Producing an easily plated rounded hull shape





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This article describes the radius chine method of metal boatbuilding as used for most of our metal designs since the mid 1980's. It allows amateur boatbuilders to produce rounded hulls which even professional yacht designers have sometimes mistaken for true round bilge boats. The first boat design in the series was the Hout Bay 30, which has been followed by many others, all available as stock boat plans. The designs are all suited to professional boatbuilders as well.

Hout Bay 33 "Alexis", to radius chine steel boat plans

Hout Bay 33 "Alexis".
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Metal boat construction has been around for a long time, long enough for its benefits and drawbacks to be common knowledge. However, with the acquisition of knowledge and technology, the methods of working with and treating materials change for the better, expanding the benefits and limiting the drawbacks. Such has been the case with improvements in paint systems which have considerably reduced the maintenance of steel boats. Developments in hull shapes and plating techniques have also gone a long way in helping to reduce the prejudice against steel yachts.

As with a boilermaker, a designer has to develop a feel for what can be done with steel plate and what cannot. Early steel hulls (and many still designed today) were of chine or multi-chine hull form to give ease of plating. However, many such designs produced hulls which were not very seakindly because the designer paid too much attention to the needs of the metal and not enough to the needs of the sea. At the other extreme are designs for which the needs of the metal have received little consideration and which have so much twist to their sections that they are not much less difficult to plate than a round bilge hull.

Hout Bay 33 "Alexis", to radius chine steel boat plans The secret of a good steel design is clean simplicity, in both the hull and the deck. The more cutting and welding that can be eliminated while maintaining good aesthetics, the more successful the design is likely to be.

A single chine hull is the ultimate in simplicity but there are few single chine designs which can be described as pretty. This applies especially to steel boats which tend to be very boxy and bulky with only one chine. A single chine enhances the performance potential of a light displacement boat but does nothing more than create drag on a heavy boat.

It is easy to shape the topsides of a single chine hull so that there is little or no twist, making it easy to plate. Not so with the bottom panel, which needs to have considerable change of angle from a deeply Veed bow to the flatter sections further aft. Unless the bow is very fine, the bottom panels will be fairly wide and will not accept twist readily, requiring a lot of fiddly cutting and fitting of the plating. Eliminating the twist will result in bow sections which will pound badly.

Hout Bay 30 "Anne M", to radius chine steel boat plansConically developed bow sections make plating very much easier but give the designer less flexibility in shaping the hull. It is necessary for all of the hull bottom in the bow area to lie on the surface of an imaginary cone and that includes the hull centreline and chine. Conical projection can force the designer to use rather fuller bow sections than he wants, particularly if the hull has a deep forefoot. As a result, the decision will often be taken to have a forefoot which is difficult to plate rather than compromise the design.

Multiple chines improve both the aesthetics and performance of a steel design. Properly placed and shaped, the topside chine can add character to a boat but, badly placed, it can ruin it. Multiple chines also allow the designer to more easily shape the hull to meet the needs of the sea. The panels are narrower so they will accept more twist without causing plating problems and the chines can be positioned to minimise pounding.

Some designs attempt to approximate a round bilge hull shape by having a multitude of chines, on the principle of the more the better. However, this just produces headaches and ugly hulls.

Bearing in mind that the bulk of the unfairness in almost any steel hull is introduced by the heat of welding, the more that the welding can be reduced, the better. Most builders use oxy-acetylene for their plate cutting which also causes some plate distortion, so plate cutting should also be reduced where possible.

Dix 43 "Starry Night II", to radius chine steel boat plansEach chine formed in a hull must have a plate cut to shape above it and one below. These must then be joined by a weld run on the outside and another on the inside. If there is a chine bar then there are a further two welds involved. If there are more than two or three chines on each side of the hull, this adds up to a lot of cutting and welding.

Another aspect of chined construction is the labour cost and the wastage of material. Everywhere that a plate must be accurately cut to shape there is cost involved in the labour of setting out, cutting and fitting and at every cut (particularly at curves) there is a certain amount of material lost in offcuts. Many offcuts can be used to make brackets , bow fittings etc but the quantity of offcut material is proportional to the amount of cutting done. A hull with six chines requires no more brackets than one with one chine but has many more offcuts.

Increasing the number of chines also reduces the change in angle from one panel to the next. While this is advantageous to a degree, it must not be taken too far. A large change in angle at a chine makes it easy to find the correct point of intersection of the two planes and gives a fair chine when viewed in profile. As the two planes become more aligned with each other it becomes progressively more difficult to locate the exact intersection point when plating and the result is a chine which shows as a wavering shadow line in profile. A hull with five or more chines each side can look like a crazy sculptor has been let loose on it and just begs for a load of putty to turn it into a round bilge hull.

Dix 38, radius chine in progress, to radius chine steel boat plansIt is often said that there is nothing new in yacht design, that whatever we come up with that is inovative has been done by someone else in the near or distant past. That applies to radiused chine hulls as much as to anything else. The technique was used by van de Stadt on the steel Doggers and has also been used by Ted Brewer and others in North America for many years. Each designer or builder has developed his own version of it which works well for his own style of boat design or construction.

To some, a radiused chine is simply a hard chine which is softened by welding in a section of the wall cut out of a large diameter pipe and welded in as part of the hull skin. Such hulls are generally of single chine form and, although they do benefit aesthetically and in performance from the softening of the chine, they are rather slab sided. Construction also needs to be rather more that of a hard chine hull than that described later for radiused construction.

