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From: Arild Jensen (no email)
Date: Tue Mar 04 2008 - 20:38:29 EST
> -----Original Message-----
> From: > Larry Zeitlin
> Trawlers tend to travel at relatively constant speeds.
>
> The possible exception to this rule would be for owners of
> semi-displacement trawlers with high power engines. The specifications for
the
> "ideal" prop for displacement speeds and those for the "ideal" high speed
prop are
> so different
> that a CPP might be an asset.
The Scandinavian predilection for CPPs that designer
> Michael Kasten cites has both a historical and practical reason. The
fishing
> industry in Norway and Sweden is a much larger portion of the boating
market than in
> the US. Most makers offered CPPs because of their practical advantages in
> handling load variation. Engines were generally less powerful and
squeezing the
> last ounce of thrust out of the prop when dragging nets made sense.
REPLY
Although I agree with most of Larry's points, he provided the relevant
comments for a dissenting opinion.
"Trawlers travel at nearly constant speed".
NOT always! Great and little Loop cruisers know first hand how often they
end up waiting for locks and bridges in their travels. This waiting is
often done in mid stream with engine running and requires constant
adjustments to speed.
In some places there is no place to tie up. If you do tie up you end up
being tail-end Charlie at the back of the pack.
Many of these places where you have to wait has a current. Not the easiest
thing to compensate for. CPP can make that easier.
Larry's mention of the "possible exception" seems to fit nicely into a
previous thread on hull forms.
The concensus from that thread seems to be that most of the popular trawler
hulls does in fact fit the description of a semi displacement hull. Combine
that with another previous thread where it was asked why the marketing
people insisted on promoting boats that seem over powered. Review the
various answers.
It appears to me that the larger portion of trawlers on the market today
fits the description of an over powered semi displacement hull. I don't see
any rush by the consumers to embrace narrow hulls with under 100 HP engines.
In other words Larry's possibel exception looks liek it may be a general
trend and likely to stay that way.
Lastly, cost effective, practical production of CPP in Scandinavia.
The original CPP drives were so simple they were manufactured in the yard by
black smiths. Not highly skilled machinists but black smiths. The only
reason the present day CPP are so complex and expensive has to do with the
demands by ever increasing horsepower and need for automated controls
remote from the helm station.
Back in the late 1960's I saw a CPP on a 15 HP Yanmar sailboat engine. The
whole mechanism was so simple you could build one with a hacksaw and a file
plus bar stock. The driveline consisted of two concentric tubes and a rack
cut into the end of one tube. A basic crown gear on the hub of the prop
blades meshed with this rack. Pitch was controlled with a lever that engaged
the shaft using a Wishbone saddle fork and a collar on the shaft.
Sadly there is no incentive for any company to offer a cheap, affordable,
CPP system. That would generate less profit for them. Being so simple it
doesn't involve specialist to fix. It would be comparable to fixing a
stuffing box on the shaft packign gland. So why would an after market dealer
want to sell something they can't expect to get lots of call backs from or
expect future service calls?
Only when the market demands an affordable CPP will builders and
manufacturers respond with a suitable product.
OR . . . . we go back to basics and use the kind of system that first came
out over a hundred years ago. <VBG>
cheers
Arild
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