flush thru-hulls

JonnyBoats · Post by **JonnyBoats** » Sun Nov 11, 2007 8:08 pm

When installing a thru-hull with a flat head (as opposed to a mushroom shaped head), how does one go about cutting the bevel into the fiberglass hull?

catamount · Post by **catamount** » Sun Nov 11, 2007 8:58 pm

I used a hole saw of appropriate diameter for the inside of the flange, drilled into the thull to a depth equal to the depth of the flange, then switched down to the hole saw for the thru-hull portion and finished the hole. Then I took a chisel and spilt out the bit of hull laminate for the flange, and finally ground the angle for edge of the flange with a dremel rotary tool. Alternatively, you can just use a hole saw for the outside diameter of the flange, drill into the depth necessary, and then split out the laminate with the chisel. This leaves you with a square-edged recess rather than a tapered one to fit your flange. You can either fill this with sealant (I used polysulfide) when you install the through hull, OR your could wax up your thru-hull and flange, fill the square edge with thickened epoxy, and then mold a tapered recess for the flange using the thru-hull flange itself for the mold.

If you don't have hole-saws that match your flange diameters, use the next smallest size and then grind out to fit with a dremel, or use the next larger size and fill with thickened epoxy molded to fit...

Good Luck,

Tim A.

Figment · Post by **Figment** » Sun Nov 11, 2007 9:33 pm

Sounds like a good way to compensate for lackluster grinding skills. ;)

(no, really, good process!)

Post by **Tim** » Mon Nov 12, 2007 7:29 am

My own opinion is that flush through hulls only make sense if you're planning on racing the boat. The amount of drag reduction is minimal and unnoticeable to the cruising sailor.

(To the racer, it's everything.)

Otherwise, save a few installation hassles and use standard mushroom heads.

JonnyBoats · Post by **JonnyBoats** » Mon Nov 12, 2007 8:03 am

Otherwise, save a few installation hassles and use standard mushroom heads.

Yes, unless you happen to have some with flush heads ;-)

Post by **dasein668** » Mon Nov 12, 2007 9:22 am

JonnyBoats wrote:
Otherwise, save a few installation hassles and use standard mushroom heads.
Yes, unless you happen to have some with flush heads ;-)

Return/exchange?

Quetzalsailor · Post by **Quetzalsailor** » Mon Nov 12, 2007 9:39 am

Tim, the LeComtes were born with flush throughhulls; no doubt the finely-tuned design took into account the assymmetries of hydrodynamics to be caused by the scattering of mushroom-headed fittings.

So, when I replaced the throughhulls on Q, I cut the stem hole with a hole saw and ground the clearance with a burr on an electric drill. This method, of course makes a lousy mess. No doubt the professional has cutters made for the purpose. I cleaned up the lousy mess by carefully taping the fitting, spraying it with teflon slip spray and assembling it into a nice thick bed of epoxy thickened with microfibers. The result, when done well, is a perfectly formed, smooth recess, with the edge of the laminate nicely sealed.

God is in the details, as Louis Sullivan used to say about his buildings. I have reported previously that I used this assembly method to locate the new backing plates to which the seacocks are bolted. It is very important to ensure that you don't epoxy the whole collection of pieces together, but only what you want to be together! I had to destroy two new through-hulls to get them apart to do a properly sealed assembly. Timing is very helpful; if you've been sloppy you can get things apart before the epoxy is rock hard.

This spring: the last two of the seven.

Figment · Post by **Figment** » Mon Nov 12, 2007 10:12 am

Tim wrote:My own opinion is that flush through hulls only make sense if you're planning on racing the boat. The amount of drag reduction is minimal and unnoticeable to the cruising sailor.

(To the racer, it's everything.)

Otherwise, save a few installation hassles and use standard mushroom heads.

SLACKER!!!!

Even if a cruising sailor doesn't love the minimal drag reduction, the hassle-free bottom cleaning and painting is another nice little side benefit.
How DO you guys paint around all those little mushrooms without tearing your hair out??!!

Post by **Tim** » Mon Nov 12, 2007 10:28 am

Painting around them? What do you mean? I paint them.

Figment · Post by **Figment** » Mon Nov 12, 2007 10:37 am

yeah, I know, but you can't just hit them with the roller, you have to break out that little brush and actually use your wrist, and then it probably starts a run down the surface of the hull... ugh.

Post by **Tim** » Mon Nov 12, 2007 11:17 am

I'm really talented and manage to keep those runs at bay. I never realized how much skill it took...

Zach · Post by **Zach** » Mon Nov 12, 2007 10:21 pm

Would a forstner bit work in this instance... given a dowel or plug is left to give it something to center on?

(Curves make my brain hurt.)

Hirilondë · Post by **Hirilondë** » Tue Nov 13, 2007 7:34 am

Zach wrote:Would a forstner bit work in this instance... given a dowel or plug is left to give it something to center on?

(Curves make my brain hurt.)

A forstner bit would work well, but fiberglass will reek havoc on it. I'm a carpenter, and damaging bits, even for a good cause, still makes me shudder. Normally, when I want a flat bottomed recess for through-bolting I make the forstner hole first. If you are fitting to an existing hole, then using a DC plug cut flush with the hull should work.

