Big windows - dangerous in an ocean-going yacht ?

1. Ya'll are getting carried away by the worst case scenario/deck prism/windows should not be allowed stuff. I have spent much time on the water, in some mean **** sometimes, and tho I once knocked ALL four windows out of the starboard side of a boat, it was my fault. Windows are fine. Make 'em thick and mount them well. I am not an engineer but would, by experience, have no problem specifying windows for a given project. The key is to match the windows to the intended task. For a small coastal powerboat, draw the boat, (this is the important step) squint, ask yourself "do those look proportional and slightly smaller than the other guys? DO they look proper?" Redraw. Use school-bus glass - one doesn't need to remember anything else. Think about potential forces when mounting. Do it well. Be a mariner and stay out of trouble.
2. The big sailboat thing, dropping off the face of a wave sideways , curled up in a ball seasick, (crying,) should tell you that your boat was not meant to be there, you weren't, or you are handling the situation wrong. If you are going to do the stupendous maneuver of lying ahull in mean ****, your side windows will get popped out. Please consider that to do this (which I have not done) you should have deadlights.
3. Bigger boats get thicker windows. Glass is amazingly tough stuff when properly mounted with no hard spots and equal loading at the perimeter. Coastal waters are generally meaner than ocean waters - it is the upwelling, currents and shoals that will stack the waves up. In areas with a lot of current, a deep pinnicle will cause an upwelling that will astound you. A prudent mariner avoids this. I find myself going into seamanship lessons deeper than ya'll want to go but the bottom line is to...
4. Don't take the Bayliner out of the bay. Don't take a beautiful eighteen meter sailboat into an opposing blow in the gulf stream then get sideways in it (the force that counters the sails, the keel, also inreases inertia and causes those windows there to be more susceptible to getting slapped. Common sence should guide everything you do on the ocean and learning (less reading - more thu the hawsepipe) is the order of the day.
5. This; "...Therefore, when the boat had been picked up by the wave it would be moving at that speed..
As the Churchill slid down the face of the wave on its side, there would be very little friction or drag, because the water supporting the boat would be moving at the same speed and would accelerate with the boat. With no friction the boat could reach a speed of 67 mph by the time it reached the trough. If we assume only half of this speed increase, the boat would strike the green water in the trough at over 50 mph.
This velocity is equivalent to a free fall from over 70 ft. This clearly explains the sequence of events which destroyed the Churchill. A boat striking green water at this speed can incur a force of over 200,000 lbs." ...is bad math.
Without knowing the actual dynamics of the sea, the wavelength, just how much it is breaking, how the vessel came off the previous wave and therefore set-up for this wave, the depth of keel, the mass of the boat, what the boat was doing (running before/agin/cross/quarter/etc., lying ahull, stabilizers, tankage free water surface area, the list goes on... one cannot say how the vessel fell. More likely than "moving at the wave speed" the vessel got momentarily shoved a bit right before the crest, then, more-or-less, fell but constrained by keel in the water, inclined angle relative to wave face ( it was a sailboat, right?) In my opinion, it could have been gently set down or acheived speeds approaching free-fall (9.8 m/s2). subtract the receding wave slope, how much of the vessel was in the water on the fall and the speed of the wave (nearly known) and the problem becomes complicated enough that guessing becomes silly but the maximum speed, rounding acceleration to 10 m/s2, figuring that the vessel was at some point other than at the top of maximim wave height when it began its fall and how it was cushioned in fall by its own bouyancy and a host of other factors including aeration, (I am imagining a worstcase scenario of accelerating for 1 second and landing window down first as the following wave is rising) the speed is only about 20 miles per hour. In other words, the velocity is NOT "equivalent to a free fall from over 70 ft" but merely is a maximum of a free fall from the wave height (did someone say 45 feet? BS, give 'em that tho - doesn't matter) minus the height at which the vessel (cleanly?) broke through the supposed breaking wave. In other words, as most things talked about ashore about the sea, an exaggeration.

 

well said...

 

In backing Mark 775's post, I'll say that if we considered only the worst-case scenarios, we'd never venture out of the house--much less go sailing. Instead, use a little common sense and design for what you're likely to run into.

If you're going on a round-the-world cruise, you're obviously going to tighten up the specs. But although many (most?) sailors like to think of themselves as the spiritual heirs to Joshua Slocum, the reality is that the average casual sailor is never going to be falling off a 45-foot wave.

The sharpie I'm designing will probably never see blue water; I'm planning to use it on the lakes along the Colorado River. But it's going to have fairly small, fixed deadlights anyway, for several reasons:

1. Simplicity and ease of construction. I'm not getting any younger, and I'm certainly not getting any richer.

2. Security. If they can't open they won't open....especially if I'm not there and someone shows up with a pry bar.

3. Unnecessary. If I'm moored, beached or putzing around in smooth water, I'll get all the ventilation I need from the forward hatch and the companionway. If I'm out screwing around or pushing my luck, I'm going to have everything buttoned up but the beer cooler anyway, and deadlights are one less thing to dog down.

4. Aesthetics. I'm hardly a romantic purist, but face it: big modern windows just don't look right on an otherwise traditionally-styled sharpie. At least not to my mind's eye....

