Convert 60/70 older "fast" motoryachts to displacement speed

Sorry if this is not a pure design question:
Im trying to figure out if this is a completely wrong idea or not.

Here in Italy we regularly have on the market very beatiful 90s motorboats (Sanlorenzo, Ferretti, Canados etc) around 60 feet - with a big "flaw" - 2 x 1000 Hp engines (MTU or MAN).

At crusing speed (20 knots or so) this would easily mean 2 x 200 lt/hour - making even the typical 200 miles Sardinia roundtrip a 4000 lt diesel affair - around 9000 Euro at this moment.

For my prospected use (2000 miles a year travelling the med) it would bump up the bill to 90K Euro.

So the idea is to operate this boats at displacement speed (7-8 kn) - and this where the discussion begins.

Mainly that the engines are not made for such a low load (maybe 50/60 hp for engine) and all sort of injector problems / cylinder temperature issues will bite us (is it true at all ?). And the high cost of mainteinance is still there - if you need a new heat exchanger its 10K+ anyway.

So our idea was to install a third "slow speed" engine (bit of the same thing some trawler does) able to deliver enough to bring the boat to 7 kn for normal cruising and keeping the old in place for emergency only (rough weather and you really want to be in port at time, strong wind pushing you back etc).

Now - does it make sense ? and what would be the best solution for this ? In my mind the simplest thing would be a hydraulic propulsion thruster hooked up to an engine (so as to have more freedom to place it somewhere in the engine room), some friend suggested go electric and use a 20kv generator (that will be in place on the boat anyway etc).

Or will be the cost of the 3rd engine plus installation, controls etc just not be worth it - just run the actual engines at the minimum load (even if it consumes maybe twice as the "small" engine in liters / hour, you would need maybe 10.000 hours of operation to recoup the cost of the 3rd engine).

Replacing the 1000 hp engines with maybe 300 hp new engines could be another idea (supposedly you can offset the cost of the new engines with the sale of the old ones) and I have seen a Canados 58 doing that - again does it make sense ?

Thanks
Matteo

 

Welcome to the Forum Matteo.

Would it be feasible to operate one of these planing craft with just one engine running while in displacement mode?
Alternatively, I have seen this mentioned by posters on the YBW Forum in England - Motor Boat Forum https://forums.ybw.com/index.php?forums/motor-boat-forum.14/
Folk on this forum who have similar boats (eg 20 metre Princesses and similar) often 'pootle' along slowly, but they will open up the throttles occasionally to give the engines a blast at their typical cruising speed, for maybe 20 minutes or so.

If you replace the big engines with smaller engines, then you will most probably need to buy new propellers, new exhausts and build new custom brackets on the engine beds for supporting the engines.

Alternatively, what about looking for a 'trawler' type of vessel instead? Although this also depends on what your budget is.
Here is a typical example -
2008 Selene 48 Trawler for sale - YachtWorld https://www.yachtworld.com/yacht/2008-selene-48-8271204/

This steel trawler burns 28 litres per hour at cruising speed -
2010 Alm 51 Trawler for sale - YachtWorld https://www.yachtworld.com/yacht/2010-alm-51-6302227/

Here is an Island Gypsy 52 in Sardinia with 2 x 435 hp Caterpillars, and an asking price of approx US$ 300,000 -
1997 Island Gypsy 52 Motor Yacht for sale - YachtWorld https://www.yachtworld.com/yacht/1997-island-gypsy-52-8396698/

 

Matteo, the biggest thing to do would be to remove the large heavy engines and possibly the large voluminous fuel tanks to reduce the weight. The next issue would be to add a hull extension so that you no longer have a submerged step transom like what you have with any planing boat at rest. This underwater step is going to add a lot of resistance at low speed. You need a hull that is nicely faired so that the new transom ends right at or slightly above the waterline. Now you would need a lower power engine and gearbox and steering system (the bigger engines should fetch a decent price on the used market, powerful engines are never cheap). Perhaps, if it doesnt already exist, add a bow thruster to help with maneuvering given the less powerful main engine. It should still be possible to do something like a 5x or more reduction in power which should dramatically reduce the fuel burn.

