For conversasion sakes only; diesel economy for displacement boat

Good point Mr Efficiency. It is a vague question, the result would be building a boat that would only be good for one thing, then useless after the contest... the 10 million would soften the blow somewhat ")

 

You are mixing issues of hull efficiency and engine efficiency. BSFC applies to engine efficiency and you will see it drops at idle rpms. The most efficient way to go slow is to use a small engine and operate at moderate rpm and load (above idle).

 

You really think manufacturers would have gone to less efficient engine technology when they are obliged to do just the opposite? Modern charged common rail diesel wins economy contest like you suggested without a question.
There are other reasons to use 'vintage' tech but fuel economy is not one of them..

 

Now pretend there is a 10,000,000$ prize, for whoever can make 1000 miles, with the least amount of fuel, all crossing the finish line at the same time.

Wrong question, the price of the engine and its maint must be a factor.

If one engine uses 3 quarts less fuel than another yet requires 8 Gal at an oil change instead of 5 Quarts , that needs to be part of the deal.

A 16,000 lb boat will only require 8 HP at displacement SL 1 cruise , so a 20 or 30 Hp engine would be fine.

Our 33ft 90/90 MS has a 35 hp antique Volvo MD 3B that runs 3/4 GPH at 6.5K 6.7K depending on other loads , battery charging and mechanical cold plate refrigeration.

Diesel engines will usually cost 2X to 3X the price of a similar gas engine , and the cost of maint and repair and replacement must also be factored in.

There are many small gas industrial engines in the 40 HP range like a Wisconson that will give almost the service life of a diesel yet cost 1/4 to purchase 1/10 to maintain thru its life.

$3. 00 spark plug $75 injector rebuild.

A better concept is to ask what would my total cost of the power package be for the first decade , with a guess at hours of use per year.

The true cost of boat ownership is the ROUND TRIP , what you pay , what it sells for .

The operating expenses are part of the round trip ,and must be calculated .

Either the 2 or 4 cylinder models would be sufficient.

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I know there are some really good points here, but let's just try to keep it a simple discussion about BSFC and the like.

Big engine might have poor BSFC at idle, but what about if you stroke one to increase compression, and run around 1000 RPM? The BSFC could change dramatically increasing compression from 17:1 to 22:1 or so

Small engines might have poor BSFC at high RPM, but what about timing and different turbocharger?

 

Looks like my big, idleing diesel isn't a hit. After re-reading the original post, I might go with an engine that makes the required power at the rpm and load shown on the BSFC charts in the sweet spot. Then use standard racing mod's to insulate and lighten everything internally to last the 1000 miles required. Then add a CPP prop to keep from under/overloading. Ideally, the hull would be purpose built for the speed and loads required.

 

A CPP wouldn't be any better than a fixed pitch prop designed for the right spped though, would it?

 

Well, I honestly don't know.

I mean, a modern engine with common rail and piezo injectors can sure adjust fueling and timing well, but how would it stack against a mechanical engine perectly set up for one speed operation?

 

In general, these things contribute to high BSFC:

High compression
High pressure direct injection
Long stroke
High piston speed
Fewer pistons
Good mixing in the cylinder (swirl)
Turbo charging

 

A lot of modern diesels are lower compression, shorter stroke.
I suspect this is to reduce harshness.

Good point with fewer pistons, it seems like 1 engine will net better economy than 2 smaller engines.

 

Yes, size, weight, noise and emissions are effecting the design of engines, sometimes in ways that hurt efficiency.

 

1300 hp,22 litres,53' overall,23 tons fully loaded,30 knots top and I can ghost along at 850 rpm with an engine out of peak efficiency and props tuned for a 24 knot cruise.... yet I get about 3 mpg at 6.9 knots.

Several hundred hours of this-with occasional blasts to fight tides and currents-and my engines are in perfect condition.

BTW a friend's 85' Sunseeker gets about .75 mpg at 7 knots or so.

 

The main issue that has do be dealt with is the trade-off between BSFC and weight of the driveline. I'm assuming that the finish line weight is 16000 + drivetrain. LWL, weight, and HP are a good fit indicating an average speed about 11 knots and a race time of 91 hours and a fuel burn of around a ton.

So, you have to decide how your choice of engine options (for which BSFC is known) affects the weight and driveline efficiency of the system. For instance, a big, slow, long stroke one cyclinder, would produce a ridiculously heavy propulsion system because long stroke = high torque and the peak torque of a one cylinder is more than 15 times average torque, making the tranny and shafting ridiculous unless you absorb that energy with a big flywheel. Massive bearings in the shafting are more lossy. Bigger drag penalty in the exposed shafting and heavier prop hubs and blade roots are lossy.

So you have a rather nasty system to optimize in general, but the equations are not hard to work out for a number of specific cases. Also, since most engines are similar in performance, you can make a very good linear approximation for several different effects - Cylinder count's effect on system wt, tranny and shafting losses; compression ratio on BSFC, Wt, and losses; the effect on tstat and cooling system mods.

Turbos are hard to say anything about, but normally, improving BSFC at a single design point isn't one of their benefits, however, their effect on weight reduction and the ability to easily tailor a motor to the specific requirements means they should be looked at. It would be a big, heavy, slow turbo since throttle-turbo response is of zero importance to the task at hand. Look to prime mover genset set-ups for ideas here (not standby gensets). All in all, a small amount of turbo boost and a slightly reduced compression ratio could be useful with a first-rate EMS.

Transmission. One option is to loose the transmission entirely for the prime mover. Reverse and maneuvering would be by a separate system or a separate powerhead clutched to the shaft. Not worth it for a 1000nm, but something I was thinking about for the round-the-world sort of distance.

Auxiliaries. Getting all the auxiliaries out of the block and optimizing them separately for the specific task can yield significant gains.

Crank case pressurization can help with torque pulses thereby lowering tranny and shafting losses. I'm surprised it isn't used more often, but seals are troublesome and complicated.

If you are able to custom build the entire drive line after the engine, you can pick your favorite 3 or 4 cyclinder job and design the tranny, shafting, and prop to suit. I've often wondered if putting the tranny in the strut and hub wouldn't be better for many displacement speed craft. If you are trying to cobble it together from off-the-shelf parts, do it the other way around. Start with prop and shaft and tranny combos and get those right. Then bolt up a suitable motor and do what you can with it.

 

Transmission. One option is to loose the transmission entirely for the prime mover. Reverse and maneuvering would be by a separate system.

CPP does this with OTS parts.

Only hassle is before the engine shutdown the RPM must be increased from idle and the prop pitched to neutral. Say 800 RPM.

The reason is a cold diesel will not idle as slowly as a warm engine , so the warm up speed must be set or the boat will move.

AS CPP are usually only on boats with substantial prop diameters , just 100RPM off is a lot of power .

 

My money would be at a healthy 3lx