Beginners guide to fast electric racing

Now a lot of people have asked for a quick orientation for newcomers to the fast electric hobby, with a view to how to start. There are a lot of points to cover and we cannot do it in a great deal of detail here. Don't let this frighten you, most of it is fairly simple - it just needs to be got right!!

Fast is a relative term when we say fast electric we are usually (but not always) talking about race boats or models of race boats or high speed planning craft. So if we are still talking the same language please read on. Further to that then there are two ways to go, the kit route is the simplest but in general kits have to be bullet proof and that costs on performance, You can hop up a kit but it will cost more than if you built for speed to start with. There are some notable exceptions in the Graupner range as they supply very high quality hull kits that look good. Just don’t buy the suggested hardware.

So you got yourself a radio controlled electric boat and now you want it to go fast.

Whatever the boat there are a number of things that you have to address to get the best out of it. The hull, balance, hardware power train and power supply all need looking at.

THE HULL

First things first - we have to sort out whether it has the potential to go fast or not. Hulls come in two forms – displacement and planeing.

A displacement hull is designed to be efficient in carrying loads and is not really suitable for going fast – so if you have a trawler, fishing boat, oil tanker, liner or a few others, it’s best to forget going very fast.

Planeing hulls are designed to ride on top of the water so reducing drag and allowing high speeds – and that is the type we are looking for. In general these hulls are typified by flat surfaces, wash strakes to remove the water, deep ‘V’ (in mono-hulls) or specially designed planeing surfaces e.g. as on hydroplanes.

Assuming that you have the correct type of hull the next thing to do is to check the hull and make sure that it has no ‘bad’ bits. What’s a bad bit – well it’s something that will have a detrimental effect on speed. Some bad bits can be fixed others can’t. If the hull is heavy then that costs in speed, at the same time it has to be rigid and not twist under the forces applied to it on the water – it’s a fine line.

Hulls that are prone to twisting can be strengthened by use of glass/carbon rods glued in a rigid frame attached to the hull. Scale hydroplanes, and Formel types, in particular can benefit from this treatment. But you cannot beat a purpose built Epoxy glass/carbon hull. (But do beware of carbon fibres (or graphite as it is called in some places) as it does conduct electricity and can cause electrical noise transmission in boats).

So we have a good twist free light hull – great of we goooooooo. No No No slow down a bit. The hull may be light and rigid but that’s not all. To work well, quite apart from the base design, the hull must be flat and true - if it’s not it can cause handling problems as dips cause the water to suck in and bumps do the opposite. If a hull is taken off, or out of, the mould too early this can happen. Grab a straight edge and run it over the flat portions of the hull and check for dips or bows. Dips can be fixed either by internal bracing or by filling the outside and sanding back bumps need bracing. I use epoxy with micro-balloons as filler.

The last step with the hull is to make sure that all water shedding edges are sharp. If they are not then water will adhere round a corner and cause an enormous amount of drag. Wash strakes that need sharpening should be filled internally first and then sanded or scraped (with a knife blade) to a sharp contour. Edges such as transoms, steps and the back edge of sponsons can be a bit more trouble but it is well worth the effort. You can use PVC tape to form a barrier and fill the curve with filler and sand back, or you can add e.g. plastic-card and fill the gap , filing to shape.

BALANCE

To run properly the forces acting on the hull must be correctly balanced for highest performance. When we talk about balance it is not just physical weight but also the hydrodynamic and aerodynamic forces that need to be considered. Now hopefully the guy that designed the boat did all this for you but what you do to the hull can cause a variance.

On an ECO boat the motor position and the point at which the shaft exits the hull both affect the angle of a straight shaft. His alters the thrust line through the boat and can alter the balance. Also the type of propeller used can affect this but we will deal with that later.

On other boats there are other considerations so unless you have a lot of experience follow the build instructions.

On mono’s – as a rule of thumb, start with the centre of gravity 30 percent of the boat length from the transom, on an 18 inch boat that’s 6 inches or if you are in to metric substitute centimetres!! If the boat is stepped then the front step is likely to be at that position. By the way in some countries a stepped mono is regarded as a hydroplane and cannot race in with the mono’s – so if you intend to race check first.

On Cats it is not quite that simple though 30% is a good starting point. In general the tunnel on a cat is designed to provide the boat with lift, so if the weight is too far back then the lift will tend to make the boat porpoise or prevent it from planeing flat. If that happens move the weight

Hydro’s are a bit different in that the front sponsons are designed to lift most of the hull, and the prop is meant to do the rest. The C of G should be in the range 6-14% of the after-plane to the rear of the rear of the sponsons. Scuse mee!!! After-plane!! Settle down it’s not that bad. The after-plane is the distance from the rear of the sponsons to the centre of the prop (That’s what I use and it works – argue if you like but it’s not broke so…..).

DRIVE HARDWARE

Still with me? Good. Now another thing that robs you of power is a poorly constructed drive train. You can measure just how much by free running the motor and measuring the current drawn with a multi-meter be warned it may be a few amps. Now connect the motor up in the boat and rerun the test. If the difference is more than say 1/10th of an amp then you need to do something.

