Have you ever regarded something and heard a little voice say, "It looks okay but is just wrong"? Our bow thruster is a 15kW unit from Sidepower. It's almost twice the size of the one on our sister yacht, Mobius, but it's ineffective for some reason.
Bow Thruster Grids
When we took over the completion of Vanguard in Antalya in April, the delivery Captain commented that the bow thruster was not big enough. I did not get along with this chap, so I ignored his comments; perhaps my bad. Later this year, Valeriy, our present Captain (and much more likable), delivered the same opinion. The power consumption was about 15kW as we measured the current draw, so what was the issue? Then, as Vanguard became reliable, I had time on the helm, freed of "fixit" duty, and experienced firsthand how miserable this unit was. Â
As luck would have it, we are visiting family in the Ozark mountains of northern Arkansas for Christmas. Not much is going on, so it's time for some reflection. I studied a video taken of a failed bow thruster blade. Naval Yachts, our builder, had fitted a well-made grid for the bow thruster. This should prevent damage from floating debris. In our yard in Fort Lauderdale, there are many large yachts on the hard. None have grids fitted. Hmm?
We used a spreadsheet to calculate the flow restriction presented by this grid, approximating 10mm aluminum for the flat bars. The total flow restriction was 25% of the full 250mm tunnel. Plus, the flow over both screens creates unwanted, energy-sapping turbulence. Being slightly inebriated, Christmas and all, I asked Chat GPT for the cumulate effect of two orifices in the flow stream. Each one reduces the flow area by 25%. This was the answer:
"When two orifices (or flow restrictions) are placed in series, each reducing the flow area by 25%, the cumulative effect is not simply additive; it depends on how the flow behavior (e.g., velocity, pressure drop) changes across both restrictions. Here's the step-by-step explanation:
Effective Area After Both Orifices: Let the original area be AA.
After the first orifice: A1=0.75×AA1​=0.75×A.
After the second orifice: A2=0.75×A1=0.75×0.75×A=0.5625×A
The effective area after both orifices is 56.25% of the original area.
Impact on Flow:
Flow restriction depends on the velocity and pressure drop, governed by the continuity equation and Bernoulli's principle.
Approximate Flow Restriction:
For incompressible flow, the cumulative reduction in flow rate will be proportional to the decrease in the effective area, resulting in ∼43.75% less flow capacity than the unrestricted condition (plus turbulence). For a detailed analysis, factors like Reynolds number and turbulence at each restriction must be considered."
So, having now returned to our yard in Fort Lauderdale, we have requested a diver to remove these guards (it's been snowing just up the coast, not aself-completee job in a pair of shorts!) Hopefully, we will be able to report back in the next blog
Other Fix-It jobs.
We were also able to complete two more small tasks on our first day back.
The Isotemp water heater was damaged when a pipe union failed on the watermaker just above this unit. The system was repaired, but we could never replace the plastic cover. Eventually, we found one in the UK using a Google Image search. It took 10 minutes to fit and now looks as good as new.
Similarly, the Airmar depth transducer on our tender lost a fight with the tender chocks some time back. It's a nice little unit, and we "Jury-Rigged" it until a better fix came around. Rytech Marine made a super strong Stainless Steel replacement bracket for this transducer, and we brought one out for that purpose. Now, it's all ship-shape again and much less damage-prone.
We have two weeks in Fort Lauderdale before venturing to the Bahamas and Abaco Islands, where we will meet up with family in Hope Town, Marsh Harbour, or similar places rapidly warming as the season begins.
Time in FL will be spent on final commissioning for the Praxis equipment, repairing some wear and tear from previous ventures, and improving what we still see as sub-par, though that list is now much reduced. 2025 should be a good year, and we are continuing a conversation with Eyos Expeditions (my long-time friend Magnus Day) about the Western Coast of Greenland.
Nothing is decided yet, but it's on the radar of possibilities. Time and tide wait for none of us in this life.
Happy New Year to all.
Chris