 

One would think after 6 pages of discussion there would be some guideline or formula that comes forth

Discression it is then. My boat hull's windows are going to be 300mm x 200mm in 8mm thick perspex, they are in a box here already. Let's not start the yellowing argument on this one again because the windows won't yellow in the next 99 years. I'll replace them after that. They will be recessed in a housing with a proper support border and a rubber foam sealer so the wind cannot blow water in.

I chose this size for the hull's because one can see through them with both eyes comfortably without being too big or too small. They will get wet, I have enough faith in the tests I have done on the perspex that it would have to be quite a blow to take them out. Very unlikely.

The cabin however must have larger windows, I expect good vision there. I have not figgered them out yet, but assuming worst comes to worst they will also be able to withstand some abuse. It's going to be a real **** if you're in a storm and a window comes out. Everything is going to be soaked in two seconds flat.

 

Fanie,

"some guideline or formula that comes forth "

Glass Physical Characteristics:

Density: approx 2.5 at 21°C
Coefficient of linear expansion: 86 x 10-7m/°C
Softening Point: 730°C
Modulus of Elasticity: 69GPa (69 x 109 Pa)
Poisson's ratio: Float Glass 0.22 to 0.23
Compressive Strength: 25mm Cube: 248MPa (248 x 106 Pa)
Tensile Strength: 19.3 to 28.4MPa for sustained loading
Tensile Strength (toughened glass): 175MPa.

The water pressure (p) is directly proportional to the Height (H) x the force of gravity (approx 10 (9.81 for people who want to be exact)).

p = H x 10 in N/mm2

The bending stress allowed (B) is equal to the Tensile Strength of glass / safety factor.

B = 19.2 / 3.8 = 5.05N/mm2 (Safety factor = 3.8)

 

http://www.allplastics.com.au/03/files/products/perspex/PerspexforglazingPXTD236.pdf

Interesting read on Perspex for those so interested.

 

Sure sure....
Though in this case knowing the exact force is not relevant to the point being made. The authors take home message it to avoid that circumstance which has proven to be most destructive to yachts and provides calculations clearly set forth as rough estimates.
Very few would place the conditions faced during the '98 Sydney Hobart as "exaggerations" BTW.

Again, I don't give the issue too much thought on my own boat outside of carrying shutters. I do believe determining appropriate port sizes should be possible if the forces faced by the panes is understood.

 

Perspex..now I remember...for several years when I was a lad I had a job at an airport FBO as an aircraft fueler on small planes and corporate jets. Almost all the Cessnas, Pipers, Mooneys and other aircraft all use perspex for their windows and I'd have to say it was pretty decent stuff and certainly fairly scratch resistant. You had to use the right cleaner though..we used this liquidy paste stuff I forget what it was called but it was somewhat waxy.

 

would looking at other boats with window installations similar to what you plan and the wave problems they may have had in the areas you plan to sail and plan accordingly be of benefit?

anyway i'd plan some sort of cover with smaller stronger ports for those occasions when you may venture out beyond normal sailing areas

 

"...conditions faced during the '98 Sydney Hobart as "exaggerations" BTW" - well, I just vaguely remember this so went to find info and didn't find much other than that the coroner stepped beyond his job description and placed blame. I've got a couple of friends that ply those waters and they tell that it is nasty, unpredictable. I watched some videos and I have to say "It looked shitty" - there's no reason to exaggerate. They shouldn't have been there in those boats - another point I was making...

 

"I do believe determining appropriate port sizes should be possible if the forces faced by the panes is understood." I gave you the answer - You cannot understand the forces faced by the panes. I above showed the lunacy of someone who hasn't been there trying to calculate them. Once you are there, you will say "Make 'em a little thicker, smaller." Look at what survives and what doesn't. Deadlights, "shutters" are appropriate for those ballsy enough to tackle the world in such a craft. Just be sensible and have another margarita in port instead of pushing it and the situation will not arise. IT IS NOT THAT COMPLICATED OF A PROBLEM - pay me and I'll construct a device to slam a framed pane with inertia (weight) into water at 20 MPH or "make 'em a little thicker, smaller" and forget about it.

 

What about a boat like this ?

It has no storm shutters , and 3/4" glass .

 

Lookup "Toplicht" they have almost all the old, good stuff!
http://www.toplicht.de/shop/fenster-bullauge-luke-und-luefter/deckslicht-und-rahmen


Mr Smith:
that one is Windhorse, she has 19mm tempered glass! (which I personally still call insufficient for that size)

Regards
Richard

 

I'd say that boat would be in big trouble in certain types of big blows...looks like an aquarium topsides...kinda like my Hunter 20 I bet it becomes an aquarium too if you got caught on a lee shore in an already established sea with 25-35 knot gusts and six-footers every 6 seconds.
What a waste of strong tin when you can still get to know Davey Jones as quickly as the guy in the 25-foot FRP boat that doesn't have the Baltimore aquarium for a cabinhouse. Did I mention it looked like an aquarium?

 

Done.

And the first round is on me.
Beer for all hands lads.