Altering a Planning Motorboatl?

[DreamMachine]

Hi,
We are considering buying an older 1990-20M planning motorboat and converting it to a semi/displacement boat by removing the current twin MAN 1200 HP beasts and fitting a pair of sensible VolvoPenta inboard diesels around 250-300HP.
Can anybody offer advise towards this idea, will it work effectively ie will fitting lighter engines cause any problems? Obviously the boat will be slower and will not plane, but will a planning hull still perform well as a semi/ displacement vessel. We are looking for reasonable fuel economy and a range of at least 300nm plus range which should be easily achievable utilising the existing 7000lts fuel tank.

Thank you for any advise offered.:encouragement:

 

[kashurst]

how about leaving the existing engines alone and just drive on one engine, swapping over every two hours.
You may end up using a bit more diesel - but will save yourselves a lot of trouble and still be able to sell the boat in future.

 

[Designo]

Most of the planning 20 m boats with MAN 1200:s are not really planning to start with, so just accept that and slow down would save you a lot of fuel. I am not sure, but perhaps the engines can get a new rating without turbos as well and with other props you have a nice semidisplacement yacht.

 

[ontheplane]

+1

Just drive with existing engines at a lower speed!!!

You'll need to give them a blast every so often (50hrs or so??) to clean out the **** but that's not a hassle) plus you'll have some emergency speed in reserve if you need it.

 

[DreamMachine]

Hi Thanks for advice, I recently read in a well known magazine about a charter company that also runs on one engine for 2-3 hours and then swaps, its a good idea but the engines are old and I think pretty inefficient, even running on one.

Do you think if the boat is converted in this way it would be difficult to sell in the future then?

 

[Designo]

Do you think if the boat is converted in this way it would be difficult to sell in the future then?

No, it will probably be impossible to sell as a yacht.

 

[Spi D]

Boats of that size are expensive to operate, even before you consider fuel cost.
How does the budget for swapping the engines look? The price of two replacement engines would buy a lot of fuel.

 

[DreamMachine]

Thanks for your input,
Im looking at spending around 35-40K on the engines and another 40K on general refurbishments throughout, teak refinishing, some new electronics, new bow thruster & a coloured hull. Im anticipating doing a lot of the works myself.

 

[Portofino]

Sunseeker made a Pred 108 with a pair of John Deers -Same idea a chap was having a sabbatical and wanted to tour the Med ,go to Greenland etc @ D speed ( with crew ) say 10-12 knots ( for a 108 ) .Great -but took 3years to shift when he was done with it -as Designo says impossible to sell ..standard Pred 108,s 2x arnesons some tripple anything up to 50 knots

Buy a 18-20 M sail boat if you want to do an extended live aboard @ 8 knots average

 

[MapisM]

Im looking at spending around 35-40K on the engines

Assuming that the existing engines, old as they might be, are not in need of replacement or a major overhauling, did you do some math on if and when you will reach the break-even?
What sort of improvement are you expecting, in terms of MPG?
You might be surprised by how little fuel also the MAN can burn when spinning at very low rpm and load.
By heart, I would say that for the average pleasure boat usage (i.e. measured in hundreds, rather than thousands of hours per year), half a century is not enough to reach the BEP.
How old are you?

And that's without even beginning to think about re-saleability, possibility to speed up if and when you fancy, the old golden rule that if you think to spend 40k in the engines replacement you're eventually going to spend double than that, etc.
There's only one advantage I can think of, with some much smaller engines: quieter cruising.
Old 12V MAN engines were not exactly built aiming at silent operation as #1 priority...
...though they are not too bad when spinning at very low rpm. And that also depends on how well the exhaust is designed, anyway.

BartW of this parish has a 22m Canados with those beasts, as you aptly called them, and he uses the boat at D speed a lot.
You might pm him and ask his opinion, if he doesn't see this thread.

Good luck, whatever your final choice.

PS: I forgot to mention: your target of 300nm out of 7k litres is what made me think that you didn't make any math on fuel burn etc.
In fact, at displacement speed I would expect the boat to have at least THREE TIMES that range, and with the existing beasts!

