Building a fast electric boat.

Introduction

This article is intended as a guide for builders and is not intended for specific models. We will cover different hull types as appropriate and different materials and provide some hints and tips that will enable you to get the most from your model. This is not a source of plans! First we will start with materials.

Wood

Wood is a a relatively cheap and easily worked and joined material but it is flexible and is affected by moisture so care needs to be used whilst building. Correct choice of wood is important and differing thicknesses and types have uses for different parts of a boat. Correctly done a wooden boat is light, strong and relatively cheap.

There are a number of different build techniques and they all have their uses and limitations.

Monocoque construction.

This is where the body panels are the only mechanical constructional parts and their shape and the way that they are joined provides the strength and shape for the whole structure.

In general panels are developed and can be tacked or sewn together to create the structure. In wood this either gives a weak joint or the joint will leak - so the joints are strengthened by glass tape and resin. Some finishing is then required to get back to the intended shape. In most cases the whole structure will be coated with resin and a very thin tissue can be used to aid the finish and add more strength. This process works well with with thin ply, light-ply (also known as balsa ply), balsa and card (not wood I know but worth inclusion).

Framed construction

This process uses internal frames to aid constructional strength. A Draw back of framed construction is that the frames intrude into the free space in the boat making fitting out more difficult. Frames should be placed with this in mind and should also be constructed to cause minimal intrusion into the useable internal space.

The frames are usually held in position by a spine (the keel) and longerons at key points (usually corners or chines). Frames can be from ply, light-ply, balsa etc. The framing is a critical part of the build and as such requires great care as any defect here is built in!! Once the frame is complete it is skinned. The skin can be constructed from planks or from sheets of suitable material. No strengthening of joints is required.

Bread and Butter

Bread and butter construction is whre planks are glued together and then carved to obtain the final hull shape - both internally and external. There is a variation on bread and butter construction where large flat surfaces are replaced by ply, leaving only the complex surfaces to be carved.

These methods of construction are mentioned here as whilst they have no merit for a fast electric model - they can be and are used to develop the moulds necessary for creating resin or plastic moulded hulls.

Pro's and Con's.

Wood and in particular plywood , is a product that is both flexible and strong, it is easily worked and glued but it is affected by moisture and must be protected from it. While it is in general easy to use and join - it is very easy to build in a warp which will destroy the effectiveness of the model. It is advisable to use a solid base to build on ad tack surfaces and check for shape and warping before final gluing.

Plastic mouldings

This really depends on where you start. If you want to produce your own boat then you are going to have to build a mould around which to form the plastic. In general a male mould is used and the mould will be need to be the thickness of the plastic smaller than the finished item. For more accurate forming a female mould can be made.

Mould materials

Moulds can be made from various materials including medium density fibre board, plaster and, for long term pulling power, aluminium filled epoxy. The epoxy solution is expensive and the mould for the mould is usually filled with scrap wood. Styrene based resins and fillers should be avoided as they gas and become unstable when hot.

Mould construction

Moulds can be made in a number of ways and cost widely varying amounts, the higher cost moulds tend to have a longer working life. If you only want to pull a few hulls then you can construct a master mould from almost any material that will take a fair edge. It is not important to have a glass smooth finish but a fair medium sanded finish is best. MDF is a good material that is easily worked but it has nasty fibres and a mask should be used when using it. Good quality would without shakes and blemishes could be used just as well.

For a very few pulls you can use plaster - moulding plaster is harder and has a longer life. Plaster can be shaped after a rough moulding - but don't be deceived - it is still a lot of work.

If you want to make a large number of moulds then the best solution is an Aluminium filled mould. This can be made from a block of material and carved to shape with a CNC machine or you can produce a mould out of one of the other materials - take a plastic mould from it and pour the Aluminium Epoxy into that. Be warned this epoxy is expensive.

Pro's and Cons

This is an extremely simple way to make a boat. You generally use a male mould as above and stretch the plastic over it so mould making can also be fairly simple. The mould does not need to be glass finished (in fact that's a disadvantage) but small bumps and lumps will be reproduced, so you don't need expert finishing capabilities but you do need to work cleanly and accurately. The reproduction is good but all the edges will be rounded so you need to think of ways to combat this before starting to build the mould. The mould needs to be the plastic thickness smaller than the finished object. The material is easy to work with, good to glue and paint (though you do have to cut the shine off) and there is a choice of plastics that the mould can be used on. ABS is the best but absorbs water and needs to be humidity controlled, Others are not quite so good. Plastics can be affected by UV and some glues (Notably Cyano's). They can also be affected by some paints. UV, Glues and Paints can all cause it to go brittle and shatter if the right ones are not used.

Fibreglass and Epoxy glass

These are constructed by laying up glass fibre cloth in a female mould. The female mould is generally made from an original or a male mould of the original. Unlike the plastic mould the male has to be perfect, any even tiny imperfections will be faithfully copied. The male moulds are polished to a glass like finish and from that the female moulds are made. Some shapes cannot be easily removed from a single part mould and the female may be made in several parts to make release easier. The female mould is then assembled before and polished before use for moulding.

