From The Boatshop
by Ron Magen

"How to Build a Better Butt"

Lately, there has been some lively discussion concerning the building of ‘larger’ boats. That is, those that require planking, bottoms, etc., of a length greater than the ‘standard’ 8 feet of sheet goods. This may be due to the onset of cooler weather and everybody getting prepared for the winter ‘building season’ as opposed to the summer ‘sailing season’.

As an adjunct to this, there has been conversations extolling the virtues & economy of ‘butting’ large pieces together, rather than the ‘modern tradition’ of scarfing.

My own preference is for the scarf joint. I know the theory of making a butt joint - from the simplest ‘plank backed’ to the more elegant & recent epoxy/glass Payson joint - and don’t disagree with it’s functionality.


Payson Joint (click to enlarge)

I guess my reluctance comes from where I first saw an example. I was visiting an individual {a frequent & valuable contributor to the on-line Groups, and may very well be a ‘duckworks’ subscriber}, who was showing me his latest ‘pride & joy’ before delivery to a customer. An original design, and really nice work. The dark finish of the hull was a smooth high gloss. As I walked around, from the Port side toward the Bow, the hull glistening in the afternoon sun, the outline of the interior butt block simply jumped out. It was if it was a raised area on the hull. I continued to walk around the boat. Then I took out and unfolded my handkerchief, put it over my palm, and gently rubbed across the area of the block. {an old trick to see if bodywork has been done on a car}. I don’t remember if I could feel anything . . . but you could DEFINITELY see it in a glancing light. This would have been invisible & inconsequential on a thick ply bottom, where the beam-to-beam butt strap would have functionally acted as a foot rest but on a side panel it made the entire job seem amateurish.

Be that as it may, and I do agree that there are places that are excellent applications, the following ‘conversation’ recently occurred . . .



From: "timk_smith"
To: <quohog@att.net>
Subject: butt joints
Date: Friday, September 10, 2004 16:37

Dear Ron,

You very kindly sent me a thorough explanation some time ago about how you make butt joints in plywood panels, glassing both the top and the bottom at once. I wonder if I could trouble you for some follow-up advice.

I made a test panel the other night using your method with pieces of half-inch ply and three layers of tape, the ends of the panels pared down a little with a router to take the tape. The joint is immensely strong, but there are some air pockets in it, small, about one inch by half an inch, and there are spots where the tape moved a bit and pulled away from the sides of its channel. I suppose I didn't get a perfect squish and should cut a hair less deeply with the router. But I also suppose that joints made this way always have a few imperfections. My question is, is it structurally sound to sand out any air pockets and those voids and any others using either bits of tape or thickened epoxy? More generally, do joints of this kind usually need a bit of such fiddling? (The panels will get layers of cloth on both sides once they are joined up.)

Best regards,

Tim Smith


Note: This is Tim's joint -
I did not see these pictures until later

From: "Ron Magen" <quohog@att.net>
To: "timk_smith"
Subject: Re: butt joints
Date: Sunday, September 12, 2004 14:27

Tim,

Glad I was able to help you. The actual credit for the concept of the joint comes from the ideas of a lot of people. It was illustrated and 'popularized' by Dynamite Payson. I got it in an addition to the
original plans for Bolger's 'Rubens Nymph'.

The caveat to this is that, to my knowledge, the method is for the joining of the thinner, more flexible, 1/4 inch ply. Although I'm sure it has been done, it is not what I would use for the much stiffer ½
inch material. ESPECIALLY if you have 'rebated' the ends of the material at the joint. While it may seem effective in a flat, or straight orientation, the square edges of the 'channel' are stress points of
incipient failure.

The joint is supposed to be made with a shallow 'swale' produced by a disk sander. {As in the simple type that are clamped in an electric drill} This is so there is a smooth transition from the surface plane of one sheet, dipping down at the actual juncture, then back up to the surface plane of the second sheet. There should be NO 'square' or sharp edges.

