Different schools of thought on which, if either, of those two through hulls to keep for your sink drain. On the one hand, it would be nice to get rid of below waterline holes; if you want to do that, keeping that hole right AT the waterline is not gaining much. How often sailing does the boat sit right on her lines athwartships?
On the other hand, if you wanted to keep a sink drain and continue to use one of those two through hulls, the lower one may drain a bit better on a heel. How far below the bottom of the sink is the upper one, anyway?
You can get rid of both, and go 'sink drainless' which has a lot of merit but introduces at least a tiny bit of inconvenience.
On patching the holes, it is not hard, but does take getting ready before you start (to save frustration). There are a couple of ways to do it that differ only in details, but since these holes are at/below waterline, you do want to proceed "properly."
Here I'll assume for completeness that you don't have any glass working materials or experience. If this is too dumbed down, sorry...just trying to be thorough.
Materials Needed:
a. Epoxy resin and appropriate hardener; WEST System works well, but is expensive compared to, say MAS. Don't use a hardener that is "too fast" for the temperature, and don't worry about 'non-blush' hardener or special finish hardeners. Get the pumps for whatever epoxy you choose.
b. Thickening agents. You'll want two. First, you need a 'bonding/filleting' thickener like colloidal silica and second, you need a fairing thickener like microballoons or WEST's 410.
c. Paper cups, single use, disposable gloves (I prefer nitrile because latex bothers me), paper towels, stirring sticks (I get paint stirrers for free, then break them into quarters for doing epoxy work...don't spend $$ on tongue depressors - and save a largish piece as a scoop for your thickeners), a can of acetone and some cloth rags.
d. Glass cloth; 6 oz is plenty heavy. If this is all you are doing, you won't need much. A 38 inch by 90 inch packaged piece from West Marine is about $45, which is way more than you need if this is your only project. If only doing this, they sell it by the yard off a big roll for about $14 / yard...1 yard will do this job and have left over.
e. Angle grinder with about 36 grit on a sanding disk if you have it.
f. 80, 100, 150 and 200 grit sandpaper for a random orbit and hand sanding blocks.
g. I suggest a full filtered respirator for the grinding stage, but I use regular air filter masks for mixing the thickeners and final sanding. If you are working outside, you don't have to worry about epoxy fumes, but the thickener "dust" is not good to breathe. - you get a lot of dust while mixing and again when sanding.
h. I like using those cheap "Chip Bristle" brushes for epoxy work...they are $1 each (or less) and use 'em once, throw the away. I've found trying to clean epoxy (or polyester) out of brushes to be a losing proposition. Also, you may want to have a few size 'putty knife' type scrapers handy, and plastic works (and cleans up) the best.
The Method I used to patch a below waterline hole on my last haul-out (after removing the through hull fitting):
(1) Grind out the glass 12:1 around the hole inside and out (I was a bit shy on the inside due to a hull liner...). That is, take the thickness of your hull (it's not cored, is it?), and multiply by 6 (essentially dividing by two then multiplying by 12). That's how much to bevel out bigger than the hole that's already there.
If anything, bevel it out bigger, not smaller; I added a slight fudge factor.
For example, if you currently have a 1 inch diameter hole in a 1/2 inch hull, the bevel should be at least 1 1/2 inches larger in radius than the current hole. Go bigger, say 2 inches, if you are doing it from both inside AND outside.
If only from the outside, make the bevel 3-4 inches larger than the existing hole.
36-ish grit sandpaper on an angle grinder makes short work of griding the bevel.
(2) Make a "plug" that just fits the hole and is pretty thin - certainly thinner than the thickness of the hull. I made my plug by pouring some epoxy resin into a small paper cup and letting it cure. After tearing the paper cup away, the plug was too large, so I 'shaped' it down by sanding against a disk sander.
(3) Cut circles of glass cloth to cover the hole. Start with the diameter of the bevel and make each circle slightly smaller until you have the diameter of the original hole. I have mostly used AT LEAST 6 layers of cloth for this, but more is better if you can fit them.
Remember, the cloth will lay up thinner once wetted out, so if it looks like your 6 (8, 12, however many) layers over fill the hole a little bit, they probably won't once you actually lay them in. In fact, I suggest you make up a few extra circles to have ready in case you have a "dimple" after laying in your last one. You want a very slight dimple to allow for fairing (so you don't have to sand down onto the cloth), but not much.
