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Water Tank Access

Started by P69, August 21, 2016, 10:51:32 PM

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P69

Use kevlar for stiffness? Or carbon fiber?

I cut open my water tanks for cleaning and to repair loose baffles this weekend and really like the 12" square holes so much that I am designing a lid for those holes. The 4", 6", 8", etc... round access hatches are really small compared to these new holes I put in the tank.

The new openings will be about 12" square, maybe a little more or a little less.

Attached is a picture of the tanks with the holes and a rough diagram of my preliminary drawings o the hatch and sealing mechanism.

Basically, the lid will overlap the tank by about a couple of inches and a bolt (or some bolts) will pull stainless steel angle iron up against the under side of the tank's top, sandwiching the tank's top between the lid and the angle iron.

I can take care of the angle iron and am looking to make the lid as thin as possible, but as rigid as possible.  Ideally, the lid should be completely inflexible, but I think that is not possible, so as little flex as possible. The more flex, the more likely that a gap will form at the perimeter of the lid, letting water out.


Primary reason for wanting a thin lid is that I am building a shallow drawer for the space between the tank's top and the underside of the berth's plywood sheet.


I plan on making the lid out of epoxy and biaxial cloth that does not have any mat in it, just two directions of cloth.


I'm going to make a female mold of the lid, including the recesses/grooves for the stiffeners and will use epoxy and yet-to-be-selected weight of cloth. I'll probably unweave some old 24oz woven roving and lay the strands in the recesses that make the stiffeners, then use some weight of biaxial.

I guess that should give me a decently stiff lid. Would laying kevlar or carbon fiber in the stiffener recesses add to the stiffness? I don't need strength, I need stiffness.

Thanks








Della and Dave

#1
Hi:  I added ports a couple of years back, and initially had trouble getting them to seal. You can see that post at:  http://www.pearson365.com/forum/index.php?topic=1408.0  Our ports were smaller than yours and still had some deflection issues.  In retrospect, it would have been easier to have added plies to the tank top before I cut the holes.  They do work now after some fussing around.  If I ever get bored or it becomes a problem, I might pull them out and add plies to the tank top.  If I get around to the starboard tank, I will defiantly do that, and use a flat caul plate to give me a flat surface for the deck plate to bolt to and seal against. 

On choices of material, I would stick to fiberglass if I were you because of the kind of stresses that can be set up if you have incompatible stiffness and thermal expansion and contraction coefficients.  Kevlar has a tendency to absorb moisture and swell, setting up stresses between the plies. 

Carbon is stiffer than glass usually, but is harder to work with and a lot more expensive.  It is also at the far end of the galvanic series from stainless steel.  There are many different types of carbon fiber available, from "Low Modulus"  to "Ultra High Modulus"  The differences are really big in stiffness.  Some of the ultra high modulus fibers are really hard to get the glue to stick to the fiber.  They come if different sizings as well that are comparable with different resin materials, so you need to be careful that you get the one that is comparable with the resin you use, and the sizing on the fiber has a shelf life and sometimes doesn't like being left open in a high humidity environment and refuses to stick to the resin as a result.  (Sizing is a chemical added to the fiber to get the fiber to stick to the resin, sort of a primer) 

If you choose unidirectional material, it is stiffer than woven material by quite a bit, but only in the direction the fibers are running.  That means you usually have to lay in a few plies.  (The over and under of a woven material lowers the stiffness)  A fairly common laminate stacking sequence would be 0 degrees, followed by + 45 degrees, then -45 degrees, then 90 degrees for the orientation of the fiber direction.  That means the thickness of four plies, but would be stiffer than two woven plies by a lot and about the same thickness.  A uni ply is thinner than a woven one of the same fiber count per tow.  (A tow is a bundle of fibers.) You also get more fiber and less resin in unidirectional material, so thickness per thickness it's stiffer for that reason as well.   

The bi-axial tape gives you the higher stiffness with the +/- 45 degree plies already built in, but you might have issues getting it in wide enough tape for your project.  I am sure some supplier has it though.  I believe Pearson used polyester resin, but I tend to use epoxy for repairs myself because it's a stronger and usually bonds to polyester well. 

