Diesel/Gas hybrid - a challenge to designers

Hello guys - I have a bit of a challenge for you technical folks to chew on:

I have a 42 foot performance cruiser that uses big gas engines to run at speeds up 50mph which is a feature a like and want tobe able to use whenever the urge hits me.

I also live in south Florida and enjoy long slow hull speed cruises when the wife and I are on vacation and not in any particular rush - like a saiboat with less work. Autopilot on and quiet solitude is the goal and frankly having one or two Big Block 502s growling, sucking fuel at idle speeds for hours on end makes no sense......I would like to find a small quiet diesel or an electric to run off my genny to provide the "limp along" propulsion but HOW DO I GET IT TO THE PROPS?

I have motors connected to Borg Warner gears and four foot U-jointed shafts going to arneson drives which attach to the transom (of course). Shafts are parallel and about 4-inches in diameter.

ANY IDEAS?

 

The cause of high consumption are the gear box ratios and the prop size. And ultimately of course your "urge".
A more efficient small engine could only marginally improve consumption.

A theoretical solution would be a 2-speed gear box....

 

Gas engines are far better than diesels at operating at a very small percentage of rated power, so your 502's are not too bad.

Why not bolt a Honda or other 4 stroke outboard to the transom?

You could borrow a couple and see what cruises your boat , at the speed you prefer at say 75% throttle. A guess a 50hp would do fine.

FF

 

An idea I have seem on some big boats. Install a jet drive between big motors. Something like the motor from a jet ski. The 50 hp should move you along. You pull up your arnesons. You can get a used jetski for little money.

Another idea that I actually did was install a electric trolling motor. I had a 27' speedboat with twin 320hp inboards. This boat would do 65knots, unfortunately it would idle with one engine at 10knots. It was a nightmare to dock or even travel down canal behing my house, to say nothing of the noise. The remote control trolling motor helped in docking and going down canal at a manatee safe 5mph.

 

I was thinking of taking out my 3-foot long u-joint drive shafts and having a shop add multi-belt pulley in the center of each and balance them.

then I could put an electric motor (belt attached) to each shaft and have them driven off of batteries and genny.

this would leave my steering as is- use the same props, and allow me to still do forward and reverse individually for each prop for proper handling in tight spaces.

Ideally I could get enough genny to drive big enough electric motors to get hull speed.

Current genny is 6.5KW gas but I could find a much higher KW diesel genny to fit in the same space (a rebuild) if needed.

So questions for the designers:
1) How much HP or thrust needed to move a 16,000 lb 42 foot long, 10 foot beam cigarette-like boat hull at hull speed?

2) What is hull speed for this vessel?

3) What is the conversion from gas HP to electric motor HP? I know HP is defined as "work" but I need to figure out how many KW I would need so I can size the genny right.

Thoughts?

 

no additions or comments???? Comon guys - think outside the box a little and help me out ;-)

Swamp

 

Hey Swamp,

A 42 footer.... say 35' on the waterline.... that's a D/L ratio of about 167 when she's in displacement mode. If she were a lightweight, displacement hull type she'd probably run up against the "hull speed" problem around a speed/length ratio of 1.6 to 1.7. But being a planing shape she'll probably start to lift her bow when you get above S/L of 1.0 or so, which on a 35' waterline would be about six knots. You've probably already noticed that this type of boat runs in poor trim and with terrible fuel economy between about 8-10 kts and whatever speed puts it fully on plane. So I wouldn't count on getting much more than 6-8 knots with your "auxiliary" motor.

A 16,000 lb boat should be able to achieve S/L 1.0, ie. 6 knots in your case, with about 15 hp, maybe 8 knots with double that. Your twin 502s are probably barely above idle at this speed- they'll run happily here, unlike an underloaded diesel, but the specific fuel consumption will probably be horrible.

I doubt you'd realize much in the way of fuel savings with a hybrid electric drive such as you describe. These are at their best when you have high house loads and hugely varying propulsion loads- such as on a cruise boat, where half the power goes into house loads, and you might be operating the drive pods at 4 knots one night, and 20 knots the next. These ships have several diesels, and will run with one, two, or maybe six engines pumping into the main power bus depending on how much power is needed at the time. Thus, the engines are always near optimum load. In a boat like yours, this same effect could be achieved with much less expense using mechanical means alone.

I would say that your cheapest, most reliable option would be Fred's idea from post #3: Bolt an outboard bracket to your transom, and slap a 4-stroke "bigfoot" or "high-thrust" series outboard on it. Power tilt on the O/B is of course necessary, you'd want to lift it clear of the water when using your main engines. I would guess, based on the estimates above, that a 25hp to 40hp unit would do the job fairly well (thanks to the planing shape and huge props, this thing will have substantially higher drag at low speed than a comparable pure-displacement hull). And this would work with your existing gasoline tanks- no need for two separate fuel systems (which would of course get mixed up by the gas dock staff, leading to one or both engines getting destroyed). Your rudders would continue to work normally with the O/B running unless they're power-hydraulic, but for extra turning ability you could tie the O/B's steering system in with them. The outboard might be $6000 new, less than half that on the used market, add another grand for a bracket and rigging, and I think the complete setup would probably be cheaper than the genset alone would be if you were to try to go hybrid-electric.

 

great response! Thanks for taking the time

 

how about a couple of Torquidos
electric 17 hp outboards

would run off your genset ( depending ) and be quiet

B

 

Torquidos? I'll have to do a search on that one.

 

Masali has a handle on them

 

another thought - the arneson surface drives are already in the water - why not use them for propulsion.....small motors turning the shafts (in engine room) seems rather eloquent to me. I need to do the math and see if I can get even 5 mph out of her this way with reasonable electrical output.

 

Swamplizard.
Your idea of electric motors on the existing drive shafts may seem elegant until you consider the ratios required to drive your big props. The electric motors need to run at very high revs, the shafts very slowly to allow the drive torque to be adequate. Now what happens when you start your gas engines to go for a blast? The electic engines need to be disengaged and the belts removed as they are now revving their little hearts out and probably destroyed!
We build little RIBs from 4 to 10 metres - see www.explorermarine.co.uk - and we can be surprised what a small auxilliary outboard - petrol or electric - will do.
That would certainly be my suggested route as the engineering is simple and you can even run for short periods without your genset started.
Good boating
HM

 

Outboard is the way to go. Lightweight and simple!

Too bad Yamaha doesn't make the high thrust 9.9 with the giant prop anymore

 

Outboards are usually hung on an adaptor and left there. They get swamped on the stern of big boats.

How about a Third shaft with a Sailboat prop. One of those centrifugal blades that only go out when the shaft is spinning.
Operate that shaft with your electric generator/motor.
It's only for puttering along anyway, there are some pretty High Output Electric motors available.

I dont like the idea at all, but I think it'd work.