Understanding Offset Tables

[180]

I've got Ted Moore's Canoecraft book, and I'm having difficulty understanding the Table of Heights for the "Bob's Special". There is a dimension for each Station for the Sheer, Butt 2", Butt 4", all the way to Butt 16", and then the Profile. I don't understand what this means. I think I understand the Table of Half-Breadths. Thanks for your help.

[Carl Simmons]

I don't have my copy of canoecraft with me but table of offsets has two dimensions (half breaths and distance from the base line (usually up).

I'll check my canoecraft tonight and see if I can better answer your question.

Carl.

[Bruce Taylor]

Butt lines are, essentially, vertical cross-sections of the hull taken at various intervals from the centerline.

Waterlines are horizontal cross-sections, taken at various intervals up from the baseline.

Diagonals are diagonal cross-sections.

[180]

I'm still confused... :confused:

[Bruce Hooke]

As Bruce Taylor noted "Butt lines are, essentially, vertical cross-sections of the hull taken at various intervals from the centerline [i.e., parallel to the centerline]." Somewhere on the plan it should tell you what the spacing is between the butt lines. I'm guessing that they are every 2 inches based on their names. Normally the heights are measured from a baseline that is ABOVE the top of the canoe when the canoe is upright. So, with the canoe upside down in the position in which it is built this baseline is underneath the canoe. Most lines plans will include a horizontal line that represents this baseline. You could think of this baseline as the floor of your shop, although in actual fact this would put the canoe too close to the floor for it to be convenient to work on it. So, these height measurements in the table of offsets are the distance from that baseline (the floor) up to the sheer, centerline or butt line in question. In other words, for the butt lines the point identified by a given height measurement is the place on the surface of the canoe where the butt line and the station line intersect. Ya follow?

If you understand the width measurements then all you really need to do is rotate the same concept by 90 degrees.

[180]

Waterlines are y coordinate and Butts are x coorodinate? So Butt +2 and a Height of 6 would be coordinate (2,6) measured from the centerline of the mold. Waterline of 10 and a half-Breadth of 14 would be coordinate (14, 10) measure from the baseline. Is that correct?

[Bruce Hooke]

Using the examples you gave: Butt+2 and height of 6 means 2 inches out from the centerline and 6 inches up from the baseline. Waterline of 10 and a half-Breadth of 14 means 10 inches up from the baseline and 14 inches out from the centerline. I should note that on the 'waterline of 10' I am assuming this is a waterline 10" above the baseline but this should be explicitly noted on the plan, or at least the waterline should be specifically labelled as the '10 inch' waterline rather than just 'waterline 10'.

[180]

Thank You. Your help is very much appreciated. smile.gif

[JimD]

'Butt' is short for 'Buttock', and like the others say, its an imaginary vertical plane running through the boat fore to aft to help get the right curve to the boat when lofting. On a hard chined boat, for example, there is no need for buttocks, because the only offsets needed are keel, chine and sheer.
jimd.

[B_B]

Hello 180, I have Ted's book, circa 1997 open in front of me and I do not see "Bob's Special"...regardless, each of the five designs shown has a set of lines plans, with a verticle look, a horizontal look and a front on look for the symetrical boats and both front and rear views for the asymetrical boats.

This last drawing shows the centreline (a stylized CL interlocking) and the baseline - your measurements for the offsets should mirror this drawing but as a lifesize version, the 'Station Numbers' are represented in this drawing also....hope this helped and didn't confuse!

Try to duplicate this little drawing on your lofting floor and transfer the lines to your molds - various ways of doing this.

The point being to visualize "fair" (boaters parlance for nice looking) lines....

[htom]

"Bob's Special" is in the year 2000 version of Canoecraft, "Revised and Expanded". I think all of the canoes are different, but didn't compare.