Deduct for plank, frame and ribbands..

[SparkG]

Planning to build the Tomcat 12.5'catboat...been studying the plans, have a question on the moulds...what method are you guys using to deduct plank thickness, frames and ribbands? I've seen many articles and plans that state the offsets are to the ouside and must be reduced but none really explain how to do that. I've seen an article in an old WB from '83 that used a calculator with sin/cos/etc...seems pretty overboard ;) I've got Lofting by Alan Vaitses, other boatbuilding books - doesn't make sense.
The Tomcats offsets are to the ouside of the planks. I'm planning to strip build the hull - 3/4" x 1/4" strips like a canoe - so I have to deduct the thickness of the strip (1/4"), frames(3/4") and ribbands(3/4"). Why not just lay out the moulds, cut them out and then make a quick tool that will glide parallel to the edge in by 1 3/4" and perpendicular to the edge, mark it all around those cut moulds and trim again? Wouldn't that be accurate?
Or what method do you guys use?
Thanks for any responses - I searched here in this forum and really didn't find an answer previously posted.

[Bill Perkins]

Sometimes just saying things in a different way can help ,so I'll give a try . The molds are perpendicular to the keel. Where a line drawn normal to the curve of the planking is also perpendicular to the keel your method will be accurate .This will happen in one spot near the center of the boat .On a catboat the planking sweeps in especially sharply toward the centerline at the bow .The thickness through plank ,frame ,and ribands to be deducted from the mold is measured perpendicular to the keel, as the mold is set .It's significantly greater than the thickness measured normal to the curve of the planking ( the simple addition you did ).

[ 06-23-2005, 10:41 PM: Message edited by: Bill Perkins ]

[John E Hardiman]

here's the easy way.

1) Lay the lines out full size and have all the planking, frame, keelson, and ribband stock faced and sided

2) Take a short piece of madeup planking/frame/ribband and lay in on the inside of the waterlines at the mould stations in the plan view . What you are looking for here is where the inner side of the ribband cuts the mould station while the outer side of the planking follows the waterline.

3) Scribe a line at the inside of the ribband stock on the plan view for each waterline on the mould station line.

4) Pick up this offest (and the bevel on a bevel board because it will not be the same as the outside plank/mould intersection) and transfer it to the section view for each mould station.

5)Do the same for the keelson structure (I assume as you are strip building it will be planking/frame/keelson right to CL with a seperate external keel) but only this time in the profile view for the keelson only (again pick up the bevel).

6) Fair the mould stations to the new inner offsets, cut the moulds and bevels (here is where the bevel board comes in).

NOTE: For moulds forward of the maximum section, the station line will be on the aft side of the mould and vice-versa for moulds aft of the maximum section.

[SparkG]

I think I've got it thanks to you guys...
Bill - what cleared it for me was your line "The thickness through plank ,frame ,and ribands to be deducted from the mold is measured perpendicular to the keel, as the mold is set".

John, your method of deducting totally concurs with that.

It all went off like flash sitting here at work!
So when these guys measure off a finished boat design (from outside the planks), they always measure, say, with a tool at waterline height, "touching" the planks at each station and perpendicular to the keel and parallel with the stations/moulds.

Also John, I presume on the profile I just need to make an adjustment from the keelson down the difference because with the boat built upside down that's the only height that changes - the waterlines are always the same...man that's all hard to describe, guess everyone visualizes it in their own way. It all has to do with how the designer came up with the offsets.
Thanks a lot fellas...you can't even start anything if you don't understand that!

[ 06-24-2005, 04:56 PM: Message edited by: SparkG ]

[John E Hardiman]

SparkG;

Just understand that the method I gives assumes that the steam bent frames are not beveled and lay along the ribbands. This also means that they are not are not going to be perpendicular to the centerline/keel at the bow and stern (they will be canted).

Just making sure that you understand this. If you want frames perpendicular to centerline, there are other methods.

[SparkG]

Thanks John...that's what I'm going to do - steam bent frames on the ribbands, not beveled and layed up to the ribbands.
Speaking of that, I presume you have to induce a little twist to those frames as they are to be parallel to the moulds/perpendicular to the keel and floor. I've read that at the bow area you can start to cant them a bit for the steeper angle but still flat to the ribbands...I forgot what they call that.
Thanks.