Vickers 45, to radius chine steel boat plansMy own experiments with radiused chine construction began purely as a means of giving a round bilge look to a hull while keeping most of the ease of construction of a multi-chine hull form. The first boat was the 64 foot staysail schooner "Rising Sun" which I designed for Marine Metals in Norfolk, Virginia. The concept was so successful that she was exhibited two years running on the Annapolis Sailboat Show and drew large crowds, being one of the most luxurious boats on show. It required more than a casual interest in hull shapes to see that she was not a round bilge boat. A string of steel boats followed with similar hull shapes, from 40 foot to 70 foot.

There is only one benefit to that hull shape and that is its round bilge appearance. It has all of the characteristics of a multi-chine hull but has more cutting, fitting and welding. It is, therefore more costly to build but that is offset by higher value. Some people do not mind owning chines as long as they cannot see them when looking at their boats on the water.

Vickers 45, to radius chine steel boat plansSince about 1986 my steel hulls have been of a completely different hull form, closely approximating a round bilge shape with far less effort. Essentially a single chine hull form, the chine is rounded off to a large radius which is constant from bow to stern. It results in a hull which is as much round bilge as some of the lighter displacement GRP boats with firm bilges. Another local designer once expressed surprise to me at how quickly the steelwork of one of my designs had been constructed from framing to completion of plating. I explained that the hull was radius chine not round bilge and he was again surprised. His trained eye had not seen the difference.

The choice of the radius to use is a matter of personal choice. Too small a radius keeps most of the radius below waterline when the hull is at rest and causes a slab sided look. The eye likes to see some curvature low down in the topsides from about amidships through to the stern.

Dix 65, to radius chine steel boat plansMost modern yachts tend to have little, if any, curvature to the topsides near to the bow. Radius chine construction gives straight sections in this area, with all of the radius below waterline. This can give a wet boat if there is not good flare to the hull to prevent the bow wave from coming straight up on deck.

The included angle between the side and bottom panels of a single chine hull gradually reduces from bow to stern. In the bow the topsides are flared and the bottom Veed, giving a large included angle. At the stern the topsides are more vertical and the bottom is more horizontal, giving an included angle nearing 90 degrees. The result of this is that the section of radiused plate at the bow has a far smaller girth than that at the stern. Viewed from the side, the radiused section of plating is wedge shaped. This means that the stern, which is the portion of the boat which the eye most expects to see as rounded, has considerable curvature to it and most of it is above waterline.

Dix 43, to radius chine steel boat plansThe stern puts the upper limit on the size of radius used because the full curve has to fit in between the sheerline and centreline of the hull. If not, some major fairness problems will result.

The twist in the bow area of the bottom panel which can cause plating problems in a single hard chine hull need not be a problem with a radiused chine. Far forward in the forefoot area the radius runs out against the centreline of the hull and so eliminates the twisted bottom plate. A little further aft, where there may still be some twist, the radius still replaces the bulk of the bottom plate and that which is left is narrow and readily able to accept the required twist.

Dix 43, to radius chine steel boat plansThe economies of this method of construction result from labour savings in easier frame construction and easier plating as well as savings in materials used. Frames are made from flat bar to topsides and bottom joined by a welded in section prerolled to the required radius. The curved sections of all of the frames are to the same radius so they can be made in bulk to a pattern then cut as required.

The curved plating is, similarly, prerolled in bulk by any engineering firm which has a three roller plate rolling machine. Sections are then cut from the rolled plates as required to plate the hull.

It seems easiest to plate the radiused sections first, trimming them neatly to a fair curve at the tangent line. All of the radiused plating should be done before the flat plating is started. It is easy then to place the flat plate in position and mark the junction with the radiused plate ready for cutting. It is not as simple if the flat plate is in position first because an oversize piece of radiused plate cannot be neatly laid against the flat plate for marking. Templates would be required.

Ancilla II, to radius chine steel boat plansIt is normal for the forward and after portions of radius to be easier to plate than the midships portion. The chine tends to be straighter towards the ends of the hull, allowing fairly long sections of radiused plate to be used. In the middle, the chine normally has a large amount of curvature on plan and in elevation. This gives a tighter compound curve which the radiused plating will not successfully cover unless it is cut into short lengths. The best lengths to use will depend on the size of the radius, the length of the boat, the length/beam ratio and the shape of the chine both on plan and in elevation.

Some builders prefer to split the radius lengthwise in the midships area of the hull. This can allow slightly longer lengths of radiused plate to be used.

In addition to the savings on plating, there is normally a saving on stringers as well. A multi-chine form forces most stringers to run full length and does not allow efficient selection of stringer spacing. The stringers must fit between the chines. With radiused chine construction, stringer spacing can be varied throughout the length of the hull to suit load requirements, although it is normal for them to run parallel to the sheer. A stringer can start in the topsides in the bow, pass through the radius and finish in the bottom plating at the stern.

Dix 57 "Majuli II", to radius chine steel boat plansSome designers combine radiused chines with developed surfaces, in an effort to more closely approximate a fully round bilge hull. Where this is most noticeable is forward of amidships where some curvature can replace the flat topsides sections. On the negative side, this somewhat reduces the simplicity of frame construction.

Boatbuilding, in whatever material, is never easy. It demands dedication to the project. Once the construction material has been chosen, the trick becomes how to optimise the combination of aesthetics, construction ease, cost, performance and resale value, some of which normally have contradicting requirements. If the material of choice is steel, radiused chine construction cannot be beaten in that optimisation.


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This page was updated 5 September 2002