Good Mastic for clinker seams

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Devon Dinghy
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Good Mastic for clinker seams

Post by Devon Dinghy » Wed May 16, 2018 10:52 pm

I was wondering if anyone has any advice on a good mastic? I want to use it to seal some seams on the inside of my Tideway.

Michael Brigg
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Re: Good Mastic for clinker seams

Post by Michael Brigg » Sun May 20, 2018 10:41 am

Sickaflex seems to me to be most boatyards favourite, though I have no personal experience with it.

https://www.screwfix.com/p/sika-sikafle ... 0wod9boF_Q
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Re: Good Mastic for clinker seams

Post by Michael4 » Mon May 21, 2018 8:54 am

I use Sikaflex 291 (I think). It comes in brown which is not not unlike cat poo but is less noticeable on a wooden boat. It works.

I use it on the outside and try to avoid using it inside where it would become quite difficult to remove should you ever want to.
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Keith66
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Re: Good Mastic for clinker seams

Post by Keith66 » Mon May 21, 2018 6:46 pm

The only mastic i would use on clinker boat seams is Evomastic, it is oil based, soft & never really goes hard. As the boat takes up in the water it will squeeze out & causes no damage. It is also cheap!

Rupert
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Re: Good Mastic for clinker seams

Post by Rupert » Mon May 21, 2018 7:19 pm

I'm with Keith on this one, use something that will squeeze as the planks swell, if you have to use anything at all.
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Devon Dinghy
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Re: Good Mastic for clinker seams

Post by Devon Dinghy » Mon May 21, 2018 11:52 pm

Thank you all for your advice. I have some Sikaflex 291 but have held off using it due to the issues you mention and specifically concern about getting it off at a later date. Would you also avoid using it to help fix down hardware such as rowlock holders etc? Regarding the seams I will get hold of some Evomastic and give that a go. There are a few varieties but the one that comes up the most is EVOMASTIC FRAME SEAL C20. Is that the one you had in mind Keith? Many thanks again for your help.

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Re: Good Mastic for clinker seams

Post by Devon Dinghy » Tue May 22, 2018 10:13 am

or the Evo-Stik Linseed Oil Putty? see: https://www.homebase.co.uk/evo-stik-lin ... kg_p067695

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Ed
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Re: Good Mastic for clinker seams

Post by Ed » Tue May 22, 2018 12:43 pm

Ha! There is no such thing as a Good Mastic for clinker seams....

But if you must, then I agree with the general concept of going towards something soft rather than too hard. I thnk I would agree with evomastic.

Does depend a bit on how big the gaps are you want to seal.

I once had an old boat builder recommend Tallow, or a mixture of Tallow and beeswax, softened a bit with turpentine. I made some up and it seemed to work, but is very soft and best for just sealing a boat whilst it is taking up, rather than trying in any way to be a permanent fix.

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Michael Brigg
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Re: Good Mastic for clinker seams

Post by Michael Brigg » Wed May 23, 2018 2:17 pm

A thought I would throw in the ring after Ed's comments is that a leaky Clinker (ie the need for mastic) implies almost inevitably that the ribs and strakes are not "tight."

It is the pulling together of the strakes and ribs, caused by the expansion of the wood as it absorbs the water, that 'locks' the interlocking structure into a tight, and importantly stiff structure.

I have noticed that a dried out hull of a clinker is noticeably, damagingly flexible, and remains so until almost the final point of take up, at which point everything "fits" and then tightens.

It is like the inflation of the airbag around the rigid inflatable Boat (RIB) which similarly will become floppy and unrigid when there is a loss of inflation (and thus reduction in tension) of the inflatable component.

Remember that in their day, clinker boats (before diagonal planking and cold /hot moulding of ply) were the dog's b******s as far as stiffness to weight ratio in racing dinghies, as carvel built boats with their non overlapping strakes would never achieve a similar stiffness or "locking" point and relied on the thickness of wood in the hull... about 3 times thicker and thus 3 times heavier for a 14foot dinghy.

They say that no (mythological) man ever suffered any loss of strength by being abandoned on the mountain side, and the same might be said for a clinker boat abandoned on the foreshore at mid-tide.

It will of course sink (initially) but in a week or two after a good hosing, it will be ready for whatever rigours you throw at it.

The theory is that the wood will expand laterally in the strakes by 4 - 8 times as much as the longtitudinal expansion of wood in the ribs, and if the fastenings are good it will emerge as a result as tight as a drum. Importantly this should be Sea water as fresh water will encourage rot.
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Re: Good Mastic for clinker seams

Post by Aquarius » Wed Jun 06, 2018 6:10 pm

There is a compound called "Slick Seam", which is intended for use on carvel boat seams before launching. The idea is that it remains soft and squeezes out as the planks "take up". It might be suitable for a clinker dinghy...
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Keith66
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Re: Good Mastic for clinker seams

Post by Keith66 » Sat Jun 09, 2018 10:04 am

The Evomastic i have always used is just frame seal natural, they might have put a number on it!
As for Linseed oil putty forget it, as the oil dries out it goes rock hard, this will destroy a clinker boat as the seams will be forced apart eventually.

chris
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Re: Good Mastic for clinker seams

Post by chris » Sat Jun 09, 2018 1:59 pm

I agree about not using linseed oil putty. It will go rock hard. I was looking at my ribbed 14 yesterday (as yet restoration is still unstarted) as I moved it from one shed to the other and noticed that many seems had been filled with what I guess it putty. In one area the skin has now bulged out very noticeably as the planks expanded and had nowhere to except bulge outwards. So one of my first jobs will be to pick out all the old filler.
One comment I would add to the discussion on clinker construction where the theory may not always hold up is with old boats rather than newer hulls. If grit in the form or mud, sand or just dirt gets in between the planking when they are on the dry side then you have introduced a grinding paste action as the hull moves about. On one hull where I had to replace part of a plank I was surprised to find how much wear the lands had suffered. Take-up may no longer seal up all old hulls.

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Re: Good Mastic for clinker seams

Post by Michael4 » Tue Jun 12, 2018 11:11 am

Just an afterthought. If you use Sikaflex and then paint/varnish it after it has skinned but before it fully cures then it stays soft-softish for a long time (5+ years in my experience). This can be a bit messy if you put your fingers in the wrong place but it is an effective bodge.
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chris
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Re: Good Mastic for clinker seams

Post by chris » Tue Jun 12, 2018 8:30 pm

Mm...after a lot of hard work I would like more than 5years life out of a hull.

Roger J Holman
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Re: Good Mastic for clinker seams

Post by Roger J Holman » Fri Sep 28, 2018 9:04 am

Mastic?! No!

The correct way to sort the problem is to use a 'dolly', (a lump hammer head is suitable), a rove driver, ( a short length of round bar with a suitable sized hole bored in one end), and a ball pein hammer;and them a fair amount of time spent tightening up the roves, properly. Any attempt to use sealant will be simply a very short term measure and will be doomed to failure.

As has been correctly pointed out, a clinker boat relies on the moisture levels within the structure to maintain the tightness of the fastenings, and its watertight integrity.

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