This is a technique that it sometimes used in lower-end double pane windows.  They will put silica gel along the edges - the same thing in the "Do Not Eat" packets that come in shipped electrical equipment.

It does a goodish job of pulling out any moisture trapped between the two panes during manufacture.  However, they become overwhelmed if the seal between the two panes leak at all, which I'm sure would happen with thin plastic bubbles and boat movement. 

Maybe a method by which, when the silica gets saturated, you can easily break the seal, clean betwixt the domes, pop the gel packs in the oven to dry them out (it works), and slap it all back together, good to go for a few more months.

To me, though, that sounds an awful lot like (shudder) added maintenance and complexity.  

Annie Hill wrote:

Michael Moore wrote: Condensation?

Well, Michael, if you stacked two together, a possibility I suggested, you would eliminate any possibility of condensation.  Otherwise, it is that very issue of condensation that makes a dome so inferior for watchkeeping, as compared with a pram hood.

However, I am suggesting using a dome here, when you want to keep the heat in.  While condensation will be a downside (although you could possibly just prop it open a wee bit), the compensations are extra headroom and increased light (often an issue on colder days.)  Moreover, your pram hood will probably be over a 'wet' area in the boat, so that a few drips won't be an issue.  An off-the-shelf alloy hatch would cause the same issues; and I've been shipmate with more than one sliding hatch that gathers condensation on its inside face.

The dome will be a perfect solution in my case: it's just one option that may be worth considering for anyone starting from scratch, or wanting to install a pram hood and/or rebuilding their companionway.

First the thickness issue, which is a judgement call. The second a very real problem sealing the two domes together so that damp cannot get between and give rise to unwipable condensation! I remember living with old double glazing in my house which had this problem.

Mike

Annie Hill wrote:Yes, I know that.  For inshore use this is possibly adequate.  However, they seem perfectly willing and able to make custom designs.  A second alternative is to buy two of these domes and to stack them with little spacers which would not only make the unit stronger, but would provide double glazing, too!

And, of course, they also make them out of polycarbonate.

I'm just moving part of another thread under a new heading, which may prove useful for other members:

Annie Hill wrote:

Darren Bos wrote:I can see where the Hasler style pramhood would suit some folks better.  However, if you have a sliding hatch, then the pramhood prevents you from having a garage to keep the hatch nice and waterproof (unless you mount the pramhood somewhere else).  I do wish I could have the flexibility of the rotating pramhood which lets you keep the spray off you whichever direction the wind is coming from.  However, we have a pilothouse and I'm 6'3", in our case the Pardey design is a better fit.  I'm surprised you can get a custom plexi bubble for $70NZ, I could barely get a thick piece of plexi the right size here for that cost.  How thick of plexi do they make the bubbles with?

According to Mr Tyler, he has designed the perfect sliding hatch cum pramhood, which he wanted me to install in SibLim.  I'm sure he'd send you a drawing of his clever concept - it may even be included in the SibLim plans in his profile.

The domes are made by Cambrian Plastics and are, indeed, astonishingly inexpensive.

Paul Thompson wrote:

The standard (cheap) dome is only 3mm thick, I'd want one at least twice that thickness. Also because it's blow moulded, it will be thinner on the top. How much thinner I don't know as that depends on a number of factors. A clever operator can keep the thinning to a minimum but it will still occur. How do I know? I write programs that control the machines that do plastic mouldings of this sort.