Question: Lofting from a half model

[Robmill0605]

I was hoping that the old school design guys/gals here on the forum could steer me in the right direction. Here is what I'm interested in doing. I have a project planned which is building a plug for a streamliner type motorcycle body.
While I'm fully aware of all the computer CAD programs out there, to be honest they are just over my head. I've tried them like Delfship etc and I just do not get it.
Which leads me back to my question.

How do I take the lines off a half model of the body which I will carve to scale ?

I have the book by Walter Simmons , Lofting and Half models. etc. I've goolged it and there seems to be no info that I can find.
I do not want to have learn a whole CAD program and besides, my wife won't let me download one since this is "her" new computer.:rolleyes:

I'm proficient at lofting, carving etc, but this computer thing .....

so what's the no tech old school way of taking a table of offsets from a model?
Thanks!

[rbgarr]

One way would be to make your carving blank out of layers ('lifts') of wood to your scale and dowel them together without glue. Carve your model and disassemble the lifts. Trace the outlines of each lift (perhaps on graph paper at the proper scale) and measure for scaling up.

Plug away!

[Jon Etheredge]

I have used a method that is basically the same as the suggestion from Dave...

I carved a model at 1 1/2" scale. The model blank was glued up of 2 pieces with the joint in the blank lying on the guesstimated LWL. After the model was carved I used a fine tooth back saw to cut the model into pieces on station lines. Then I layed each section on a piece of paper and used an architects scale to lift the offsets from the traced section. The offsets were measured relative to the glue line (LWL) to provide a common reference for lofting.

I have also heard of using a small lead bar or copper wire to lift the sections from the model. You press the bar/wire against the model at the station line to pick up the shape. Then lay the bar/wire on paper to trace the section and measure the offsets. I haven't tried this myself.

This is the boat that resulted from my experiment with building from a model:

http://www.intergate.com/~jethered/images/catboat.jpg

[Robmill0605]

Thanks for the suggestions,I seem to recall that there was an article on taking offsets from a model in Woodenboat from a shop made machine a long time ago?, I thought of the suggestions above and that I also might use a molding gauge which is used in carpentry to take a molding profile from an old crown molding etc. It's bascially a series of very closely fitted wires that when pressed against the half model should take off an accurate station shape from the model without cutting it up.
I cannot find an article online about the methods used in the days before the computer.
I know that modeling this in CAD has made this skill go the way of the doe-doe bird, but I thought it would be fun to do it this way and I would add to my learning curve. as soon a I can I'll take a class on CAD to take advantage of the great moldeling programs out there, but in the meantime
I guess I'm just stubborn enough to want to do it the old way.

Do you think a1 1/2 scale would be the best?

[Jay Greer]

Nice looking cat boat! It reminds me of a Herreshoff
12 1/2. Actually Nathaniel Herreshoff designed all of his boats from models. His son L. Francis mentions, in his writings, that his father carved the model for one of the cup defenders in one evening! His method for taking off lines consisted of a
pattern tracing device, of his own design, that could trace the sections and water lines on to a drawing surface. I have used the strip lead method as well as water line lifts on my own designs. I am toying with the idea of lifts that will allow the sections to be drawn as well. The strip lead method does require a certain amount of correction before and accurate set of lines can be drawn

[John Meachen]

One of L. Francis Hereshoff's books contains a photograph of the device his father used for taking sections.There is another way but it takes a little time and I will try to describe it.You need to build a vertical drawing board,large enough to contain any transverse section you wish to record.It will need to be on legs that are higher than the highest part of the model.You then add a traversing carriage that contains a vertical pointer which could be likened to a slat from a picket fence with either a very fine end or one half of the tip removed parallel to the centre of the slat.You need to drill a small hole that just exceeds the height of the bottom edge of the drawing board with the pointer touching the base that the model rests on.
The procedure is that you place the half model securely on the base and draw round it.You then select the points that you need a section for and mark where they occur on the outline you have drawn.It's a good idea to label them at this time and then you tape or clip a suitable piece of paper on the drawing board.You then position the vertical drawing board over the section you are interested in and trace it with the vertical pointer while pressing a pencil,located by the hole in the pointer, against the paper on the drawing board.Repeat the process for all the sections you believe you need.
Having extracted the sections,you can enlarge them either by photocopying (beware,the lines get fat and fuzzy) or by building a pantograph.Good luck and if you need a sketch or more detail,let me know but its getting late here and it may be a while before I check back.

[Jon Etheredge]

I thought of the suggestions above and that I also might use a molding gauge which is used in carpentry to take a molding profile from an old crown molding etc.


This would certainly work as far as picking up the shape.

You do need some way to align each station relative to the others on the paper though. With a lift model or model that is sawn up you will have an edge that is straight (the back of the model) that you can use as a reference.

