Author Topic: In-hull transducers  (Read 265 times)

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json

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In-hull transducers
« on: August 12, 2019, 12:57:14 PM »
I read in rbob's build thread a bit ago where Brian mentioned that you could make a fiberglass layup and replace part of the hull to be able to use an in-hull style transducer. I would love to go this route, but am questioning a couple aspects of it.

First is that it needs to be bubble free. I have access to a vacuum chamber so I could likely make a bubble-free fiberglass plug without too much trouble, but adhering it into the hull has me wondering how I would keep it bubble free. For instance, if I had an 18 mm thick plug that I dropped into a precut hole in the bottom assembly, I assume I would have to glass over it, which would introduce bubbles. Do I need the ability to vacuum bag right on the boat hull? Maybe I can make the plug bigger than the area the transducer will be transmitting through and then just glass around the edges leaving the center only the originally vacuum-chamber bubble free material? Once I have the plug in place can I coat the outside of it as normal with something such as graphite impregnated epoxy? How bubble free does it have to be?

Brian.Dixon

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Re: In-hull transducers
« Reply #1 on: August 18, 2019, 08:20:29 AM »

There are more than one type of thru-hull transducer that will work at speed, and some external choices that either go on the transom or on the bottom of the boat.  For the thru-hull ones, there are bronze ones that require a hole in the bottom and are installed similar to a seacock (clamping the hull, caulk etc).  I think the type you are referring to are designed to mount on the inside of the hull, no holes required ... but require an air-free 'window' to look through.  Wood is NOT it!  A bubble-free fiberglass lay-up works though, and no, you don't need to use vacuum bagging in order to achieve this.  Just use lightweight glass (4-oz or 6-oz), lots of layers, saturate with enough epoxy on each layer to nearly float the glass.  In between layers, apply fill coats until smooth and give the cured fill coats a light sanding (and clean it) before adding the next layer.  There won't be any bubbles and you can do the whole thing on your bench ... nice and flat and level.

To install it, make your lay-up bigger than required.  You'll want to cut a rectangular hole in the bottom of the boat (aft 1/3rd of the hull where the water runs smooth), and make artful use of a sharp chisel to make bevel the perimeter ... think "picture frame".  You want the beveled side facing down, towards the water.  No you need to cut your fiberglass 'window' layup down to size, giving it a matching bevel around the perimeter to fit the hole in the boat.  The reason for the bevel in the boat's hole facing down is so that water pressure will be in the direction that forces the glass window into the 'picture frame' against the bevel in the hull's opening.  In other words, water pressure is in the direction to encourage 'assembly' not 'disassembly'.  The glass window gets epoxied into place so that it's flush with the exterior hull.  You can, BTW, use an oval or circular hole in the boat and perhaps a 45 degree or higher angle around the perimeter ... but it'll be a large oval or circle because the rectangular area needed by the transducer will require it.  See Airmar's choices and installation instructions for more info.

So yeah ... the above procedure might be hassle and that's why you rarely see people do it .... but if you want the BEST deep-water high-speed transducer, this is the way to go.  Personally, I'd look for one that mounts on the transom and is advertised to work when on plane ... then fart around with installation until it works as advertised.  Finding smooth water flow and having the transducer at an optimal depth is the key.  Halfway between the keel and side strakes is a good pace to start.

Brian

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json

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Re: In-hull transducers
« Reply #2 on: August 20, 2019, 08:53:30 AM »
Ya, seems like a hassle. I have been looking more at thru-hulls and might go with one of those. It seems like those would have the best performance since they are just shooting through a small purpose built window into the water. I largely was trying to avoid putting another hole below the waterline. I got to thinking though that I can build a chamber in the bilge that houses all the thru-hulls and either doesn't drain below the waterline (ala the titanic) and/or is basically sealed around the plumbing and access, to which it wouldn't matter then really if a thru-hull fitting was breached. Does this approach sound reasonable?

Brian.Dixon

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Re: In-hull transducers
« Reply #3 on: August 22, 2019, 06:44:46 AM »
Ya, seems like a hassle. I have been looking more at thru-hulls and might go with one of those. It seems like those would have the best performance since they are just shooting through a small purpose built window into the water. I largely was trying to avoid putting another hole below the waterline. I got to thinking though that I can build a chamber in the bilge that houses all the thru-hulls and either doesn't drain below the waterline (ala the titanic) and/or is basically sealed around the plumbing and access, to which it wouldn't matter then really if a thru-hull fitting was breached. Does this approach sound reasonable?

Under the deck, walled in area, 15/16" holes front and aft walls for drain plugs (which stay in until the boat's in storage), screwed-down aluminum or plexi gasketed deck plate for access.  In other words, it's a small compartment that provides no way for water to get anywhere else, and has drain plugs for drainage in front of the compartment towards the bilge (through the 2 drain plug holes) and an 'not used often' deck plate that doesn't leak.  The commercial ones with lids that rotate (screw) into place leak big time ... even if the manufacturer says not.  A 6" or so plate screwed down onto a neoprene gasket with 6 screws around it's perimeter is waterproof ... like you want.  And you'll rarely open it, so who cares if you use stainless wood screws to hold it in place?   And if those thru-hulls ever leak, the deck plate and drain plugs limit the water ingress to a small compartment ... no problem.

Brian

><((((> .`.><((((> .`.><((((>  The Great Alaskan - Professional grade offshore performance - Designed to be built by anyone! -
 http://www.glacierboats.com  ><((((> .`.><((((> .`.><((((>