 

Look for some low speed fuel burn numbers with the existing motors, you may be pleasantly surprised.
Engine temps can be controlled by changing out thermostats, or adding throttling valves to cooling system.
Removing some weight will help in any instance!
Next best would be to change to lower powered diesels, utilizing existing running gear, but almost certainly changing propellers.
Adding a third engine might be least satisfactory, the weight penalty of retaining the old motors is steep.

 

Depending on the power plant some do idle relatively well. Mtu and Man would not be my first choice, but they can do alright.

Fellow a few docks down bough a big beautiful Hatteras with hopped up 8v92 mains with really high hrs. He got the boat really cheap about 10 years ago. They rebuilt the mains to a different arraingment from 715, detuned to around the high 500s with smaller injectors and a smaller turbo. It's worked really well at hull speed.

I've put a lot of hrs on some pumped up bay boats, idling around isn't ideal but the mains do last longer. Most of the parts like aftercoolers and such are based off of btu absorption and can last a lot longer idling around. I put 10,900 on a 550hp 6v92 over about 8 years. Probably 500 at speed the rest idling to 1100. Did injectors at 8k and turbo at 9500. Next owner ran it to 14 with some valve work then re-powered it. I've seen some 1000 hp c18s currently at 10k with minimal repair 95% 1000 rpm a d 5% high speed.

 

Its one of those "if you want to get to there,don't start from here" situations.It always has and always will take a good deal of power to make a 60 foot boat plane and reducing the power so that it only just planes will still take a lot more than would be needed to push a displacement hull at hull speed.The cost of new engines and all the associated work would be significant.Either travel at a reduced speed with the current engines or choose a displacement hull that doesn't have the drag of an immersed transom to begin with.It wouldn't be a big surprise if some new designs come on to the market in the next year or two as fuel price increases this year are getting noticed.Some marinas are reporting a lot less boating taking place as the boats are being used more as floating weekend cottages.Our American friends might be horrified at the cost of fuel in Europe.

 

It seems a lot of these proposals are going to cost a fair amount of money. I have no idea of the answer, but would they be cheaper than fixing up a displacement hull fishing boat as a cruiser? A dedicated displacement hull might use a bit less fuel. Know any big sailing yachts with broken rigs and an auxiliary engine?

If you were going for the third engine, leaving the two props stationary in the water is going to cause a lot of drag. I wonder if it would be feasible to have a clutch, disconnect the two big props from the big engines and drive them with the third? You might need some kind of gear reduction, but I imagine that's not a problem if you're using hydraulic drives. Swinging big props slowly is more efficient than swinging little ones quickly, unless you're going pretty fast.

Selling the two big engines might finance smaller ones. If gear reductions are a regular thing, you might also use them in this case, keeping the existing props. Not sure which is cheaper.

I don't know how practical it was, but at one point there were American cars with, I think, V-8 engines which shut off 4 of the cylinders unless high power was needed. I think around 1970's or 1980's. There was probably some problem, because you don't see them around any more. I imagine turbocharged engines might still run efficiently with the turbocharging shut off.

Another way to save fuel, if it doesn't look too horrible, is some kind of sailing rig that you can put up and take down. Or how about a large power kite?

I admit to no familiarity with big, fancy yachts, though if someone wants to fund it I could charter some and do some research. ;-)

 

Thanks for all the suggestions,

> Or how about a large power kite?

That would be easy to install and silent. There is even some example here: Wingit Kite-Boat-Systems - Kite-Sailing with a 56ft motor boat | Facebook | By Wingit Kite-Boat-Systems | Check out the latest video of a nice cruise down wind with a 56ft, 25 tons Motor boat. Kite:11sqm, Wind: 10-12knt, Boat Speed: 4,5knt. Great performance... https://www.facebook.com/kiteboatsystems/videos/kite-sailing-with-a-56ft-motor-boat/1369244829753131/

However: they are cruising at 4.1 kn and with the wind on their back. If I calculate the power needed to move that boot at 4.1 kn its something around 10kW. Maybe get a larger kite ?

