Fabricating
Metal
Hull Components
When building a 'Bezier Designed' True Round hull, itself, all the component of the hull were pre-defined to fit seamless together, no Lofting. I think, t maybe I spoiled you a little!
It is now time to fit out that hull with thru-deck coamings onto which we will mount various types of Hatches, Window Ports, Companionway openings, tanks, and even Steel Masts and Booms for those so inclined.
Such custom fabricated steel components are also more in keeping with the hulls construction material ensuing a harmonious steel design that is not attainable using components designed for another construction material.
Components such as these can be fabricated for nickels and dimes on the dollar over purchased manufactured versions of the same product that are designed for fiberglass or wood construction.
Somehow a deck hatch and Port Lights designed for fiberglass construction just does not ring true when fitted to a steel hull! Also take into account that you will have have to fabricate some type of component to transition a purchased component, anyway.
Therefore, I wrote ‘Applied Metal Boatbuilding Methods’ – ‘Sheetmetal Pattern Development’. In essence it is a Metal Fabricator's course in ‘Sheet Metal Layout’, teaching the theory, principles and application of ‘Bend Allowance’ and ‘Bend Deduction’ to calculate the cut sizes, locations of bend lines and interior features through the fabrication of marine components typical to steel and aluminum pleasure craft.
Good Buy, Trial and Error and Tuck and Fit.
The following illustrates the Layout and fabrication for a Thru-Deck coaming for a Deck Hatch
Architectural drawings routinely represent only the size and location for features such as hatches and their coaming by way of a single drawn line as shown in the ‘Construction Plan and Profile' for the Bezier 35 n below.
The first thought should be how you will approach to the actual fabrication. For example, our Thru-Deck Coaming will be fabricated in two sections from sheet material and formed in a 'Press Brake'. After which the two fabricated sections will be welded together before installation in the deck structure.
With the fabrication and installation methods straight in you mind your mind gather all the dimensional information provided in the architectural drawings and or specifications.
- I scaled the outside dimension of the coaming off the 'Construction Plan and Profile' drawing at 16.375" in both directions.
- I found the thickness of the coaming in the Specification at 0.187" thick.
- In a section drawing I found that the coaming at the centerline of the deck would be 4.186" in height and the height at the ends would be 4.344".
- I than determined that the 'Inside Radius' should be 0.937". This radius is somewhat arbitrary, usually determined by the availability of 'Press Brake' tooling.
To begin the layout process for any component and in this case a Fore-Deck Hatch Coaming, I usually start out with a rough 'Hand Sketch'.
Below is my hand sketch showing all the principle Apex dimensions, Inside Radius dimension, Weld seam location, and material thickness in a Plan View. For now knowing the term Apex dimensions sufficient. You can learn more about the Apex dimensions in the book.
Plan View of the coaming - Hand drawn:
Plan View of the coaming - Computer drawn:
A Computer drawn version is not necessary it does have advantages over a 'Hand Sketch' in that dimensions can be verified easily were 'Hand Sketch' dimensions need to be verified by Math. The below drawing is included her for only for clarity.
Note the location of the hinge tabs, which are welded to the deck and the hinge rod which will align and connect the hatch cover, not shown to the coaming. Also see the hinge tabs in the profile view.
Profile View of the coaming - Hand drawn:
Notice the crown in the coaming which follows the curve of the deck transversely. The coaming widest point is 4.344” to 4.146” at the centerline, these dimensions were taken from the transverse frame at the hatch location.
From these drawings all the dimensional information necessary to layout a flat pattern for the coaming can be entered into the development spreadsheet, which is free to download from the below link.
Profile View of the coaming - Computer drawn:
Fore Deck Hatch Coaming Flat Pattern
Layout Method
Before we even begin, while the calculations to determine the 'Cut-size' for the example coaming is simple math, the calculation are long and numerious, therefore prone to simple errors.
I am not going to include those calculation in this illustration, therefore the following screenshot of the 'OD Apex' Excel spread sheet will be used.
You can download the the Excel spreadsheet used to calculate the Cut-size and Layout for 'Fore-deck coaming and all the other spreadsheets used in 'Applied Metal Boat Building Methods' by Clicking on the below link.
The Inputs:
- The first column is where you enter the Apex Dimensions. In this case the Apex Dimensions are the same as the Outside Dimensions - 8.187" x 13.375" x 8.187". See my book 'Applied Metal Boat Building Methods' for more details.
- The next column is where you would specify the 'Bend Angle' for each bend. Here it is 90-degrees for both bends.
- In the next column is where you would specify the 'Inside Radius' for each bend- 0.937" for both.
- The next column shows the 'Material Thickness'. This is entered in a Cell not show here. You will see it in the Excel Spreadsheets that when you download the.
The Outputs: For now we are only interested in the last three Outputs. All other outputs are fully described in my book 'Applied Metal Boat Build Methods'.
- Cut Size - Is the calculated length or width for the component.
- Bend Lines from 0 - The distance from the starting edge of the flat layout to each Bend Line.
- Between Bend Lines - The distance between each bend Lines.
The hand drawing below represents the flat patternfor half of the Fore Deck hatch coaming.
- The overall cut-size is 31.455" x 4.343". This cut-size is for one half of the coaming. Two Pieces are required. They will be welded together to complete the finish coaming.
- There is a 90-degree bend 7.864" in from each end of the 31.455' dimension.
- a 1.875" diameter top punch will be use to form the radius corners for this coaming layout.
- Notice that the radius of the deck is cut between the two bend lines.
This last drawing shows the full-size layout on the material from which it will be fabricated.
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