To A New Paradigm With FPB

FPB 97 – The Foundation On Which Successful Cruising Is Built: Part One

FPB 97 61 Hull shape 106

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.

 

 

 

FPB 97 61 Hull shape 104

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
  • Draft
  • Hull shape
  • Structure
  • Drive line geometry and propulsion efficiency
  • Tankage
  • 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

Cruising Speed

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

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.

FPB 97 61 Hull shape 100 2

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.

FPB 97 61 Hull shape 101

Hull Shape

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.

FPB 97 61 Hull shape 100

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.

FPB 97 61 Hull shape 102

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.

FPB 97 61 Hull shape 105

There is flare of course, but this is relatively minor.

FPB 97 61 Hull shape 103

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

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.

FPB 97 forepeak 1

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.

FPB 97 Forepeak structure 200

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.

FPB 97 61 Buttock shaft angles 100

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.

Tankage

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.

Samoa Kodiac

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.

Kodiac Pssg 107

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.

Secluded 2 066 Edit

Part two follows.


Posted by Steve Dashew  (February 15, 2012)




29 Responses to “FPB 97 – The Foundation On Which Successful Cruising Is Built: Part One”

  1. CJ Says:
    Brilliant as always. Only one nitpick though, as someone who used to live in Alaska it’s ‘Kodiak.’ Looking forward to the interiors…..

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  2. Carl Nostrand Says:
    Wicked….. What a fine design. I long for a Wicked ride….with a running tide. ITs niceto see you have Alaska on your mind with this vessel. Steve, you must come to Homer and explore our beautiful coast line. Homer would be a good location to build Wicked vessels of your design. Lots of talented welders, sparkies, and craftsman and women. We just need to start off on the right foot building the Wicked designs. Maybe, your NZ builders will want to set up shop in our country. These vessels have to be built in USA to prove there earning potential. Private yachts are very nice, but a working vessel working the coastline making a profit proves the design……. Maybe I’m just a industrealist at heart, with a romantic nature wanting a job at sea…… Can you blame me for the desire to work on a Wicked vessel!! Kind Regards, Love your vision, Carl

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    Wolf Reply:

    Just a suggestion from abroad to instantly improve the situation of your local marine industry in Alaska: Get rid of your strange pilot requirements for non-US flagged pleasure craft larger than 65ft loa, and many more yachts will come your way. Today many of these yachts prefer not to continue north, but stay south of the border in BC in a much more cruiser-friendly set of regulations.

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    Steve Dashew Reply:

    Could not agree more. The rules are there to feather the nest of the pilots association, and is costing the State and the marine industry a lot.

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    Carl Nostrand Reply:

    Thanks for the heads up about our pilot requirements for 65 foot vessels and over. I was not aware of such behavior. Can I ask what the charge is? Is this a one time fee once you enter US ports, or do you get hit every time you enter a new port? There are many contentious issues with our maritime polices in the USA. I will talk to our political leaders in our state and try too get the ball rolling for change in this unjust issue.

    Steve Dashew Reply:

    You would have to check current, but for Wind Horse it was several thousand dollars. For details on our “welcome” see http://setsail.com/s_logs-dashew-dashew397/

  3. Michael Golden Says:
    I’ve been following this blog since Wind Horse was in the design stage. It’s been nothing short of an excellent on line course in boat design. I’ve learned a lot. Thank you. Are you planning to build Wicked and replace Wind Horse?

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    Steve Dashew Reply:

    Hi Michel: Right now we are too busy to even think about changing horses.

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  4. Johan Says:
    Steve, An extra (important) point of efficiency in your hull lines, as I see, is beside the sleek stern, the not immerged transom. As an immerged transom has an important impackt on resistant.

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    Steve Dashew Reply:

    The amount of immersion depends on the speed length ratio for which you optimize.

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  5. JKH Says:
    As much as I love the Wicked one (and the 64, 83 and 115), please move on and design a 40-something that I can actually afford… PLEASE! :-)

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  6. jorgen johnsson Says:
    Steve, I am a longtime fan of yours, visited some years ago Windhorse in CA. Have you considered using a KITE, see http://www.skysails.com and diesel/electric drives i.e. pods like VAN DER VELDEN Marine System EPS silent thrusters Thanks for great things \Jörgen

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    Steve Dashew Reply:

    Kites have potential. The issue is launching, retrieval, and control. We fully expect this to be a viable option some day.

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  7. Mark Says:
    I TOTALLY agree with JKH….. A nice little 48 to 52 foot FPB that 2 can easily live aboard. I know it won’t be as fast as the 64 but more people can get into it. I would be one of the first in line….. Mark

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  8. Alain M Says:
    Hi Steve, Nice clean hull! Really like it! Thinking about Ice, tankage and range, means thinking about right insulation too… As a good insulation brings energy saving in hot as in cold climate, it also adds some time and energy range. More long time off the grid capability… What about insulation? But I think you will come with this in the time of the great room and below deck arrangement. Waiting for the next!!! Regards Alain PS: For those who want a shorter version, in range 40 to 50 feet, more short you go, more you have to write off capabilities in all aspect of range and sea going comfort. I think the 64 is near the limit to keep the whole concept. Going more short in LOA and WL will make you lose most part of the concept as the WL is one important part to hull shape and load capability. You change one characteristic, you will impact all other! Owners of the 64 said it is a hell of a boat, I agree, that’s because it is a hell of a good balance between the different characteristics… But I will be happy too if Steve can prove I am wrong, I wish I am wrong, but….

