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Old 20-01-2022, 19:22   #1
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Making a tranducer fairing block & Routering fiberglass?

Question: how do you cut a rebate around a hole in fiberglass hull? Can you use a regular router and bits?

I'm unexepctantly facing the usual problem of transducers and deep bilge hulls. I built a fairing block, and have the mushroom head of the tranducer pointed right down as well as pressed up against the hull as close as it can get from the outside, but leaving barely enough threads on the inside to secure it. ( I was misadvised that it would fit just fine where I drilled the 2" hole.) I'd like to router a rebate around the transducer hole on the interior side of the hull, to fit the 1/2" washer and get it to a depth of at least 1/2" -3/4" too so I can get the washer down tight. (I have a little 1/4" shank Makita router which I can just barely fit and turn about 280 degrees around in the 2" hole before it bumps against an edge of interior joinery so that's another complication.) The thickness of the hull is 2" down to 1.5", so there's plenty thickness to dig into, plus the fairing block on the outside is pretty heavy and will be glassed in place. I'd like to leave as much material as possible of course but I can just enlarge the 2" hole to a 3" one, install the tranducer on to the back of the tranducer fairing block instead of interior of hull, knowing that the fairing block is epoxied into place and thicker than the hull itself.

Also here's how I made a transducer fairing block in case it helpscanyone, it it like grade school arts n crafts class
1- I used sculpting clay to shape it to sorta reduce drag maybe who knows I'm no water engineer...
2- taped over the area around the tranducer hole on the hull exterior and slapped the clay sculpture up against hull (water makes it sticky) and further shaped it
3- waxed the area with mold release wax but not necessary since clay will just wash out of mold, and tape ensures nothing sticks to hull
4- a few layers of fiberglass material over it
5- peel whole thing off hull after curing, wash out the clay, cut off sharp pokey bits and now you have a mold for a fairing block that perfectly matches the contours of your hull.
6- wax the inside of the mold well, fill with epoxy filler, let cure. After cured,, sand exposed area flattish back of the block to remove any bumps ( easier while in mold) and pop it out of mold (might have to use chisels, and cut the mold. If there are voids in the casting, fill and sand until fair.
7- line tranducer fairing block up to tranducer hole and adhere to hull exterior. The tranducers itself will go in with 4200 but the fairing block will never be removed so it is glassed.
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Old 20-01-2022, 20:20   #2
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Re: Making a tranducer fairing block & Routering fiberglass?

Yes, this will work just fine. A carbide bit will cut the fiberglass but it should be a new bit and it will dull very quickly.
I would enlarge the hole with a hole saw. Just use the larger hole saw to cut a hole in a piece of plywood, then lay the plywood on the hull to be a guide for the larger hole saw when you cut through the hull.
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Old 20-01-2022, 20:26   #3
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Re: Making a tranducer fairing block & Routering fiberglass?

I like your 'pod' thanks for the write-up.
I think I would start with carbide blades in a oscillating multi-tool. Not the fastest way but safer in close quarters. Mark it out, cut slots down into the glass and then chip or cut the rest out flat to the hole. Wear all forms of personal protective equipment. Die grinder would be faster, more chance for collateral damage, but I would not go more than 1/8" deep at a time.
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Old 20-01-2022, 20:33   #4
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Re: Making a tranducer fairing block & Routering fiberglass?

Thanks, that is a cool alternative way to enlarging an existing hole (the other is to wedge a piece of wood into the hole and use it to center the hole saw drill)
OK we'll try some carbide router bits then!
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Old 20-01-2022, 20:37   #5
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Re: Making a tranducer fairing block & Routering fiberglass?

Quote:
Originally Posted by Spot View Post
I like your 'pod' thanks for the write-up.
I think I would start with carbide blades in a oscillating multi-tool. Not the fastest way but safer in close quarters. Mark it out, cut slots down into the glass and then chip or cut the rest out flat to the hole. Wear all forms of personal protective equipment. Die grinder would be faster, more chance for collateral damage, but I would not go more than 1/8" deep at a time.
Cut slots ! Good idea! Circular saw 3" wide to create out limit of rabbet,, cut only 3/4" down so now you have a circular cut around the hole. Next use oscillating tool to cut diagonal slot from out to inner holes, then chip it all out. Seems like a good idea mentally, more work than router, option 2
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Old 20-01-2022, 21:37   #6
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Re: Making a tranducer fairing block & Routering fiberglass?

