Notes on Metal Boat Building Methods

Steel and aluminum boatbuilding. See: "Boatbuilding Methods", in left-hand column of the Home page, for information about alloys.

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Kevin Morin
Posts: 665
Joined: Thu Jan 24, 2008 11:36 am
Location: Kenai, Alaska

Re: Notes on Metal Boat Building Methods

Postby Kevin Morin » Thu Feb 18, 2016 8:50 pm

We've spent some time building a plate model (Station Card Model) or builder's plate model that would allow us to use a strip, which is like the middle of an entire sheet of material, to pattern a shape defined by the outside edges of the Body Section/Body Plan hull intersections of a hull. Using the techniques shown above, a 3' or 4' plate model can accurately reveal the exact shape of hull panels 10-15X that size or 30' to 60' boats could be plated using this method of taking off model to final shape.

Now its time to adjust some of the pattern making techniques to other places in out boats' hulls. This post just restates what has been shown in work methods- as words. This is an introduction to the main principle of pattern making surfaces outlines. Please recall this series of exercises, like the boats they model, can only be used to pattern developable surfaces, not compound surfaces which require plate/sheet metal goods to be distorted which is called forming.

We're concerned ONLY with planes which, when rolled along a single centerline with a uniform radius become cylinders or, if rolled with a graduated radius around a line or axis become cones. These three geometric forms are all that sheet metal goods can form without stretching of one form or another. (English Wheels, press braking, planishing and contraction are all examples of stretch forming) We are not addressing forming tools, techniques or hulls formed from that wider scope of shapes; our discussions are confined to the three cold formed shapes that make up almost all small welded aluminum boats - not stretch formed pressed formed factory products like; Lunds, Gregors, Starcraft, Smokercraft and dozens of other small boat manufacturers who use tooling and engineering not included in our home built boat shapes.

One way to say this same thing is; "If you can't form it from paper without tearing- we're not discussing that set of shapes."

Because of the limited number of shapes that make up these hulls, we could review what patterning techniques we've used. So far, we've defined a vector and a distance from a pattern "take off piece" and then reversed the points taken off. With the information of vector and distance on the pattern piece- all we did was put the pattern take off record on new material and the drew vectors and marked distances to recreate the shape.

If the pattern take off piece was in a plane, it took off the planar intersection points, and if it was warped to a cylinder's surface or conic surface the pattern take off piece reliably recorded the surface changes for those two changes of the surface of the hull. For example, the topsides was modeled from a Station Card builder's plate model by laying a strip along the cards' edges. This model may have had a flat or cylindrical topsides aft... but the topsides surely became a conic section int he forward 1/3 of the hull..... We recorded a vector along each (card) edge by drawing on the pattern strip; associated distances to the chine (above on our inverted, keel up model)or downward to the sheer line were noted. When we were finished, a take off strip represented the entire sheet as though it were held to the topsides and indeed it could be held up and marked then battened an cut! however instead of the entire sheet being held and clamped to the frames or Station Card model- all we handled was a strip of metal (paper).

Almost all patterning techniques can be done in exactly the same way. If you place any take off (pattern piece) in the surface that will be recorded and record vectors and distances- reverse the technique on the new material where the final outline will be cut then you have the basis for patterning surfaces.

If the surface is a plane, record in that geometric figure. If cylinder or cone record in those surfaces and the result will be an accurate 'take off and lay down' of that targeted surface.

Next we'll move to two separate tools that are widely used to take off patterns using the Rules of Thumb in we've already explored and described.

Cheers,
Kevin Morin
Kenai, AK
Kevin Morin

Kevin Morin
Posts: 665
Joined: Thu Jan 24, 2008 11:36 am
Location: Kenai, Alaska

Re: Notes on Metal Boat Building Methods

Postby Kevin Morin » Sun Feb 28, 2016 2:14 pm

Reviewing the patterning concept briefly; patterns have directions and distances of points on the outline of the piece to be patterned or 'taken off' of the model or the finished frames if they're assembled and faired?

