I think I am confused. Paul Kruse mentioned the formula HP = GPH * 16.5. Let's consider an old Grand banks with a Lehman diesel at 120 HP. Specs say she should consume 2.5 GPH at 220 rpm at 8.5 kts cruise. That is a speed/length ratio of 1.14 ( hull speed = 7.43 kts). Using the Kruse's formula, it would suggest that at this performance point, there would be only 41.25 HP being consumed from a 120 HP engine. There can't be that much losses in efficiency from the flywheel to the water. What gives? Let's suppose the factor is 20. That would still only mean there is 50 HP consumed at what should be close to full load rpm of the engine. What am I missing??? It should be possible to install a fuel rate meter to measure GPH accurately; then refer to the specific fuel consumption curves for a particular engine to determine HP being used. Using Kruse's formula, one could then determine the factor for a particular vessel. Assuming the factor remains constant over the HP curve, one could then use GPH to determine where the engine is operating in its HP envelope at any given time. I believe this would be useful to determine load factor on the engine. All this information would be useful to predict maintenance intervals, etc, etc. Having worked 35 years with diesel engines, I have never heard of this simplified formula, and suspect the real world is not that simple. I suspect the factor may vary considerably depending on rpm, load factor, torque, and the duration of steady state condition the engine may or may not be operating in. Help! Maurice Marwood

> I think I am confused. Paul Kruse mentioned the formula HP = GPH * 16.5. > Let's consider an old Grand banks with a Lehman diesel at 120 HP. Specs say > she should consume 2.5 GPH at 220 rpm at 8.5 kts cruise. I'm guilty of not explaining the first post in enough detail. I'll try to make it clearer. The real way to estimate how much fuel a diesel engine will burn is to look at an engine power graft. That graft will normally plot two curves related to RPM; Horse Power and Torque. A third curve will be plotted which is related to the horse power and torque: It will be the Specific Fuel Consumption. Its values are normally in pounds of fuel per hour per horsepower. To get to gallons per hour require converting pounds of diesel to gallons of fuel. It should be noted that this graft shows the horse power and torque available at different RPM and that the rated horsepower is only available at maximum RPM. Take note of the word available. The actual horsepower the engine develops is determined by the load. It is never more than the amount shown on the graft at any particular RPM, but may be any, or almost any, amount less than the curve displays. If one can define the load at a particular RPM, the specific fuel curve can be used to estimate the amount of fuel that will be burned. The load business takes some doing so this graft is not convenient for amateurs to make fuel consumption assumptions from. Good yacht manufacturers develop and publish Speed/Fuel grafts that do compute loads and measure fuel consumption at different RPM with a specific engine. For example, the curve for the Krogen-42 developed by Mr. Krogen for the Lehman 135 shows that it burns 2.0 gph at 7.5 knots/1600 RPM, and 2.6 gph at 8.0 knots/2000 RPM. These are indeed less than the 135 rated horsepower of the engine and much less than the available horsepower at those RPM. By the way, those figures are right on the money. All those curves are great for many purposes and actually form the basis for the generalizations presented by CaptnWil and Paul to solve a different problem. The information presented was to turn the problem around. Most of us get to know how much fuel we burn per hour because if we don't, we will likely run out of fuel. If we don't know, we can generate that information easily. We can take that knowledge and get a good idea about how much horsepower we are using. That ball park can be helpful to us to select various pieces of equipment, etc. The purpose of the factor in my first post was to help reduce the cost of installing a piece of equipment if one actually uses less horsepower than the rated engine horsepower. I think all the responses to this thread show that almost all of us do use much less horsepower that the engine is rated for. It is interesting to note that Mr. Krogen's curve shows that the 135 horsepower engine burns 7.0 gph at the top 2600 RPM. If we use the factor of 16.5 to estimate the load we get 7 * 16.5 = 115.5 HP. If we use Paul's factor of 20 we get 7 * 20 = 140 HP. The 115.5 could be obtained by this engine, but the 140 could not. Both figures are in the ball park of the actual performance of this engine -- when used for the purpose proposed. Remember that these factors are generalizations. CaptnWil's generalization is, "Generalizations are generally wrong about specific things." Some engines will do better and some engines will do worse. It is likely that no engine will hit either figure right on the mark. Even though these generalizations will serve many purposes, those purposes must be chosen with care. I believe that they are fine to select the correct size bypass oil filter for your engine. You might be interested to know that after reading the post, GCF agrees. CaptnWil 40 Pier Pointe New Bern NC 29562 (252) 636-3601 captnwil@coastalnet.com