![\"Albermarle](\"https:\/\/setsail.com\/wp-content\/uploads\/2011\/10\/Albermarle-Sound-Prop-Test-100.jpg\" "\\"Albermarle-Sound-Prop-Test-100.jpg\\"")
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Predicting performance under power is relatively simple if you have a hull form that fits standard models (the definition of standard here covers a gamut of fishing trawlers to high speed destroyer hulls). In the olden days you would look up David Taylor model test data for something similar and then go to work with your slide rule. More recently, this process has become quicker, with the ubitquitous computer filling in for the retired slide rule. But if you pick the wrong model data, or miss something in your hull characteristics, the resulting calcs can be off, sometimes by a lot.<\/p>\n
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Tank testing can be a help, but the tank drag data has to be scaled from the towed model to full size, and this process is frought with potential error. Having data on similar models that turned out to tie to the real world is critical to even getting into the ballpark with the projections. A major impact on accuracy is the size of the test model. The bigger the model in scale, the better the results are likely to be. But big models are very costly, so only the well heeled programs, looking for that last bit of performance, can afford anything larger than one fifth size and most are more in the range of 1\/10th scale.<\/p>\n
Over the years we have accumulated data from our prediction software in the context of a relatively consistent family of hulls, with plenty of opportunity to observe the results in the real world. When we did the design work on the FPB prototype, WInd Horse<\/em>, we did go into the tank, but this was more to check the release of the stern wave and to see if squatting might be a problem, rather than looking for accurate drag numbers.<\/p>\n Correlating tank and VPP data with smooth water operation is one thing. Doing it for rough water is something altogether different. It is the latter envrionment where real world data becomes so critical.<\/p>\n We’ve got more than 50,000 miles of performance information on WInd Horse<\/em> so we have a pretty good idea of the power required for normal operation. However, since we’ve never come close to running out of top end, even in the gnarliest of up wind sea states, we have had, in effect, excess power, and so we were without a data set of which we were highly desirous.<\/p>\n As previously reported, we had the diesels on Wind Horse<\/em> derated by resetting the injection pumps from M4 to M1\/3. Prop pitch was altered as well to reduce RPM, giving us an M1 rating of 105 HP at 2300 RPM.<\/p>\n Wind Horse<\/em> is close to a scale model of the FPB 115, and as such, in effect, a real world test bed, far more valuable than anything we could do in the tank, or with computers. Our goal with the mods has been to get closer to the answer of what sort of power we need for the FPB 115, beyond normal cruise, for difficult sea states. We finally got a chance to test this set up crossing Albemarle Sound today.<\/p>\n Albemarle has a reputation for kicking up a vicous chop when the wind blows. With a post frontal breeze at a steady 25 knots plus, gusting higher, three to six foot (.9 to 1.8m) short coupled waves were making their presence known. In the interest of science, this was enough incentive to get us to move from a snug anchorage just north of the Sound.<\/p>\n Now these seas and the wind force in and of themselves would not be a sufficient test, except fore one other factor – water depth, or more precisely a lack thereof. Depths averaged 15 to 20 feet (4.5 to 6m) during our journey which is shallow enough for the hull to feel an increase in drag. While we don’t have precise numbers in smooth water shallow depths like this will increase drag by ten to twenty percent.<\/p>\n We are pleased to report that Wind Horse<\/em> was able to achieve the M1 rated 2300 RPM in these conditions, with a speed \u00a0over the ground of 11.7 knots. Between alternators and stabilizer hydraulics there was a load of roughly ten horsepower per engine, leaving approximately 95HP from each engine, or 190 HP total, to overcome propulsion drag, sea state, and windage. We have more testing to do once Wind Horse<\/em> has her swim step extension fitted (similar to the FPB64s), but for now we know we are on the right track.<\/p>\n <\/p>\n <\/p>\n","protected":false},"excerpt":{"rendered":"![\"Albermarle](\"https:\/\/setsail.com\/wp-content\/uploads\/2011\/10\/Albermarle-Sound-Prop-Test-101.jpg\" "\\"Albermarle-Sound-Prop-Test-101.jpg\\"")
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