When it comes to creating a successful yacht for long distance voyaging, you have to start with the fundamentals, and build from there. Get the foundation right, and everything else falls into place. Get it wrong, and regardless of how cool the boat looks, or how much you like the interior, the real world experience is guaranteed to be less than optimal.
We’ve disclosed the exterior of this Wicked new FPB early because it is fundamental to how the boat functions in a holistic systems engineering context. Likewise the Matrix deck, which is also fundamental. Now come the details about what makes possible the cruising dream to which we all aspire.
When we are working up the preliminaries on a new project, the design spiral touches on a series of specific disciplines, coming back to each as other aspects are refined. This circular creative process takes into account the following:
- Cruising speed
- Range at cruising speed
- Hull shape
- Drive line geometry and propulsion efficiency
- Range under power
- Basic systems approach
- Systems impact on structure and interior
- Dinghy storage and handling
- Boat handling in traffic
- Bulk storage
- Seagoing interior requirements
- At anchor interior requirements
We begin with speed and range targets because every other facet of the design affects their outcome. And it is the speed target, in our case the sustainable ocean-crossing speed, that drives waterline length. This starts out as a non-dimensional speed length ratio (SLR), or the multiple of the square root of the waterline, at which you can efficiently operate. SLR is a function of the distribution of volume in the hull, and the displacement. As displacement length ratio (DLR) drops, the boat gets lighter for its length, efficient SLR can increase. To a lesser degree, as you push volume into the ends – increase the prismatic coefficient – the SLR can also go higher. Obviously, there are other factors like motion, steering control, and heavy weather handling that affect hull shape as well.
As the DLR drops, you have more flexibility with hull shape and a greater range of efficient speeds.
Since we have a long history with powering at speed length ratios well under 100 with our sailing and FPB designs, we can start from known, real world values. We know that Wind Horse has her sweet spot for long passages at 11 knots. Given her 81 foot waterline, this is a SLR of 1.22. The 64 is a little heavier for her length, being shorter a somewhat predictable outcome, and has her voyaging sweet spot pegged at 9.7 knots. That is a SLR of 1.175 with the swim step extension.
As a start, this tells us we can count on an SLR of 1.22 for the FPB 97. But let’s back off a touch for now and say 1.205, or halfway between the FPB 64 and FPB 83. Based on a 94 foot waterline, this equates to 11.7 knots.
There doesn’t seem to be a great deal of difference between the three sets of speeds, so what makes this so important? Simply put, the 17 nautical miles a day gain over the FPB 83 buys you an hour and a half of daylight each 24 hours, making passage planning that much easier. It also buys you additional weather insurance. And with FPBs, the faster you cruise the more comfortable you are going to be in almost all sea states.
Draft is important in two regards: cruising grounds, and piloting risk. It is a function of the hull, its projections, and where they occur. With the FPB 97 the hull, what we refer to as the canoe body, is just 42″/1.1 meters deep at full load, based on our preliminary shapes. The stabilizer fins are above the hull bottom. It is only the skeg that projects to the full draft of five feet/1.53 meters.
From a thin water perspective you could have the bow in depth barely beyond three feet/90cm, and with a little angle to the beach have the skegs easily clear.
We should point out that the FPB is designed to dry out on a tidal grid, something unique for larger yachts. That this opens a vast area of tidal waterways to exploration goes without saying.
With a ballpark length established and a guess at displacement (in this case relatively easily determined), we then begin to explore the hull shape. In order of priority the factors we want to target are:
- Steering control (a key comfort factor on passage, the major determinant in heavy weather tactics, and necessary when maneuvering in close quarters)
- Skid factor (how the hull reacts to wave impacts, by skidding to leeward and reducing heel and rotational acceleration)
- Uphill motion optimization (as long as it does not interfere with steering)
- Stability (so the stability curve has the correct shape for comfort and safety in bigger seas)
- Powering efficiency
Some of the hull design can be done with numeric modeling, trading off various parameters for efficiency. But much of this is based on real world experience, logging hours ourselves and seeing what our clients are doing.
Each of the three preceding FPB designs, 115, 83, and 64, are their own mix of ingredients. The same holds true for the FPB 97. The closest approximation would be a combination of the soft ride of the FPB 64 melded with the relative pitch stability of the FPB 83. Bottom line: very comfortable.