[Frames are faired by using a flat bar of sheet strip to lay on the frames, then rotated 45 or 35 degrees tot he keel in both directions to simulate the sheet material and insure there are no high or low (proud or shy) frames' edges]

We'll explore on way to take off very accurate patterns of other hull panels, and this method works for many different locations in the boat- especially effective on bulkheads, frames or other transverse sheet materials if the hull is made skin first- which is just another alternative means of building in sheet metal due to the very large sheets available 6'x25', for example.

The boat we'll use in this example is in the first sketch below.

Image
This set of skiffs with identical lines shows a scale 20' planing hull with a reverse chine or chine strake; sometimes called the chine flat. To the right is the full hull, inverted with both the bottom and topsides plates shown. The left hand image removes the adjacent hull panels to show just the chine strake on the starboard side. The shape of that hull panel begins to show more clearly in the next two images.

Image
In this first of 2 multiple view images of the green chine flat in a 3D grid box; there are two views of the same surface of metal. The upper view was taken from the top and rotated so we could seen inside that top view box from the bow end. The lower grid box holds a copy of the chine surface in the same orientation as the first sketch above, where the boat is inverted.

Image
2nd of the two 3D grid box chine surface views, here above, moves the point of view of the two grid boxes and their chine surface copies to the stern quarter. The reason to take all this effort to show the true shape of the finished surface is to see that patterning this piece requires putting some "take off strip" on the same plane or surface as the final shape will be taken out of blank aluminum sheet or plate.

We'll concentrate on the bow as that is the area where most curvature is and may be the most inconvenient to 'take off'.

Image
The view here in this sketch, above, is like the first image in this post, looking down on the inverted forefoot of the skiff with the chines colored green/yellow and pink/red. A purple/lavender 'spine' strip of off cut aluminum is laid on the chine flat area. The frames are not shown because I thought it better imagery to show the final shape tapered surface in relationship to the technique used.

The purple spine is tacked at both ends or clamped to the frames, on the card model is can be taped simply to the Station Cards, being sure it lays in the same curve/flats/surfaces as the chine flat. This is critical to the method being shown- if the pattern's spine is not in the surface this method will not work. In this images all the "slivers" used to "take off" points are under the surface so they're nearest the plane of the chine flat- they all touch the bottom panels' outer chine edge and are spot tacked to the spine.

The slivers can be on top, but the ends will need to be bent down, to touch the sheet edges or a gap may form in the patterning process. By tacking strips along the chine flat spine- they will be held in place, and they each provide a direction and distance to an exact point on the inner chine's curve.

These points are shown taken at random points but if you're taking off the frames or Station Card builder's plate model- these points can be along the frame/cards' edges and can be marked under the 'spine' strip of take off strip just like the previous work methods shown to take off the larger hull panels. The point in illustrating the welding tack slivers w/ spine is to show a full size method of work on the metal frames than can be used in other locations.

Image
As with any pattern, shown above, the spine and tacked sliver pattern is laid down flat on the new stock and the tips of each pattern point marked on the new material.

Image
And last, above, the same battening fair techniques are used to clean up the curve and 'best fit' the points of the curve. In some bow chine curves the sheets may be flattened between the frames- so battening those flats out of the final inner chine curve will allow the builder to pull the bottom's outer chine sheet edge out to a fair curve that has been battened into the chine flat. The chine flat is essentially a 'flat bar' being tacked on edge- so the width of the chine will easily overcome and tendency of the bottom panel's chine edge to pull in- leaving a segmented or unfair curve to the weld seam.

Once the inner chine curve is cut, the blank material can simply be laid to the card frame or metal frames and the points of the outer curve marked directly onto the surface. This also leaves the outer curve until any curve filing or sanding is done on the inner chine in case your pattern work or cutting techniques are still gaining in proficiency. The outer curve being marked in place means that a framed boat may already have the topsides sheets on the frames therefore an entire curve cutline may be drawn inside the topsides sheeting.

