Installation of a 12v circuit

[Nev]

Hi,

 

We now have our first boat (Comet) back at her moorings. She is an old and basic boat and as far as I can see she has a lighting circuit and a 12v cigaret lighter socket wired in. I would like to add a 12v circuit (the boat came with a generator for 240v) for a 12v tv I have. The boat currently has 2x 6v batteries in series (I'm sure this is a limiting factor) The dropping in of a socket or two and wiring them back is within my abilities -what do I wire them back to and do I need to change the batteries for a couple of 12v ones......is it that simple?

 

The boat does not seem to have a starter battery seperate from the 'leisure' bateries (read lights) I could see this being a problem - I was advised there was a starter handle for the engine (that was not a wind up (ouch) was it? - its a lister 2cy air cooled

 

Nev

[Liam]

Sorry I can't help with the electrical side of things..

 

But the comment about your Lister having a starting handle is probably true. We had an SR2 in our old boat which had the hand starting gear on. Like you we had a very basic 12v system and were always having problems with the batteries if one us had a shower or fell asleep with the light on! Easily modifying the back deck boards enabled us to have enough space to swing the handle to start the engine ... which it did, first time every time which was a handy backup.

[alan_fincher]

But the comment about your Lister having a starting handle is probably true. We had an SR2 in our old boat which had the hand starting gear on. Like you we had a very basic 12v system and were always having problems with the batteries if one us had a shower or fell asleep with the light on! Easily modifying the back deck boards enabled us to have enough space to swing the handle to start the engine ... which it did, first time every time which was a handy backup.

Yes, absolutely !

 

But as Liam says, you need the necessary access to where it goes on, to be able to use it.

 

Also there have to be "decompression levers" on each rocker cover, which allows valves to be held open so you can actually turn the bu**er over. The idea is you throw the levers so you are not trying to compress whats in the cylinders, then try to get the engine turning at a reasonable speed. If all goes well, when you then throw the levers to reestablish the compression it should fire. (If it doesn't, try again!).

 

Not too bad on an SR2 - a lot worse on bigger older engines sometimes.

[Nev]

Yes, absolutely !

 

But as Liam says, you need the necessary access to where it goes on, to be able to use it.

 

Also there have to be "decompression levers" on each rocker cover, which allows valves to be held open so you can actually turn the bu**er over. The idea is you throw the levers so you are not trying to compress whats in the cylinders, then try to get the engine turning at a reasonable speed. If all goes well, when you then throw the levers to reestablish the compression it should fire. (If it doesn't, try again!).

 

Not too bad on an SR2 - a lot worse on bigger older engines sometimes.

 

Cheers gents for the quick response re the starting - sounds like I may need to get the technique. There are two levers on top of each cylinder head, both upright at the moment so I presume this is the default. Just got to find the starting handle and match it with the right part of the engine.

 

Nev

[Keeping Up]

Hi,

 

We now have our first boat (Comet) back at her moorings. She is an old and basic boat and as far as I can see she has a lighting circuit and a 12v cigaret lighter socket wired in. I would like to add a 12v circuit (the boat came with a generator for 240v) for a 12v tv I have. The boat currently has 2x 6v batteries in series (I'm sure this is a limiting factor) The dropping in of a socket or two and wiring them back is within my abilities -what do I wire them back to and do I need to change the batteries for a couple of 12v ones......is it that simple?

 

The boat does not seem to have a starter battery seperate from the 'leisure' bateries (read lights) I could see this being a problem - I was advised there was a starter handle for the engine (that was not a wind up (ouch) was it? - its a lister 2cy air cooled

 

Nev

If you've got two 6v batteries in series that gives you 12 volts. That would run your TV, the big problem is that TV's flatten batteries so if you have to rely on the same battery for starting you may have a problem if you haven't yet found the starting handle.

 

The other problem is that those TV's need a really good 12 volt supply. So you need to have it near to the battery unless you've got very thick wires. It just needs to be wired back to the battery, via a fuse, making sure you get the +/- the right way round; if you've got an existing fuse box (for the lights) that would be the best place.

 

Allan

 

Not too bad on an SR2 - a lot worse on bigger older engines sometimes.

Yes I remember all those times I tried starting my friend's old Lister FR3, I could just about do it on a warm day but in the winter there was no chance. I could spin it over OR throw the decompressors but never both.

