Zygote: 11 years later: Hull moisture

Zygote is scheduled to today be re-launched, after more weeks than I want to remember of yard work. We had the bulwarks, cover board, and taff rail sprayed with Alexseal. And the rudder cheeks and boomkin coated with an 6-coating system from Milux, which is apparently only sold to professionals.

The manager of Scarborough Marina kindly lent me a moisture meter to run over the hull (after what we’ll call 4 weeks of being on the hard). Other than yard work, Zygote has been in tropical or sub-tropical waters for all except a few weeks of its 11 years.

The hull, which had a coat of vinylester before leaving the Sam L Morse Co yard, registered at 0% (or less than 5% - the needle of the instrument did not move) all over except for one patch about the size of my hand, about 20 cm vertically below the prop shaft/cutless bearing on starboard. That patch read 7 - 9% moisture. No detectable moisture on port side. My guess is some water enters the hull beside the cutless bearing but does not travel far.

The rudder, which is GRP, registered an even 17% moisture. I don’t know how Zygote’s rudder was constructed - I’m guessing it has a wooden core or a foam core, probably the latter. No blisters in the gelcoat.

Zygote has a Freehand self-steering trim tab aft of the rudder. The trim tab is teak. About 6 years ago, while we were still in South-east Asia, I noticed some worm in the teak. So I ordered a new trim tab from Mike Anderson but, before it arrived, I had a yard in Singapore coat the old trim tab with a high density epoxy. The trim tab was no doubt moist when it was clad in epoxy. And the trim tab reads maximum moisture (25% or greater)!

I had the moisture meter set to read surface moisture.

I’ve no time to do anything about the water in the hull today (a reading of 7 - 9% is acceptable according to some), but I will think about whether there’s a way to stop more water entering via the same route. Your suggestions are welcome. Douglas? anyone with hull moisture figures?



Yes , Bil , Please stand by . Need time to gather photos !

Ahoy Bil , more info will follow , but for now , the “deadwood” area, just ahead of the prop and below it, can be suspected to absorb heaps of moisture , Ouch !

During hull construction , there was a plywood dam placed into the hull. There were two such dams, one aft , and one forward at the aft end of the lead balast.

The filler resin , behind the dams, maybe could be called an “end pour”, was poured into a mish-mash of floor sweepings and f/g rolled up cloth .

This could later be considered an undesirable hull fabrication practice !

On BCC Calliste hull # 72 , the moisture readings, in the aft “end pour” have always been high .

The real key , is does this high moisture reading cause a blistering concern, in that “dead wood” area or not ?

Sorry , that I am not familiar with ether Alexseal , or Milux , ?

If Fletch is still at Scarbourough , he would be a most realiable consultant, for advice.

Please do understand , that even wood in your house furniture , will measure any where from 10% (very dry) to 18 % a bit wet , moisture content.

For us boat owners we most be concerned about the difference of moisture readings in our boat hulls , those above the water line, and those below the water line !

Marine surveyors suggest taking a moisture reading at our cabin sides, then compare that to the underwater areas, when on the hardstand .

This reading may or may not show a problem area with excessive moisture absorbution.

Moisture absorbution in our hull f/g laminates , may or may not be a concern !

I have seen epoxy resin laminated boat hulls with 20% or higher hull moisture readings , but zero amount of blisters, anywhere on those hulls !

You may remember that on my boat , the f/g shaft log broke loose and allowed moisture to migrate down into the “dead wood” , below the shaft , an area that was riddled with air passages and voids , just ripe to absorb and hold moisture, thus creating high moisture readings from the outside, in those areas .

More to follow ,

Thanks, Douglas.

We’re not worried about the moisture in the hull in the deadwood area. A reading of 7 - 9% is nothing and still in the ‘green’ zone (I did run the meter over the hull above water line to calibrate the meter - Zygote’s hull above WL reads 0% water. And below WL is 0% too, except for that one patch in the deadwood on starboard).

The rudder moisture level is higher. The meter is marked to call that the yellow zone. No gelcoat blisters. No cause for concern yet (as you say, many GRP boats show 20% moisture. A worker at the yard in Scarborough said their operating guidelines ‘allow’ them to lay glass and resin over GRP with moisture up to 16%)

Thanks for the recap on how the deadwood area was built. One of the Sam L Morse Co videos showed a shipwright with a hole saw cutting the aperture for the shaft - and I guess that only the heat generated by the hole saw would seal the cut edges.

I guess that there are two routes by which Zygote’s deadwood has collected a little water:

  1. a small localised failure of the vinylester barrier coat (which would explain why the moisture meter only shows water on the starboard side); and/or

  2. moisture entering cut edges of the shaft aperture.

What do you think of those two routes? Would it anyway be practical to seal the cut edges of the shaft aperture?

As I understand it, Alexseal Premium Topcoat is a composite of polyester and polyurethane, resulting in a thicker and more flexible coating than Awlgrip (and other linear polyurethane LPU coatings). Claimed by some to be more suitable to the marine environment than LPU. And can be repaired more easily than LPU (runs can be sanded flat, nicks can be filled flat etc). We paid for a spray application, but roll and tip is said to deliver acceptable quality. See www.alexseal.com

Milux is an Italian company that produces, among other things, this particular 6-layer coating system. In Australia, it’s only applied by professionals (ie the distributor keeps it away from us full-time sail boat maintainers who reward ourselves with sailing every now and again). I only had it applied to woodwork that is difficult to work on while Zygote is in the water (rudder cheeks, boomkin, staysail halyard turning block) as a test to see if it’s worth using. Jolly expensive. Application is uneven - the worker who did the boomkin left brush marks (spit! I can lay varnish with brush marks any day of the week, without trying! Mostly I try hard to avoid brush marks, so I’m amused that the yard worker thought I wanted a non-skid pattern on Zygote’s boomkin.) but the worker who coated the rudder cheeks and the staysail halyard turning block did a superb job free of brushmarks.



Hi Bil, Oh, v good memory, you have, about seeing the BCC construction video, and the hole-saw drilling for the shaft log.

If the SLM workers used the next larger size hole saw, then fiberglassed that cut-out hole, then epoxyed in the shaft log, we wouldn’t be talking about it now!

Sorry, but I am having trouble adapting to Lang’s new puter, so not able to attach pics yet !

When I first discovered high moisture readings in the deadwood area, my yard guy advised to just drill a 1/2" hole at the lower point of the high reading. Not all the way through the deadwood, just about 2/3rds the way through, thwartships.

Out of this hole came about a cup of salty water, ouch !

He then advised to tap the hole and screw in a nipple, and then attached an air hose onto the fitting.

Result was v slow air coming out from the shaft log to deadwood joint area, both inside the bilge and outside the boat.

Ah Hah, me says , so just how far down the deadwood filled cavity will I find another “glory hole, fountain” ?

We plugged the higher-up hole, and began an air pressure assisted drying of that dead wood-filled cavity, we also pressure injected acetone then styrene, then filtered dry air.

You can now guess our next step ,