For a smooth ride uphill you want a sharp, deep, entry. But if this is taken too far the bow locks in, and begins to over-steer when heading downhill. Aside from major comfort issues, at some point this becomes dangerous, forcing you to slow down to avoid an out of control broach. Slowing down means more motion as the swells sweep under the hull. And at some point, you can no longer run and must turn and face into the waves. In many heavy weather scenarios this is exactly the wrong tactic for avoiding the storm center.
The bow shape of the FPB 97 is undoubtedly knife-like in plan (looking down) and body section (looking bow on). That it will knife through seas does not take a lot of imagination to see. But it is also very shallow, and coupled with two enormous rudders, is easily steered. Locking in is not going to be a problem.
You are used to seeing lots of reserve buoyancy in bows, in the form of flare. We believe it is better to build that reserve in with waterline length. This makes for a much smother, faster ride, and is safer in heavy weather.
There is flare of course, but this is relatively minor.
What happens forward has to be balanced by what happens aft. Buoyant bows require fat sterns. If you have a fine bow, then a svelte stern naturally goes with it. As the wave passes down the hull it has less impact on a narrow stern, so there is less force shoving the bow down into the oncoming wave by the stern.
If you watch our many videos of the FPB 64 and 83 at sea, you will note very little longitudinal movement. Now you know the secret.
Structure impacts displacement, interior, systems, tankage and therefore range. The first decision is factor of safety. With the FPBs, we have specified framing equal to twice the requirements of the already conservative Lloyd’s Special Service Rule (SSR). The forepeak area is spec’d at three times the rule, to help with pushing through ice. In addition, regardless of what the rule calls out, bottom plating is no less than 12mm (15/32″) and topsides 8mm (5/16″). In the bow the 12mm bottom plate, normally 300mm/1 foot above the waterline, is 600mm/two feet up. Topside plate is 10mm in the forepeak area. In addition, there is a 25mm/one inch doubler plate 400mm wide (16″) down the center line from the stembar forward to the aft watertight bulkhead. This is a structure designed for a tough life.
Tanks are of course integral. Tank tops are within 50mm/two inches of the waterline. This means a breach of the hull into the interior would accumulate very little water. The cabin soles sit at the specified full load waterline.
Having such high tank tops also creates a substantial volume to be allocated between fuel, fresh water, ballast, and cooling tanks.
How does this manifest itself in the structure? Let’s take a minute and look at the forepeak. This forward quarter of the hull is where the heaviest loads at sea occur, and where the odds are highest of having to absorb impact from logs, containers, ice, or other floating objects. We’ve detailed the basic approach used in the FPB 64 here. For the FPB 97, the approach to bow impact is similar, just scaled up.
There is a huge stembar, reinforced with horizontal breast hooks. The first frame aft forms a collision bulkhead. Note the self draining/self cleaning chain locker in the next section aft.
The forepeak sole shown here is up a foot/30mm from the waterline, with the structure between frames another foot higher. This shortens the topside frame span. As mentioned before, to a height two feet/60cm above the waterline there is 12mm plate and solid structure (called floors) across every frame. This is not for normal collisions, groundings, or even floating containers. We are thinking here about sea ice.
Drive Line Geometry and Efficiency
The overall propulsion efficiency, how good a job the drive line does turning the energy provided by the prop into forward motion, is a very complex subject, not fully understood even today. However, we’ve learned from experience what works and what doesn’t with FPB style hulls.
The key ingredients are:
- Prop shaft to hull buttock angle
- Propeller tip clearance from the canoe body
- Propeller diameter and reduction gear ratio
- Disturbance ahead of the prop by shafting support and protection structure
- Water flow in the after sections of the hull, including boundary layer and propeller wake friction
There are many tradeoffs between these factors, and every time you vary one element it affects everything else. A key facet is access to running gear (flanges, CV axles, thrust bearings, and shaft seals), cleaning around the engine pan, and natural drainage from propeller shaft seals to canoe body center.
The shape of the canoe body in the aft section, how it works with the volume of the skegs, and the flow along the hull and into the props is the most complex part of the design package. There is some science of course, but also a lot of black art, and gut instinct. In the end, one looks at the computer numbers, chats with consultants, and rolls the dice. In our game, at least, the dice are rigged. We know what works on Wind Horse and the FPB 64, and we can infer what to expect in the new FPB 97, with efficiency increases, from a flatter shaft to buttock line angle, held in reserve as a safety factor.
