Glue/sealant question

[Graham W]

I'm in the process of replacing my depth transducer.  It used to be standard on BayRaiders to attach a transom-mounted type of transducer just ahead of the centreboard slot to a specially shaped block of wood that fixes inside the very front of the slot.  Photos of the sloping fixing position and of the block of wood with attached transducer below.  Note the screws into bare GRP!

For various reasons to do with lack of decent access to fixings on this particular transducer, the only way of installing it is first to attach it to the piece of wood and then to bond the wood to the GRP surface inside the centreboard slot.

The question is, what sort of bond/sealant/glue is it best to use to ensure a good strong bond between the piece of wood and the GRP, given that the two of them are going to be exposed to seawater?  It wasn't that difficult to unstick the wood from how it was originally installed by the yard.  Obviously I will need to clean up the two surfaces before applying anything.  Looking at the sealant remaining on the piece of wood, it seems to be a mixture of some cream-coloured stuff and some black sealant, which may or may not be Sikaflex.

All suggestions gratefully received.

[garethrow]

Hmm, have you got any way of clamping / applying pressure to the block whilst it is glueing in place Graham? If so two choices spring to mind: a) epoxy it in place as a fairly permenant / bomb proof solution, or b) use the stuff that the yard use which is a black bostick marine consruction adhesive: https://www.sealantsandtoolsdirect.co.uk/bostik-simson-msr-marine-construction-adhesive-sealant-black

If you can't get a way of applying pressure during the bonding process - may have to resort to some screws as well?

Regards

Gareth Rowlands
GRP BR20 Halen Y Mor

[Graham W]

Thanks Gareth.  I think epoxy is a bit too permanent so I’ll try the Bostik.

I’m thinking of temporarily attaching a jig to the underside of the piece of wood that will allow me to apply pressure by bracing it against the outside of the hull.  Once the adhesive has cured, I can unscrew the jig and hopefully everything will stay in place. 

[Sea Simon]

"Glue"
Sikaflex 291 is excellent for this sort of thing, and widely available too.
You can sometimes  get it in less wasteful (and cheaper!) smaller tubes, but check expiry dates.
I've had mixed results  with CT1 construction adhesive  outdoors/underwater. But otherwise an excellent (relatively) economical alternative.

Clamping.
Try wedges etc..I'm  sure you've done it all before?
I have found these mini air bag/air wedges really useful for this sort of clamping job (in holes/crevices).
https://www.screwfix.com/p/winbag-inflatable-air-wedge-160mm-160mm-x-160mm/1363k
Mine were from Aldi, and about 1/3 the price!

[Graham W]

Thanks Simon.

Does anyone know if there is much to choose (apart from smaller sizes available) between the Bostik and the Sikaflex?  And can Bostik shelf life be extended, as it can with Sikaflex, by putting it in the deep freeze?

I could get much better access to the transducer installation if I was prepared to remove the centreboard and/or the centreboard case capping.  However both are big jobs, so some sort of temporary jig seems to be the easiest way of applying pressure to the installation while the sealant cures.

If you look at the first photo above, I think an inflatable wedge would be in mortal danger from all those bare screws!

[Charles Scott]

Sea Simon, I am interested in your remarks about CT1, as I have just bought some to fix new rubbing strakes on my Bayraider 20. May I ask in what way were your results mixed? Removing the remains of the old strakes has been ( is being ) quite a job, whatever they were stuck on with before ( black colour, presumably Sikaflex or the Bostik mentioned by you and Graham ) being the very devil to take off.  The remains of the old strakes ( partially rotted from years of neglect , some mine, but most by the previous owner, Plymouth university) having to be chiselled off and destroyed in the process. In my naivete, I had thought I would take out the screws, and then simply peel off the strakes . Ha Ha!  It proves the yard's black stuff is very effective anyway, but I hope I haven't done the wrong thing buying CT1 instead.  I also have to replace the old white sealant in the join between the cockpit floor and sides, as this has degraded after being exposed to UV light during its University years living on a  pontoon, in the Plymouth sun ( a lot closer to the equator than Co.Mayo where I keep my boat.)

[Sea Simon]

Interesting...Plymouth Uni (my v v long ago Alma Mater, a humble Poly back then...).
I viewed these three then very much unloved boats at QAB marina,  Plymouth in 2017. All a bleddy disgrace, and I told the Uni bosun so. I offered to buy a melange of the best of the 3 (having picked thru the Sails, spars and trailers too) but my low offer was somewhat "ungraciously refused"...I  left all 3 behind and brought a BRe! Have mentioned these boats on here before!
Would never have happened in my day, etc, etc.....

CT1. I've had good results bonding and sealing internally, but this formulation seems perhaps less tolerant of seawater and/or UV when used externally? The "beige" shade is usefully tonal with the off white yard finishes.
The yard Bostik stuff seems good?

Latest experience was a pal who used CT1 to fix his saildrive fairing diaphragm to the Hull (not a Swallow boat). Very  poor results, failed in weeks. I did mine with RHiB repair materials and no sign of movement, peeling or detachment of any sort at the end of season. Of course, any adhesive application to used items as a repair (even more so than production use, on new materials in more controlled conditions) can be very variable.

Others on here will be better placed than me to advise on your particular application, I suspect?

PS . I'm closer even than Plymouth  to the equator 

[Charles Scott]

Thank you Sea Simon.  I have now sent off for the Bostik.   
   Anyone need 2 tubes of CT 1 going cheap ?!

[garethrow]

Charles
If you haven't finished removing your rubbing strakes / batteling with the Bostick glue, a good tip I picked up from the  yard is to use a sawing motion with long pieces of cheese wire to cut through the glue. This does work but you get through a lot of wire as it gets very hot - hot enough to alter the tempering of the wire which then tends to kink and snap. I did this to get my locker fender tie off rails off for varnishing.

Regards
Gareth Rowlands
GRP BR20 Halen Y Mor

[Charles Scott]

Thank you for that tip Gareth.  I had thought along those lines, but decided instead to sharpen a wallpaper stripping tool and used that to chisel off the second strake, walloping it with a mallet.  This was much quicker  than using a regular chisel, and I was able to remove the strake complete, unlike on the other side where my chiselling completely destroyed it. ( I say complete but actually some of it fell apart , rot having got in to several places where it had been damaged by careless university people)

[Graham W]

I've managed to apply pressure to the glued-up wooden block to which the depth transducer is fixed and it seems to be holding well.  I used a combination of wedges underneath (thanks for the tip Simon) and a toy sword arrangement through the top of the centreboard slot, held down by two clamps (photo).

In the end I used Sikaflex 291i.  Despite experiences posted here, according to PBO it is not the strongest marine adhesive sealant on the market by any means.  However, as my transducer doesn't have a flip-up mechanism for when I hit the rocks with it, I'm hoping that the sealant will partially fail instead.  Whatever adhesive the yard used back in 2010, it was still quite sticky when I removed the wooden block the other day.  They seem to have applied less pressure post application, as the places where it was applied still stood proud in squiggly lines.