Work done by Storer Boat Plans
- Change scantlings for simpler construction in Cedar Strip
- Optimize Foils
- Design a Birdsmouth hollow mast with optimised wall thickness
- Rig using a range of modern low stretch ropes
- Set up rig controls for ease of adjustment.
- Design a tabernacle for the almost unstayed mast
The performance and building stuff in a moment, but some lurve for the builder and owners first.
David Wilson, who mostly works at Duck Flat Wooden Boats is about the best boatbuilder I know.
A strong sense of aesthetics plus a practical streak. It means that every individual detail of the boats he builds are perfect to look at and usually work very well too.
This particular project was undertaken as a partnership between David Wilson and Winemaker Rob O’Callaghan of Rockford’s Winery (champion of old vines in the Barossa and the wonderful Basket Press Shiraz).
The catboat has taken several years in the building as more urgent work kept being put to the top of the queue, but this is a real “Gold Plater” as Phil Bolger would say, or a special vintage in Rob O’Callaghan’s language
Everything is simple, which is appropriate given the workboat background of Cape Cod Catboats, but it all works very well indeed.
And as you can see from the pics she is just gorgeous.
(More Pictures – click here for the other Pictures Page)
OK, now the bit the technical types are interested in.
Hull Scantlings from Fenwick’s original to modern simplified
This boat is a good case study for the simplifications that are possible using modern materials. They enable to boat to keep the original character, but eliminate a lot of the downsides of traditional building.
First the boat is very much simpler. All Ribs, knees, bilge stringers are eliminated giving a much cleaner interior and cutting the labour required to build the hull to a fraction of traditional methods. Many other parts can be combined compared to the original design – sheer clamp and deck clamp can be combined, the stem and backbone can be simplified into a simple scarfed structure with the hull skin itself acting as a knee between the two members.
The result is an immensely strong monocoque construction with loads from one area being dissipated into many others. There are no lazy bits of boat. The cabin and cockpit seat tops and fronts stiffen and support the hull skin, transferring loads into the bulkheads and centrecase. This boat is much stronger than the original design and much faster and cheaper to build because of the structural simplification.
Plus it is trailerable and able to be stored without any detriment from shaking on the trailer or drying out in storage.
Also see the EPOXY and Boatbuilding FAQ for more detailed info on building methods and materials that make this Catboat possible – also good info there if you are building any boat.
David is like many experienced builders and likes to start with the smaller components before starting the hull. The job seems to go faster and takes up less room in the early stages. His first job was to make the Centreboard and Rudder.
The original centreboard was a galvanised plate with an offset case that dropped the board to one side of the keel and skeg. This was retained, but we changed the board to a built up foil of timber, glassed with two heavy coats of biaxial glass. This allowed us to use the simplified foil shape derived by Melbourne aerodynamicist Neil Pollock some years ago.
It allows an efficient section without the foil becoming too thick, which is often a problem with the basic NACA00XX profiles on traditional boats.
Thicker is better for efficiency, but the thickness required for the long chord of the raked board would be around the 4 or 5 inch mark, which even with an interior that is as wide as this one (almost 9ft) does take up an undue amount of centreline space and doesn’t help companionway ergonomics.
We also took the opportunity to reduce the width of the board slightly to bring the top of the centrecase down to a sensible table height on the assumption that we were gaining a lot of efficiency from an accurate foil shape of a known family.
New ways with a Birdsmouth Mast
Birdsmouth masts are currently flavour of the month on many of the internet boating forums. They certainly are an elegant way to get a hollow mast.
However they have a performance damaging DARK SIDE.
Basically the wall thickness stays the same all the way to the top. This means that while the wall thickness may be a nicely balanced 15 to 20% of the mast diameter at the base, but as the spar tapers up to the head the wall thickness may start to approach 50% as it was promising to do if we had just used the original mast taper. The result could have been that the top third of the mast weighs about twice as much as actually required.
This type of tip weight is particularly damaging to performance as being at the top of the mast it contributes a great deal toward making the boat heel excessively as well as increasing the pitching of the boat when encountering head seas.
It becomes a lot of weight in the case of a bulky catboat mast.
