Fast Electric Model Hydroplanes

By Paul Williams.
Article posted 17th January 2009.

Hydroplanes

Hydroplanes are the fastest type of racing boat. Full size, Unlimited hydroplane racing on the lakes and rivers of the USA is probably the most spectacular form of boat racing you can watch. With speeds sometimes in excess of two hundred miles per hour, the boats are made from composite materials and powered by multi-thousand horsepower gas turbine engines.

Bluebird K7.

Possibly the most famous hydroplane ever is Donald Campbell's "Bluebird". Bluebird was a jet-powered, three-point outrigger hydroplane built specifically to set world water speed records, which Campbell did numerous times before Bluebird's last and fatal run in January 1967. Much could be written about Bluebird; I would suggest checking out the following links:

The Bluebird Project
Gina Campbell
Donald Campbell CBE

Prop Riding

A hydroplane design attempts to reduce wetted area and drag to the absolute minimum. It was realised many years ago in full size boat design that the propeller could act as a planing surface, or a point of suspension, by using blades shaped in such a way as to create lift as well as thrust. A boat, unless it's jet powered like Bluebird, must have the propeller running in the water - the designer has no choice in this. Dispensing with a rear planing surface and using the propeller instead offers a significant reduction in wetted area and thus drag.

For this reason, prop-riding, three-point (three points of suspension) hydroplanes are the fastest type of racing boat at both full size and model scales. There are essentially two types of model racing hydroplane: scale (or semi-scale) and outrigger.

Scale Hydroplanes

As the name would suggest, a scale hydroplane attempts to emulate the look and feel of a full size boat. A scale hydroplane, like a catamaran, traps air in a tunnel between the sponsons to generate aerodynamic lift to support part of the boat's weight when planing.

Section through a typical scale hydroplane.

Scale hydroplane wetted area. A scale hydro that prop-rides (right) will have much less wetted area than one that does not and drags the rear of the hull in the water (left).

In common with catamarans, the width of the tunnel on a scale hydro is a compromise between a wide, stable track for cornering at high speed, and a tunnel that is not excessively wide and which generates far too much lift, rendering the boat unusable in windy or rough conditions. This problem is solved in the "Outrigger" hydroplane form where the lifting tunnel is removed, and the sponsons are mounted on booms or outriggers as far from the centre "tub" as desired, with no increase in lift. Scale hydroplanes can be as fast as an outrigger, but often lose out in stability and high speed cornering.

Darin Jordan's ProBoat Miss Elam semi-scale sport hydroplane. You can see a build thread for this boat on Offshore Electrics. Thanks to Jim Clark for the photo - you can see more of Jim's excellent FE photography here.

Mark Shipman's "Miss Thriftway" 1/10th scale unlimited hydroplane, built from plans by Gary Finlay. Mark and his Dad have done a fine job, you can read the build thread on Astec here, and see videos here and here.

Another shot of Mark Shipman's Miss Thriftway at speed.

Outrigger Hydroplanes

Outrigger hydroplanes are so-called because the two forward planing surfaces, or sponsons, are mounted to outrigger booms. Model outriggers more often than not use carbon fibre tube for the outrigger booms for high strength, rigidity and light weight.

Typical outrigger hydroplane section.

Wetted area of a typical outrigger hydroplane.

The outrigger hydroplane is the fastest model racing boat design, both in terms of straightline speed thanks to a low drag profile, and in cornering speed, where the much wider track of the outrigger allows it to corner at near maximum speed. If the outrigger has one flaw, it must be looks. They're not really scale (although full size oval racing outrigger unlimited hydroplanes have been built and raced), and to be honest they are a bit plain.

1969 Pride of Pay 'N Pak Unlimited hydroplane.

Outrigger Construction

When I first started racing outrigger hydroplanes, we did not have the abundant power of brushless motors and lipo cell packs that we have now. My first outrigger was powered by seven Sanyo 1700 SCRC nicad cells and a modified 540 motor mounted to a Hughey reduction gearbox. We had to run for 3 minutes, and our boats had to be very light. As the weight of cell pack, motor, drive system and radio were pretty much fixed, the only weight savings to be had were in the construction of the hull itself.