For the thicker ½ inch material I would use a scarf joint. More so, if you are willing to go to the time & trouble to rout a flat on each sheet. A decent scarf joint {8:1 or 12:1}is much easier to produce. Simply 'stair-step' the two sheets, then hand plane or power-sand them to a uniform angle. Gluing them is simply a matter of clamping one sheet down, painting on some unthickened epoxy, mixing some filler into the remaining, 'buttering' that on, lining up the second sheet with the first, and clamping that one.

As far as your specific problem . . . the 'channel' gave room for the glass & epoxy to move. When in it's fluid state, the epoxy acts like a lubricant. When the 'epoxy butt' joint is used with thicker materials, the first layer is a piece of EPOXY matting or woven roving. This is worked into place with a 'bubble roller' to drive out air & fully saturate the stuff. Then a piece of cloth is rolled into place, followed by a wider piece that overlaps the whole. This way the bubbles and wrinkles are worked out, and 'escape' via the sloping sides.

The joints shouldn't need any 'fiddling', they are so simple {by using the router, you did more work than is necessary}. A 'bubble' roller is handy, but a simple plastic squeegee will do the job. Depending on the size of the bubbles, the quantity, their location, and your sensibilities, you can dig them out/open. Fill them with a mixture of epoxy, fumed silica, and milled fibers. 'Mayonnaise' of a thick cream consistency will give 'self leveling'.

I'm trying to diagnose the problem without actually seeing the 'patient'. Any number of the boat suppliers have catalogs with much, if not all, of this stuff listed & illustrated. If you need more detail,
I'll try to dig out the page with Payson's sketch and scan it.

Regards & Good Luck,
Ron Magen
Backyard Boatshop
- - - -

From: <timk_smith@

Ron,

Thanks for this. All very sensible. The panels are for a Bolger Topaz, and in the building guide Phil and Suzanne actually specify a Payson-type butt joint for half-inch ply. Did one last night with a slightly shallower rout, will see how it turned out when I go home.

Thanks again,

Tim.
- - - - -

From: "Ron Magen" <quohog@att.net>

Tim,
WHERE is that butt joint going? If it is for a FLAT 'surface' like a bottom, then I would have no concern. However, if it will be in a CURVED panel it will give a 'flat spot' in the fairness of the shape. Also a stress point. The greater the curve . . . the more of a 'hitch' and greater the stress. It will become more obvious if a fine, high gloss finish is applied.

Bolger's 'attitude' has always been toward MAXIMUM efficiency. "Use everything but the squeal", so to speak. Therefore he gives layouts for 4x8 sheet goods that use as much of the material as possible. This often means butt splicing the forward 4 feet of a bottom to the aft 8 feet - where the aft part is almost as wide as the sheet, and the fore part is the corner of another where many other smaller parts are laid out.

This is opposed to the 'loss' of a couple of inches of sheet goods in the scarfing process. About 95 percent of the people on this forum will agree that to skimp on materials is often foolhardy. All of them combined are usually only a fraction of the cost & value of the time & energy put into the total. If I was working with $200 a sheet ply . . . I might think about it. Then on the other hand, if I was putting that kind of material into a project, I would want to do the ABSOLUTE BEST work I was capable of. Only ONE joint? If you are using ½ inch ACX . . . we're speaking about what - $25 for the less then half a sheet 'remainder'? There are a LOT of 'odds & ends' that can be made from that. A bit of stain, epoxy, & varnish and 6inch square pieces become nice looking backing blocks for deck hardware, a larger piece becomes a motor mount board, etc.

Don't worry about the router, get a $5.00 'disk sanding attachment' for your drill and make another test piece. {Of course if you have an 'angle grinder' or a Random Orbit Sander . . . use that with some 60 grit}.

Regards & Good Luck,
Ron Magen
Backyard Boatshop
- - - - -

From: <timk_smith@

Ron,

The good news is that I unpacked the dried joint last night-made with a slightly shallower rout-and it came out very nicely. No air bubbles to speak of, three layers of tape nestled neatly in glue on both sides.

