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Metal Boat Final Weld-up Sequence

A Strategy to Help Prevent Distortion

Copyright 2012 Michael Kasten

Recently I was asked to prepare a few notes on weld sequence for an owner-builder. What follows is the core of my notes about the final weld-up, using a large aluminum hull as an example. The following notes are also applicable to steel boats, though weld sizes will be much smaller and spacing will likely be greater. The overall objective with the following weld-up strategy is to limit or eliminate distortion, and to thus avoid use of fillers in the final paint coatings.

Most of what follows is probably common knowledge to professional metal boat builders, but maybe not in all cases. Many builders will have a different approach... Extra good, I say...! Anyway, here are the notes I prepared.


In general, the welds that will fixture the plate perimeters should be made first (perimeter tack welds) then those welds that will help hold the plates fair should be made (long'l stringer chain welds). After that, the welds that will cause the most distortion should be tackled first (e.g. butt welds), working toward those that will be relatively benign in terms of distortion.

Overall, there should never be a series of welds made "all in one place." Welds should be balanced port to starb'd and top to bottom, and should be made in a controlled sequence.

Assuming that the frames, long'l stringers, and plates are all in place, using an aluminum hull as an example, the weld-up sequence will ideally be as follows:

1. Tack weld all plate perimeters in place, i.e. to trim pipes, long'l seams, butts, edges, etc. This should be a regular sequence that is the minimum to assure that the plate edges are securely fixtured, but not so much as to induce any 'weld shrinkage' distortion of the plate. For example, on an 0.375" plate, this possibly means a 1" tack weld every 6 or 8 inches around the perimeter, more if needed, less if possible. This 'tack weld' sequence should ideally relate to the eventual weld length layout, i.e. if you eventually will make 3" final welds, then a 3; 6; 9; 12 increment would be appropriate, etc. In steel of course, the tack welds will be much smaller, and the final welding will probably not involve welds any longer than 1 inch (more or less, depending on plate thickness).

2. Chain weld all long'l stringers to the hull plate, allowing a distance from any butt welds to the nearest long'l chain weld - say 12 inches to allow for shrinkage at the butt weld.

3. Fixture all plate butts inside (with sister long's if needed) and outside (with temporary long's).

4. Achieve all transverse butt welds, working in a measured incremental pattern over the whole vessel, much as is done when torqueing down a head. Usually this means working from the middle toward the perimeter (of the whole vessel). Within each butt, ordinarily it is most favorable to achieve the inside seam weld first - because the tendency is for the plate to shrink, causing an indent, but having been welded on the inside, there is some tendency to counter that force. Plus... you then have the whole outside weld to accomplish afterward, which allows you to thoroughly back-gouge down to the weld root.

5. Fixture all longitudinal butt welds with internal and external fixturing as needed (usually outside on both sides of each long'l butt seam). This is especially important where long'l seam welds converge and the welds have the possibility of overcoming the local plate strength (causing distortion). In general, we prefer to allow the long's to converge in order to help resist that tendency.

6. Treat all "insert plate" welds as being locally "butts" or "long'l seams" and include them in the weld sequence accordingly - inside first, then outside - fixturing as needed, inside and / or outside.

7. After all plate butt welds have been accomplished, then begin welding the lengthwise edge seams using a regular, pre-planned sequence. This will also be like torqueing a head. It is usually best to start in the middle and work outward both lengthwise and vertically. Thus a radius chine or hard chine would receive the first sequence, followed by the sheer and rabbet line. Ordinarily it seems best to start amidships, working toward the ends for each sequence. Once you determine the best weld length, assuming it might be, say 3 inches on 3/8" aluminum plate, it is best to do a "back-step" sequence. Welds should be spread apart some distance, therefore something like making a 3 inch weld, then skip 24 inches, make another back-step weld, and so forth down the line. Next time you pass through, you can fill in at the 12 inch increment (but still laying down new welds at 24 inch increments). The next series would be at the 6 inch location, spaced 24 inches, etc. The specific layout increment you decide will be up to your own experience and what you observe locally. Plate of lesser thickness will require shorter welds, and possibly a longer interval between them. Steel will have less thickness, and requires shorter welds spaced farther apart.

8. After all edge seam welds are completed, welds can be made from the long'l stringers to the frames - also introducing heat minimally at each joint so as to minimize the tendency of the long's to 'collapse' when heated. This could require local fixturing on the outside, in particular near any butt joints in the plating.

9. At the very last, welds that attach the plating to the frames can be made. This must be done carefully, since these welds will cause more distortion than might be imagined, and whatever distortion they do cause will be much more visible to the eye. It is an advantage at this point to have made all the other "problem" welds first. It is helpful in terms of weld shrinkage / distortion to bias more of the welding toward the obtuse angle rather than the acute angle (the angle of frames to plates). This is most pronounced toward the bow and stern where the intersect between plate and frame deviates the greatest from being a 90 degree angle. In other words, in the ends of the vessel the welding schedule should not be too strictly adhered to in terms of exact weld lengths. In other words, if you need to bias more welds toward the open angle and fewer toward the acute angle side of the frame, that's expected, and very beneficial in terms of distortion. The goal will be to try to get the same overall amount of welding accomplished after adding both sides of the frame together. This is not strictly correct in terms of engineering, but it will help quite a lot in terms of eventual fairness.

