Through hull fittings - How much "play/distance" is ok?

[Cpt. Black SOL]

Hi!

Im in the process of changing my through hulls for the first time.

1. Im wondering how much play/distance around the skin fittings that is acceptable.

So im going to try and explain this in the best way i can and hopefully some will understand.

I have two skin fittings which are 1 1/4 in size, these have some distance around them. At the same time the whole in the hull is too small for a 1 1/2 fitting. What would you do in this situation?

2. How much thread on the fitting should be left for the valve? see picture. is that ok?

3. I cant find 3m 5200 where i live(Malmö Sweden), Sikaflex 291i is easy to find around here. Whats your go to sealant? and why?

[skipmac]

Hello,

If I understand correctly, you have a through hull that is 1 1/4" size but the hole in the boat hull is a little larger than the fitting so there is a little gap around threaded shaft where it passes through the boat hull.

You say a 1 1/2" fitting is too large and will not fit the hole. This would mean the gap around the 1 1/4" shaft is 1/8" or less which should be fine.

Regarding 5200 I would not use it for this application for several reasons.

- 5200 is very permanent and more adhesive than a caulk. If you ever need to remove that fitting again (or any other fitting you attach with 5200) it may be impossible without destroying the fitting.

- For a through hull you need a caulk and not an adhesive since the fitting is held in place by the flange that is screwed in on the inside of the hull.

NOTE - also avoid using silicone caulks for this or most other bedding applications because the silicone oils penetrate the fiberglass and prevent any caulk from ever sticking to that surface in the future, even more silicone.

For this application any good marine caulk that will Sika makes a lot of very good product but regret I don't recall the specific types.

[Bruce Beard]

So, the first question I have is, what size thru hull size do you want or need?
If it were my boat and I needed a 1 1/2” thru hull, I would open the hole to the correct size. I have “stacked” 2 hole saw bits together on a single mandrel in order to use the smaller saw as a guide for the larger saw. Then it is a simple matter to open up the hole. If you are unsure of your technique, practice on a scrap piece of materiel first...
If the 1 1/4” thru hull is the size you want then you have to decide whether you you need to do something about filling that hole up and starting again. Is this a fiberglass boat?
I personally get nervous about thru hull fittings and believe that they should be very carefully executed. A sinking boat isn’t something I ever want to be responsible for.
Here as always is a good source of information for you on the subject of thru hulls and seacocks...it does not answer your question about the size of the hole but it is good reading for you before you begin your project. Maybe he will see this post and comment...
Bruce

https://marinehowto.com/seacock-thru-hull-primer/

[Cpt. Black SOL]

Originally Posted by skipmac

Hello,

If I understand correctly, you have a through hull that is 1 1/4" size but the hole in the boat hull is a little larger than the fitting so there is a little gap around threaded shaft where it passes through the boat hull.

You say a 1 1/2" fitting is too large and will not fit the hole. This would mean the gap around the 1 1/4" shaft is 1/8" or less which should be fine.

Regarding 5200 I would not use it for this application for several reasons.

- 5200 is very permanent and more adhesive than a caulk. If you ever need to remove that fitting again (or any other fitting you attach with 5200) it may be impossible without destroying the fitting.

- For a through hull you need a caulk and not an adhesive since the fitting is held in place by the flange that is screwed in on the inside of the hull.

NOTE - also avoid using silicone caulks for this or most other bedding applications because the silicone oils penetrate the fiberglass and prevent any caulk from ever sticking to that surface in the future, even more silicone.

For this application any good marine caulk that will Sika makes a lot of very good product but regret I don't recall the specific types.

Hi Skipmac!

Thank you for your input. Yes you understood me correct. The hole in the hull is a a bit bigger than the 1 1/4 threaded shaft. It doesnt slide in tightly. Id say that the 1 1/2 is proabably 1-2 mm to big atleast.

As for the permanent fixture of the through hull. It makes a good point. Ive been reading about Sikaflex 291i too , whats your experience with that?

