Re: [PCW] 'Sno Dog performance
Hi, Henry and all.
I did some calculations from your performance figures
because it appeared to me that the speed/rpm ratio
was surprisingly linear. I have converted your chart
to SPEED/1000 RPM ratios shown below. Although not
perfectly linear, they show up much more so than most
boats. Very interesting indeed. I know that these don't
translate at all to fuel economy due to the degree of
loading of the engines, it would appear that increasing
speed when it is needed at least won't put you into the
bankruptcy courts!!!
..
| RPM SPEED RATIO |
| (1000) (Knots) |
||
| 1.0 4.8 4.80 |
| 1.2 5.6 4.67 |
| 1.4 6.6 4.71 |
| 1.6 7.5 4.69 |
| 1.8 7.8 4.33 |
| 2.0 8.1 4.05 |
| 2.2 8.6 3.91 |
| 2.4 9.2 3.83 |
| 2.6 10.9 4.19 |
| 2.8 12.7 4.54 |
| 3.0 14.5 4.83 |
| 3.2 15.7 4.91 |
| 3.4 17.0 5.00 |
| 3.6 17.9 4.97 |
| 3.8 18.6 4.89 |
|________________________________|
D C "Mac" Macdonald
m/v Another Adventure
Grand Lake - Oklahoma
. ----Original Message Follows----
From: HClews@aol.com
Reply-To: Power Catamaran List power-catamaran@lists.samurai.com
To: power-catamaran@lists.samurai.com
Subject: [PCW] Power cats - performance
Date: Tue, 1 Aug 2006 17:20:26 EDT
Thank you Malcolm for your response - very insightful, as usual!
I found the graph you mentioned (at
http://www.tennantdesign.co.nz/news.php?story=33) very interesting.
I am glad that both you, and your graph, refute my contention that
powercats are no more economical at displacement speeds
than monohulls. That was only a suspicion on my part, I'm happy to
be proven wrong!
I also computed the Froude numbers (useful website:
http://www.processassociates.com/process/dimen/dn_fro.htm )
for various speeds on my PDQ 34 Sno' Dog and found that,
sure enough, the speed corresponding to a Froude No. of 0.4
on my boat is about 12 knots. This is exactly the speed I normally
try avoid as it is quite obvious that extra power is required without
a commensurate speed increase.
I have just added a "Performance" page to my website which includes
a speed vs RPM chart for Sno' Dog. It shows that our two preferred
speeds are 7.5 knots (at 1600 RPM) and about 16 kts (at 3200 RPM)
This page can be accessed directly at:
http://www.geocities.com/snodoglog/Performance.html
Also, your article (the one with your photo) at:
http://www.catamarans.com/news/2006/04/CatComparison.asp
is very useful background for anyone interested in powercat performance.
Thanks again,
Henry
aboard Sno' Dog
www.geocities.com/snodoglog
In a message dated 8/1/2006 12:53:24 P.M. Eastern Daylight Time,
malcolm@tennantdesign.co.nz writes:
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
Mac,
I'm wondering what portion of your ratio is a result of the prop
(pitch/dia/cup...) and engine (torque/hp) characteristics as compared to
the hull characteristics?
Is the linearity due to a careful match of hull, engine, and prop? Or
is it just happenstance? Or is it maybe just a meaningless number?
Seems to me that this number gives some indication of the performance of
the whole propulsion system (hull/engine/prop) but I'm not sure what to
make of it.
Any insights you can share?
Bob Deering
Juneau Alaska
-----Original Message-----
From: power-catamaran-bounces@lists.samurai.com
[mailto:power-catamaran-bounces@lists.samurai.com] On Behalf Of D C
Mac Macdonald
Sent: Tuesday, August 01, 2006 5:12 PM
To: power-catamaran@lists.samurai.com
Subject: Re: [PCW] 'Sno Dog performance
Hi, Henry and all.
I did some calculations from your performance figures
because it appeared to me that the speed/rpm ratio
was surprisingly linear. I have converted your chart
to SPEED/1000 RPM ratios shown below. Although not
perfectly linear, they show up much more so than most
boats. Very interesting indeed. I know that these don't
translate at all to fuel economy due to the degree of
loading of the engines, it would appear that increasing
speed when it is needed at least won't put you into the
bankruptcy courts!!!
