Obviously I don't know about the boat building industry but I do know that in the construction equipment industry, engine manufacturers do tailor the torque/power characteristics of their engines to suit particular applications which I guess is fairly easy to do with electronically managed engines.

Good point, M.
I agree that it shouldn't be a big deal nowadays to tweak even one specific engine, depending on its application requirements.
Otoh, I never heard of such type of customization, neither offered by engine builders nor demanded by boat builders.
I suppose it's just a matter of not being something worth bothering, because once the size/weight/power is fine for a given hull, that's all a boat builder is really interested in, at the end of the day. Well, that and the price, of course!

Incidentally, what Hurric said re. his MTUs not being very happy with 1900rpm/22kts seems to prove that they weren't tuned specifically for a P67.
I mean, both Princess and MTU are among the most respectable companies in this industry, and it's easy to guess that the P67 hull would cruise very nicely at 22 knots, if it weren't for this (very minor as it might be) inconvenience.
So, if Princess/MTU didn't bother ironing that out, I'd be surprised if anyone else did.
But glad to stand corrected, if anyone knows better!

 

[rafiki_]

Did JFM have the Cat callibrators on M's 1 and 2 on there handover voyages? Or have I gone mad again? I know that this would not have been a full cal, but some tweaks around the props, lift etc. IIRC M2 had to be lifted for prop re-pitching at Southampton after her trip down from the Orient.

 

[Deleted User YDKXO]

At displacement speed, yes, you DO gain proportionally between (say) 10 knots vs 5 knots but is it really worth it - thats double the passage time.
If you are going to save fuel, the big one is between displacement and planing.

For my boat, the big difference in nmpg is between minimum in gear idle speed, which is 7kts, and our maximum displacement speed, which is 10kts. Over this range of speeds, the fuel consumption more than doubles. Between 10kts and 25kts, the consumption increases again by about 60%. We do sometimes cruise at 7kts but I agree, that feels painfully slow and if I was going to cruise at this speed all the time, I've got the wrong boat. For us a good compromise between speed and time is 9.5kts which, length for length, is equivalent to the 10kts at which your boat feels comfortable