By Ross Lillistone - Esk, Queensland - Australia

Stems and Trailers

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High in a beautiful Blue Gum tree, a dignified Magpie viewed the activity taking place outside a country workshop.

The bird was familiar with this place, and with its human inhabitants, so he showed no hesitation as he swooped down to harvest insects from the grass just feet from two men who were struggling to winch a small wooden boat onto a galvanised road trailer.

Now, you would think that getting a light fourteen-foot lapstrake (clinker) dinghy onto a trailer would be a simple matter, and so it should be. The problem was that she suffered from having come from a mixed background.

No-one could complain about the looks of this boat – she had the shape of a classic plumb-stemmed clinker dinghy or ship’s boat, and she showed all the characteristics of the type – plumb stem, burdensome hull-shape, firm turn-of-bilge, buttock lines sweeping up to a transom stern above the waterline, external keel-batten and prominent skeg. This boat would fit perfectly into an Arthur Ransome novel from the “Swallows and Amazons” series.

As handsome and light as this boat appeared, she was very difficult to get onto the trailer. Why? Because her plumb stem (hyrodynamically very efficient as it may be) would not ride smoothly up and over the first trailer roller. So instead of the boat automatically lifting up onto the trailer as would be the case if she had an angled, swept-back stem, the bow came up against the trailer roller and just stopped dead until someone physically lifted the bow up and onto the trailer. A two-person job – one to lift and one to winch.

The difference in stems.

Not only that, but when the bow was eventually manoeuvered onto the first trailer roller, the shape of the bottom of the boat meant that she kept falling to one side or the other.

The problem is that many of us seem to have lost sight of the proper application of the “form follows function” adage. Yes, the boat’s primary function is to be an efficient shape from a hydrodynamic point-of-view, but she also needs to be practical in other modes of operation. Examples include efficient utilisation of building materials, comfortable internal arrangement, suitability for trailer loading, ease of rigging and un-rigging – the list goes on…

The shape of boats (for general use, that is – not racing) has evolved because of three primary influences: -
1. the sorts of materials available for construction;
2. the location and mode of operation;
3. the load to be carried.

Obviously these are not the only influences, but they are very important. In the case of the dinghy being described, her design goes against the common-sense application of the above rules.

She was built using the glued-lapstrake (glued-clinker) method of construction. This is a wonderful way of building wooden boats which will be stored out-of-water, because the glued plank overlaps will not open up as the timber dries out in storage. Also, the fact that glued-lapstrake produces a stressed-skin hull means that the boat will be lighter than one of traditional construction, further enhancing ease of loading and unloading from a trailer.

But in order for this method to work, the planking needs to be made from high-quality plywood or some other system where cross-grain strength in the planking is increased over what is available in natural timber. If ordinary timber is used in a glued-lapstrake boat, the planks will inevitably crack along the line where the planking changes from double-thickness to single-thickness.

Plywood planks do not like being forced to take the compound curvature that is required to produce fair plank runs in a plumb-stemmed boat which has the beautiful hollow waterlines we find so attractive. Some skilled designers have produced good examples, but the successful examples still show some unfairness in the last few inches before the stem. They also require many narrow planks rather than being able to capitalise on the nice wide planks which can be cut from good plywood.

Bolger Hope

In the old days, when the plumb-stemmed lapstrake (clinker) boats which we all admire were being built commercially, there were a number of different conditions in play. Firstly, the boats were usually left in the water (ship’s boats being an exception), so the planks were always wet and swollen. Therefore, there was no need for the laps to be glued to keep the water out - but the boats did require scores of small steam-bent ribs rivetted or clenched across the planking to provide the cross-grain strength.

Secondly, the narrow, natural timber planks were steamed in the area where lots of twist and compound curvature was required. The result was the beautiful sweeping plank lines which characterise the best examples of this construction. Plywood does not respond well to steaming, and therefore the forefoot of a plumb-stemmed ply dinghy never quite looks the part.

So, the design of traditionally-built lapstrake boats was dictated by the fact that they were:-
• kept in the water (no need for glued laps and no need for trailer-friendly characteristics);
• built from narrow, steamed planking which could easily take a compound curve.

In contrast, modern lapstrake boats should be designed with different factors in mind: -
• kept out of water – therefore the plank laps benefit from being glued, and the planks are best made from quality plywood;
• transported on a trailer – more easily loaded if the bow stem is raked rather than being plumb. This is fortuitous, as a raked bow stem is the natural termination of wide plywood planks which have a minimum of compound curvature.

Given that the function of a modern trailerable lapstrake boat is suited to wide planks and raking stems, lets have a look at good-looking examples of similar boats from history. The obvious ones which come to mind are the various Dory-styles, and the Scandinavian Oselvers. There are others, but these will do as examples.

The Dories and Oselvers have very sharply raked bow stems and have narrow, flat bottoms (in the case of the Dory), or very shallow ‘V’ bottoms (in the case of the Oselvers and similar styles). Notably, both types evolved from circumstances which supplied wide planks – in this day and age we can very effectively substitute high-quality plywood for the wide pine planks - without having to compromise the designs or force the plywood to do something it can’t.

“Here is a glued-lapstrake boat which uses only five, wide planks. Note the sharply raked bow stem which is the natural termination of such planking”.

Ross' Periwinkle

Back to the country workshop. If the Magpie had been watching a Dory or Oselver-derived design being loaded onto a trailer, he would have seen that it took one human, not two, and that the boat kept herself upright as soon as the bottom hit the trailer rollers. Not only that, but an intelligent bird would also have seen that the shape of the boat was perfectly suited to modern timber construction, and that the design did not look pretentious.

There are many areas where common-sense has been overlooked in design. One very common example is the design of centreboards or daggerboards. The theorists will tell you that the best design has a high aspect-ratio board, made with a high-tech foil section, and set so the leading edge is vertical.

Now that is fine if you are sailing in a competitive class such as the Eighteen Foot Skiff where the rules allow such things – not to go for the last fraction of a second means being a looser. But for cruising there is no need for such impractical things.

I recently had a wonderful days sailing on a stretch of water which was somewhat infested with weed. The boat which was accompanying me had a vertical daggerboard, and she just kept stopping, and then drifting off sideways as relatively small amounts of weed caught around the leading edge of the board and destroyed the hydrodynamics. All that was required was for the board to be lifted and the weed was cleared – but it had to be done dozens of times.

In contrast, the boat I was sailing had a pivoting centreboard which was swept back at 45 degrees. Not once did I have to stop. The boat just kept on sailing through the weed patches, and the weeds slid back along the angled leading edge and swept clear of the board. A similar situation occurs when encountering sand bars and mud-banks. For cruising, just apply a bit of commonsense to the design equations!

“Section of boat showing angled centreboard”

There are many, many other examples of situations where common-sense has been left to fall by the wayside in boat design. Consider carefully before committing yourself to the task of building (or buying) – think about the practical matters of operating the boat, and trust you own judgement.


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