Osmosis Blisters and Epoxy Filling

[Refueler]

I don't want to cast aspersions on your knowledge of your boat, but in view of how she is of chine construction I am wondering if maybe she was built of plywood with a heavy layer of fibreglass on the outside?

That did cross my mind - the heavy chine hull. Reminiscent of Waterwitch / Barge style ply plate construction. But I stayed silent based on OP's saying its GRP.
I'm wary of it being a ply hull though - the photos first shown - do have deep b listers, deeper than I would expect to find on a glassed ply hull.

 

[Tranona]

That is something I thought of, as the shots of the blisters in the first post seem to show a different material at the bottom where one would expect to see glass mat. If it is GRP sheathed ply then the blisters could be pockets when the GRP skin has not adhered properly and the flexible area delamination of the outer sheathing as you suggest.

I do not recognise the boat at all and I have a pretty good memory (although not infallible!) of DIY designs from that period because that is when I was into kit boat building - and of course owned a sheathed hard chine boat for nigh on 40 years. Although it is possible to build hard chine boats of GRP, usually it was by taking a female mould tool off a ply built boat. The Falmouth Gypsy was an excellent example of this wales.boatshed.com/falmouth_gypsy_mk2-boat-176525.html However building a one off hull and superstructure in GRP would be unusual - although Tinker's superstructure does look moulded, but may have come from another boat. All sorts of strange things went on in those days.. The other construction it could be is Airex foam sheathed inside and out which could be built over a fairly basic male mould.

More questions than answers.

 

[Caraway]

Test core with a hole saw. It'll be easy enough to reinstate and at least the doubt will be removed.

 

[Boathook]

It does look like a Maurice Griffiths design boat and they were based on ply. My parents first boat was ply and years later I saw her advertised as a GRP boat. I found out that she had been sheathed in GRP. Enjoyable times on board in the late 60s and early 70s based near Faversham, Kent or to be precise, Shipwright Arms, Hollowshore where I drank sludge? as a 12 year old ....

 

[Tranona]

Definitely not MG design. too narrow, wrong sort of keel and he would never have designed a rudder like that! Pretty familiar with all his designs as I owned the original Eventide 26 for nearly 40 years. He designed really only 3 basic ply hard chine boats, Eventide, Golden Hind and Waterwitch although there were many variations on the basic designs, such as stretched versions or redrawn for building in steel. Only the double chine Golden Hind 26 and 31 were built in GRP, and all with wooden decks and coachroofs.

GRP sheathing was always problematic, and often done to hide problems with the ply. Many of the early eventides built like mine by Hartwells (or later by Terry Erskine) were Cascover sheathed which was a success - even if very expensive and not really suitable for home builders.

 

[Restoration man]

I’m currently fixing some , well a “lot “blisters on my 1970’s grp motor boat , fist thing I would do is get screwdriver with hard handle and tap the handle on a blister then tap it on some solid fiberglass you will then get idea of what it should sound like ,then tap over the Entire hull , you are looking for sharp tap not dull thud ,hopefully only the blisters will sound dull the rest of the hull should be nice sharp sound , if you have got large areas of dull sounds this is delamination , and I would get some professional advise before going any further,
. I then scraped back to bare grp then sanded to remove all the remaining anti foul , this highlights the blisters you can’t see ,I then got some hole saws of various sizes to Match the approx size of the blisters, I then got a scrap piece of 6mm ply and cut some holes in it with each of the different size hole saws , then remove the drill from the center of the arbour and place the guide over the blister and chose hole saw that bigger than blister and carefully drill trough the gel cote and poss the first coat of csm ( chop strand mat) just be careful to go to deep but it’s quite easy when get the hang of it ,then a get sharp 5mm chisel and remove the disk of fiberglass , I then used a pistol air grinder with 36grit rolock disks to feather the edge into sound gelcote, sometimes they chase off in any direction, I then hot washed the blisters out once week for as long as possible,I’m lucky as the yard where keep my boat has a hot pressure washer I can use for free just a donation for the diesel, make sure your bilges are totally clean and dry this is very important and get big fan from screwfix and run it on timer everyday inside the boat leaving the door open if possible, I plan on leaving mine till next March and then fill the holes , when this time comes im
Going to wipe out blisters with alcohol ready for filling ,you must use an epoxy filler and as others have said you need to prime the blisters first this needs to be done by painting the exposed matting with solvent free epoxy, as this doesn’t shrink ,I’m then going to let this go tacky and fill the shallow ones with international water tight epoxy filler , this is apparently very water resistant and easy to sand and mix , deeper blisters I’m going cut some small disks of matting and re laminate before finishing off with international water tight , I might paint some penguard epoxy on the repairs, but only on the repairs, don’t seal the hole bottom up as apparently it can cause more blisters ,I then plan on painting bottom with international primorcon and anti fouling, then each year do same to whatever re appears hopefully not to many , I’ve done months of research into this and apparently this is the way to go , this is NOT a cure ,but it should help, the other key to slowing it down is to get the boat out water every winter , and always keep the bilges 100% dry and this should slow the process down , and hopefully put the boat in remission

 

[slawosz]

When pushed the hull of that section flexes inwards by a few millimetres. Someone else in the shipyard suggested that it is due to delamination.

As someone suggested, you can do 'inspection' cut with holesaw. This will give information about material and possible damage? Do you own her for long time or this is new buy?

 

[Eero Gaffney]

Hello all, thank you a lot for your responses. I very much appreciate the advise. For the time being I ended up leaving that section of the hull as I wanted to get back into the water. I filled the blisters that were opened with epoxy and painted over them. As for the boat model I think it's entirely possible that it is a ply boat coated with GRP. As I mentioned before the previous owner didn't have much information on it so I was just trusting what he said but of course it could've been wrong. For the flexible part I have measured it and will routinely check whether it expands at all.