 

[MapisM]

Most of the planning 20 m boats with MAN 1200:s are not really planning to start with

Mmm.... The Canados I just mentioned above is 22m, weighs 60T, has actually 2x1100 and makes 25/26 knots.
Wouldn't you call such vessel a planing one?

 

[Designo]

Mmm.... The Canados I just mentioned above is 22m, weighs 60T, has actually 2x1100 and makes 25/26 knots.
Wouldn't you call such vessel a planing one?

Not really, as I said. You can not compare it with a small runabout where you have a hump speed and then you can reduce the throttle and still maintain a high speed on plane. The Canados and most boats of that kind are like plowing water and have not travelled over the hump so to speak, it is more like a semi displacement boat even in higher speeds.

 

[Bertramdriver]

Lots of focus on the lumps but have you considered the way the hull handles? Low speeds in a moderate slop driving a planing hull can be very uncomfortable, and most of your sea time will be spent in sloppy seas. At the very least you need to think about adding a three quarter length keel, and then add some spray rails on the bow to spread the bow wave to give stability. One of the benefits of a SD hull is that when you hit a slop you can add revs, which puts the bow up and pushes the bow wave under the hull, smoothing out the ride.
An interesting project but better to buy an SD hull from the outset. Plenty of larger, older SD's around especially in Greece and Italy.

 

[Piers]

Play d'eau (Fleming55) has a SD hull which we drive at D speeds of 7 to 8 knots. Economy is good at 1.7nm/UK gallon, but without the stabilisers the handling would become dreadful in anything but a flattish sea.

So my qn would be 'does your boat have stabilisers?' If so, keep old MANs and drive slowly. If not, still keep old MANs and fit stabilisers just like BartW did.

Job done.

 

[henryf]

You've pretty much taken the words out of my mouth. The first thing I would be doing is fitting stabilisers (assuming not already fitted).

Providing you don't try to drive the boat above the natural hull speed any engine including the Mann is going to be extremely frugal.

Henry

 

[Bertramdriver]

Bertram 47' with two 1980 Detroit diesels 430hp. Semi D hull, all up @20 tons
8 to 9 knots displacement using 4litres/nm. 12 knots when rough 8 litres/nm. No need for stabs.
But"....if we need to run for cover..... 18 knots.
Best of both?

 

[DreamMachine]

Thanks for your input, this is exactly the type of information I was looking for, obviously I can make fuel calculations on existing engines and proposed engines and then make a judgement call to go ahead with the project or not but if the ride is going to be sloppy cruising at say around 10-15 knots the project may be a no go without even looking at fuel usage, as hull modifications sound expensive. in your option would the installation of stablisers alone work sufficiently.

I need to weigh up purchase price and costs and compare as you say against an existing SD's for sale, I just like the look of this boat and with some money spent on refinishing the teak and a coloured hull if could look super.

 

[Piers]

Thanks for your input, this is exactly the type of information I was looking for, obviously I can make fuel calculations on existing engines and proposed engines and then make a judgement call to go ahead with the project or not but if the ride is going to be sloppy cruising at say around 10-15 knots the project may be a no go without even looking at fuel usage, as hull modifications sound expensive. in your option would the installation of stablisers alone work sufficiently.

I need to weigh up purchase price and costs and compare as you say against an existing SD's for sale, I just like the look of this boat and with some money spent on refinishing the teak and a coloured hull if could look super.View attachment 41088

Hi DreamMachine. The max displacement speed will be determined by the length of the waterline. Pushing beyond this will increase consumption hugely.

I suspect you'll find not a great deal of difference in DS cruising consumption from modern engines - at least, none that warrants an engine change.

My last comment would be about a coloured hull. These always oxidise far quicker than an owner would like, and you should expect to need full hull polishing to remove the whiteish oxidation and restore the colour certainly by the fourth year, if not by year two.

I come back to stabilisers as being the most important additive - all IMHO.

 

[DreamMachine]

Hi Play d'eau, thank you for your constructive comments,
I'm also not excluding a vinyl wrap of the hull, we had our Sunseeker done last year and it came out superb, they now can come in a large variety of colour's now and minor repairs are almost Invisible.

By the way I like your blog, we are from St Martins, Guernsey, a few of my school friends work at M&G..

 

[DreamMachine]

Old school friends I should say!