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Replacing old thru-hull transducers, hole size difference.

Jul 26, 2020
Hunter 27 Pepin, Wi
My 27' 1978 Hunter came equipped with SR Mariner instruments. The transducers for the knot and depth meters were mounted beneath the v-berth near the fresh water tank.

We bought the boat last year and neither instrument worked at the time. Neither was properly wired (or wired at all). The knot meter's paddle wheel has suffered damage and lost a magnet in its life.

We've thought about replacing this equipment with newer electronics, but here's the catch. The SR transducers used a 2 1/8" hole for mounting their thru-hull fittings. Everything I can find new uses a 2" hole. I haven't removed the old thru-hull yet to verify the exact diameter of the hole in the hull, it could be slightly more than 2 1/8.

I'd like to avoid having to dish out and glass over these holes to start over. Is it safe to install a 2" transducer thru-hull in a slightly larger hole? I've seen a lot of mixed opinion in similar threads here through my searching. I'm also hesitant to simply re-install these 43 year old plastic parts and hope they don't fail.
Sep 25, 2008
Alden 50 Sarasota, Florida
. Is it safe to install a 2" transducer thru-hull in a slightly larger hole?
No, it’s not safe. If you’re asking if some fool did it and managed to not sink, probably yes. The better question is - Why would anyone take the chance?
My opinion is that feathering the glass back and properly closing the hole seems a reasonable effort given the peace of mind.
Jan 11, 2014
Sabre 362 113 Fair Haven, NY
Just sticking the new through hull in the oversized hole is not a good idea. It will be difficult to seal.

There will be 1/16" gap around the new fitting and this will be captured between the flange on the transducer and the nut. This gap is large enough to allow water to enter, but small enough that grinding out the bevel and laying up multiple layers cloth and epoxy will be overkill.

In the past, I have filled the hole with cloth and epoxy, let it cure and then drilled a new hole in the middle of the plug. Make certain all of the old caulking compound is removed and the epoxy has a good clean surface to bond to. It is important to put the new hole in the center of the plug, so the compression from the through hull contributes to the plug's strength.

Fair warning, this absolutely not the right way to seal hole permanently, it is only good for making a hole a little smaller. If the hole is going to be sealed permanently then it must be ground out to a 12:1 bevel and new glass and epoxy laid up.
Nov 30, 2015
Hunter 1978 H30 Cherubini, Treman Marina, Ithaca, NY
Hey there @Jerdavis77, welcome to the forum. I feel your pain. I’ve replaced the knot meter transducer twice since purchasing my ’78 Hunter 30 in 2014. Replacement paddle wheels were not available. The travel lift strap, when seasonally launching or retrieving boat, slides right over the paddle wheel and has crushed magnets and vanes...:yikes: Can you tell me what the inner diameter of your through hull is? I may have a complete transducer with cable that might work for you...but maybe knot? I will give it you for the cost of shipping. It’s brand new, old stock. Never seen the light of day or water.

The OD of the transducer is slightly tapered from insert end at 1.72“ to 1.81” at the seating end. It came with through hull seal and nut. I think esgdirect.com is where I got it from and of course they no longer carry these items. The filler/sealant for these transducers was color coded. This one is red, of course my limited knowledge of such things was Wayneing at that point in tIme, I needed blue. Slightly larger in diameter. Here’s a pic.

Jul 26, 2020
Hunter 27 Pepin, Wi
I got this response from Airmar support.

There is no issue with the 1/8" difference as you are correct that the sealant can be used to fill the gap.
With that in mind I'm going to investigate further this weekend when I do some work on the boat. I'll likely pull one of the two thru hulls and inspect it and confirm the condition and size of the hole in the hull.

@BlowMeAway I'll have to see what the dimensions are on my transducer. Yours looks slightly different and mine were from SR Marine Instruments, but thanks for the offer!