[hpv-boats] Prop

[Rick Willoughby]

Kevin
To get any advantage using hydrofoils each rider of average weight  
would need to be able to get to 250W at a miniumum.  Most people can  
only get this power for very short periods.  If you could produce  
this power level then the design speed would be something over  
14kph.  You can get lift off using bigger foils at lower speed and  
less power but what is the point when you could go faster with less  
power and much less complexity in displacement mode!

Hydrofoils increase the susceptibility to fouling with weeds and  
other litter.  They also require considerably greater draft such that  
beach launchings becomes a wet affair.

A key step is to decide what you intend to do with the boat.  If it  
is purely sprinting then hydrofoils would be worth consideration.  If  
you want to do longer distance cruising (anything over 5km at a time)  
then hydrofoils would not be in the mix unless you are a trained  
athlete.

An important element in the design is to establish the power output  
of the engines.  If you do not have this from bike data then it can  
be determined in a gym on a recumbent cycling machine.  You will then  
have a better idea of the possibilities.

Setting up any pedal boat for two people involves more complexity  
than a single person drive.  The best configuration from a hull  
efficiency point of view is the stabilised monohull or possibly a  
monohull without stabilisers with a single drive shaft.  Setting up  
the mechanical system to get the two cranks to drive the common shaft  
is a challenge.

I personally think two boats would be better than one but with two  
people in a single boat you will be able to go a bit faster.  The  
problem is you will most likely need two people all the time because  
the weight of the boat will be greater than a single person can  
manage.  Also performance with one person will be down on what could  
be achieved with a single person in a boat specifically designed for  
the person.

I am currently making my own 1:4 gearboxes.  The first of them are on  
the boat I made for my sister seen here:
http://www.youtube.com/watch?v=_fBLi40q4pU
The box is painted yellow so it is easy to see in the clip.  The  
parts for this box cost a little over AUD300 including the cranks and  
pedals.  It weighs 2.8kg.  I have now made a second one with more  
expensive gear set that weighs 2.3kg.

The selection of the prop shaft is quite important.  If you use high  
aspect props to get the efficiency then they need to be run in line  
with water flow to avoid vibration.  I use mostly spring steel or  
machinable aluminium or a combination of these in a curved  
arrangement.  I have a yet to be tested shaft made from carbon fibre  
and wound to my specification to get high torsional stiffness but  
compliant in bending.

I have tested most possibilities over the years and can give you a  
reasonable idea of the weaknesses or advantages of any set up.  More  
importantly I can determine the performance you can expect in  
whatever configuration you come up with providing you know the weight  
of the engines, what you want to carry and the output of the  
engines.  This can save a lot of trial and error with the associated  
cost and time.  I provide free design for non-commercial applications.

Rick

Re: Prop

Hi Rick,
thanks for the info.It is very helpful. I really like the folding  
prop and the
boat prop system. I have built a few recumbent bikes in the past and  
would like
to build a recumbent for two on water. Something not to complicated.  
We live in
the Dominican Republic at present and would use it on the ocean or  
lakes near
by. Things are not so easy to find here but some things can be built  
fairly
cheap. Where can I find information on the drive shaft and prop  
system. Much
like what you have done.  I was looking at this hydrofoil bike but  
thought I
should start with something without hydrofoils and try to get the  
prop system
first.
http://www.human-powered-hydrofoils.com/hydrofoils/project-pedal- 
aquaskipper/

What kind of drive shaft and prop system was Alexander Gäbler using  
at this site
http://www.human-powered-hydrofoils.com/hydrofoils/haiflyike/

Thanks again for your help. Kevin