how to work out fuel consumption ?

[grumpyoldman]

Thanks JFM for the info

We are going to sit back for a day or two for the vendors to come back with the answers we need, after which we will decide the next steps

 

[MapisM]

I'd then have a hunch that if the engines are doing 7@1200 and 21LPH at 1750rpm, both numbers being 3/4 of the JD full power curve values, the thing might be under propped for 8knot running and you'd be better adding an inch to the pitch, but that is another subject and anyway is upside for you in the fuel burn stakes

Hang on, maybe I wasn't clear enough. When I said "the prop demand figures can vary depending on the hull", it wasn't because the numbers I posted were for full power - in fact, I also said "according to the JD prop demand curves".
My caveat was because, even if the propeller curves are usually rather accurate for D hulls, I wouldn't be surprised by differences up to 30% or so, based on actual results I've seen (also on my own boat, btw).
As a rule of thumb, with overpowered D hulls (as Bonker slightly appears to be, on paper), actual fuel burn numbers are better than theoretical prop demand curves.
Which is also intuitive (well, sort of) because with oversized engines it takes a lower than "normal" engine load to move the boat, at any given RPM (other than WOT).
Bearing this in mind, I don't think she's actually under propped.
Also because there's another whopping 450 rpm to go, from 1750 to WOT, while there's not much room above 8.5 knots in terms of potential speed increase, for a full D hull of that size.
Anyhow, yes, that's definitely another subject, which has nothing to see with the 6.5 vs. 8.5 knots fuel burn comparison.

In the meantime, just in case anyone else would be interested in more details, I thought to upload the JD specs sheet:
https://www.dropbox.com/s/jtuw0g0tna2mw8e/6081AFM75-300.pdf

And while I was at that, I also checked whether those engines are electronically controlled, and indeed they are.
In fact, I didn't see Floscan mentioned anywhere in the ad (jfm, did you?), so it's sensible to assume that it wasn't a calibration problem.
This is the JD diagnostic display, which can show the fuel burn, among other numbers:
http://manuals.deere.com/omview/OMRG35860_19/OURGP11_00000A7_19_03SEP03_1.htm
And while in the advert I couldn't find a pic of the dashboard good enough to see such display, it is clearly visible in this e/r pic, above the run/stop buttons:

Therefore, I would expect the same display to be installed also somewhere on the dashboard.
Regardless, the boat indeed has a reliable mean of controlling the fuel burn in real time, so re-checking the numbers at any rpm/speed should be easy peasy.

Btw, while I re-checked the ad, I looked at the specs and pics more carefully, and I can only think of "WOW!" to summarize my impressions.
GOM, if you will go ahead with the purchase, and if she's currently in the Med as jfm guessed, I would strongly recommend to do yourself a favour.
Keep here down here for one season, before bringing her back to the UK. My bet is that you will change your plans for good.
Mind, I'm not saying that she wouldn't be a great boat for exploring the UK coastline, but she screams "designed for the Med" in so many aspects, that I'm going to swallow my previous comment re. the different design which I would prefer for a steel trawler of that size. At least for Med cruising, that is.
With her one level main deck, the gorgeous airy saloon with opening windows all around, the foldable stern platform, and bimini coverage all around, she's the ultimate cruising machine for warm weather, imho.
The obvious capacity to handle also the rough stuff is of course a big bonus, but while I can think of several other boats which I would choose for ocean crossings (not that I would be interested in it, being possibly the most boring boating activity on the planet, but each to their own on that), at the moment I can't think of any other mobo on the market, in the 60 to 70' bracket, which would be better for Med cruising - even assuming money no object. 'nuff said.....

Looking forward to seeing her soon in flesh, during your maiden Med cruise!

PS: btw, if she's really in the Balearics, as jfm guessed, I'm only a couple of hundred miles away.
Just good enough for stretching her legs a bit....!

 

[jfm]

I agree all your prop curve analysis there MapisM, but I don't agree your conclusion. Everything you say, with which I agree, tells me she is under propped. If you put coarser/bigger props on her you'd get higher cylinder pressures and lower rpm for any given D speed, and that's nicer for the engines and nicer for her passengers. But I'm happy to take that debate to another place sometime as you say. And it's upside - if you're right then the boat is great and if I'm right it's even better! Not relevant to the buy/don't buy decision therefore.

See pic below showing the JD electronic displays installed on dashboard at the lower helm . They will read out LPH as you say. (My brother just got his delivered, btw, deal with that on the other thread). No I didn't see any mention of floscan (sorry if I started a hare running there...) and as you say the LPH data will have been taken straight from the JD computers. If it were my boat I'd bridge these to the Raymarine stng network then you get instantaneous LPM on your screens which helps you compute and manage range much more easily

In your pic above, what is the red thing top right? Hot water tank/boiler? (The rest of the background in that pic is easy to work out. Mostly hydraulics obviously; the electric motor is for zero speed stabs I expect)

 

[MapisM]

In your pic above, what is the red thing top right? Hot water tank/boiler?

