Installing a Flush Through Hull Fitting

[John Stone]

I am in the process of installing an inboard diesel in the Far Reach. You can find that thread here:

http://www.capedory.org/board/viewtopic.php?f=9&t=36770

During my six year rebuild, which I completed five years ago, I eliminated 7 of 10 through-hulls and sailed for the next five years with only three of them (two scuppers and the galley sink drain—all 1.5”). They are your standard bronze mushroom style through-hulls. I always wanted the them to be flush mounted but just couldn’t determine a good way to do it. But, I was determined to figure it out for the engine intake 3/4” through-hull. I have read about several techniques but didn’t really like any of them. I practiced in my shop on plywood with various jig over the past couple months but was never satisfied with the results. Then, yesterday I read an article in an old Practical Sailor magazine from 2011. It made sense to me.

https://www.practical-sailor.com/boat-m ... oth-bottom

I did not need to increase the thickness of the hull since the through-hull is 5/16” thick and the hull where in installed it is 5/8” thick. So, I set my small trim router about 1/32” deeper than the head of through-hull is thick to allow for bedding compound. I traced the outside circumference of the through hull on the hull and just routered it out. Took maybe two minutes. I test fit it. I sanded the mortised portion with forty grit abrasive. Then I wrapped the threaded part with rigger tape (duct tape) and waxed up up thoroughly. I wetted out the mortised hole with neat epoxy. I mixed up epoxy thickened with 404 high density filler and just a little 406. I slathered the thickened epoxy on the through-hull head and pushed it in. I installed the nut on the threaded barrel and snugged it down. I cleaned up the squeeze out. When it cures, but is still green, I will use a mallet to pop it out being careful to back the nut out to the the very end first so I am not hammering on the threads.

Later I will epoxy in the 1/2” thick G10 backing plate (which by the way effectively adds 1/2” to the hull thickness). Then, I’ll be ready to drill the three holes for the seacock mounting bolts and apply bedding, etc and install the seacock.

If this works as well as I think it will I’ll remove the three 1.5”’mushroom head through-hulls and replace with flush ones using the same technique.

Cored hulls and hulls that are thin require some additional steps to use this technique. The PS article covers those steps too. Fortunately for us our Cape Dorys have thick solid glass hulls.

[John Stone]

The additional pictures.

[Jim Walsh]

Very cool. Flush throughhulls say “I care about performance”.

[Keith]

I must admit the thought of flush or faired through hulls never crossed my mind. But since I just launched MOONDANCE I guess I'll ponder on it for next winters projects. Maybe try fairing the two head through hulls since the article mentioned that further forward had the most impact.

Keith

PS I still have to post my 2020 winter project results.

[John Stone]

A lot of people don’t think about it but our boats have lovely underwater profiles and with a little work can perform much better than they doing left as is. If you take the time to make some minor mods, the difference in performance when sailing is eye opening. You simply can’t believe the difference not having a prop aperture makes.

I know most people will not want to go to that extreme but little things add up and can make a big difference too. There is a guy name Dave King who made a name for himself a while back racing a West Sail 32 in California in the Pacific Cup and the singlehanded TransPac. In fact his boat Saraband won first in class in a number of those high visibility and very competitive races. The SH TransPac win shocked the west coast sailing community. The thing is he took the time to streamline and modify his boat in small but important ways. The difference was startling. And he keep his propeller aperture! My sister had a WS 32 and I sailed on it a couple times with her. If well handled they are quite capable. But when modified in certain ways they are remarkable given their design and often undeserved reputation. Our boats are much faster than a WS 32. And, it’s not just speed. They handle so much better when the underway profile is as smooth as possible. Small things can add up: like eliminating unnecessary through hulls, or making them flush fitting, or fairing in around the leading edge of the rudder, or squaring off the trailing edge, fairing in around the prop aperture, or rethinking how you apply bottom paint, etc...all these things can make a difference in how your boat sails.

Anyway, for those interested I have linked below to an article written by Dave King on what mods he made to his and other WS 32s to improve their performance. Maybe it will give you some ideas, or inspire you to improve your boat’s underwater profile, or perhaps will just serve as interesting reading. It’s all good.

https://docs.google.com/file/d/0BxGyXPo ... y=CMqlo-sJ

A website photo album of Dave King making his hull modifications to a group of WS 32’s:
https://get.google.com/albumarchive/118 ... zNulNLN9CH

[Frenchy]

Nice job, John. I think you probably plunge-cut most of the glass and then carefully guided the tool around
the circumference, taking small cuts. It's not easy guiding a router by hand; the few times I've tried it usually
ended badly.
After that, I'm sure you won't add a pick up screen over the intake-that would defeat the whole
purpose- but rather rely on the internal strainer to take care of weeds and such. I've wondered how
really useful the external screen is. It's great at accumulating barnacles, if nothing else.
I'll have to think about installing flush thru-hulls. - Jean

[John Stone]

Thanks Jean. No screen. As you say, defeats the whole purpose.

