Ed Firth, one of Circa’s engineers, has been with us in Arizona the past week, refining the preliminary FPB 112 design. We thought a few comments on the process might be of interest.
As we have discussed before, capsize resistance and recovery are critical components of yachts intended for offshore work. This is a function of the center of the gravity and bouyancy as the hull rolls through various heel angles. So our first step with Ed is to complete a detailed weight analysis to confirm displacement and VCG. There are inumerable items to look at in terms of weight and position. The spread sheet, a small sample of which is above, has thousands of cells of data. The earlier work was done with educated guesses, but at this point in the cycle we need confirmation of the numbers.
With displacement and CG in hand, we do roll over simulations in different load conditions, varying liquids, and looking at lead ballast options (including no ballast). The image below shows the boat at three quarter load, heeled 45 degrees.
Note how she floats with the deck edge barely awash. Even at this heel angle Dorade vents, hatches, and doors are still above the static waterline. As we have detailed before, the FPBs are designed to skid easily, which helps to absorb wave impact minimizing extreme heel risks in the first place. There is little to impede sideways motion in in this attitude.
At 90 degrees the Dorade vents on the coamings are wet, but of course they have closure plates to secure them, which are operable from inside the boat.
And 120 degrees. Note that most of the deck is still dry. These images and their accompanying stability curve help us fine tune the heeled volumes, and define what is and is not practical within the design envelope.
With the basics now defined the hull design cycle is restarted to see what can be done to improve sea keeping and efficiency. SInce the vertical center of gravity is a bit lower than originally estimated – a bonus we are happy to take – freeboard can be increased if desired. Adding to the topsides means more weight up high, so amendments are made to the spreadsheet, and new models with higher freeboard/VCG are tested for ultimate stability.
The stability curve above is a worst case scenario, half tanks and no lead ballast. The data on the left has been omitted – there is a limit to what we will share. Data across the bottom represents heel angle in degrees. Note there is no stability indicated below the horizontal line. This indicates that the FPB 112 in this trim has positive stability through 180 degrees of heel.
Here is an updated (to June 20 2011) look at the direction we are heading. Freeboard (topside height) has been raised 12 inches/300mm. The coamings and window mullions are up a slightly smaller amount. From which we derive the following benefits:
- Additional tankage (now 40,000+ liters/10,500 US gallons).
- Extra interior width from raising the cabin sole and refining the hull shape.
- Higher factors of safety in regard to damage and water containment.
- Full headroom in the basement, making some of the basement usable for living quarters (perhaps with expansion of the owner’s suite or another cabin) if required.
In order to accurately evaluate these changes the weight budget has to be modified again. Decks, systems, and interior have their VCGs raised. Topside weight increases. The fact that VCG is up can be mitigated by adding lead or water ballast, but these increase weight and drag. So we need to look at the roll over simulations again, and then check the drag and horsepower numbers.
New hull shape and displacement data is keyed into VPP programs. A recent drag curve is above. Note the shape (data across the bottom is speed in knots). From 12.5 knots and on the increase in drag with speed is linear, not geometric, as you are used to seeing. We are still within our original allowances on fuel burn at various speeds, and this includes an assumption of lead in the weight budget. In other words, we are in good shape with the higher freeboard and revised hull shape.
Numbers and simulations are one thing. Gut instinct, based on experience, is another. When it comes to evaluating how this combination of design factors works at sea the computer is at a disadvantage. That is where the offshore miles come into play. Just as sailing designs like Sundeer and Beowulf lead us to the FPB prototype WInd Horse, WInd Horse and the the FPB 64s have given us the background to optimize the FPB 112.
Speaking of which, one of the goals for the FPB 112 was to make possible the use of tidal grids for hull maintenance, and to be able t o dry out on a smooth beach or mud flat. This requires the stabilizer fins, when centered, be at or above the bottom of the hull. We have this feature on the FPB 64s, but were frankly dubious about making it work with the longer hull.
However, we are happy to report that the current configuration is indeed grid/dry out friendly, an advantage we suspect is unique in this size of yacht.
We will let all of this sit for a few days, allow excitement about to cool, and then have another look. If we still like the results, working drawings will commence.
Posted by Steve Dashew (March 13, 2011)
March 13th, 2011 at 11:30 pm
Steve,
Thanks for sharing these ‘behind the scenes’ processes.
