Drive Systems

By Paul Williams.
Article posted 2nd March 2008.

The drive system of an electric model powerboat transfers power and torque from the motor to the propeller. There are two basic types of drive: submerged drive and surface drive.

Submerged Drive

Submerged drive is the type of drive fitted to "ECO"-type racing boats, and in this system the propeller is mounted underneath the boat, most often at an angle to the horizontal using a solid straight drive shaft. Submerged drive has a number of things to recommend it. Acceleration from rest is pretty much instantaneous, and tight, full speed turns in both directions are possible with minimal speed loss. However, there are also a number of serious disadvantages to submerged drive, including:

• Appendage drag
• Angled drive
• Maximum propeller size is limited
• Cavitation

Appendage drag is the drag caused by the propeller, the propeller shaft and all the mounting brackets and struts that attach the drive to the boat. If the drive is mounted at an angle in the boat, then this causes two problems. Firstly, angling the drive away from the horizontal reduces forward thrust, and secondly, the lift caused by angling the drive upwards often causes handling problems.

Typical submerged drive setup.

A submerged drive has a fixed amount of clearance between the end of the drive shaft and the underside of the hull, which limits the maximum size propeller that can be fitted without the blade tips impacting the hull. When installing the propeller shaft and outer tube, the angle at which the drive is placed should be kept to a minimum to reduce hull lift from propeller shaft upthrust - unfortunately, keeping this angle to a minimum also restricts the largest size propeller you can fit.

Replacing a solid straight drive with a flexi-cable allows the drive to be run parallel, increasing drive efficiency and eliminating the lift caused by angling the drive.

An alternative type of submerged drive uses a flexible cable shaft in place of a solid straight drive. This allows the drive to be run parallel, instead of angled, which increases forward thrust and reduces or eliminates the handling problems caused by the lift of an angled drive.

Cavitation

A submerged drive propeller generates thrust by creating a pressure difference across the propeller. Water flowing over the forward face of the propeller has further to travel than water flowing over the rear face, forcing it to increase velocity and thus drop in pressure. If this drop in pressure is too great, an effect called cavitation can occur.

Cavitation is the formation of low pressure vapour cavities or voids on the propeller blades. The boiling point of water is directly related to pressure. Drop the pressure hard enough, and water will boil at ambient temperatures - cavitation is effectively the water boiling for a tiny fraction of a second, before the cavitation void collapses against the propeller blade.

Cavitation on a model propeller merely reduces thrust and therefore speed, and is usually caused by turning a small propeller at excessive rpm. On a full-size propeller, cavitation can cause serious damage, as the cavitation voids collapse against the propeller with considerable force and eat away at the blades.

Surface Drive

The alternative type of drive for model power boats is surface drive. Surface drive moves the propeller out from under the hull and mounts it behind the boat. The propellers used in surface drive applications are designed to run with one blade in the water and one blade in air, or ventilated. There are several advantages to surface drive over submerged drive. Appendage drag is reduced to a minimum, with only the propeller itself and the rudder blade in the water - all brackets and struts are moved out of the water and onto the transom. The drive thrust line is parallel, and there is no limit to the size of propeller that can be used (well, there is a limit, but that limit is dictated by other factors).

Surface drive moves the propeller out from under the hull and mounts it behind the boat.

The drive system must be robust enough to handle the power output of your chosen cell and motor setup, and must transmit power to the prop efficiently. Any binding or friction will simply waste motor power in heat instead of useful thrust. The most widely used surface drive system is the flexi-cable type.

Flexible Cable Drives

A flexi-cable uses spiral-wound steel power transmission cable that is glued or soldered into the stub shaft (the shaft to which the propeller is fixed). Flexi-cable drives have been around for many years. They are easy to install, robust and allow the drive to be installed parallel to the waterline. Once installed, a flexi-cable has a small amount of adjustment vertically and in angle relative to the boat. This allows for some fine-tuning. So, flex-cables have much to recommend them.

There are four sizes of flexi-cable used in model powerboats: .098", 0.130" (1/8th" or 3.15mm), 0.150" (3.6mm) and 0.187" (3/16" or 4.75mm). .098" (refered to as "98 thou") is used in the lightest, small low-powered boats, for example mini oval racing. 130 thou is used for everything from submerged drive ECO boats all the way to 4S Hydro 2/Mono 2. 150 thou can be used in place of 130 thou for more powerful models, and for the most powerful models 3/16" cable is used. This is the cable used most commonly in IC powerboats.

Flexi-cable surface drive set.

The main problem with flexi-cable drives is making sure the motor/flexi coupling securely grips the flexi-cable. If the motor coupling lets go of the flexi-cable, you risk losing not only the flexi-cable, but also an expensive propeller that might have taken 2-3 hours to sharpen and balance. With the power and torque of modern brushless motors, you need a substantial coupling to adequately grip the flexi-cable.

