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Old 12-08-2018, 12:30 PM
LordGothington LordGothington is offline
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Atomic 4 -- generator conversion

Ahoy!

Like most sailboats, the engine in my Tartan 27 is designed to provide a lot of propulsion and a little bit of electricity.

But a vast majority of the time, I only use the engine to motor in and out of the harbor. But I do want to have a lot of electricity for things like radar, an induction stove, refrigeration, computers, maybe even HVAC or a dive compressor, etc.

There ain't room for an engine *and* a generator on this boat. So, it is tempting to go diesel electric. Install an electric motor for propulsion, and a diesel generator (plus some solar/wind) for generating electricity. For long distance motoring that is clearly less efficient. However I do not usually motor long distances, but I always want a lot of electricity. And, unlike pure solar electric boats, I still have the option to motor long distances by running the generator.

But that is a pretty major overhaul to commit to in terms of time and money -- especially since it is unproven. That has got me wondering if I can do a quick and dirty trial run.

Here is my thought.

Disconnect the drive shaft from the A4 and install a 5-10KW motor to drive the propeller. I believe I have enough room to do this with out moving anything -- I just need a shorter propeller shaft. I would, of course, need to add mounts for the electric motor.

Install a big 100-200A 48V alternator using the hanley method,

http://www.moyermarineforum.com/foru...2&postcount=12

Make sure the transmission is 'locked' in neutral. (I am assuming the flywheel spins whether or not the transmission is in gear). Disconnect the existing transmission gear shifter and convert it to control the electric motor.

Install a 48V LiFePO4 battery bank and charge controller. One advantage of LiFePO4 batteries is that you can go from fully discharged to fully charged in an hour. (Lead Acid batteries prefer a much longer charge time). So, the engine only needs a short run time to top off the batteries.

The advantage of this hack job is that it is trivial to undo. If it turns out I it is a terrible idea, I can reverse the changes and sell off the alternator and electric motor. I'd probably keep the 48V battery bank, since I'll still be power hungry. And I'd probably just install a smaller 48V alternator.

If it mostly works great but the A4 isn't really up to the job, I can fully commit by replacing the A4 with a proper diesel generator. This also allows me to break up the costs of two time periods. A high output alternator is a lot cheaper than a whole generator. And quicker to install despite the custom mods required to the A4.

Another Tartan 27 owner did do an electric motor conversion and used a 5KW electric motor. They claimed it was strong enough, but that seems underpowered to me. Later models of the Tartan 27 used a 12HP diesel instead of the 30HP atomic 4. So I think 10KW is probably the upper limit of what is required.

In a perfect world, a 200A 48V alternator would require (200A * 48V) / (750 W/HP) = 12.8 HP to drive it. So that A4 should be plenty. But I am wondering what a real world limit would be. Keeping in mind that the A4 is not *also* (directly) driving the propeller anymore.

What else am I overlooking? It is a bad idea to run the A4 for hours on end with out engaging the transmission?

Does this seem like a viable experiment? Or am I overlooking a huge glaring flaw? Has someone already tried this?

(The battery system will be hella expensive, but let's assume I am going to end up buying that either way).

Also, I just want to reiterate that I am fully aware that when motoring this will absolutely be less efficient than a direct drive system. I am willing to make that sacrifice because I want to have a generator on board but don't want to install a second internal combustion engine. And, a majority of the time, motoring will just use the batteries and be topped off using solar or shore power. I am also aware that this will be less efficient than a true generator -- but this is just a stepping stone to that.

Another shortcoming is that a true generator will have a feedback loop that adjusts the throttle to match the current load. In this hack job, I'd have to control the throttle manually. While annoying, I don't think it is a deal breaker. The batteries will make up for any temporary shortcoming. After all, in theory, everything can be powered off the battery bank alone. Though, running the HVAC while motoring is probably a bit excessive.

Last edited by LordGothington; 12-08-2018 at 12:34 PM.
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