Hello again, Michael, and thanks for the note. I greatly appreciate the time it took to help me check my numbers. You and I are actually in agreement on most things. I have snipped those parts on which we agree. At 04:23 PM 7/1/98 -0700, Michael Kasten wrote in part: >>when they saw my numbers based upon Gerr, their reactions were all the same. >>They believe that these numbers show the power requirements to be much too >>high. > >Gerr's algorithm seems to be geared toward determining engine size, prop >parameters, etc. rather than being necessarily intended for range >prediction. Still, with HP and fuel consumption plus the vessel >parameters, we can easily derive a range prediction via Gerr. That is pretty much what I endeavored to do. >If you were to tow a boat, and measure the force on the towing line, that >is considered to be the actual resistance of the hull. Yes, I have done this before with other boats; and will very likely do it again with Doulos once I get her finished and in the water. Until that time, I will always have some degree of uncertainty in my range estimates. It is really silly for me to talk about range at this time, since my accessories will burn more fuel than my propellers. This is common for fishing boats of this sort. This is also the reason I am carrying so much fuel. My actual range with the accessories running will be much less. On the other hand, I can easily add 7500 gallons of fuel to the boat in temporary tanks, if the freezers are empty. >What you have encountered among other designers is that, based on overall >propulsive efficiency, Gerr's algorithm gives what looks like a very >pessimistic view. Whether his is or is not a true picture can only be >determined via a rigorous feedback program, using reliable data for a >given boat type and for a given use. When I told you that these other multihull designers had this attitude towards my numbers based upon Gerr, perhaps I should have told you that they believe the numbers to be too conservative by a factor of three or four. I discount their input completely, however; since the data I got from actual boats that are similar to mine, and which are being powered at speeds close to what I am interested in, actually support Gerr fairly well. At this point, I should mention that these designers are all designing sailboats that they hope to drive at speeds much faster than I. I suspect that for their boats running at these high S/L ratios, they are right. For real displacement catamarans in the 60-90 foot range running at S/L ratios between 0.7 and 1.2, Gerr's numbers correlate fairly well. At these slow speeds, you can design a more efficient monohull passage maker than you can a catamaran. The efficiency advantage of a cat is only seen at the higher speeds. You would actually be better off with a trimaran if you wanted a passage maker to run between an S/L of 0.7 and 1.2. >>I used long tons for the purpose of figuring the D/L, but I used short tons >>for everything else. > >>What I used was one gallon per hour per twenty hp. > >For things nautical, it is typical to use long tons everywhere, unless >using metric tons. I understand this, but I decided to do it differently. I have been involved with industrial cranes and lifting equipment too long, and have come to appreciate the 2000 pound ton. >Converting 38.5 short tons into long tons for DOULOS' >average displacement gives 34.38 LT. Using half of that displacement for >each hull, half of the total fuel amount, and using 20 hp / gal / hr, >your numbers match the Gerr curves exactly. This is true. Actually, if you were to treat both hulls together, they would still yield the same result. I refer to Formula 2-1 and Chart 2-1 in his "Propeller Handbook." This gives simply a "pounds per hp" number, which will yield the same result if you treat a cat as two half weight boats, or one full weight boat. This is how I worked the data from the real boats, and this is also how I worked up the table for Doulos. Once I had hp required at a particular speed and displacement, I used specific fuel consumption to kick out a range number. >The Gerr prediction without any fuel reserve and presuming 20 hp / gal / >hr, as you have done, using an available fuel amount of 2,500 gallons per >hull, at 34.38 LT or 17.19 LT per hull, your range and power numbers >would be as follows PER HULL using half the total fuel (so multiply HP >and GPH times two for the whole boat, and consider range as is): > > > S/L Knots HP GPH Range > >0.70 5.4 10.9 0.54 22,414 >0.80 6.2 16.2 0.81 17,161 >0.90 7.0 23.1 1.16 13,559 >1.00 7.7 31.7 1.59 10,983 >1.10 8.5 42.2 2.11 9,077 >1.20 9.3 54.8 2.74 7,627 >1.25 9.7 62.0 3.10 7,029 >1.30 10.1 69.7 3.49 6,499 >1.40 10.8 87.1 4.35 5,603 >1.50 11.6 107.1 5.36 4,881 >1.60 12.4 130.0 6.50 4,290 I guess you have lost me here. Perhaps it might be possible that you have another Gerr book and are