Patterns

PATTERNS

HOW TO MAKE FULL-SIZE BOAT HULL PATTERNS.

One of the new services that I have developed here at SYDI is the creation of full-size building patterns for boat hulls. Add to this, too, full-size patterns for keels and rudders. We get inquiries from time to time from people who want to build a boat, but either can't or don't want to loft the lines to full scale. If they had patterns, they could simply trace them onto plywood or have them cut out with a numerically controlled (NC) router. Making patterns can be done in different ways.

The three examples given below show some of the projects we have worked on in the last few years. The first is a set of partial hull patterns for a Crosswater 54 power catamaran designed by John Marples and built by Felix Herrin at Catman Cats in Urbana, VA. The second is an SYDI custom 24' sailing catamaran for a sailor in Jacksonville, FL, who wants to build his own boat. The final example is for a production sailboat for Hake Yachts in Stuart, FL, the Seaward 46. Each example shows different design starting points, different features of the boats, and different aspects of the kinds of patterns that we can do.

Elsewhere on this website there are stories of other designs that we have done in which we have designed the boat and/or provided full-size building patterns. Two of these are Bagatelle, my 44' lightweight sailing yacht that is for sale as stock plans, patterns included. You can get to that story here: Bagatelle. Another example is the Chris Craft Cobra that I redesigned for a classic boat enthusiast who likes to build his own classic-styled speedboats. That story is here: Cobra.

If you would like SYDI to provide you with full-size patterns for your boat project, you can reach us at the Contact Us link below. In the meantime, follow on with these interesting case studies.

Crosswater 54, Mk III

Multihull designer John Marples approached SYDI to create new hull patterns for the Crosswater 54, Mk III that was to be built by Felix Herrin--The Catman--at Catman Cats in Urbana, VA, who had also done the Mk I and Mk II versions. These are wood-epoxy vessels, and the hulls are very flat sided above the waterline, but below the waterline and the chine they have some shape to them. The picture below is a Mk I version of the Crosswater 54, Mare Bello.

Fig. 1. The Crosswater 54 Mk I, Mare Bello. Note the chine strake between the green topsides and the white anti-fouling. The green topsides are plywood-epoxy, and the white underbodies are a combination of plywood and strip-planking with epoxy, all covered over with fiberglass. SYDI provided full-size patterns for the white underbody area on the revised Mk III design.

John Marples gave me hand-drawn scale drawings that showed the lines to the outside surface of the hull as well as laminate and construction notes. The patterns were to be cut and built to the inside surface of the wood-epoxy structure, so I had to calculate the thickness of the hull skin and reduce the sizes of the patterns accordingly.

Fig. 2. Crosswater 54 bow lines (L), and stern lines (R). These are the AutoCad versions that I redrew at full scale from the original paper lines plan, which was drawn at 3/4" to the foot. This AutoCad version has been reduced by the thickness of the hull planking and laminate.

For each of these stations, I copied them over to another part of the AutoCad drawing file and lined up all the patterns and alignment holes which can be seen in the next figure.

Fig. 3. This is a typical full-size pattern at Station 30'. It shows the reference lines for centerline and design waterline, and alignment slots so that each station can be aligned on the strongback and in line with every other station both vertically and horizontally.

Every pattern for every station looks pretty much like Fig. 3. In this case, there were 34 station patterns, including the stem profile through the bulbous bow. What is not shown on this example is the fact that each station is larger than a full-size sheet of plywood. Therefore, Catman Cats had to figure out how to join multiple pieces of plywood to create a full pattern.

The total cost for these patterns was $3,000 which is quite reasonable for this size boat, considering how much labor was saved by having the patterns in a drawing file. From the AutoCad file which I delivered by email, Catman Cats either used the file to drive an NC cutting machine, or they plotted full-size Mylars and traced each station onto plywood, I am not sure which. Later, some additional patterns were required for a half dozen additional bulkheads, and those were provided for an additional $500.

Richard Smythe 24' Custom Catamaran

Richard Smythe caught the sailing bug in catamarans a while back, and he likes to go really fast. He is a youngish (early 30s) sailor who has an ambition to build his own catamarans. The 24'er is a first-step on the way to a grander project. Step two will be a 35' catamaran, and the final step will be a 70' catamaran. Richard does not know too much about how to build boats, so this is definitely a learning experience that we are helping him with.

To begin, Richard didn't really have a design--just an idea. What he described was pretty simple in shape, really, and since this was going to be a slow, piece-meal development process, I offered to shape one hull and deck combination, with patterns, for $1,900. I spent a little more time than that, but it is a fun project.

Fig. 4. Smythe 24' custom catamaran, one hull only, totally symmetric, rendered in Rhino.

