DESIGN AND CONSTRUCTION OF THE SCOW

1 Introduction

5 Overall Design - Rig

9 Rig Drawings

2 Design Objectives

6 Design Drawings

10 Assembly New

3 Dimensions

7 Construction Illustrations

11 Materials and Nesting 

4 Overall Design - Hull

8 Panel/Part Drawings

12 CAD Files Download 

1 Introduction

This section aims to give enough detail on the scow:

2 Design Objectives

The scow was designed to meet several objectives:

The end result doesn't look a lot like other dinghies - I wasn't too concerned with its looks, just how it worked. However I do acknowledge that one design heavily influenced this scow - it is a Chesapeake Bay scow, 'Tubby', from a book entitled 'How To Build 20 Boats', published by Mechanix Illustrated in the US in the 1950s (my late father worked there for a year and must have brought it back with him). The drawings for Tubby may be found here.

3 Dimensions

All dimensions given on this web site are in millimetre, with some conversions into inches.

On the sizing of individual parts, particularly for solid wood, I have tried to use a convention of giving the actual (ie, finished, after planing) dimensions of the stock in millimetre, with the nominal (ie, before planing) dimensions in inches.

To avoid endless repetition, plywood thickness is given only in millimetre. Only two thicknesses are used: 6mm = 1/4" and 9mm = 3/8".

4 Overall Design - Hull

The overall dimensions of the scow are:

 

 

Hull Length

3600

11' 10"

Hull Beam (excluding gunwhales and leeboards)

1200

3' 11"

Draught, leeboards down/up

600/120

2' 0"/5"

Hull Weight, estimated*

60-80 kg

130-180 lbs

*The scow was not weighed on completion, so this weight is estimated from the area of ply used. The higher estimate would be appropriate for a scow with all solid wood in mahogany, the lower estimate for a scow with the minimum of solid wood (eg, without bottom rubbing strips) in softwood.

The scow is a simple shape with a flat (transversely) bottom and vertical sides throughout its length. The vertical sides are required where the leeboards pivot, so it seemed sensible to keep them vertical elsewhere - it also meant that the panel dimensions could be worked out without a panel-expansion program.

The scow has three seats which double up as buoyancy tanks that give a flooded buoyancy of about 400kg (850lbs) - enough for four adults. The forward seat/tank incorporates the mast step for an unstayed mast.

As the scow was designed to be 'bottom-bumped', it was given three rubbing strakes under its bottom, capped by metal strips. The centre one runs the full length of the bottom and merges into the skeg at the back. The two side strakes run a little over half the length of the hull and end on the forward and aft seat/tank bulkheads. The three strakes form a significant part of the strength of the bottom shell between the mid and aft seats/tanks - if they were left off, the scow would probably sail better, but their strength would need to be replaced.

5 Overall Design - Rig

A balanced lug sail was chosen for the scow, because:

I got the sail made to my design by a local sailmaker - it cost GBP 175 (about USD 250) in 1999 which was about half the total cost of the scow, bearing in mind I got the spars for free.

The mast is a simple anodised aluminium tube of 66mm (2.6") diameter. Not being sure if this was really strong enough, it was decided to sleeve it over about the bottom 800mm (31") by a 70mm (2.75") diameter tube. For sure, the end result is amply strong enough and isn't at all heavy.

There were obviously several benefits to working in a boatbuilding company and making the spars for the scow from various bits of scrap lying around was a big one. As noted on the main scow page, the gaff and boom were made from some damaged carbon-fibre top mast sections for the 49er dinghy - these are tapered, from about 60mm (2.4") diameter at the front end to about 35mm (1.4") at the rear/top.

6 Design Drawings

I designed the scow using TurboCAD v5 and it has not been too easy to convert these into a format which can be viewed over the Internet. Believe it or not, the GIF files are made by copying the contents of the TurboCAD drawings into a Visio file, saving that as a TIFF and then using MS Photo Editor to convert the TIFF into a GIF. Phew!

Sail plan

Profile and plan

Offsets and panel dimensions

Midship section and leeboard pivots

If you have a CAD program, see below for downloads of CAD files.

7 Construction Illustrations

Here are some drawings of various parts of the scow. They are only intended to show the structural arrangement - the panel/part drawings show the detail necessary to build the scow.

Aft seat/tank

Mid seat/tank

Forward seat/tank

Mast step

8 Panel/Part Drawings

These drawings show the dimensions and assembly instructions for each part of the scow and are used to cut and make the parts before starting assembly of the hull. Comments from readers on how easy these drawings are to use would be welcome.

If you have a CAD program, see below for downloads of CAD files.

Side shell panels

Bulkheads in aft and mid seats/tanks

Seat/tank tops

Mast step

Bottom panel

Bulkheads in bow seat/tank

Leeboards

Leeboard pivots

Stern transom

Bow transom

Rudder and tiller

 

9 Rig Drawings

These drawings show the sail, spars and the various sailing controls (ie, bits of string).

If you have a CAD program, see below for downloads of CAD files.

Sail

Spars

Strings

10 Assembly

This page describes the assembly process and a drawing shows the building jig I used to assemble the scow.

The last two links are to a description of the build process in pictures. These files are fairly large so may be slow to load. The one-page description is extremely large (387K), but might be useful for downloading for viewing off-line.

Assembly

Building Jig

Build Process in Pictures
(step by step)

Build Process in Pictures
(one page)

11 Materials List and Nesting Diagrams

The materials needed to build the scow are listed on the first of these pages. The second page shows the 'nesting diagrams' - how to lay the parts into standard sheets of plywood with minimum waste.

Material List

Nesting Diagrams

12 CAD Files Download

If you have a CAD program, here are groups of files, WINZIPped together, which you can download in three alternative formats:

Format:

Original TurboCAD format

DXF format

AutoCad DWG format

All four design drawings -
1 Sail plan
2 Profile and plan drawing
3 Offsets and panel dimensions
4 Midship section

Download (73KB)

Download (43KB)

Download (51KB)

All thirteen panel drawings:
1 Side shell panels
2 Bottom panel
3 Stern transom
4 Bhds in aft and mid tank/seat
5 Bulkheads in bow tank/seat
6 Bow transom
7 Seat/tank tops
8 Leeboards
9 Leeboard fittings
10 Leeboard pivots
11 Rudder and tiller
12 Mast step 1/2
13 Mast step 2/2

Download (239KB)

Download (109KB)

Download (133KB)

All three rig drawings:
1 Sail
2 Spars
3 Strings

Download (62KB)

Download (35KB)

Download (38KB)

Building jig

Download (19KB)

Download (10KB)

Download (11KB)

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