Real benefits of hull/bridgedeck streamlining?

Discussion in 'Hydrodynamics and Aerodynamics' started by Autodafe, Aug 10, 2011.

  1. Autodafe
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    Autodafe Senior Member

    Many catamaran designers/manufacturers make much of the fact that their boats topsides and superstructure shape is carefully optimised (on a computer) to minimise air drag and improve windward performance.
    John Shuttleworth goes on at some length about the benefits of this on his "Multihull design considerations for seaworthiness" essay.

    However, after the careful shaping has been completed, lots of odd projections get stuck onto the outside of this "perfect shape". Winches, sheet tracks, toerails, stanchions, jacklines, hatch-trims etc...

    No doubt the serious racing designers take the aerodynamics of all this cruft into account when designing the deck layout, but I don't believe this is true of the average "cruising" design.

    So my question is, do all these deck parasites ruin the airflow around the streamlined hulls and bridgedeck?

    Obviously they do add their own drag, but is there a larger detrimental effect due to the turbulence downstream?

    Any thoughts appreciated.

    For those who only deal in specifics, I'm thinking about partial and full bridgedeck cruising catamarans in the 35'-45' range.
     
  2. BATAAN
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    BATAAN Senior Member

    Screw wind flow over the hull, make the rig big enough and none of that matters. What do you think a crewman's body does if you're worrying about winches? Big sails=satisfactory performance and they will make a sailor out of you.
     
  3. brian eiland
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    brian eiland Senior Member

    Deckhouse Drag

    On cruising multihulls, I've always thought that the drag created by the deckhouse, both in size and shape is a much bigger than all the smaller parasitic items. I prefer to streamline the front of deckhouse if possible. On larger vessels this is possible. On smaller vessels you might end up losing too much interior in the deckhouse.

    I've never seen any definitive figures, but I have to believe that 'vertical wall of windows' on the Lagoon boats is terrible for drag....but it sure makes a nice interior effect....and without some of the solar gain of the slanted windows.
     

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  4. Autodafe
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    Autodafe Senior Member

    That is the sort of comparison I was considering. Everyone points to the vertical windows and shakes their head, but no-one has actual numbers.

    On the pod cat I'm designing for myself I can get all the accommodation I want into a small brick-shape cabin, or I can streamline the cabin shape, theoretically reducing Cd from ~0.8 to ~0.35, but adding frontal area, skin area and weight in the process.
    It the theoretical drag coefficient improvement isn't realised on a real sailing boat then it could be a great leap backward.
     
  5. DCockey
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    DCockey Senior Member

    On bluff bodies such as automobiles the drag doesn't change appreciably with changes to the front IF the flow is fully attached. Relatively bluff front but with nice large radius curves gives the same drag as a "streamlined" front. The rear end is what matters.
     
  6. masalai
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    masalai masalai

    In a cruising boat why the streamlining? - Do you plan to sail at 150mph? or are you such a dedicated racer that you must sail at 99.99% ??? Crew perfection and sail trim need to be PERFECT first...

    Set up for utulity and useful comfort and your time on board will be lots better... I travel at 5+knots on one engine and 8+ flat out on twin 20hp saildrives... I can still surf on a good swell and on a recent trip of about 700 miles surfed at 19 knots :D :D :D , (and that is without a mast and sails, - yet to be fitted) ... just before anchoring up in the lee of South Percy Island, (Southern islands in the Whitsunday Islands, Central East Coast of Queensland)...
     

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  7. Autodafe
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    Autodafe Senior Member

    In general I agree Mas.

    However, in the specific I plan to go cruising in a boat with no engine, so I want to be able to sail to windward in a gale with only storm sails up.

    The smaller the sail area, the more significant the drag becomes...
     
  8. Autodafe
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    Autodafe Senior Member

    Thanks, good to know.

    Where designers are talking Cd of around 0.35 for a cabin structure I have to think that they are assuming flow remains attached over most of the leeward area, and so upstream turbulence could spoil the party.
    My impression is that only rounding the front is unlikely to produce Cd better than about 0.48.
     
  9. masalai
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    masalai masalai

    Autodafe,
    Where do you plan to cruise? - Many marinas will not be happy about your "no engine" rig if you seek to enter overnight, to resupply or whatever, ESPECIALLY, if you cannot get the Au$10-million-public-liability-&-third-party-insurance demanded before allowing you in...

    For cruising the Barrier Reef, Islands and other waterways, or periphery of the Coral Sea, to go engine-less is downright dangerous as there are tidal streams in this region of 2 to 4 knots and in some regions LOTS faster flows... When a storm warning is issued, you need to be able to be anchored up as soon as possible, or getting the heck out of the vicinity of the bad weather - FAST (under power if necessary)... I know my anchor/chain is good and held me comfortably in 40+knots and cross seas in the lee of South Percy Island...