Good job getting creative to fix the issue.

Incase anyone else is reading this and unaware, most modern transducers are tilted element style and can be purchased for a large deadrise angle (up to the mid twenties). And some - like the DST110 - are wide beam and adjust from 0-20+ by themselves.

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Old 20-01-2022, 22:23   #7
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Re: Making a tranducer fairing block & Routering fiberglass?

Indeed but on my boat, it was 40 degrees
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Old 21-01-2022, 05:31   #8
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Re: Making a tranducer fairing block & Routering fiberglass?

Most Airmar transducers are supplied with fairing blocks ➥ https://www.p2marine.com/airmar-fairing-blocks
INSTALLATION INSTRUCTIONS ➥ https://www.airmar.com/uploads/Insta.../17-276-01.pdf
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Old 21-01-2022, 07:20   #9
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Re: Making a tranducer fairing block & Routering fiberglass?

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Indeed but on my boat, it was 40 degrees
Very nice job with the block, but (too late) my suggestion would be:
- a combination of a tilted transducer and a lesser fairing block, say 20 degrees of each, which would have reduced the need to cut away the interior.

- have a new through-hull sleeve machined with a longer threaded shaft, again to reduce the need to cut into the hull. You’d also need to extend the upper part transducer element as well, but your epoxy skills are clearly up to that ;-)
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Old 21-01-2022, 11:33   #10
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Re: Making a tranducer fairing block & Routering fiberglass?

They do indeed sell blank fairing block which you'd have to cut to conform to the compound curves of the hull and use lots of filler to fill in gaps, honestly making one that already conforms is quicker than ordering it and waiting for delivery nevermind finding a bandsaw etc. And how often do you get to play with clay?

I would have used the tilted element tranducer but I'd still have to make a fairing block and due to location, 20 degrees more or less results in less than half inch difference in height if the block. Getting tranducers to point down on deep v shaped hulls, is just always going to be more fun in practice than the theory of just making a hole and shoving it in there with sealant.

Gonna try routering fb, will let you know how it comes out
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Old 21-01-2022, 13:20   #11
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Re: Making a tranducer fairing block & Routering fiberglass?

If there is access from within, simplest is wooden bung in the hole to center 3” hole saw. Can use intermediate sizes on same mandrel to make material removal easier
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Old 21-01-2022, 14:56   #12
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Re: Making a tranducer fairing block & Routering fiberglass?

what is the fairing block made of? Nice job on the mold!

I layered 1/2" FRP board glued with epoxy and cut on the correct angle and used the other half inside. It took a little shaping but worked nice.

You didn't post a pic of the inside of the hull so this may not apply. I have simply used an angle grinder or a Dremal tool with a sanding drum. It is messy but using a shop vac cuts down on the dust.
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Old 21-01-2022, 15:41   #13
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Re: Making a tranducer fairing block & Routering fiberglass?

I'm facing the exact same situation, the deadrise is very steep, but I will install the transducer to shoot thru the hull instead.
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Old 21-01-2022, 19:28   #14
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Re: Making a tranducer fairing block & Routering fiberglass?

Why not cut a 4",or so, hole in the hull & a 2" hole in the fairing.
Mount the ducer down in the fairing as pic 4 shows the fairing face parallel to the water.
You might want to add a few layers of glas to the fairing to strengthen it?
Cheers/Len


PS: the best place for a t-ducer is about 1/2 way,or so,back from the bow & within a foot ,or so, out from the keel. This helps avoid air bubbles from the bow rolling over the ducer face which can cause picture breakup/erratic depth readings.
Just my experience as an old tech.
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Old 21-01-2022, 19:38   #15
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Re: Making a tranducer fairing block & Routering fiberglass?

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Originally Posted by SVTatia View Post
I'm facing the exact same situation, the deadrise is very steep, but I will install the transducer to shoot thru the hull instead.

The face of the ducer still needs to be parallel to the water. Perhaps you can build a "chest" using 3-4" pvc pipe cut to the hull slope so it will stand vertical.Glas this pipe to the hull.
Put a pipe cap on it,not glued, and mount the ducer in the cap.
Cut this pipe so that the ducer face will be approx 1/4" from touching the hull. Add oil to cover the ducer.
Cheers/Len
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