Which brings us around to tankage. Fuel tanks are clustered near the center of buoyancy, to minimize impact on trim with change in loading. Fresh water tanks are located either side of the fuel array. Between fuel and fresh water there is an empty space called a coffer dam. Finally, there are two large diesel tanks in the engine room. These provide a simple way to tune longitudinal trim, as well as compensate for heel with or without the dinghy on deck.
You start out with a gross volume, then deduct for structure, the coffer dams, stabilizer compartments, and throw in a fudge factor. We end up with a nominal 5500 US gallons/20,800 liters for fuel, and 2500 gallons/9400 liters for fresh water. These are preliminary, of course, but based on experience we would expect this to be pretty close.
You may be wondering: why so much water if we have a watermaker? Several reasons. First, it is a backup in case the watermaker fails. Second, if we are light on fuel we can ballast the boat with fresh water for comfort in offshore scenarios. Third, by running the watermaker for a day or two prior to making port, we can arrive with full tanks so the watermaker does not have to be run at anchor.
A note on filling tanks is probably in order. Theoretical volume and what actually can be pumped into or out of a tank is subject to several factors. With fuel, it is critical that both filling and breather systems are done correctly, or you end up with a foaming mess and lose 15/20% of capacity. Our standard practice with fills and breathers has proven effective at minimizing the foaming diesel problem.
Range Under Power
Range is based on need, desired flexibility in fuel purchases, and supply disruption insurance. We look at 5,000 to 6,000 nautical miles as a magic number.
That gets you across the really long hauls – Auckland to Puerto Monte, Chile for example – with reserve, and without the use of always dangerous drums or bladders. And it provides enough fuel to avoid having your own plans disrupted if there is a fuel supply problem due to politics.
Range includes a host of variables that are hard to precisely predict. These include sea state, windage, bottom and prop condition, vessel loading of course, and how much air conditioning is being used.
We know that on average the FPB 83 Wind Horse uses .7 US gallons/2.6 liters of fuel per hour for auxiliary loads on passage. This covers AC, DC, and hydraulic needs. For the FPB 97 we are assuming this will increase 50% to one gallon/3.8 liters per hour. The rest of the hourly burn is for propulsion.
On the assumption that the boat is getting lighter as it progresses, there is a theoretical smooth water range of 6000 nautical miles at 11.7 knots. Drop back to 10.5 knots, as slow as you would ever want to go, and range increases another thousand miles. If you are cruising without the air conditioning, add another six to percent eight percent range to these figures.
Downwind mileage will go up five to ten percent depending on wind and seas. Uphill, just the opposite, with 15% being a good average deduction.
When we talk about the flexibility the range confers, think about the trip depicted above. Let’s say you are in the South Pacific, maybe in New Zealand after sea trials. You have an easy four day trip up to Fiji, and begin to enjoy the comraderie with other cruisers at Malololailai. You are having drinks at Dick’s Place talking about dreams. Somebody mentions bears, then the clam digging grizzlies of Geographic Harbor on Alaksa’s Kenai Penninsula. Suddenly you realize you have dreamed since you were a kid of visiting Kodiac Island and the Alaskan Panhandle.
It is May, the cruising season is just beginning here. What would it take to get to Kodiac? Turn on the iPhone, look up Google Earth, and check the mileage. If you top off the tanks in Samoa, directly on the path, it is less than 5000 miles to Kodiac. Halfway there, if you need a pit stop, sits Hawaii. But the trip up to Hawaii is easy (being early winter in the S. Hemisphere the trades are not yet blowing strongly). By backing off to 11.2 knots you have a 25% reserve for the leg to Kodiac without refueling. You have only been on passage for nine pleasant days, and are well-rested and enjoying the sea-going routine.
When you call Rick Shema, your weather router, he tells you there is a blocking high strengthening over the North Pacific. This occasional early summer phenomenon is indicative of a really easy passage. Stop, kill a week at least, and the weather is not going to be as nice. You have the option of continuing on. If Rick is right, and he usually is, 19 days after leaving Samaoa, 5000 miles south, you are sitting in Kodiac.
And the odds are you won’t be in a hurry to get off the boat. For some real world data on long distance fuel burn crossing the Atlantic click here.
Part two follows.