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twotails

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Mizzen Tabernacle Hole

Steve Abbott
Oct 10, 2008 - 12:17PM
Mizzen mast "hole" at rear

I've recently fitted a solar panel across the grab rails on top of the aft cabin (yes, I know that it's partially shaded by the mizzen boom!) and run the cable through the 3 inch gap at the rear of the mizzen mast tabernacle, below the foot of the mast. To make it a neat job, and having always been concerned about water ingress (when we're away from the boat, we cover it up with plastic bin bags and masking tape), I've run the cable through a cable gland fitted to a wooden blanking plate which I've glued in place to completely seal the gap.

So, my question is this: was this a good idea, or is that gap a deliberate feature to do with engine room ventilation, and consequently liable to have an adverse effect?

It's fair to say that my cockpit floor currently is a poor fit, and has no real seal, so providing a small amount of air escape if needed - that is until I get around to doing something about it. I guess that my worry is really about air coming in, possibly at pressure, from the intake scoop in front of the windscreen, and having nowhere to dissipate; opinions gratefully received!

SeaDog Name (if owner or crew) Arethusa
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Graham
Oct 10, 2008 - 1:02PM
Re: Mizzen mast "hole" at rear

Steve,
The hole in the top is intended to provide ventilation to the engine compartment, the plan is that it comes in through the 'letterbox' in front of the wheel and exits through the tabernacle. Some SeaDogs have both sealed up, but at least one of these (April Dawn) has problems with excessive engine smoking.
I am concerned you may have wasted your time re the solar panel position, ours is across the davits on Dougal and we found that the slightest shadow (from the ensign, or mizzen boom, caused the output do drop tenfold. We got round this by removing the ensign or by moving the mizzen sheet to the guardwire.
Graham

SeaDog Name (if owner or crew) Dougal
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Steve Abbott
Oct 10, 2008 - 1:57PM
Re: Mizzen mast "hole" at rear

Dear Graham

Aren't you prompt!!

Sadly, you've confirmed my worry about ventilation, although I guess that I'll have to "suck it and see" (I deliberately only used glue, so that it could be removed if necessary).

Whilst it's true that we don't get maximum efficiency from the panel in that position, it's primary purpose is only to trickle charge the batteries in our abscence (it's a 20watt panel so will still deliver a measurable dose in reduced light). My next step is to buy additional semi-flexible panels, and attach them to our sun-awning (a necessity in Greece), and wire them via a "cigarette-lighter" type of plug and socket recessed into the hollow mizzen mast step (hence the beauty of running the permanent panel's wire down there; I can pick it up and tap into it) so that we can get maximum power and thereby run the fridge, whenever we're at anchor in a harbour (i.e most of the time!).

Thanks for your rapid, and - as ever - useful comments; anyone else got something to add?!

SeaDog Name (if owner or crew) Arethusa
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Eric Richardson
Oct 10, 2008 - 2:06PM
Re: Mizzen mast "hole" at rear

Hi Steve
I am with Graham on this one, it not only lets air circulate, but helps to keep the heat down in the engine bay.

I well remeber one Seadog owner saying that it was the smoke coming out of there that aleted him to an engine room fire before it got out of hand.

In an ideal world we should be able to seal the engine compartment and have an automatic fire extinguisher and alarm system set up.

Regards Eric

SeaDog Name (if owner or crew) Ex Taliesin
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Graham
Oct 10, 2008 - 2:08PM
Re: Re: Mizzen mast "hole" at rear

Hi Steve,
Not sure if you have done any testing re the solar panel, we found that out 120 watt panel would deliver up to 6 amps when at right angles to full sun, this would drop to abot half an amp or so if I held my hand over it. Also found if the sun struck at 45 degrees output would be halved. It is mounted so that it can swivel fore and aft to get round the latter problem but when at anchor we have to put up with the sun rising and setting from port or starboard sometimes (when te panel is very little use). Overall we have found our installation excellent, and it allowed our freidge to stay on, and making ice, from April through to the end of September. Never switched it off.
Graham

SeaDog Name (if owner or crew) Dougal
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mike
Oct 11, 2008 - 9:59PM
Re: Mizzen mast "hole" at rear

Couple of vague thoughts on the solar panel.

