Bow Thruster Cables

[Mat-C]

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...?

[Stumble]

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.

[Submarine Tom]

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

[CDK]

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.

[Bglad]

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.

[Stumble]

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.

[Submarine Tom]

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

-Tom

[CDK]

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.

[CDK]

I will back-up CDK's comments (as if he needs it:)

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.

[Landlubber]

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....

[Mat-C]

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

[pistnbroke]

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 ?

[Mat-C]

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

[pistnbroke]

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

[mydauphin]

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.

[Bglad]

Mat-C is there any way to shorten the cable run by rerouting them? When I looked at the Lewmar installation information they indicated 1/0 was good for a 50' run. You wouldn't need to find much to get there...

[Mat-C]

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.

The thruster (and windlass) run off the house battery bank. There is a separate battery for the engine. Though having said that, I would usually only operate the windlass (and thus thruster) with the switch to 'both' so that the engine is covering at least some of the discharge.

Bglad - no I can't shorten things up - at least not with the existing set-up...

[Landlubber]

"I have fixed many corrode/undersized cables."

...I'll second that, and also had to do a febuild under the fwd berth when it all caught fire on one boat too.......

...if the boat is so small that a battery is a problem, why have a thruster anyhow....

[Submarine Tom]

That is an excellent question.

If you really need a thruster, then you had best install it properly.

If it's not worth the money to do the job right then perhaps you don't really need it.

-Tom

[Scott Carter]

Adding a second smaller cable in parallel to an existing under-sized cable is a poor approach to solving this problem. If you opt to run a new conductor (two actually, + and -) then why not size them appropriately instead of band-aiding? The cost will be negligible when compared to raising the charred carcass from the bottom.
An alternative to installing an additional battery at the bow in the near vicinity of the thruster might be to add it somewhere along the way, the distance to the thruster thus being reduced to a length appropriate for the existing 1/0 cables.

[Submarine Tom]

"If you think safety is expensive, try having an accident (fire)."

[FAST FRED]

Lets get real, when using a bow thruster you are not playing with 12V, most likely the engine will be operating so the voltage will start at 14.4V.

Do the voltage drop math with 14.4V as the start and see how much better the numbers come out.

FF

[mydauphin]

Lets get real, when using a bow thruster you are not playing with 12V, most likely the engine will be operating so the voltage will start at 14.4V.

Do the voltage drop math with 14.4V as the start and see how much better the numbers come out.

FF

But when you overuse the thruster a couple of times and everything is not perfect, then your back to 12 volts or even below and the whole thing overheats and catches on fire....

Murphy is really a bitch...

[Submarine Tom]

For the run times allowed by the manufacturer (they are rediculously low) are they really of benefit?

Put the dock on your leeward side if you're landing and the whole boat gets pushed in!

Back in and use your prop-walk to your advantage.

Nose in, tie a bow spring line and rotate around it onto the dock.

I'll ask one final time, is a thruster really necessary?

If so, then size and install it properly.

Murphy is a bitch, and an opportunist!

-Tom

[mydauphin]

A thruster for most people is a crutch and a marketing ploy. On a small boat they are really not necessary, and honestly useless because if current or wind is so strong that you can't dock it won't help you either. Thruster make sense on much larger vessels like 45' and above and those should have what it takes to make it work properly.

Take classes on proper operation of a motorboat.

[Submarine Tom]

Agreed. Look at a cruise liner. Three of the biggest bow thrusters I've ever seen but no need for a tug boat assist anymore.

-Tom

[Scott Carter]

Back to the point of the thread:
Relying on alternator power alone as the power source for a thruster isn't generally (in most applications) advisable. What would be the point of wiring through the house battery at all, then? Assuming the alternator is sized appropriately for this load (which very well may not be the case) what happens on that occasion when the thruster is used on batteries alone? Even worse, on batteries with a less than fresh charge? Contingencies need to be planned, and needing to use a thruster without the engine running is not far fetched. Even if it were, prudence requires that any reasonably likely scenario be planned for.

[Mat-C]

Ok - let me say from the outset, that if the larger cables are required, that's what I'd fit. I'm not into short-cuts or dodgy installations...

