D e s i g n Your Own Motor Boat

Continuing his series on designing an environmentally-friendly motor boat,

naval architect Ian Nicolson develops the mid-section.

Figure 1: Once the elevation outline, the waterline and the deck line have been finalised, the mid-section is drawn using the establishing point at A to E as shown on the drawing. On the right is the midship section plotted from these six points. The width of the waterline is AB, the width at the deck-edge is A-C, the height at the deck edge is E-F and the depth below the waterline is D-E.

The mid-section of a boat is the view at half her length looking aft. It is easy to rough in on the lines plan once the basic elevation and plan views have been drawn – as described in the last issue – because we have three exact points on the section. Figure 1 shows how we get the distances off the centreline and the distances above and below the Design Water Line for the deck edge and the width at the Design Water Line.

We also know from the elevation the height where the midship section meets the centreline. Having established these three points, we have two choices:

1: We can accept them and draw in the midship section and once we like what we see, we can harden in the line.

2: We can decide we do not like what we see and make a change which has to be reflected in the elevation or the plan view or both.

At this stage, some designers agonise long and hard, making changes galore and sometimes coming back to the original shape, sometimes altering the elevation and/or the plan lines. The late Alfred Mylne ll seemed never to have second thoughts and he drew lines plans with the swift certainty of a master craftsman.

At this stage, we have only three points on the curve of the midship section. The way the line goes between these points can vary considerably, as Figure 2 shows. A naval architect will select the final curve bearing in mind the need for easy progress in spite of low power propulsion, the requirements of the accommodation and a good appearance. Sometimes, ease of construction will be a factor and also the way materials can be coaxed into shape.

When designing an easily-driven motorboat with a titchy engine, the designer has stability problems because the waterline beam amidships must be narrow. A cunning way to get around this disadvantage is shown in Figure 3. Here there is a knuckle just above the Design Water Line so that when the boat heels a little, the extra hull width is quickly immersed and suddenly there is a valuable increase in stability.

When assessing the stability, the designer notes the difference in volume between the immersed wedge and the emerged one which are shown cross-hatched. The greater the difference in area between these two shapes, the better the stability. Here, the area on the right is appreciably bigger than the area on the left. the area on the right is going into the water and here the buoyancy is pushing upwards, tending to shove the boat back upright.

Some shipwrights will protest that this knuckle is a hard shape to build but they are too pessimistic. When building in wood, one just fits an external stringer – a longitudinal

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www.watercraft-magazine.com Figure 2: Having fixed the three basic points of the mid-section, the designer has a choice of shapes. Three possibilities are shown: a full section below the waterline and flared topsides.; a more moderate easier to build; shape and one which is hollow below the waterline and full above it, with tumblehome giving extra space inside the hull.

Figure 3: To get extra stability in spite of a narrow waterline beam, have a knuckle just above the LWL. As the boat heels, a large amount of hull volume is immersed, much larger than the volume which emerges from below the waterline, as shown by the two hatched areas. The extra buoyancy thrusts upwards, pushing the boat upright.

Figure 4: In extreme conditions, the boat may heel beyond the usual angle, so stability is vital. T he dotted line shows a normal hull shape but there's a knuckle high up, giving extra immersed volume when heeled. If the deck edge is going under, extra stability is gained from the full width.

www.watercraft-magazine.com length of timber – outside the lower planking, tapering it away at each end. The planking extends up from the keel to the inboard side of the stringer, then restarts going upwards on its outside.

Forming a metal hull is just as easy, only this time instead of the stringer one fits a horizontal plate which tapers away at the fore and aft ends. In practice, the underside of the wood stringer or the metal plate will almost always slope upwards as it goes forwards. This is to reduce pounding and ease progress through rough water. It also tends to lift the bow as the knuckle slices into the waves, which reduces the immersion forward and – with luck and good design – slightly reduces the resistance to forward motion by keeping the bow high, so there is less wetted surface and a little less volume to shove though the water.

Whatever the hull material, this shape has ancillary assets. It tends to keep down the amount of spray coming over up over the deck, it provides a longitudinal strength member and also a little extra space in the hull. However, it can be vulnerable, especially when coming into a marina with low walk-ways and so it needs notched or joggled frames to support it. In rugged conditions, it can pound noisily and since this is an indication of the loads imposed by waves, it needs to be strong. Also, sharp changes of hull shape are notoriously weak and here we have two violent changes of shape, so the internal structure must be extra rugged.

With this in mind, I like to do at least a preliminary midships construction plan once I have finalised the midship section. I want to check that there is ample space below the sole – the cabin floor –for deep floors and perhaps for tanks. The floors are the bottom parts of the frames and they are important because they tie together the lower ends of the frames and the bottom planks or plates and help secure the hull shell to the keel or hog. On ballasted boats, they help support the heavy weight.

On modern boats, there is a tendency for floors to be too small, too thin and above all too shallow. More than once I’ve extended them on old boats up through the cabin sole at each side to give extra life and critical extra strength to an otherwise sound craft. However, some owners object to these protrusions through the sole. One complained he stubbed his bare feet on them; “Buy some deck shoes” was a comment I heard at the club bar.

Every vessel needs all the stability it can get when the weather turns nasty. Another device to get extra stability is to have a knuckle high up, with lots of flare, as shown in Figure 4. This only comes into operation when the vessel is heeled a great deal but by then things may be getting serious, so extra hull volume well off the centreline can help save the situation. If the boat suffers a knockdown, perhaps by a rogue wave or a hurricane blast, then a cabin top which extends upwards from the deck edge also gives buoyancy where it is most needed.

Keeping the water off the sidedeck and supplying hull volume when the craft is lying over at about 90˚ will give the boat a good chance of self-righting. However, this technique of dealing with extreme condtions only works if the cabin top is strong enough to withstand the waves and has windows which are shatter-proof.

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