Richard knew the overall dimensions and appproximate weight of the total boat, but had only the sketchiest of sketches on what the boat was supposed to look like. I transfered those notes into a 3D surface model in ProSurf, then translated hull station cuts to a dxf file that can be read into AutoCad. By starting with the surface model in ProSurf, I automatically have the ability to calculate hydrostatics and stability which will be needed later on. From the hull cuts I created the full-size plug patterns in AutoCad for the hull and deck. In this case, I still had to reduce the thickness at each station according to the plug skin thickness. This is done automatically in ProSurf. I had to have Richard build me some sample laminates with the materials he is going to use so that I could be as accurate as possible on this thickness dimension. Single hull and deck plugs are going to be built on flat tables, and molds will be taken directly from them. So the plugs are not going to be a finished boat, just a shape. Richard has in mind that he might build more than one boat, trying out different features, and maybe selling each previous version to finance the next version.

Fig. 5. A perspective view of the hull and deck for the Smythe custom 24' catamaran. The hull has to be deep enough to carry surfboards inside.

Just to make sure this new boat would have the overall dimensions that he would need to achieve the speeds he is after, I took it upon myself to do a parametric study of other catamarans. There really aren't too many in this size range--most are smaller. The only ones that come close are the Stillettos, 23', 27', and 30', which actually are quite heavy despite their advance composite construction, and the RC 27 and RC 30 by Aquarius Sail Inc., Bill Roberts designs. These are shown on the accompanying spreadsheet graph.

Fig. 6. A plot of Sail Area/Displacement ratio versus Bruce Number for a population of 46 catamaran designs. Bruce number is like a sail area/displacement ratio, so one would expect a pretty uniform curve such as this.

Since this design is a trial horse for Richard, he'll want to go fast, but we don't really want him to break his boat or his neck. So we have tentatively targeted a Bruce Number of 2.50, which puts him squarely between the RC 30 and RC 27.

The next step is the patterns themselves, and these are a bit like the Crosswater patterns above.

Fig. 7. This is the master plan for the full-size hull and deck patterns for the Smythe 24 catamaran. There are 15 stations including the stem profile and transom, each station at 2' centers. This drawing tells how to interpret the patterns which are shown in another drawing in the same drawing file.

Since these patterns are for male plugs which will create female molds, I had to take into account the shape of the hull-deck joint and work that into the construction details for the plug. Just below the middle of the plan above can be seen a detail of the coffee can type of joint. The deck molding comes down over the hull molding and is glued into the rabbet. The joint is then taped over with fiberglass on the inside. It can also be taped over on the outside, faired out, and painted over. Alternatively, the joint line can be covered with a decorative vinyl style stripe, or it can be covered with a rubber bumper strip. That detail will be determined later.

We also have to take into acount that these are two-part molds with a ventral seam (on the centerline). The flanges that will be required in the tooling are not shown, but that will have to be addressed after the plugs are built. That is going on now as this is being written, so there are no photos of this project at this time. As this project develops, progress photos can be posted.

Hake Yachts Seaward 46

SYDI has been consulting with Hake Yachts since about 1992 when the 17' Seaward Fox came out with its free-standing mast on which I had rendered design and building advice. Currently, Nick Hake builds the Seaward 26 and Seaward 32, both shallow-draft cruisers with lifting keels and stayed rigs. They have a distinctive sheer which makes them easily identifiable from a distance. The newest member of this stable is going to be the Seaward 46, which isn't out yet, but the first boat is nearing completion. Nick Hake came to SYDI to develop the hull shape from his original sketches, which was a 45'er actually, and create the patterns for building the hull plug.

Fig. 8. Nick Hake gave me this drawing to start with on shaping the hull. My notes are in pencil. Knowing his basic dimensions and the salient design aspects, I developed a 3D surface model to create full-size patterns and calculate hydrostatics and stability.

The distinctive sheer is there with a deep low point aft, and the hull is quite shallow. The lifting keel will come up into the hull such that the bulb recesses to half its depth. Nick takes that into account in the tooling by making a removable half-bulb insert that is keyed in place prior to hull lay-up. In this way, Hake Yachts can fine-tune the precise placement of the bulb should anything need to be modified for weight and balance. In addition, should the bulb shape ever change in design at a later date, only the insert has to be modified, not the entire hull mold.

Fig. 9. The profile of the Hake 45 hull from ProSurf, translated to Rhino for rendering.

Fig. 10. This perspective shows the very flat bottom for shallow water cruising. No keel bulb recess is created in the hull computer files or the hull mold proper; that is done by removable bulb insert built by hand after the tooling is built.

