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By - Michael Storer - South Australia - Australia

Rules of thumb for boat and yacht design - are they legitimate?

A creak of leather as he climbs on his hobby horse

I sort of have a mission against using rules of thumb.

The problem is that often they have been stripped of the original context. A particular type of boat or a particular situation.

I haven’t really seen any that hold up in all situations – or at least very few.

Example 1 – Sail Area to Keel area

One rule of thumb that crawls around the net is that the centreboard area should be a particular proportion of the sail area. It is set up to work for yachts, but by the time you get down to small boats they travel so much slower that you need a lot of extra area.

My take is that for dinghies and other light boats (take that as missing context in the original) the centreboard or leeboard should be getting up toward 3ft deep and 10 inches wide. If a boat carries a trapeze or leaning plank or is flared widely the area will need to be tweaked up more the closer you get to 16ft.

Preliminary sketch for Son of Goat designed by Michael Storer

Inevitably if I write this often enough it will become a rule of thumb itself – but will be presented as an absolute size as the contextualising information is gradually stripped away.

The original “rule of thumb” is even wrong for yachts as we know now that the depth of the centreboard, leeboard or keel is much more important than the area.

Example 2 – that you can stretch or shrink an existing boat by X%

One “rule of thumb” for boat design that continually comes up – and I got bitten by it once when a shipwright suggested to the customer that it was OK – which resulted in a boat that wouldn’t sail at all because it was too unstable and a customer unhappy with me.

A builder contacts me and says “something wrong with the boat” he built to my plan. Later I found that the 15.5ft Goat Island Skiff had been shrunk to 14ft.

That “rule of thumb” that is quoted is that you can generally shrink or expand a boat 10 or 15% with little problem – this one is so wrong that it is a real danger.

15% reduction in overall dims and stability is halved.

The source often quoted for this is Bolger – who once said that a cargo boat could be increased in length by adding a 10 or 15% section in the middle – with a possible improvement in fuel consumption. Completely different – not only is the body of a cargo boat mostly parallel sided in the middle, making the mod easier, but Bolger was tempering it with his vast experience.

He mentioned it only as a “notion” and certainly didn’t just add a bit in the middle – but drew it out carefully and properly as a new design.

Example 3 – the mast on the small boat should be three times the beam

Now for the Chappelle comment that mast length being 3 times the beam. Clearly he is talking about stability. So when he is talking about a specific model and size of boat he might be right – but what rig is he talking about – does he mean the peak/head of the sail?  A lug sail could almost have double the area of a triangular sail under this “rule of thumb”.

These three boats have about the same mast length.  The lug is 105 sq ft and the triangular sails are 82 sq ft.

Three masts about the same size.

But there is a more serious part to the argument.

In general if you double the size of the boat the stability goes up by a factor of 16 but the heeling forces go up by a factor of 8. This is why a maxi yacht can either have a lighter keel – or like the modern ones be somewhat narrower than a smaller boat of roughly the same proportions – note the really long keels on model boats.

This pic shows a keel depth about half the length of the hull – the full size boat would need a keel 35ft (10m) deep – not necessary though

A model boat needs a much longer keel than a full size boat - boat design

The best guide is looking at existing boats built to a similar purpose to see which ones are good sailors and keeping to the same rough proportions.

Beyond that you can make sensible conclusions – like a goose is more stable than a pointy nosed boat and put a bigger mast and sail in it. That is assuming it has a similar position of the centre of gravity.

Or that a new design will sail a little faster so maybe the centreboard can be made a little narrower.

Howard Chapelle knew exactly what he meant by the comment, but it has been pulled out of context by being turned into a “rule of thumb”.

Legitimate rules of thumb?

There are a couple of areas where I use rules of thumb.

Francis Herreshoff once said that the rake (angle back) of each mast on a boat needs to be 2 degrees more than the mast further forward.  This is to simply prevent the masts look like their tips are converging.  It is simple, it works and is not very open to interpretation.  It also doesn’t have any effects beyond its brief.

Another place I use rules of thumb is in construction.  For plywoods under 9mm an epoxy fillet needs to have a radius three times the plywood thickness.  Same if you use a piece of timber in the join – the glue cleat needs to be have three times the gluing area to the ply of three times the ply thickness.

This is another rule that falls apart if taken too far.  Testing shows that the fillet radius actually needs to be about 2.2 times the ply thickness, so there is a safety factor already applied (someone is always ready to “add a bit” for safety.  But as the ply thickness gets bigger the rule falls apart as you need to add glass cloth to get enough strength – and by that stage if you rely on fillets only then the fillet has so much volume that it has become crazy expensive.

So, I would suggest that rules of thumb are very useful for professional designers because they know the ramifications – it is easy for Bolger or Chapelle to know what they mean and know when the “rule” breaks down.  But a better guide for amateur designers is to look closely at similar sized boats to see which ones perform well in terms of meeting the required use patterns and which ones do not.

Cautions on copying current practice – beware of creep

A problem with looking at current practice as a standard is that it can go badly wrong if you just look in one place for information.

One example is plywood canoes and kayaks.

When I got into wooden boat building in the late 1980s it was common for a stitch and glue kayak to be built of 4mm (3/16″) ply and glass taped with 50mm (2″) tape inside and outside the seams.  This was also common for plywood racing dinghies in Australia and New Zealand – which are subject to very much greater loads.

By 2000 the standard way was to use glass tape on the inside of the boat but do the whole outside of the boat in 4 or 6 oz (125 or 200gsm) glass cloth.

By 2010 several major manufacturers of kits are saying you have to build the boat with the boat completely sheathed with glass inside and out.  As any builder knows – glassing the inside of a built hull is a cow of a job.

But it adds significantly to the expense, the cost and weight of the structure.  Some say that glass has negligible weight – they obviously haven’t had to lug a boat very far!

Others say it is to get an “I-beam” or “composite sandwich” effect.  This is misunderstanding how materials work.  ALL materials loaded in bending, whether by having weight in the middle, or by water pressure work by the same method.  It just has to be strong enough for purpose.

If you are expeditioning and doing rock shore landings it might be worth going to glass inside and outside – but this is not how 95% of the market is going to use their boats.

One last parting shot … this is a Jarcat – a very clever range of trailer catamarans designed by Ross Turner in OZ.  There are hundreds launched.

Jarcat trailer cabin catamaran is built lighter than many kit kayaks

The Jarcats in the 16ft range is built of the same 4mm ply – with 20z glass on the outside only.  The cabin and other structure still needs to go on this one, but you can see how much bigger than a kayak it is but built out of lighter materials than a standard kayak is. It is a case of “creep” a small step by step change which is not perceptible but ends up a long way from the original point with little real justification.

Either the kayak designers or the Jarcat is right – the other wrong.  With 500 or more Jarcats on the water …

I only found this out in the last couple of years – despite the Jarcat being around for a couple of dozen – my bad.  It has made me think that for most boats in dinghy size that are going to be glassed that some glass is more important than the actual weight per metre of the glass – and also to think WHERE the glass is really needed.  Alternatively the earlier practice from the 60s and 70s might indicate that glass taping is all that is really needed for most boats and most normal uses.

So it is sensible for amateur – and all designers – to look broadly and find out what is the actual best practice across a wide range of boats rather than accept things that are touted on the internet.  Do your own research and share your conclusions.

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