Altering a Planning Motorboatl?

[Searush]

I love these threads. It's like a lesson in the Dark Arts at Hogwarts for a Muggle

I learn so much about a class of boat I will never ever get the chance to drive, never mind own. It is just so utterly different from mud plugging at 3-4 kts at 20deg off the vertical. :encouragement:

 

[MapisM]

a class of boat I will never ever get the chance to drive, never mind own

Well, cost of ownership aside, of course there are also advantages in owning big vessels.
But since you mention also "driving", rest assured that in this respect they are no more fun than smaller stuff, by all means.
If anything, the opposite, in fact!

 

[BartW]

Hi there
Sorry I’m late to this thread,
Owning a similar vessel as what you are looking at,
a 1991 22m Canados, with V12’s 1100Hp Man’s, (same base block like the 1200Hp)
Here is my view

I would never consider changing these engines,
Actually I’m very glad with this type,
Because they are still popular models, all spares generally available at reasonable prices,
Same engines are used in zillions of busses and trucks.
Maintenance is very easy, all straightforward, mostly done by myself or friends
The engines can (relatively) easily be rebuild, if ever needed,
All very basic mechanical parts, injection, turbo, intercooler, etc.
the cylinders have separate liners, all very easy acces,
no computer needed for diagnoses.

Recently I had a friend onboard, who owns a mobile generator rental company,
they own about 200 mobile genny’s used for entertainment shows, etc..
His biggest generator models are driven with this type of MAN engines, and these are his favorite machines.
They run at high duty, (only at 1500 RPM continuously, 600 KW iirc) not like mine, specced light duty 1100HP
But actually I use them a lot at displacement speed, 10kn, 1100 rpm, = high duty (?)
And in his opinion, these engines can take up to 10.000hrs in that usage,
Mine have about 2800hrs, so they are still in their early years  

In my experience the consumption of the engines is not too bad, and exactly according the curves that are published on the spec sheet.
For sure with a pair of very modern common rail, and a state of the art-designed drive train and a more modern hull, you could improve consumption,
But Just replacing the engines is only a partial improvement, and absolutely un-economic. imho

THE most easy method to reduce consumption is reducing speed.


We use the boat in two modes,
10Kn D speed (<=70l/hr)
or
20kn P speed (<= 300l/hr)

But I could easily navigate at 8kn if I would like to decrease consumption more.
In D speed the curve is fairly linear: consumption versus speed

We have been using the boat,
1the season 100% planning mode (180 hrs)
2nd season 90% displacement mode (200 hrs)
3the season 70% displacement mode (220 hrs)
And plan the same for the next season.
But we could aswell sail her at 8kn fe.

I’m a bit confused (and not convinced) with the article in MBY about reducing consumption by driving on one engine,
The writer compares between using twin engines at SD speed 12…15kn (?iirc) compared to using one engine on slow D speed 6kn,
I would be interested in the figures with twin engine at 6kn,
I don’t expect too much difference compared to one engine
At 8kn fe I expect the difference between 1 or 2 engines to be even smaller

Replacing the engines for economic reason doesn’t make any sense to me,
The investment would be a total loss,
The resale of Such a boat is difficult, but such a replacement would make the boat almost unsellable
The fuel cost is only a small part in the total scheme of costs owning such a vessel,

In the thread above came up the issue of fitting stabilizers,
And this is absolutely a good investment,
Not in the sense of the resale value of the boat, but in the usability and the comfort onboard, a totally different world,
You have to experience to understand what a huge difference this makes.
Recently I had the opportunity to navigate quite a distance at 15…10kn, in a F4…F5 swell,
And I can assure you that this was stunning ,
A lot more exiting then navigating this heavy boat at 20kn in flat water. ;-)

regarding the debate about calling this a planing boat or not at 20kn,
indeed during acceleration, I don't experience the moment of going over the humb as on a small planing sport boat,
but when decreasing speed from lets say 18kn to 12kn you can easyly see and feel the moment that she goes off the plane, (approx 15kn)
meaning sinking deeper in the water, not glyding anymore, and going to a displacement mode... (or whatever people feel they would call this mode)

Good luck with the surch,
You’re in a very nice and interesting boat range (to me)
Keep us updated, thanks

Bart

 

[MapisM]

Very well summarised B, I couldn't agree more.
Also on the 1 vs. 2 engines bit, btw.

