half models for hard chines?

[Murray Campbell]

...so the instructions i've found for working up a half model have all started with gluing up a sandwich of 'lifts'. To me, the method doesn't make sense for a hard chined boat, especially where the chine follows the sweep of the sheer and would cross over lines set parallel to the water.

Does anyone know if there is a different way of half modelling boats with hard chines? Or maybe no-one bothers?

( mostly i wanted a model to help me see how wide my garboard plank would have to be, how much planking stock i would need, etc.)

ps please god i don't want to have to wrestle with learning any hull design software

[mmd]

How about building a half-hull with "planking" from balsa on half-moulds. Backbone, chine logs, and sheer inwale can be of boxwood. All supplies should be available at your local hobby store. That will give you the form you want plus plank runs & spiling, too.

[Beowolf]

Sam Devlin's book includes a chapter on building half models for chined boats. I realize that all of his are stich and glue, but I would think that you could modify the process a bit to get it to work for you.

Jeff

[John Meachen]

mmd makes a good suggestion about building the model at scale with planking.If you really do want to carve a model,I would offer the following suggestion.Take as your cue the feature on "Maid of Endor" in WB182,specifically the way in which a contrasting boot-top section is included in the hull .
What I would suggest is that you make up two blocks for the boat.Each should be rectangular and the upper block should be deep enough to make the section from the lowest part of the chine to the highest point of the sheerline and then a little more.The lower block should be deep enough to reach from the highest point of the chine to the lowest part of the hull plus a bit.Mark them both with station lines and mark the chine line on both of them.Before cutting anything,cramp them together with the back face and the stations aligned and use a drill press to put a couple of dowel holes from the top down into the bottom to assist with alignment once shaped.
Seperate the blocks and cut the chine curve in each as accurately as possible,a bandsaw makes life a lot easier in this respect.Keep the offcuts as they will support the block when you bandsaw the sheer.Mark the sheer in plan view on the top face and the half breadth of the chine line on the bottom face of the upper section.Cut the sheerline in profile first,clean the surface and pin the offcut back in place to use your line while cutting the sheerline in plan view.
Turn your attention to the lower section and go through the same exercise with marking and cutting the chine in half breadth and the profile of the keel and stem.Glue the blocks together using some of the offcuts for cramp pads and take off the corners as required.It may be that your chosen design features conic sections to make it suitable for sheet materials.In which case you will have to make templates for the sections rather than just joining the dots.

[Tom Lathrop]

For a hard chine model, the lift method shown in Sam Devlin's book is by far the easiest to do. All you need is a plan and profile drawing, no offsets or station sections needed. I used this method recently to make a model that I took the panel shapes from to build a full size S&G boat. Came out fine too. I did loft (foreshorten method) the panels to fair inaccuraces caused by the scaled up dimensions.

[John Bell]

My only comment on Devlin's method is that looks like the way to go for developable shapes. However, remeber not all hard chine designs have developable panels.

[Murray Campbell]

sorry, developable means you can make the chine from one panel?? simple curve or very slight compound curve?

[Buddy Sharpton]

Developable means the panel is curved only along one axis, something that can be "developed"by bending in one direction. It will be a section of a cone or cylinder, but not I believe necessarily a cylinder or cone with a circular base-that could be an elipse or oblong. Something (in miniature) you could cut out of an oatmeal box or a paper cup. By chosing the right apex for that cone or right angle of incidence of the cylinder. a designer can use descriptive geometry ( or a CAD program) to "develop" a series of such sections that can be joined to get a boat length panel because somewhere along they can share a common straight edge to butt. This will not necessarily be vertical in a side view or occur at a mold station.

This design method has long been used for welded metal ships, but it's the same technique for designing tack and tape panel boats taken a step farther and requiring less skeletal structure like backbones, bulkheads and ribs being completed to fasten panels to ( where metal boat builders would weld on plating) and no full size lofting. The very stiffness of plywood panels makes possible a lighter, stressed skin monocoque structure for the hull.

[Bob Cleek]

Building a model is a very good way to get the feel of a boat one contemplates building. If done to scale exactly, you should get an idea of what the panel shapes or planks or whatever will look like. Particularly with panel stock like plywood (or steel), you will be able to see whether you have problems developing the shapes. (Compound curves will not work with plywood... hence hard chines.)

