Flush Fit Thru Hull Install Question

sailnoproblem · Feb 27, 2019

Try a router with a 1/2" chamfer bit. Just a thought, you might be able to make it work.

dlochner · Feb 27, 2019

Ward, I had a similar problem when I upgraded a speed transducer. I tried using a hole saw to enlarge the hole, let's just stay it didn't go as planned. The hole saw will walk and the the resulting hole is a mess.

The hole saw needs a pilot hole. Eventually, I cut a plug for the hole from a piece of scrap 2x4 and epoxied that into the original hole, then is was easy to drill a new larger hole.

As for the chamfer, a dremel tool with a grinding bit would work. After plugging the original hole, scribe a circle for the outside diameter of the flange. Drill the hole and then use the grinding stone to cut the chamfer. A little imperfection will be OK because the caulking compound will seal minor imperfections.

SG · Feb 27, 2019

I think you have to put a plywood sandwich drill through to "enlarge" the hole. If the hole is on a part of the hull which doesn't allow to do that, then I think you have to "fill" then redrill.

I thin the idea of carefully grinding our the edges is a lot tougher and "less likely to succeed.

I have not done this myself on a boat -- but I've had to drill enlarged holes in doors for hardware and through plywood pieces for furniture.

EV21 · Feb 27, 2019

Was thinking for the chamfer if your fiberglass is thick enough you might be able to use a chamfer bit with a ball bearing pilot or possibly fix a layer inside with a 1.5 inch hole aligned with your hull hole to guide the pilot bearing. Hmm, hull hole -- could have some fun with that.

Hayden Watson · Feb 27, 2019

What I do whenever I need to enlarge a hole with a hole saw is cut the required size hole in a piece of plywood that is at lease a couple of inches larger than the diameter of the hole saw. I then glue, screw or sometimes hand hold the plywood over the new hole until the saw cuts a full groove. Then you can remove the plywood guide and finish the hole.
+1 on using a router and chamfer bit with bearing. https://www.lowes.com/pd/IRWIN-Carbide-Tipped-Chamfer-Bit/1000234599

LeslieTroyer · Feb 27, 2019

Ward use the 1 1/2 hole saw to cut a hole in a 2” square of plywood. Use double stick tape to mount square over existing hole. Remove pilot drill from hole saw. Carefully start you hole. After about 1/8” your golden and don’t even need the plywood anymore

Here is an example of how I put the SG200 in with no center hole. Did same thing on pressure water input in fiberglass Worked great

Les

FastOlson · Feb 27, 2019

It's easier than you might imagine. Put your new backing plate on the inside - G10 would be best but you could use epoxy-coated wood if you want. Next drill the hole for the size of the threaded thru hull part, and plan on letting the outside lip just stay proud on the surface. Next, after a dry fit, place the new Forespar part on the inside, and set the base into thickened epoxy. put thickened epoxy around the thru hull and screw it in from the outside and bring it up tight with excess epoxy oozing out on the outside and hardening. Last, grind off the outside flange lip. You end up with a smooth 'hole' in outside surface and the whole composite thru hull is solid as the rest of your frp boat.
We did this in the late 90's for all 5 of the thru hulls in our '88 boat. They all look and work like new. The model 93 is a pretty good product. I do not dislike bronze for the purpose, but it is really nice not to have to worry about electrolysis at some other moorage with sketchy wiring, when out cruising.
Good picture of two of ours in reply #4 of this thread:
http://www.ericsonyachts.org/infoex...-seacocks-vs-gate-valves-anyone&referrerid=28

mdidomenico · Feb 28, 2019

Ward H said:
The flush fit requires a 45 degree chamfer 1/4" deep. Do they make chamfer bits that large or is there another way to make the chamfer?

does your C30 already have flush thru-hulls? are you sure the flanges match? if they do you should already have the chamfer.

see this thread where i did mine. if you want to see it in person i can show you next time i'm down at the marina.

https://c34.org/bbs/index.php/topic,9777.msg74568.html#msg74568

Maine Sail · Feb 28, 2019

If the thru-hulls were not flush from the factory the hull thickness will need to be increased at each seacock in order to flush fit them. Not a small project.

SG · Feb 28, 2019

Maine Sail said:
If the thru-hulls were not flush from the factory the hull thickness will need to be increased at each seacock in order to flush fit them. Not a small project.

Beyond Maine Sails observation, I think that the Marelon Forespar theuhulls are best installed with reinforcement on the inside. The are pretty flangeless compared with a traditional bronze Groco (for example). If you use the thruhull as a hand or foot brace, you don’t want it moving around. The installation suggestions from previous posters alluded to this need to have a mounting base which is glassed in and rot proof.

LeslieTroyer · Feb 28, 2019

Back in 1983 I rented a chamfer drill bit to install my sounder and paddle wheel as I remember that was one big all in one bit - drilled the hole and made the chamfer all in one pass - first holes I put in the hull of any boat - brand new C22 fresh off boat show floor.

MitchK · Feb 28, 2019

There are a lot of different way to accomplish the tapered hole. What I did was make a bevel tool out of some wood and cut up grinder sanding disk. 80 grit if I remember. I undercut the center piece so the sanding disk would be able to cut all the way to the inside edge of the hole. Chucked it in a drill , stick in hole, spin up, and it made short work out of making the proper taper. Be careful, it will cut pretty fast. Here is a picture of what I made. Not pretty, but worked great. I made the wood part on a small metal lathe. Used a 1/4-20 bolt for the arbor.

dlochner · Mar 1, 2019

Ward H said:
Does a flush thru hull really make that much of a difference in speed?

If you are racing, every little bit helps. For the rest of us, it doesn't matter as much. There is also the aesthetics. A smooth flush hull is more attractive than one with multiple fitting sticking out.

shemandr · Mar 1, 2019

The only racing I could imagine in which it would matter to have flush fit thru-hulls is one design. All the boats would need to be nearly identical before flush fit would make a difference. Even then, a moment of inattention to the helm would negate any advantage. And, not to be a project killer but someone locally put one in for his head and it shot across the cabin when he hit a wave the next season. You'll need a plug wired there (Not to the thru -hull) that will close the hole in the hull that you want to cut.

sailnoproblem · Mar 1, 2019

dlochner said:
If you are racing, every little bit helps. For the rest of us, it doesn't matter as much. There is also the aesthetics. A smooth flush hull is more attractive than one with multiple fitting sticking out.

I am sure the snappers in Barnegat Bay will appreciate the aesthetics of Wards smooth hull. Sorry Dave I'm in one of those moods this morning. It's snowing here and it ruined my boat plans.

dlochner · Mar 1, 2019

sailnoproblem said:
I am sure the snappers in Barnegat Bay will appreciate the aesthetics of Wards smooth hull. Sorry Dave I'm in one of those moods this morning. It's snowing here and it ruined my boat plans.

It is important to humor and please the local fauna. Snappers and their colleagues know a boat with a smooth bottom is not likely to be dragging metal hooks with tempting treats attached. They feel safe and secure.

Yesterday's 8 inches is still waiting for me in the driveway. More on the way this weekend. I feel your pain.

Flush Fit Thru Hull Install Question

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