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    Pat Reply:

    I do think the concept can be scaled down to 50 feet or even less with a catamaran. Cruising speeds and fuel consumption could be comparable to larger FPB’s. Only fuel tankage (due to weight restrictions) will be reduced, so less range as a consequence. But as a coastal cruiser it could be awesome. Of couse, a self-righting catamaran is impossible (so ultimate seaworthiness is compromised), but when turned upside down it is virtually unsinkable. A catamaran makes a excellent platform for a ‘great room’, ‘matrix deck’ and even dinghy stowage between the hulls. One can extend the range by adding a sailing rig, and have the ultimate motorsailer. Pat

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    Steve Dashew Reply:

    Sorry Pat, but there is a basic flaw in the cat logic. To be efficient they have to be light. And cruising cats are almost always heavy. And from a cost perspective, you get a lot more monohull waterline for your budget. At t he end of the day, the cat might make sense if you are willing to pay a premium for the boat and dockage, and then put up with a quick motion. But it is not a panacea.

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    Vincent Cate Reply:

    A catamaran is a way to combat roll compared to a simple monohull design. But the FBP gets much better roll stability than any cat using stabilizers. Stabilizers add much less to the cost than switching to a catamaran design. So FBP really does seem the better approach.

  9. Henry Says:
    Steve, I was interested in your comments about large bow depth at the stem and broaching risks while hard running/surfing. Perhaps there is a case for stern ballast tanks which could be filled when hard running/surfing so as to lift or partially lift the bow out of the water. Make any sense?

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    Steve Dashew Reply:

    Depends on boat and conditions. Wind Horse is set up for this, but it has never been necessary.

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  10. Don Dahl Says:
    Hi Steve, Having read every entry, I believe that this is one of your best posts to date about the FPB “process”. What a pleasure it is to see the design of a new product being based on the real life experiences of the designer. Also, not holding my breath, but have to agree with JKH above…a 40-something would be wonderful for near coastal cruising. There is always something magical about owning the absolute best, and an FPB of any size certainly qualifies. An exclusive “club”, to be sure, but one that many folks would love to join. Thank you for writng about your journey. We are along for the ride as long as you continue to share. Don and Cindy

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    Steve Dashew Reply:

    Thanks Don (and others) for the kind words: I should remind all that this is a team effort. Shortly we’ll get into this in more detail.

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  11. Vincent Cate Says:
    With solar power, running a watermaker at anchor is not really a problem, is it?

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    Steve Dashew Reply:

    No problem at all.

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  12. David Sutton Says:
    Hello Steve, It is wonderful that you bring us all along on your journeys, be it cruising or yacht design. I know you’re fond of aft engine rooms in yachts, and thus have had to use V-drives to get a reasonable shaft angle. I was wondering if you have ever considers pod drives such as the ones made by ZF. I hope you are able put your mouse aside for a bit and get back to boat soon. Cheers, David

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    Steve Dashew Reply:

    Azimuth drives and pods all have issues, David, for our usage. The drives we are using are more reliable, cost efficient, with better prop efficiency, than anything else we have seen for our application.

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  13. John Ozechowski Says:
    I figure this is as good a place to ask as any. In my dreams I’d like to do the great loop. I know these boats (the FPB64 will) will not be able to complete the loop due to that one bridge in Chicago. How would the 97 do along the rest of the inland waterways (tenn-tom, Mississippi, Missouri etc). I figure it would probably be fine on the intra-coastal.

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    Steve Dashew Reply:

    The ICW is not a problem. The Great Loop I can’t comment on as have never researched it. Note that the FPB 64 mast folds down.

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  14. Carl Nostrand Says:
    OK, i’m a little bit guilty of showing off this Wicked design at the local coffee shop that I ski down too most days from my house in Homer Alaska. Well, we have are share of experts in boat design as you can imagian in a commericial fishing town in Alaska. The Wicked designs brings up some raw emotions I must say. We have many proud boat owners that don’t take kindly to a new and progressive design which they know nothing about. Usually they say its uglee right off the bat. Then they laph at all the glass and call it a poor design for safty reasons. I try and tell them that she is not going to be winter fishing in the Bearing Sea……. After a while some of them get it, and come around to the unbelieveable economy and much higher earning capasity used in passinger operations. We don’t have a working vessels of this size running folks around Cook Innlet. There are no efficent vessel going from Homer to Anchorage, Seward, Kodiak, or ports in Prince William Sound. I’m thinking that a vessel like the Wicked could be designed to carry 20-30 passangers or ocean traveling students with out compromizing the comfort for all. I for one would surely love to see the Wicked designed for adventureous passengers looking to experance something out of the usual ferry voage. I hope one day to ski down to the K-Bay Coffee shop and over here people talking about what a wicked ride they had on a Wicked Vessel. WU li wa

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