Rachel · Post by **Rachel** » Tue Nov 13, 2007 8:33 am

Hirilondë wrote: If you are fitting to an existing hole, then using a DC plug cut flush with the hull should work.

I know this will probably be totally obvious once you tell me (and I get the concept already), but what's a "DC plug"?

keelbolts · Post by **keelbolts** » Tue Nov 13, 2007 9:36 am

Good morning Rachel,
DC is for damage control. They're the conical wood plugs you should have attached to each of your thru hulls, but like most of us probably don't.

catamount · Post by **catamount** » Tue Nov 13, 2007 1:00 pm

Another approach is to use nested hole-saws, i.e. use a hole saw that matches the diameter of the existing hole to center the bigger hole saw that you are using to cut the recess for your flange.

Zach · Post by **Zach** » Tue Nov 13, 2007 2:30 pm

Two very cool ideas.

Thanks!

Rachel · Post by **Rachel** » Tue Nov 13, 2007 6:30 pm

keelbolts wrote:Good morning Rachel,
DC is for damage control. They're the conical wood plugs you should have attached to each of your thru hulls, but like most of us probably don't.

Thanks for the clarification.

I've on purpose tended to keep those plugs in an easy-to-access "damage-control" locker, along with a hammer and assorted other items. I did actually have to use one once, about 150 miles offshore, and it worked out okay.

Cruising buddy: "Um... help! Bring a plug and the hammer! We're sinking!"
Me: "What!?! Okay!"

I quickly grabbed a plug and the hammer and hastened to the head (where the voice had come from), only to see my cruising buddy with a dumbfounded look on his face, a transducer through-hull in his hand, and a round spot of blue-green Caribbean showing in the bottom of the sink cabinet. Remarkably, even though this through hull was probably 3-feet below the waterline, there was only a low stream of water coming in (that is, no "Old Faithful"). We were trundling along at 4-5 knots (the problem side was not "high"), so maybe that's why? I bet we didn't take on more than a couple of gallons of water, all told.

Anyway, we tapped the plug into the hole so that it projected just a bit past the hull (didn't want it too long, but did want some wood on the outside to swell to hold it in place), and there it stayed until we hauled in Virginia two months later (had to split it out with a hammer and chisel).

I have great faith in the plugs :-)

I have read that you aren't supposed to tie them to through-hulls because that area might be damp and then they might swell ahead of time (instead of when you want them to), but that's not actually why I had them in a locker. It just liked the idea of always going to that one locker when there was a problem. I don't know that they'd swell that much unless one had a lot of extra water around anyway.

By the way, that knotmeter through-hull was the one underwater fitting that was installed by a yard (and not DIY). When we investigated we found that there was only a wee bit of thread engagement (which had corroded) and the bonding wire had only been shoved under a piece of plastic and had made no contact (although it had an eye connector on the end). Sheesh.

Not that yards don't do good work, of course, but it never hurts to check a given installation. The owner had had misgivings about the fellow working on the boat, but hadn't wanted to "be a pain."

Okay, back to your previously scheduled topic.

Hirilondë · Post by **Hirilondë** » Wed Nov 14, 2007 7:29 am

Rachel wrote:I have read that you aren't supposed to tie them to through-hulls because that area might be damp and then they might swell ahead of time (instead of when you want them to), but that's not actually why I had them in a locker. It just liked the idea of always going to that one locker when there was a problem. I don't know that they'd swell that much unless one had a lot of extra water around anyway.

It is likely true that the plugs would have a higher moisture content in the bilge. So yes, they would swell less after being used then dry ones still in the bag, or in a drawer. But I'm not so sure that is the most critical component in an emergency leak situation. ORC requires that every through-hull have a DC plug on a leash at and designated for it. Even if 2 or more through hulls are in one location, each must have its own plug. The reasoning is that there is very often very little time to secure a 1 1/2" hole before it is below the water enough to be hard if not possible to find. So even though you still have time to plug the hole, you can't find it any more. This was how an inspector for the Marion/Bermuda Race explained it to me.

I keep one tethered to each seacock and I have a couple more in the drawer with the hammer. I never thought of it, but I suppose I could secure the leak with the tethered one, then swap to a fresh one as soon as all was secure.

What really amazes me, and it seems most common on large roomy boats is seacocks in tight and remote places that I doubt could ever be emergency plugged. Now that scares me!

Rachel · Post by **Rachel** » Wed Nov 14, 2007 1:54 pm

Hirilondë wrote:What really amazes me, and it seems most common on large roomy boats is seacocks in tight and remote places that I doubt could ever be emergency plugged. Now that scares me!

Yep, we had one of those at the very bottom of a galley "dry locker" that was accessed by a lift-off top (picture an icebox, only not an icebox). We couldn't not use it for stowage, but we literally told ourselves (out loud, to make sure it sunk in) that if the time came when we needed to get to that seacock, we would quickly toss everything out of that locker in a heartbeat, into the sinks, without worrying about the contents or the consequences thereto. At least for me, I find that being careful when I unload a locker is ingrained, and so I had to make a mental point that it did not matter on that locker.

It is crazy where some seacocks end up.

Rachel

Zach · Post by **Zach** » Wed Nov 14, 2007 5:16 pm

Yeah...

I can't believe the lack of access to the sink drain... and doesn't even have a seacock! (Can't decide to do a pump or an access panel with a seacock behind it.)