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Deck construction ?

I just had my 1984 C36 Mk 1 surveyed  and the surveyor used a combination of a moisture meter and tapping to report there is moisture present in the side decks which could be a problem. I have often been down on my hands and knees examining the decks and tapping them for any signs of a dullness to the ring of the non-skid surfaces, swelling or sponginess and have found no apparent issues.The decks feel solid.
Does anybody have any experience with how moisture presents itself, what signs could go undetected and are these so-called moisture meters of any real diagnostic value? I've heard they require great skill to  interpret the results because  it's conductivity changes they are detecting NOT percentage of moisture.
Also does anybody know what the construction of the side decks is made of? Is it a GRP/plywood/GRP laminate or a GRP/end-grain balsa/GRP laminate?
What could be the effects on hull rigidity and ultimate strength of the boat if there is some rot starting. Is there a simple, minimally invasive technique to test the core to verify if there is a real problem developing?
I live in Ontario Canada where we sail May - October then the boats are laid up during our frigid winters.
Many thanks.
PS - He couldn't find the hull identification number said it's 12-14 characters long but all my boat has is the aluminum plate at the transom marked hull #288, Does anyone know if there's another number tucked away somewhere out of sight?

KevinLenard's picture
Last seen: 1 day 6 hours ago
Joined: 1/28/15
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In 1991 the hull serial number was stamped into the gelcoat on the starboard side at the stern just below the toerail.

For some minimal info, see JPG attached.  There are also several YouTube videos from 2011 by Catalina illustrating their current build techniques.  I'm sure other owners will have more info for you. 


Kevin Lenard
'91 C-36 Tall Rig, Fin Keel, Hull #1120, Universal M-35 original (not "A" or "B")
CBYC, Scarborough, Ontario, Canada

Last seen: 1 week 2 days ago
Joined: 4/2/16
Posts: 8

Hi Kevin - Thanks for this but it is really the decks to the side of the cabin where the moisture issues have been detected - basically form the cockpit forward to the aft lower shrouds and undermining the double stanchions. My hull so far seems ok execept some osmosis blisters.

IslandTime's picture
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Jeff D
Island Time
'84 MK-I SR/FK #273
Kemah, TX
Last seen: 1 week 2 days ago
Joined: 4/2/16
Posts: 8

Hi Jeff, thanks for the diagram - once again this is helpful but I'm still not seeing exactly what  the side deck composition is from the cockpit forward to the aft lower shroud areas.The surveyor indicated 15 sq ft was affected each side which is almost the whole deck form cockpit to shrouds.I am able to hear slight differences in the percussive sounds when tapped but the decks are in no way spongey underfoot. There a few stanchions that seem to move a little with side to side pressure but only the port double has slight signs of upper deck skin movemment. I am thinking I will have to remove the genoa track and several stanchions and all 3 shroud plates to fully access the damaged areas. I am assuming there is vertical grain balsa between the upper and lower deck skins but again the  PDF you sent does not quite seem to clarify this?​ I am hoping I do not have to take up a significant area of the non-skid side decks to access the water damaged core material, because relaying it could be difficult to get cosmetically perfect. I believe it is possible to get the mold for the diamond pattern non-skid gel coat and I may repaint all the non-skid areas after repair. 
 Just wondering if anybody else has had to tackle this type of repair or if there are any other articles on the process?

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