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Old 05-23-2022, 10:30 AM   #1
City: Anacortes
Vessel Name: Harvest Moon
Vessel Model: 1981 Grand Banks Classic 42’
Join Date: Apr 2017
Posts: 18
Cored or Not? Grand Banks

I installed a new Raymarine GPS and Iím having a guy install a thru hull for the depth finder. He says if itís not a cored hull there is another way to go about it. Itís a 1981 42í Grand Banks Classic. Does anyone know if my hull is cored?, anyone know what this guy is referring to?, he says Iíd just need a narrow hole for the wire. Do you think a thru hull is the better way to go? Thanks in advance, by the way Iím cruising while I peck on this phone right now. A bit choppy in Rosario Strait as I type.
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Old 05-23-2022, 12:07 PM   #2
City: Boston
Vessel Name: Adelante
Vessel Model: IG 30
Join Date: Nov 2016
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I can't answer question re. cored or not but I'm a little confused about the need for a narrow hole for a wire. A transom mounted xdcr needs a small hole for a wire. A thru hull mount needs a large hole for the xdcr. It can be done in either cored or solid.

In solid fiberglass, drill the hole, smear everything with 4200, you're done.

In a cored boat, an additional step is required. Drill the hole oversized. dig out 1/4 of the core, epoxy the hole edges, then repair the hole using std fiberglass techniques. Now you can drill the hole for the xdcr in solid fiberglass.

A little more work but very standard, easy to do, and will give best resolution for depth sounder.
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Old 05-23-2022, 12:18 PM   #3
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A cored hull has 2 layers of fiberglass, one on each side of the core. If you put a hole in a cored hull or cored deck you must deal with sealing the core or eventually the core will get wet and fail, fixing that is very expensive. I doubt the hull is cored, but that is a guess. If it isnt cored then you drill the correct size hole in the hull and mount the thru hull after using the appropriate sealant. If the hull is cored you still drill the correct size hole in the hull but then you need to reef out the core all around the hull. Rod Collins has great instructions on Then you fill the reefed out area with thickened epoxy and proceed with the install of the thru hull.

What he is talking about is a shoot through the hull transducer. Where the transducer is mounted inside the hull and the signal goes through the hull. Upside is no hole in the hull. Downside is the depth sounder may not work as well or as deep. The small hole he is referring to, I believe, would be for the cable from the transducer to the MFD. shoot through the hull doesnít work on cored hulls, only solid glass hulls.
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Old 05-24-2022, 05:17 AM   #4
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City: Narragansett Bay
Vessel Model: Grand Banks 36
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Hull is solid glass.
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Old 05-24-2022, 09:45 AM   #5
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City: Bayview
Vessel Name: Puffin
Vessel Model: Willard Vega 30
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Originally Posted by Cigatoo View Post
Hull is solid glass.
^what he said^
What kind of boat is that?
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Old 05-24-2022, 10:29 AM   #6
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Vessel Name: Retreat
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In a solid glass hull, all you need to do is epoxy a piece of ABS pipe of large enough diameter to the inside of the hull and insert your transducer in that pipe, in a bath of light oil. I use hydraulic fluid,10w, just because I have some. The performance of the T-ducer in this installation is plenty good enough for the waters I sail in, as the deepest important thing is Prawn trap location, where I need accuracy at 400'. If I needed 2000' accuracy I would need to go to a through hull installation.
The pipe needs to be cut to the angle of deadrise of that location on your hull.
I have 2 such sounder installations in my boat, each going on 20 years and working perfectly.
There are no, count em, NO holes through the hull.
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Old 05-24-2022, 10:39 AM   #7
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I once had a transom mount T-ducer, installed on the transom, which on my boat is cored. at least above the WL. The T-ducer mount was screwed into the transom at the bottom, so the paddle wheel (speedo) would get speed through the water, in addition to the depth reading. The wire from that unit then penetrated the transom above the WL and wound its way up to the instrument head. Who needs a paddle wheel any way?
A subsequent transom mount T-ducer went into an ABS pipe as I mentioned previously, so as to avoid all Hull penetrations that weren't really required. If you need more room for physically larger T-ducer, get a larger diameter pipe.
One of mine is a Garmin product, so it has a variable angle gizmo on the pipe to eliminate error on getting the angle of deadrise cut. Expensive if your T-ducer doesn't already come with one, but not necessary if you have a lumber store nearby.
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Old 05-25-2022, 09:59 AM   #8
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If you have some time , not in a rush, many depth finders offer shoot through the hull transducers which negate the need for any holes. Ask the equipment supplier. You do lose some depth range but not accuracy.

Just follow Kolivers lead.

Mine is the same way, shoot through a solid glass hull. no holes.
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