These are the boat's side panels, and they will be curved-four sheets joined end-to-end. I am a little concerned about flat spots, though when I picked up one end of the two-sheet assembly last night it seemed to curve fairly. Is the width of the tape in a joint like this a factor? For this design Bolger says to make taped butt joints using a power plane of three inches; that seemed so narrow to me that I went with six inches instead.

I made a couple of the butt joints for the roof panel using an angle grinder. Because the grinder gives the irregularities of freehand work, I switched over to the router and got more predictable results. That's why I wanted to use it applying your description of the Payson joint-since the underside is assembled "blind," I thought it would be a good idea to build predictability into the process. (The router is no more trouble to use; I have a good edge guide and it takes only a few passes).

But if you think flat spots will be a problem, maybe I should switch back to the grinder and use narrower tape.

--Tim.
- - - -

From: "Ron Magen" <quohog@att.net>

Tim,
I'm really at a loss here. There are no pictures, so I can only go by your verbal descriptions as to what you are doing.

It is only a guess, but the probable reason for no bubbles in the shallower 'channel' was that you were able to apply more pressure on the epoxy saturated material. This squeezed out the air.

Regarding the 'flat spots'. They become more noticeable with the severity of the curve, and the smoothness & gloss of the final finished applied. The width of the tape is a factor - the narrower the tape the more abrupt the transition between the sheets. In addition, from a structural standpoint the greater the stress on the actual joint. The wider the tape and application the more contact area for the 'glue' and the more the stresses are spread out. This is one of the reasons a scarf joint is done on an 8 to 1, or 12 to 1, slope - rather then an abrupt 45 degree cut.

I'd really like to see exactly what Phil is saying, and in what context, when he refers to a, ". . . using a 3 inch power plane . . .". The reason is that a lot of people recommend the use of a power plane to CUT SCARF JOINTS. {Plus, 12 x 0.25 = 3 . . . the length of a long scarf in 1/4 inch material.} I don't own a power plane. If I had to make a large number of joints, my tool of choice is an in-line belt sander. This is much more of a multi-tasking device, as opposed to the purpose built plane. That being said, I do my scarfing with a Surform tool and a sharp Jack plane.

I mentioned the angle grinder because it is another handy tool. Like the belt sander & power plane, it takes a bit of practice & a LIGHT touch to do finesse work. Also the CORRECT 'head'. There is a specific type of 'disk' called a 'flap disk' {NOT the square edged 'flap sanders}. Some of the people who make Windsor chairs use them to make the 'hollows' in the seats. The same technique would be used here. The disk held at an angle to the surface & 'rocked' back & forth in a gently curving motion.
A Random-Orbit sander could also be used, or the simplest disk sander attachment for an electric drill. Where did the plans come from? The reason I ask is that I dug up my drawing from Dynamite Payson illustrating the method - it came with either a 'Study Plans' packet, or with a set of plans.

You shouldn't use narrower tape, actually the opposite. "If stronger joint is wanted, hollow ends of plywood across slightly with disk sander and make with mat, tape, and epoxy." These are the EXACT words on the drawing. Because the instruction above this talks about an "over the knee" test with 1/4 ply, 3 INCH tape, and polyester resin . . . while the hollow technique specifies epoxy . . . I believe the drawing is *relatively* recent. If I was working with ½ inch sheet goods - and couldn't use a scarf - I'd be using 4 to 6 inch wide pieces of glass fabric for my top layer.

Remember, there is NO PROBLEM with ' freehand irregularities' when working with epoxy. In fact, joints that are 'too tight' can cause epoxy starvation. The top surface is what you will be manipulating, and that should become smooth. 'Predictability' comes from the testing you are doing, plus thinking about how you are going to handle the full scale pieces, then arranging everything at hand before you actually begin. Maybe even a few 'dry runs'.

