Bow Thruster Cables

I'm thinking of installing one of Lewmar's smaller bow thrusters to my boat. It's a 2kW, 12v unit.
Lewmar suggest that the required cable size for the length run that I have (65' ) is size 00.
Unfortunately, the existing cables that run to my windlass are size 0.
I was wondering what would be the outcome of just running the existing cabling...?

 

Matt,

The easy answer is overheating, fire hazard, voltage drop, couldn't pass survey, if it caught fire insurance could refuse the claim (and win), ect...

The long answer... Go read the effects of under sized wiring in any application and come to your own solution. But any yard or Marine electricians would probably refuse to do the work.

 

How much are you out by, 2 feet, 10 feet, 30 feet?

Voltage drop would mean the thruster would not perform as well. Lower RPM, less thrust.

Increased resistance from undersized cables would mean potential heat build up in the cable resulting in increased resistance, resulting in heat build up, etc., etc.

Fire hazard. All the things previously mentioned.

Although, you've got the wiring already in place. Test it and take some measurements. How long and at what "throttle" setting will you run it for?

Maybe you can get away with it? Only you can make this decision of course so do your homework, but generally, it's not a good idea and I would not recommend it.

-Tom

 

The difference between AWG 0 and 00 in cross section is only 20%.
100 ft of AWG 0 has a resistance of 0.01 ohms, causing a voltage drop of 1.5 volts @ 150 Amps. I guess your boat is shorter than 100 ft., so the voltage drop will by all means be acceptable.

 

I will back-up CDK's comments (as if he needs it I ran calculations using the formula provided in ABYC (couldn't get it to upload). This type circuit is permitted 10% voltage drop. According to the formula the length of the circuit is the distance to and from the device. If yours totals 65' then 1/0 should work. If it is 130' then it needs to be 3/0!

It tried to upload it three times if you want the formula and relevant table PM me and I will try and email it to you.

 

Just as a caution here, any time you are doing something contrary to the recomedation of a manufacturer you are risking an insurance problem. If CDK says the numbers support a smaller wire I would believe him, however in the unlikely event of a fire, your insurance company will look very closely at the size of wiring and may deny a claim based on the fact that the wiring was not up to the manufacturers recomendations. Certainly you can argue that the wire used was ABYC compliant (and probably win), but this will mean you are going to hire me to fight the case for you.

Unless there is a good reason to use smaller wiring I wouldn't do it, just to avoid the potential problem. I would also be concerned about having both a bow thruster and a windlass hooked up to the same circuit unless there is some override that prevents them from being used at the same time. But I would ask CDK's advice to be sure.

 

It may bolster your insurance position if you install a switch that would only allow one unit to be used at a time.

-Tom

 

That made me think about the phrasing of Mat-C's question.
In my boat, the windlass relay is in the aft cabin against the bulkhead, close to the batteries. The cables running towards the bow are only powered when the anchor up/down switch is pressed, so can impossibly be used for anything else.
If in his case the cable is always powered, that would mean he has the relay near the windlass, probably in the chain locker. That of course is a very electricity-unfriendly location.

Just like the windlass, the bow thruster is also relay-operated, so both have a control wire going to the helm. It is possible to wire the switches in such a way that enabling one function excludes the other.

 

Sometimes it's good to know someone agrees with you.

My data comes from the 1970 edition of "Reference data for radio engineers", Howard W. Sams &Co, Inc. was the publisher.

 

You can also place a battery at the bow thruster, that may be far cheaper (and electrically better) than very expensive 00 cable. Remember to install a fuse at the battery. The charge cables for the battery need fuses at both ends (the battery is a power source and the charger is also a power source)
My amp metre reads over 450 amps on the bow thruster of a 50 footer, so be careful....and also do NOT use the thruster longer than the recommended times, sometimes it is only 4 minutes in AN HOUR......(cos the whole kebang may go kebang).

Bow thrusters are the biggest users of amps in all the boat...take care doing it right and it will be....

 

Thanks for all the replies guys - sorry I had to go away for a couple of days...

A little more info about my system.
There are two 120 AH house batteries, the cables then pass through a battery switch, then an ANL type fuse.
There is a post near the main DC panel and then the windlass has it's own circuit breaker. The cable then runs forward to the windlass's relay (located just aft of the anchor locker, and in the same compartment as the thruster will be located). All of this, of course in the +ve side of the circuit.

The total run is - just as Bglad guessed - approx 65'

I had checked it against the ABYC guidelines and the 1/0 that is installed would seem to be ok. It was the manufacturer sizing that had me worried...

My intention was to replaced the existing windlass cicuit breaker with a higher amp unit as it is only 150A, then install the 150A one between the thruster and windlass.

I had considered installing a 2nd battery, but would really prefer to avoid the additional weight

 

if the cable is only 20% undersize you dont need to replace it to comply only add another thinner cable in parallel to bring the cross sectional area up by 20% to match 00
Using 65ft cables on 12v at 2 kw is stupid anyway ....would you put your starter motor on the end of 65 ft of cable ???/..you need a battery near the thruster and a solenoid and have 65 ft of thin control cables ....how much does 65 ft of cable weigh ?

 

On the face of it, what you say makes perfect sense, but how many smaller boats do you see with separate batteries to run their windlass? I can't say I've ever seen one.
And, of course, as a planing hull, I'm trying to keep weight to a minimum.
If the extra is a requirement, running an extra smaller wire is a great idea though

 

remember though if the main thick cable were to come disconnected then the smaller one would take all the current and certainly burn if not properly fused.. a 80A fusible link on each of the thin cables at the battery end would be the way to go

 

This is why electric bow thrusters are problematic. You do want a separate battery. You can live without a bow thruster, but if if you use main battery and kill it. What are you going to use to start engine. Don't underestimate load from Thruster. When it overheats, the cable heats up and resistance can increase tremendously very quickly drawing down battery or causing at least a fuse break or fire. I have fixed many corrode/undersized cables.