Model Ship Secrets

Tips, tricks and techniques to transform model ship building into a SUPER FUN hobby!

Tuesday, September 05, 2006

How To Read A Table Of Offsets..

Can anyone explain, or point me to an explanation, of how to read a table of offsets? I am looking at the Table of Offsets for the Clipper Ship Sea Witch in Davis' Ship Models - How to Build Them. Thanks.

You should see the table divided by station lines, sections from bow to stern. Then for each waterline you will see a half-breadth dimension at each of the stations. This gives you the plot points for drawing all the baterlines. Then same thing for the buttock lines, but these will be given in heights above the waterline.

The table should also give the heights of the decks sheer line and the decks heights which will be different from the waterlines as it is a curved line usually.

All the points will be plotted on the waterlines, buttock lines, and on a body plan at each station. Once the points are plotted the waterlines, buttock lines, and station line points should be fair. However, because of measuring errors they will probably not be fair. Consequently after you have all the l ine drawn you need to "fair" all the lines; that is, check all the points and correct any line not correct.

The table of offsets may also have dimensions for "diagonals" which are planes cut diagonally thru the hull.

If you don't know how to read a table of offsets, chances are you don't know how to draw and fair lines either. It takes a little knowledge and experience. You probably will need to read a book on the subject, or in a basic book of naval architecture or boatbuilding.
{Ben Langford}

The table of offsets is used to 'loft' - draw the plans fullsize on the loft floor. It's a complicated process, and you'd need a book on "lofting", which you can find at marine bookstores or at Woodenboat. But realistically, since the book also includes the plans for the Sea Witch, you don't really need the table of offsets. Work from the plans.

If you look at the plans, you'll see that the profile (side) view and section (fore/aft) views have lines marked with numbers (aft of midpoint) and letters (forward of midpoint). These mark the moulds (the slices through the ship that define its shape) and correspond to the numbers/letters in the leftmost column in the table of offsets. The figures in the grid are measurements that correspond to points on the lines, which are then joined to make the smooth curves that make up the plan. The figures in the grid are in the form "3-8-4" which if you look in the lower left corner of the table of offsets, you can see means "Ft. Ins. 8ths", so 3-8-4 means "3 feet + 8 inches + 4/8 inches".
{David Hill}

Lofting a hull from a table of offsets, if you have the gumption to do it, is a wonderfully rewarding exercise that will immeasurably increase your knowledge of and appreciation for hull design.

You can do it to some comfortable scale that fits your available drawing gear. You don't even need to do the whole thing, a few waterlines, a few stations, buttocks, and diagonals and you will soon "understand" the process. Don't have ships curves? Don't need 'em, use those little sticks we call modelling supplies as battens. You can even free hand the curves if all you want to do is learn the process.

If you complete the drawing, you can always overlay and trace a cleaned up neat version. These make wonderful wall hangings, and you can puff out your chest and say "I did it myself"

For the mathematically inclined, the table of offsets is nothing more than the cartesian coordinates that define an ordered set of points on the ships hull. The waterlines, buttocks and stations are an ordered set of fair curves that flow through the points. {Robert M. Crane}

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