Regards & Good Luck,
Ron Magen
Backyard Boatshop
{I'm going to try and scan or photograph the sketch and attach it}
[insert ‘Payson Joint’ drawing here]
- - - -

From: <timk_smith@

Ron,

I don't mean to keep lobbing puzzles at you, sorry. I've made Payson-type joints before, successfully, albeit in thinner ply--it's just that I usually make them one side at a time, so I can see what I'm doing. I have the right flap wheel for the grinder and made the Payson joints for the roof panel with it. Came out fine.

The joints in the half-inch ply seem to be coming out fine, too. I've bent the panel pretty severely and find no flat spots. I felt the same way you do about the width of the tape, so widened it out to six inches in three layers--inside layer of 12-oz tape, followed by two layers of 6-oz. So far so good.

The study guide is from Bolger, though I'm pretty sure Suzanne wrote it--Topaz, I'm told, is really the first design from PB&F that's "hers." if you're really interested I'll copy out the portion I'm describing and e-mail it to you.

Thanks for all your help with this.

Best,
Tim.
- - - - -

From: "Ron Magen" <quohog@att.net>

Tim,
It's your boat, and you are the one who has to have confidence in it's construction. It's no 'puzzle' - you have well described what you are doing - it's just that the methodology seems awkward to me. It almost could be called a 'double lap' joint. The concept of a single pass with a 3 inch power plane {with the sheet junction centered}, seems to be a condescension to a 'quickie' way of doing it. I don't know of 'Topaz', but anything that has ½ inch thick side panels and cabin with a 'roof', certainly isn't an 'Instant Boat'.

Once again . . . *MY* personal preference would be to use a scarf joint. Probably no more effort and maybe only a touch more time. {Using the 'stair-step' technique, they would be all cut at once} It might 'cost' a couple of more sheets of ply - due to the few inches of 'shortening' in the overall sheet length - but it would be minimal compared to the total cost of the completed boat.

If this is what 'they' *SPECIFY* {and illustrate with sketches}, and you would rather not question them about it, then it's your choice. No matter 'who' drew/designed it, "Phil & Co.' stands behind the work.

Regards & Good Luck,
Ron Magen
Backyard Boatshop
{PS - could you send me the EXACT wording - and context - plus any sketches of the joint}
- - - - -

From: <timk_smith@

Ron,

Topaz is a "poor man's picnic boat," 31 feet long with an octagonal house and a forward cockpit (there's a lines drawing in one of the Bolger group folders).

The exact wording for the sides is "assemble with Payson-type butts (if they're scarfed they won't come out of four panels)." That is the second reference in the building guide to joining half-inch panels with Payson butts. The first is in the guide to building the bottom, which is made of two courses of half-inch ply and takes a strong curve forward. The wording for that, referring to a single course, is: "...Lay out flat...with Payson-type taped butts as diagramed. [they enclose the same diagram you sent me]. Use a power-plane of 3" to hollow out the plywood to take two layers of tape bedded in epoxy flush, with no protrusion especially on the side that will take the second course."

I've made mine a little differently, six inches wide, with an extra layer of biaxial tape under two layers of lighter woven stuff. If it would help for the column, I could send a couple of pictures.

Best and thanks again,
Tim.
- - - - -
From: "Ron Magen" <quohog@att.net>

Tim,
'Topaz' is certainly an ambitious project !!

However, the wording of 'their' instructions proves my original supposition. The individual who wrote them is either NOT a 'tool user', or had fallen into the 'More Power !!' trap in trying to help you . . . with an incomplete explanation.

The reason for choosing the 'Butt' over the 'Scarf' is as I first suggested - it fits with their {?} philosophy of efficiency - ' . . .if they're scarfed they won't come out of four panels'. The suggestion that you use the 'Payson Joint' {and including his son's sketch} was to illustrate the proper technique. Where it became confusing was in the 'instruction' to use a 3 inch power plane. To sand a 'swale' across a sheet of ½ inch sheet goods takes a bit of time & effort. Plus, the specific note on Payson's diagram {about a drill-mounted disk} may be cheap, and effective for 1/4 inch material, but is not the most efficient for ½ inch. In addition, what *may* have been clear in their mind, WASN'T complete on the page. This led to confusion for the reader {YOU} who wasn't intimately involved with the varied use of many tools.