If the inside is not accessible, but the hull plate is on the hull's frames? then an outside line can be drawn and using an offset block, or a finger tip technique, equal to the thickness of the topsides; the outer chine's cut line can be marked to fit inside the lower edge of the topsides. This curve should be checked for fair by visual inspection in case there are flats in the lower topsides when the line is marked under the new chine blank plate. Once in a while, even if the topsides are in tension due to the cold formed cone around the frames- there can be a flat in the lower edge (chine) of the hull panel.

This technique (tack strips on strong back) works on bulkheads, frames, fore decks, hatches, and tanks' transverse profiles if those edges have to fit into a complex or odd shaped cross section. The tacks needed are very small, temporary and can be taken loose by hand or a tap with a cold chisel so the same 'parts' or slivers can be used over and over. The remaining tacks on the used slivers can be dressed off very quickly on a belt sander or with a hand held sanding disk on the bench. Sometimes the tips of used slivers need sharpening; again sharpening on the belt is fast and leaves enough point to make very accurate take off's from most areas in the hull.

To summarize; if a strip, bar or other rigid shape of aluminum is held into the same surface as a final hull plate, in this case a combination flats and cylindrical surfaces AND a series of small pointed pieces of aluminum scrap are tacked to that spine/strong back/pattern piece where they touch the edges of the final part to be cut; and accurate patten of the shape can be lifted/taken/removed from the boundaries of that final hull panel. Once the true shape is taken off, it can be laid down on the plate stock where the final hull material will be cut out. Once marked and battened the final piece will accurately reflect the outlines of the part needed to join each edge of the adjoining hull panels.

This technique would work fine for the entire hull panel, buttttttt! that would be one big and unwieldy pattern piece.

There are other tools for this work as well. One traditional method is still about as accurate as anyone can work, so we'll explore it next. The dividers can take off the pattern of this shape, which is like spiling planks on wooden boats.

Cheers,
Kevin Morin
Kenai, AK
Kevin Morin

Kevin Morin
Posts: 665
Joined: Thu Jan 24, 2008 11:36 am
Location: Kenai, Alaska

Re: Notes on Metal Boat Building Methods

Postby Kevin Morin » Mon May 07, 2018 12:32 pm

Bumping this thread to remark about a boat I've recently viewed with some interior hull corrosion and pitting in the bilge.

In this thread and others, I've been on my soap box about the need to remove mill scale from all sheet and plate aluminum used in boats- at the time of construction or after building and before fit out. I've also noted that acid etching an entire finished boat- inside and out is pretty hard- since many designs provide a deck that is welded in- making bilge or sub-deck access pretty tough to do a good etch and rinse job in that void.

On the other hand, I've also ranted about my many experiences of trying to help repair boats where the mill scale is left intact after the build and logged many details of the 'white flower' corrosion sites that begin what will become full scale pitting if the mill scale is left on the sheet goods as they come from the mill.

I had the opportunity to look into the bilge of a 13 year old welded boat in the last few days. The builder did many things I can't agree are good practice in welded metal boat construction; and specifically the mill scale was left inside the hull AND.... oddly enough- the outer hull was partly etched- by someone? - in the last 13 years.

The outside of the hull was not etched very thoroughly- perhaps with a cold or weak acid solution- but in the areas where the etchant was applied and the mill scale removed? the aluminum was nicely uniform surface with no corrosion flowers and no pitting. However- that is not the case with the rest of the boat.

There was bottom paint added OVER the mill scale and that has created beginning pitting outside- this bottom paint was applied over the mill scale so the water vapor trapped in that process - absorbed into the porous scale layer- eventually formed acidic cells and you can easily see the paint lifted locations AS well AS the small powdery pockets of by-products of corrosion.

Also this hull has a band of paint just below the sheer's rub rail and above a side rail from bow to stern. This too has many sites of paint lifting- small corrosion pockets below the paint film - creating a reactive cell that lifts the paint which was applied to mill scale NOT to the underlying aluminum- and that paint band is showing the pitting and chalky corrosion sites as well.

