Dom
The main issue with weight is transport. Life is so much easier when
you can carry the complete boat. The 16kg is probably right for an
older style surfski. Does not leave you much to play with. I doubt
you will get under 20kg!
It actually is worthwhile taking the time to plan out what you intend
to use to build it.
Target weight for stabilisers is 1kg each. I have used 100gsm cloth
on 3mm Klegecell foam. If wood, 3-ply is ample. Be careful with
fairing and painting. They hardly ever touch the water so finish is
only related to looks not performance.
A seat can weigh as little as 300g. An aluminium frame taped up with
glass, no more than 2kg. Need to be mindful of the load paths.
There is high force between backside and cranks. There is also high
force on the tension side of the chain. The prop thrust is
relatively small compared with the crank forces and chain force.
Having frame located to most directly resolve the forces will help
keep weight down.
You can use some parts of the frame for multiple purposes. The
support for the back of the seat can double as a mounting point for
the prop shaft strut if you have the shaft side mounted. If you make
a separate seat then the part of the frame that the seat bottom is
mounted to can act as grab handle for bracing against, mounting the
frame to the hull and provide a place to insert the stabiliser beam.
A little gearbox is around 1kg.
Rick Willoughby
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