Don’t rush on those flush through-hulls. Working on a technique for the other three that uses the existing mushroom through-hulls. If it works, could save a lot of time and money and be dead flush.

[John Stone]

So...there is simpler and less expensive way to achieve flush mounted through-hulls.

In an effort to convert the three remaining mushroom head through-hulls to flush through-hulls I decided on a different approach than that described at the beginning of this thread. I reasoned that my installing purpose made flush mount through-hulls, as described above, would prove problematic since I didn’t want to remove the seacocks. Why is that important you may ask? Well, I wouldn’t be pressing the through-hulls into the thickened epoxy to create the chamfered mold as I did with the 3/4” through-hull. I would have to turn these against their threads since the seacocks are already installed. My concern was the squeeze out of thickened epoxy to make the beveled mold would be uncontrolled when turning the through-hull against the epoxy. Would the thickened epoxy be forced up into the seacock to get ensnarled in the threads, or worse? Plus, 1.5” bronze Groco flush through-hulls run about $85 each. No good I decided. To many unknowns.

But, as I pondered options I recalled reading about using West Systems epoxy on metal. So I read did more research last night. If it sticks to bronze then there is another way.

Today, I removed the three through hulls easily with a through-hull wrench which I recently purchased for $38. A marvelous tool. Will I ever need it again? Who can say. I added a little heat with a propane torch and along with a 12” crescent wrench they came right out.

I cleaned up the old dried bedding compound in the holes and on the through-hulls. I made a 2” diameter arbor I cut from a 2 1/8” hole saw. I placed the arbor on the pilot bit for a 3” hole saw. The arbor allowed me to use the existing hole as a guide for the 3” hole saw. No chatter. A clean cut. I marked the 3” hole saw with tape for a 5/16” deep cut. Nothing to it. With that complete I used the laminate router with a straight shank bit, much as I did when installing the 3/4” flush through-hull at the top of this thread. But, this time I had only to cut to the circumscribed 3” diameter cut. It was simple and left me with a perfectly round, clean, flat, recess 5/16” deep and 3” in diameter. I cut 5/16” off each through-hull using a vise and a hack saw. I cleaned up the cuts with a mill file. I then screwed in the mushroom head through-hull to test fit it. The top was flush with the hull of the boat. In the next few days I’ll chamfer the edge of the 3” hole and burnish the through hull. After bedding it, I’ll fill the exterior bevel with thickened epoxy and Bob’s your uncle. If I ever have to remove the through hull it will be a simple matter to grind back to the edge. Same process as we do to remove the rudder gudgeon.

The through hull will need to be thoroughly cleaned and heavily burnished before applying epoxy. Additional reading informs me that West Systems GFlex may be well suited to this project. I’ll call the tech line tomorrow to confirm.

Bottom line—I believe this is a viable technique to flush mounting existing through-hulls for very little investment. A word of warning. Your hull needs to be thick enough to accommodate the recess. I also have 1/2” G10 seacock backing plates epoxied to the inside of the hull which essential acts as additional hull thickness.

MTF.

[John Stone]

A few more pictures.

[John Stone]

The last photo.

[Steve Laume]

When I purchased Raven, she had a gigantic wooden block mounted on the side of her hull for the depth sounder transducer. It bugged me for a few years much like those little mushroom through hull fittings must be bothering some folks. The difference is that this thing was BIG and must have created some significant drag.

Those little through hulls just can't make that much difference in a heavy cruising boat. I faired Raven's bottom, barrier coated it and keep the paint application nice and smooth. Keeping the slime layer and any other marine growth off of the bottom is also a priority. Good crisp sails and even smaller diameter running rigging will help a good bit. Learning proper sail trim and adding things like better traveler controls and a boom vang will also improve performance. If money is not an issue, then a folding prop would be an advantage.

My point is, there are lots of things you can do to improve the sailing characteristics of your boat without cutting through a significant amount of your hull in a crucial area and then burring perfectly good through hulls in epoxy. If anyone ever has to remove them, it will be a nightmare. I doubt there is any messurable difference in preformance. It seems like a great deal of trouble for little gain and creates a weaker area around the trough hull that if the full hull thickness was left intact.

I don't see the point of doing all of this. Will it be worth the lost sailing time? Don't get me wrong, I want a fast boat but there are limits. If speed was that important to me, I would buy a faster boat or better sails, Steve.

[John Stone]

When I purchased Raven, she had a gigantic wooden block mounted on the side of her hull for the depth sounder transducer. It bugged me for a few years much like those little mushroom through hull fittings must be bothering some folks. The difference is that this thing was BIG and must have created some significant drag.

