Simple Steam Launch - Boat Design

This boat hull was constructed to take the simple steam plant.

The design shown here was made to take the 2.5kg weight of the steam plant fully fueled. It was equip with a single radio channel to control a rudder with the intention that the motor would run continuously but the direction could be controlled whilst sailing.


The proposed long Sleek hull profile

The boat dimensions were set out to be 1000mm long with a 200mm beam and a Vee bottom hull.
To aid assembly a flat deck would be used to connect the 10 rib panels along with a solid keel running bow to stern.
All planking would consist of 2 dimensional curves and twists to simplify the process and no tight curves would be used.
The majority of the boat would be an open section to house the steam plant, but a water tight rear section would cover the radio gear.
The hull was drawn out full size on the back of a sheet of wall paper. The steam plant was used as a guide to ensure the boat would be wide enough and deep enough.

A plan view was drawn to create a line for the deck of the boat using slight curves where possible to simplify construction.

A side elevation was drawn directly below made up of a flat deck line, a keel profile and a Chine line.

Click for larger view of hull profile
With the side and top profile of the boat complete end views were projected to provide an outline for each of the ribs. The projection process used a 45 construction line to allow projection lines to meet at the end view. The side plan provided the height of features and the top view the width.
See below for the construction lines required for the first Rib on the hull.

Constructing a rib profile (click to enlarge)
The resulting profile for the first rib is shown on the left.

The same process was used to create rib profile drawings for the other 9 parts.

The full size drawings were then glued to plywood for cutting out.
With the the basic hull construction complete.
The engine and propeller shaft were coupled before installing either of them permanently in the boat. This allowed the best fit positions to be determined with the engine turned by hand to find position of minimum friction. The shaft was then glued to the hull and the engine secured to the boat floor.

The table below gives some ideas for possible shaft couplings depending of the angle and offset of the engine and propeller.
Coupling Type Image Installation (click for larger image). Comments
Straight coupling Very strong but shafts have to be exactly lined up to avoid bearing wear.
Tube Very simple and suitable for low torque applications such as this. Requires shafts of similar diameters
Spring Drive More capable than the tube drive in terms of torque.
Universal Joint Excellent solution to torque transmission through slight angles. Two joints can be used in series to coupling misaligned shafts
Oldham coupling Sliding disc couples misaligned shafts and helps isolate vibration, but is only really suited to low speed applications and slight offsets.
Pulley system Can be used where shafts are parallel but offset. Easy to make but some power is lost in the tension required to maintain drive. Toothed belts can help and speeds can be changed using different pulley ratios.
Gears Another solution to offset parallel shafts. Capable of transmitting high torque but can be noisy with straight cut teeth and direction is reversed at each mesh. Once again ratio can be changed.
Pin Drive Simple toy boat solution. Works well through slight angles but can be noisy.
Disc and Pin Another version of the pin drive. noise can be a problem unless pin has a rubber sleeve.
Bellows Similar to the spring drive but coupling contains slotted cuts rather than a helical body. Only suitable for small angle changes but helps reduce vibration.
Flexi Drive Good at absorbing vibration or cushioning speed changes. Not good for large angle changes
CV Joint Improved universal joint that transmits a constant speed. Can be used for angle and offset torque transmission.
Schmidt Coupling Complex coupling design for high torque transmission and can even have dynamic shaft spacing. Complex to make, especially in this scale.
Visit this page to see more details on the hull constriction.

To copy this hull design shown here, the full size plans can be downloaded using the link on the right.
The parts are split so that they should fit onto a normal sized office paper for printing and can then be assembled matching the join lines as appropriate.

Click to download the hull plans