using a different algorithm than I? >You appear not to have considered any reserve fuel amount for those >figures. It would be perhaps more typical to use around 90 percent of >the available fuel amount before calculating range. I would use 100 percent, but then I would add a safety margin on the other end. In other words, if I knew that my range was 1000 miles under a set of circumstances, then I would not attempt a passage of more than 900 under those same conditions. I also agree that ten percent is reasonable, since it is always so easy to track your actual fuel consumption, and then to slow down to extend range if required. >You have used a somewhat optimistic 20 hp / gal / hr. If this is a known >efficiency, you are doing very well indeed!! The Beebe method (and most >other conservative methods) use around 16 hp / gal / hr. I've done a lot of work with diesel engines in trucks, tractors, heavy equipment, generators, and hydraulic power units. In the past, I have found 20 to be a good number to work with; though recent improvements in diesel engine technology will beat this in almost every application. This is only possible, however; if you are able to match the engine and the load effectively. This means that you must either design the system to run at a constant speed and a constant load, or else you need a variable speed transmission. Since you are using a single speed transmission, you have done very well to get a 16 average. I would have expected less. When I ran the numbers for Doulos with a fixed speed transmission, they came out at about 12 or a little less for the type of duty cycle I plan to fish her with. This is the entire reason for the fluid power drive: to be able to use the variable speed transmission to optimize specific fuel consumption. As you can see, this will more than make up for the loses in fluid power between the engine and the propeller shaft. While I have told everyone that I will put a Deutz 913 engine onto my boat, that will not be a final decision until I actually buy the engine. I'm actually watching several very closely. Right now, they range in specific fuel consumption between 19.8 and 24.8 hp per gpm. The Deutz is at 20.8, and I will most likely reject it if Deutz does not improve on that to compete with the other engines. I know how to get 25 out of any diesel, and so do the manufacturers. You are beginning to see it offered in a stock engines in Europe and in other parts of the world where fuel costs are high. In the USA, you are only seeing it in larger industrial diesels, starting at about 500 hp. The reason for this is that this sort of fuel efficiency cost lots of extra money, adding as much as a third to the cost of an engine. The fuel prices in the USA simply do not justify the costs--except in the larger engines. I have already figured out how to put a European engine into my boat if I have to, but I believe that by the time I need to buy the engines that they will be available in the USA in the smaller sizes that I need for Doulos. Mercedes claims that they will soon be able to greatly improve over the 25, but are only going to offer that engine in Europe. They are not even going to try to make an American version that will meed our clean air requirements. >An interesting anomaly, observed by Charles Vollum when analyzing data >from the Sink's various crossings, is that the Beebe range prediction >algorithm, as can be seen from the above numbers, seems to become wildly >optimistic at speeds below S/L 1.0 when compared with actual ocean voyage >fuel use feedback. For example, one would be hard pressed to imagine a >real-world range for DOULOS of twice around the globe if you simply back >off to 5.4 knots... Actually, I know of fishing boats that can do this--if they were to shut down all their accessories during the trip; but then that is neither here nor there. I'm not about to try to make a passage that slow, unless I have a very good reason to. >I have replied in depth here mainly because I would like to encourage >others with sufficient data to contribute their "real-world" findings, >and because I believe the TW list is the ideal forum for making such data >more public. I'd love to see others post their data, too. >Predicting resistance and therefore range is indeed not a simple trick. >Without good feedback from vessels in use, it is, at best, approximate. That sums it up pretty well. Thanks for the input. I think that perhaps we have both taught each other something new. +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ :: Paul and Cindy Kruse :: KJV Joh 14:27 Peace I leave with you, 165 South Kenneth Court :: my peace I give unto you: Merritt Island, FL 32952 :: not as the world giveth, give I unto you. E-mail: plkruse@iu.net :: Let not your heart be troubled, 407-453-6206 :: neither let it be afraid. :: +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++