With GRP a gel coat is normally applied to the mould before laying up with clothe and normal resin. With GRE a GRP gel coat can be used but some manufacturers use a normal paint in lieu of gel - and some use no gel coat at all. The gel coat is normally coloured to make the hull opaque. With GRP the noraml resin can also be coloured but many craftsmen will use it in it's normal state as the couloring adds significantly to the weight. With both GRP and GRE there is an optimum amount of resin. This amount is enough to fully wet the glass cloth out, any more does not add to strength but does add to weight. Professional establishments will use a vacuum baging technique that ensures that the layers maintain a close contact whilst curing. The bag will also have an absorbent layer that soaks up excess resin. Both GRP and GRE hulls are generally supplied joined and most will be joined in the mould to ensure accuracy. Some will be paste joined and some are butt joined with tape over the join.

Pro's and Con's

This method of construction results in an extremely accurate reproducible replica of the original, it requires little or no finishing but can be painted. Finished hulls will have a waxy coating on both sides and this must be effectively removed before any glue will bond to it. This is especially true with epoxy to epoxy. Epoxy hulls will reject many glues (e.g. the one on the rear of velcro) and special care must be taken. The high gloss finish means that care must be taken to stop toold slipping when construction is under way. GRE is the strongest and lightest easily available mould construction, though carbon and kevlar can be obtained they are not generally available. A twisted hull is virtually impossible to correct and is an expensive mistake.

Truing the hull

Before you do anything you need to check that the hull is straight and true, has sharp edges where the water runs off (edges and strakes) and that it has no dips or bulges. If you continue the build with a crooked hull then you will lock the shape in forever and the boat will not perform at optimum. Even a carefully moulded epoxy or glass hull can take on a warp as it cures so do not assume that it is straight.

You will be able to lay most hulls on a flat surface in a way which will allow you to check them out side to side. Plastic hulls and fairly freshly moulded GRP hulls can be reformed by gently heating with a hair dryer and "untwisting" them. Any hull with multiple deformations should be returned to the manufacturer/ supplier for replacement. Most Epoxy hulls are joined and cured in the mould and should not be twisted - if it is send it back or better still don't buy it in the first place.

The running surfaces need to be flat and true, no lumps bumps or hollows. Remove all deformities by judicious use of a light filler and sandpaper. Strakes and wet edges of a hull need to be sharp in order to shed water. If they are not sharp then the water will run round the edges and create drag. On a transom this can cause the boat to have a different and unpredictable ride. This is not generally a problem on glass or epoxy boats but plastic mouldings are well known for curved edges and they need fixing.

Steps and Transoms and Sponsons

Some manufacturers have developed and use techniques to help with obtaining sharp edges on the larger surfaces. These include:

• adding a bump on critical edges, the bumps are then filled internally with a light filler and scraped or filed flat externally.
• Moulding a recess in the hull which is then filled with sheet material
• Covering the surface with sheet material.
• Arranging the mouldings so that edges are clean cuts in the moulding

The choice will be made on ease of implementation, required finish and cost.

Strakes

None of the above will work well with strakes and we are left with two options, leaving them off or leaving rounded edges that the modeller will fix.

Strakes can be applied as part of the build by gluing an extruded plastic mouldings or wood shapes, to the outside of the hull. This gives good sharp strakes but requires a lot of care in application. Available plastic extrusions are good but tend to be of softer plastic, wooden extrusions of the right size and shape are hard to come by.

This leaves us with fixing moulded in strakes. Whilst this can be tricky it's not necessarily that difficult. The easiest way is to fill the inside of the strake with a light filler mix and then carefully scrape, file or sand the exterior surface to the desired shape. Another method involves using sticky tape to build a dam and then using cyano-acrylate glue as a filler to produce the edge. Edges produced with cyano are hard and durable but they still need finishing and the glue can adversely affect plastics.

The effect of sharpening strakes will only be truly felt when racing at a high level. Detecting the difference in performance of a plastic moulded eagle without sharp strakes to one with unsharpened ones is a close call. The choice is yours as the say.

Adding the hardware

You now need to decide where everything will go. To do this you may draw on your experience or use a suggestion that came with the hull, whichever it is important to get this right as often it is difficult to change it significantly at a later date. Lay out all your components within the hull to get a rough idea of where everything will fit so that the correct centre of gravity and drive thrust line can be achieved.

When doing this you need to:

• try and keep the weight low and near the centre line (on mono's).
• Keep the power and radio leads as far apart as possible.
• Keep the power leads as short as possible
• Make sure that the aerial has as much vertical length as possible.
• Take account of the weight of the running gear
• Make sure that all items that will need servicing are accessible.

