A response to the letters of Henry Clews and Pat Reischmann regarding
performance.

If you go to our web site and look under the "latest news" section for 14th
July 2004, well it was the latest news back then! you will see a discussion
of fuel consumption/hull resistance curves. The graph shown there
illustrates perfectly what both Pat and Henry are talking about.

Pat is correct; the graph clearly shows that even at "displacement" speeds
the displacement catamaran still has less resistance. Although there is  not
as much difference as at higher speeds [higher Froude numbers]. Pat is also
correct in saying that the displacement is an important variable which is
why we tried to get the displacements of the boats that are plotted as
similar as we could. We further normalised the data by using litres per
tonne. This decreased the differences in resistance between the boats due to
the slight differences in displacement.

The high point in the resistance curve for all these boats is at Froude
number 0.4. This is the point where a monohull displacment boats speed
stops. This is "hull speed". However the displacement catamaran using our
"CS" hull shape can economically go up to a Froude number of approx 1.0.
After that the power required to go faster becomes economically unfeasible.
Given that Manta power catamarans use the "CS" hull shape and the PDQs is
very, very similar, these curves would appear to be applicable to both of
these boats.

Henry is correct: off the shelf performance prediction models are unreliable
because of the wide variety of hull shapes that are used. However our
in-house performance prediction model based on the "CS" hull form is very
reliable and valid for our particular hulls. If you are designing a multi
million dollar boat then it is only good business sense to confirm your
performance preditions with tank testing.

Given that the PDQ hull is virtually a "CS' hull we can see from the graphs
exactly why Henrys boat behaves the way it does. The salient point is that
you need to stay away from a Froude number of 0.4 for your boat ie: the
position of maximum resistance. Either side of that will show much lower
resistance and less wave making and this is exactly what "Sno Dog" is
showing two "optimum" speeds. So if possible your length/speed ratio [Froude
number] needs to be tailored to fit the speed at which you will normally be
travelling so you are not travelling at the speed of maximum resistance.

Henry; our boats are still operating as displacements vessels at 35 knots.
You can see from the resistance curves that a displacement hull can
sometimes perform just as fast as a planing hull. Of course it all depends
on whether your definition of planing is the mathematical one, or the
physical one.

I hope this has helped.

Regards.

Malcolm Tennant.  ARINA  MA

Malcolm Tennant Multihull Design Ltd
PO Box 60513 Titirangi,
Waitakere 0642
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

Good morning to you, Mr Tennant.

I am wondering if I may be the only person
who does not know what a "CS" hull shape is.

I have the same difficulty with "Froude" number.
Is this number that depends on the square root
of the waterline length (I have forgotten the
correction multiplier <1.23? 1.34?>)?

Sincerely,

D C "Mac" Macdonald
m/v Another Adventure
(non trawler/catamaran)

----Original Message Follows----
From: "Malcolm Tennant" malcolm@tennantdesign.co.nz (by way of
GeorgsKolesnikovs)
Reply-To: Power Catamaran List power-catamaran@lists.samurai.com
To: Power Catamaran List power-catamaran@lists.samurai.com
Subject: [PCW] Power cats
Date: Tue, 1 Aug 2006 13:13:01 -0400

A response to the letters of Henry Clews and Pat Reischmann regarding
performance.

If you go to our web site and look under the "latest news" section for 14th
July 2004, well it was the latest news back then! you will see a discussion
of fuel consumption/hull resistance curves. The graph shown there
illustrates perfectly what both Pat and Henry are talking about.

Pat is correct; the graph clearly shows that even at "displacement" speeds
the displacement catamaran still has less resistance. Although there is  not
as much difference as at higher speeds [higher Froude numbers]. Pat is also
correct in saying that the displacement is an important variable which is
why we tried to get the displacements of the boats that are plotted as
similar as we could. We further normalised the data by using litres per
tonne. This decreased the differences in resistance between the boats due to
the slight differences in displacement.

The high point in the resistance curve for all these boats is at Froude
number 0.4. This is the point where a monohull displacment boats speed
stops. This is "hull speed". However the displacement catamaran using our
"CS" hull shape can economically go up to a Froude number of approx 1.0.
After that the power required to go faster becomes economically unfeasible.
Given that Manta power catamarans use the "CS" hull shape and the PDQs is
very, very similar, these curves would appear to be applicable to both of
these boats.

Henry is correct: off the shelf performance prediction models are unreliable
because of the wide variety of hull shapes that are used. However our
in-house performance prediction model based on the "CS" hull form is very
reliable and valid for our particular hulls. If you are designing a multi
million dollar boat then it is only good business sense to confirm your
performance preditions with tank testing.

