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 

Solid trailerable shallow draught coastal keelyacht, ready to sail. Overcomplete with Yamaha 6, GPS, tiller pilot, toilet, galley, good sails and spars, unbraked trailer, electrics, solar etc.. € 7.000.00
Sneem, Co. Kerry
(Photo shows my other Drifter, No. 27. Fitted with BayRaider rig.)

BayRaider 17 "Eliza" For Sale - Built 2015 - Located Jupiter, Florida USA

Excellent Condition with Aluminum Trailer $15,000.00 US Dollars. Please contact Lee & Carol Gwatkin at carolgwatkin@icloud.com for photos and more information. Includes 5 hp 4-stroke Mercury Outboard, Garmin nav, wind speed/direction indicator, solar panel for charging battery.

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. 

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.