Blowdown Skin Fitting

[Sandles100]

I'm seeking advice on what to use as a blowdown skin fitting for my boat. Hull is glassed ply, max boiler pressure is 125 psi, so max temp in the blowdown line could be up to 350 F/177 C, so I think I need to somehow insulate the line from the hull. Any guidance or good designs out there?

[Lopez Mike]

My take would be to just use an ordinary bronze through hull and not worry about the temperature. By the time the steam and spray get to the hull, the temps (and pressure) will have dropped enormously. Also, fiberglass can take a lot of heat.

I'm assuming that the blow down valve is at the boiler end of the hose and that you use steam rated hose.

Mike

[PeteThePen1]

For a blow down fitting through the hull I was advised to acquire one or more old fshioned long stem taps. These usually have a good big base and a long threaded setion. The tap can be cut off near the base and the part that will be the visible fitting can be turned down in the lathe to a suitable shape. The threaded part can be cut down to suit the application.

The picture below is stolen from eBay since my internet searches did not turn up a suitable photo. However it does illustrate the sort of tap that was intended. The best source would probably be a scrap yard or municipal re-cycling depot.

Old Tap.png (52.87 KiB) Viewed 2346 times

Adding to Mik'e comment I would expect that the fitting, assuming that it will be fitted to be under water, will be chilled by the water in which the boat floats so the heating issue should not be too much of a problem.

Regards

Pete

[barts]

A below-surface discharge for a blow down should have a shutoff valve at the through-hull fitting; as with all through hulls this should be closed if the boat is left moored somewhere. If the boat is going to be operated in salt water or other waters unsuitable for the boiler, I'd fit a check valve to make sure leaving the discharge valve open doesn't result in a boiler full of salt water when the boiler develops a vacuum on cooling. Also, keep in the mind that the fitting & hull will be subject to boiler temp if blown down on a trailer; waiting until the boiler is down to 10 or 20 psi before blowing down on the trailer will help keep temperatures reasonable.

I've found a steam hose that can be directed overboard connected to the blow down valve is a reasonable compromise.

- Bart

[Lopez Mike]

I agree with Bart on this one. Keep it simple. After all, how many times a day or even week does one need to blow down. I'm assuming that your system is condensing. I choose not to put lake or stream water in my rather expensive system!

I use common 3/4 turn 1/2" water valves just about everywhere in my boat including the throttle and have not had a one fail. Looking closely you will find that they are rated at 150 w.s.p. By the end of the steam hose (Yes! Above the water line!) the temps have cooled down a good deal. I'll check my temps for you when I creep back from Baja in the Spring. Cough, cough.

The handles get rusty over time. I replace then with S.S.

And do keep as much of your blow down system above the water line. Peace of mine matters.

Mike

[Sandles100]

Hey Friends,
Thanks for the advice and tips. I'm sorry I can't deal fully with it at the moment - the family insists I take part in Christmas and New Year stuff - Bah Humbug - steamers are much more fun than reindeer, sleighs, eggnog and moose's milk - maybe not the latter. I'm planning to sneak off for thinking time later. Merry Christmas and Happy New Year to you all.

[barts]

I'm planning to sneak off for thinking time later. Merry Christmas and Happy New Year to you all.

And a Merry Christmas & whatever other holidays is being celebrated to one and all!

- Bart

[fredrosse]

I use the blowdown system recommended by BARTS. Coming off the boiler is a 1/2 inch ball valve, followed by a steam service angle valve (double valving required here), followed by a common hose connection, and 4 feet of proper rated washing machine water hose. After I put the boat on the trailer I dump steam down to 15 PSIG (250F saturation temperature), open the angle valve, and blow dirty water overboard with the ball valve until the blowdown is clean. This takes about 2-3 seconds on each blowdown connection, three in all.

After that I can very easily steam clean my engine where oil and grime have accumulated. Finally a coating of WD-40 on the exposed iron/steel, and the plant is stored until the next outing. Oil absorbing pads in the bilge catch the oil and grime, and I vacuum out the bilges when I get home.

Maintain the boiler water at a Ph of 9 to 11, by adding Tri-Sodium Phosphate, usually about a tablespoon full in 10 gallons of water will do. This provides corrosion protection for a steel boiler, and keeps impurities from forming hard scale. At pet stores you can buy Ph test strips (for fish tank maintenance) at about $5 per 100 test strips.

[Sandles100]

Ok, Thanks for all the advice and information. I think I'll go for the standard bronze skin fitting, with a ptfe disk inside the hull, Ball vv and NRV at the hull end. Half inch blowdown valve on the boiler, (Ofeldt type) with a copper blowdown line and a good sized expansion loop. I'm not a fan of steam flexibles unless they are the really pricey ones.
Again, thanks, and Happy New Year steamies.