Plank (and ribband and frame) deductions

I understand the technique of using 'perceived' plank thickness rather than actual nominal thickness when deducting from lofted sections to get your mold shapes. Recall that you draw in normal lines on your lofted body plan sections and use triginometry with frame spacing and distances between adjacent station lines in the body plan. But when one is going to build upside down and bend frames outside and therefore needs to deduct frame and ribband thicknesses as well as planking thickness, the information seem a bit unclear. The way I read Chapelle and Stweart, you just deduct these nominal thicknesses but it seems to me that I should go through the exact same same process as when deducting plank thickness and trig it out with this TOTAL thickness.
Comments please.
Ed

 

The fool proof method is to skip the math and just use actual samples of the stock as marking guides.

 

Sorry folks for posting too short of a question for anyone to offer real advice. When I say "trig" I only mean the drawing of a simple graph on a separate sheet of plywood and using the station spacing along the horizontal and the actual measured distance in the body plan up the vertical as my two legs of a right triangle and drawing the hypotenous. I then just use a piece of wood to simulate the combined thicknesses of plank, frame, and ribband to walk down this new line to determine the perceived thickness at this particular spot on the mold of interest and deduct accordingly. I did this at enough places along the station to get a fair curve and built the mold accordingly. My question was just whether all three members should be included in the deduction if my boat is lofted to the outside of the hull and I am building her by bending frames outside of the ribbands.
Come on guys. This is a real "salt in your hair" question for real boatbuilders and I was hoping that there were more of you out there to help out an amateur in real need. Where are all the wooden builders (Probably out there building boats which is what I should be doing right now I suppose). Is there another forum that I should be using? I am making quite a leap in project size and investment and can't really consider this secoond boat another learning tool. Thanks again folks. I hope that this expanded description of my question helps.
Thanks PAR. I know you know the answer but coundn't comment very well with my incomplete description of my technique.

 

I'm still not completely sure of your needs, but on the lofting, you should have deducted for planking thickness at least. A builder showing signs of forethought and planning will also make their molds to the inside of the bands. You have fair molds, so what's the issue? Can you post a sketch of your dilemma? It's quite common to have to shim bands on the molds to produce fair sweeps. Is this your question?

 

I too am having trouble understanding the question. I think what you want to know is, how to draw the station molds that you will then set up on a strongback with ribbands bent around them, so you can bend the hull frames over them and have the outside of the skin end up per the design stations which are specified to the outside surface of the hull.

If that's right, then the obvious, simplest and most direct way is to draw the station, then the skin, then the frame thickness, then deduct ribband thickness and transfer the resulting mold shape to the wood.

Are you trying to go through the above route using a spreadsheet, or maybe bypass the drawing steps and use trig? I think that, by the time you have done one station as above, you will see how that works out. But of course I could be totally out in left field ...

 

draw the station etc, i agree its simple & clear & well put

 

It is important at the bow and starn if it is a double ender to calculate the thickness the plank will have at 90* from the centerline. Because the plank is at a increased angle the measured thickness will be greater, therefore the deduction needs to be greater

 

longfellow.

The mould frames can be simply cut to the lofted outside line as per your drawings, the moulded depth is subtracted, as are the ribbands of the mould, and in your case the statiion frames, using actual pieces of the build wood as edges to draw onto the station frames that you have created. Before these frames are put in situ, do the deductions, just scribe the wood pieces as you are going to use them, cut to the insid line, there is NO maths involved in the process...forget the squares and hypotenuse crap, you are somehow trying to complicate a very basic and simple construction it seems to me.

in real life, the ribbands will be adjusted to become fair, that is what we are trying to do eventually, that is to have a fair surface, sometimes this will result in wedges or subtractions of the inner frames to make the job fair. Yes the outside drawn lines will be altered, so therefore the shape will be altered, but it does not matter, what matters is that the end build looks nice and true, bugger the numbers.

 

Listening longer before a contribution makes sense........... ever so often.
Thanks Lubs.
btw: I have a really nice qoutation if the engine is still at your order list.

Regards
Richard

 

Apex,

I would dearly love to buy it, but for now, we have to do some "money gathering"...you know the story. Ta.