Given that the PDQ hull is virtually a "CS' hull we can see from the graphs
exactly why Henrys boat behaves the way it does. The salient point is that
you need to stay away from a Froude number of 0.4 for your boat ie: the
position of maximum resistance. Either side of that will show much lower
resistance and less wave making and this is exactly what "Sno Dog" is
showing two "optimum" speeds. So if possible your length/speed ratio [Froude
number] needs to be tailored to fit the speed at which you will normally be
travelling so you are not travelling at the speed of maximum resistance.

Henry; our boats are still operating as displacements vessels at 35 knots.
You can see from the resistance curves that a displacement hull can
sometimes perform just as fast as a planing hull. Of course it all depends
on whether your definition of planing is the mathematical one, or the
physical one.

I hope this has helped.

Regards.

Malcolm Tennant.  ARINA  MA

Malcolm Tennant Multihull Design Ltd
PO Box 60513 Titirangi,
Waitakere 0642
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

Power-Catamaran Mailing List

I'm sure Malcolm can add color commentary, but to get your started:

Froude Number: http://en.wikipedia.org/wiki/Froude_number
(just a slightly different scaling constant, the typical Speed-Length-Ration (S/L) ratio isn't properly dimensionless)

CS: Canoe Sterned - a good overview is here:
Comparing Catamarans:
http://www.catamarans.com/news/2006/04/CatComparison.asp
Also, lots of good material on Malcolm's website.

D C Mac Macdonald k2gkk@hotmail.com wrote: Good morning to you, Mr Tennant.

I am wondering if I may be the only person
who does not know what a "CS" hull shape is.

I have the same difficulty with "Froude" number.
Is this number that depends on the square root
of the waterline length (I have forgotten the
correction multiplier <1.23? 1.34?>)?

Sincerely,

D C "Mac" Macdonald
m/v Another Adventure
(non trawler/catamaran)

----Original Message Follows----
From: "Malcolm Tennant"  (by way of
GeorgsKolesnikovs)
Reply-To: Power Catamaran List

To: Power Catamaran List

Subject: [PCW] Power cats
Date: Tue, 1 Aug 2006 13:13:01 -0400

A response to the letters of Henry Clews and Pat Reischmann regarding
performance.

If you go to our web site and look under the "latest news" section for 14th
July 2004, well it was the latest news back then! you will see a discussion
of fuel consumption/hull resistance curves. The graph shown there
illustrates perfectly what both Pat and Henry are talking about.

Pat is correct; the graph clearly shows that even at "displacement" speeds
the displacement catamaran still has less resistance. Although there is  not
as much difference as at higher speeds [higher Froude numbers]. Pat is also
correct in saying that the displacement is an important variable which is
why we tried to get the displacements of the boats that are plotted as
similar as we could. We further normalised the data by using litres per
tonne. This decreased the differences in resistance between the boats due to
the slight differences in displacement.

The high point in the resistance curve for all these boats is at Froude
number 0.4. This is the point where a monohull displacment boats speed
stops. This is "hull speed". However the displacement catamaran using our
"CS" hull shape can economically go up to a Froude number of approx 1.0.
After that the power required to go faster becomes economically unfeasible.
Given that Manta power catamarans use the "CS" hull shape and the PDQs is
very, very similar, these curves would appear to be applicable to both of
these boats.

Henry is correct: off the shelf performance prediction models are unreliable
because of the wide variety of hull shapes that are used. However our
in-house performance prediction model based on the "CS" hull form is very
reliable and valid for our particular hulls. If you are designing a multi
million dollar boat then it is only good business sense to confirm your
performance preditions with tank testing.

Given that the PDQ hull is virtually a "CS' hull we can see from the graphs
exactly why Henrys boat behaves the way it does. The salient point is that
you need to stay away from a Froude number of 0.4 for your boat ie: the
position of maximum resistance. Either side of that will show much lower
resistance and less wave making and this is exactly what "Sno Dog" is
showing two "optimum" speeds. So if possible your length/speed ratio [Froude
number] needs to be tailored to fit the speed at which you will normally be
travelling so you are not travelling at the speed of maximum resistance.

Henry; our boats are still operating as displacements vessels at 35 knots.
You can see from the resistance curves that a displacement hull can
sometimes perform just as fast as a planing hull. Of course it all depends
on whether your definition of planing is the mathematical one, or the
physical one.

I hope this has helped.

Regards.

Malcolm Tennant.  ARINA  MA

Malcolm Tennant Multihull Design Ltd
PO Box 60513 Titirangi,
Waitakere 0642
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

Power-Catamaran Mailing List

Power-Catamaran Mailing List