If the hull is not joined already then you can do most of the build before joining, this gives you room to work and is therefore easier BUT you have to make sure that you do not introduce a twist into the hull and that you make sure that everything is accessible once built.

Sometimes the potential for adding a twist can be easily countered by adding dowel or carbon rod. This can be either bonded to the hull as you would bearers, giving longitudinal stiffness, or can be constructed as a sort of space frame that will give multidirectional stiffness to the hull. The type of solution that you use will depend on the hull and hardware arrangements.

Next step is to provide some additional strength to the hull where the hardware (prop shaft and rudder) will run through it and/or attach to it. Doubling the area around the fixing points is the solution of choice. Doubling adds strength and spreads the loads across a larger hull area. In Eco type boats this can take the form of small patches around the propeller shaft and rudder shaft entry points. In surface drive boats a plate is secured to the inside of the transom - often the whole of the transom is doubled in this way but sometimes a smaller area is done in order to save weight.

Where a turn fin is fitted additional strength must be provided to spread the loads and to allow for the effects if impacts.

Having added all the necessary support you can now make the holes for access and fixings as needed.

Holes should be a close a close but not tight fight. Tight fits cause deformations in the hull, which will cause performance problems.

The fit for a subsurface mounted shaft and propeller should allow for the largest prop that you will use. With a straight shaft, the shaft normally enters the hull at or close to the longitudinal centre of gravity (LCG) and at as low an angle as possible. This is worth some discussion as there are several forces at work and it pays to understand the effects of each. The complete loaded hull will tend to rotate around the centre of buoyancy and the centre of gravity. We will hope the design is correct as that determines the centre of buoyancy and there is not a lot we can do about that.

Rotating the propeller produces four effects:

• Forward motion
• Longitudinal moment
• Rotational effect

The forward motion and the lift are directly affected by the shaft angle and the force applied. I cannot provide a full explanation in this small space nor would it be good as it is complex - so I hope I have given enough information to provide a general and high level description of what goes on.

If the shaft to the running water line is 'a' degrees and the force applied is F then the forward motion is proportional to Fcos(a) and the lift is proportional to Fsin(a)

If 'a' is zero cos(a) = 1 and sine(a) = 0 so all the thrust goes along the running water line

If 'a' is 5 degrees then cos(5) = 0.996 Sin(a) = 0.087 or 99.6 percent of the force is along the running waterline. At 10 degrees it is down to 98.5% and at 15 it is 96. So a small angle is OK but the effect grows rapidly as the angle gets larger.

At 15 degrees the lift is 4 percent - this is directly lifting the transom and altering the running angle to bows down increasing drag and slowing the boat. This effect is much more significant than the forward thrust.

The longitudinal moment is a twisting motion that is caused because the boat will try and rotate about the centre of gravity. This is a generalisation as the twist will move the centre of buoyancy applying a correcting force. We need to balance these forces as much as possible and that is easier when they are all close together. One way to do this is make sure that the thrust line passes close to the centre of gravity and centre of buoyancy. If tis line is above the c of g then the bows will be forced down. The reverse is true if the thrust line runs under the c of g.

The rotational effect is caused as every action has an equal and opposite reaction, spinning the propeller and motor armature causes the motor case to try and rotate in the opposite direction. This phenomenon is generally called torque roll. Torque roll can be offset by:

• Design (built in anti roll features in the hull)
• Additional anti torque wedges
• By simply offsetting the mass of the boat to counteract it.

The Motor Mount

Ahh - a special section for motor mounts, that's because they need care and attention. The motor mount is provides a solid location point for the motor and so it needs to be strong, that much is obvious but there is more to it than that. When the motor spins it turns the prop, and turning the propeller produces a number of forces as described above. The motor mount must be designed and placed so that these forces do not distort the hulls running surfaces that we so lovingly ensured were straight earlier. This means that the mount must spread the tortional load, and any end load transferred up the drive shaft. End load is not a big problem with straight shafts but can be a problem with flexible shafts. The mount needs to be designed to effectively spread the loads, so with a straight shaft the load needs to be spread around the hull to prevent twist, and with a flex shaft (unless a thrust bearing is fitted) it needs to spread the load across and along the hull as appropriate. Hull features such as steps provide well braced and solid points for attaching motors. Long flat un-braced sections of hull need a larger motor mount to prevent distortion, whilst at the same time minimising weight. Also remember to allow enough room to fit the water-cooling.

For solid shaft and motor combinations the shaft and motor should be fitted as a unit, these units can be relatively heavy and you need to be careful when fitting to ensure that he hull is not stressed and deformed. Hanging the hull up by the shaft so that the shaft is more or less vertical is one approach that minimises the effect of the weight of the motor. Tack the assembly and make sure it all sits true before finally gluing it all together.

Cells radio, speed controller etc

All these items are need to be accessible so whatever method you use to fix them has to be secure but temporary. Velcro is one solution and re-useable tie wraps are another. Velcro is suitable for batteries and radio whilst tie wraps can be really useful for battery packs.