Square peg, round hole. Round peg, square hole.
by John Curnow, Global Editor, Powerboat.World 23 Sep 2021 07:00 UTC
The 86 footer, AmnesiaIV - from Vancouver to San Diego in 7.5 days, averaging 9.6 knots over the ground, all at 3l/nm, including generator time!!! Now that's efficiency © Pat Bray
If you think about fluid dynamics for just a second, one thing is pretty clear. Smooth and flowing wins the day. Be it aerodynamics, and how the little probe goes out in front of the car to show the way the smoke moves around the car in the wind tunnel, or hydrodynamics where charts plot the waves heights, and tank testing shows the results in real time.
It could even be more low tech, like the late and great Kiwi yacht designer, Jim Young, who used to tow freshly caught fish along in the shallows. His aim was to study the results over their bodies, and more importantly their appendages, like fins and tails, for they resembled keels and rudders.
Now when I got a note that said we have empirical proof that our hard-chined hull did better than a round bilge in a full suite of tests, well the first thing that came to my mind was the very heading above. Next I had to evaluate in my mind the thought that a brick with a nose cone had done better in a tank than a whale, whose only straight edges exist on whatever form of teeth it has, and then the leading edges of fins and tails (and there's that appendage word again...)
Here's the excerpt from the CFD report 2017 - Mavi Innovations Inc., Vancouver, B.C., Canada. "The CFD results showed a marginal increase in resistance for the round bilge hull, along with a change in trim angle. This was not the anticipated result, as round hulls typically outperform hard chine hulls."
"The important conclusion to draw from this analysis is that the chines on the vessel are shown to be well designed. The added cost of fabricating this hull form with round bilges is therefore not warranted, as it would not lead to substantial reductions in fuel consumption."
As it says, add in that a hard-chined vessel with its square corners is both easier and cheaper to build to the better fuel economy and range from the tests, and you had to wonder about why was this coming about now. How could this be? It literally flies in the face of the norm. When additional stability is also on offer, as well as easier placement of machinery and tankage, it not just piqued one's curiosity it demanded close attention.
This all came about after an earlier editorial on bulbous bows, called It ain't necessarily so. Once again, Canadian Naval Architect Pat Bray has offered some illumination over the matter, and proffered this analogous situation as a way to begin to contemplate it all.
"Take the progression of the internal combustion engine, from the first inefficient version to its modern-day efficiency. One of the first engines (1890) was a 1.1 litre motor capable of 4hp @ 900 rpm (Daimler/Maybach). Today that same size production motor with today's efficient carburettors, turbochargers, header exhaust, fuel injection, and careful intake and exhaust porting can develop up to 100 bhp @ 6000 rpm, whilst still being lighter and more fuel efficient than the original model."
Still not a believer, then just look at the production Toyota Yaris GR which will make 268hp (200kW) from just 1.6 litres!
Stay with us now
Bray added, "The basic principles of combustion and even the general design of these motors have not changed greatly from the initial concept, but fine-tuning the design has brought huge gains. Much of this has been brought about by bolting on better carburettors, tuned air intake and exhaust systems, etc. In that same spirit, the principles of a good, basic hull design were evaluated and then means reviewed, so as to increase efficiency by utilising enhanced 'bolt-on' appendages."
This all means it is a case of refinement, refinement, and refinement, rather than the light bulb paradigm moment. We also have to thank the Canadian Government at this point, for in the 80s they funded all the modelling and testing that delivered these results, as they aimed to achieve better outcomes for their vast fishing fleet in the face of rampaging fuel costs. It was all open source material, and the era of the slippery hull form and bulbous bow had dawned.
All of the work was done in Vancouver, at speeds of up to 24 knots in a smaller type tank, for the larger ones get under the auspices of the military PDQ. Now when the facility at Vancouver was closed, the Dean of Naval Architecture at the University of British Columbia just happened to be Turkish, and knew of a facility in Turkey that was small enough to handle the project, and offered faster speeds.
