building a model before the real thing

I am in the process of disigning a 20foot open boat to be built from laminate wood and epoxy on oak frames.

my plan is to build a scale model first to test the disign. However i am not quite sure of the best way to go about this?

Also how do i go about scaling the weight to see where the water line will be? Since the model will probably not be made of the same material as the final boat.

Any idias or other guidance greatly appreciated.

 

Get Weston Farmer's book "From My Old Boat Shop" from a library, Amazon or ABE Book Exchange. He goes into great detail on the subject. Usual scale is 3/4" = 1'.

 

20' is already a model length!
just kidding.. but you won't get much info from a tiny model. by the way, what kind of tests do you plan to perform??

the weight of the model can be obtained dividing the weight by (length scale)^3

 

From Westy Farmer's many years of this practice:
Use 1/16" photo mounting board, a stiff cardboard available at photo stores.
Plank with 1/8" balsa.
Usual size is 3/4"=1' or 1/16th scale, or linear ratio.
Scale speed is square root of linear ratio. Sq root of 16 is 4 so towing behavior matches at one quarter real speed. Tow model at 4 mph to simulate 16 mph.
Scale displacement varies as to cube of the linear ratio, on 3/4" scale is 16x16x16=4,096. If after trimming your model weighs one pound, the real thing is 4,096 pounds. At this scale a Lincoln penny is 25 pounds, so it's easy to make a scale engine. Tanks are stiff paper full of paraffin wax.

 

These shots show the process.

 

More on displacement models at 1:16 scale.

 

1/16th scale will not make a good model for a 20' boat as it will be only 15" LOA. Most who make towing models say that you need a model length of about 4' to keep scaling effects from distorting the results too much. That is the length I use and let the scale vary to make it so, which for your 20' boat would be 1/5.

Of course if this is a building model only and not for towing, it does not matter much what it is made of or what scale it is built to. Getting the weight and distribution right for a non working model hardly matters. Trying to get the scale components weight proportional to the real boat would be nigh impossible and is not useful as far as I can see.

Good and durable results can be had with 1/32" and 1/16" plywood available from Aircraft Specialty Co. 1/8" doorskin plywood can be used for hull panels that are not too curved for it to conform. Much more durable than cardboard or balsa and the 4' size usually offers enough weight allowance in the model for protective paint finishes and corrective lead weights to trim the hull to its lines and proper displacement for towing testing.

 

I use a variation of the Farmer method as shown to me by Bruce Bingham many years ago:

www.pkboatplans.com/Display_Model.html

 

Cheapest way to get doorskin is to go to your local recycled building materials store and buy hollow core doors. They can usually be had for a couple of bucks and a little work will give you two large sheets of usable skin.

I'm getting ready to do a model for my kayak tender design and am going with 1/12 scale for simplicity. The simpler you can make the math the less opportunities there are to screw something up.

 

This is not a towing model but a flotational model, a proven and ***-saving technique to back up normal calculations practiced by a pioneering NA for over 50 years, that he used on everything from 20' motorboats to large aluminum yachts to get accurate loaded waterline, CB and CG. When finished vessel was actually launched (we're talking hundreds of vessels here) he said often the corrected model figures wound up being more accurate than the most painstaking calculation, and this from a man who worked as a practicing NA in the offices of Elco, Newport News Shipbuilding, Annapolis-Vosper, Burger and Palmer Johnson, long-time design writer for the National Fisherman etc. This is credibility, something to listen to and a valuable adjunct to drawings and calculations.
A towing model is a different thing, and not what the poster asked about.
All square and cubing rules of scale apply to whatever you do, in any scale.

 

Thanks everyone for a great response, real helpfull!
I will definetly try to get my hands on Weston Farmer's book "From My Old Boat Shop".

Although my main purpose is a flotation model (to establish where the waterline will be and position the engine), i would like to be able to tow it too.

The disign contains a lot of curves and would be extreamly difficult to plank conventionly. Therfore my plan is to two layers of diagonal 5mm larch planking (at 90 degrees to eachother which should bend easily) laminated and coated in epoxy and glass fiber.

For the model to give accurate results it presumably needs to be made from a material of similar density.
I can make a fiber glass model but how do i calculate the thickniss necessary and how accurate dose this realy have to be?

Alternativly can i calculate a correction to apply to the draft to allow for the actual calculated weight of the compleated hull, based on its surfase area and the density and thickniss of the shell?

 

For towing, you do NOT have to make the model in the same density and weight distribution as the full scale boat. Just make it light enough and correct trim and displacement with small weights distributed to float the boat on the designed waterline. This allows for adjust of displacement over and under the design value. Very few final builds come out to the original displacement for many reasons, including additions, redesign and miscalculation.

 

Thanks Tom,
Any idias on a float test; i know where i would like the waterline to be but i need the model to show me where it will be in reality so that i can adjust the design if necessary?

 

Making a model of a proposed design is a sensible thing to do and is often resorted to by professionals. I’m not a professional boat designer myself but for one of my early ideas I made a 1/12 model which worked, so did the full-sized effort. I would not have dared go full-size without the insight from the model.

Most things scale as you would expect, weight as the cube and so forth. Be aware of the importance of getting the CoG in the right place. Note that hydrostatic drag, sail area and wind speed do not scale simply.

If you are inclined to use a computer you can use free software like FreeShip; it takes a bit of practice but will tell you the most important things like displacement, draft and stability, and also provide an estimate drag vs speed, as well as useful stuff like the plank developments - which are essential if you plan to build using the stitch and glue method. Carene2008 is more limited with regard to hull shape but is easier to use (if you can find it on the Web) handles stuff like positioning cargo, passengers and engines. Either will allow you to enter skin density to estimate the build weight. Learning time for Carene is probably less than model build time.

 

The waterline is usually part of the design which offers the performance goal and the weight distribution adjusted to make it so. If designed trim cannot be achieved within the designed weight parameters, then then some redesign is necessary. In other words both CB and CG should be together as part of the design not a mystery to be solved after the hull floats. You get there by calculation, models per Bataan (which I have not seen) or computer programs. All of these mean several trips through the design spiral. There are too many down by the bow, down by the stern or listing boats already. No need for any more.