[Cpt. Black SOL]

Originally Posted by Bruce Beard

So, the first question I have is, what size thru hull size do you want or need?
If it were my boat and I needed a 1 1/2” thru hull, I would open the hole to the correct size. I have “stacked” 2 hole saw bits together on a single mandrel in order to use the smaller saw as a guide for the larger saw. Then it is a simple matter to open up the hole. If you are unsure of your technique, practice on a scrap piece of materiel first...
If the 1 1/4” thru hull is the size you want then you have to decide whether you you need to do something about filling that hole up and starting again. Is this a fiberglass boat?
I personally get nervous about thru hull fittings and believe that they should be very carefully executed. A sinking boat isn’t something I ever want to be responsible for.
Here as always is a good source of information for you on the subject of thru hulls and seacocks...it does not answer your question about the size of the hole but it is good reading for you before you begin your project. Maybe he will see this post and comment...
Bruce

https://marinehowto.com/seacock-thru-hull-primer/

Thanks for the response Bruce!

The boat in question is a Hallberg Rassy 35 Rasmus. Fibre glass. The skin fittings are for a vareity of utilities, since im changing all of them. I agree with you on the responsibility for the sinking boat. Im double checking and tripple checking with different sources around to be sure, before i execute. So far i think the 1 1/4 will do. the biggest ones are toilet-out and kitchen sink-out.

Should i completly stuff the "empty space/ play" with sikaflex then?

I read somwhere that it can be a good idea to paint the hull /hole with exoxy paint to protect it from moisture etc before you mount the sealant/ skin fitting (obviously the pain will have to dry in good time before) Any ideas on this?

[skipmac]

Originally Posted by Cpt. Black SOL

Hi Skipmac!

Thank you for your input. Yes you understood me correct. The hole in the hull is a a bit bigger than the 1 1/4 threaded shaft. It doesnt slide in tightly. Id say that the 1 1/2 is proabably 1-2 mm to big atleast.

As for the permanent fixture of the through hull. It makes a good point. Ive been reading about Sikaflex 291i too , whats your experience with that?

Again I am not expert on all the different types of Sikaflex products but I believe the 291 would be a good choice. It is rated for use below the waterline and I think not so strong that it would be impossible to remove in the future.

By the way, there is a benefit to the small gap around the 1 1/4" through hull, that is you fill that gap with the caulk when installing to make a better seal. Be careful to keep the caulk off the threads on the inside where you will attach the inner flange.

[Bruce Beard]

Originally Posted by Cpt. Black SOL

Thanks for the response Bruce!

The boat in question is a Hallberg Rassy 35 Rasmus. Fibre glass. The skin fittings are for a vareity of utilities, since im changing all of them. I agree with you on the responsibility for the sinking boat. Im double checking and tripple checking with different sources around to be sure, before i execute. So far i think the 1 1/4 will do. the biggest ones are toilet-out and kitchen sink-out.

Should i completly stuff the "empty space/ play" with sikaflex then?

I read somwhere that it can be a good idea to paint the hull /hole with exoxy paint to protect it from moisture etc before you mount the sealant/ skin fitting (obviously the pain will have to dry in good time before) Any ideas on this?

First of all, what type of toilet? Most of the manual flush toilets I have encountered require a 1 1/2” hose not 1 1/4”. A sink drain is another matter...
I still think that I would tend to move up to a 1 1/2” hole and a tight fitting thru hull...
As for painting the area, I wouldn’t. If the hole exposes a coring material, it is always a good idea to remove some of the exposed core and seal with an appropriate epoxy-with-filler mixture to seal the core before proceeding. That is very different than paint.

Sicaflex 291 has about 1/3 the tensile strength of 5200 and is closer to 3m 4200. It is fine for use as a sealant for your thru hull. It would also prove to be dificult to remove, which is a good quality when it comes to seacocks and thru hulls as far as I am concerned.
Bruce

[Cpt. Black SOL]

Once again thanks for the responses. Its a rough world out here for newbie boat-owners :P

So ive concluded that i will use sikaflex 291i instead of 5200. it will be easier to make mods if needed and if it seals as well. Then why not.

