The Offshore Voyaging Reference Site

Q&A: Safety of Large Pilothouse Windows

JHH5_3562

Question:

Member Tim asks [edited for brevity]: I notice you have not mentioned [in this Online Book] the integrity of windows during a knockdown and the possibility of storm covers. (I don’t mean something to fix a broken window afterwards.)

Maybe this not a significant issue for many boats but ours is a pilot house/deck saloon design…I worry that should we get knocked down and smash a window, the volume of water that could enter would be colossal.


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Marc Dacey

Well, this is a good discussion, because I have a boat with no hull portlights at all, but with large pilothouse windows, or (technically) “deadlights”, which confusingly is not only a fixed portlight, but the storm cover that can go over it. Storm covers/shutters are a necessary thing to carry, I think, but are only part of the solution. We also have the means to seal off the “downstairs” aft cabin and saloon with gasketed storm doors (also a security measure when away) and we are putting in a gasketed engine bay “clamshell” hatch: if we take a wave over the stern, the water should not get far. Lastly, the pilothouse itself has scuppers formed into the hull: if we got two feet of water inside in a worst-case scenario, it would (eventually) drain out, although I would hope that a well-designed set of storm covers and a doggable companionway hatch would keep 99% of the sea out, even in a roll. If the site allows, here’s a recent shot of the pilothouse to show what I’m talking about. The port scupper is just below the long white fender. http://3.bp.blogspot.com/-vkM8UZxqoMc/VE6ZXDGmkCI/AAAAAAAACUg/bzZs-WdUQu0/s1600/WP_20141025_10_17_30_Smart.jpg

You can’t plan for everything, but I am often surprised at what isn’t planned for: storm shutters mounted on four or six stand-offs, threaded and backed, are really cheap insurance. Many boats seem to suffer when a portlight is stove in and it becomes impossible to keep up with water ingress. Even wooden shutters (although I prefer 1/4″ aluminum as has been suggested) could mitigate a big ‘un hitting the boat squarely.

Tim

Marc,

Do you have covers that go over the forward pilothouse windows also? I was told by someone in a yard, who was relatively expeirenced, that boards were only really necessary for the p+s windows in the case of a broach / knockdown.

Tim

Marc Dacey

I do not yet have boards for the forward pilothouse windows, two of which are tempered, fixed glass and the center one, which opens up and out. I am considering Lexan boards for the fixed panes, and leaving the center pane to its own devices as it is well-blocked by the pilothouse overhang and by the substantial mast tabernacle.

But it’s definitely a consideration.

Eric Klem

Hi John,

A very worthwhile discussion that you have framed well. I actually have always considered the portholes and large forward hatch on our boat to be the weakest component structurally and am working on a plan to replace them.

A couple of thoughts on the stresses in windows/stormcovers. I suspect that the load on these can be approximated by a pressure. Unfortunately, I don’t know what pressure would be appropriate to use but there are probably people here who already know the answer to this. From there, you can take a given geometry and look at what the stress should be. In general terms, the strength in bending of a material is proportional to the square of its thickness so you would need a material that is 4 times as strong if you are going to halve the thickness. How the material is constrained at the edge makes a huge different in the stresses. Also, the geometry of the area covered is very important. If you look at a cantilevered 2D beam with a constant force per length acting on it, the stress will quadruple if you double the length. This is important because bigger windows will require thicker material or a separate support structure.

I have only ever sailed with stormcovers on once. It was on a boat that was well thought out and had clear covers so we were able to easily put them on while the weather was still nice and then there was no rush to take them off afterwards. It would seem to me that using a clear material would be advantageous as you could put them on well in advance of poor weather.

Eric

Trevor Robertson

John,
As I understand it (and I am not an engineer), if a window or storm cover is uniformly constrained around its edge, the critical dimension for calculating the stress on it is its minimum dimension. A square window is much weaker than a long, narrow window of the same area. If I have got this right, the most important variable in specifying the strength of a deadlight is its height, assuming the fastenings are stronger than the deadlight. Perhaps an engineer could confirm this?

Given this, and the fact that most windows are rectangular rather than square, a deadlight probably only needs only to be attached along its top and bottom. However if the plate has a strongback attached along its long axis, the strongback effectively divides the deadlight into two pieces. These probably have to be attached at the ends where the strongback laps on the cabin structure. A series of short reinforcing pieces at right angle to the long axis would probably be more effective. Again, perhaps an engineer could comment?