The first is that some designs of panels suffer heavily from shadows causing power loss. Others are only slightly affected. It all depends on what type of crystal structure the panel is made from - err, but I can't remember which - not very helpful. But if you know a shadow will fall on the panel, its important to buy the right type of panel.

Also on a wiring thought. From Steve's email about direct wiring to the fridge. If this is done before the battery switch and the battery can be isolated from the circuit but leaving the panel connected to the fridge - be aware the voltage can be 20 - 30v. I once connected a 12 volt bulb to a large solar panel out of curiosity and it literally exploded - the bulb, not the panel! The voltage was about 23 volts at 6 amps. You need the battery resistance to drop the voltage down - at least that's my understanding of these things. Just a thought.

I'm just jealous that you're sailing in conditions requiring a fridge!
Graham
Oct 12, 2008 - 8:09AM
Efficiency of solar panels

I have located the following information about the efficiency of the 3 available types. Although possible with low output panels I would never connect one without a proper regulator. Certainly wouldn't connect one to the fridge without a battery.

There are basically 3 technologies to choose from:

monocrystal (high output) -- the most efficient
polycrystal (high output) -- almost as efficient as monocrystal
thin film / amorphous (heat- and shade-tolerant) -- less efficient

A high output module on each side of the boat will usually guarantee that at least one of them will be in full sun; and at anchor, you can settle the boom to one side or the other for maximum sun exposure. If one solar cell is fully shadowed, you're down to 50% power output. If a whole row of cells is shadowed, you're getting nothing.
A shade tolerant module on each side of the boom will give you charging capability even when a portion of the surface is shadowed. The percentage of output is equal to the percentage of surface exposed to sun. When 40% is shadowed and 60% is in the sun, you'll get 60% of the rated capacity of output.

What makes the monocrystal and polycrystal output drop so much in shadows?

The high output modules have individual cells, and each cell can be thought of as a small battery. When you shade one cell, it's like taking a battery out of the (series) circuit. Think of the old Christmas tree lights - when one went out they all did. Thin film modules don't have individual cells, it's more like one big cell.

The larger high output modules also have a bypass diode so that the cells are essentially divided into two separate circuits. When one cell in one of the circuits is shadowed, that whole circuit is down. But because the disabled circuit can be bypassed, the other circuit can still function.

Why would anybody buy such shade-sensitive stuff?

They're much higher output than shade-tolerant modules. Shell has a 55 watt monocrystal module that's in the exact same frame as their 40 watt shade-tolerant module! On a sailboat, space is an issue. I want more power from the space the module will occupy!

If monocrystal is the most efficient, why would I even think about buying polycrystalline?

They come in different shapes and sizes. The difference in efficiency is small enough that available space is the more important issue here. Measure your space and choose the module that fits best.

The most popular polycrystaline (Kyocera) tend to be more square, while the most popular mono-crystal (Shell) tend to be more rectangular.

Also, some polycrystaline modules such as those made by Solara, are thin, lightweight and flexible. They designed to be glued or screwed to the deck of your boat, giving you more options in positioning and mounting panels.

SeaDog Name (if owner or crew) Dougal
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Steve Abbott
Oct 12, 2008 - 8:31PM
Re: Mizzen mast "hole" at rear

Dear Graham, Eric et al,

Appreciating that this thread was diversified in to a much more valuable discussion about solar panels, just to let you know that in consideration of your helpful response to my initial query, I've decided (and bought the parts), that I'm going to fit a hinge, catch, and rubberl sealing strip to my wooden cover, such that it can normally be close, thereby looking neat and keeping the water out, but hinged open whilst the engine is running, thereby fulfilling the original ventilation function. Fortunately I threaded sufficient slack cable down the mizzen mast "shell" for this to happen.

Incidentally, I'm aware and fully endorse the comments about the need for a regulator for use with solar panels; I've fitted mine inside the aft cabin, at the top of the bulkhead, just to the port side of the hatchway. In that position, the cable goes through the mizzen mast aperture (as discussed); down a few inches; out through the bulkhead and into the regulator; then out of the regulator and back through the bulkhead into the mizzen mast "box", thus down to the batteries in the engine compartment. This has made a very neat and practical job, and will enable me to easily add additional panels by plugging them into a socket on the outside of the mizzen box, which will connect with the cable between the aperture and regulator - which is why I stuffed a fair bit of extra cable down there.