HOWEVER, don't think that by building a model you will be able to simply scale up shapes and build a boat full size. (Somebody above opined about "spiling.") There is no avoiding lofting the boat full size to get the right shapes for your patterns. If you want a general idea of how wide your garboard stock has to be, your best bet is to measure it off of the lines. In any event, anything more than eight or ten inches is going to require a splice or butt, since almost always when you get into widths beyond that in grown stock you have problems with grain splitting at the ends, or with plywood you are into wasting a lot of expensive material. A model is no more accurate than drawn lines and the errors creep in when the model is expanded to full size, just as they do with drawn lines. If you are sure you wan to build the boat and the model is only to develop patterns, forget it. Building a model is great entertainment and a good learning process, but not a substitute for proper lofting. It won't work that way.

[ 02-24-2005, 09:54 PM: Message edited by: Bob Cleek ]

[Tom Lathrop]

Originally posted by Bob Cleek:
HOWEVER, don't think that by building a model you will be able to simply scale up shapes and build a boat full size. I'm not certain just what you mean Bob. If the boat has developable surfaces to be formed of sheet plywood, it is certainly possible to scale up the shapes from a lift model and build full size. Some form of lofting is an asset but full scale lofting is not necessary. Either that or I have just been lucky since it works for me and for others who have been building boats that way.

[Neville Striker]

Any one ever hear of a Hawks Nest Model?

[Murray Campbell]

mostly for fun and learning, bob...i would plan on lofting too, though it's a Very Simple Boat

i also wanted to evaluate the planking options, (trad plank, lapstrake ply or maybe just one piece panels.....i would really like to go the traditional planking route if it can be done with reasonable stock i can afford....but if single panels would work there would be a terrible temptation to try something quick and easy first)

i must be missing something, i didn't realize i could get the garboard width off the lines plan..

[ 02-25-2005, 01:13 AM: Message edited by: Murray Campbell ]

[Bruce Hooke]

Originally posted by Murray Campbell:
i must be missing something, i didn't realize i could get the garboard width off the lines plan..As far as I know the best you can do is get the garboard width at a given point. This does not allow you to figure out how much sweep there is to the plank and how wide a plank you will ultimately need. However, Bob's basic point is that if you know the width at any one point is in the 8-10" range then you can pretty much count on having to do some splicing. BUT, you may not know what the width is at the bow and stern without having some way to work out how much the top edge of the plank sweeps up. This may be shown on some plans but often it is not.

As to Bob's comments about lofting not being avoidable -- I think this is a fairly reasonable comment for traditional contstruction methods. Once you start moving into stitch-and-glue territory the rules change a bit and it seems to be fairly common to get the panel shapes from some sort of model. Part of the reason for this difference is that with stitch-and-glue the shape of the boat is defined by the panels rather than the panels having to be laid over some sort of pre-built framework that must be fair for the boat to be fair. One consquence is that a stitch-and-glue boat will likely diverge more from the original lines plan that a traditionally built boat would, but as long as the boat is fair this doesn't really matter.

Getting back to the original question...I've built a couple of half models of hard chine boats. One I made by simply cutting it out of a solid block. It's long enough ago that I don't remember just how I did it but I probably flattened the back, laid out and cut the sheer and bottom and then laid out the widths on those surfaces and cut the side. This worked fine and gave me a model that was accurate enough for me to figure out whether I could get the sides out of one sheet of plywood, which is what I wanted to know. This model was small enough to be gotten out of a 2" thick piece of wood. If I had wanted a larger model I would have glued together more pieces of wood. This is roughly the "lift method" but the shaping process is different so I would simply make the lifts somewhat oversize and roughly cut them to shape rather than trying to precisely work out the shapes of the lifts so that you can use their edges as guides to the shaping process.

Another time, more recently, I built a hard chine model using 1/32" plywood. I used the lines on the body plan as the templates to cut out station molds and glued them down to a backing board and then cut and fit panels to these molds for the top (deck), bottom and sides. This gave me an accurate enough model to get a feel for whether I was happy with the shape of the boat from a visual perspective and it gave me a feel for how the plywood would bend around the curves of the boat.