Tuesday, August 21, 2007

A Beautiful Solution

I'm all about simplicity. I have a low tolerance for frustration. And I am easily surprised by engineering decisions that are obviously bad.

That said, boating offers some spectacularly horrendous examples of what I call 'That Should Work' solutions - usually taken as an expedient fix with the idea that it'll get taken care of correctly later. But when a boat is built with these 'Gotchas' I am truly amazed.

Here's a case in point - Pelican has three water tanks, one each 50 gallons under the port and starboard settees in the main cabin and one 60 gallon tank under the v-berth. So far, so good. But the vents are not overboard - that's also a pretty good idea except in this case when the port tank vents into a hidden space, the starboard into a hanging locker, and I haven't found the vent for the bow tank yet.

Rerouting the vent is no problem - I've rerouted them to the bilge which is not the best idea but is better than venting into your clothes or somewhere else. I may change them to vent into the forward cabin's sink, which is the best solution. But that was a stupid engineering/builder decision.

The next issue is one of inappropriate engineering. When you're venting into hidden spaces, you need to know how much water is in the tank to prevent an overflow. So someone - Either Pearson or a previous owner decided to put a clear plastic deckplate into the tanks so they could ostensibly clean the tank and see how much water was in it during filling. Sounds like an idea, right? Except those plates are designed around keeping water out of something, not in.

More to the point, to get to it you have to remove all the cushions and slide the berth extension out. How about not needing to do that each time you fill the tank, eh?

If you inadvertently fill the tank above the top but not enough to spill out the vent some three feet above the tank, you put approximately 1.3 psi on the tank, and that results on a total force of about 16 pounds over a 4" diameter inspection plate. That is enough to make it leak. And that doesn't take into effect the dynamic loading of water splashing about.

Ok, you say, you've lowered the vent. That should help, but the fill is 1.5" and the vent 5/8", so you could have a standing column that can essentially pressurize the tank to some 3psi including the dynamic loading of the force of the water - so even with the vent there will be leaking around the plate.

So what's the solution? This is great. Brewer's Post Road Marina's maintenance manager turned me onto this product that will work on any tank that doesn't involve explosive liquids like gasoline. SeaBuilt makes real, honest inspection plates out of stainless steel or aluminum.

They consist of a split back plate held together with a rubber gasket, a separate gasket for the top, and of course, a plate that's bolted on.

Installation is as simple as simple can be. If you don't have a hole in the tank, cut one or drill it with a hole saw, drill the 3/8" holes in the pattern of the top plate, fold the internal ring at the gasket and insert it into the hole, push 1/2 up through the holes you drilled and place the top gasket over the bolts to hold the thing up. Then you unfold the bottom plate, do the same with those bolts. Finally put the plate on with the supplied washers and nuts (they even supply an extra set to replace the ones your going to drop in the bilge), snug them down and you're done.

If you're handy with power tools, it'll take you about an hour. If you're not, maybe an hour and a half. It's that easy and you will have an real inspection plate that can take the same or more pressure than the tank it's in. Highly recommended.

Next: Installing instruments, frustration associated with that and living out of the water, and the good people at Raymarine.

My cruise starts next weekend, and I'll be seeing you on the water!

1 comment:

Unknown said...

Your math is wrong as to psi on the tank. having a vent run 3 feet above a tank would indeed raise the pressure by 1.3 psi, however, you would be raising it for the square footage of the VENT LINE ID - it it's a 5/8" line, thats .3 square inches, or roughly 8 OZ of pressure, total

Applying this to your 4" deck plate, which has an area of roughly 12 .6square inches, equals a pressure of .03 psi.

In other words, the vent height adds no appreciable pressure to your tank whatsoever.

Heeling over at 25 degrees, however, places many gallons of water above the port, at 8 pounds per, puts considerable pressure on the inspection port.