T-Pain Is On A Boat: Re: [Electric Boats] Intro

Sunday, December 29, 2013

Re: [Electric Boats] Intro

"I'm contemplating building a small DC generator instead of an AC charger run off a gas generator."

Mornin' Bob. Here's some electrical "how it works" that might help with your decision on what charger to use.

All rotating generators work by initially generating an AC wave. There really isn't anything that rotates that is truly generating DC. The difference between a "DC generator" and an "AC generator is simply in how the AC wave that both make is rectified into DC that can be used to charge a battery.

The "DC generators" are an older technology and were designed back when solid state rectifying diodes were inefficient and expensive. So the designers used geometry instead....they made a slotted rotating commutator that was oriented relative to the field coils so that each brush riding on that commutator would pick up only the positive or the negative side of the AC wave. In effect, they were using geometry and mechanics to only pick off the part of the AC wave that was wanted....very clever way to get around the need for an outboard rectifier. Those old "DC generators" put out a useful - but pulsating - DC current.

Then along came good small affordable rectifier diodes. In fact, a stack of diodes was small enough to be mounted right to or even inside the rotating generator. So that meant that the commutator no longer needed to have slots in it. The brush could ride right on a continuous unslotted commutator, pick up ALL of the AC wave instead of just the best parts of it, and leave the conversion to DC to the rectifier diodes. Much more efficient to use all the wave and getting rid of those slots meant the brushes could last longer too. This type of rotating generator became known as an "alternator". It's better in all respects for anywhere batteries are used for storage.

Bottom line: You'll do better with the modern "AC generator" - also called an alternator. It's hard to beat that Honda. If you can, get it with 110vac out as well as 12vdc battery voltage out. There are lots of uses for 110v on a boat.

That other question:

"Has anyone tried to install a sheave on the prop shaft so that it turns an alternator when in EP mode to help recharge the battery bank?"

Doing that simply won't work. I mean....it's true that hookup will charge the battery somewhat, but it won't charge the battery as much as is drained from the battery doing the extra work of turning the alternator.

If it worked, it would violate the whole idea of conservation of energy. If it worked that way, so would perpetual motion machines..... And they don't.

In a perfect world the alternator rotation and battery charging would balance equally. In our imperfect world there are always small frictional and resistive losses to keep things from coming out equal.

Bottom line:

No advantage whatsoever, and some disadvantages.

Good luck with the conversion. Building a boat is huge. While it's all coming together, sticking a gas or diesel in there isn't a bad option at all.

luck, Roger L. - 28 foot sailboat, F28cc

----- Original Message -----

From: bob jennings

To: electricboats@yahoogroups.com

Sent: Saturday, December 28, 2013 9:51 AM

Subject: [Electric Boats] Intro

Hi, new member here.

EP for recreational boats is a pretty interesting concept.

I have a 28' project sailboat & want to repower the old Volvo before I put her in the water. I've pretty much decided to go the electric route. There is a lot of information out there & decided to join the group before undertaking my science experiment. I am leaning towards the 5KW electric yacht system.

My boat will be on a mooring so I'm contemplating building a small DC generator instead of an AC charger run off a gas generator. Seems many that go this route have a slip with shore power or use the Honda 2000 as part of their hybrid but from what I've read they have to run her for 3-4 hours when recharging the battery bank due to amp limitations of the generator. Coming from the power boat crowd I don't mind a little noise but up here in the northeast the sailors go nuts when you run a generator in an anchorage....

Is a DC generator fueled by gas or diesel a viable solution for quickly recharging a bank compared to an AC charger & Honda generator?

Another thought I had. Has anyone tried to install a sheave on the prop shaft so that it turns an alternator when in EP mode to help recharge the battery bank? I wasn't sure if the addition of a sheave & belt put so much additional torque on the electric motor that it was counter productive.

Thanks & Happy New Year!

Bob Jennings