{"id":28964,"date":"2013-07-07T19:36:44","date_gmt":"2013-07-08T00:36:44","guid":{"rendered":"https:\/\/setsail.com\/?p=28964"},"modified":"2013-12-27T12:37:28","modified_gmt":"2013-12-27T17:37:28","slug":"fpb-cruising-speed-range-under-power-and-the-real-world","status":"publish","type":"post","link":"https:\/\/setsail.com\/fpb-cruising-speed-range-under-power-and-the-real-world\/","title":{"rendered":"FPB: Cruising Speed, Range Under Power, And The Real World"},"content":{"rendered":"

\"AVATAR<\/p>\n

Those of you familiar with our work will know that we consider being able to maintain comparatively fast cruising speeds the most<\/strong> <\/em>important factor in safe, comfortable ocean crossing. Get this right and you enjoy making passages. Get it wrong and you will prefer sitting at the dock reading about the folks who are really out there cruising.<\/p>\n

Our theoretical numbers now have the benefit of many thousands of miles of real world experience, both our own with FPB 83 Wind Horse,<\/em> and those of the seven FPB 64 owners now actively cruising. As we are re-checking our weight, balance, drag, and power numbers for the FPB 78, four recent equinoctial gale season ocean passages with FPB 64s in the South Pacific have provided us with a new batch of excellent real world data. We thought you might like to see some of the numbers. This is the first in a series of very detailed passage posts over the next few days, each of which is a combination of the Owners’ comments and ours.<\/p>\n

\"\"<\/p>\n

We know from long experience that 9.5 to 9.75 knots is a magic number. If you can average<\/em> <\/strong>this, day in and day out, you can avoid really dangerous, if not unpleasant conditions, with a dose of luck, and judicious timing. And faster is even better. Speed reduces passage time, allows more flexibility in weather tactics, makes stabilizer systems more effective, and is more fun.<\/p>\n

Directly related to speed is range under power: taking into account sea state, how the yacht handles sea state, including windage, and allowing a fudge factor for safety. If your fuel burn is high and tank capacity low, then you are forced to slow down. This reduces stabilizer effectiveness and increases exposure to weather risks.<\/p>\n

Equally important in the speed equation is the physical capability of the yacht to absorb what may be a punishing sea state, and how the crew fares in these sub optimal conditions. This is where layout, a sea kindly hull, and details like hand and fiddle rails, position of sleeping and watch keeping stations and even seat belts come into play.<\/p>\n

We hear a lot about weather forecasting, modern routing, and the use of private forecasters. These are wonderful tools, but those who put their faith here hoping this will keep them out of harm’s way should investigate further. Simply put, the best forecasters, working with the very latest models, make mistakes<\/em><\/strong> – all of them. Weather patterns change once you have departed, and being able to maintain that high average speed is the key to making the best of the changing situation.<\/p>\n

In the end, all design factors come into play in the performance equation. It starts with hull shape, interior volume and where it is positioned relative to the motion centers, goes on through weight distribution, into the engine room and the efficiency of the drive line, through to noise, vibration, and motion under way.<\/p>\n

In the examples that follow in this series of passages you will see all of these factors at play, and hear from these voyagers just how well the weather forecasting is, or is not, working. Mixed in are compelling comments on average speed, fuel burn, and life aboard an FPB crossing the Tasman Sea and South Pacific ocean, with a slew of gales enlivening the contest.<\/p>\n

Before we get into the projections for the FPB 78, let’s have a look at what we calculated for the FPB 64, and then what we have seen in an ocean crossing context.<\/p>\n

\"64<\/p>\n

\u00a0We will start with this table from a post several years ago, where we interpolated from smooth water to what we’d expect in an ocean crossing context. This table includes allowances for hydraulics, DC power generation, and adverse ambient conditions. At 9.7 knots we are indicating five US gallons\/19L of diesel per hour plus hydraulic and electrical loads, or a total of 5.7 USg\/21.57L per hour, all things considered.<\/p>\n

Recap of the four FPB 64 voyages<\/em><\/strong><\/p>\n

This is a long post, filled with details from four different FPB 64s, on long South Pacific passages. We have a great deal of data to share, from which there are many lessons to learn. Our owners have been generous with their time, and their permission to share their comments, for which we are grateful. In addition, you will find quite a bit of detailed weather information, some from the FPB 64 crews, and quite a bit from South Pacific weather routing expert Bob McDavitt.<\/p>\n

Before you get into the details we thought a short recap of what the boats encountered would be of interest.<\/p>\n

FPB 64 Grey Wolf<\/em><\/strong><\/p>\n