Pete Wilkinson's ultralight hydroplane from circa 1995. This boat was built from 1/64th" plywood, and while super quick and a testimony to Pete's skill as a builder, it did not survive for very long when raced - it was simply too light.

This obsession with light weight meant that we became paranoid about every gram that went into the boat. Some people, notably Pete Wilkinson, sometimes went a bit too far, building boats that were so light they simply would not survive a whole race meeting in one piece.

Today, the situation is very different. Brushless motors and lipo cells mean we have a lot of power to play with, and now the primary goal is to build a boat that is strong enough to cope with speeds of 50 mph or more. It is still true that a lightweight boat should, in theory, be faster than a heavier boat on the same power. However, very light boats will break more easily, and will be affected by aerodynamic forces much more than a heavier boat.

Outriggers are traditionally made from plywood and balsa, in much the same way as a model aircraft. You can buy epoxy-glass outriggers like the Hydro and Marine Excess boats or those from Team Lindenau, and these can be very fast, but for many racers nothing beats the traditional materials. Plywood, balsa and foam can be used to make boats that are light, strong and very rigid, and which are easy to repair if damaged. I've fixed up some very badly mangled outriggers at race meetings which have been crashed (due to bad luck I say, others say it's my driving...), using nothing more than some glue and a few scraps of balsa and ply.

A foam/ply outrigger hydroplane. This boat is over 700mm long, yet the hull as shown in these pictures weighs only 350 grams.

Plywood/balsa construction also means you will have a boat with razor sharp running surfaces. Some epoxy boats I've seen have required the planing surfaces to be sharpened up with filler. There is one other aspect to building a plywood 'rigger. If you build your own design, then you have a unique boat that no one else in the world has, and there is also the pleasure of building in wood.

Epoxy-glass hulls are very convenient, as the serious building has been done for you, and if you really do not fancy the idea of building from scratch then an epoxy-glass hull is an option. However, consider this: for the price of one epoxy boat, I can buy enough materials to make perhaps as many as ten ply/balsa boats. Epoxy outriggers are not cheap. The H&M Excess and Team Lindenau Executive are €123 and €145 respectively. A ply/balsa boat would cost a fraction of this. To get an idea of what is involved in building a ply/balsa/foam outrigger, check out this article.

Turn Fins

All hydroplanes, whether scale or outrigger, usually require a turn fin to provide a fulcrum for the boat to turn against. Without a turn fin, a hydroplane would simply yaw sideways instead of turning. A hydroplane normally only has a single turn fin, meaning it can only really turn in one direction.

Turn fin on an outrigger hydroplane, fitted to the inside of the right hand sponson. Angling the blade off the vertical generates a small amount of downthrust, stopping the sponson from lifting out of the water. Too much angle can cause the sponson to stall and dig in.

For a model, the turn fin is mounted to the right side of the boat, and the model turns right on a clockwise course. Left turns are possible - just. Turning left causes the boat to ride up on the turn fin, which causes a lot of drag and consequent loss of speed. Without a turn fin on the left, a hydroplane is extremely reluctant to turn left.

There are numerous different ideas on what is the perfect shape for a turn fin. Ask three people who race outriggers what the best type is, and you'll more than likely get six answers. Broadly speaking, there are two basic types of fin, one that pivots upwards if the blade strikes something in the water, and one that is fixed and shaped to ride over obstructions. The last thing you want is to hit something with the turn fin and have the sponson wrecked because the turn fin either does not pivot or cannot ride over whatever you hit.

Turn fin shapes. Top: a pivoting blade mounted with a single fixing will hinge up out of the way if struck. Bottom: a fixed blade, shaped to ride over obstructions.

The position of the turn fin relative to the boat's centre of gravity, or balance point, can be critical. A turn fin mounted in the wrong place can ruin the handling of a hydroplane, causing it to snatch or spin in turns and be totally unpredictable. In my experience, the best position for a turn fin is on the inside of the right hand sponson, just forward of the centre of gravity. I've tried mounting the turn fin to the tub, with frankly not much success.

Turn fin location - typically on the inside face of the right sponson, forward of the Centre of Gravity.