[Nev]

Allan,

 

Thanks for the advice re the 12v circuit - really thick wire, what does that mean in spec terms so I can start haranging the sparkies at work for some offcuts?? I would think the tv will be about 35 feet from the batteries

 

TIA

 

Nev

[Keeping Up]

Allan,

 

Thanks for the advice re the 12v circuit - really thick wire, what does that mean in spec terms so I can start haranging the sparkies at work for some offcuts?? I would think the tv will be about 35 feet from the batteries

 

TIA

 

Nev

Nev,

 

That's quite a long run. Do you happen to know what current the TV takes? Even for a fairly small TV I'd be thinking in terms of 5 sq mm cable (that's a conductor diameter of 2.5 mm). I wonder if the other electrical people here can find their calculators to confirm (or challenge) this figure?

 

Allan

[chris w]

Nev,

 

That's quite a long run. Do you happen to know what current the TV takes? Even for a fairly small TV I'd be thinking in terms of 5 sq mm cable (that's a conductor diameter of 2.5 mm). I wonder if the other electrical people here can find their calculators to confirm (or challenge) this figure?

 

Allan

 

Assuming the following:

 

1. Total cable length = 70' (21 metres) ie: 35ft out and 35ft return

2. TV consumes 60W; typical 14" TV (therefore current draw = 5 amps)

3. Allowable volts drop on the cable = 0.3v

 

Therefore, running the maths, the minimum recommended cable cross-section = 6 sq mm. Allan's suggestion of 5 sq mm would be OK too giving a slightly higher volts drop of ~0.4v. 6 sq mm cable happens to be a standard size so may be the one to go for.

 

Chris

[blackrose]

Hi,

 

We now have our first boat (Comet) back at her moorings. She is an old and basic boat and as far as I can see she has a lighting circuit and a 12v cigaret lighter socket wired in. I would like to add a 12v circuit (the boat came with a generator for 240v) for a 12v tv I have. The boat currently has 2x 6v batteries in series (I'm sure this is a limiting factor) The dropping in of a socket or two and wiring them back is within my abilities -what do I wire them back to and do I need to change the batteries for a couple of 12v ones......is it that simple?

 

The boat does not seem to have a starter battery seperate from the 'leisure' bateries (read lights) I could see this being a problem - I was advised there was a starter handle for the engine (that was not a wind up (ouch) was it? - its a lister 2cy air cooled

 

Nev

 

Before you go to all the trouble & expense of installing a starter battery, motor, etc., try to get the technique of the hand crank.

 

I used to have a hand crank on an old Petter and the first few times I tried it I thought I was going to have a heart attack! It's basically a knack and once you get it you'll realise you don't need an electric start. You need to find the decompression lever(s), set them and then knock them over when you've got the crank going fastest (which means you need 3 hands!)

 

It's much harder in winter than in summer because your cold oil is at a higher viscosity.

[BlueStringPudding]

I want to buy some "fat cable" - 8 or 10mm diameter so I can extend the 12v electrics down one side of the boat. I need about 12m (black & red) but can't find anywhere that sells it in less than 30m or 100m reels.

 

Can anyone recommend somewhere I can buy it in this sort of thickness - pref. online (but that's not essential if there's a shop willing to post it out to me.)

 

Thanks

 

Lisa

[bottle]

Hi Lisa

 

Cannot help in where to buy it but if you do you will probably find it to be cheaper to buy a roll than an odd length.

 

You could always sell the excess on E-bay or here.

[Tim]

Cannot help in where to buy it but if you do you will probably find it to be cheaper to buy a roll than an odd length.

 

Vehicle Wiring Products Ltd, 0115 9305454 will sell you any length of just about anything.

[chris w]

Cannot help in where to buy it but if you do you will probably find it to be cheaper to buy a roll than an odd length.

 

Vehicle Wiring Products Ltd, 0115 9305454 will sell you any length of just about anything.

 

 

They also have a website with all the cables etc which can be ordered online. They are a good bunch of guys and tend to always deliver next day if you order in the morning.

 

http://www.vehicle-wiring-products.eu/VWP-...me/homepage.php

 

Chris

[Liam]

Talking about decompression levers on the rocker covers on an SR2... we had slight leak so removed the rocker cover to replace the gasket, but when it was replaced I don't think it caught the decompression thingy properly. It was a bit of a bugger to get it going over as only one cylinder was decompressed. I managed it though, wasn't as bad as I thought it would be, because as people have said, there's a knack to it and it's only a light engine anyway. On this occasion I pushed the handle forward and waited for it to bounce back which gave me the momentum to swing it round, thus starting the engine.