What should I check? Well – I’m glad you asked that!!

First check the stern shaft – if it is a plain tube with no bearings it needs replacing. If it has plain bush bearings check that they are a good running fit and lube them correct. Do not fill the shaft with grease – it only causes power loss. If you are getting water in replace the bearings for ones that fit properly. Check the prop shaft for straightness and refit in the tube and check that there is no bind and it runs freely (The same applies to flexible shafts). It is important that the motor shaft is in a direct line with the prop shaft, flexible couplings allow for errors in this line up but they soak up power, if you have a flexible one then get rid of it and replace it with a solid coupling.

The same goes at the prop end, many kits supply a flexible coupling to change the drive line or for a steerable out-drive. If you have a steerable out-drive think about replacing it. If you elect to keep it then we need to do a bit more work. To minimise the losses the pivot on the shaft must be in line with the pivot on the steerable out-drive. You can usually tell if it’s not as it will float around. So the prop shaft needs to be aligned to bring the pivot point to the right place. Use a long pin in the outdrive pivot to point at the correct position for the coupling. Place a hard nylon thrust washer on the shaft to take the end loads and get the whole thing lined up properly. Wee need zero play in the prop shaft and no bind so use another thrust washer and a collar to fix the position – but make sure it moves freely. The stub shaft for the prop needs similar treatment. If you get it right you will be able to move the whole assembly and rotate the prop shaft without significant extra bind. Yes it’s hard so think about replacing it with a solid or flex shaft.

On a surface drive boat I like to position the bottom of the boss of the prop on an extended line from the hull – if the hull is stepped then it will be from the peak of the front step. I would suggest that the right prop position is trailing the hull by 10% of the hull length. 30 inch hull – prop 3 inches back.

On a subsurface hull there is usually no adjustment so the prop centre needs to be far enough below the hull to allow you to fit the largest prop that you want – plus clearance (say 1mm). Earlier I mentioned lift due to prop shaft angle – this can help in flat calm and is no help in the rough. Some people prefer to run a flex drive and apply power parallel to the hull. My feelings are mixed in that if you can apply the power through the centre of gravity then no torque is generated, The centre of gravity also needs to be the same point as all the hull dynamic forces operate on….. the designer should help you with this!! Here is not an appropriate place to go into it except top make you aware that the other forces acting on your boat need to be taken into consideration.

The propeller is a key component of the drive chain. To work properly it must be statically and dynamically balanced – if it is not then it will destroy your bearings. This is not really and issue with glass filled nylon props, unless they are damaged. Plastic props flex, and you cannot predict how – so avoid them. Metal props need balancing and sharpening. We already discussed unbalanced props but blunt ones act as paddle wheels and destroy the handling of a boat. Unbalanced blade shapes can also cause vibration and destroy bearing. The only solution for metal props is to invest in a balancer – they range in cost from £10.00 to £30.00 the choice is yours but it is money well spent.

We should just mention rudders, my take is that the rudder should be vertical to the waterline when running. A trailing rudder angle will lift the stern and drop the bow in the turns. A leading rudder angle will help keep the bow up in turns. This applies to all models.

One last point on the drive line. A lifting prop or a downward point thrust line on the prop will cause the bow to drop. An upward thrust line will pull the rear down and lift the bow. My preference is in line.

THE POWER SUPPLY.

Now that the rest of the boat is in good order our attention moves to the power supply, cells, speed controller and motor. The motors job is to convert electrical power into mechanical power, the prop converts that into thrust. Motors range in efficiency and if you are racing you need to ensure that yours is as efficient as possible. Standard motors are machine wound, have plain bearings and probably the wrong brushes and springs. All these bits cost money so consider how much you want to spend. Ball bearings, medium brushes , medium springs, water cooling and hand wound and balanced armatures are all there for the taking. Water cooling? Yep hot motors run less efficiently so cool the motor and the brushes. Also clean the motor and oil the bearings regularly.

The speed controller meters the power supplied to the motor if it is inefficient it wastes power or limits the amount available to the motor get the best you can.

Batteries store the power so we need good ones, there is no point in buying high end matched cells if you only intend to have sport use though!

And all this is to apply power to the prop which converts mechanical motion into propulsion – its critical so ignore it at your peril. You need to match the prop to the motor combination so ask or experiment. BIGGER PROPS DO NOT NECESSARILY GO FASTER!! To run the motor efficiently you need to allow it to run at as high a speed as possible there comes a point where a bigger prop will slow the boat and may damage motor ESC and batteries.

CONCLUSION

I hope this has helped – Next step is a book there are two available but I think the best is Fast Electrics by Paul Williams.

Be warned everything is interrelated. If you make changes do them one at a time and measure the effect. For example, you may want to try a number of props on different motors but make the changes in a controlled manner and note the results. Also remember that a change to one set up may have a negative impact but with a slightly different settings elsewhere may work better. So keep an open mind….. And above all have fun.

Oh and if you have a question try the qanda lots of people there to help.