masthead light wiring

[samwise]

I intend to change the masthead tricolour for a combined tricolour / mooring light. Obviously I will need to be able to switch between the units. I have no idea how these things are wired. Is it possible to use the existing wiring? Or do I have to try some of the techniques kindly offered by R to R posters in response to an earlier post from me around the same subject. I had dallied with the idea of a a mega light but it seems like a half-baked solution. As I have the rig down I may as well do "a proper job."

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[bedouin]

Unfortunately you will need to run a new cable from the switch panel to the masthead light. This will be 3 core, with a separate positive lead for each light, plus a common negative return.

I have considered a design that used only a twin core cable up the mast, and selecting the light by switching the polarity of the wires - but on reflection I decided it wasn't worth it, and it would be better to do it the conventional way.

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[neil_s]

You could use a diode in series with each of your lamps. Make sure that the polarities of the diodes are in the opposite sense. Swapping the polarity of your 12v supply will control which of the two lights comes on. You could arrange to swap the polarity of the supply using a two pole double throw switch. If you found one with centre off, that would complete the job.

This would allow you to retain your two core wiring, at the expense of a diode loss (about 0.7volts) in your light circuit.

Neil


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[VicS]

I assume you will be looking at a tricolour plus a white which can be used on full power in place of the masthead and stern lights when under power or on reduced power as an anchor light. In which case it should come with a resistance to reduce the power in the last application. This means exta switches or a change over switch but still a three core cable in place of the existing.

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[LadyInBed]

No, 3 core not required. A second on/off switch with a resistor across the contacts will do, or even a resistor in series with the forward bias diode that puts the anchor light on would be better.

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[HaraldS]

not sure the resistor is such a good idea: Let's assume a 10W nav-light bulb. It gives 10W at 12.5V. The resistance drops a bit when the filament gets cooler, so you get half the light power at about 8V. Assuming you want to drop the anchor light to half power, you'll have to 'burn' the other 4V in the resistor, which would be 2.5 - 3W, so your real saving is just 20% -25%. Not worthwhile in my mind.
Also, with no engine running, your batery voltage will soon be down to 12V, depending on the wire, you loose about half a Volt there and another 0.7 V in the diode. So the bulb gets about 10.8V which translates into 8.5W, so it's already 'dimmed' that way. Under engine, with about 14V battery voltage, it will be just right and enjoy a long life.

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[bedouin]

Yes Nigel, the 3 core is still required because you need one feed for the tricolour and another for the anchor light.

However I would agree with the comment that putting a resistor in series is a poor idea and pointless. I think the required range for the masthead light is the same as for an anchor light, so there is no point in adjusting the brightness.

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[chriscallender]

I would agree no need for resistors - presumably the manufacturer of the combined light designs it to give both the tricolour and the anchor light the correct intensities when a nominal 12v is connected to the appropriate bulb - so no additional resistors are needed. If you do want to dissipate significant power in them they'll need to be big ones (5W or more) not the little 1/8W ones that are used in a lot of electronic circuits, and they will get quite hot, enough to burn so give some thought to where you put them. But I'd assume that they sell the things with bulbs that give the right brightness in the first place.

I agree that the simplest way to wire it is with a 3 core cable, a common ground and use a core for the +12v to each of the bulbs.

If by any chance you have a spare wire or even a spare core within a wire you could use that, or possibly you could share a common ground between the light and some other masthead electrical equipment (but not VHF antenna!) and free up a core that way. Maybe you could even use the grounded aluminium mast itself as one of the conductors!

Otherwise arrangements of diodes would be feasible if you really don't want to replace the existing 2 core cable with 3 core It will make the wiring arrangement on the switchboard a bit more complex since you need to be able to reverse the polarity on the cable (can do this using 2 pole switch with 3 positions (off, anchor, tricolour) or else two seperate switches, a simple on/off and another (2 pole 2 position) that swaps polarity to select anchor or tri). The diode voltage drop will reduce the intensity of both lights a little but probably not that noticably in practice. One benefit of this approach is that its impossible to accidentally switch both lights to be on at the same time ... unless a diode fails in an unusual manner.



Chris

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[bedouin]

I would worry a bit about the voltage drop caused by the diodes - especially on a masthead light where the cable may already by 30-40metres (round trip). The same effect can be created using a relay and a diode without causing any voltage drop.

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[HaraldS]

would strongly discourage using the mast as conductor because of the potential to get electrolytic corrosion going that way. In a marine environment it is a good practise to have full control over where the return currents go, in other words separate wirering and only ground connected in one place or none.

If the 0.7V drop in the diodes is too much of a concern, there is the possibility to take low loss MOS-FET transistors, they'd usually drop 0.2 -0.3V. In this case take pole A to the lamp and to the Gate, the second feed of the lamp to the Drain and the other Pol to Source.

Relais are a possibility, but I'm not sure I like the thought of soething that "mechanical" up at the mast top.

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[Chris_Robb]

Steaming Light???

Can a mast head light be used as a steaming light??? I thought that they had to be separate, with their own ark of visibility, so that the watching boat can judge the direction of the vessel.



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[bedouin]

Re: Steaming Light???

Yes, for vessels of less than 12m (I think) it is permitted to combine the stern light and masthead light into a single all round light (i.e. the anchor light) when under power.

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[chriscallender]

I'm sure you're right about the dangers of electrolytic corrosion - I did think that it must be a silly idea when I posted it, otherwise everyone would do it. And it did occur to me that electrolysis was the reason why ... so lets forget that one!... Since the mast is unstepped, 3 core cable is surely the way to go.

Chris

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[LadyInBed]

No no no no no. 2 wires.
Change over switch (with centre off) reverses polarity up the 2 wires.
At the top you have a diode in series with a filament.
In parallel with that you have a diode (pointing the other way) in series with a second filament.

Switch to one side - one filament is lit through the forward bias diode.
Switch to other side - polarity reversed - the other filament is forward biased.

Totally agree about the resistor, I wouldn’t put one in.


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[samwise]

Re: Steaming Light???

Many thanks to all who responded to my query. The amount of knowledge on tap through this forum never fails to amaze me.
One final question - possibly for Kim -- At one point does a "New User" become " A Regular." Is there an initiation ceremony?

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[bedouin]

Oh, I agree you can, but surely you wouldn't? The voltage drop would be significant, the system would be less reliable, and even fitting the switch would not be trivial - not many switch panels have two pole changeover switches.

Given the mast is down and accessible, replacing the two core with three core is only half an hours work. KISS

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[LadyInBed]

On my last boat the light was wired with diodes in (it was manufactured that way). When I replaced it, I cut the diodes out and used them on the new light. I can't see any real problem with this configuration. I was working with what I had, putting a new cable up the mast would also mean new 3 pin deck plug, and associated wires to switch panel.

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[rex_seadog]

Re: Steaming Light???

Although a combined stern and mathead light is permitted I'm not convinced that it's very satisfactory. We have this arrangement and when motoring it means that in order to show a steaming light we have to extinguish the separate transom mounted stern light. In confined waters such as on the river this results in the stern light being "lost" 30/35 feet or so above the water. Out at sea this isn't really a problem.
Similarly when used as an anchor light it is also too high - better to use a separate one lower down.

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