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Old 26-11-2014, 11:49   #16
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Re: Thru hull backing blocks

No, there is no compression on uncured caulk. It squeezes out.
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Old 26-11-2014, 12:24   #17
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Re: Thru hull backing blocks

Nobody seems to be making a big deal of it, but there is a new sealant that does stick to starboard. Even the maker doesn't make a big deal out of it.


It's Sudbury Elastomeric Sealant. I haven't tried it on Starboard myself, but I have a small sample in my store and the bond seems quite strong.
Link: Sudbury Elastomeric Sealant
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Old 26-11-2014, 12:29   #18
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Re: Thru hull backing blocks

Hop--thanks for that. It says "Starboard to Starboard". The site shows "Adheres tenaciously without primers to fiberglass, glass, plastic, metal and wood." Sounds good. Not sure I'd recommend it for below the waterline until it has sufficiently proven itself.
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Old 26-11-2014, 12:37   #19
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Re: Thru hull backing blocks

I didn't think anything stuck to starboard, OK maybe dried fish guts, but nothing else.
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Old 26-11-2014, 12:47   #20
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Re: Thru hull backing blocks

Tera Nova, I'm using it to bed the hardware, rub rail and drain tubes on a little plywood skiff I'm building. At this point all I can say is it's pleasant to work with and it doesn't seem to be able to jump through the air and land on you like 5200 does.
I don't think it is possible to walk within three feet of wet 5200 without it getting on you!
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Old 27-11-2014, 03:08   #21
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Re: Thru hull backing blocks

Quote:
Originally Posted by gbgreen59 View Post
I am just about finished doing all of my thru-hulls. I used 1/2" G-10 and bedded the G-10 plate in Epoxy. The Epoxy was thickened with a West Systems product for adhesive applications. The G-10 base plates can be removed by hitting laterally with a heavy hammer. It breaks loose, but with much force.

The G10 was tapped for 5/16 machine screws. The bronze screws are bolted in through the base of the seacock. So, the screw heads are inside the boat. The seacock base is attached to the G-10 with screws prior to installation. A good amount of thickened epoxy is laid down and then the based is pulled down by screwing the thru-hull into the seacock base. Keep the thru-hull flush with the hull...this is important, since the outside of the hull does not necessarily match the contour of the inside hull.

Once the epoxy is set, then pull the thru-hull out, caulk it (SikaFlex 291 used caulk), put Teflon tape on the threads and screw back into the base. Works great.

Disclaimer: Almost all of the information is well documented with pictures and text in Maine Sails "How_To" articles.

A side note: Fiberglass release wax was used in places where contact with epoxy was not desired. So, the threads of the screws and the threads of the thru-hulls had release was applied. This way, these items are easily removed just in case epoxy might get on them.



I much prefer to glass my plates in instead of just bonding them. Then the hammer doesn't work anymore.
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Old 27-11-2014, 04:32   #22
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Re: Thru hull backing blocks

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Here is a photo of one of my boat's t-ducers installed past summer.
I have used clear pine for a "leveller block"& 4200 for many yrs on many comm.fishing boats & yachts. Use a matching piece of pine inside.
Bevel/champfer the outside of hull hole,to allow sealant to form an "o-ring" around t-ducer stem,& apply sealant to the area where stem joins t-ducer.
Avoid getting sealant on threads where nut travels.Apply thin layer of sealant to both sides of outer wood leveller block,just to keep t-ducer from turning in future.No sealant req'd on inside Just install a "hard washer" made from starboard,plexiglas,or other hard non-corroding tuff material,to preven nut from digging in to inside block & to spread the load. Paint the outside wood leveller with bottom paint.
Advantage of pine-it swells when wet & really tightens things up.
I've never had a problem with this method. Just my experience over 30 yrs.

Note-I have seen quite a few bronze t-ducers with built in Temp sensors corrode & fall off.The stem always stayed in the hole,so no leaks-I suppose due to sealant "O-ring"effect. Temp sensor is a chip,embedded in ducer,& is supplied with +5VDC from sounder.Inevitably,water gets in,& u get fast electrolysis. Solution is to cut the 2 Temp wires @ sounder end.
Again-just my experiences.
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Old 27-11-2014, 09:48   #23
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Re: Thru hull backing blocks

I would avoid using pine anywhere on a boat, especially below the waterline.
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Old 27-11-2014, 10:50   #24
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Re: Thru hull backing blocks

I suggest 1/2"-3/4" electrical grade fiberglass (GPO3), bedded with polysulfide (e.g., Lifecaulk) or epoxy or polyesther resin or polyurethane (3M 5200 or 4200). Nothing really sticks to Starboard (HDPE) unless you flame treat it first, and it's not really much less expensive than fiberglass. GPO3 has good structural properties and low water absorption, and the cost is reasonable. Marine grade plywood has been used on countless boats, but it is susceptible to rot and delamination if it gets wet. The premade backing plates like these:
Seacock backing plates look great but fairly expensive.
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Old 27-11-2014, 11:11   #25
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Re: Thru hull backing blocks