On the toileat/head outlet hole. at this point i have it down to a 1 1/4 fitting. But as said with conciderable play. Definitly to small for a 1 1/2. Although, as some background info. This boat hasnt been in use in around 8-9 years. Its toilet could be of an older generation. How problematic would it be to stick with the 1 1/4. is it really that rare?

[Bruce Beard]

Originally Posted by Cpt. Black SOL

Once again thanks for the responses. Its a rough world out here for newbie boat-owners :P

So ive concluded that i will use sikaflex 291i instead of 5200. it will be easier to make mods if needed and if it seals as well. Then why not.

On the toileat/head outlet hole. at this point i have it down to a 1 1/4 fitting. But as said with conciderable play. Definitly to small for a 1 1/2. Although, as some background info. This boat hasnt been in use in around 8-9 years. Its toilet could be of an older generation. How problematic would it be to stick with the 1 1/4. is it really that rare?

Look at the toilet manufacturers recommended hose and stick to it. In general terms, restriction is bad.

[Terra Nova]

Originally Posted by Cpt. Black SOL

...a 1 1/4 fitting...with conciderable play. Definitly to small for a 1 1/2...

Originally Posted by Cpt. Black SOL

...Should i completly stuff the "empty space/ play" with sikaflex then?...

Originally Posted by Cpt. Black SOL

...The hole in the hull is a a bit bigger than the 1 1/4 threaded shaft. It doesnt slide in tightly. Id say that the 1 1/2 is proabably 1-2 mm to big atleast...

Originally Posted by Cpt. Black SOL

...I have two skin fittings which are 1 1/4 in size, these have some distance around them. At the same time the whole in the hull is too small for a 1 1/2 fitting...

Why so vague about the hole size? It is a very simple matter to measure it precisely. Then you could get reliable advice.

There should be very little clearance between the hole and a properly fitted thru-hull fitting. Not more than perhaps 1.5 mm on the diameter.

3M 5200 is an excellent caulk for thru-hull installation on fiberglass hulls. Yes, it is more difficult to remove.

[foufou]

5200 and other polyurethane caulks ca be easily removed by using a heat gun. By applying the heat to the metal thru hull fitting damage to the fiberglass hull can be avoided. Still be careful not to over heat

[guyrj33]

I don't know if this idea will work with your boat.
You could add a backing plate for the thru hull with a smaller hole sized to fit the thru hull tightly. Then the extra space in the original hole can be filled with caulk.

[Fuss]

If using Sika, I would use 292 not 291. However at the moment I am using 3m 4200 and this seems fine.
Sika recommends 291 I think but this is not as strong as 292 so I don't really understand this.

[skipmac]

A few points

The OP said a 1/14" fitting has a little play but a 1 1/2" fitting is too large for the hole. That means the gap around the edge of the 1 1/4" fitting is less than 1/8" or approximately 2mm. That is not a large gap and I wouldn't waste a second on trying enlarge the hole for the 1 1/2' fitting out of any safety concerns.

Second, the strength of the installation is not or at least should not be from the caulk. The strength of the through hull should come from a correct installation using a flanged fitting on the inside, insuring the outer mushroom fitting is fully screwed into the flange, and making sure the area is well reinforced, adding a backing plate on the inside if not.

The caulk is there strictly for sealing the hole and preventing leaks.

By the way, in theory one can remove 5200 with heat or various solvents. Perhaps if it is a recent install but I just replaced two 30 year old seacocks on my boat and instead of putting a new seacock on the existing, perfectly good through hull mushroom I had to cut the whole assembly apart with a grinder because it was bedded with 5200. And yes I tried heat, a special solvent guaranteed to remove 5200, a through hull wrench and as much force as I could apply and still had to cut them out.

[Fuss]

i agree that 2mm of play should be ok... i am going to replace my through hulls with tru-design ones that have the supporting flange