Thru Hulls: Seacock bolted through the hull or ball valve??

[Chris "Tako" Green]

1974 Ericson 35-II

Preparing for my first haul out for the bottom paint and I thought this was a good opportunity to get rid of these thru-hulls with wood backing blocks. I talked to my yard and its $1000 per hole to do the work. I planned on replacing like 5 thru-hulls but $5000 for this job is not what I wanted.

The real question. If the job was just to remove the original thru-hull, add a "plastic" backing plate and tighten, obviously using maybe 5200 I would feel fairly confident in doing this, and then just spinning on a new bronze in-line ball valve by Groco.

I have read that the only way to do this job anymore is to use a seacock which requires drilling through the backing plate, the hull and bolts to the outside of the vessel. I get this is a much more secure install but is the first method wrong or dangerous? Its whats in my boat now and seems to have held for 40 years.

Thanks in advance for any answers!

 

[Loren Beach]

That yard must want about $200/hour per thruhull. Seems quite pricy to me.

For a backing plate I am a fan of something that is strong and rot proof, like G10 composite. Note that terminology matters. A "seacock" fitting is a valve body with molded-on flange and that flange or base is attached to the hull. i.e. one piece.
This is much much stronger than having a ball valve screwed onto a thruhull fitting, whether it's made of bronze or Marelon.

I know of three ways to attach a seacock.
1) traditional way of boring thru the backing plate and the hull. Chamfer the hole on the outside so that a bronze flathead bolt is flush with the hull surface. Set in a lot of 5200, this should never leak and you can fair over the bolt heads before applying bottom paint to that area. Ideally the actual bronze thruhull fitting is also flush mounted to the outside hull.
2) Same except that the inside plate of G10 (about 1/2" thick) is glassed to the hull after having the valve mounting holes drilled and tapped into it - but not extending thru the hull itself.
3) Use a Forespar Series 93 seacock with the flange base. Screw in the outside Marelon thruhull fitting, set in epoxy mush. Also have epoxy mush under the G10 plate inside and under the seacock. After the epoxy cures grind off the outside lip, flush with the bottom. FWIW, this is how all of our Forespar Marelon (series 93) seacocks were installed in 1995. Still work and look like new. Since the Marelon material is a fiber-reinforced resin product, I figure that it is a strong as our FRP hull, and the fittings and the hull are become "one."

(3a) There is a competitor for Forespar nowadays and they seem to also have a decent rep, although I forget their name.

The reason for using a true seacock is that ABYC requires these fittings to withstand a side shock load of something like 200#, like when a lurching tool box whacks into it when the boat does a hard roll to one side or the other. I believe that Rod Collins has done some testing of thruhull fittings and written it up for his site. https://marinehowto.com/replacing-thru-hulls-and-seacocks/

Regards,
Loren

 

[Frank Langer]

I think the other similar company to Forespar is called Trudesign, and they make a collar that goes around the seacock and will apparently withstand a 500 lbs. force to comply with the ABYC standard. They have Teflon inside, so don't need greasing, and mine have worked as advertised for the past three years. They are alot cheaper than bronze, and don't corrode.
Frank

 

[Christian Williams]

General alert to thread compatibility:

Sites-WestMarine-Site

West Marine

www.westmarine.com

 

[nukey99]

General alert to thread compatibility:

Sites-WestMarine-Site

West Marine

www.westmarine.com

I found that out 35 years ago, when I tried to screw a ball valve on to a thru hull and it would not thread all the way down. A diver had put a plug in the hull on the outside while I did the work. This lead to a frantic trip back to the local boat yard to get the correct (and more expensive fitting).

 

[bigd14]

Here is another option that eliminates the mismatched thread issue:

Bronze NPT to NPS Flange Adaptor | GROCO

GROCO Bronze flange adaptor eliminates dangerous thread mis-match when NPT in-line valves are screwed onto NPS thru hulls. Meets ABYC H-27 standard

www.groco.net

I replaced all thru hulls and backing plates with new bronze fittings, 1/2” G10 backing plates, these flange adapters and bronze ball valves. Now if a ball valve fails I can easily replace it without disturbing the thru hulls. As per Loren’s #2 point above, you don’t have to drill through the entire hull, you can simply tap threads into the G10 plate and use bronze studs (cut off the heads of bronze hex nuts). You have to cut the thru hulls to proper length too but that’s easy with a hacksaw. It’s a bit of work but eliminates a major worry factor.

Here’s a photo of the sink drain thru hull assembly (before installing all the hose clamps).

 

[JSM]

Groco makes a backing plate for use with their flange adaptors. Looks like it would make it a fairly straight foreword job to install a new thru hull and valve assembly.

BB-1 Seacock Backing Block | GROCO

GROCO BB-1 Backing Block allows the installer to gain the benefits of a backing block with proper material without the hassle of making their own.

www.groco.net

 

[G Kiba]

I had thru-hulls and valves replaced in 2005 on my 73' Ericson 27. Labor costs and parts have become outrageous in the last 2 years.
This is true for so many recreational activities. Have you seen the price of bicycles?!
I have found that... If you can't do it yourself; or if you are not willing to learn; or if you don't want to take a risk, or if you can't or don't have the time to spend doing it yourself; or don't have the right tools; then, the best choice might be to have it done. And even that is no guarantee that all will go well these days. At least by having it done, you are supporting the people in the marine industry. I expect that as prices get out of our reach, there will be fewer of us spending and the prices will continue to climb.

 

[Chris "Tako" Green]

Ok! These are all great responses. It seems the best route is to use the real deal seacocks. I always can get the tools for the job. This one specifically sounded tedious and I have other tasks I wanted to attend to while the yard worked the thru hulls. I think it is going to be cheaper to keep it dry an extra week and just do it all myself and pay some folks in beer to lend a hand. Everything I have done to this point has been a learning experience and probably the main reason I love having a boat. Wish me luck! If I can remember I will update this thread with the results but that’s in a couple months.