Access hatches

[p webster]

I am still building my Engel Type IX sub and I am worried about access to servos etc. and would like to put in another access hatch at the rear of the middle deck and also change the existing aluminium hatch to make them both polycarbonate. My question is how thick would I have to go, Ideally 4 mm (3/16in.) or 6 mm (1/4in.) .
the 1/4inch would be the maximum thickness I could use dependant on what type of seal I use, any suggestions there would also help.
Please help as I want to get this part finished as it has been 2 1/2 years since I started it.
Thanks for any help received

[Guest]

Re: Access hatches

Personally I would not dispense with the aluminium hatch. It is considerably more rigid than even quite thick polycarbonate or other unreinforced plastics.

However if you're dead set on installing a clear hatch, then I would say 6mm would be a minimum thickness, 4mm would be very squishy.

[Wheelerdealer]

Re: Access hatches

I used 8mm, it's a little bit of an overkill, so 6mm should be fine. You may want to increase the number of bolts though to get a better seal- as a Andy mentions, polycarbonate flexes a bit more.

[Guest]

Re: Access hatches

Polycarbonate needs to be about three times the thickness of aluminium for equivalent strength. It should weigh almost the same, as polycarbonate is about a third the weight of aluminium, assuming it doesn't have any glass in it.

[JWLaRue]

Re: Access hatches

Let's not go too far overboard here. As SubCulture stated, we're talking about is making sure that the polycarbonate has enough rigidity for it to maintain the seal with the top of the dry space....but we only need enough rigidity for the depth at which we intend to be operating.

Plus we should take into consideration that when an Engel piston-based boat is submerged there is something very close to positive pressure in the dry space, which helps offset the external pressure.

-Jeff

[Guest]

Re: Access hatches

Just quoting what he needs if he is intending a like for like.

Pressurization from the retracted piston tanks will tense up the hull, which is beneficial. Not sure what the internal volume is of the Type IX's box, but I believe that Engel tend to engineer their boats to squeeze up to about 2psi. So beyond about four feet the boat will start to compress.

Again not sure what the hatch size is on the Type IX, but it will be fairly large. Let's say about 4"x36". At about six feet, assuming the tub is pressurized to 2 psi internally by the tanks, the boat will be experiencing 1psi of pressure bearing down on the dry space, added up over the area of the lid, that represents 144 pounds pushing down on that lid. At nine feet (often the deep end of a swimming pool) that increases to 360 pounds. As the lid bends under the pressure it puts a bending load on the studs holding it snug- almost all the failures I've seen on boxes have been down to bolts broken from their seating. The best boxes have sturdy bolts tapped into metal plate then soldered or brazed over, the plates are then bonded into the box.

They tend to give good service, but at the expense of having to make a much heavier enclosure than would be required from a cylinder.

I am not a fan of boxes, but if you must have one, then build it very sturdily if you want your boat to return from depths.

[p webster]

Re: Access hatches

Thanks for all the replies and assistance. It looks as if I will be ok with 6mm or preferably 1/4 " (6.35mm).
This is the maximum thickness I can use or I will have to use aluminium on this one also. The size I need will be 4" x 6", the maximum depth the sub will be set for is about 6 feet ( recommended by Engel) as the pressure switch is set for surfacing, once this depth is reached.
I will be setting the bolts in a metal strip the same as the other s are done.
Once again thanks to all who answered.
Peter Webster

[Guest]

Re: Access hatches

With subs it's always worth building in the largest safety factor possible.

In practice most submarines rarely dive beyond three or four feet, however if something was to go wrong e.g. the throttle got stuck on for some reason, the boat could easily and quickly dive beyond six feet if the waters where you sail permit it.

The pressure switch might well activate, but it takes time for the piston tanks to actuate and for the boat to rise.

Cylindrical pressure hulls can easily withstand quite large forces, even fairly thin walled ones. That's why I prefer them to boxes, which need to be made very stoutly built in comparison.

[subwrecker]

Re: Access hatches

I have two engels(typeIX,Gato) with 3/8"or 10mm lexan access hatches.I cut dado on bottom 1/8"edge approx 1/2" in.This still allows same clearance of 6mm and stiffness of 10mm thickness.Keep in mine, lexan contains a percentage of rubber which makesit more difficult to machine but alot less prone to cracking. . Christopher

[p webster]

Re: Access hatches

I like the idea of the 10mm Lexan which I canget locally. Can you supply a few details as to how you fitted it on your Type IX Engel
I an sorry I took so long to answer as I have just come out of hospital.