This post describes how to design steps for a boat. Before we go too far I’ll do a quick explanation of the key terms used when designing steps.
- A Tread is what you step on.
- A String is the side of the step. There are usually two strings in a set of steps.
- Rise is the vertical height from the top face of a tread to the top face of the next tread. Also the height from the cabin sole to the bottom tread, and the height from the top tread to the cockpit sole.
- Going is the depth of the tread.
To draw your design you can use the old fashioned but reliable: paper, pencil, rule and eraser; or you can use your computer. In the past I’ve used AutoCAD, but recently changed to Google Sketchup. Purchasing AutoCAD will require a second mortgage on the house, whereas Sketchup is a free download from Google.
Step 1: Decide the position of the top tread. There is no specific rule for positioning this tread, but it needs to be a comfortable step from the tread to the cockpit sole. Keep in mind the height of the entrance coaming and whether you normally sail with the bottom washboards in or not. You will be stepping over these and you don’t want it to be too big a step in rough weather. One rule of thumb is that the top tread should be positioned roughly half way between the height of the cockpit sole and the entrance coaming. In the example, I’ve placed the top tread at the same height as the top of the coaming.
Step 2: Measure the height from the cabin sole to where you’ve decided to position the top tread. In the example, this is 921mm. At this point also decide how wide you will make your steps.
Step 3: Divide the height by the number of treads required to achieve a rise close to 225mm. In our example, a height of 921mm divided by 225mm gives 4.1, so we can do 4 treads with a rise of 230mm.
Step 4: Decide what size material you will use. As a guide, if the steps will be less than 450mm wide and there will be 3 or less treads, then 150x25mm (unplaned) timber will suffice for both treads and strings. Larger steps will require thicker timber. I use the building code span tables as a guide – a copy can be found here on the Australian Hardwood Network website. In our example, we decide to use 175×32 rough sawn Rosewood. The timber was very roughly sawn and had a slight bow to it – so after dressing it came down to 170×25, which is still a good size for this size steps.
Step 5: Add lines to represent the tread thickness. Draw these lines below the existing tread lines from step 3. In the example the tread thickness is 25mm (from step 4). Also in our example, I’ve decided to remove the top tread as this tread is nearly level with the cockpit sole.
Step 6: Now decide how far out the foot of the steps will be and mark this distance on the drawing. There is no strict rule here, except for the fact that space is precious: angling the steps out too far will consume too much space. Usually this is going to be determined by what else is around the steps. In the example, we’ve determined that the foot of the steps can be out 360mm before the steps start to interfere with other things. Now that all the critical measurements have been made, it’s a simple matter of finsihing of the drawing to get the final shape of the strings. Follow these steps against the example, and hopefully by the end it will make sense enough to do the same for your own steps.
Step 7: A. From the mark at the foot of the steps, measure back 100mm. Mark this point and draw a line between the two points.
B. From the mark just made, draw a line to the top left corner of the top tread going past the corner. We’ll erase the excess of this line, later. We now have the foot and back edge of the strings.
Step 8: Using the back edge of the string, draw the treads in place. Each tread’s top edge lines up with the string’s back edge, and from step 4 the tread will have a going (depth) of 170mm. Erase excess tread lines as necessary.
Step 9: Now draw a line connecting the front, bottom edge of the treads. Extend the line past the bottom of the string and up to the level of the coaming – we’ll erase the excess later. This is the front edge of the string, and if you’ve been drawing to scale you can now measure the string width, which in our example is 150mm.
Step 10: Draw a vertical line from the bottom edge of the string to the front edge, to finish of the foot. At the head of the string, you’ll need a vertical face to rest against the bulkhead, and a radius between the top and the front edges. There are no specific dimensions for these – whatever looks good and fits for your particular installation. I’ve reduced the height of the string to fit underneath the coaming.
You now have the design and basic dimensions for the steps. From here you can finish off the drawing, or you can move straight on to building. A further post will look at the building process in some detail, but if you want to start building now here’s a quick summary.
- Consider how you will keep the steps in place. This may affect the dimensions.
- Ensure you have enough timber for the job. The example requires approx 3.3m of timber:- 2 x strings @ 955mm = 1910mm- 3 x treads @ 412mm (450-2×25+12) = 1236mm
- Cut the five blanks: 2 strings and 3 treads. Note that the string length is the lateral length (955mm), which is longer than the finished height of the steps (881mm). And the treads are shorter than the finished width. Tread length = step width (450) – string width (2 x 25) + 12 (for the housing). Treads can be cut to the finished length, but the strings should be cut a tiny bit longer.
- Transfer the string drawing onto the timber. If you use right angle triangles you can draw the string outline without the need for measuring angles.
- Remember the strings are a mirror pair.
- Cut the string to shape. Cut out the tread housings to a depth of 6mm.
- Sandpaper strings and treads.
- Assemble with screws and glue. Plug the screw holes.
- Chisel off the tread corners.
- Final sandpaper and coat.
If you want a Sketchup design for a set of steps then send me a message using the Contact Us form, and I’ll email you the file.
Copyright (c) 2011 Glenn Andrew Ludlow.