> I notice from this image:
>
> http://www.geocities.com/jeremiahobrien/dry1.jpg
>
> that the rudder is split into two halfs. The top is rotated to the
> right in comparison to the bottom. I presume this is to offset paddle
> wheel effects, so I wonder why smaller boats don't use this idea?
I'm sure they could but can't be bothered - as you can see from the pic,
the hull form and swim are beautifully shaped - better than even a
working boat. Bearing in mind that the ships were built for getting as
much cargo across as possible on the single voyage they were expected to
last, this is a striking contrast to modern expensive nbs - of course in
this size it is possible to produce a well-faired shape with basically
flat plates.
>
> Also how many running steam narrowboats are on the system at the
> moment?
President, Adamant, Emily Anne, Laplander and, I think, Tixall and
Swallow - three ancient and three modern.
>
> Mike
>
> ps. Nice big prop on the O'Brien, I notice a lot of modern ships use
> far smaller screws for their comparitive size.
>
Shaft rpm is about 75. Rpm has a big effect e.g. for the wartime Bangor
minesweepers, there were diesel and steam versions of the same power and
speed.
Diesel - 6'7" diameter, 5'1' pitch, area 12.5 sq ft @ 360 rpm
Steam - 8'8" dia, 9'5" pitch, area 22 sq ft @ 210 rpm
Makes clear the effect of rpm - of course with diesels the rpm can be
chosen to trade off efficiency against draught.
> --
> Michael Askin
Sean
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