10. In some cases if there is noticeable distortion caused by the rigidity of the frames (the hungry horse look) it can be useful to locally 'relax' the frames. In steel, this can be done by 'line heating' across the frame locally so that it will shrink some to accommodate the overall plate shrinkage. In aluminum, this is not advised due to locally weakening the frame. Builders have developed a variety of strategies to address this particular issue, some even going so far as to very slightly "oversize" the plate girth when cutting out the shape, i.e. so the plate will 'shrink to fit' and end up snug against the frames without noticeable distortion. Whether this trick is used, and how much extra to allow along the plate edges will vary according to one's experience as well as the plate thickness.

In the design of a metal boat's shape, it is highly beneficial to design in a certain amount of "belly" to the hull plates, i.e. some transverse curvature. If the hull shape is developable (or nearly so) there will naturally be some belly to the shape. It is never advantageous to have the plate be totally straight in the transverse direction.

Since it is ordinarily a requirement that the plates be fully in contact with and welded to the transverse frames, this means some curvature must be introduced to the frames. When frames are NC cut, the frame curvature automatically follows the plate belly. When building frames using flat bar there should always be some transverse curvature introduced in order to accommodate the natural "belly" that the plate will develop when placed on the boat. In other words, the frames should be brought out to meet the natural shape of the plate, rather than forcing the plate to lie flat against straight frames.


In the distortion battle, it is interesting to note that in The Netherlands where metal boat building is both common and extremely well developed, and where NC cutting is the norm, many builders prefer to actually begin with the plating...! By this method, all plates are pre-cut, so when they are placed the plates will naturally assume the correct position and belly. To accomplish this, the plates are ordinarily held in place using a few temporary *external* mould frames.

When building upright, that means the keel bottom and sides will be placed first, with floors then inserted and fitted to the plates as needed. The bottom plates are then offered up and attached along the rabbet, and then the internal transverse frames fitted to the installed plate. Then the side plates, then side frames, etc. If there are longitudinal stringers, they will usually be pre-planned and attached to the plates before they are offered up to the boat, i.e. while the plates are still lying flat.

In The Netherlands it is fairly common to see heavier displacement metal boats. With the added displacement, they are able to make use of slightly greater plate thickness and fewer or possibly even no longitudinal stringers, in which case the frames are placed closer together. This is vastly simpler, in particular when using the 'plates first' strategy. Since the frames will in this case be placed closer together, the frames are able to have a slightly smaller size. If frames are "fitted into" the already placed plates, distortion is kept to an absolute minimum. If long'l stringers are used, they are intercostal, i.e. fitted in between the frames as needed.

From a design strength point of view, it is favorable to have longitudinal stringers since they contribute greatly to plate stiffness and to the vessel's overall longitudinal strength. This is especially so in way of transverse butt welds. In general it is possible to create a lighter structure using a combination of long'l stringers to support the plate and transverse frames to support the long's. Where it is necessary to create the lightest possible structure, the combination of transverse frames and long'l stringers is nearly always favored.


The above notes are mainly aimed at controlling distortion during the weld-up of the hull structure. The deck and house structures, while similar, might impose other demands regarding weld-up sequence.

The above also does not take into account any tank faces or tops, nor any WT bulkheads. It is usually best to fit the tank faces into the fully welded-up hull. However if the tank faces are NC cut and fitted in advance, then it is best to fixture the long'l tank faces with tack welds to the exterior plate, much as with a long'l stringer, so that the tank edges are not displaced by overall shrinkage of the hull plate. In so doing it is always favorable to place the tack welds in a location that is easy to access with a grinding wheel in case the tacks need to be released later.

It is best to make the tank perimeter welds according to the orientation of the tank face, i.e. whether the local weld is long'l or transverse. When the external hull plating has been welded up, the tank long'l plates can have their remaining welds completed. The transverse tank ends can then be included along with the frame welding sequence. In other words, the transverse tank-end welds are saved for last. Each incremental weld should be as short and as far apart as can be justified, while still achieving good weld quality.

In general, if distortion becomes evident at ANY time, it is prudent to stop right away and see what can be done to correct it, then determine what caused it to happen, then determine how to prevent it from occurring again or elsewhere.


The American Welding Society's Guide to Hull Welding and their Guide to Welding Aluminum Hulls have very good information regarding:


Of course you can ask any metal boat builder these same "weld sequence" questions and you'll probably get quite a variety of answers..! It all comes down to the battle against distortion. In pursuit of that goal each builder will have their own strategy.

Unfortunately, many builders will not discuss what they regard as being their 'trade secrets' openly. It often seems the more secretive the individual, the less they know about it all, or the less confident they may be about the approach they've developed.

There should be no bloody secrets...!

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