Reducing hole size for thru hull transducer

[finefurn]

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 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 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

 

[walmsleyc]

I would not trust a simple plug or fill solution. Any failure of the bond could sink your boat. A lifting strap could accidentally be placed on the transducer and cause stress, and break the bond. I would effectively bevel the edge of the old hole from the outside (some suggest up to 12:1 the thickness of your hull) so you have new glass exposed, as if you were going to fill it completely, and build up layers of glass + epoxy/resin and then fill and fair and then re-drill the proper size.

If it were just 1/8" difference, you might be able to just fill or caulk/sealant around it.

Just my thoughts.

Chris

 

[rgranger]

I'm with @walmsleyc

 

[Captain Larry-DH]

I agree with the others, you need to bevel the repair. If you want to cut corners; leave the old speed sensor "dummy plug" in place and make your new hole of the proper size nearby.
However, I have one question: You mentioned the old "speed sensor" but you're putting in a CP, and CP's generally come with depth sensors, not speed sensors. Are you putting in a depth sensor? If so, and you have a solid layup in that location (not cored) you can simply "glue" the new depth sensor to the inside of the hull (using a blob of silicone) and shoot through the hull with negligible signal loss. No hole required.
I have done that and my CP has full fishfinder functionality, and I get clear readings to 200' depths and beyond. I also have my original Raymarine ST60 depth instrument sensor and it was originally installed through the hull. I get depth-only from that at the same time as I can get added graphic bottom contour info from the CP.
It's a nice way to go, just keep in mind that you can not have any air bubbles (or core) in the fiberglass layup or in the blob of silicone -- so be very careful to shape the top of the blob to form a dome that can't trap any air as you press the transducer onto it.

 

[Terry Cox]

I'm with @walmsleyc

Hi Marty, Chris has it about right. Altering/reducing the size of the hole needs to be treated the same way as any other in the hull below the waterline. Most yards will grind out and taper the fiber glass to at least a ten to one ratio in order to prepare the hull in that area structurally equivalent to the entire hull. They will then layer in FRP to the ground out area to bring back the hulls original construction integrity finish including new gelcoat. At that point you can drill a new hole of the correct size wherever you want.

In 2004 I chose instead to use a shoot-through-the-hull type transducer for our chart plotter rather than drill a new hole for a through-hull type and it has worked out great. I kept our original through hull speed transducer, which gives me speed through the water and the new chart plotter transducer that gives me speed over ground, both very useful information.

 

[shemandr]

For the shoot thru the hull transducer, I think you can test the location by placing in a bag of water to verify if the hull location has any bubbles. At least that's what I remember the instructions for mine Rx'ed.
As for the hull repair I have this story: A friend put a new thru hull for conversion from a porta potty to a flushing head. He did the fiberglass work. There was no exterior flange. Early in the season on a rough Wed. PM race he hit a wave and the whole thru hull and surrounding fiberglass repair shot across the cabin. He couldn't even use a plug. He stuffed whatever in there and had his wife stand on it. He made it back to the travel lift and someone was there to lift the boat out of the water. He was lucky a yard worker lives on the property and that it wasn't very far to get back.

 

[rgranger]

..... he hit a wave and the whole thru hull and surrounding fiberglass repair shot across the cabin. He couldn't even use a plug.

Dang! That made my stomach clench just reading about it. I bet your friend also needed new shorts.

 

[shemandr]

If he didn't the Mrs. did. He was going to ground the boat which he could have done quickly. But imagine half way to Block Island.

 

[rgranger]

If he didn't the Mrs. did. He was going to ground the boat which he could have done quickly. But imagine half way to Block Island.

Are they still married?

I can hear the conversation in my head... "Honey.... would you mind taking that seat cushion, shove it in that hole and stand in the hole for bit....?

 

[shemandr]

Ha! Yeah, they're still married. I notice they don't have that boat anymore.

 

[rgranger]

.......I notice they don't have that boat anymore.

This thread is killing me... funniest thing all year....seriously

 

[RoyS]

You may be able to purchase a substitute transducer of the correct size from Airmar. I had a similar issue and they had the solution. Good luck.

 

[Maine Sail]

This is what a proper prep for repair of a transducer hole will look like. It is often referred to as a 12:1 bevel repair...

 

[dscribner]

Looks more like a primer job on the Death Star.

We replaced our sounder but instead of disturbing an apparently watertight connection on the old thru-hull, we opted to leave it in place and use a shoot-thru transducer in mineral oil instead.