Some people didn't seam to appreciate my hydraulic air compressor and alternator concept, so I thought I'd provide a quantitative example to illustrate the point. Executive summary: Since the DC system is sized to the average load and the AC generator is sized to the peak loads, the AC generator requires over twice the horsepower of the DC system, which offsets the added cost of the extra inverters required for the DC system. If hydraulic are already onboard, say for the get-home-drive, then the DC system is $3,463 or 20.3% cheaper than an AC generator. The DC system will provide lower operation and maintenance cost, so it will save money in the long run. In very hot conditions the AC generator will need to run almost continuously to provide air conditioning, whereas the DC system will run 8.5 hours per day in 2 hour runs separated by 4 hour calms with the A/C running off the battery bank. Although the hydraulic system seams complex, it will be purchased as a palletized and pre-tested system and maintenance and operation will be no more complex than a conventional generator albeit less conventional. Hydraulic drives on the alternators allow then to produce maximum power independent of engine speed (provided it is producing enough power to run the alternators) and more power at low engine speed than belt driven alternators. The gory detail: The smallest air compressor for the SCUBA tanks that I can find is the Bauer Junior Compressor with a 3 HP electric motor. According to Bauer it takes 17 amps steady state and 90 amps for startup which is a ratio of 5.2 and in the normal range for induction motors. They also claim it will operate on an 8 KW generator although I calculate that it should have a 10KW generator ideally. An example of an 8 KW generator is the 8 KW Northern Lights M753Ka which costs $8,021 at http://www.clis.com/mmarine/NorthernLights.htm and weighs 529 LBS. Another option is the 8 kW Spartan Series Marine Generators with an Isuzu Diesel Engine which costs $4,299 @ (http://www.americasgenerators.com/products/product_view.php?ProductID=8 73 ) and weighs 446 lbs. This engine is rated at 22.4 BHP Continuous @ 3000 RPM (http://www.frontierequip.com/isuzu/isuzu.htm). Now if you are wondering why it takes a 22.4 HP engine to drive an 8 KW generator i.e. 10.7 HP output, there are three reasons. First the generator runs at 1800 RPM to produce 60 Hz power and the power rating at 1800 RPM is 15.4 HP continuous (http://www.tuban.net/Model8_5Isuzu60HzDataSheet.html ). Second most generators operate at ~10 to 20% power margin to assure they can maintain the required speed (this is where many manufacturers cheat to get cost down). I calculate the Spartan power margin at 10.7 /(15.4*.85) = .817 or 18.3%. Third the typical AC generator efficiency is ~85% so output or 7.8 KW. Using a hydraulic pump and motor with a 70% power transmission efficiency (typical for a well designed system) would require a 4.3 HP diesel engine minimum. One example of a liquid cooled marine diesel is the Kubota Z482E which is rated at 10.4 HP continuous and weighs 117 lbs with an end plate and 179 with SAE flywheel housing (http://www.frontierequip.com/kubota/kubota.htm ). The Z482E would need to run at 1800 RPM to produce 4.2 BHP. The Z482 cost US$2,617 (http://www.rs-refrigeration.co.uk/Kubo.asp ). Without taking the time to engineer the hydraulics system, I anticipate the cost of the hydraulics will be far less than the cost difference between the Northern Lights 8KW generator and the Kubota and a little higher than the cost differential relative to the 8 kW Spartan generator. However operation and maintenance costs for the smaller Kubota should be significantly lower. Even though I am a Mechanical Engineer and I could design this system myself, I would contract the design and assembly of the complete system to professionals such at Frontier Equipment who provide custom auxiliary design assembly services (http://www.frontierequip.com/powerunit/marineaux.htm ). I would have it built-up, tested and delivered as a palletized subassembly. Take a look at some of the examples they provide, some of them are close to what I am proposing. Next I'd like to consider the total electrical requirements of the boat. I've started a generator load estimating spread sheet and uploaded it to my web site www.portager.info the direct link is http://www.portager.info/generator_loads.htm . I also provided links to down load Excel 2002 or Excel 5/95 versions of the workbook. I used a list of loads intended for home use, so there are still some items in the list that don't belong. These loads are only preliminary estimates intended for comparison purposes. I sized the DC refrigerator/freezer load at 65 watts/hr