EDIT: are the propeller free-spinning in this case ? Or just dragging.

 

Planing hulls are not very efficient or comfortable at displacement speed, so you can forget powering them with existing generators. They will always use more power then the equivalent size displacement hull.
Installing a third engine is primarily a function of space. Removing the existing engines will destroy whatever value the boat still holds. I see two realistic options:
1. Derate one engine to the manufacturers minimum, change transmission and prop. This will give you a single engined boat with offset propeller, and no ability to plane.
2. Install a third shaft and go diesel electric using one of the existing engines. This will retain the ability to plane, but is a much more involved operation.

In the end it's a question of money. Whatever you do it's going to cost a minimum 50-100k€ and a lot of time, so the question is why not take that money, add it to the original purchase price and buy a displacement boat instead. The only reason I see is that there are few displacement boats finished to the standard you like in that price bracket.

 

> The only reason I see is that there are few displacement boats finished to the standard

The other reason was still retaining the ability of doing 20 kn when the weather suddendly changes, I had a trawler (50') for many years and the crawling back to port 3 hours for covering 15 miles when the weather suddenly changed is not something Im keen to repeat.

I also have seen things like this Auxiliary Propulsion Unit - WESMAR - Western Marine Electronics https://www.wesmar.com/commercial-auxiliary-propulsion-units , looks something "exotic" (and probably priced accordingly).

Maybe there is simply no pratical (economical) solution at the end.

 

In the end "economical" is subjective, 90k€ is the projected fuel cost for 1 year, spending 100k€ outfront is not outrageous. However you approach it the conversion it's not a cheap project, if it would cost 10k€ the boats would not be for sale. The material price itself is not all, you need to budget a NA for the planning, instalation costs, etc.

To retain the ability to plane a third prop is a must. As said I prefer the electric option, the technology can be had cheaply from industrial or light rail applications, there are plenty of vendors.
How to do it all depends on the available space, straight shaft, Z drive, I/O drive, etc. Then it's adding another engine, exchanging the exiting generators for much bigger ones or fitting a generating head onto one of the main engines.

 

Its not just the fuel cost, the cost of maintenance on the big engines is still very substantial and usually would have to be hired out because of the size and weight of parts etc. They say the cost of the hull itself is something like 25% of the cost of a finished boat. The attraction of these boats is their exquisitely fitted out interiors etc. Whereas most passengers would have no interest walking around the engine room. I think removing and selling the large engines would likely cover the cost of fitting a smaller engine and if the worst features of the hull were improved by adding to the rear of the hull (not terribly complex and most of the work is on the outside of the hull) one could potentially end up with a vessel that is much less expensive to operate. On the other hand, some of the fundamental characteristics of the original vessel would not be changed (potentially high profile, high windage, behavior in unfavorable seas).

Trying to add to the installed equipment would mean loading the vessel even more, not being able to recover any funding from selling the original high power engines and still being stuck with the high maintenance cost of the original equipment. Seems an awful final product.

 

Many planing type hulls do operate with good economy at below”hump” speeds.
Short, fat hulls are less likely to be satisfactory, and wander incessantly at low speeds, but not those in the size range being discussed here.
Derating the motors is cheapest option, or simply blocking the travel of the throttles.
Anything beyond that is going to cost big money, and will probably hurt resale value.

 

Boats with 2000hp installed are already virtually worthless when fuel costs as much as it currently does. So the resale argument is way off base. Its just the same as what happens to cars that use 8-10mpg in the same market. So one can buy one of these boats for way under true value for pennies on the $ and so long as you can recover money from selling the original running gear to cover the cost of a smaller single engine install, you can end up with a much more usable asset with reduced maintenance cost also. If the new engine is sized to get you to 80% of theoretical hull speed you will achieve the best efficiency.

 

Buy 2. Cut off the forward halves and fasten them together. You now have a displacement hull and you can install a small engine. ;-)

For going upwind against a storm, it may be worthwhile to gear down that small engine and use the largest prop that will fit.