Hull pressure

A quick question please. I opened my drain plugs this past weekend and while both were nice and dry, I had some air pressure in the starbord hull. I'd guess around 20-30# of pressure. In reading other posts I've learned there are breathers under the front crossbar to prevent pressure buildup. I have been trying to locate these holes for inspection but so far i havn't found them. Is 20-30# of positive air pressure normal or harmful? If needed, can these breather holes be accessed with the boat assembled?

Thanks

Whip,

20 to 30 pounds of pressure in a Hobie Hull is highly unlikely. that is as high as some car tires. that kind of pressure in a Hobie hull would cause it to literally excplode. It does not have the structual integrity to hold that kind of internal pressure.

anything over 2 or 3 pounds of pressure would cause the hulls to ballon and pressures higher than that will cause the beginnings of seperation at the hulls weakest points usually the seam where the top is joined to the bottom.

I have heard many others talk about breather holes located under the front crossbars of the Hobie 18, but I have never seen them myself, in any of the boats I have owned. If I understand what others have described correctly, you would have to unlace the trampoline, unbolt and lift the front crossbar out of its normal location in order to access any breather holes that may be there.

I Have indeed seen the breather tubes located in the front pylons of the Hobie 16's, but the Hobie 16 is built with no deck access like the H-18.

Speaking for myself, I wouild not create any breather holes. There is just something about drilling a hole in a boat that offends my common sense.

If I had a problem that caused me to want to drill a pressure relief hole anywhere in a Hobie 18, I would drill a VERY SMALL hole, in the porthole that screws in right behind the front cross bar. That way it is easy to maintain, and easy to seal with silicone if you decide that you have made a mistake. Also, it does not create a place for water to get into the "fabric or core" of the boat itself. Worst case scenario, the porthole is cheap and easy to replace.

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If the grass is greener on the other side of the fence, maybe it is time to water your own lawn.

All I can add is that I have never had pressure build up in my 18. I sail in Oklahoma so there can be wild temperature variations between the water and on the beach. One would imagine a cooled hull in the water, then pulled on the beach and checked for water (resealed out of habit) which is allowed to sit in the sun while we play around would build some pressure.

How would you get a significant amount of pressure in the hull? Unless you sealed it nice and tight in the mountains then drove down to the sea. Guess I'm just confused.

If this buildup is while you are sailing, perhaps crack the port-hole covers every now and then. Maybe I have these magical pressure holes and just don't know it. I'd be interested to hear what you think caused such significant pressure.

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Proud new owner of a 1991 Hobie 18.

Hmmm, very interesting. While I have some experience with pressure in commercial situations, and have witnessed escaping pressure from a number of sources, I could be wrong regarding the strength of pressure escaping when the plug was pulled.

I thought 20# of pressure is very little in relation to what might be considered criticle or hull damaging, and I seriously doubt 2-3# of air pressure in a hull will damage it, but I could be wrong.

I suppose it may have been a vacuum, but from the sound it seemed like a blow. I'll check it often to find out for sure. In any event I doubt very much the amount of pressure I witnessed escape the hull was sufficient to damage the hull, but again I may be wrong. I guess I'll pull the plugs when not using the boat and check for pressure after a day of sailing.

As for what caused the pressure/vaccum? I have no idea, except the boat was on a very long solo sail accross 100 mi of lake Michigan through several storm blows for 4 days, then towed by the Coast Guard and then put up on the Station lawn for 2 days till I came to retrieve it.

The difference in water temp and air temp can be very extreme in the middle of Lake Michigan, like 50 degrees water to 80 degrees air, maybe this was the cause.

That's likely.

I've blown up deck seams at 12 PSI. It's cool that your boat is so tight. If it were leaky on that journey and the hull filled up instead, when the Coasties towed it it would have put so much pressure on the x bars and fittings, something would've broken. I saw an 18 get towed by the Coatsies in the late 90's and by the time they got it into the harbor, the decks around the x bars were all but broken completely off.

Crack the ports when the boat is stored and you'll be fine. I've seen people drill small holes in the port lids. Like on the Tiger and FX.

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The first Hobie I bought back in "81 had rope handles on the caps of the inspection holes for removal. The dealer told me those holes with the rope in them allow hull pressure to remain equal. The 18 I bought used a few years ago has the threaded caps that are water tight.

Just got back from 5 days of solid sailing on the big lake. Put her through some pretty good surf and wind. I used the small drill hole through the cap trick and both hulls are bone dry, and no pressure difference. THANKS for the tip!!!!!!!