Large flexi-drive coupling for 3/16" flexi-cable in an 8S Maratimo offshore catamaran. This coupling has six M4 socket set-screws to grip the flexi, and four to grip the motor shaft.

Flexi-cable Installation

Flexi-cables drives will break if the bend you run them through is too severe. The following diagrams are a guide to what's ok and what isn't:

Flexi installed in a mild curve - no problems.

Flexi installed in a mild S-curve - again, this is fine.

Flexi installed in a very tight S-bend - this will break.

Flexi-cable should be run either in a PTFE liner or direct in a brass tube outer. For example, the most common size flexi used in FE racing is 130 thou (3.15mm) which can be run in a PTFE liner inside 3/16" OD brass tube, or directly in 5/32nd" brass tube. In both cases, you must ensure the flexi is kept lubricated and is removed, dried and re-lubricated after each session racing or testing. Left wet in the boat the flexi-cable will rust quickly, as it's made from carbon steel. If it rusts, it will swell up, possibly seizing inside the stuffing tube. If this happens, you may be faced with having to remove the whole drive to get the rusted cable out of the boat.

.130" flexi in 5/32nd" brass tube (left) or PTFE liner and 3/16" brass tube (right).

Whether you choose to run in PTFE or direct in brass tube, the flexi must be a good fit. Any amount of slop will allow the flexi to helix up under load, or thrash around inside the tube. Finally, flexi-cable shortens under load. The more power you put through a flexi, the more it shortens, so leave a generous gap between the dog-drive/propeller boss and the strut, such that when running under full power and torque there is still a gap so the dog drive or propeller isn't rubbing against the strut.

Wiredrives

A wiredrive uses thin, spring-treated wire run through a mild curve to transmit motor power to the propeller. The wire used is typically piano wire of 62 thou (thousandths of an inch), or approximately 1.6mm, and 78 thou (approximately 2mm) diameters. A wiredrive offers better efficiency than a flexi-cable for two reasons. Firstly, flexi-cable has an inherent amount of friction due to its construction from multiple strands of spiral-wound cable. As the flexi-cable rotates, the strands of wire move around by a small amount and rub against each other. Secondly, a flexi-cable must be fully supported throughout its entire length from the motor coupling to stub shaft, to prevent the cable winding up, or "helixing", under high power outputs. A wiredrive, in contrast, only needs supporting where it exits the boat (the underneath of the hull in a hydroplane or cat, the transom of a monohull) and only has a single wire strand, so has no in-built friction like a flexi.

A helixed flexi-cable. You can clearly see in this wrecked flexi the cable construction - longitudinal strands held in a spiral-wound outer. This is what can happen if the flexi-cable is not fully supported thoughout its entire length.

Wiredrives are also much stronger than flexi-cable, and are the drive of choice for most very high power fast electric boats. Wiredrives, however, require much more accurate installation than a flexi-cable drive. A flexi-cable obviously benefits from being installed in the boat as accurately as possible, but a flexi can tolerate a small amount of misalignment - a wiredrive most certainly cannot. Any amount of misalignment with a wiredrive will result in broken wires, so if you choose to install a wiredrive, take the time to get it right.

Wiredrive strut and shaft assembly to suit 3/16" bore propellers. This design won't lose the propeller and shaft if the shaft breaks or the coupling works loose.

Wiredrive and coupling. This coupling has two M4 carbon steel set screws to clamp the wire.

The components: stainless steel strut with lead/teflon plain bearings, 1/2"∅ aluminium coupling, countersunk prop retaining screw, prop retaining collar, stainless dog drive, 3/16" shaft assembly, brass stuffing tube and molybdenum-nylon thrust washers.

Wiredrive as fitted to my FM4 12 cell hydroplane. The propeller is an Octura P747. Note the water cooling pickup made from some scraps of brass tube and strip.

View inside the boat. Only a small length of brass tube is required to support the wire, hence the very low frictional losses.

Downsides to wiredrives are that the carbon steel wire will rust if you don't dry and lubricate it, and at high power outputs you will need motor couplings with more than one grubscrew to properly grip the wire. The range of adjustment with a wiredrive is less than a flex-cable, so you need to think carefully before installing a wiredrive about the props you want to use and the height and angle you want to run them at. Get this wrong and you are faced with having to remove the wiredrive from the boat and try again.

This outrigger was built with a tunnel between the motor mount and transom to allow the whole wire to run without a support, to see if it was possible to run a wire unsupported without resonance. Unfortunately, the wire did resonate, and required the fitting of a support, in this case a length of 3/8ths" aluminium bar, cross-drilled and sleeved with brass tube.

Solder or Glue?

