lofting basics help

Yes, lofting is usually full scale, but can be done at half length and full beam. During lofting the fairing errors left at the design stage are corrected.

 

I believe the fairing error is amplified when the CAD model is scaled down; cluing the designer to his/her error.

RX Thanks for the link, I will tinker with the software.

Off to the shop
Cheers,
Dave

 

It is the opposite, as the drawing is enlarged so are the errors. Computer screens don't have enough definition.

 

oops... my mistake the % of error remains the same

Dave

 

HO229...That is the reference to the navigation tables that we used to use with the sextant, I thought that you may be an old sailor.....not to worry mate.

Make your sections on the computer to the same scale that you are making your model, print out a million copies and then cut then out so that they are a set. You can then glue each section onto the plywood frames and cut them to exact size, taking regard to the thickness of the plywood being used, the cutout is done from the bow to the centre and then from the centre aft so that you do not get the plywood in the road. Also remember the thickness of the planking material, it is called the moulded depth, but it has to be withdrawn from the sectional frames so that the job ends up the overall width required.

 

If you can live with lumps and waves that is a possible method. Printers add a fair amount of error, paper distorts (more errors) and lining up the sheets of paper will be approximate at best (even more error). All these errors will accumulate and multiply each other.

 

....depends on how big the scale is gonzo, many parts will actually only be A3 and A4

 

If it is so small it is different. I was thinking of something bigger. The accuracy of printers is usually not more than fair though. I suggest you add a horizontal and vertical grid to verify distortion.

 

I want to thank everyone for all the helpful comments and add a few closing thoughts


Quotes from the book Lofting by Allen H. Vaitses

Quote
“Unfortunately, the potential for perfection has to be shared with the designer and the boatbuilding crew. The loftsman cannot make a good boat from a bad design, nor can he ensure against sloppy workmanship.” End Quote

Been in aviation and manufacturing for 30+ years the statement is spot on regardless of what discipline you practice.
Brings to mind the whole dimensional stability of the design and materials used; how fair can it really be when temperature, humidity, and how the lumber is sawn.

Quote
“I remember feeling a little let down when, after years of carefully lofting each custom boat I built, I read a remark by L. Francis Herresholff that it is not really necessary to draw all the long lines of a boat and fair them; one could just draw the profile and the body plan, pick up the backbone and moulds of the stations and go ahead with the building.” End Quote

I have been building models per Herresholff comment for some time prior to reading his comment; I found it to be fairly accurate.

Cheers,
Dave

 

Herreshoff supervised the construction and corrected errors. They owned the yard.

 

Landlubber in post #6 had it about right if it is a model, just build it because no one is going to get a micrometer to see if it is "true" scale. At this scale, a 1" radius on the skin is just a roundover of a sharp edge and any "small" differences curves in the full size design will not effect the aerohydro of the model, especally considering the huge increase in power to weight provided by the model. Just remember scale effects of the empanage which will need a check.

What I would do is take the stations you have at the correct scale, subtract the planking thickness from them, add a keel and stringer cutouts, but not chine stringers. Set up, stringer, and skin the fusalage allowing the skin to butt at the chines using carved blocks for the tight curved areas (this is very similar to making a ship model which can have the same problems). Paint it with a couple of coats of autobody filling primer paint. Knock off the hard edges at the chines and sand smooth then wax the fusalage and splash a female mold.

 

Hello all,
I took some of your advice and built a half model; what may I ask is the best jig to take the lines off the model?
The book "Lines lofting and half models" has an example but the drawing is a bit difficult to interpret?

Thanks in advance
Dave

 

My idea would be to setup some kind of parallel sliding arm in a bit of U section aluminium, with a depth marker mounted on a vertical arm - measure the depth marker with a Vernier Caliper or similar instrument for best accuracy, or you could mount a caliper on the vertical arm directly. Dont try and just use a rule or similar 'measuring stick" as the inaccuracies would be too great.

Simplistic sketch attached in case it helps

 

Long lines, (waterlines, butts, diagonals) are all, at simplest, most useful for fairing. The smaller the scale (at the drafting table) the easier it is to induce, or miss, any errors. That's why lofting, to full scale, is more accurate. A computer screen is about as small a scale as a human can draw in. Years ago, i visited the office of a shop that was pioneering in the field of computer drafted tooling drawings for custom yacht yards. As i spoke with the owner of the company, i looked over his shoulder and one of his draftsmen was sighting his computer screen across the edge of a straightedge! In otherwords, at the scale he was working at, he couldn't tell if the straight line he'd just composed by punching keys was truly straight, or not.
The critical shapes in any form, are the transverse sections - body plan lines - add the stem profile - the transom profile and it's expanded shape, and no, technically, you don't need full development of your long fairing lines. But settle for that, and you miss the intimate relationship that you can establish with so many of the other details of your drawing, not to mention the wealth of information, and the "problems" you can solve during development of the construction drawings.
From your photo posts, it's pretty obvious that you have a handle on construction, sculpture, and developing fair shapes in 3 dimensions.
Try leaving off the computer, for a while and committing yourself to hands-on lines drawing, at whatever largest scale you have room for. I suspect, again from your photos that you can already do whatever you need to do to get to the physical construction of whatever object you want to build - but you're "letting" the computer do some of the most rewarding work of developing your drawings.
As to the best method for taking lines off of a model - flat surface with centerline and station lines. Model centered. Bridge frame, with centerline and butts drawn on overhead portion of bridge. Set frame over each station.
Record keel hts. and butt hts. from overhead. W.L. half-breadths from legs of bridge in. Take the offsets, use to draw profile and body plan. Use tick sticks, or dividers, to pick up offsets from body plan, use these to fair W.L.s and Butts, As you fair the long lines, correct the body plan. Use faired diagonals to justify final shapes in body plan. Try it, you'll like it.
To clear up any questions, find a lofting class to attend. Hands on almost always works. Been teaching this stuff for 12 years and only 2 students managed to grasp it all, unaided, out of the books you mentioned.

 

I have noticed that the control points in FreeShip, which is a NURBS-based application, effects the shape in the same way as applying force to several places along a batten, with the ends fixed. It is so alike that I can use a batten to determine how many longitudinal points I require to define a given hull shape.

In FreeShip at least, if you have too few points it can't be done and if you have too many it becomes almost impossible to handle the redundancy. It is much quicker to establish the optimum number of points with a batten than trying to get it right in the application.

I don't know how well this translates to the applications you are using but it might help ...