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: 636
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: 636
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


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