..
| RPM SPEED RATIO |
| (1000) (Knots) |
||
| 1.0 4.8 4.80 |
| 1.2 5.6 4.67 |
| 1.4 6.6 4.71 |
| 1.6 7.5 4.69 |
| 1.8 7.8 4.33 |
| 2.0 8.1 4.05 |
| 2.2 8.6 3.91 |
| 2.4 9.2 3.83 |
| 2.6 10.9 4.19 |
| 2.8 12.7 4.54 |
| 3.0 14.5 4.83 |
| 3.2 15.7 4.91 |
| 3.4 17.0 5.00 |
| 3.6 17.9 4.97 |
| 3.8 18.6 4.89 |
|________________________________|
D C "Mac" Macdonald
m/v Another Adventure
Grand Lake - Oklahoma
. ----Original Message Follows----
From: HClews@aol.com
Reply-To: Power Catamaran List power-catamaran@lists.samurai.com
To: power-catamaran@lists.samurai.com
Subject: [PCW] Power cats - performance
Date: Tue, 1 Aug 2006 17:20:26 EDT
Thank you Malcolm for your response - very insightful, as usual!
I found the graph you mentioned (at
http://www.tennantdesign.co.nz/news.php?story=33) very interesting.
I am glad that both you, and your graph, refute my contention that
powercats are no more economical at displacement speeds
than monohulls. That was only a suspicion on my part, I'm happy to
be proven wrong!
I also computed the Froude numbers (useful website:
http://www.processassociates.com/process/dimen/dn_fro.htm )
for various speeds on my PDQ 34 Sno' Dog and found that,
sure enough, the speed corresponding to a Froude No. of 0.4
on my boat is about 12 knots. This is exactly the speed I normally
try avoid as it is quite obvious that extra power is required without
a commensurate speed increase.
I have just added a "Performance" page to my website which includes
a speed vs RPM chart for Sno' Dog. It shows that our two preferred
speeds are 7.5 knots (at 1600 RPM) and about 16 kts (at 3200 RPM)
This page can be accessed directly at:
http://www.geocities.com/snodoglog/Performance.html
Also, your article (the one with your photo) at:
http://www.catamarans.com/news/2006/04/CatComparison.asp
is very useful background for anyone interested in powercat
performance.
Thanks again,
Henry
aboard Sno' Dog
www.geocities.com/snodoglog
In a message dated 8/1/2006 12:53:24 P.M. Eastern Daylight Time,
malcolm@tennantdesign.co.nz writes:
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
Hi, Bob.
I have no idea as to why the performance of Henry Clew's
PDQ 34 seems to be so linear. I am just mathematically
inclined and I noticed an apparent linearity. I just used a
calculator to check out the extent of linearity and decided
to share it with the group.
I would say that the linearity indicates a happy marriage
of design elements, whether intentional or accidental.
In any event, I would say that the linearity in 'Sno Dog's
performance is highly desirable.
Mac in Oklahoma City
----Original Message Follows----
From: Robert Deering deering@ak.net
Reply-To: Power Catamaran List power-catamaran@lists.samurai.com
To: 'Power Catamaran List' power-catamaran@lists.samurai.com
Subject: Re: [PCW] 'Sno Dog performance
Date: Tue, 01 Aug 2006 20:16:44 -0800
Mac,
I'm wondering what portion of your ratio is a result of the prop
(pitch/dia/cup...) and engine (torque/hp) characteristics as compared to
the hull characteristics?
Is the linearity due to a careful match of hull, engine, and prop? Or
is it just happenstance? Or is it maybe just a meaningless number?
Seems to me that this number gives some indication of the performance of
the whole propulsion system (hull/engine/prop) but I'm not sure what to
make of it.
Any insights you can share?
Bob Deering
Juneau Alaska
-----Original Message-----
From: power-catamaran-bounces@lists.samurai.com
[mailto:power-catamaran-bounces@lists.samurai.com] On Behalf Of D C
Mac Macdonald
Sent: Tuesday, August 01, 2006 5:12 PM
To: power-catamaran@lists.samurai.com
Subject: Re: [PCW] 'Sno Dog performance
Hi, Henry and all.
I did some calculations from your performance figures
because it appeared to me that the speed/rpm ratio
was surprisingly linear. I have converted your chart
to SPEED/1000 RPM ratios shown below. Although not
perfectly linear, they show up much more so than most
boats. Very interesting indeed. I know that these don't
translate at all to fuel economy due to the degree of
loading of the engines, it would appear that increasing
speed when it is needed at least won't put you into the
bankruptcy courts!!!
