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Through hull transducer levelling block
Anne Kidd and Mark Shallow
25 Posts
9th December 2023 - 10:37 pm

Good evening, 

I am going to fit a through hull tranducer/log impeller. I have worked out roughly where I need to cut out the hole in the GRP hull. My question is: What should I use for a levelling block?

I had a transducer fitted on a boat in France once and the yard cast a block out of a plastic / resin type substance then cut it at the angle of the hull and fixed it on with sealent. This worked well but I've searched on the Internet and can't find any information on how to do this.  I have read that wood is not suitable with a plastic through hull as it can swell and damage that tube.

Any insights would be appreciated 


Thanks in advance 


Anne & Mark


John MacMullen and Ann Musgrave
197 Posts
13th December 2023 - 6:45 pm

Hi Anne and Mark,

I have, in the distant past, made fairing blocks out of body filler which is easily worked and faired to shape. I'd agree that wood is a no-no as any swelling is likely to overstress the transducer and, unlike the old ones which were bronze, the plastic ones won't like being under too much tension.

Our current installation is minimalist; A log impeller on the 'turn of the bilge' about 'midships No fairing block, so it follows the hull lines. The results correlate well with distance made good in non-tidal waters. We also have an echosounder transducer on either side in a similar position, also without fairing blocks. We had hoped that the beam width would negate the angle it's mounted at, and again that seems to be the case. The electrical outputs are switched via a little double pole waterproof switch from Radiospares) mounted in the companionway which works fine and every time I switch it I get a warm feeling thinking of the £hundreds saved over a pukka gravity switch!

Happy refitting and Happy Christmas.


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