P.S.:  Think about what is going to happen if the boat really starts to flex in heavy seas, not just what it does at the dock......  One reason deck plates are round and smaller than anyone wants for access reasons. 
Della and Dave
S/V Polaris

P69

Della & Dave,

Thanks for the good points. I'll rework my design and look into making them round, then make a test hatch with just fiberglass and epoxy, then test stiffness.


P69

I could not find any access hatches that were 1) big enough to reach all corners of each chamber and 2) not several hundreds of dollars each. So, I made my own.

It will be interesting to see if the lids leak when the boat flexes.  I'll have to develop my plan B if that problem arises.

The tank interiors were a mess. Baffles were separating and  the surfaces were covered with various colored mold that different concentrations of bleachy water would not remove. I had to reach into each chamber to scrub the mold off.  The remaining gel coated surfaces imparted a strong fiberglass odor. I cut square holes to do the cleaning, repair, and sealed with potable water epoxy. Closed them up, then cut round holes (10" dia) and made lids for the tanks.

I finished my lids for the settee water tanks (bow tank removed).
Each of the three chambers has its own lid, big enough for me to get a good look inside and reach in to repair.  All the baffles needed to be reattached.

I made the lids out of fiberglass with polyester resin with sandwich of balsa core, two 1/4" layers, separated by one layer mat

I made a mold so all the lids would be identical.  Male plug made out of blue foam, shaped by hand, faired with epoxy (polyester dissolves blue foam. Made female mold out of gelcoat/poly/mat from the male plug (black object in attached picture).

From there, I made my six lids. I sprayed polyester gelcoat, then layered mat/balsa/mat/1708.

The lid has two diameters:
  Inner, 10" diameter
  Outer, 15" diameter
The inner lid forms a plug for the hole to minimize water contact with the gaskets. I wanted to reduce the chance of water dislodging the gaskets.

The smaller diameter has the balsa core (to reduce weight, increase rigitity).
Larger diameter is 1708 and this is what overlaps the tank body.

Attached is a diagram of the lid design.

The lid has two rings of 1/2" EDPM foam glued to the lids with 3M Weatherstrip Adhesive #8008

Each lid is held down with eight 5/16" x 18  bolts that are threaded into 1 1/4" x 1 1/2" x 2" pieces of G10. These eight pieces of G10 are epoxied to the ceiling of the tank about an inch away from each opening. That puts the bolt hole roughly in the center  of the epoxy block.  Epoxy blocks are drilled and tapped for the bolts. Ring of bolts is outside of the two gasket rings and the bolt holes do not go all the way through the G10 blocks (keep water out of the bolt holes).

Holes in the tank are 10" diameter, big enough for me to get my head in there and look around and reach all areas of the tank for cleaning. I originally cut square holes to get in the tanks and repair the baffles/coat interior with potable water epoxy. Sealed tanks backup, then cut out circular holes

The interior of the tank is coated with potable water epoxy.

In order to get a perfect fit between lid and tank, I put a layer of thickened epoxy on the tank around the hole, then carefully put the lid into that paste. A few days later, after the epoxy was good and cured, I popped the lid off the  tank. First one was scary because I thought I coudl just gently tap the lid edge and expect it to  pop out. Even though I waxed the hell out of the lid, I still had to pry the lids off. Ended up temporarily glassing a 1" x 9" galvanized pipe to each lid, then lashing a 1 1/4" x 3/16" wall square tube (that was all I had lying around). I pried with all my might and it popped right off.  That gave me a perfect fit.

I have not 'water tested' them yet. It'll be months before I get the plumbing back together, then fill the tanks.

More pictures are at:
http://bodylens.com/Gallery/thumbnails.php?album=41
http://bodylens.com/Gallery/thumbnails.php?album=42
Attached are a few sample pictures



1 1/4" G10: https://www.mcmaster.com/#8557k72/=1e1ne8p
Potable water epoxy: https://www.epoxyusa.com/coat_tar_p/ep34.htm
Gasket material: https://www.mcmaster.com/#8605k44/=1e1mnb0