As in the other boats described above, in the ProSurf 3D surface model I created hull cuts at each construction station for the plug, less the thickness of the plug laminate. In this case, the stations are all notched for ribands, the positions of which the tooling workers at Hake Yachts layed out and made themselves--I did not specify those. But Nick Hake and I decided on the best plug skin laminate of two layers of fiberglass woven roving and one layer of cloth over 1/8" plywood for a total thickness of 1/4".

The other thing we refined on these patterns was the jointing of the plywood stations. Just about every station is larger than a standard 4' x 8' sheet of plywood, so multiple pieces had to be joined together. These joints have to be tight, so that when put together, the separate pieces don't actually make the station a touch larger. The stations were all cut from 1/2" thick plywood by John Welling at CamCarved in Stuart, FL (web link: CamCarved.) He had developed a keyway, seen on the pattern drawing, that he said works very well. I have adopted this keyway shape now for our SYDI patterns. When I drew the patterns, I had to make sure that the pieces could individually fit on 4' x 8' sheets using these guidelines: the cutting head is 3/8" in diameter; any cut edge had to be at least 3/4" from the edge of the plywood; and the minimum distance between cut pieces had to be 5/8" or greater.

In addition, the line defining the shape of the station had to be a continuous polyline--it could not be broken into segments or groups of lines, circles, arcs, and other polylines. In the AutoCad pattern file, I had to make sure all the line segments were joined in continuous closed loops. This is because the cutting head will stop at the beginnings and ends of line segments, and if I make sure all lines are continuous, then the cutting head will move continuously without stopping. The NC cutting technician then has only to define the path from one whole piece to the next. These are the rules for laying out the patterns. They may be different for other pattern suppliers, so it is important to know this information prior to laying out the pattern design. The next figure shows a drawing of one of the station patterns as finally finished. These were exported from my original AutoCad drawing to an eps file format which is what the NC cutting machine uses.

Fig. 11. The master hull pattern for the Hake 45 describing the salient features common to each pattern. There were 24 patterns total, including the stem profile. Note the jigsaw puzzle-like keyway pattern to join separate pieces together.

The next series of pictures show the plug and mold being built. One can appreicate the amount of work saved by not having to loft each and every station.

Fig. 12. The plywood stations set up for the Seaward 46. Note the alignment slots for the centerline, reference waterline, and sheer line.

Fig. 13. The ribands have been notched into each station, and the 1/8" plywood hull skin is going on. Over this will be two layers of fiberglass woven roving and one layer of glass cloth.

Fig. 14. The plug complete, filled and faired, polished, and ready for making the mold. Note that the bowsprit platform has been added. Although not easily seen, the stem is actually sloped forward at the deck about 6" to alter the aesthetics and reduce the bow radius.

Fig. 15. The mold is finished and being removed from the plug.

After the tooling for the Seaward 46 was complete, I continued with details for the shape of the keel and the bulb. Again, I produced lines and patterns for the keel blade, which has a constant chord length, plus the bulb shape at regular stations. The 3D surface file was sent directly to keel makers Mars Metal in Burlington, Ontario, Canada, who made their own patterns and cast the bulb (web link: Mars Keel.)

Fig. 14. This is a Rhino rendering of the hull with the keel and bulb in its final design position. This works for level trim on the water and is balanced with the rig.

Fig. 15. This is a close-up of the bulb on the Seaward 46. The blue line is the approximate position of the joint between the lead ballast casting and the composite blade.

Nick Hake liked this bulb shape enough to commission two more bulbs, one each for the Seaward 26 and Seaward 32 so that all three designs would have the same bulb family. This was easily done by scaling the Seaward 46 bulb shape to suitably smaller sizes for the correct weight. Again, the 3D models were sent directly to Mars Metal for patterning and tooling.

Fig. 16. The Seaward 26 bulb (top) and the Seaward 32 bulb (bottom.)

Recently, Nick Hake sent me some publicity photos of the Seaward 46 finished and sailing, a few of which I include here:

The cost of the hull and keel design and pattern work for all of the above came to about $4,000, spread out over time as it accummulated, and which was paid in progress payments. Actually, the easy part of this type of work is generating the shapes themselves--that goes fairly quickly. What takes some time is to draw out the patterns in AutoCad so that they are accurate, readable, and orthogonally correct to each other. If the pattern parts are made up of separate pieces, it is important that each is numbered in a coordinated way so that they can be assembled quickly and accurately. There is also considerable coordination that goes on with the client and the various third parties who are responsible for actually fabricating the patterns and molds. I have to know how everyone is going to do their jobs so that the drawings will reflect what actually gets done. That's what naval architects do.

If you have a new boat project for which you would like full-size patterns, you may reach us through the Contact Us link below. I'd be happy to learn about your project and tell you what we can do for you to make appropriate patterns.