Regardless of shape or location, a turn fin is a source of drag and you should use the smallest blade that allows effective turning. My personal preference with regard to turn fins is a simple pivoting stainless steel plate blade, mounted to a hardwood dowel glued into the sponson and secured with an M4 stainless bolt. This arrangement is strong enough for Hydro 1, for higher speeds I would use a curved fixed blade that has more than one attachment to the sponson, or a separate mounting bracket and pivoting blade setup.

Aerodynamics

The speeds now easily attainable from cheap brushless motors and lipo cells mean that fast electric modellers who build, run and race high speed models need to be aware of the aerodynamic effects that can influence the way a boat runs. For example: when turning, a hydroplane (scale or outrigger) will have a significant flow of air across the boat.

Airflow over sponson edge when turning. Bevelling the top face of the sponson allows the air to flow over the sponson, instead of under it, and kills any lift produced.

Airflow in this direction can get trapped under the edge of the sponson and cause a significant lifting force, perhaps enough to cause the boat to take off or "blow over", especially if there is a sudden gust of wind during the turn. Bevelling the top face of the sponson allows this airflow to release and flow over the top of the sponson, killing any lift.

Bouncing Hydroplanes

Prop-riding hydroplanes are very sensitive to the depth the propeller is set at. A common problem with prop-riders is for the transom to bounce up and down as the boat runs, with the result that the boat never achieves its potential top speed.

Effect of propeller height on a hydroplane. X represents wetted area, which increases as the transom rises out of the water, due to the propeller being set too deep. The bottom image shows where the propeller should be; set well out of the water.

The usual cause of transom bounce on a hydroplane is running the propeller too deep. There is a point at which the lift from the propeller balances the lift generated by the front sponsons, and the boat will ride level. If the propeller is set too deep, it will try to lift itself out of the water, which raises the transom and reduces the effective angle of attack of the front sponsons, making them run very wet. This causes drag to shoot up, which makes the propeller slip. When the propeller slips, it stops lifting and the transom drops back down again. The cycle then repeats.

Asymmetry

Outriggers are the most extreme form of racing boat, and are usually designed to turn in one direction only. For this reason, it is not uncommon for an outrigger to be designed with different angles on the left and right hand planing surfaces, for the planing surfaces to be of different widths, with different angles of attack and possibly set at different distances from the tub. In other words, the design is asymmetric, in order to maximise turning ability, and stability, in one direction.

Asymmetric outrigger design.

For example, I usually design my own outrigger hydroplanes with more anti-trip angle on the left than the right, and often set the rear sponsons at different heights, so that most of the time the right hand, inner sponson is kept out of the water. Some designs have anti-trip angle on the inside faces of the right hand sponsons, or may have left sponsons with some deadrise angle.

If you look at a 60 metre oval course, you have two straights of 60 metres, and two corners of radius 15 metres. Each lap, a hydroplane must turn a circle of pi x 2r, or 3.14159 x 15 x 2 = 94 metres. 94 ÷ 214 = 44. For a minimum of 44% of each lap, your boat is turning. Realistically, turns probably make up a half or more of each lap. So, anything that you can do with the design to minimise speed loss when turning will have a dramatic effect on your lap times. Your boat can be slower on the straights, but if it turns faster and is faster out of a turn through losing less speed, you have a very good chance of beating boats that are faster on the straights but less good at turning.

Sponson Width

When I first started building outrigger hydroplanes, I made the reasonable assumption that if a hydroplane went fast because it had very little wetted area, then the more you reduced the wetted area, the faster it would go, leading me to build 'riggers with increasingly narrower sponsons. I built a few boats that were absolute dogs - they would barely plane, and took forever to break free, and would drop off the plane as soon as you thought about touching the rudder, or would dig in and spin, sometimes without touching the rudder at all. They also suffered from appalling torque steer.

Sponsons too narrow. Without sufficient planing area proportional to the weight of the boat, a hydroplane will never run as intended.

I went back to the workshop, and made some new sponsons, this time with much wider planing surfaces. With wider sponsons, the boats worked exactly as an outrigger should. I made some more sponsons, each time increasing the width of the planing surfaces a little bit, and each time the boat would be a little quicker and would handle better. I realised that a hydroplane needs a minimum amount of planing area for it to work at all, and that the width of the planing surfaces was somewhat akin to the wing loading on an aircraft.

