# Multiple BLDC per shaft for marine propulsion

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1. Apr 23, 2015

### Dostetres

New to the forum, and can't find this info or examples on the web. I'm interested in the possibility of coupling multiple DC brushless motors to a single drive shaft for main propulsion on a 45-50', twin screw boat. Lots of sail boat examples are given, but my interest is in a power trawler-style boat. Twin diesels of 150-250hp are standard on these vessels, but I want to consider using multiple smaller electric motors, say 3 x 50 kW motors per shaft. The motors would need to be assembled linear along the shaft to avoid major retrofit of the boat. My interest is in redundant systems, ability to carry spar parts, ease of installation, etc. Rather than contracting an engineer to design it, are there any gaping holes in my plan?

Thanks!

2. Apr 23, 2015

### SteamKing

Staff Emeritus
It's not clear how you plan to provide power for 6x50 kW motors. Batteries?

Unless you have a friend in the industry, these motors aren't cheap.

Most motors are not set up to permit multiple units on the same shaft, and the shaft size for a single 50 KW motor is probably too small for three times the power input.

Some of these motors operate at very high RPM (7,000 depending on model), so you can plan on throwing in a couple of reduction gears as well (which also aren't cheap).

3. Apr 23, 2015

### OldEngr63

This sort of thing is done in the Navy using specially designed motors at extremely high capital cost. The worst part, as SteamKing has already suggested is where are you going to get the electrical power? The US Navy is currently wasting billions of dollars trying to design all electric warships (I worked on this boondoggle!). The weight and volume of the electrical power source plus the heat generated is killing them, but hey ... its only tax money!!

4. Apr 23, 2015

### Dostetres

Steam King, Yes, LiFePO4 battery banks, with diesel gen-sets. I'm looking at older (but proven and well designed) hulls, so engines will need to be torn own anyway. With the savings on the older hull, I'll have $60,000 to$80,000 for renovations. It sounds like I would need to have multiple motors driving a transfer case, due to shaft size, agree? Would the transfer case act as the reduction?

Thanks!!

5. Apr 23, 2015

### Dostetres

Thanks OldEngr63, LiFePO4 battery banks, with gen-sets. The hull speed on these trawlers is 7-8 knots, thus the lower hp diesels. The research I've seen utilizes the torque of the electrics as an advantage by increasing prop size. The result is a lower kW requirement as compared to direct drive diesel. There are propulsion units available with up to 100kW motors, and all have rave reviews from their captains. I guess my questions end up with "will this work" as a primary propulsion system? Thanks for the input.

Dos

6. Apr 24, 2015

### SteamKing

Staff Emeritus
It's not clear that a transfer case will also serve as a reduction gear. The internal gear ratio could be 1:1.

Let's see: Diesel generators, battery banks, electric motors, switchboards, gearboxes, etc.; that's a lot of stuff to manage in one small hull. Were you planning on doing anything else on the boat besides keeping the machinery running? We haven't even discussed if the internal arrangement of the boat will permit the installation of all this equipment, building new foundations to support it, connecting the auxiliary systems (cooling, fuel, lube, electrical, etc.)

All of these parts are not going together on their own. Without someone who is able to look at what you wish to do realistically, I fear you are going to sink a lot of money into this project and not get a functioning vessel in return. Doing this without the help of an engineer just increases the odds you'll sink your money into buying all the parts and then not be able to get everything to work, assuming that possibility even exists.

By some miracle, should all of the machinery go together without a hitch, there is still the matter of the stability of the vessel to consider. This is a much heavier installation than just a couple of diesels, and it's not clear that you would be able to do anything with your boat except carry this machinery around (with how much fuel?) and pray you don't encounter any problems.

In the OP, you said you wanted to avoid a major retrofit of your boat and you wanted ease of installation. I don't see either of these things happening, unfortunately.

7. Apr 24, 2015

### Baluncore

Yes. Motor speed reduction and a cost of over US\$100,000 for the conversion
If possible stick with two diesel engines, each driving a propeller shaft.
Why do you need electric drive?

8. Apr 24, 2015

### Dostetres

Exactly the reality check I need. Thanks for the time you spent on this!