While I would approach it a different way, here's what I THINK was meant {and it's what I would do if I HAD to use a 3 inch power plane}.

1 - Use the power plane to make a 3 inch wide cut on the end of EACH SHEET !! Make the cuts on an angle, so that when they are butted together there will be a 6 inch wide, shallow 'V'.
{1a - at this point I would butt the pieces together and use a flap disk & angle grinder to 'soften' the angular 'V' into a swale}

2 - Butt the pieces together and 'paint' the cut area with unthickened epoxy

3 - Lay a 3 inch wide strip of cloth - centered over the joint - across the panel. Roll down with a 'bubble roller' and add more epoxy as necessary to saturate.

4 - Repeat, with strips 4 and 6 inches wide - the final piece covering the entire cut area. Roll down & add epoxy as necessary. {you used 3 pieces - I would have used light mat, then 4, and 6 inches wide strips, respectively}

5 - Lay a wide piece of heavy plastic over the lot and roll down with a length of heavy PVC pipe {use like a big rolling pin}

6 - Place a plank wide enough to cover all {or a piece of ply with a heavy plank on top} and clamp in place.

As I stated at the beginning of this note - it's an ambitious project. She WILL NOT be 'built in a day'. Plan ahead how you will fabricate each segment . . . and don't scrimp or cut corners with time. Rather then concern yourself with doing both sides perfectly AND simultaneously - do ONE side at a time. OR, consider one side 'primary' {the exterior side that will be seen by the 'public'} and don't concern yourself with whatever cosmetic imperfections occur.

From your last paragraph, I get the impression that our discussion is moot. You seem to have already done the job. If so, why question the technique now? I don't think you are about to do the work over - no matter what I say.

Thanks for the offer of the photo's

Regards,
Ron Magen
Backyard Boatshop
- - - -

From: "Ron Magen" <quohog@att.net>
Date: Wednesday, September 29, 2004 10:18

Tim,
Haven't heard from you for a while.

As I said, an ambitious project . . . how far have you progressed ?

Regards,
Ron Magen
Backyard Boatshop
{PS - how has the remnants of Jeanne affected you ? We got about 10 inches of rain yesterday night. Basement stayed dry - so all I did to the house & surroundings worked . . . Interestingly, Joanne mentioned that having two boats in the yard was ‘nice’}
- - - - -

From: <timk_smith@

Ron. I'm grateful as can be for your advice.

I'm plugging away, but haven't gone 3-D yet. Have fabricated the bulkheads, house top, house sides, and most of one side panel (that's the one I queried you about). Still getting good results with my wacky method, which turns out to have an advantage: the hard corners of the routed declivity keep the glue in the joint when the whole works is compressed. Another thing I forgot to mention is the importance of doing both sides of the joint at once when working alone. When there are multiple butt joints in a panel on just one side, flipping the panel without cracking the joint can be quite a project.

At any rate, I enjoy our correspondence (and your columns). If it would be any use to you, I could send you some photos of the joints.

best,

tim

And there it stands . . . so far. As Tim indicates, while a number of parts have been fabricated they are still in ‘2-D’. That is, as cut out & shaped flat surfaces. While they obviously have their epoxy & glass butt joints, I don’t know if they are also glass sheathed. While Tim may decide to sheath them ‘in the flat’ {a rather neat & efficient way to get a good, level job}, the final finish will be applied when the boat is in the last stages of construction. THAT will be the absolute proof.

Ron Magen
Backyard Boatshop
[PS - I recently used long, horizontal butt block joints to fabricate the sides of a raised flower bed, for Joanne. Rather then buy ‘new’ planking, I was ‘instructed’ to use some old, odd-sized, trim edged planks that have been laying in the shop overhead for YEARS. She got inspired when ‘we’ decided to clean out that area. She was fascinated by the method and liked my Stainless screws so much . . . told me to ORDER MORE !! . . . a) I guess she has plans, and b) you never know where you’ll be using those boatbuilding skills & techniques]