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Fairing external transponder shims

1246 Views 7 Replies 4 Participants Last post by mitiempo, Oct 16, 2009

cghubbell

Discussion Starter · Oct 16, 2009

Ok, I've been using up my question cards pretty quickly this week, but try to forgive a guy who's cramming in projects before the snow flies

This time my question is about the shims for the transponders on my hull. It looks like it was made from wood, which doesn't directly concern me. But, it also appears that it wasn't well sealed with epoxy. It's not rotten if I poke it, but it was definitely a little damp. I'd like to leave it in place and improve it rather than starting over.

Here's a picture... There's one on each side (depth, speed).

I'm not happy about the [1] sealing, and [2] slight gap between the hull and the shim. Oh yeah, and the blisters I haven't ground out yet!

My question is... How SHOULD this look? Do I just need to sand it down and coat it with unthickened epoxy, then make a fillet to join it to the hull better? Do I need to glass it to the hull? Should I be concerned about it having screwy water flow that messes with the sensor?

I just haven't seen enough of these things to know what to do, and my bible, "This Old Boat" doesn't have the answer for me

Thanks in advance,
Chris

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sailingdog

· #2 · Oct 16, 2009

Why not build up a shim using either fiberglass or thickened epoxy instead???

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cghubbell

Discussion Starter · #3 · Oct 16, 2009

sailingdog said:
Why not build up a shim using either fiberglass or thickened epoxy instead???

I was just thinking that it might save me a lot of time if the current configuration wasn't a bad one. Rather than trying to remove and re-create it, I could improve it. But I really don't know if this is a good or bad idea. I don't mind the time if that's what needs to happen.

Building up a whole new one sounds like considerably more work than sealing an existing one but I could be wrong...

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mitiempo

· #4 · Oct 16, 2009 (Edited)

If you remove the old shims in one piece you can use them to mold new ones out of epoxy thickened with colloidal silica. If you cast them in say plaster of paris after waxing them, you have the mold. Wax the mold heavily to allow removal. Attach them to the hull (sand it first) with more thickened epoxy and create a fillet where they join the hull. And the best part is most can be done indoors! You will probably never seal the existing ones very well as there will be water between the hull and shim and they have a high moisture content anyway - epoxy doesn't stick to wet wood.
Brian

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pedcab

· #5 · Oct 16, 2009

Here's what I've came up with to solve a similar problem.

http://www.sailnet.com/forums/gear-maintenance/58350-dyi-depth-souder-hull-fairing.html

Everything is epoxied together and has been underwater for quite a while now with no problems

Best regards!

Pedro

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cghubbell

Discussion Starter · #6 · Oct 16, 2009

pedcab said:
Here's what I've came up with to solve a similar problem.

http://www.sailnet.com/forums/gear-maintenance/58350-dyi-depth-souder-hull-fairing.html

Everything is epoxied together and has been underwater for quite a while now with no problems

Best regards!

Pedro

That looks pretty similar to mine, so it's good to see my configuration isn't too far off base. Also good to know you have no problems because my speed instrument is completely flaky and I had suspected the fairing blocks might be causing it.

I'm not really sure why they used a block for the speed log since I believe they can just be installed perpendicular to the hull. Seems like a lot of work when it wasn't necessary.

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pedcab

· #7 · Oct 16, 2009 (Edited)

Well, I made mine for the depth sounder transducer because it has a maximum allowable tilt of 18º, wich is much less than the angle of my boat's hull.

My speed transducer is mounted directly to one side of the hull but that isn't a perfect solution. The thing is that when sailing upwind in such a way that the transducer stays on the windward side of the boat the paddlewheel stalls due to the underwater turbulence cause by the leeward movement of the boat.

Keep in mind that paddlewheel speed transducers are very sensitive to water turbulence and make sure that there aren't any obstructions "upstream" of it, such as other transducers.

Also, the fairing must be as streamlined as possible because of the previously mentioned reason...

Regards from Portugal!

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mitiempo

· #8 · Oct 16, 2009 (Edited)

The best place for the speed transducer on a CS27 is centreline, just inside the V-berth aft wall which is just forward of the keel - the recommendation of most manufacturers. If mounted on one side you will likely get a different readfing on either tack. But you already have this other hole....
Brian

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