[PCW] 'Sno Dog performance

Wed Aug 2 12:32:56 EDT 2006

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
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