Actually, I just hyperlinked the pic from the Elburg website which you posted.
All I know about the boat is what I read on that webpage and on this thread...
...but if the red thingie isn't a Kabola diesel heating system, it sure looks like one!

Good to hear that SD got his engines, are you (or him) going to post a "repowering project" thread?
I for one would read it with interest. All the very best anyway!

Re. the props debate, apologies to GOM for going slightly O/T, but after all it might be relevant for the boat he's interested in....
...or maybe not.
In fact, my trouble is that I'm not sure to see your point, unless you have some other numbers I'm not aware of.
My (very simple) train of thought was as follows:
1) for that type of boat, it wouldn't make sense - regardless of power available - to aim for a max speed above 10/10.5 knots.
Let's make it 11 for good measure. After all, some folks like living on the fast lane....
2) The engines are rated for 2200 rpm max
3) Fuel burn aside, she achieves 6.5 knots @ 1200 and 8.5 @ 1750 (which I guess they quote as the optimal/suggested cruise speed - not that this matters in this context, anyway)
4) based on these numbers, and on some experience on D hulls behaviour, I think it's plausible to extrapolate that at 2200 the boat should achieve somewhere near 11 Kts
5) IF this guesstimate is correct, the boat is not underpropped

Now, I have zero problems to admit that this is not a scientific analysis by all means.
Otoh, I struggle to think of any other/better evaluations which could lead to different conclusions.
Unless other numbers are available, of course.
Which is very likely btw - I'd expect a detailed sea trial report, with speed and engines load at any given rpm, to exist.
But it would be unusual for any builder to disclose them publicly...
...though I never really understood why, TBH.

 

[jfm]

Actually, I just hyperlinked the pic from the Elburg website which you posted.
All I know about the boat is what I read on that webpage and on this thread...
...but if the red thingie isn't a Kabola diesel heating system, it sure looks like one!

Good to hear that SD got his engines, are you (or him) going to post a "repowering project" thread?
I for one would read it with interest. All the very best anyway!

Re. the props debate, apologies to GOM for going slightly O/T, but after all it might be relevant for the boat he's interested in....
...or maybe not.
In fact, my trouble is that I'm not sure to see your point, unless you have some other numbers I'm not aware of.
My (very simple) train of thought was as follows:
1) for that type of boat, it wouldn't make sense - regardless of power available - to aim for a max speed above 10/10.5 knots.
Let's make it 11 for good measure. After all, some folks like living on the fast lane....
2) The engines are rated for 2200 rpm max
3) Fuel burn aside, she achieves 6.5 knots @ 1200 and 8.5 @ 1750 (which I guess they quote as the optimal/suggested cruise speed - not that this matters in this context, anyway)
4) based on these numbers, and on some experience on D hulls behaviour, I think it's plausible to extrapolate that at 2200 the boat should achieve somewhere near 11 Kts
5) IF this guesstimate is correct, the boat is not underpropped

Now, I have zero problems to admit that this is not a scientific analysis by all means.
Otoh, I struggle to think of any other/better evaluations which could lead to different conclusions.
Unless other numbers are available, of course.
Which is very likely btw - I'd expect a detailed sea trial report, with speed and engines load at any given rpm, to exist.
But it would be unusual for any builder to disclose them publicly...
...though I never really understood why, TBH.

Let's assume that 11kts is the correct max hull speed (before you start riding up the bow wave and all that) and the max speed the owner wants.
Then, by mistake or design the boat has been built with engines 2x the power needed.
You have 2 choices (and I'm cheating by keeping the maths proportional and simple)-
(A) you can prop the boat so that at 2200max rpm the boat is doing 11kts; or
(B) You can prop it more coarsely so that the engines are doing 1100 rpm at 11kts, and therefore never ever get run at max RPM (though they could, and would make say 14 knots or whatever if they were run at max RPM)

With A, you are getting much less than the max fuel flow in each injection cycle; with B you are getting the full fuel flow as per the curve. Neither is right or wrong; both will "work". I would put coarser props on and keep the cylinder pressure high and the rpm low. In other words I would tend to be at the B end of the spectrum. As I understand it, you're recommending A, and indeed Bonker seems to be built with some A. No worries, but I am a B man!

A is easier in a marina of course, but for passage making I'd want B

You are proposing I think that the prop that gives you desired Vmax at engine's RPM-max is the correct one, even if the engine is giving <100% rated power, and I'm disagreeing that in general. But in some cases it could be ok: suppose you had bought the "short life rated" version of an engine, E-rated (to use Cat jargon). By doing what you say you'd have in effect converted it to A rated, and that would be a good thing. If it is already A rated you might as well do what I say as it is nicer for the passengers.

Just briefly, yes my brother's 2x JDeere's finally arrived last week, at boatyard in Scotland. Far too late for him to make 2014 Med seasons sadly. We are not loving JD's broken promises on delivery times right now I can tell you. I'll encourage him to post more on his re-engine thread soon. Old engines are out, new Onan genset is going in, a GRP radar mast is being made, loads of things. He really ought to post a load of pictures but is a bit busy with work and 2x 5 year olds (twins!)