If the deadlight is of aluminium 6061 T4 or T6 (the obvious material to use) and requires reinforcing, it may be sensible to do it with an angle section bolted or riveted in place, not welded, given the considerable loss of local strength that occurs when that alloy is welded.

As you point out John, the amount of water than comes in though a broken window is considerable. It has only happened to me once, in a previous boat off the South Island of New Zealand. I was running before a gale when a wave picked Salvation Jane up and dumped her on her beam ends, breaking the galley window on the lee side. The window was about 60cm x 12cm (2ft x 5 in). It took me about 30 minutes to get a plywood patch in place and by that time the water in the cabin was over the bunks. Salvation Jane did not have an inboard engine so at least the interior was not covered with emulsified oil, which happens quite quickly once an engine is flooded. Even without a layer of oil on everything, getting the patch in place was difficult due to the irregular, violent motion, perhaps caused by the free surface effect of a boatload of water. I baled her out and carried on to Australia, but it was a close run thing for a while.

In addition to deadlights for saloon windows, it is probably a good idea to have a precut aluminium or plywood panel to replace the acrylic section of each deck hatch. These panels are for repair only and not routinely fitted in heavy weather.

Trevor Robertson
Iron Bark
Stornoway, Scotland

George

Hi John: I faced this same decision when designing a 55 ft aluminun sloop with a i closed pilot light. I decided on 3/8 inch thick tempered glass windows not over 3 sf in area. Glass so they would remain clear. I know that any glass can be broken given the conditions at sea so my solution was to separate the pilot house from the interior. A self draining pilot house floor, additional conpainionway enclosure and port lights to the interior. Happy to report that after 15 years of offshore sailing they have not been needed but I know that they are there. George

CaperAsh

I was looking at this for sale listing recently in which some of these issues seem to have been addressed for a pilothouse in a 2011 William Gardner-designed build done by a ‘professional mariner’ owner.
Listing: http://www.popyachts.com/Boat/37303/For-Sale/Oregon/William-Garden/40-Custom-Ketch-Deep-Keel.html

“From the aircraft windshields in the house to the military grade portholes in the stern..”

Images 13-15 show what appears to be a ruggedly constructed double-layered porthole (also a sealed hatch?), and image 38 shows a large part of the large ‘aircraft windshield’ comprising the front part of the pilot-house.

( I find his chart table arrangement in Images 70-71 really nifty.)

Clearly this is a custom-made job, but it appears this builder worked hard to make this a very strong ocean-going vessel. Of course this doesn’t mean he got it right, but presumably he researched this a little and might be able to contribute some specs regarding the aircraft windshield material and ‘military grade portholes’ such that an engineer contributing here might be able to evaluate if he has come up with a good pilothouse window solution.

Marc Dacey

Wow…look at that price!

RDE

That would make a great Alaska boat. Think of it as a trawler with steadying sails built like a fish boat instead of a yachty toy.
Friends who have gone to Alaska for the summer come back with spider’s nests under their sail covers——.

Marc Dacey

I forwarded the ad to some sailing friends as an example of “a bridge too far” in terms of customization. Those are definitely safe portlights, however…

CaperAsh

Marc,
I am curious to learn what aspects specifically you found ‘a bridge too far’. I have no attachment to the vessel, particularly, but am generally looking at different boats/designs in order to see different designs and solutions simply in order to learn more about how the many different issues involved are addressed. The main takeaway I got from this one is that it seems more like a working boat than a pleasure yacht, almost military-like. It also seems massively overbuilt compared to most 40′ vessels. And interesting that the hull is wooden. Looks seaworthy, but probably quite slow under sail.

Marc Dacey

CaperAsh, by “too far”, I suppose I meant “an intensely personal vision fully realized that is not going to be broadly shared”. I can see great care went into this boat structurally and (as far as I can tell) in term of equipment, access and other considerations. But it’s a damned ugly thing, is “needlessly wooden” in the sense that I associate wooden construction with beauty and “showing off that it’s wood in the first place”. Here, there’s a lot of rough lumber showing, aged three-strand for halyards, there’s no gunwhales where I would expect them, and instruments are labelled (and misspelled) with Dymo tape. It looks as if large parts were salvaged from the bins behind Home Depot, and while I suspect it’s quite strong and seaworthy, it bears the emphases of the one guy who built it to his own particular tastes. In this, it resembles a LOT of “modified Roberts homebuilt” designs I’ve seen quietly rusting in yards. *Definitely* safe portlights, though!