Mark, this is an interesting point. I remember when the auto makers changed the HP rating method. Then there is the 'at the wheels' figure as well.
With smaller ICE engines, the difference might be significant. The alternator alone can be a big drag, especially if the battery(s) is less than fully charged.
This would also explain why what seems to be a puny electric outboard can push quite a proportionally large boat. Not being a sailor yet myself, I was surprised how easily a boat can be moved while sitting at a dock, although wind and currents have to be allowed for.
I have to assume that your maintenance costs are without battery replacement. That could place a big no-no onto a project if added in, but at the same time might not be too bad - I am totally convinced that once the ICE mindset is eliminated, then probably 99% of all sailboats would be better off with electric, because of the usage type. Even with a 'summer' lasting 6 months, not all weekends would be spent sailing.
And on a weekend basis, solar charging surely becomes more useful, and large battery banks more optional.
Looking to the future, there will no doubt be, at some point, a whole industry based on refurbishing road vehicle batteries, bringing costs down for boat applications as well.
And for many locations, the power usage would be for a few minutes only, leaving and returning to a dock or mooring. With a low percentage battery drain the battery life increases. A lot.
The exception would/might be where the boat dock location was a long trip from the cruising ground, in high traffic, maybe a river, where ICE might be better for the main distance part.
Another less thought of item is prop shaft size. The actual true (as opposed to what boat manufacturers have historically allowed for, due to massive ICE HP) thrust and torque involved surely doesn't warrant a shaft much larger than 3/4" ? This would be for a protected prop that is unlikely to hit ground. Just guessing.......So this means larger bearings that drag more, and all because of the ICE culture of overpowering boats to ensure competitive original sales.
Another YES I might add is the lack of stink and oil that has to be cleaned up from the bilges.
Having said that, I remain convinced that a unit that was, say, an 8 or 10HP diesel with built-in 5 or 6 "HP" electric drive, that was a bolt-in as a replacement (as well as within the comfort zone of original builds) would capture the sailboat market until such time as the sailing public got more comfortable with electric alone, at which time the diesel gets eliminated. Having said that, an often undiscussed situation is time. With a good electric drive, possibly small-ish battery bank, and a small ICE powered means of re-charging, there would always be a way home, the only issue being a need to start the ICE once in a while as a preventative maintenance item, as in most installations I am guessing that it is more likely to never be needed, other than for peace of mind, primarily to fulfil the ICE mindset.
John
2b. Re: Power Relationship
Posted by: "Mark n Angela" mstafford@natca.net markscottstafford
Date: Mon Jan 24, 2011 9:03 am ((PST))
Roger,
Welcome. As Steve indicated, your 6hp motor is mostly a 6hp motor,
not a 6hp propeller. Manufacturers usually measure the motor's
horsepower without gears and oil pumps and water pumps and
alternators and any other incidental engine load. So electric
repowers are usually about 1/2 the watts (horsepower), a little more
or less depending on the guessed old efficiency and the guessed or
calculated new efficiency. Propeller diameter and RPM make the
biggest difference (bigger and slower are most efficient).
The other part of your question: "is electric power more efficient?"
There are several answers:
1. YES, because there is no idling wasted energy.
2. YES, because you use exactly the power you want, when you want
(you are not confined to minimum RPMs).
3. YES, because internal combustion engines (ICE) can only harness
10% - 30% (ballpark) of the energy of the liquid fuel, and electric
ones get 70% - 90% (ballpark) harness.
4. YES, because the maintenance cost of electric is roughly one
tenth the yearly cost of ICE maintenance.
5. YES, because sailors end up sailing much more and motoring much
less (starting an ICE is problematic, starting an electric is quick).
Questions are good,
Mark Stafford
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