Those little through hulls just can't make that much difference in a heavy cruising boat. I faired Raven's bottom, barrier coated it and keep the paint application nice and smooth. Keeping the slime layer and any other marine growth off of the bottom is also a priority. Good crisp sails and even smaller diameter running rigging will help a good bit. Learning proper sail trim and adding things like better traveler controls and a boom vang will also improve performance. If money is not an issue, then a folding prop would be an advantage.

My point is, there are lots of things you can do to improve the sailing characteristics of your boat without cutting through a significant amount of your hull in a crucial area and then burring perfectly good through hulls in epoxy. If anyone ever has to remove them, it will be a nightmare. I doubt there is any messurable difference in preformance. It seems like a great deal of trouble for little gain and creates a weaker area around the trough hull that if the full hull thickness was left intact.

I don't see the point of doing all of this. Will it be worth the lost sailing time? Don't get me wrong, I want a fast boat but there are limits. If speed was that important to me, I would buy a faster boat or better sails, Steve.

Well, ya know, that’s just like your opinion man—The Dude

The short reply: Choose what works for you.

The long reply: I disagree. Our boats are fast if they are set up right and well handled. That’s the whole point. But, there is nothing that says anyone has to do any of these things to change or improve their boat. If you are happy with your boat as is and it brings you pleasure then by all means stay the course. If you prioritize the modifications as only some being worth it to you, then “right on.” But, if you want to improve your boat in lots of ways and that brings you pleasure then that’s cool too. One man’s waste of time is another’s great joy.

After all the things I changed/modified/improved on the Far Reach, why would I stop at such a simple modifications as making the under body more stream lined? That does not make any sense to me...but, that’s just my opinion....

I don’t see the “nightmare” here. Simple to grind back that small amount of epoxy if the through-hull ever has to be replaced. Or better yet, hit the through hull with a little heat from a propane torch and you can probably scrape it off with a metal putty knife. I just don’t see the issue there. I submit it is far easier to do that than the contortions required to say, replace the rubber hose on the stuffing box on a Cape Dory. And the hulls on our boats are very thick—certainly the case on my boat with minimum 3/8” of fiberglass remaining after I cut the recess. As mentioned, every one of my four seacocks are further backed with 1/2” thick G10 epoxied to the hull and through bolted between seacock and the external hull with three 5/16” bronze bolts. No structural issues there at all. Zero.

It’s true that probably no one mod by itself offers a “dramatic” improvement in performance...except perhaps new sails or eliminating the prop aperture. But, all of them together can make a big difference, especially in light air as attested to by Dave King and many more.

Underwater drag plays significant role on boat speed...and handling. That’s one reason why modern race boats have such small under water appendages and their crews go to great extremes to achieve a fair smooth underbody in all areas that make contact with the water. I’m operating at the maximum level of my math/engineering skills but water is something like 700 times more dense than air. Swipe the palm of your hand through the air then try that in the water. Big difference. So, sure, eliminating the friction associated with numerous mushroom through-hulls helps. But if that mods not worth the effort to you then by all means don’t do it.

However, if you enjoy sailing in light air and loathe cranking on the engine like I do then maybe these kinds of mods might bring you pleasure too. Sure, they don’t change your max “hull speed” but they can help you achieve it in a lot less wind.

So carry-on...or not...as you wish....

[John Stone]

My wife is good with a dremel and grinding requiring detail work so she beveled the edges of the three recessed holes for flush mounting the standard through-hulls. Then we bedded the 3/4” flush through-hull and seacock for the engine intake and called it a day.

I’ll give the bedding compound a couple days to cure then sand and apply GFlex and fairing compound to fair in the through-hulls.

[John Stone]

I called West Systems tech line the other day and they gave me some tips about fairing over the bronze. Make sure the metal is wiped down with acetone. Abrade it with 40-80 grit till shiny. Clean and wipe down with acetone. Apply GFlex 650 (without thickeners). Immediately abrade it with a SS wire brush. Let it cure till very tacky. Apply West 105/205 or 206 if it’s hot. Add normal fairing compound.

That’s what I did. Once the Gflex tacked up I mixed up some 105/205 with colloidal silica (406) then added 407 medium density filler and a little 404 high density applied the thickened epoxy with a plastic stir stick then faired across with a 6” plastic trowel. Took less than 10 min to fair all four through-hulls.

I’ll let it cure for a couple days. Then scrub off the amine blush with water and scrub pad and wipe dry with plain white paper towel. Sand it with 80 grit and apply a light coat of fairing compound to fill in any pin holes or low spots. Very happy to have a completely fair hull.

[Jim Walsh]

Great post John. Your step by step documentation would make it relatively easy for anyone to arrive at similar results.