I find these updates incredibly interesting. It’s nice to live the process along with you.
If you have the time, please keep ’em comming! 🙂
Cheers,
Anthony
March 14th, 2011 at 8:51 am
Thanks Anthony:
Explaining the process helps us clarify our thinking, and often elicits suggestions or questions which help the design in question.
March 13th, 2011 at 11:53 pm
“The stability curve above is a worst case scenario, half tanks and no lead ballast. Note there is no area below the line. ”
I don’t understand this statement. There appears to be lots of area under the line on the stability curve graph. What am I missing?
Thanks
John
March 14th, 2011 at 9:07 am
Hi John:
We should have explained what a stability curve shows, which we will do in detail another time. For now, let us refer you back to a recent post on stability (click here). The first stability curve is of a sailboat. You will see that the curved line representing stability crosses the horizontal line at the bottom of the graph at 130 degrees. This is the point where all upright stability, or the LPS (limit of positive stability) occurs. The Area below the horizontal line represents negative stability, and will tend to keep the boat inverted. This requires wave energy to flip the boat back to upright.
The simulations we show for the FPB 112 indicate positive stability at all angles of heel. No wave energy is required to insure return to upright.
With regard to the comments on loading, anything done to lower the vertical center of gravity, filling low tanks, or adding lead ballast, enhances the range of positive stability. Conversely, raising the VCG with gear stowed high, or light tanks, reduces positive stability and adds to negative. With the simulation presented we had a worst case situation of no ballast and light tanks, and still have a very good stability curve.
March 14th, 2011 at 12:17 pm
Very interesting posting. I know a lot of people who are quite interested in the concept and are patiently awaiting being able to “participate” in the process; so keep the info coming. By the way- I sent the URL to Ray Richards (NA) who promptly fired back back that if I ever: “wondered what Stan looked like- there he is!” (Thought you’d enjoy that comment!)
March 14th, 2011 at 3:16 pm
Hi Steve,
New tankage would make a nonstop circumnavigation a possibility, yes?
March 14th, 2011 at 10:49 pm
Interesting thought, Max:
If the ballast tanks were used for diesel, the engines were detuned to M1 rating (CPU adjustment these days), props adjusted to M1, and you ran at ten knots, range would be about 25,000 miles. So, a non stop circumnavigation is theoretically possible. But the crew would miss some nice anchorages along the way.
March 15th, 2011 at 10:08 am
Hello.
Someone was questioning the use of cowls on the 64 and you said words to the effect that there is an effective alternative to cowl vents now. What did you mean? I’m not keen on Nicro Fico type vents because they don’t include dorading that is effective regarding green water.
Thanks in anticipation and for a wonderfully interesting site.
Bob N.
March 16th, 2011 at 3:59 pm
Hi Bob:
The alternate at sea is air conditioning. The alternator/battery/inverter system allows the air conditioning system to function without running a genset when passaging. At anchor, 99% of the time the hatches do a great job, at least that has been our experience with Wind Horse. Under sail, without a readily available source of electrical power for the air con, Dorade vents are the answer.
March 17th, 2011 at 4:34 pm
I say “No lead ballast.” If you need more wt down low, add battery capacity.
March 17th, 2011 at 11:19 pm
It many scenarios we’d trade lead for batteries, Daryl:
However, the lead has a VCG on the order of four feet/1.2m lower than the batteries, so in this case it does not compute.
March 29th, 2011 at 12:46 am
Steve, I have been a fan early on when the FPB 83 was first introduced to me by Kelly Archer. It has captivated my attention for many weeks at a time. From day one I always thought that at least a twenty foot stretch would be ideal. Just when I got used to the sixty four being my only option your design gets better still. I am happy to see the FPB 112 development, its awesome. Your latest top deck configuration is much better too. Would it be possible to extend the range of travel and be performance efficient at sea to utilize two Mercedes Benz generators with two large variable speed electric motors as drive units. In general how many dollars must I have to claim my own FPB 112. Best regards, James
March 29th, 2011 at 12:59 am
Howdy James:
Re diesel electric, we have yet to see any systems in our size with appropriate history for an ocean girdling yacht. These boats are so efficient and have such enormous range that even if there were proven gear the extra cost would not pencil out. Re pricing, we should have this in another few weeks.