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In a month I plan to haul the boat and do a lot of maintenance. One item will be to replace the existing thru hulls with new. Install will be done like Maine Sail advises (G10 backing plate method).

Problem is the existing thru hulls are totally glassed into the hull. The thru hull is either a bronze pipe (threaded on inside of hull where a ball valve would screw on) or maybe they are a true thru hull but the water side and bilge side is all glassed so the only thing visible are threads inside and a hole in the boat outside.

My first plan was to just grind the thru hull out with a 1:12 slope and reglass and drill for the new 1-1/2 inch thru hull. But now thinking maybe I should just try ream out (by grinding, or cutting or both) the existing thru hull so I have a hole slightly bigger than 1-1/2 inch. Then I could just install a new thru hull. Maybe if the new hole were buggered some I may need to put some new epoxy to fill and the drill to 1-1/2 inch. But this would be a lot less work than grinding to the 1:12 slope and reglassing the hull.

The hull is solid glass and the hull is thickened at the existing thru hull so guessing total thickness is 3 inches of solid glass.

Will the method of reaming the hole work? What would you use to cut away the existing thru hull? Maybe a cylidrical grinding wheel?

Regards
 

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Do the inside of the "thru-hulls" have the short stubs or "ears" that a thru-hull tool can lock on to?

Are you sure the outside has been glassed over? Or could it be a flush headed thru-hull?

My mushroom head bronze thru-hull accidentally got epoxied in place. I took a small torch to the inside surface of the thru-hull to soften the bond and was able to extract using a thru-hull tool.
 

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I have a 1979 Sabre MKI and the original flush mount thru-hulls were glassed over on the outside of the hull. The interior/bilge side has a wood block glassed over. To access the outside I used a 4.5 inch flap disc on my grinder to take down about 3/8 ths to 1/2 inch of glass directly over the thru hull. Be careful with the flap disc it takes a lot of material quickly. I then used a Festool 150 sander with 80 and 120 grit rubin sandpaper and vacuum to fair the area to about 12:1 from the center of the thru-hull. The hull/hole was shaped to fit a flush mount thru-hull which provided a good seat for the back of the thru-hull head into the hull.

I have replaced with the Sparten bronze thru-hull which more or less matched the original thru hull(s). Then glassed and epoxied the area, barrier painted then bottom painted. Essentially I used Maine Sails methods as you plan. I did thru bolt where I had access and some I reused the original method of two 1.25 inch stainless lag screws into the glassed block. I filled the original holes of the lag screws with thickened epoxy. did the same for the new holes and set the screws in the thickened epoxy.

I would be hesitant to ream out the hole but rather slowly and carefully remove the glass around the thru hull (i.e., on the outside and inside). I would start on the outside. You also might find once you remove the outside glass the thruhull might just spin free with a step wrench, again using Maine Sails methods.

I had trouble with one thru hull spinning so I used a narrow demolition blade on a saz-all and made four cuts (i.e., north, south, east and west) from inside the 1.5 inch diameter thru hull (that is stick the blade in the thru hull hole) and stopped immediately as I cut through through the metal or nearly so , since I (or you will) wanted to preserve the integrity of the thru hull hole. A pry bar and hammer used gently, with a wood shim against the hull to protect from damage, got the beast out of the hole.
 

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In a month I plan to haul the boat and do a lot of maintenance. One item will be to replace the existing thru hulls with new. Install will be done like Maine Sail advises (G10 backing plate method).

Problem is the existing thru hulls are totally glassed into the hull. The thru hull is either a bronze pipe (threaded on inside of hull where a ball valve would screw on) or maybe they are a true thru hull but the water side and bilge side is all glassed so the only thing visible are threads inside and a hole in the boat outside.

My first plan was to just grind the thru hull out with a 1:12 slope and reglass and drill for the new 1-1/2 inch thru hull. But now thinking maybe I should just try ream out (by grinding, or cutting or both) the existing thru hull so I have a hole slightly bigger than 1-1/2 inch. Then I could just install a new thru hull. Maybe if the new hole were buggered some I may need to put some new epoxy to fill and the drill to 1-1/2 inch. But this would be a lot less work than grinding to the 1:12 slope and reglassing the hull.

The hull is solid glass and the hull is thickened at the existing thru hull so guessing total thickness is 3 inches of solid glass.

Will the method of reaming the hole work? What would you use to cut away the existing thru hull? Maybe a cylidrical grinding wheel?

Regards
When we replaced the through hulls we used HSS bi-metal hole saws with same OD as the through hull from the outside.

For the small ones we used gaffer tape on the center bit to match the ID of the through hull.
For the larger ones we made wood plugs with center holes that matched the ID of the through hull.

This way you can get a nice clean cut with no damage to the surrounding areas.
After removing the "pipe" like this you should be able to find out what you must do to remove the outside flange.
 

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Here is a link to a video I made about removing some Thru hulls maybe there will be something in there that will prove helpful. I like the hole saw idea above as well. You just have to get creative depending on your situation. If your hull is truly 3" thick there is no need for backing plates. If fact I do not use backup plates on any FRP over 1/2" as there really is no need.

 

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I removed the thru hull for the head on our classic Pearson using the hole saw method. I cut from the inside. I cut the old thru hull nearly flush with the hull using an angle grinder with a cut off blade. Fit a hole saw just slightly larger than the existing thru hull right over the stub. Worked like a charm.
 

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On a friends boat (Jeanneau Sun Fizz, I believe) we used my Dremel with a cutting disc, into the through-hull from the outside, to cut the sleeve of the through-hull so the frozen inside nut/collar could be separated from the outside flange. The inside part was then removed from the inside, and the flange tapped out from inside. Took a while, and several exploded cutting discs, but it worked with no damage to the fibreglass.
 
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