Pylon racers and fighter jets, which have very high wing loadings, stall very easily - in other words, there is a very small difference between the wing working and producing lift, and it not working at all, and the plane falling out of the sky. Hydroplanes behave in a similar way. Without sufficient planing area proportional to the weight of the boat, a hydroplane will struggle to break free and plane at all, and will drop off the plane very quickly, and be very badly affected by propeller torque effects and prop walk. Any rudder input, with the consequent increase in drag, will cause the boat to stall, dig in and spin out. If this describes your boat, then the answer is simple: wider sponsons.

The easiest way to widen the planing surfaces on a set of sponsons is to add ride pads or plates. These are simply strips of plastic sheet stuck onto the planing surfaces:

Ride plates can be used to widen a sponson planing area, up to a point. If ride plates don't cure the problem, you need wider sponsons.

Touching on the subject of asymmetric design again, I always make the right hand sponsons on my designs wider than the left, for two reasons. The right hand sponson is being pushed downwards into the water by the torque of the propeller, and is also pulled downwards by the turn fin when turning. So, the right hand side benefits from a little more planing area to help keep it out of the water.

Asymmetric, wider sponsons. X represents the combined downwards forces of propeller torque and the turn fin working during a turn.

A well designed hydroplane should not need the sponson widths altering. However, if you're designing and building your own hydroplanes, or have bought/built a boat that behaves as described above, then maybe you need more planing area to allow the boat to work properly. However, there is a point at which increasing the width of the planing surfaces begins to slow the boat down again, and sponsons which are excessively wide not only suffer high levels of drag, you will find they are more sensitive to rough water. A wider sponson will be kicked upwards by a wave where a narrower sponson would tend to carve through. So, a compromise must be found. Or, better still, build a number of different sponsons suited to differing water conditions.

Sponson Angle ("AoA")

The angle at which the sponson planing surfaces are presented to the water is usually called the "angle of attack", often abbreviated to "AoA". This angle is one of the critical aspects of a hydroplane design, and can drastically affect both speed, handling and the ability to cope with rougher water conditions.

Sponson Angle of Attack or "AoA".

On my own designs of outrigger, I have used angles of three, four and five degrees. Slower, less powerful boats tend to favour sponsons with more AoA, and faster boats and boats which are used in rougher water are, in my experience, better with less AoA. The AoA governs how "hard" the boat rides. In calm conditions you can use more AoA, but when it chops up, you are better with a softer riding boat that is less likely to be thrown around as it impacts waves. AoA is another compromise, in this case between outright speed and stability.

IC outriggers, with their open design, often incorporate a mechanism for altering the AoA with packers under one of the outrigger booms. FE outriggers, like any FE boat, require that the whole boat be sealed against water ingress while running, so it is usually impractical to use such an adjustable system on an electric boat. Instead, I prefer to make extra sponsons with different AoAs to suit calm and rough water.

Hydroplane Balance Point

The longitudinal balance point, or "centre of gravity" (CofG), is one of the most important aspects of the setup of an outrigger or semi-scale hydroplane. If the CofG is in the wrong place, the boat can become very unstable due to the planing surfaces running light. The boat can suffer from a bouncing transom, or the transom may fail to lift from the water and simply drag, killing speed. To understand why, we need to appreciate the concept of leverage.

Hydroplane CofG location. X and Y added together is the afterplane, the distance between the rear of the forward planing surfaces and the rearmost point of suspension, usually the propeller. The top boat has the CofG nearer to the sponsons, resulting in a relatively larger moment arm - the propeller will not have to work so hard to lift the back of the boat. The lower diagram shows the sponsons moved forwards, reducing the moment arm and loading the propeller more, reducing some of the propeller lift.

The propeller on a prop-riding hydroplane provides both forward thrust and the rearmost point of suspension, by virtue of creating a lifting force due to the shape of the propeller thrust cone. The location of the CofG fore and aft can make it easier or more difficult for the propeller to lift from the water, by reducing or increasing the moment arm, or lever length, relative to the propeller, the CofG and the forward planing surfaces.

