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Old 16-02-2017, 21:24   #1
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Reducing hole size for thru hull

I am adding a chart plotter to my Hunter 27. I intend to use the current thru hull speed sensor location for the chart plotters transducer. The question is that the current sensor has a hole diameter of 2", the hole diameter for the*chart plotters transducer is 1 1/4". What is the best or preferred way to make up the difference in hole size. The dead rise block that the transducer will be mounted to measures 2" x 8" so it will just barley cover the original hole. Would epoxying in a spacer made of fiberglass or some sort of acrylic to act as a reducer bushing work. Would there be a need to bevel the sides of the bushing/hull before epoxying in.

Thanks for your help
Marty
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Old 17-02-2017, 06:47   #2
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Re: Reducing hole size for thru hull

Take out the old fitting, bevel the edge inside and outside and use fiberglass cloth and epoxy to close the hole. Plenty sources on how to do that like from West System.

Now you can use a hole saw for the new transducer.
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Old 17-02-2017, 07:02   #3
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Re: Reducing hole size for thru hull

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Originally Posted by s/v Jedi View Post
Take out the old fitting, bevel the edge inside and outside and use fiberglass cloth and epoxy to close the hole. Plenty sources on how to do that like from West System.

Now you can use a hole saw for the new transducer.
That's the correct way to do it but it's far more difficult than it might seem. You can't just plug it, it needs to be ground out quite a ways and patched with several layers of fiberglass cloth and epoxy. And then you have to paint it.

An easier way is to leave the existing device in place and mount your new transducer somewhere else. If you're worried about the drag from the existing device, replace it with a thru hull fitting and cap it off on the inside.
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Old 17-02-2017, 07:11   #4
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Re: Reducing hole size for thru hull

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That's the correct way to do it but it's far more difficult than it might seem. You can't just plug it, it needs to be ground out quite a ways and patched with several layers of fiberglass cloth and epoxy. And then you have to paint it.

An easier way is to leave the existing device in place and mount your new transducer somewhere else. If you're worried about the drag from the existing device, replace it with a thru hull fitting and cap it off on the inside.
Its really not that difficult. I've done 7 so far on my current boat (with 6 more to go!). please don't abandon the existing thruhull and cut a new hole, eventually you or the next owner will be cursing that move when it needs fixing . I had a similar situation with an old Depth transducer (bronze one) that my boat came with. A PO of the past just installed a new transducer in a new location and ignored this one. when I inspected it, the bronze had deteriourated so badly that the threaded part of the transducer that goes through the hull just crumbled and fell off. Fix it the right way and sleep better. Its really not hard to do with epoxy!
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Old 17-02-2017, 07:17   #5
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Re: Reducing hole size for thru hull

I second PCMM. Not that hard. You can do most of it in one day, and finish it off the next with some fairing compound and a few layers of epoxy with some barrier compound mixed-in.
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Old 17-02-2017, 07:36   #6
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Re: Reducing hole size for thru hull

I'm surprised there isn't a bushing made for this, have you looked?
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Old 17-02-2017, 08:25   #7
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Re: Reducing hole size for thru hull

Before you begin, lay out and indelibly mark, THE CENTER, so you can locate it afterwards. Then go with the fix that Jedi proposed. If you center it correctly, in effect, you'll be making a 3/8" bushing, bonded into the hull. In this case, the forces, if any, will be negligible.
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Old 17-02-2017, 08:31   #8
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Re: Reducing hole size for thru hull

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I second PCMM. Not that hard. You can do most of it in one day, and finish it off the next with some fairing compound and a few layers of epoxy with some barrier compound mixed-in.
The trick is doing this correctly. It may not be that hard for an experienced fiberglass person, but for someone who has never worked with fiberglass before, that repair could be a serious liability below the waterline.
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Old 17-02-2017, 08:33   #9
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Re: Reducing hole size for thru hull

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Originally Posted by rwidman View Post
That's the correct way to do it but it's far more difficult than it might seem. You can't just plug it, it needs to be ground out quite a ways and patched with several layers of fiberglass cloth and epoxy. And then you have to paint it.

An easier way is to leave the existing device in place and mount your new transducer somewhere else. If you're worried about the drag from the existing device, replace it with a thru hull fitting and cap it off on the inside.
Ron has the right idea! Doing a closing off correctly on a relatively thin hull would be a major undertaking. Not impossible but a bunch of work for little practical reason and frankly not for a novice with glass. JMHO
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Old 17-02-2017, 08:48   #10
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Re: Reducing hole size for thru hull

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Its really not that difficult. I've done 7 so far on my current boat (with 6 more to go!). please don't abandon the existing thruhull and cut a new hole, eventually you or the next owner will be cursing that move when it needs fixing . I had a similar situation with an old Depth transducer (bronze one) that my boat came with. A PO of the past just installed a new transducer in a new location and ignored this one. when I inspected it, the bronze had deteriourated so badly that the threaded part of the transducer that goes through the hull just crumbled and fell off. Fix it the right way and sleep better. Its really not hard to do with epoxy!
I'm a third vote on this, I believe. don't half-ass it. I've done a few of these in my refits and it is not hard.

