making a scale cardboard model

I would never do this. I would always buy the plans or buy the boat. There is too much at risk even after extensive mathamatics the boat can be down on the nose or some thing else equally as horrible.

Like said many plans to buy.

A long boat would be be more forgiving in this respect, the types of boat to be left on the beach but a small dumpy trailer boat is full of risk and could easily build a dog.

I bought a small fibre glass dinghy from the local fishing boat builder . It was oustide in the car park and was the last the made years ago --I paid them 30 dollars for it. I can offer a whole string of similar purchases during my lifes boating experience.

30 dollar will not cover your epoxy.

 

- I don't think any reasonably sized model would be accurate enough for plank developments to use for butted seams like S&G. Lapstrake maybe but they're usually built over a mold anyway. I have read of half models being used to plan the plank lines though.

- Like you I use FreeShip; the local printshop couldn't work with the files that FreeShip put out but they did full-size blow-ups of my plank development printout and dimensionally they were spot on.

I sometimes use FreeShip's plank developments to make a very small card model as a sanity check, but a larger model is needed to make reliable conclusions. Currently I am developing a design using radius chine planking which is new to me; not satisfied with that I have a new approach to the method. Before I proceed with a full-sized build, I will make certainly an accurate scale model to check out the entire concept, about 1/6 scale, using 0.8mm model aircraft plywood. The hardest part of modeling at this scale is cutting darts or slits which are needed for this design to aid bending, but I now have a saw with a 0.010 kerf which is 1/6 the kerf width of the saw blade I normally use.


Getting back to original poster's questions, slight modifications of a design - perhaps 10-15% dimension changes - is probably as far as you should go. A bigger change may well ruin the behaviour of a sound boat design.

 

$30 would not cover the chipboard for the moulds.

Put me down for a dozen !!

 

Hi Frosty, from what i've seen boats are much cheaper that side of the pond. To give you some idea of second hand boat prices over here.

8ft punt/tender = $500-$600
12ft lake boat with small engine = $1500-$2000
17ft Dory style boat with 50hp engine = $4000-$5000
23ft inboard diesel fishing boat (fairly old) = $15,000-$20,000

As you can see building your own boat is a much cheaper option!

 

Terry, The fact remains that many of us have done exactly what you say is not reasonable. It does require some care and often lofting is needed but these things have been worked out long ago. Some of us just don't do computer design and will not change. Nothing against software, but we are old dogs and that is a new trick.

 

Im not over the pond you can drive here from London.

No No No it was in the car park --it was old -- it was unfinished,-- it needed painting, they didnt want it, only a ***** would buy it, it was the last one -- I negotiated.

Ive bought many in Uk the same. And Dubai, Hong kong, Singapore.

Every factory has ---un-perfects, bubbles, inperfections in sparkle.

I know of a free Roberts in Brighton marina engine sails take it...

I'lle give you his e mail



You need to look. ---up to you....

 

Were you were able to cut planks for a successful S&G starting from developments obtained from a card model? I wouldn't have thought that . . . I know a gap of a millimeter or so can easily be bridged with good 'ole epoxy, but I've found if the plank has to be trimmed more than that with a plane to close up a seam the shape of the boat starts to change. Of course, my interest is kayaks and canoes and anything long and skinny is extra sensitive to errors.

 

Terry, Its likely that slightly different procedures are used by different modelers but you are correct in that taking the shape of model panels straight to layout for full size can make for unfair shapes at the chine. That is where the lofting batten comes in. I use the foreshortened method of lofting to get fair panel edges. Combined with some practice working with plywood panels, this works very well. Lofting is usually done before the model is built also.

 

The skiffs in the Outer Banks were built with slab sides. They first cut the angle for the bow and stern and attached the bowstem and transom to it. Then they spread the sheer and chine with a couple of sticks until the shape was right. Finally, the chine got beveled and a plywood bottom got added. I worked with Sonny Briggs and Cannady many years ago on some. Cannady would sometimes make flared bows though. You can play with these kind of shapes cheaply by building them in cardboard or thin plywood.

 

Cheers for the offer Frosty, but i'm not in the market for a sail boat, I'm sure someone out there will gladly take it!

 

Very good designs. Good idea to remember them. But they are displacement boats.

 

As some have said, the best is to buy plans. Some members of the forum have very nice plans at a very moderate price. Wooden boat mag has plenty also.
If you absoutely to try models, use aviation plywood 0.8, 1 and 1.2 mm. It's the lone material to behave similarly to full scale plywood at a scale of 1/12.
0.8= 3mm 1/8" full scale
1= 4 and 5 mm 3/16" full scale
1.2 = around 6 mm 1/4" full scale
Card board, paper, sheet metal and others are not accurate, as they do not ploy like plywood.
Download the Gougeon Brothers on boat construction and read it!!!
You'll be surprised by the amount of knowledge and experience needed for designing even a small boat. Mistakes are pretty costly...

 

For small boats in hard chine plywood, the "big" softwares are useless and clumsy. I't's faster to make a hand drawing, make the calculations with a spread sheet and verify the whole with a model. After all, the design types that work in this kind of boat are not many. If I remember well Hull Designer deals rather well with hardchine plywood boats.

I do use sometimes softwares, but instead of struggling with the nurbs for a useless fairing, I prefer to go to real life and make a model at 1/12. Very few softs can use real splines, handle pressure curves and be able to predict the behavior of a plywood panel or even a composite panel in the case of a GRP hard chine boat made without mold.

Plywood may ploy and compound badly or nicely for a few millimeters at the frames. No software will tell you than a panel at the bow has no intention to lay on the frame unless using 125 screws with a resultant "can oil"...And if you stay at the "only true developable" panel the boat will be truly ugly, very boxy.

I was loftman and naval carpenter long time ago, and experience showed me to not trust blindly any plan. But lofting a motor hard chine boat is easy.

 

You can get a lot of information from a accurate model even 18 inches long. The true waterlines with different displacements and distribution of weights, the Bonjean curves, and if you're careful the static stability curves.

Herreshoff worked with models and half hulls. Nigel Irens drawn some of his trimarans from a piece of wood modeled at scale. I saw him modelling Ilan Voyager in 1987 at Saint Philibert, the final drawings were made from the sculpted model... A model tells a lot to a "visual" guy .
In old times the developped panels of the ships where drawn from a model made by qualified modellers in the shipyards. They didn't work at 1/5 or 1/10 for a 200 meters ship but at 1/50 or 1/40...

Speed, seaworthiness and consumption are verified with models in water tanks, with very accurate results. So models are not toys!!!

Delt Ship is useless for a small plywood boat, it's made for ships in ductile or molded materials with a lot of curves like a tug boat. The time needed to learn this soft is so long, that is meaningless to use Delf Ship or Free ship for such simple boats. Hull Designer is more adapted for this task...

 

Does DelftShip define developable/conic surfaces? Does it unwrap them accurately? It appears (at another thread) the answer is yes, but it would be of interest to me if you know.

It seems to me once you have panel shapes defined it makes sense to take a few hours to build model both to confirm that the panel shapes are accurate and to confirm you like what you see before building full scale. Historically a number of top drawer designers, including Nat Herreshoff (who was MIT trained and no slouch as a draftsman) designed using models.