..
| RPM SPEED RATIO |
| (1000) (Knots) |
||
| 1.0 4.8 4.80 |
| 1.2 5.6 4.67 |
| 1.4 6.6 4.71 |
| 1.6 7.5 4.69 |
| 1.8 7.8 4.33 |
| 2.0 8.1 4.05 |
| 2.2 8.6 3.91 |
| 2.4 9.2 3.83 |
| 2.6 10.9 4.19 |
| 2.8 12.7 4.54 |
| 3.0 14.5 4.83 |
| 3.2 15.7 4.91 |
| 3.4 17.0 5.00 |
| 3.6 17.9 4.97 |
| 3.8 18.6 4.89 |
|________________________________|
D C "Mac" Macdonald
m/v Another Adventure
Grand Lake - Oklahoma
A little bit of a tangent, but I always wondered why some designs use high RPM diesels, e.g. WOT=3800, vs many sailboats or passamakers with the high torque low rpm diesels - I think these guys redline at <2500 rpm and generally cruise in the 1200-1800 rpm range.
It would seem you'd get better efficieny with a slower turning prop with large diameter and/or more pitch, especially if your wot speed is <18-20. Also less noise, stress on the bearings, etc. Is it cost, power-to-weight, or ??
Thanks.
D C Mac Macdonald k2gkk@hotmail.com wrote: Hi, Henry and all.
I did some calculations from your performance figures
because it appeared to me that the speed/rpm ratio
was surprisingly linear. I have converted your chart
to SPEED/1000 RPM ratios shown below. Although not
perfectly linear, they show up much more so than most
boats. Very interesting indeed. I know that these don't
translate at all to fuel economy due to the degree of
loading of the engines, it would appear that increasing
speed when it is needed at least won't put you into the
bankruptcy courts!!!
..
| RPM SPEED RATIO |
| (1000) (Knots) |
||
| 1.0 4.8 4.80 |
| 1.2 5.6 4.67 |
| 1.4 6.6 4.71 |
| 1.6 7.5 4.69 |
| 1.8 7.8 4.33 |
| 2.0 8.1 4.05 |
| 2.2 8.6 3.91 |
| 2.4 9.2 3.83 |
| 2.6 10.9 4.19 |
| 2.8 12.7 4.54 |
| 3.0 14.5 4.83 |
| 3.2 15.7 4.91 |
| 3.4 17.0 5.00 |
| 3.6 17.9 4.97 |
| 3.8 18.6 4.89 |
|________________________________|
D C "Mac" Macdonald
m/v Another Adventure
Grand Lake - Oklahoma
. ----Original Message Follows----
From: HClews@aol.com
Reply-To: Power Catamaran List
To: power-catamaran@lists.samurai.com
Subject: [PCW] Power cats - performance
Date: Tue, 1 Aug 2006 17:20:26 EDT
Thank you Malcolm for your response - very insightful, as usual!
I found the graph you mentioned (at
http://www.tennantdesign.co.nz/news.php?story=33) very interesting.
I am glad that both you, and your graph, refute my contention that
powercats are no more economical at displacement speeds
than monohulls. That was only a suspicion on my part, I'm happy to
be proven wrong!
I also computed the Froude numbers (useful website:
http://www.processassociates.com/process/dimen/dn_fro.htm )
for various speeds on my PDQ 34 Sno' Dog and found that,
sure enough, the speed corresponding to a Froude No. of 0.4
on my boat is about 12 knots. This is exactly the speed I normally
try avoid as it is quite obvious that extra power is required without
a commensurate speed increase.
I have just added a "Performance" page to my website which includes
a speed vs RPM chart for Sno' Dog. It shows that our two preferred
speeds are 7.5 knots (at 1600 RPM) and about 16 kts (at 3200 RPM)
This page can be accessed directly at:
http://www.geocities.com/snodoglog/Performance.html
Also, your article (the one with your photo) at:
http://www.catamarans.com/news/2006/04/CatComparison.asp
is very useful background for anyone interested in powercat performance.
Thanks again,
Henry
aboard Sno' Dog
www.geocities.com/snodoglog
In a message dated 8/1/2006 12:53:24 P.M. Eastern Daylight Time,
malcolm@tennantdesign.co.nz writes:
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