 

My mate has a Kelvin K3 and I've turned that over with the starting handle, but we kept it decompressed as we were only moving the oil around the engine, it didn't have a diesel supply

 

Oh, and on another occasion, I managed to hand start the JP2 which my uncle has in his Large Northwich. I'm not so sure about other engines but this one in particular... the decompression levers are at 90° to the main starting crank whatsit, so this enabled you to get some shoe laces... tie one end round the compression lever and one end round the hand crank shaft.. you've then both hands to concentrate on swinging the handle over, and when you get to the end of the shoe lace, the compression levers are pulled forward, which compresses the engine and it starts, and because the compression levers fell flat towards the hand crank shaft, the shoe lace popped off, and carried on being wrapped round the hand crank shaft so it didn't pull the levers off.

 

Yes, slightly more akward in the winter as the oil is thicker. My uncle used to get around this by starting the petrol genny in the engine room, and jumping out of the engine room and shutting the side hatches. The only problem was that when you returned to it 20 - 30 mins later to start the engine you got a lung full of petrol genny fumes

 

I even managed to start the JP2 in gear aswell... so not only was I swinging a big lump of an engine I was also swinging 30 inches of prop on the end too lol.

 

When I get my "ideal" boat, there won't be a starter motor in sight, only the hand starting gear. I think it's great, that, and speelwheel and gearshift!

 

 

Edited to ask: By the way, if you also have an SR2 I'm sure we've still got the spare starting handle lying about on the boat somewhere. I'm off to the boat shortly so I'll check.

Edited August 31, 2007 by Liam

[1066]

I want to buy some "fat cable" - 8 or 10mm diameter so I can extend the 12v electrics down one side of the boat. I need about 12m (black & red) but can't find anywhere that sells it in less than 30m or 100m reels.

 

Can anyone recommend somewhere I can buy it in this sort of thickness - pref. online (but that's not essential if there's a shop willing to post it out to me.)

 

Thanks

 

Lisa

 

Most chandlers, such as Midland etc, should be able to supply cable by the metre. I would guesstimate that 8 or 10 mm cable will cost you close to £1 per metre, tho'

[BlueStringPudding]

Checking out the VWP website (thanks for the link) there are various cables under names other than I'm used to (e.g not "Single Core" or "Twin-core zippable"). Things like "Thinwall" and "Standard" and "Battery Cable". Midland Chandlers do single core by the roll but it's more than I need and £55 a shot.

 

Please could any of you decifer the terminology on the VWP website for me? This company seems ideal as they offer cable cut to length rather than just by the reel.

 

(I'm after 8-10mm 12v cable to extend the boat electrics down the length of the boat (about 12m each of black and red))

 

Thanks!

[reg]

I want to buy some "fat cable" - 8 or 10mm diameter so I can extend the 12v electrics down one side of the boat. I need about 12m (black & red) but can't find anywhere that sells it in less than 30m or 100m reels.

 

Can anyone recommend somewhere I can buy it in this sort of thickness - pref. online (but that's not essential if there's a shop willing to post it out to me.)

 

Thanks

 

Lisa

I use OnBoard Energy in Nuneaton, very helpfull and nice people to deal with

Not to far from Stratford

 

I'm happy to recommend them to anyone

 

-Can be found here

http://www.onboardenergy.com/contactus.html

 

Hope this helps

 

Edited To give some calulations

For 70ft return run and Max 60watt Load

Key

A-Size mm sqr

B-Amps rating (Manufactures suplied data)

C-Volt Drop per Amp Per Metre (Manufactures suplied data)

D-Result

Calculations

--A-----B-------C-----------D

4.00 - 39 - 0.00471 - 0.5024628

6.00 - 50 - 0.00314 - 0.3349752 - Possible

8.50 - 63 - 0.00220 - 0.2346960 - O.K

 

2 Years Ago(only figures I have) these would of cost, rounded up, approx

£12(4mm),£16(6mm)and£22(8.50mm)

However these prices have risen sharply over the last couple of years so you may have to double or even triple them

 

Hope this helps

 

Edited to correct above table keys and descriptions as pointed out by Chris w

Edited September 3, 2007 by reg

[bottle]

Hi Blue

 

Just go for the 'standard' in the size you require.

 

I did not see 'thinwall' at the size you require.

[chris w]

Edited To give some calulations

For 70ft return run and Max 60watt Load

Key

A-Size mm sqr

B-Strands/mm

C-Amps rating

D-Calculated Volt Drop per Amp Per Metre

E-Result

Calculations

--A-----B-------C-----------D------------E

4.00 - 39 - 0.00471 - 0.5024628

6.00 - 50 - 0.00314 - 0.3349752 - Possible

8.50 - 63 - 0.00220 - 0.2346960 - O.K

 

2 Years Ago(only figures I have) these would of cost, rounded up, approx

£12(4mm),£16(6mm)and£22(8.50mm)

However these prices have risen sharply over the last couple of years so you may have to double or even triple them

 

Hope this helps

 

 

Your units for "D" are incorrect. You state D is in "Volt Drop per Amp Per Metre" when actually it is in volts if you work through the maths. (You have assumed 5A (60W) and 24metres length.)