If the lever is too short, the propeller will struggle to lift the transom clear of the water, and will load the motor more due to increased propeller immersion. If the lever is too long, the propeller will lift too far out of the water, possibly causing the transom to bounce up and down, or magnifying torque effects and prop walk. Both of these scenarios can be partially or wholly cured by changing the propeller type, to take advantage of the differing levels of lift from the various propellers available. For example, if you have a boat that drags the transom, try swapping to a lifting hydroplane propeller like Octura's 17 series. Conversely, Octura's X6 series have the lowest lift, and could cure a bouncing transom.

In practice, however, if the CofG is in the wrong place you will almost certainly have to either move the CofG, which can be difficult to achieve in the limited space of an outrigger, or move the front planing surfaces fore or aft. The layout of the components in a hydroplane is often dictated by the installation of the drive system, especially if using a wiredrive as the curve on the wire must not be too tight a radius, nor must the wire be excessively long, to reduce the possibility of resonance or whipping.

When building an outrigger, all the hard work goes into the tub - installation of motor and drive system, radio, isolation loop, rudder and steering servo etc. In contrast, making a new set of sponsons is relatively easy. Obviously, with a semi-scale hydroplane, the sponsons are an integral part of the hull and cannot be moved. In a scale boat, you usually have a great deal more room to move the cell pack around to achieve the correct CofG. As a last resort, you can add lead ballast to get the trim right. Years ago, in the days of limited power brushed motors and sub-C cells, the idea of adding lead weight to a boat would have been unthinkable. Nowadays, with lipos and brushless power, adding lead ballast is a common practice.

Rear Sponsons

Rear sponsons - some people do, some don't. There are pros and cons to fitting them, but, for me, the benefits of having them outweigh the drawbacks. So, what are the arguments for and against?

• Rear sponsons damp down the back of the boat, and keep the boat riding more or less level therefore maintaining a more constant load on the propeller
• They keep the boat level when turning, preventing the propeller from running too deep and bogging down
• They add a large amount of useful extra bouyancy to the hull
• Aesthetics - to me, an outrigger without rear sponsons looks unfinished and unbalanced

• Extra drag

Rear sponsons are not intended to run in contact with the water all the time - they should only touch the water when the transom drops, for example due to the nose pitching up, or when the boat is turning. Supporting the back of the boat clear of the water and making the hull run flat is the job of the propeller, not the rear sponsons. If you need rear sponsons to make an outrigger run flat (as opposed to "uphill"), then something is wrong with the design or setup of the boat, and fitting rear sponsons is no cure. Try moving the boat's balance point further forward, or swap to a lifting hydro propeller.

Sometimes, a rigger won't run flat purely because of a lack of motor power; the propeller is not being turned fast enough to generate any lift, and the boat drags its backside in the water. Try a motor with a higher Kv and a smaller propeller, and make sure any binding in the driveline is eliminated.

If you are not sure about fitting rear sponsons, and are reluctant to glue them in place in case you decide to remove them completely, or you are not 100% sure as to what depth to run them and might want to try them in another position, then attach them to the side of the tub with servo tape. That way, you can easily remove them using dental floss to saw through the foam of the servo tape.

Conclusion

There are many forces acting on a model hydroplane. These include:

• Propeller torque effects and prop walk
• Propeller lift
• Aerodynamic lift
• Hydrodynamic lift from the planing surfaces
• Aerodynamic drag
• Hydrodynamic drag, from the planing surfaces, rudder blade and turn fin

All these forces must balance out for a hydroplane to run at high speed, and to be stable both in a straight line and in the turns. If you build a boat that does not perform "out of the box", do not despair. Small changes to the setup and trim can dramatically improve a boat that seems at first to be a dead loss.

For less experienced modellers, the best advice is to change one thing at a time, to learn what it is that makes your boat better or worse. If you're totally at a loss as to what to do, try uploading a video of your problem boat to YouTube and asking questions on forums like Offshore Electrics and Astec Models. A video will allow more experienced modellers to pinpoint exactly what the problem is and suggest possible cures.

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

This article may not be reproduced wholly or in part without the written permission of the author and www.fastelectrics.net. If you would like to use this article or the accompanying pictures/diagrams please email articles@fastelectrics.net.

Last modified: 08th July 2010 @ 09:06