Might require some ingenuity if using a molding gauge or the lead strip method. I'd be interested in hearing from Jay Greer on how he handles this part of the process when he uses the lead strip method.





I guess I'm just stubborn enough to want to do it the old way.


Yeah, I did it just for fun myself. And to get a feel for how it used to be done.

Both goals were met.

I'd encourage you to try it out since you have the interest.




Do you think a1 1/2 scale would be the best?


The scale doesn't really matter (within reason).

My model at 1 1/2" scale ended up being about 18" long and that was a convenient size to work with.

A bigger scale will increase the accuracy of the offsets somewhat. A larger model may also be easier to fair.

In my opinion a model in the 18-36" size range would probably be easiest to work on. Oh, and you also want to choose a scale that is easy to work with when picking up the offsets. For me that meant choosing a scale that was on a standard architects scale.

You will need to loft the boat to pick up the mold shapes anyway so you can make any needed corrections to the offsets on the loft floor.

[Robmill0605]

Great info everyone, thanks so much. After the points made here, I think I'll build from a scale model and use a fine saw to saw it into the stations and then scale from there. Should be interesting.
Thanks !

[rbgarr]

Be sure you have a fine panel saw and guide like a mitre box provides for sawing the stations. I tried doing what you suggest once freehand and it didn't come out as well as I would have liked. It was a half model and would have lent itself to a homemade mitre box arrangement.

Will your carving be a 'half model' or a full carving at a smaller scale? I'm not exactly sure what you mean by a 'streamliner type motor cycle body'.

[Paul Pless]

this is a streamliner
http://www.streamliner.com/bonn1.jpg

[Paul Pless]

Rob, what are your intentions? Are you planning an assault on Bonneville? How 'bout some engine details, etc, etc?

[Robmill0605]

this is a streamliner
http://www.streamliner.com/bonn1.jpg


Pretty close.

The project is a reverse trike. The front suspension will be indy car stye. It's powered by a motorcycle that has been modified to a custom built chassis. The 'trike' will seat two people andhave a streamliner body but instead of a canopy will be open like an indy car.
Take a look at this video and you will see a reverse trike that this guy built
http://youtube.com/watch?v=LxjfvH8P7ZA

He has another video of the trike at speed.

[Robmill0605]

The trike at speed
http://youtube.com/watch?v=WdB51jLD1dE&feature=related

[Paul Pless]

Interesting project Rob. What model motorcycle are you using for a donor? Will it be streetable? Registerable?

[Robmill0605]

Interesting project Rob. What model motorcycle are you using for a donor? Will it be streetable? Registerable?

I'm building the front suspension now, and looking for a donor bike. There are a lot of the 'rice rockect' bikes that have been wrecked in the front end. :rolleyes:
Since I do not need the front end, I'm waiting for the right bike which has to be over 1100cc. The motorcycle acts only as a power source.
I intend the streamliner to be fully streetable and because it is still considered a motorcycle because of the 3 wheels, registration is no problem here in Fl. This thing should be a real head turner and a lot of fun to drive. I have a set of plans from the Indycyle for the geometry and construction of the suspension and framework, so I'm really just modifying it to fit a body of my own design which will be based on a Streamliner.
Should look like an F-16 on wheels!
I'm toying with the idea of a removable canopy so that it can be driven anytime.

[soba]

I work in the motorcycle business, and let me make some powerplant suggestions...

You definitely want an inline 4. Twins shake alot, and while some of the litre-sized v-twins (rotax, ducati, maybe an RC 51 motor) have sufficient power, they are just not as smooth as a 4.

Price per pound, the best bet is a Suzuki long-stroke 1100 cc motor. 1127 cc, to be exact. Out of a GSX-R 1100, not a Bandit (GS1200F). They are plentiful, easy to work on, and proven in your application. If you have the money, spring for the Hayabusa version (1340 cc) which is used for small race cars (such as Caterham Super 7's and older mini's).

Failing that, I'd look for a Kawasaki ZX1200R.

Pushing 2 people on a trike, you want the longest legs you can get with a close-ratio gearbox. That's why I have suggested these bikes rather than touring bike engines. A GL1500 engine or GL1800 (goldwings) would probably also work, but you won't have the F-15 effect anymore. And chain drive will serve you better than shaft. Lighter, too.

Remember if you are going to put in a carburated motor (rather than fuel injected) a smaller carb bore will give you more torque than a big bore, so don't go swapping GSXR 750 carbs onto a GSXR 1100...won't work. At any rate, those are the motors I'd look for.

Have you seen the Can Am Spyder? Couple of them around here...


soba

[Sailor]

Just make sure the model is big enough to be accurate.