by DrPR » Mon Nov 21, 2016 12:16 am
Joe,
Here are my guesses:
Data in the Table of Offsets (TOO) is based upon "stations" or transverse cross sections of the hull. No distance is given for the spacing of the stations along the length of the hull. You will have to get this from the drawings.
NOTE: The distance between stations is not the same for all ship and boat types. For larger ships the length between the "perpendiculars" (where the design waterline crosses the fore and aft end of the ship) was divided by 40 and this was the distance between the stations. But the dividing number can be anything.
Station positions are not necessarily related to the frame spacing in the hull. Stations are just used to calculate the hull shape as defined in the Table of Offsets. After the hull shape is defined the actual shapes of individual frames are calculated or determined in the hull loft floor. In some cases the actual frames were assembled to fit inside hull plating that was already in place in the ship.
The top half of this table contains elevations above the base line. The bottom two "Sheer" and "Chine" rows contain the transverse offset from the center line. Since these boats had slab sides the sheer and chine define the top and bottom of the sides.
Without the drawings I can only guess what B1 and B2 are. But these appear to be butt lines between the keel and the chine, perhaps defining a slightly curved bottom. Maybe the transverse distance between the keel and chine is divided into thirds (keel to B1, B1 to B2, and B2 to chine).
Maybe the "raised sheer" is a splash board mounted on the sides above the top edge of the sides. I seem to recall that the Higgins boats had something like this.
The table doesn't give any water line dimensions. They really aren't needed on a slab sided boat. Just draw lines between points on the sheer and chine lines and use these more or less vertical lines as template lines to stretch a surface on. After you have the side surfaces you can generate water lines if you want them by finding the intersection of the side surfaces and horizontal planes at various elevations. But I think you'll find you don't need them.
Ditto for the butt lines. Other than B1 and B2 for the curved bottom surfaces, butt lines for the flat sides really are unnecessary.
****
I see three things in the table that are different from larger ships that I have studied:
1. The Base Line is somewhere below the bottom of the keel. The Base Line is just an imaginary horizontal longitudinal reference line, so it can be anywhere and not necessarily related to any actual piece of the construction. All vertical dimensions are referenced to it. In this case it may be the bottom of a framing fixture that the hulls were assembled on - something easy to measure from.
On larger ships the Base Line is often on top of one of the keel plates - actually above the bottom of the keel. All construction vertical offsets are referenced to the top of this special keel plate. This provides a fairly easy place to measure from.
2. The transverse offsets are to the outside of the hull planking. On larger steel framed ships the transverse offset is often to the inside of the hull plating.
3. this table actually calls out 1/2 fractions of the eighths part of the distance, instead of having a "+" after the eighths as some Table of Offsets do.
Phil
Joe,
Here are my guesses:
Data in the Table of Offsets (TOO) is based upon "stations" or transverse cross sections of the hull. No distance is given for the spacing of the stations along the length of the hull. You will have to get this from the drawings.
NOTE: [b]The distance between stations is not the same for all ship and boat types.[/b] For larger ships the length between the "perpendiculars" (where the design waterline crosses the fore and aft end of the ship) was divided by 40 and this was the distance between the stations. But the dividing number can be anything.
Station positions are not necessarily related to the frame spacing in the hull. Stations are just used to calculate the hull shape as defined in the Table of Offsets. After the hull shape is defined the actual shapes of individual frames are calculated or determined in the hull loft floor. In some cases the actual frames were assembled to fit inside hull plating that was already in place in the ship.
The top half of this table contains elevations above the base line. The bottom two "Sheer" and "Chine" rows contain the transverse offset from the center line. Since these boats had slab sides the sheer and chine define the top and bottom of the sides.
Without the drawings I can only guess what B1 and B2 are. But these appear to be butt lines between the keel and the chine, perhaps defining a slightly curved bottom. Maybe the transverse distance between the keel and chine is divided into thirds (keel to B1, B1 to B2, and B2 to chine).
Maybe the "raised sheer" is a splash board mounted on the sides above the top edge of the sides. I seem to recall that the Higgins boats had something like this.
The table doesn't give any water line dimensions. They really aren't needed on a slab sided boat. Just draw lines between points on the sheer and chine lines and use these more or less vertical lines as template lines to stretch a surface on. After you have the side surfaces you can generate water lines if you want them by finding the intersection of the side surfaces and horizontal planes at various elevations. But I think you'll find you don't need them.
Ditto for the butt lines. Other than B1 and B2 for the curved bottom surfaces, butt lines for the flat sides really are unnecessary.
****
I see three things in the table that are different from larger ships that I have studied:
1. The Base Line is somewhere below the bottom of the keel. The Base Line is just an imaginary horizontal longitudinal reference line, so it can be anywhere and not necessarily related to any actual piece of the construction. All vertical dimensions are referenced to it. In this case it may be the bottom of a framing fixture that the hulls were assembled on - something easy to measure from.
On larger ships the Base Line is often on top of one of the keel plates - actually above the bottom of the keel. All construction vertical offsets are referenced to the top of this special keel plate. This provides a fairly easy place to measure from.
2. The transverse offsets are to the [i]outside[/i] of the hull planking. On larger steel framed ships the transverse offset is often to the [i]inside[/i] of the hull plating.
3. this table actually calls out 1/2 fractions of the eighths part of the distance, instead of having a "+" after the eighths as some Table of Offsets do.
Phil