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 _______________________________________________