Bray's Bulb#2, as it was known, was created in 2012 and tested across the same, efficient 45m displacement hull to provide continuity. Assoc. Prof. Emin Korkut, Superintendent of Ata Nutku Ship Model Testing Laboratory, Technical University Faculty of Naval Architecture and Ocean Engineering, Istanbul, Turkey, stated, "Model testing and CFD - OS 45m (Enigma). - I should tell you that your form has been one of the best forms we have tested recently in our towing tank."
Thereafter, the research moved back to Canada and crossed over to Computational Fluid Dynamics (CFD) as the march of computational power meant more reliability and faster turnarounds. Again, the support of the Canadian Government through the National Research Council of Canada, and the Industrial Research Assistance Program (NRC IRAP), showed that appendages were the answer.
Here's the deal
The mid-ship bulb had been under consideration since 2006, and they had done a run at one speed only, and found it offered a 6% reduction in power. There was no more budget to pursue this at the time, nor take on the stern bulb the Japanese had discovered much earlier, but for which there had not been much appetite given fuel was comparatively cheap at the time.
Bray added, "This is how I met Gabor Karafiath. He researched stern bulbs, as he'd worked at Cardrock Annapolis Naval Facility. He now works for DARPA - the Defense Advanced Research Projects Agency. The thing is that you start with a good basic hull, place all three bulbs on one vessel - and it works - it's actually cumulative. Being 40% more fuel efficient is on the cards because you require a smaller engine and carry less fuel to feed it, all for the same speed and often more range."
"Interestingly, at 12 knots there is a 25% improvement, where as at 16 knots it down to 16%, but by the time we are back up at 20 knots it is 20%. The bow is creating a wave, with the pressure overlapping or out of sync, and thereby flattening the whole wave train."
Additional benefits are a reduction in pitch by 30%; the bow can work well in new ice for Explorer class vessels; you have a quieter acoustic pattern for all on board and undersea; and the smaller engines need smaller tanks, create less noise and pollutions, and deliver more room for the all important accommodations.
Bray also added, "When looking at the vessel afloat, the chine is almost entirely below the waterline, unseen to enquiring eyes. Use the glitz and the glamorous styling for the topsides, and go with the efficient, cost-effective form of the chined hull below the water, thereby enjoying its many benefits."
So the extensive research and testing have proven that as long as the chines follow the flow of the water (flow lines), then there is no powering penalty over the traditional round bilge shape. "The amount of stern immersion is far more critical in determining performance, so that must be kept in mind when designing the hull", said Bray in conclusion.
It gets even cooler from here, for soon all the material from the extensive, years long research will get revealed. Stay tuned... it's going to be good!
Congratulations!
You've made it this far... So as a reward, this videos will help you see for real some of the elements we have just been banging on about. the rest we'll all have to wait for. Enjoy -
High speed in flat water to taking a pounding
The rise of the Day Boat
Jeanneau's new DB/43 looks pretty good, and is certainly part of the burgeoning category, of simpler, easier to run boating that is appealing to many who don't see how a Convertible/Sportsfisherman/Flybridge craft applies to them and their lifestyle. Outboard Express and Inboard Efficiency are on offer to go with her modern and clean lines.
Not entirely sure when a dealer can take an order from you, and there is not any more information than that already released, so it looks like we're all on the hook together for now...
Off we go!
The Amels/Damen Yachting SeaXplorer 105 with its twin helo hanger certainly was one to ponder over extensively. Recently, the new SeaXplorer 60 was announced and continued the theme with hybrid propulsion, a matching limousine tender, along with that clever for'ard observation lounge. She's under way now, with a target delivery in 2024. That leaves plenty of time to work out which souls will be on your 100-person guest list for the Christening Party that you'll stage on her helo deck.
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John Curnow
Global Editor, Powerboat.World