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Tuesday, October 26, 2010

Need to Vent?

Despite being knocked on my back with illness late last week, I managed to get some stuff done toward the end of the weekend.

The logical follower to fuel fill lines is, fuel vent lines! All tank vent lines lead up to a small manifold mounted on the bulked near the deckfill. Each vent line has an inline "fuel stop" that prevents overfilling of the tank by keeping the vent line clear of fuel. Basically a check valve.

The combined "out" portion of the manifold leads back to the engine room, to which another hose which snakes up inside the coaming and down to a thru-hull vent fitting in the cockpit, just above one of the cockpit drains. This setup follows "all the rules" with the vents leading to the highest point (coaming) for the loop (reducing the chance of anything starting a siphon), down to an inconspicuous discharge outlet (kick panel of the cockpit), with no sags anywhere in the lines to trap air or fuel.

As each tank has its own vent hose that leads up near the deck fill, there is a greatly reduced chance of any cross-contamination of fuel between tanks. The only way it could happen is if any of the "fuel stop" valves fail and the boat is nearly upside down. In which case, we would have larger problems to worry about.

Also, to keep things tidy and organized, I machined a couple "hose mounting clamps" out of coosa.


Rhys said...

The things you find with ultra-specific Google search terms...

Robert, that's an excellent installation. I am currently grappling with the issue of tank vents, specifically where to run them.

I'm installing (finally) my Filter Boss after launch, and to lessen the odds of water getting in to the tanks, I am choosing to reroute ALL tank vent line runs from the typical gunwhales or cabin sides to the pilothouse, either to a gooseneck or two on the roof or under the roof overhang. I got this idea from the same old book on North Sea trawlers where I first saw a transverse exhaust that dispenses with a siphon break (a particular bugbear of mine).

For the exhaust, see Dave Gerr’s drawing of what I mean here:

It strikes me that if one can run the vents higher, and T in a petcock for drainage at the bottom of the vertical run, you stand a far better shot at keeping water out of the tanks completely without the use of problematic (to me) check valves. Your thoughts would be appreciated, although really, I can't fault your decision to run one "master vent" at the bottom of a undercoaming loop to your cockpit.

Robert Sutton said...

Certainly the higher the loop/vent the better.

I don't care for check valves either. In my case, the goal was to keep tanks from being overfilled. Vent-stops just above the tank was the easiest solution, while still keeping the devices accessible.

Vent discharge in the cockpit footwell is right above the cockpit drain, so any fuel discharge discreetly goes overboard. As we are a center-cockpit, getting water up in the vent seems unlikely enough for me.

I was a board a local pilothouse sloop where the fellow ran the vent loop up inside the stainless steel framing supports for the roof, then down onto the deck. Completely contained in the wall and hidden from view. You could not get the loop any higher unless you went up the mast.

Rhys said...

Because I intend to install a Tank Tender setup, and will therefore have closely monitored capacity (along with fuel cross-transfer pumps if I need to pump out or "pump forward"), I'm going to skip the check valves and do something like what you describe in my pilothouse.

My concept is to get brass poles from pilothouse deck to the pilothouse roof as (on one side) engine fresh air intake, and on the other, a single open vent line going to a vent line manifold much like your own.

So it's not dissimilar, which is the case the deeper I get into your blog: a lot of similar preferences and concepts executed in different orders and timeframes. You've got the far bigger to-do list.