Once they are all cut, arrange your circles so you can lay on the largest one first, then the next largest one, etc.
If doing from inside and out, you'll obviously need two sets of cloth circles.
(4) THOROUGHLY clean the bevel and the plug with acetone before starting the actual glass work.
(5) Wet out the area of the bevel (or both, inside and out if doing that way) and the plug, and stick the plug in the hole. Try to center it and get is straight, though I don't think this is super critical or anything.
(6) Mix up some thickened epoxy. Begin by mixing the resin and hardener and stirring them together really good. You have to sneak up on the correct thickness, and it's easier to mix in a little at a time anyway. It's usually described as equivalent to something familiar like "ketchup" (pretty thin and runny), "mayonaise" (thicker, but still pretty thin) and "peanut butter" (thick enough to support it's own weight).
I prefer the thick side of peanut butter for most things. So, add some silica, stir it in and test the thickness. If you want it thicker, add some more, stir, etc. Once you think you are getting CLOSE to the thickness you want to be, slow down...it is VERY easy to 'overshoot' and end up with what amounts to wet powder that won't work. I've done that a few times trying to rush (this is why you choose your hardener to give you time to work).
Once you have a good peanut butter thickness, dab it around the plug to form a fillet all the way around, thus bonding the plug to the middle section of the hole. You can use your (glove covered) finger to dab and form this fillet. Don't worry about making it shaped perfectly...no one's ever going to see this handiwork.
You need a fillet inside and out if you beveled from both sides.
Let the fillet cure while you go do something else.
(7) After the fillet is cured, sand around the fillet if necessary to smooth any sharp bumps; sand it with 'rough' paper so the next layer can 'bight.'
(
Clean the area thoroughly; you can wash with water and acetone, but before proceeding, make sure things are both clean and dry.
(9) Begin laying in the cloth. There are a couple of methods to doing this: (a) you can hold up the dry cloth and add resin, (b) wet out the cloth "off job" and lay it in wet, (c) some combination.
For a small layup like this, I prefer to wet out on a plastic covered board (or in one of those few cent throw-away paint trays) and lay the cloth in wet. For large areas, like hull sheathing, it's probably better to fix the cloth in place, then brush/roll/squeegee on the resin. Find what works best for you to get the layup you need.
You don't want too much resin in your cloth...try to keep the resin:cloth ratio low. But that said, you want the cloth fully impregnated. You can tell there's enough if the cloth is clear (dry, it looks white). Scrape or squeegee off the excess, though.
So, start by wetting out the whole bevel area with straight resin. *IF* your fillet is too bumpy or has 'void' areas, mix a little thickened epoxy (mayonaise is good here) to glop on and fill those voids. Remember, the cloth WON'T do it; cloth lays pretty much flat or conforms ONLY to very gentle concave curves...if a void area has sides that are too steep and you don't "fill" them with thickened resin, you'll have a void in your final layup which is BAD.
Once you have a good "fair" surface for the cloth (even if it's wet thickened epoxy), begin laying in your circles, whether wetting off the hull or on it, and again begin with the largest and working down progressively smaller circles. After all the circles are in place, let it cure.
Do inside and out.
(10) After this cures, sand it fairly lightly just to rough it up (the fairing needs something to "bite"). Again, after sanding, thoroughly clean with acetone.
(11) Mix up some resin and wet out the entire area with straight resin. With the resin left over, add some fairing thickener, enough again for peanut butter (but not TOO thick). Dab it on and spread evenly over the slight 'dimple' you left with a putty knife (or similar).
(12) After that cures, sand it fair to the hull with 150 ish grit. It's not a bad idea (and highly recommended with some fairing thickeners such as microballoons) to overcoat the fairing compound with non-thickened resin just to seal the fairing material. You can resand (to keep fair) the resin and paint.
(13) One less hole in the boat!!
It's harder to describe than it is to DO. All of this sounded like a LOT of work and DETAIL to me the first time I did it. Once you get into it, and the steps make sense for what you are doing, it's actually NOT hard and goes pretty smoothly.
Sorry so long...and I hope this helps at least a little bit.
Input from other, more experienced, glass workers ALWAYS welcome.
--John
Links: sailFar Gallery , sailFar Home page -->> sailFar Gallery Sign Up - Click Here & Read 
Author