The actual volts drop per metre per amp for 8.5mm² cable as an example is actually 0.002

 

I also don't understand what units "C" is measured in. The amps rating of say 8.5mm² cable will be of the order of 50 - 60 amps. It's still not correct even if it's meant to be the amps rating per strand.

 

Chris

[reg]

Your units for "D" are incorrect. You state D is in "Volt Drop per Amp Per Metre" when actually it is in volts if you work through the maths. (You have assumed 5A (60W) and 24metres length.)

The actual volts drop per metre per amp for 8.5mm² cable as an example is actually 0.002

 

I also don't understand what units "C" is measured in. The amps rating of say 8.5mm² cable will be of the order of 50 - 60 amps. It's still not correct even if it's meant to be the amps rating per strand.

 

Chris

 

Thanks Chris I'm afraid a

errors creep in when I tried to cut and paste from my spreadsheet, completly dropped one data colum which meant the table did not match up with the Key guide I provided, To use a phrase I don't like My bad.

 

I have edited my previous post to correct this.

 

You did raise another important point in that my calcualtions were done based on the manufatures supplied data for the wire I used i.e 8.5mm2 = 63a and 6mm2 =50a. These amp ratings are quite a bit higher than for normal 'Auto cable' and are more in line with 'tri-rated' cable values. I personnaly think it is wise to obtain and use the manufactures specifications whenever an of accurate calculation is required, however if the data is not available then I tend to use rule of thumb values and then step up one cable size.

Again thanks for pointing out the previous error in my table data

regards

[chris w]

In 12v circuits it would be very unusual for current rating to be the limiting factor in cable size. It will be the volts drop along the total length (out & return) which should be limited to no more than 0.4v and which can be accurately calculated mathematically to give the required cross-sectional cable area for any length and current.

 

This is to ensure that the power available to the device is still around 90-95%. (Power being proportional to volts², a small loss of voltage will result in a higher loss of power). So if there were a 10% loss of voltage the available power would be reduced to 80%, viz: a 20% loss which may cause some appliances to operate uncertainly.

 

Chris

Edited September 3, 2007 by chris w

[ChrisPy]

all this is good stuff, however if the lady really intends to modify her 12V electrical wiring can I suggest she checks <http://www.tb-training.co.uk/> where all will be revealed and the background, including the need for circuit ptotection, etc. is well described.

 

Tony - your free technical pages are a godsend and full of practical common sense advice - better than several glossy how to build a canal boat books put together - thankyou.

[reg]

all this is good stuff, however if the lady really intends to modify her 12V electrical wiring can I suggest she checks <http://www.tb-training.co.uk/> where all will be revealed and the background, including the need for circuit ptotection, etc. is well described.

 

Tony - your free technical pages are a godsend and full of practical common sense advice - better than several glossy how to build a canal boat books put together - thankyou.

 

 

May I add my vote of thanks to Tony's site, a great resource much appreciated

 

The specific link relating to this thread is

http://www.tb-training.co.uk/MarineE03.html

 

 

On a purely practical note The above discussion has reminded of another useful resource which is the Acorn Engineering 'Chandlery and Fit Out catalogue' this can be ordered from here

http://www.acornengineer.com/show_cat.php?id=248

 

Amongst other things the catalouge contains useful general and specific information on all the cable they supply

 

Acorn are another supplier who will sell cable by the metre

details on pricing here: http://www.acornengineer.com/show_cat.php?id=3

 

Hope this helps

[BlueStringPudding]

all this is good stuff, however if the lady really intends to modify her 12V electrical wiring can I suggest she checks <http://www.tb-training.co.uk/> where all will be revealed and the background, including the need for circuit ptotection, etc. is well described.

 

Great website. Thanks.

[chris w]

May I add my vote of thanks to Tony's site, a great resource much appreciated

 

The specific link relating to this thread is

http://www.tb-training.co.uk/MarineE03.html

 

 

Note that the cable area in Tony's equation has to be in mm² which is not stated and the length in metres (which is stated).

 

Also, the example given, where the volts drop is 1.2v (which is huge), ignores the point I made about its not being the voltage drop but the power drop which is the important factor as power is proportional to volts². So in Tony's example the lamp would only receive 80% of its rated power. While this may or may not be bright enough for a tunnel light (it would be equivalent to having a 40W bulb), it will certainly affect other devices, like your 12v fridge.

 

The implication is that a 1.2v drop is OK - IMHO it isn't. You should aim for a maximum of not more than 0.4v drop.

 

Chris