Thread: GIS: The hollow square mast and big temperature changes

I haven't had any cracks in my hollow spars which are sealed for flotation. But they are covered with fiberglass and carbon sleeves in epoxy. Peter

Considering the forces that the mast takes when sailing and the mast having been designed for 3 persons sitting on the rail to keep the boat upright in very windy conditions, I would not worry about this at all.

Joost

I imagine the mast will constantly bend to the south, trying to find more stable weather...

I don't get temperature swings quite that large, but I've had no problem with my mast. There must be little pinholes somewhere allowing the pressure to equalize over time.

I am a little more concerned about my seat tanks. On a couple of occasions, I have forgotten to open the hatches after pulling the boat from the water. The pressure builds quickly in the hot sun.

Didn't you have a black GIS? It must get really warm inside of the tanks. I even read an article claiming that black painted epoxy becomes hot enough to weaken the epoxy over the time.

Originally Posted by engblom

Didn't you have a black GIS? It must get really warm inside of the tanks. I even read an article claiming that black painted epoxy becomes hot enough to weaken the epoxy over the time.

If the boat is in the sun on a hot day, it is upright, so the sun shines only on the varnished parts.

(Okay. There has been a capsize or two, leaving the black parts on top for short periods. But only very short periods. )

I did leave it upside-down outside over the winter, but in a very shady spot. It hardly ever gets direct sun beside the garage. I plan on building it a better shelter this winter.

There is an epoxy strengthening process called postcuring - basically baking the part at an elevated temperature.

However there can be "creep" ... that the epoxy becomes a fraction softer and there is some movement in the joins. That's the way you spot the result of "creep" ... you start to be able to see the joins in the underlying structure in the paintwork. It's not like a crack at all just a stretchy movement.

In Oz we might have the real upper end compared to almost anyone else but the Texans, but we don't have the cold end. I do often think that freezing of liquid water would be about the worst thing.. but the epoxy in tests brings the moisture in the timber down to an equilibrium with the average humidity of the area the spar is in.

If I could store the spar in a garage or a roof or up under an eave or somewhere there might be a bit of extra heat ... that would be good. But it does look like that there are few masts that actually seem to have a problem.

MIK

So, you seal up the mast, then raise the temperature say 30 degrees celsius. Thats about a 10% increase in the Kelvin temperature, so pressure will go up 10%. 1 atmosphere is about 100kPa, so the excess pressure is about 10kPa. (Assuming you sealed it up at a temperature somewhere in the middle of your 60 degree celsius temperature range).

Pulling out my engineering books, stress around the circumference of a cylinder s =rp/t where p = pressure inside the cylinder, r = cylinder radius, t is wall thickness. Wooden spars have 15-20% wall thickness relative to diameter, 17% is about 1/6 so cylinder radius is about 3 times wall thickness for these spars, so circumferential stress is about three times the excess pressure inside the spar. Circumferential stress on the timber therefore about 30 kPa.

Cross grain strength of timber (table 4-3a.) is about 2000 - 3000 kPa, so pressure from the inside of the spar getting hot produces about one hundredth the strain needed to burst the spar open.

Picture is different with large, thin walled tanks (e.g. sealed buoyancy compartments), but I wouldn't worry about the effect of pressure on a hollow, sealed spar.

Ian

Thanks!

You are a good man to have around Ian!

MIK