MotorBoating magazine has made an attempt to compare the pros and
cons of outboard and inboard power:

http://www.motorboating.com/motorboat/features/article/0,12696,1074541,00.html

Your comments are invited.

--Georgs

Georgs Kolesnikovs
Power Catamaran World
http://www.powercatamaranworld.com

Dear Georgs,

Rod Gibbons expresses bewilderment about the performance differences etc of
a "scaled" design.  NEVER,NEVER,NEVER scale a boat. There is no such thing
as a "simple" scaling up or down. If we take two boats of the same hull
shape, one "bigger" than the other. Then for the performance [resistance] of
them to be the same they must have same Reynolds number and the same Froude
number. If this condition is not met then you cannot expect the same
performance. This is largely because length scales linealy but the area
increases by the square and the volume and the displacement by the cube. And
of course a lot of the loads increase by the fourth power so the structure
must be stronger/heavier. The power to weight ratio is just too simple. Much
more useful is the displacement/ length ratio where you use the square root
of the LWL. You do need to get the numbers right.

We have designed many of hundreds of catamarans now and have a very large
data base concerning a large number of our power catamarans. Consequently we
have no problem with guaranteeing performance and fuel consumption
parameters as long as the displacement does not change from the design
displacement. ie: as long as the client does not keep on loading on more and
more "necessary" items. If he does then a new set of performance and fuel
consumption parameters will be required. The performance and fuel
consumption parameters can only be for a particular displacement. This is a
major reason for quoting the light ship, half load and full load
displacements when describing a vessel and then stating in which condition
the performance and fuel consumption figures were obtained.

As Rod points out even a well designed and tested design can be turned into
a "dog" by increasing the displacement beyond what it is meant to be. In
real life this means that you must be very cautious when making changes to
an existing boat.

Malcolm Tennant Multihull Design Ltd
PO Box 60513 Titirangi,
Auckland 1007
NEW ZEALAND

ph +64 9 817 1988
fax +64 9 817 6080

e-mail malcolm@tennantdesign.co.nz
www.tennantdesign.co.nz
www.catdesigners.com

Dear Malcolm

Interested in what you have written. Have you any data on effect of
different types of antifouling on speed, along with beaching shoes,
canopies on say a 30ft power catamaran. We have speed differences on
similar 30ft power cats we import. Have you any data on what percentage
a cat could be slowed down by application of antifouling with a normal
speed of say 25 knots. Cold we be losing as much as 3 +++ knots? Are any
types of antifouling better than others?

Regards

Victor Aspey

-----Original Message-----
From: power-catamaran-bounces@lists.samurai.com
[mailto:power-catamaran-bounces@lists.samurai.com] On Behalf Of Malcolm
Tennant
Sent: 30 September 2005 03:15
To: Power Catamaran List
Subject: Re: [PCW] Rodd Gibbons on "scaling"

Dear Georgs,

Rod Gibbons expresses bewilderment about the performance differences etc
of a "scaled" design.  NEVER,NEVER,NEVER scale a boat. There is no such
thing as a "simple" scaling up or down. If we take two boats of the same
hull shape, one "bigger" than the other. Then for the performance
[resistance] of them to be the same they must have same Reynolds number
and the same Froude number. If this condition is not met then you cannot
expect the same performance. This is largely because length scales
linealy but the area increases by the square and the volume and the
displacement by the cube. And of course a lot of the loads increase by
the fourth power so the structure must be stronger/heavier. The power to
weight ratio is just too simple. Much more useful is the displacement/
length ratio where you use the square root of the LWL. You do need to
get the numbers right.

We have designed many of hundreds of catamarans now and have a very
large data base concerning a large number of our power catamarans.
Consequently we have no problem with guaranteeing performance and fuel
consumption parameters as long as the displacement does not change from
the design displacement. ie: as long as the client does not keep on
loading on more and more "necessary" items. If he does then a new set of
performance and fuel consumption parameters will be required. The
performance and fuel consumption parameters can only be for a particular
displacement. This is a major reason for quoting the light ship, half
load and full load displacements when describing a vessel and then
stating in which condition the performance and fuel consumption figures
were obtained.

As Rod points out even a well designed and tested design can be turned
into a "dog" by increasing the displacement beyond what it is meant to
be. In real life this means that you must be very cautious when making
changes to an existing boat.

Malcolm Tennant Multihull Design Ltd
PO Box 60513 Titirangi,
Auckland 1007
NEW ZEALAND

ph +64 9 817 1988
fax +64 9 817 6080

e-mail malcolm@tennantdesign.co.nz
www.tennantdesign.co.nz
www.catdesigners.com

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