Hull inversion hotspots

[geoffr]

Hi folks:

This morning I noticed two large inversion areas on my hull, in the rear quarters just aft of the two rear rollers on the trailer.

One is quite large: about the shape of a football, with the hull seemingly sucked in by about 1-2 cm. The inversion on the other side is not as big.

I'm wondering if the 40 degree day we've just had here in Victoria has caused these, given they sit directly under the sealed floatation areas, accessed through the port and starboard cabin lockers. (My boat also sits outside, under a tarp.)

I'm thinking that these floatation areas are sealed so well that in the heat, the air trapped in both areas has swelled the hull out; then, with today's markedly cooler conditions here, the air has contracted and sucked the hull back in with it.

If that's the case, then the probable remedy is to drill an air escape hole down through the locker floors to allow the pressure to equalise, then seal the hole with an inspection lid of some sort.

Has anyone else had this issue? Always something new to worry about and fiddle with!

Cheers,

[Yara50]

If it aint bust don't fix it. I have noticed the same on Yara. As long as the hull is sound I would leave well alone.

[geoffr]

Thanks Ian - I'm glad I'm not going crazy here.

The hull in both areas certainly seems sound enough, and after drilling two small holes into the port side floor area (and nothing happening, I.e no sudden 'pop'), then I'll be leaving as is, and filling the holes again with some bog.

Cheers,

[Steve]

So what do reckon these spots are Geoff, would they be something to do with the molding of the hull?. I have found that alot of the basic fitting out of my hull is as rough as guts.

[Yara50]

WARNING! I think these may be caused by wobble rollers. Definitely do lower the rear trailer supports during launch /retrieve. I am going to buy a dent puller to see if i can pull them out.
I am beginning to hate wobble rollers on our boat trailers.

[geoffr]

Hi all:

I agree with Ian that the pressure of incorrectly set rear or front rollers can dent the hull over time, especially as the centre rollers (that carry the boat's weight) wear or disintegrate (and we don't notice in time).

In my boat's case, this incorrect setting has caused significant hull damage which I am currently fixing.

I think too that if you tow a boat any distance over bumby roads, the up and down movement would also flex and weaken the hull where it makes contact with the rollers. The previous owner of my boat towed it regularly on country roads out to Lake Eppalock and back to Bendigo, so perhaps that's where the original damage was done.

Good tie downs help address this, but I imagine even they flex under great pressure, and you would only need the boat to go up and down about 10-20 mm over a prolonged period to start flexing the hull spots. (I'm basing my views on observation, but would welcome the views of others.)

I'm currently replacing the original 30+ year old black rubber rollers on my trailer (that mark my hull every time I load or unload) with new grey plastic models. During this process, I'm resetting their heights so they only just sit against the hull. However, as I remove each roller, I'm finding more hull damage beneath -- some is just worn gelcoat, but other areas run much deeper.

In an ideal world, I'd replace the rear rollers with pads, but I don't have welding gear. Perhaps lowering the rear rollers during launch and retrieval might work, but who could bother doing this? It would add an extra 20 minutes of frigging about with a ring spanner, which is all you want at the end of a day's sailing.

The other indents I mentioned at the start of this thread remain a mystery. I doubt very much that they were caused by rollers, as the hull strength over these areas is otherwise very tough, with no flex at all. I still believe they sucked in due to some vacuum issue with the sealed areas on the inside of the hull.

I'd be interested in hearing if Ian or anyone else has some luck in pulling these dents out as panel beaters do.

Cheers,

Geoff

[Yara50]

Still working on the theory. For a 30 degree temperature difference, the pressure would be roughly 10% of atmospheric, say equal to 1m water. (1 atmosphere =10m head). Over an area of say half square metre, thats roughly 500 kg force. Add some water load in a seaway, and yes, maybe it could pop inwards. I pulled with a load of roughly 100 kg, and it did not pop out.

[camanche73]

I have had the sealed compartments of my investigator cut open for storage. The glass there is not very thick. I would imagine if it was due from a pressure change, the inside of the compartment would dent in and collapse as opposed to the hull which is a lot stronger.

[Yara50]

Good point Ron. So there goes the temperature change theory. The dents do not line up with the wobble rollers, so I might forgive them at least this transgression. Could be that that some stresses were created during the lay-up due to shrinkage, and that a good whack from a wave is enough to flip it to a stable position in a negative curve. Funny thing is, I never noticed it before on the starboard side, only the port side. Now the starboard side looks worse.

[Steve]

Photo's anyone? I only seem to have trailer damage in that area of the hull.