[hpv-boats] Flex Shaft Material Selection (Dom Rumbolo)

[Rick Willoughby]

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