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Old 04-19-2013
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Cutting a through-hull for bilge pump outlet

Ok, going to cut a through-hull for the automatic bilge pump (3/4" outlet). Guessing I should place it somewhat near and parallel to the manual pump outlet. This will be a plastic fitting, to match the other one. Please pardon the many questions below!
3/4" hole saw?
Drill from inside or outside?
Drill in reverse when it first touches the fiberglass?
Tape off the target area to protect the gelcoat?
Do you put any epoxy or sealant inside the hole prior to installing the through-hull fitting?
Thanks in advance. This is a nerve-wracking procedure for me.
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Re: Cutting a through-hull for bilge pump outlet

My answers in blue
Quote:
Originally Posted by FirstCandC View Post
Ok, going to cut a through-hull for the automatic bilge pump (3/4" outlet). Guessing I should place it somewhat near and parallel to the manual pump outlet. This will be a plastic fitting, to match the other one. Please pardon the many questions below!
3/4" hole saw?
- I would prefer a hole saw
Drill from inside or outside?
-both - start from one side, when the center bit is through and you have a good grove, switch side
Drill in reverse when it first touches the fiberglass?
- Yes, less chance of damaging the gelcoat, drill pilot hole first with a normal drill bit if necessary. You can do the whole cut in reverse - less chance of the drill starting to kick
Tape off the target area to protect the gelcoat?
- No I don't do that
Do you put any epoxy or sealant inside the hole prior to installing the through-hull fitting?
- If it is only fiberglass it is not necessary, if cored then Yes
Thanks in advance. This is a nerve-wracking procedure for me.
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Last edited by knuterikt; 04-19-2013 at 10:40 AM.
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Old 04-19-2013
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Re: Cutting a through-hull for bilge pump outlet

Drill an 1/8" hole first. If you make a mistake, it's really easy to fix with a dab of Splash Zone or similar.
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Re: Cutting a through-hull for bilge pump outlet

I do tape the outside to prevent splintering and dust
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Re: Cutting a through-hull for bilge pump outlet

I have done this once each last year and this year. I use the next-size-up hole saw from the thru hull side of the thru hull. I think 3/4 is too tight, a one inch leaves only 1/8" on either side of the hole, which allows for a little caulk to squeeze into that gap. I also use a backing plate for good measure, though for a small thru hull under little stress it is not strictly necessary. Last year was a piece of 3/8 ply pre-holed and liberally coated with epoxy. This year was a piece of 3/8 fibreglass sheet, again pre holed. These will later be bedded in caulk.

Drill your hole. You can go backward, or just start slow. I go from outside and go slow. Gel coat is relatively soft, and cuts easily. Once through the gel coat, (my hull is solid glass) I stop and remove the saw frequently to let it cool - it's a high friction situation. Going slowly I have not had any tear-out or splintering experience.

Once the hole is cut, mask all around the hole a couple inches. Place the thru hull in place and hold it firmly with one hand while you trace around it with a razor knife into the masking tape. Remove the thru hull and remove the tape you've scribed around the hole. Goop up the thru hull, flange and shaft with caulk. Put it in place. Have a friend you don't like very much or a teenager that needs to learn a lesson climb into that tiny locker to bed the backing plate and the shaft and spin on the nut. If it's a nylon or marelon thru hull, don't overtighten and deform the threads. Even if it's bronze don't overtighten, you don't want to squeeze all the caulk from between flange and the hull, or between the backing plate and the hull. Enough caulk has to remain to do its job.

Clean up the excess caulk with paper towels, then a rag and some solvent. Stand back and admire your manliness.
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Last edited by Ritchard; 04-19-2013 at 12:31 PM.
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Re: Cutting a through-hull for bilge pump outlet

Thanks to all. Ritchard, which caulk do you prefer?
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Re: Cutting a through-hull for bilge pump outlet

Ideally, you want the outlet to remain above the waterline when heeled over. Not sure if your current manual outlet is.

If you have a cored hull, you are going to want to dig out some of the core, leaving the glass at the correct size and then fill the core back with thickened epoxy.

Unrelated to your question, but be sure to use smooth bilge hose or you lose much of the pumping ability. You also lose flow rate, the higher you have to lift it to the outlet.
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Re: Cutting a through-hull for bilge pump outlet

Quote:
Originally Posted by Minnewaska View Post
...Unrelated to your question, but be sure to use smooth bilge hose or you lose much of the pumping ability. You also lose flow rate, the higher you have to lift it to the outlet.

It sounds like you are installing a little automatic pump to take care of the stray drips, et cetera. If so, and to the extant that you can, route the hose so that it's highest point is higher than the thru-hull and as near as possible to the pump (i.e., the section of hose between the pump and the high point should be as short as possible). Then route the hose so that the water continuously flows down to the thru-hull from the high point. That way you will have the minimum amount of "back-slosh" going back into the bilge when the pump turns off. If the thru-hull is the highest point in the system the contents of the entire run of hose are likely to wind up back in the bilge at the end of each cycle.
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Re: Cutting a through-hull for bilge pump outlet

Be absolutely sure that the outlet is above the water line at all points of sail, or a least there is a loop inside the boat the accomplishes the same.

If you don't, what happens is one day you are out sailing healed over on a beautiful day and the bilge starts filling with water as a nice siphon sets up.

Don't ask me how I learned this please
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Re: Cutting a through-hull for bilge pump outlet

Per your question above, I would use 3M 4200. For me it has worked well, and it's readily available. Others may have better suggestions.

I second the ideas about routing a loop in the plumbing, and perhaps more crucial would be excavating a half-inch of core material around the hole you've just cut should you in fact have a cored hull. I just did a little reading, and I think you'll find that a '77 C&C 27 will have a solid core.

http://www.cncphotoalbum.com/reviews/c&c27.htm
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