Quote:
Originally Posted by LargeMarge
This may sound wacky... but hear me.
If your project has a genset and batteries, could your outboards be electric?
As I understand the electric outboard concept, the engineers use common lower-legs from Yamaha, Honda, etc.
The power head consists of an enclosed motor with external cooling, in most cases, a radiator or similar heat-exchange.
This might sound nutty... but this's the way we are headed.
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An electric outboard is indeed viable. However, it is not efficient. Using a diesel engine to turn a
generator to produce electricity to charge a battery to power an
electric motor to turn a propellor can never be as efficient as an optimized setup of diesel engine mechanically driving a propellor. Never. Can you come close? Sorta. Especially when you need to operate at extremely
low power levels at times, and extremely high levels of power at other times, and you factor in the wear and tear on the engine or the need for a variable ratio
gearbox and possibly controllable
pitch prop. So when extreme maneuvering is the order of the day every day, straight diesel electric can be the way to go. This is why there are so many diesel electric
docking assist tugs, dredges, ferries,
cruise ships, (most no longer
anchor... they use DP to maintain position when
docking facilities are unsuitable and passengers must be tendered ashore) and the like. For steady state cruising, it is on the order of twice as efficient to simply couple the appropriate diesel to a prop shaft turning the appropriate prop via the appropriate reduction
gear.
Where you can still operate more economically in spite of all the energy conversion losses is when you get "free" energy from the sun or
wind or the boat's motion through the
water. If you are running a 10kw motor at a 2kw power level and have 2kw of
solar charging capability, you can reduce reliance on the diesel genset considerably or even eliminate it completely if you do not motor for long periods. Also, if you only make day trips you can charge via
shore power more cheaply than
charging with the genset.
For extremely short operation such as docking/undocking and power tacking, efficiency is not the issue anyway. And in this scenario, the freedom from
buying fuel, liability of spilling it in the water, cost of upkeeping a diesel engine including perfectly normal wear, the quiet operation of electric, not having a minimum idle speed, instant on and instant torque, ease of
maintenance and customization, all point a favoring finger toward electric, even if it is an electric outboard, a definite step downward from a proper electric inboard
installation.
The ordinary 12v bank is not what you want for electric drive, though. For small boats you want the highest voltage without going over 50V where there are different
electrical standards due to over 50V being arbitrarily considered lethal and under 50V non lethal voltage. If 48V is not enough then smart
money goes straight to the 144 to 300V range. Most boats under 35' or so go with 48v and there is nothing to be gained from using lower voltages except trouble. Higher voltage systems are more efficient. A 48v system operated at 5kw requires
current of about 100A. A 12V system at the same power level has a
current of 400A. Line losses vary with the square of the current. Every wire or cable has resistance. Cable over about 2/0 can be very difficult to work with, and you can't increase the size of the wire in the motor windings, anyway. So you can see how much energy is simply wasted as heat in a low voltage system compared to a higher voltage system. Also let's hypothesize a voltage drop of 2V. This is of little consequence in a 48V system, but is VERY significant in a 12V system. So, the smart
money wanting to remain within the "non lethal"
electrical codes and standards goes 48v and would never even consider 12V except for a toy.
Electric inboards are fairly simple to set up, and can be inexpensive for a
small boat. A 5kw system can be DIY'ed by a knowledgeable person for around $2k using all brand new components, exclusive of batteries. A 10.5kwhr bank can be put together out of 8 6V golf cart batteries bought at Sams Club for $85/each. Add
charger for mains charging,
inverter for 110VAC
appliances, DC/DC converters to eliminate the 12V bank, and you are still in the
game pretty cheap. A pair of 300w
solar panels and the appropriate
MPPT charge controller is a smart addition, and a good base on which to build a bigger
solar capability as funds allow.
For the average person who really doesn't feel like
learning stuff or is easily intimidated by tech stuff, it makes sense to just
rebuild the old diesel, or plop in a new Beta diesel, or hang an outboard on the stern. Alternately pick up a used running diesel but too often you are just inheriting someone else's problem basket when you do that. But I saw a 3cyl Universal diesel of indeterminate age and provenance go for $800 last year, after a run demo bolted down to a
bed of 2x12 and railroad ties. It ran, not much smoke, pumped water and didn't leak
oil or fuel. It's not unheard of to find a salvaged diesel that is cheap enough that you don't really care if you get 6000 more hours out of it or 600 more hours. Swapping a small diesel really is not that big a technical challenge. The result will be a lot more useful under varied conditions than an outboard.
BUT, an outboard on a transom bracket has a certain Redneck simplicity and appeal. It is right THERE. You can take it off and use it to push your
dinghy around. Remove it to take to a shop. Swap it out for a new or newer one in minutes. Depending on your usage profile, the reduced efficiency might not be significant at all. You can get a decent one on
sale for $1500 or so, a pawn shop special for $200. (be sure to get a long shaft!) And if you remove the old diesel and put nothing in its place, you have a lot more room below.
A clever shade tree
engineer can build an electric outboard. Most such projects involve simply swapping a busted power head out for an
electric motor. Turnkey outboards are mostly woefully overpriced IMHO.