Also consider that this boat will be kept on the trailer and it was intended that the mast pivots in a tabernacle to allow the crew to simply push it up into place. Every kilo we could get out of the tip made the job easier and eliminated the need for winches, derricks or other things that get in the way of sailing.
Another bit of engineering we did was to substitute the normal density timbers with a much higher density timber. Rather than adding weight it allows the walls to be considerably thinner for the same strength. The risk is that the glue joins become too small in relation to spar diameter.
Timber stiffness is more or less in proportion to the timber density. This is also true of most normal engineering materials.
The same reason that an aluminium mast is lighter than a normal wooden one plays out here. The aluminium is heavier of course .. so the wall is thinner and that moves more of the material to where it does more work. If you consider the principle of a hollow mast is that material inside the mast which is not supporting much load is removed.
So the higher the density, the more material can be moved out closer to the surface of the mast where it contributes much more to stiffness. This excessive stiffness is then brought down for a weight saving by thinning the wall further to bring the stiffness in line with the original
This saved around 6 percent in overall weight.
Concern about glue join surface area was unfounded some 15 years later 🙂
By keeping the mast wall thickness at around 20% by designing the staves that make up the mast to give the correct internal diameter of the mast at all heights and working out a schedule for removing timber from the outside of the spar once it was glued we managed to save a great deal of weight throughout the length of the spar, and in particular the tip. That is, we designed the hole up the middle then planed the excess off the outside.
Some further information about birdsmouth mast construction is here
Low stretch ropes
I have a pet theory that Gaff and Lug rigs are considered slow compared to Bermudan because they have big rig stretch problems. There are such enormous lengths of rope involved in the halyards for example that a few percentage points result in the gaff sagging off to leeward and killing off the upwind performance.
Using modern low stretch ropes (various tradenames – Spectra, Dyneema, Vectran, Dynex and others) can reduce these problems significantly with a significant increase in boat performance. Also the price is starting to drop away. And they are a hell of a lot nicer to handle than using wire for halyards, which is the usual method of reducing stretch for boats of this era.
One thing to be aware of is that while some materials are very UV stable (spectra), others are not (vectran). So if the boat is going to spend a lot of time out in the sun – eg be kept at a marina, then some care should be taken to make sure the best type is selected.
Generally it is simple to choose a rope with the same breaking strain as the wire.
Setting up sail controls to be accessible and low friction.
Pretty straightforward. Carefully plan out the route for the sail controls in advance to get a good clear run for all the control lines. No twisting and make sure the blocks align well with the direction the rope needs to run in.
If you have to economise put the lowest friction (ball or roller bearing) blocks in the highest load areas and plain sheaves elsewhere.
One of the biggest improvements has been through moving to smaller diameter ropes running through the same sheaves. Can cut friction significantly.
Tabernacle for raising and lowering the mast.
Catboat masts are beasts.
They have to be to stand up to the loads imposed by such a stable hull. Not many boats are half as wide as they are long. Only catboats and catamarans!!!
Neither David nor Rob was particularly keen to have to lift such a heavy mast into place, so they wanted to fit a tabernacle to allow the mast to pivot up into place.
One thing with normal tabernacles is that they are pretty ugly. The normal form is to have a vertical rectangular post either side of the mast.
This approach could make the mast appear about 2.5 times wider than the mast just by itself.
The turning point was looking at the “half tube” technology developed by Robert Ayliffe (of NISboats.com) and Randall Cooper (Goolwa Masts and Welding) for the Norwalk Island Sharpies.
Robert spent many years working out the best way to do it and Randall tranformed the ideas into an elegant alloy structure with minimum impact on the appearance of those boats.
This allows their masts to be raised in seconds rather than in scores of minutes. But unfortunately that technology was in aluminium – not really suitable for a piece of craft like this catboat!
We decided to use the birdsmouth technology to reduce the visual size of the tabernacle from most angles. it has a rounded back and an open front. As the mast goes up it is snugly slots into place inside the tabernacle.
The gooseneck is attached to the tabernacle to allow the boom to remain in place whether the mast is up, down or in the process of moving to the other. It also means that all the sail controls can remain attached.
Raise the mast and go sailing …
…Which is how it was meant to be.