Wires can be glued or silver soldered into the stub shaft. In practice, almost everyone uses the glue method. The favoured glue is Loctite 603, an oil-tolerant, anaerobic high strength engineering adhesive. Anaerobic means that it starts to cure when you exclude oxygen from the joint. An alternative is Loctite 648, which tolerates higher temperatures than 603, but is not oil-tolerant. Successful glue joints require close fitting, clean components and you need to let the joint cure for at least 24 hours before it achieves full strength.

Why is glue favoured over silver soldering? Loctite works at room temperature, silver soldering requires temperatures in excess of 700° Celsius. If you heat piano wire to the temperatures needed for silver soldering, it can warp and twist out of shape, and the heat affected zone of the wire will lose its spring temper and strength.

Loctite is perfectly adequate, if applied correctly. Loctite has a limited shelf life. Even if you make a lot of wiredrives, you are unlikely to use up a bottle of 603 before it expires. The typical shelf life of 603 is about a year to eighteen months. Basically, the older the 603 gets, the more likely it is to fail in use, so use fresh 603 that's in date.

Bearings

Finally, a brief word about bearings. There are two types of bearing one can use to support the driven shaft in a model powerboat drive system, these being plain bearings and ball-race bearings. For some reason, many modellers seem to look down their nose at plain bearings, considering them inferior to ball-race bearings. This I find very strange, as I take the opposite view: in my opinion, plain bearings, in a marine environment, are vastly superior to ball-race bearings for a number of reasons. To explain this further, let's explode a few myths.

A selection of plain PTFE shell bearings and miniature flanged ball-race bearings.

Myth: ball-race bearings have less friction than a plain bearing. This is true, if the bearing is run in a dry environment. It is not true when water is present. A ball-race bearing always has the friction of the inner and outer races running against the ball bearings. With plain bearings, however, above a certain shaft speed, if water is present and the propeller is balanced, the shaft will float on a film of water without touching the bearing surface at all. A thin film of water must be lower in friction than a ball-race bearing.

PTFE plain bearings. Above a certain shaft speed, if water is present and the propeller is balanced, the shaft will float on a film of water without touching the bearing surface at all.

If the propeller is not balanced, then the spinning drive shaft will rattle around inside the bearings and will not float on a water film. Regardless of whether you have plain or ball-race bearings, a propeller with any significant amount of imbalance is a problem you must rectify if you want high speeds.

If the propeller is not balanced, the spinning shaft will rattle around inside the bearings, causing very rapid wearing of the PTFE material. You will never see anywhere near top rpm if this happens.

Myth: ball-race bearings last longer. I usually hear this one from people who run propellers that are badly out of balance. If your props are balanced, and you never rev up the boat dry, plain bearings will last forever. Ball-race bearings, once you get them wet, have a very short lifespan. The water we run our boats may look pretty clean, but in reality it's a soup of mineral and particulate matter suspended in the water column. Once this muck gets inside a miniature ball-race, the precision machined parts of the bearing wear very quickly. Plain bearings, however, can tolerate large amounts of contamination in the water and still work perfectly, as long as the propeller is balanced and the shaft is floating. So, no, ball-race bearings do not last longer; quite the reverse in fact.

Myth: you need ball-race bearings for high rpm. Plain bearings will work at very, very high rpms, if the propeller is finely balanced. If you do something idiotic, like rev up your motor to maximum throttle with plain bearings that are dry, then yes, you will destroy them in seconds, as without the cooling and lubrication of water, PTFE plain bearings will overheat and melt in a second. So, don't do this and there's not a problem.

A ball-race bearing consists of inner and outer rings, or races, separated by miniature ball bearings.

Ball-race bearings have several other drawbacks that make them less attractive to use in place of plain bearings. A ball-race bearing relies on the shaft that it supports maintaining solid contact with the inner race. This usually means you must use a bearing retainer compound, for example Loctite 243, to glue the driven shaft to the inner bearing race.

At high power outputs and high rpm, unless the driven shaft is fixed securely to the inner race with retainer, it is likely to slip and spin faster than the ball-race, meaning you have metal-to-metal contact where the shaft touches the inner race. You are now spending the energy stored in the cells heating two pieces of metal to high temperatures, the result being the hardened ball-race will start to machine away the softer shaft. The shaft is now no longer a sliding fit in the bearing race, and must be replaced. You are more likely to experience the driven shaft slipping inside a ball-race as the bearing wears and its friction increases.

Another drawback to using ball-race bearings is cost. Plain PTFE shell bearings literally cost pennies, but a miniature ball-race bearing will cost £3-4 to replace. If your propellers are balanced, plain bearings last indefinitely, and cost very little to replace, whereas ball-race bearings begin to wear out as soon as you get them wet and replacing them costs a significant amount of money. So, which type of bearing do I choose to run? Well, no prizes for guessing that I run plain PTFE bearings!

© Copyright Paul Williams and www.fastelectrics.net, 2010.

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Last modified: 08th July 2010 @ 09:05