Robby, Date: Fri, 24 Nov 2000 10:34:37 -0800 From: Robby & Dolores <robbyr@pacbell.net> Subject: TWL: Suggestion needed for increasing speed. >> Need suggestions/comments on how to increase the displacement speed of my fishing troller, Dolores E. The vessel is 37' long, waterline is approximately 34-35 feet, beam is 9.5 feet, draft is 4.5 feet and displacement is 24,000 lbs. She is powered with a 43HP CAT Industrial diesel with max RPM of 1500 with a 3:1 twin disc transmission turning a three blade RH 25 X 25 prop. The Power-Speed-Range calculator on http://fmyers.com/BCalcs.html web site tells me that I should be able to travel at 7.8 knots using 36HP while using 2.2 gallons per hour. My experience with our sailboat, Rolling Stone, confirms this. At 1000 RPM of engine speed I can do about 5.75 knots in smooth water. Offshore in any sea and wind (which we have here in SF) the speed drops below 4 knots. It is obvious that the prop is too small for the shaft speed available. I've taken these number to a local prop shop and they say I should have a 31 inch wheel (don't remember the pitch). A simple fix, unfortunately the 25" prop completely fills the aperture available for the prop. My questions: Since I can only change the pitch of the prop, how much (or how oversquare) can (or should) a prop be on a displacement vessel. The prop shop I discussed this with seemed very uncertain. At $1000.00 a prop, I don't want to experiment. I'll pursue this with other shops in the area, but thought someone might have some experience on the list. The second possibility would be to increase the shaft speed by reducing the reduction ratio. Does anyone know whether the shaft speed ratio on Twin Disc transmissions can be changed? I have no idea what model Twin Disc I have since there are no identifying plates on it. I know the engine is very old (perhaps pre W.W.II) and I assume the transmission is of similar age. I'll discuss this with a dealer, of course, but I wondered if there is any experience on the list or alternate suggestions. As a sidebar, the engine tach shows exactly double the RPM of the actual engine speed. I have accurately confirmed this. I assume a previous owner simply used that RPM to select the incorrect prop size. Regards Robby [robbyr@pacbell.net] S/V Rolling Stone (In Scotland) http://www.selfsteer.com/monitor/covers/MonitorCOV.jpg F/V Dolores E. (In San Francisco Bay) http://www.wingo.com/sailing/dolores.jpg<< Propeller specification is an arcane art. Boats and conditions differ by so much that even the most "scientific" approaches can only give you an estimate within plus or minus 10% of the correct value. (Sort of like an election.) However, I've run a few calculations on a prop specification program we used for trawler sized environmental research vessels. Given the specs of your boat, you should reach hull speed of 7.8 kts with 34.23 hp at the prop. Assuming a 5% loss in the drive line that works out to about 36 hp needed from the engine. Using Crouch's method of calculating prop specs, as described in Skene, you need a 31.9" diameter three blade prop with a pitch of 33.5" to reach hull speed with that power. In case you want to pull any stumps, your static thrust should be 1450 lbs. Your top speed at full engine output should be about 8.4 kt. So your prop shop was about right as far as the diameter goes. Using the Bp method of prop specification that most big ship naval architects use, and described fully in Dave Gerr's book, your prop diameter should be 36" with a pitch of 27". The efficiency will be quite high at 61%. The bollard pull will drop slightly to 1398 lbs, restricting you to pulling slightly smaller stumps. Slip will be only 17%. If you want to keep your 25" prop, you can keep the 25" pitch but the efficiency drops to 41% and the static thrust drops to 1088 lbs, nearly 40% less than with the optimum diameter prop. As you surmised, the main problem is that the shaft rpm is far too low for the maximum prop size you can fit. Repitching will not help much. Dropping the reduction ratio to 2:1 will enable you to keep your same prop and should improve overall performance under power. At that reduction ratio, the ideal prop size is 25" with a 23" pitch, right in the ballpark! I don't know if a simple gear replacement in a Twin Disc geabox can give you that result but the engine power is low enough to let you use a rebuilt or used box without too much concern. Your slow speed diesel is probably continuously rated at 1500 rpm so I would run it at 1300 rpm at cruising speed. About 200 rpm less than peak power strikes a nice balance between longevity and low speed fouling. Good Luck, Larry Zeitlin PUFFIN Cortlandt Manor, NY