 

[MapisM]

As I understand it, you're recommending A, and indeed Bonker seems to be built with some A. No worries, but I am a B man!
...
You are proposing I think that the prop that gives you desired Vmax at engine's RPM-max is the correct one, even if the engine is giving <100% rated power, and I'm disagreeing that in general.

Aha, ok, now I see what you mean.
Funny that you should say that, because actually I'm also on B camp.
In fact, my boat is propped very much in a B way, to the point that at WOT the engines can't reach the max rated rpm.
That's how she was propped when I bought her, and initially I thought to have the props shortened a bit, but after a while I realized that the advantages of low rpm cruising were interesting enough to forget the cons of never being able to run the engines at their max rated speed (other than out of gear, obviously).

Re. what I was proposing, nope, actually what I suggested is that Bonker (for all I know, which is not a lot) might well achieve 11kts at 2200 with 100% load, hence "using" the whole rated power.
And IF she would, then the boat would be propped correctly.
At least, correctly according to the academic principles, as I understand them (now, where's Latestarter when you need him? ).
Goes without saying that in this respect, my boat is NOT propped correctly.

Anyhow, in all the heavy trawlers I've seen, the power absorption increased exponentially, after reaching their hull speed.
In other words - and I'm now trying to use meaningful (albeit obviously rough) numbers, based on the boat hull/size/weight - 450hp might well be sufficient to push Bonker at 10.5 knots (WOT, rated RPM, 100% load).
And this result would be fine, for a heavy steel trawler which will cruise at 8 to 9 knots 99% of the time (ocean crossing aside, where the speed would be even lower).
Therefore, with 600hp, the boat is arguably overpowered.
But this doesn't mean that 150 additional ponies could significantly increase her max speed.
In fact, an increase of only 0.5 kts (again, WOT, rated RPM, 100% load - but with 600hp instead of 450) would be perfectly plausible, in my experience.

Anyway, these are all just speculations, of course.
GOM, I'm afraid you MUST now purchase the boat, invite us at the sea trial, and put us out of the misery of these armchair debates.
I'll gladly bring a bottle or three of Franciacorta Gran Cuvée Brut for the occasion.
Good for a blind tasting with some French bubbles which I'm sure jfm would bring to the party!

PS: J, sorry to hear of SD troubles/delays, all the best for a successful conversion anyhow.
I guess it's not much of a consolation, but I'd rather miss some boating than have such complex job done in a hurry... :ambivalence:

 

[Cumba]

In your pic above, what is the red thing top right? Hot water tank/boiler? (The rest of the background in that pic is easy to work out. Mostly hydraulics obviously; the electric motor is for zero speed stabs I expect)

It's a Kabola Heater, I think the HR500. It's for warming the water and providing heat through a CV system, more info over here: http://www.kabola.nl/
Lot's of Dutch builders install these on their boats, (Elling, Super Vancraft, Linssen etc).

 

[jfm]

Ah ok Mapism. All 100% agreed. We now each understand what the other was saying. We're not disagreeing any principles; we were just making different assumptions to fill in the huuuge gaps in our knowledge about Bonker. (Forums are good for that!)

Yep my brother is resigned to the loss of 2014 season. As you say, the re-engine job cannot be hurried and must be done right. I'll pick up the story on another thread in a while (I'm off to Corsica for 10 days tmrw)

 

[MapisM]

Excellent timing. I wouldn't have wanted to deal with the mistral blow that kept bashing the Corsican sea during the last week or so.
Looking much better now, though I'd expect water to be a bit colder than normal, for mid of July...
No chance to head further S, after Corsica? Fair winds anyway!

PS: yep, all understood/agreed re. propping etc.
Looking forward to hearing from GOM about his actual findings, if he'll go ahead.

 

[jfm]

Only doing Corsica. But there is a bigger forum fleet in last 2 weeks August and we are definitely doing olbia and madellenas. Not sure yet if we will get further south. We are making it up as we go along rather than having a proper plan, as there are 7 boats at various different times and itineraries

 

[grumpyoldman]

just updating position on bonker

We are waiting for engine and fuel consumption test results - which are arranged and are assured will shortly be available.

Apparently no one thought these would be a requirement by any prospective purchaser?.

Will update when full range figures are available which will tell us apart from fuel whether she is propped A or B (i think for comfort i favour B) - thanks for your guidance and help gentlemen - thankfully no longer buy with my heart - the wait would be frustrating in the extreme

 

[MapisM]

thankfully no longer buy with my heart

LOL, naaah.
No matter how carefully you consider all the pros and cons, eventually the last word in the purchase of a pleasure boat is always heart-driven.
And rightly so, if you think about it.
You don't want to spend your quality time onboard something which makes sense but you don't love....
And I'm sure that the boat you're looking at will be a love affair indeed.
Also because, even in the worst case of a radically wrong propping, that's a relatively easy fix, and surely not a deal breaker, in the big picture.
All the very best, and keep us posted - possibly with pics!