 

[Divemaster1]

..............I’m a bit confused (and not convinced) with the article in MBY about reducing consumption by driving on one engine,
The writer compares between using twin engines at SD speed 12…15kn (?iirc) compared to using one engine on slow D speed 6kn, ....

Bart, the MBY article boat had two stroke Detroit Diesel (DD's) V12/71's @ 670 hp each (delivered those in 730 Hp as well, so not overly stressed) and comparison graphs focussing on range vs RPM at 8 knots (where steering became OK), not because it provided the best results....(knowing the engine as I do, I think they did right)..

On the lower RPM's, the two stroke DD's loose out any time against a four stroke... simpler design, but more mechanical parts to pull along .... blower, individual injectors, etc., plus some loss at the bottom of the stroke when the exhaust valves open .... makes the DD's use a fair bit of HP just to keep the parts moving (mechanical energy loss)... more percentage wise than an equivalent Hp four stroke that loose on every second, non-power, stroke at the lower RPM's we are talking about here

That percentage loss is starting to balance out at higher rev's and as the low-end torque benefits of the two stroke kicks in..... hence it making more sense running a single DD at 11 - 1200 RPM, than two at 700 ... but for a similar boat with two four stroke's, you may have been better (from a fuel consumption perspective) running both engines at 7 - 900 RPM..... it's all about how the engines convert energy (fuel) to the propeller(s) that move the boat and the energy losses experienced.... but I am sure someone more knowledgeable than me can come along and correct/add to my ramblings...

Anyhoo ... I agree ... taking those lovely V12's out of the boat makes no sense from a boat balance, weight distribution etc., or re-sale value ..... unless they are donated for spares for Blue Angel of course...

 

[kashurst]

+1 per bartW's post - buy a boat in this range is a big commitment re ongoing costs. Planing will be expensive but available should the need arise (misread weather forecast - **** get me out of here moments). Stabilisers are a great investment long term.
However displacement mode in a boat this big, in mild conditions (say @ 10kns F4 or less) will be lovely. If you want a liveaboard/long distance crusier its a lovely way to travel, but don't underestimate mooring charges re fuel costs. I was offered a beautiful Maiora 70 for the same price as my 50 footer. I was very tempted as it is imaculate but the engine servicing, mooring fees, a reducing number of marinas that it would fit, in decided me against such a large vessel.

 

[RogerRat]

Someone will be long shortly with the calculation for hull speed but you won't be cruising at 15 knots. That will cost you a fortune in fuel. You're talking under 10 knots, I'll guess 8-9 knots plucked out of the sky. At this speed your range will be measured in 1,000 plus miles, I bet at 15 knots it would be 2-300 miles depending on the tank size(s).

You simply can't defy the laws of physics unless you throw a lot of energy at the problem.

Henry

Nice one Henry, you're on the right track with Sq. root and ratio of 1.34 as a factor.

A 20Metre boat with roughly 64' (easy No.) length of water line should have theoretical hull 'displacement' speed of 10.72 knots. The speed will almost certainly be a bit less than 10 knots in practice as these calc factors are for heavy weight rounded displacement yachts etc.

This is a theoretical maximum 'hull' speed where all the displaced water can move outwards and around the hull. Above this speed, the water cannot escape quickly enough and a bow wave begins to build. Increasing speed through the water from here on requires exponential increases in power.

Unless the vessel is light and then the boat can be thrust hard enough to create lift and then climb over its own bow wave and then be said to be truly planing. If no complete 'hump' then this is 'semi planing' as most cruising shaft driven Mobos' do over 15-20 knots.:encouragement:

RR