as opposed to speculation of various posters I'll take the liberty to describe how we actually put a throughhull transducer into our 37' aluminium keelboat while at anchor
in Fannie Bay, Darwin, in a place where there was no hole:
3 people, 2 in the bilge
, one in the water
1 plumbers friend
1 stainless saladbowl, flat rim, diameter ~10", bead of sikaflex sealant
on the rim
1: drill 3mm hole from inside, when drillbit becomes visible under water
put plumbers friend over it from outside, drillbit withdrawn, waterpressure pushes rubber bell strongly against hull
. waterentry so far 3 thimblefuls.
2: cover hole from inside with thumb, after knocking from inside pf replaced by saladbowl, knock from outside, thumb removed from hole, waterpressure presses saladbowl strongly against hull
(bottom of bowl visibly dished)
3: centered on the 3mm hole 2" hole cut with an electric
holesaw (from inside of course), waterentry into boat additional 5 thimblefuls; flat piece of ply with towel pressed against hole from inside, knocking, saladbowl removed from outside (against surprisingly strong waterpressure), saladbowl handed up into boat
4: transducer w sikabead on sealing surfaces put into saladbowl & cable coiled nicely into bowl, sika-bead on bowlrim, cover with clingfoil, second sikabead on bowlrim, bowl handed overboard
5: bowl positioned over 2" hole, knocking, sealing plate inside the hull removed, area dried with papertowels, clingfoil on bowl broken, cable carefully fished from bowl & pulled through hole, transducer pulled up by cable & secured with it's mounting nut from inside, knock
6: bowl pried lose, sika on hull cleaned with spatula
surfaced relieved & proceeded to connect transducer up
altogether abt 50ml of water entered
comments: boat yawing at anchor
& slight chop didn't help a lot
only on an aluminium boat would I ever do it exactly that way!
on fibreglass: the water would absolutely have to be kept away from the fibreglass edge & the edge sealed with epoxi! could be done but would be more work, entail a hairdryer to make sure the exposed edge is absolutely dry.
same on a steel
or wood hull; on the steel hull
of course the cutting of the 2" hole itself would be more troublesome.
As our procedure had worked so well we did it a second time as we discovered that the transducer was faulty (from new...). secondtime around we had a little bit more water enter (1/2 pint) as the faulty transducer had to be pushed out & recovered (warranty)
btw I guess a plasticbuckt would not stand up to the waterpressure of 50cm (20") depth
that the transducer had to be mounted in