so I just entered it as a 65 watt DC load with a 100% duty cycle. Load case #1 shows everything I think I might want to be able to run at the same time with an electric air compressor. This is a worst case day where both A/C systems are running with a 50% duty cycle, i.e. the hottest day the boat should ever see. You will notice that I have included a combo washer/dryer, dishwasher, convection oven, ... I figure if I'm going to have my air compressor how can a deny the Admirable her quality of life? In the startup load calculations, I assumed that the startup loads are short in duration and therefore they never occur simultaneously (not necessarily a good assumption). In this case the steady state power load is 12.3 KW and the peak load is 20.4 KW so I rounded down to 20 KW. Load 2 is the same except the air compressor is hydraulic. In this case the steady state load is 10.4 KW and the peak load is 13.2 KW. So using the hydraulically powered air compressor allows the generator to be reduced from 20 KW to 12 KW (I rounded down to 12 KW since the next step up was 16 KW). Using Northern Lights generators the cost savings is $4,616 and the weight savings is 262 lbs. Load case #3 uses the 18,000 BTU Flagship Marine heat pumps instead of the HFL 16,000 BTU air conditioners. I used the cooling steady state power requirement instead of the greater of the two (heating), because the duty cycle is worse in the cooling mode. Despite the slightly higher steady state power requirement, due to the lower startup load, the required generator size decreases 1 KW, however this benefit just adds reduces the negative margin. The primary feature of the Flagship Marine heat pump is that it does double duty by providing both heating and cooling. Column "K" provides an estimated duty cycle and column "L" provides the average power per day, which is used to determine the DC generator run time. Cell L158 provides the DC power input to the invertors to run the AC loads and cell O159 provides the total DC load. Note that in load case #1, the AC generator required a 20 KW capacity while the DC load is only 2.5 KW after accounting for the inverter efficiency. Adding in the DC load brings the total to 3.2 KW. I know this seams like I'm an incredible power hog, but it is the hottest day of the year and it is only an example. Load case #2 requires 13.2 KW AC versus 2.4 KW DC and load case #3 requires 12.3 KW AC versus 2.8 KW DC. In calculating the alternator output, I deducted 10% for hot operating conditions. To determine the alternator output of the belt driven alternators I set the drive ratio so that the alternator speed is = the rated speed when the engine is running at 3,600 rpm. I then determined the output with the engine operating at 1,800 rpm. For the hydraulic alternator I assumed the alternator would operate at 5,000 rpm. Note that since the 98-24-220-BL has a maximum speed of 6,000 rpm, the hydraulic alternator has a 22% higher output when the engine is running at 1800 rpm than the same alternator with belt drive. If the engine speed drops to 1200 rpm the hydraulic alternator produces 52% more power than the belt drive alternator. At 900 rpm the difference is 78% (assuming the engine produces enough power at 900 rpm to drive the alternator). Since the 97-24-140-BL alternator has a maximum speed of 10,000 lbs the maximum drive ratio is 2.78:1, so with the engine operating at 1800 rpm it produces its full rated 140 amps but 39% loss than the hydraulic 98-24-220-BL. At 1200 rpm the 97-24-140-BL produces 44% less power and at 900 rpm 50% less. The real benefit of the hydraulic alternator is if the engine is oversized for the alternator. In other words if the auxiliary is sized at 40 HP for the get home drive and the maximum alternator load is 12.3 HP, when only the alternators are on the engine will be able to slow down and still provide maximum power output. With one 98-24-220-BL alternators the output is 5.5 KW of 24 VDC and the run time is 17 hours o the hottest day of the year. With two 98-24-220-BL alternators the output is 11 KW and the engine run time is 8.5 hours and the time to recharge the battery bank is <1 hour. Finally, I estimated the weight and cost of the all AC and all DC alternatives. The DC system turned out $1,062 or 6.2% higher cost which is ~8.1% higher than the AC system, however the DC system will have significantly lower operating cost and shorter run time. In addition, the DC system is 358 lbs or 34% lighter than the AC system. Finally, cell E47 shows that if hydraulic system is already onboard, say for a get-home-drive, then the DC system is $3,463 or 20.3% cheaper than an AC generator. Regards; Mike Schooley Designing "Portager" a transportable trawler