Bevel-out inside and out about 3 times the diameter of the existing hole. Cut glass cloth in increasing circles to the thickness of the hull and epoxy in place inside and out at the same time. Use a backing of stiff cardboard with plastic to hold the inside while you layup the outside. be sure to squeeze the layers together to get the excess epoxy out for a strong layer. Build the layers up so they are a little thicker then the hull.

The next day sand smooth and holesaw new hole. done. easy 1-1/2 hr job.
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Old 17-02-2017, 09:11   #11
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Re: Reducing hole size for thru hull

Unless you are a top class racer and think the drag from a little repair is going to slow you down, then remove the old fitting and rough up the surface inside and out and tape a bit of smooth plastic to the outside of the hole.Working from the inside apply about 6 inch squares of fibreglass over hole.3 or 4 should do. (years ago I used to build canoes and 3 to 4 laid up sheets of chop strand was good enough)If you want to be really fussy cut some 2 inch discs to fill the hole making sure the glass is fully saturated with resin before inserting. Depending on the amount of catalyst and air temp you should be able to remove the plastic from the outside in a couple of hours. I would apply a square of woven cloth on the out side and when dry fair it off with filler which if done carefully will be very hard to see once the paint/antifouling is applied. You should be able to get a car body repair kit which will have all you need + instructions on how to do it. Best advice get some good surgical styles gloves as resin is hellish hard to get off your hands without solvent although I have resorted to large quantities of clothes washing powder as a last resort. ps. ideal ratio of resin to cloth is 1 to 1 by weight but is hard to achieve on small scale. Too much resin makes the repair brittle and more prone to cracking.
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Old 17-02-2017, 09:12   #12
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Re: Reducing hole size for thru hull

I agree with those who recommend to fill and redrill.

If the hull is polyester FRP to begin with, polyester can be used for the repair. We use a high grade unwaxed Isophthalic resin with barrier coat.

Frp repairs under the waterline should only be performed by qualified personnel.

While a failure may be unlikely, it would most certainly be catastrophic.

Some pros use plugs, but I won't due to reduced secondary bond surface area.

Never forget that the hull can be subject to a lot stress and flexure.
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Old 17-02-2017, 09:17   #13
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Re: Reducing hole size for thru hull

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I'm a third vote on this, I believe. don't half-ass it. I've done a few of these in my refits and it is not hard.

Bevel-out inside and out about 3 times the diameter of the existing hole. Cut glass cloth in increasing circles to the thickness of the hull and epoxy in place inside and out at the same time. Use a backing of stiff cardboard with plastic to hold the inside while you layup the outside. be sure to squeeze the layers together to get the excess epoxy out for a strong layer. Build the layers up so they are a little thicker then the hull.

The next day sand smooth and holesaw new hole. done. easy 1-1/2 hr job.
I do this for a living and the average transducer removal, proper hole fill, new hole drill, transducer install, barrier and anti-foul takes about 6 hours.
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Old 17-02-2017, 09:44   #14
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Re: Reducing hole size for thru hull

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I do this for a living and the average transducer removal, proper hole fill, new hole drill, transducer install, barrier and anti-foul takes about 6 hours.
I agree, I was only referring to the filling of the hole.
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Old 17-02-2017, 10:04   #15
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Re: Reducing hole size for thru hull

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Originally Posted by ramblinrod View Post
I agree with those who recommend to fill and redrill.

If the hull is polyester FRP to begin with, polyester can be used for the repair. We use a high grade unwaxed Isophthalic resin with barrier coat.

Frp repairs under the waterline should only be performed by qualified personnel.

While a failure may be unlikely, it would most certainly be catastrophic.

Some pros use plugs, but I won't due to reduced secondary bond surface area.

Never forget that the hull can be subject to a lot stress and flexure.
Unfortunately finding a GOOD fiberglass pro is hard at the best of times, On Lake ontario... I haven't seen a fiberglass repair yet done by a pro that I couldn't do at least as well myself, and some of them have been just shameful! Epoxy is your friend for underwater amateur repairs. take your time and do a proper job and there won't be any issues. Poly while good takes alot more experience to get a good job out of!

There are no shortcuts to repairs ( specially underwater) remeber, your work is keeping the water OUT! West has some great youtube videos on how to make hull repairs. I personally like the lay the patch in one go method with peel-ply to help keep a smooth surface!

Use this method:
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