I wouldn't use wood of any kind, and especially not solid teak! Use something synthetic. A solid bit of GRP works very well indeed. Just find an old boat at the dump and cut some bits out of it or lay some up. 5200 will work fine as a sealant. This is the method i've found to work very well indeed:

1) Grease your through-hull fitting
2) Drill the hole in the new fibreglass backing plate
3) Mix up some high-density epoxy filler and slather it around the hole in the hull on the inside.
4) Install your through-hull fitting and backing plate, screwing it down only lightly; enough that the epoxy squirts out a bit and that the backing plate is seated.
5) Clean up the edges with a rag and some acetone.
6) Once the epoxy is set, remove the through-hull fitting. The grease is critical to achieving this!
7) Clean up the grease thoroughly with some acetone and re-install the fitting using 5200 as a sealant.

The benefits of this method are:
1) The fitting can be removed if necessary in future without disturbing the backing plate (which is essentially part of the hull now!)
2) The backing plate is perfectly seated parallel to the hole in the hull, so the pressure on the flange is equal all the way around and the seal is perfect.
3) It'll last forever!
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Old 27-11-2014, 11:28   #26
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Re: Thru hull backing blocks

Def--good post.
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Old 27-11-2014, 11:36   #27
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Re: Thru hull backing blocks

I agree with Denverd0n. The illustrated" Compass Marine" solution is easy to follow and
is a slick solution to your problem-it worked well for me.
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Old 27-11-2014, 11:47   #28
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Re: Thru hull backing blocks

Thinking out of the box;
Sooooo.... backing blocks started with wooden boats.... if fiberglass hull is 1"= thick or so.. do you really need them?
If you do need them, aren't they supposed to distribute the load wider than the seacock base?
Those Groco ones shown above don't really look much bigger than the seacock base..?
If you want load distribution... looks to me like a triangular seacock base should be avoided and would be better with the traditional round base....
If you use fiberglass blocks, glassed to the hull... what is the point? Concerned about breaking your hull when turning a stuck seacock?
BTW: I was able to see the 31 footer I built in 1981 summer before last. The one seacock I observed (Spartan) that I bedded on epoxied Marine play with 5200 still looked as new visually... hard to say I guess inside....
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Old 27-11-2014, 11:54   #29
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Re: Thru hull backing blocks

Quote:
Originally Posted by deblen View Post
Attachment 92397

Here is a photo of one of my boat's t-ducers installed past summer.
I have used clear pine for a "leveller block"& 4200 for many yrs on many comm.fishing boats & yachts. Use a matching piece of pine inside.
Bevel/champfer the outside of hull hole,to allow sealant to form an "o-ring" around t-ducer stem,& apply sealant to the area where stem joins t-ducer.
Avoid getting sealant on threads where nut travels.Apply thin layer of sealant to both sides of outer wood leveller block,just to keep t-ducer from turning in future.No sealant req'd on inside Just install a "hard washer" made from starboard,plexiglas,or other hard non-corroding tuff material,to preven nut from digging in to inside block & to spread the load. Paint the outside wood leveller with bottom paint.
Advantage of pine-it swells when wet & really tightens things up.
I've never had a problem with this method. Just my experience over 30 yrs.

Note-I have seen quite a few bronze t-ducers with built in Temp sensors corrode & fall off.The stem always stayed in the hole,so no leaks-I suppose due to sealant "O-ring"effect. Temp sensor is a chip,embedded in ducer,& is supplied with +5VDC from sounder.Inevitably,water gets in,& u get fast electrolysis. Solution is to cut the 2 Temp wires @ sounder end.
Again-just my experiences.



For transducers, high performance fairing blocks are standard in the boatyard anymore. All synthetic, just measure the angle and cut the block in half on a table saw or band saw with fence. Installs with a bronze anti rotation bolt. Billed 1.5 hrs for my last remove & replace in a sailboat.




Airmar 33-476-01 High Speed Fairing Block for B744V $65


Obviously a thru hull is a different story. But most modern combination knotmeter/transducer units are using these now.
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Old 27-11-2014, 11:59   #30
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Re: Thru hull backing blocks

Quote:
Originally Posted by Cheechako View Post
...backing blocks... if fiberglass hull is 1"= thick or so.. do you really need them?
If you do need them, aren't they supposed to distribute the load wider than the seacock base?...
Good questions.

The hull was OK at 1" thick (or whatever) before you started running hole saws through it. A bonded doubler (backing block) reinforces the hull's skin in the area around the hole so that this hull piercing does not propagate. This block also facilitates fairing the hull's shaped surface into a flat surface against which a seacock can be mounted.

Bruynzeel plywood is what I prefer because it is durable, is slightly compressible to allow for inaccuracies, accepts epoxy well, and is easy to shape to the inside of the hull, in situ.
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