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Weekly Update - Understanding Our Longitudinal Stability.

Dunning Kruger - "When a person's lack of knowledge or skill in a certain area causes them to overestimate their competence." I once worked on a research project adjacent to Newcastle University School of Naval Architecture, home of Froude and Reynolds numbers, Parsons, Turbinia, and all that followed. That does not necessarily make me much of a Naval Architect.


So, we faced the question of stability for our floating investment of 3 years. Our fallback is twofold, "Show me the science" and "Who has existing knowledge in this field?" This learning stage is probably not for the faint of heart. These long, skinny, round bilge hulls don't behave like your typical "Down Easter." They may also look like a yacht hull, but there is no keel; our points of reference are few and far between.


So let's start with the "Show Me the Science."

Our Yard uses two hull design programs, Orca3D (same as some FPB designs), and Maxsurf. The issue was that the two reports we had did not agree with each other; the end result was consternation. Examination (by Naval) revealed one had set the baseline at the bottom of the skegs and one at the bottom of the canoe form body of the main underwater hull. As the skegs played little role in overall stability, this report was rejected. Also, we always expected to add an amount of permanent ballast and fluids to increase stability from the true as-launched "lightship" condition. Any fluid added to tanks will lower the CoG and reduce the roll period (see drawing below).



So, having established an agreed way forward, we need two pieces of factual data.

  1. Firstly, to undertake a preliminary "Inclining test" to establish a longitudinal Center of Gravity (CoG). An official inclining test will be undertaken later, under the watchful eye of our UK MCA Surveyor.

  2. Secondly, we will have a close approximation for the distance between the CoG and the metacentric height (GM) for this load condition.

Once we have that data, then revert to the CAD model of the underwater hull and superstructure, input the known conditions for GM and CoG, and then extrapolate stability data for various other load conditions, including Arrival, Departure, permanent ballast, and swinging the tender outboard in whichever load conditions is the least stable. Autnautica has also agreed to model this for us as an allied third party.


Who has prior knowledge?

For this, we turned to Eyos Expeditions, specifically my friend Magnus Day and, latterly, his buddy Steve Parsons. Magnus has spent long voyages on FPB 78, and Steve was the commissioning engineer/captain for many of the FPB Explorer Yacht hulls, continuing his involvement to the present day. Talking about a subject will gain some understanding, but more clarity will be needed to set targets. Our initial intent was to develop three stability conditions.

  1. Lightship

  2. Loaded/Arrival

  3. Loaded/Departure

Like FPB from Dashew Offshore, Artnautica designs use water ballast compensation. Too little, and she becomes tender; too much becomes stiff with an added fuel consumption penalty due to increased draft. So we studied the trim "envelope" with knowledge of a good starting point and some hard physical data on trim and draft for her present, unoptimized condition.


Lightship

No fluids, no lead ballast, Vanguard's launch condition. It is quite a scary place, like a balloon floating on water, but we also know it's unrepresentative of any "at sea" condition we may encounter.


Loaded - Arrival

This would be our condition after a long passage. Minimum fuel and sufficient water ballast compensation to maintain adequate stability and efficient trim. The input was arrival conditions from FPB 78 with an estimated 20% fuel reserve remaining in our center tank (we might push this to 10% to see what happens), and around 50% water ballast capacity. Vanguard trims naturally by the stern due to my overindulgence in propulsion machinery. To counter this, we will add permanent ballast to restore trim closer to an even keel and alleviate over-reliance on pressing the forward water tanks. We have yet to determine exactly where we will add the ballast as all in the forepeak may bring its own performance issues (Polar moment being proportional to distance^2).


Loaded - Departure


This will be the third condition we will examine. Full fuel tanks for maximum range, including 2-day tanks, four wing tanks, and our main central double-bottom tank. Marine diesel in each, 0.85 Relative Density. We will investigate the freshwater ballast requirement under these conditions to establish a good trim while minimizing our draft penalties. The calculations will include any permanent ballast, a full equipment compliment, and the tender.


Overview of preliminary loading conditions

Tank ID

Lightship

Arrival

Departure

FW Fore P

0%

??

??

FW Fore S

0%

??

??

FW Aft P

0%

??

??

FW Aft S

0%

??

??

GW Fore

0%

10%

10%

BW Fore

0%

10%

80%

Fuel Fore P

0%

0%

100%

Fuel Fore S

0%

0%

100%

Fuel Center

0%

50% (approx 20% total remaining)

100%

Fuel Aft P

0%

0%

100%

Fuel Aft S

0%

0%

100%

Fuel Day P

0%

100%

100%

Fuel Day S

0%

100%

100%

GW Aft

0%

10%

10%

BW Aft

0%

10%

80%

Glycol P

0%

100%

100%

Glycol S

0%

100%

100%

Ballast (Lead)

0

??

??

Note - the designs use water ballast to compensate for fuel usage. FPB 78 used approximately 50% water capacity for fuel bunkers between 14,000 and 4000l. The roll performance at the lower end of this was towards "tender".


The output from this work should be threefold:

  • We will know our extreme conditions and always be able to operate within a safe envelope of vessel and crew safety.

  • We will automatically obtain the data we need for our Stability Book. This is a commercial requirement for MCA Category (0) Certification, not strictly necessary for private operation, but we want it anyhow.

  • We will know what to expect as we launch the tender over the side of a rather slender hull and can take some precautionary measures as needed.

We hope to report back on the results of this work in the coming weeks. In the meantime, my thanks for their indulgence to:

Chris Leigh-Jones



I was talking today to my very patient wife, Sebrina. I'm old, I'm retired from house building, scared hands and old bug bites. I should perhaps be joining a few men's clubs and playing golf, but every morning seems to bring a new challenge, demanding use of my remaining grey matter. Adventures are best in the telling, and this has been quite the adventure. Close now, so close (and I can't play golf!).


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