A few moments to spare from earning money to pay for the boat ?

[MapisM]

Generally speaking and still sitting on the fence, I run a lot at D speeds these days thanks to my gyro but I still try to ensure I spend a few minutes running at high P speeds every trip just to make sure that the engines do some work. In any case is that not why we buy P boats in the first place?

Well, not necessarily.
I for one maintain that the replacement of a D boat with a P boat of similar size can be aptly defined a downgrade, in more ways than one. And as you know, I went that route mostly on the basis of VFM considerations, as opposed to speed or anything else.

That said, you might well be right.
In fact, what would happen using the engines of my old lady to repower a P boat, and then running them at much higher load, is anybody's guess.
Btw, by using the DP (as well as any other P boats) extensively at D speed, the underload would be even worse than in my old lady, for obvious reasons.
That doesn't worry me though, because I can't see myself using her that way for more than 50% of the time (if that), for two reasons:
1) cruise comfort at D speed doesn't hold a candle to a proper D boat, and I'm sure that's not just due to the lack of stabs, because also when I tested her in a head sea, where rolling was not an issue, the hectic pitching was still MUCH worse than in my old lady. So, fuel saving aside, the incentive to slow down is somewhat lost.
2) the hull really comes into its own anywhere north of the low 20s, with a very smooth, flat and stable attitude, even in the same head seas which made D cruising uncomfortable. That's bound to be true only up to a point, I guess - and I've yet to find what her limits for a reasonably comfortable cruising at P speed are. But gut feeling tells me that whenever sea conditions are bad enough to restrict her from cruising at P speed, I'll rather not be out there at all...

Bottom line, time will tell, but I believe that excess of low load usage is unlikely to ever become an issue, with the DP!

PS: sorry M, I forgot to answer your second to last Q: nope, the boat was actually overpropped - as most overpowered D boats are.
In fact, she could exceed 12 kts, if it weren't that nobody in his right mind would have kept her screaming "slow down, you idiot!" for more than a few mins....

 

[Deleted User YDKXO]

Bottom line, time will tell, but I believe that excess of low load usage is unlikely to ever become an issue, with the DP!

I wouldnt be so sure. For starters, you will find the fuel consumption of a 60ft P boat at 20kts to be eye wateringly expensive. Its not much fun putting your hand in your pocket for €3000 worth of diesel every 300nm even if you can afford it. I'm guessing you'll adopt the same kind of cruising style as me which is to potter along at D speeds when the sea conditions are relatively calm and allow you to do so in comfort; and only speed up to planing speed when its a bit lumpy and you know that the extra stability at P speeds will make it more comfortable. Before I got the gyro I was probably cruising at 50/50 D/P speeds and I guess now its more like 70/30. Yes the DP will have a more lively motion at D speeds compared to Ant but you'll soon get used to it especially when you realise how much less fuel you are using. FWIW and just as a guess I reckon the DP will be using 3x as much fuel per nm at 20kts than at 8kts

 

[Portofino]

There's a 2004 ish SUNY Pred 108 in the Med with a pair of Factory fit when new John Deeres
D only -- but running @ correct EGT + load+ stabilised too
Huge tankage too -- very self sufficient.

STD spec MTU 2 or triple 2000 ++ annesons
Guy was having a gap year touring the Med with family etc

Took ages to sell on -- back then . Wonder how quick something like that would linger on brokerage today ? -- not 3 years - which it did back then. Nearly new .
You see that's the point correct rated engines for the job
I think the EDC and CR guys will get away with running low loads
Not convinced none EDC / CR

Ideally use em as intended - planing
Easy for me to say n do - agree

 

[MapisM]

FWIW and just as a guess I reckon the DP will be using 3x as much fuel per nm at 20kts than at 8kts

Spot on. Ballpark is 2L/Nm vs. 6.
But yes, 50/50 is probably a good estimate of what also my usage will be - and which I wouldn't call an excess of low load usage, anyway - as well as I wouldn't for the 70/30 you're now envisaging.
99% as in my old lady, THAT would probably be excessive for engines sized for a P boat.
Btw, since I suppose that by 70D/30P you are talking of hours, bearing in mind that engine usage should rather be measured by fuel burnt rather than running hours, your engines are actually doing their work the other way round - i.e. 70% (if not more!) at high load and 30% at low load... :encouragement:

 

[Portofino]

Yeh buts it's the time spent at low loads particularly in none fancy EDC - CR --- computer chip controlled diesel squirt -- @ low EGT that the killer

Sticky rings = fracture when you " some hrs " later Italian tune it
$hitted up injector tips = duff spray pattern ---v important in CR

Poor fitting valve stem seals at low cylinder temps /pressure


Remember you are burning oil here not a volatile gas -- it needs huge temps / pressure to burn --causing least harm
In a P boat the only way to get enough load to do that is plane

That's the application builder / MAN -- picked off the self

 

[Deleted User YDKXO]

Btw, since I suppose that by 70D/30P you are talking of hours, bearing in mind that engine usage should rather be measured by fuel burnt rather than running hours, your engines are actually doing their work the other way round - i.e. 70% (if not more!) at high load and 30% at low load... :encouragement:

Good point!

 

[MapisM]

That's the application builder / MAN -- picked off the self

I don't think it's correct to say that light duty rated engines are designed from scratch to run at 80% or whatever load, and they suffer at anything below.
Arguably, the opposite is true - most of modern high output engines were originally designed for much lower power, and their output was constantly increased just because most boaters (and in turn boatbuilders) are willing to pay for the power of one engine, rather than its sheer weight - which in many ways would be a more accurate durability indicator.

But as a matter of fact, light duty engines are not only meant to power Itama and the likes: they are also used in fisherman boats, where they spend many hours of operation at or just a tad above idle, before being hammered to death when rushing back home at the end of a fishing day, with Bertram-style hulls, which are VERY demanding in terms of power, in order to go fast.
And that's an application which was popular well before electronic engines were around...

Remember - from the horse's mouth (and I have that in written): MAN gives ZERO restrictions about how long to run my mechanical engines at very low load.
They just suggest to spin them at higher rpm/load every now and then, during long passages.
Which was already understood and agreed, anyway.

 

[Portofino]

https://daim.idi.ntnu.no/masteroppgaver/011/11083/masteroppgave.pdf


Bit of a tome ?