2nd - the batteries that I have installed are twice the required size as specified by the manufacturer. So, whilst I would generally only operate the thruster whilst the engine is running (just as I do with my windlass), if the need ever arose to do so, I could

3rd - I consider myself an experienced boat operator, with over 40 years operating all manner of boats. I can and do do all the things you suggest to get by without the thruster. However, the boat in question has quite high windage and very little draft. I would challenge just about anybody to dock it on a windy day without embarrasing consequences. Further, my marina berth doesn't allow me dock on the other side of the pier. On a number of occaisions, I've done the prudent thing and tied up elsewhere until the breeze has dropped out and come back later.
If a thruster is a viable option, then why not use one?

[Willallison]

I'd like to back Mat-C up on this one. The notion that if you can't easily berth your boat without thrusters regardless of the situation, then you aren't a competent operator, is just plain nonsense. Otherwise those ships that were mentioned wouldn't need them either....
Yes a sterndrive allows you to vector thrust, giving greater control over the back end of proceedings. But it provides precious little help for the pointy end and if there is little in the way of lateral resistance then the boat can be quite a handful.

Back to the question at hand, and I'm with the others here: stick rigidly to the manufacturers instructions. Or at least get their approval in writing for any variations. The cost of arguing with your insurance company will far outweigh the cost of new cables....

[Landlubber]

..there are many boats that are "top heavy"...massive windage, and some then even have stern drives...an accident waiting to happen....yes, the bow thruster is well worth the effort under the circumstances.....again,, why not just use one of theose batteries and stick it under the berth with the motor.

[CDK]

Mat-C's question was whether or not he could use the existing AWG 0 cable, not if it would be wiser to abandon the idea, how to moor without a thruster etc.

I calculated a 1.5 V drop for 100 ft. of AWG 0 in post #4.
The actual length is 65 ft., so the drop will be 0.975 V; should AWG 00 be used, the voltage drop would be 0.78 V, all based on 150 A being drawn by the thruster.

To assess the danger of overheating, we must look at the dissipation in the cable.
That is 146 W for an AWG 0 cable, or approx. 2.25 W per foot (1.8 W for AWG 00).
It is safe to say that in both cases this is a very light load and the cable will not warm up noticeably during normal use of the thruster. It would take hours of continuous use for the rise in temperature to become significant.

The dissipation in the thruster motor is 1650 W: prolonged use can and will quickly lead to overheating there. For that reason the manufacturers state a duty cycle of max. 5% and a maximum duration of just a few minutes.

[Submarine Tom]

Okay, we now have some information about if it is really necessary and it appears to be.

Is it sized properly for the job (windage)? Lets hope so.

I agree with CDK's logic. Go for it.

-Tom

[FAST FRED]

A 6-8 HP outboard doesn't weigh much make up a bow bracket and simply plop it in the water 90deg to the keel,before docking.

FWD is Right , Reverse is Left . Electric start or pull , your choice.

IF the boat is a real nightmare a 40hp could be used.

FF

[Submarine Tom]

That's what I've done on the bow of my houseboat. A Merc 3.5 with built in fuel tank simple installed "backwards" on the bow. I never use reverse, just position it as necessary and work it against the Merc BigFoot 25 on the stern. I have a bow steering station by rope and tiller for the transon mounted 25. It works great and it's nice to have a spare motor onboard too.

-Tom

[Landlubber]

Fast Fred and Sub Tom...you guys need a life fellas....I am surprised that you do not start the motors with a crank handle....

[mydauphin]

Fast Fred and Sub Tom...you guys need a wife ?
Does she operate bow outboard when docking?
:P

[Submarine Tom]

"Happy wife, happy life"

-Tom

[WickedGood]

An old rope pull outboard is all that is really required to more your boat.

Actually a true Sailor would not need an engine at all.

The Salty method is to develop as much headway as possible and then unleash your BoomVang and push the Boom into the wind oppisate of the direction that you wish to sideslip. Be sure to hard over your rudder anticlockwise and dogg off the cleat.

Try this method in a crowded commercial harbor for best learning satisfaction. Other Yachtsman and Tug Captain will let you know if you have a learning curve to overcom.

http://www.deletetheweb.com/unstuck/archives/bath-444.jpg