My reason to mention this again, is to confirm the facts of marine aluminum preparation for paints, coatings, and other films that may be considered. Getting the mill scale off (it ain't aluminum!) the aluminum and following one of the two or three accepted methods of properly to bond films to the aluminum is worth the effort.

#1 acid wash followed (while wet) with a fresh water rinse and then allodyne (chromic acid to obtain a bonded layer of chromium oxide on the etched aluminum) followed by priming and topcoats- still very reliable method with consistently good results.

#2 OR, as alternative, mechanically preparing an anchor pattern (micro level surface roughness) with rotary or belt abrasive tools OR sand blasting (including soda, beads, silica sands) then..... using a self etching primer to allow the primer a highly mechanical adhesion to the metal- followed by compatible topcoats.

There are combinations of these methods used but... if the mill scale is left intact- then whatever adhered to that corrosion susceptible film of material will lift off the metal. If water is allowed in (even condensate from the evening dew) these sites will end up pitting the parent metal. IF the films added to the metal are chemically bonded after the removal of millscale- the results are quite different.

Last remarks: about dry bilges/draining bilges/standing bilge water.

In this particular hull, at the forefoot, was a small "sealed" compartment formed below a foc'sle cabin sole that was welded in. However, water could and did collect below the welded deck- there were no provisions to pump this bilge OR to drain it aft to any hull drains. And as it happens the "seal" wasn't- so water collected in the forefoot of this hull under a welded down deck plate/cabin sole.

Just forward of the forward berth compartment is an anchor locker that was not sealed properly- welded but evidently not thoroughly OR worse yet, its possible this locker's drain hole, near the bow stem and keel bar, is plugged or inadequately sized to actually flush the mud/debris that comes aboard with the anchor rode?

Further, the anchor chain's eyelet and clevis w/ pin and (maybe retaining wire?) where the chain is made up to the rode; were apparently immersed in salt water for prolonged periods of time? The anchor is deck mounted to a chock but even that clevis and pin is rusted - think what might be possible if the entire chain and locker were awash because the drain/limber hole plugged?

This fluid, from the anchor locker found a leak path to the small void under the cabin sole plate and created a solution with significant galvanic differences to the inner hull! The corrosion sites are located in a roughly waterline located ring around this compartment -like drawing a very shallow waterline on the hull at this depth - about 6" to 8" above the bottom & keel intersection.

This indicates the approximate fill level of this compartment- and the location where the top of the galvanically different bilge water could react with the hull, and then re-wet and dry out in a continuous cycle for as long as this problem has been going on?

Some of the pits in this section of the hull have actually penetrated the hull's 1/4" thickness in the 13 years since the hull was constructed. I think the pits can be drilled out- backed with SS plate or bars and TIG welded from outside the hull- once the pit sites are drilled and cleaned to bare metal?

I'll probably get the opportunity to try that exercise in Michelangelo Welding in the next few days once the clean up and inspections are done?

Bumping the thread to repeat my previous expressions of the need to treat marine aluminum for the long haul- yes it takes more effort- but what of a Glen-L builders' efforts to build a welded metal boat that began to deteriorate from ill-advised or NOT informed building techniques?

Nobody would even consider leaving mill scale on steel products used to build a boat- why on earth would they consider leaving mill scale on aluminum- just because it looks shiny? Pretty short sited in my view.

Hope those who read these posts recognize that welded aluminum boats are actually a new product compared to wooden boats which have been around for countless years. The methods, techniques and 'best practices' of welded aluminum are still being debated (or ignored by some) even among many builders of widely recognized brand name boats.

I'm not trying to condemn the builder who skipped steps, made errors and ignored chemistry (!) as much as I'd like to caution new builders about surface treatments for this material- a few extra steps in the build will definitely allow your welded aluminum boat to last a life-time and be valuable after many decades of reliable use.

Hope this makes consistent sense to those reading the series? If not? Always willing to try to clarify what's posted.

Cheers,
Kevin Morin
Kenai, AK
Kevin Morin


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