STIRLING MARBLEHEAD YACHT RACING CLUB Inc.

Woorabinda Lake - Stirling South Australia

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Designing a Rescue Boat for RC Yachts

 

Eventually I succumbed to a desire to create an electric powered rescue boat for radio yachts.  I started with a set of design criterion and produced a result which while it doesn't match all criterion,  certainly performs OK and is definitely worth a look to see how it could be improved.

Design Criterion

  • Must be a fairly heavy platform with significant inertia as it will be towing yachts which can weigh up to many Kgms.  It needs to be able to control a large RC yacht despite its likelihood of having significant sail area.

  • Must be stable as pushing a large RC yacht may put significant tipping forces on the rescue boat when working from the side.

  • Must have protection around the propeller.  Any tipping of the rescue boat could result in the propeller gauging the side of the yacht particularly when driving from the side.

  • Must have sufficient power to deal with the forces to be encountered in driving a disabled yacht with sails against wind.

  • Must have a reasonable turn of speed to catch up with a runaway yacht.  

  • Must have a means of capturing the disabled yacht and remaining hooked onto it.

  • Must protect the disabled yacht from physical damage.

  • Must be maneuverable ie turn quickly and have excellent reverse.

  • Maybe required to 'clean' a pond or sailing area of debris such as reeds, small twigs or even the occasional floating log.

Hull choice and issues addressed

There are many designs of boats that could be suitable for a rescue boat.  The simplest would be a hard chine power boat shape.  This could satisfy many of the design criterion.  I chose a one metre hull as I had a mold of a 'Triple Crown' and it was easy to pop out another hull.  While easily driven, it lacks stability both static and at speed. To deal with the static stability the boat hull was fitted with a short IOM keel of about 10 cms depth and fitted with a 1800g bulb.  To deal with the speed stability, a flat planing surface was added to the rear of the hull extending a few cms beyond the transom with a width similar to that of the maximum beam.  The keel/bulb addition has some advantages giving great stability and able to catch reeds and small sticks or branches but makes it difficult to power through reeds that are actually growing near the edge of the water which is often where disabled yachts end up.  I figured more power should solve this.

Power Train

To give the rescue boat plenty of power and to trade top speed off for more thrust, I chose a low revving motor with a large diameter propeller running at a low voltage to keep the revs down.  Hobby King have many examples of motors to use though many are designed for max revs rather than max torque at low revs which is more suited to the rescue boat needs.  The particular model I chose was 3660-1460.  The propeller I used is 65mm diam. which gives plenty of thrust.   An appropriate speed controller is essential and should be programmable to ensure it can be made to start quickly, have forward and reverse.  The programmer can make adjustments to increase reverse power and make forward - reverse changes happen quickly.  I chose the Turnigy Aquastar 120A controller and the appropriate Turnigy Aquastar programming card.  These items from Hobby King are not cheap but as they are the key to success be prepared to spend here.  I chose to connect the motor and shaft with a double universal coupling ensuring quiet running without the need to perfectly align them.  The motor is supplied with power from a 3 cell 4500 mAh LiPo battery.  Both the motor and controller have water jackets and I fitted a scoop beneath the hull to force water through them.

Propeller Protection and steering

To protect the propeller and to enable maximum thrust while giving good maneuverability I produced a carbon fibre shroud to surround it.  This shroud turns as well as an aft mounted rudder giving quite good turning with just a single drive train.  It would have been better maybe to have a dual drive system but the cost doubles so I opted for simplicity.  Both thrust and speed have proved more than adequate.

Protection for Rescued yachts

The rescue boat is surrounded by two rubber bumpers.  One on the gunwale region and a second near the water line.  A front bumper on the stem is large and matches the side ones while a rear bumper on the aft edge of the planing extension completes the protection for RC  yachts.  The lower bumper near the water line also acts as and excellent planing strake by throwing the water out from the hull.  The rear bumper is angle out beyond the planing surface and generates lift when reversing so keeping the rear of the boat form submarining.

Yacht Catching Arms

Two arms controlled by a simple yacht sail winch open outwards to a position at right angles to the hull.  The arms are sculptured to catch on a vertical object on the disabled yacht.  It works by hooking on to shrouds, backstay, jib boom swivel, mast, mainsheet post or main or jib sheets.  When disabled a yacht will often have the rudder off centre so causing it to sail in a circle.  Placing the rescue boat on the inside of the turn can often create enough friction on the side bumpers to guide the yacht home.

Radio Programming

I use a mode 2 spectrum dx6i because I had one.  The controls though suggest that you could use either a mode 1 or 2 radio effectively.  The throttle action (left knob) is used to control the movement of the catching arms while the speed control is governed by the rudder action(side movement of left knob)  This takes a little getting used to but the speed control needs a self centering action while the arms need an in or out position which holds.  The normal aileron control on the right knob is used to control the rudder and propeller shroud.  (The speed control could have been set up on the right knob on the elevator control but I found the interaction between steering and speed a little dangerous as I often added throttle when steering - up to you). In addition two mixes feeding to the rudder action (really speed) are set up on two switches to allow a three position fixed speed. Slow ahead with switch one on, medium ahead with switch two on and fast with both on.  As well the throttle control was set up with dual rates to allow finer control when making the final approach to the disabled yacht.

 

How does it perform

Considering the many possible options that could have been chosen, I have been more than happy.  Power and speed are more than adequate but occasionally necessary.  Maneuverability is good forward though not so good in reverse.  A dual drive system or a side thruster would be a bonus but more complex.  Capturing disabled yachts with mast up is good to excellent but care has to be taken if attempting a retrieval of a yacht with the mast down (I have damaged one sail and jammed up the shaft with a backstay).  Retrieval from reeds is problematic - it depends on how many loose reeds there are in the water.  Usually brute force wins out even with the fin / bulb getting caught.  Maybe an old style keel with sloping front and weight molded onto the bottom would be better?  Picking up reeds, sticks and even floating branches from the pond are a doddle as the bulb and fin catch most readily.  We seldom need to get our dinghy to rescue yachts and on one occasion when forced to use a very hard to row flat bottom aluminium dinghy, the rescue boat adequately pushed it into a twenty knot wind when the rower became tired.

 

Power

It certainly has more than enough power.  It pushed this dinghy into a 20 knot headwind with no problems!

 

Video

See a short video of the rescue boat in operation on YouTube at https://youtu.be/BEQy-AMAo_8

 

 

I would love to hear comments and examples of other rescue boats people have used and will add them to this page

  e-mail Ben Morris