Bedding in new transducers - what's best?

[ExcaliburII]

I'm facing a similar problem on an old sportscruiser I am refurbishing to replace Excalibur II, sold this year after 12 years of happy ownership. (Note to self: must update my user name etc!)

Anyway, the hull on this new (to me) boat is massively thick: grp inner and outer skins with a balsa core. Like the OP I am fitting NASA speed and depth transducers and I have two existing hull penetrations which I aim to use. I need to enlarge them both slightly to accept he NASA transducers, so the plan is to go right through the hull with the right size hole cutter in each case, dig out a bit of the core around the holes and fill with resin, then rebore with the hole cutters. The paddle wheel carrier will go straight in, sealed with silicone as recommended by NASA.

Because of the hull construction an in-hull depth transducer mounting is not an option, and given that 1) I already have a large hole in the hull which will either need to be used or filled, and 2) I don't want the body of the depth transducer mounted outside the hull in a big blister as it will cause too much drag and turbulence, I have come up with the idea of mounting the depth transducer in an adapted skin fitting.

The plan is to enlarge the existing hole in the hull slightly to accommodate a 2"BSP Tru-Design skin fitting. The internal diameter of this fitting is slightly larger than the diameter of the head of the NASA depth transducer. A 2"BSP blanking cap from my local plumbers merchant will be drilled to accept the shaft of the NASA depth transducer and screwed on to the skin fitting. I will then mount the depth transducer on the blanking cap with two nuts and adjust them to bring its working face flush with the outside of the skin fitting. Once in position, I intend to fill the void between the transducer and the bore of the skin fitting with silicone to give a smooth profile. Hopefully this will allow the depth transducer to work as intended with much less drag than if it were mounted in the conventional 'through-hull' manner. Time will tell . . .

 

[salar]

What would you put in as oil? Is olive oil OK? Or baby oil?

I used antifreeze on an Airmar P79. Water will do, but that stuff freezes so the recommendation is to use something that doesn't, like oil or antifreeze.

 

[salar]

In the case of the hole for the depth, the new unit is much smaller diameter than the old. Old about 40mm? new about 15mm shaft with a bigger head.
In effect I need to fill the hole that is left and bore a new hole for the new unit - or attach it inside perhaps?

You will need to do a very thorough job of filling the hole with a 1:12 taper and all that, with epoxy resin and mat. Then you can bore a new hole of the correct size.

 

[pvb]

I used antifreeze on an Airmar P79. Water will do, but that stuff freezes so the recommendation is to use something that doesn't, like oil or antifreeze.

Wouldn't the weather have to be awfully cold for the liquid to freeze next to the hull?

 

[prv]

Wouldn't the weather have to be awfully cold for the liquid to freeze next to the hull?

Not if you're up on sticks in a boatyard.

Pete

 

[pvb]

Not if you're up on sticks in a boatyard.

Do people still do that? How quaintly traditional.

 

[BERT T]

The plan is to enlarge the existing hole in the hull slightly to accommodate a 2"BSP Tru-Design skin fitting. The internal diameter of this fitting is slightly larger than the diameter of the head of the NASA depth transducer. A 2"BSP blanking cap from my local plumbers merchant will be drilled to accept the shaft of the NASA depth transducer and screwed on to the skin fitting. I will then mount the depth transducer on the blanking cap with two nuts and adjust them to bring its working face flush with the outside of the skin fitting. Once in position, I intend to fill the void between the transducer and the bore of the skin fitting with silicone to give a smooth profile. Hopefully this will allow the depth transducer to work as intended with much less drag than if it were mounted in the conventional 'through-hull' manner. Time will tell . . .

I've done a similar job myself but made up a plastic core for the skin fitting with a hole through the centre for the stem of the transducer. The transducer face sits flush with the outside flange of the skin fitting. The core was machined slightly smaller than the ID of the skin fitting and two grooves cut for O rings, a fibre washer was put inside the cap to form a seal. Doing this enables you to change the transducer without taking the boat out of the water ( similar to the speed log ) by simply screwing on a spare cap.

 

[ExcaliburII]

I've done a similar job myself but made up a plastic core for the skin fitting with a hole through the centre for the stem of the transducer. The transducer face sits flush with the outside flange of the skin fitting. The core was machined slightly smaller than the ID of the skin fitting and two grooves cut for O rings, a fibre washer was put inside the cap to form a seal. Doing this enables you to change the transducer without taking the boat out of the water ( similar to the speed log ) by simply screwing on a spare cap.

Nice idea. I will have to look for a suitable bit of plastic and some O rings. Thanks for the tip.

 

[duncan99210]

On a previous boat with an Autohelm depth and speed log, they recommended castor oil for the through hull depth transducer.

 

[Norman_E]

http://www.airmartechnology.com/uploads/InstallGuide/17-217-01.pdf

Those instructions are for the in hull transducer. The through hull Airmar one that I fitted was sealed in with Sikaflex 291i.

I share the view that replacing Airmar transducers with Nasa ones is a step backwards.

 

[pvb]

Those instructions are for the in hull transducer.

Yes, you need to read the whole thread. I was responding to Lazy Kipper's question "What would you use for an in hull Raymarine depth?"

However, the OP's question was about sealing Nasa through-hull transducers, and Nasa specify silicone, as other sealants may attack whatever plastic Nasa use.

Silicone sealant is perfectly adequate for transducers. There's no great pressure or mechanical stress involved. My last boat was a 24 year old HR, and the transducers were originally fitted in the factory with silicone and never leaked.

 

[vyv_cox]

My last boat was a 24 year old HR, and the transducers were originally fitted in the factory with silicone and never leaked.

There is a great deal of difference between factory fitted silicone sealant and a tube of bathroom goo. My windows were rebedded in their frames by a company in Athens using an engineering grade silicone. The frames and glass were primed, the sealant applied with a pressure gun and the whole lot was baked for 24 hours in an oven (not that I am suggesting that your hull was ) They gave a five year warranty.

 

[ghostlymoron]

I have just bedded my NASA thru hull transducer with Puraflex 40 http://www.everbuild.co.uk/Puraflex-40 although the fitting instructions say Silicone. I'll let you know if the fitting disintegrates.

 

[martinriches]

I fitted a NASA thru hull transducer last spring. It was supplied with a small tube of silicone. I think it was described as a adhesive.

Martin