    Far better you sort out the below waterline, ensure you can comfortably rest the boat on its bottom (vertically) and have a draft of around 700mm for gunk-holing and hiding from storms and cyclones up little mangrove creeks... Tie off to several mangroves, and be lots more secure than any anchor, As you can re-position your lines to allow for a 9 meter storm surge; So the flow does not carry flotsam, uprooted trees and other debris and rubbish to foul your anchor line and pull the boat under...

    My boat is built light, The bottom is very slippery, (When I clean the growth of grass off), Will happily take a grounding and stay upright... With a mast, (The mast sits where the radar is at present and extends 12m upwards), and sails, (a pair of 45 square metre genoa, to each bow...), are designed to ghost along in the early morning 'lack-of-breezes'... When the wind over the deck exceeds 20knots, the sails are down and I can motor for as little as 2 miles per litre !!!

    The advantage of any aerodynamic effort is lost as soon as a sail is hoisted... That is, aerodynamic consideration of the coach-house etc., mostly bull5hit to sell to the boy racers who know no better... A good sailor will make better way in a bathtub, than a smart-*** in the most flash newest "go-fast" design...

    Spend your effort in making sure the boat is ergonomic and easy to handle, has space for you and companion, you can have a hot meal cooked in a rough overnight seaway... It is important that you can remain comfortable, warm, dry and use the toilet at any time...
     
  10. Autodafe
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    Autodafe Senior Member

    Hi Mas,

    Thanks for your interest.
    No engine is a slight exaggeration - I will have two electric motors for docking etc, but I don't plan to have huge batteries, so my practical range under motor power will only be about 2 hours - pretty useless in a prolonged blow.

    I haven't shopped around for insurance but plan to have third party if I can get it. However I don't plan to spend much (any) time in marinas.

    Most cruising time will be in home waters down here in Tassie. I would like to do longer trips AU/NZ and Chile as well, so need to design for offshore in bad weather.

    I agree that for performance, hull design below the water is more important than deckhouse design, but I might as well get the whole package as good as I can.

    As far as ergonomics go, you're preaching to the converted.

    If you're picturing me as one of the boy racer set, looking for an extra 0.1knot, then you're far off.
    Not installing engines saves me roughly $35,000 and 1000kg. Having chucked the engines, improving windward ability under sail is a matter of safety and comfort, not winning races.

    George
     
  11. Tackwise
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    Tackwise Member

    "Deck parasites" or deck appendages can be a substancial factor in the overall resistance. However as Brian Eiland correctly states the effect of the bridgedeck is far greater than that of the deck appendages. It is also much easier to optimize the bridge deck as opposed to the deck appendages.

    Once a significant effort has been put into optimizing the bridge deck, there will come a point where it will pay to give more attention to optimizing the deck appendages as opposed to the bridge deck. When this point is reached is very difficult to asses/quantify.

    The difficulty in quantifying this point has at least two major reasons:
    1) Although the total resistance of the deck appendages may be significant, the resistance factor of each individual winch, hatch, etc is still relativally small. For instance replacing all the winches with an aerodynamically foil :D shaped winch, may seem like a good idea. The foil shaped winches may result in less than 1% or even less than 0,1% reduction of the overall resistance, while costing you a fortune.
    2) It is very difficult (= impossible) to isolate the individual resistance factor of each appendage. There is always an interaction between the parts so the simple sum R1+R2=R3 therefore R1=R3-R2 is therefore seldomly true!

    That said, it does not mean it will not pay to give some attention to the aerodynamics of the deck appendages. Just don't lose yourself in it! Also on the point of ergonomic design, I believe that every second you give in optimizing the ergonomic design will pay itself back tenfold as opposed to any effort put into the aerodynamic design of deck appendages!
     
  12. Autodafe
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    Autodafe Senior Member

    Streamlined winches are well outside my scope :)

    My question was really whether the interactions between the deck appendages and streamlined bridgedeck would be highly detrimental to the drag of the bridgedeck.
     
  13. DCockey
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    DCockey Senior Member

    Depends on where the winches are located. If they interfere with the flow over the upstream portion of the bridgedeck then they may cause increased separation. If downstream then they should have little or no detrimental effect on the flow around the bridgedeck. If they happen to be located in a region of separated flow their contribution to overall drag will be lessened.
     
  14. Autodafe
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    Autodafe Senior Member

    Winches I can move. Stanchions, lifelines, toerails and jacklines I'm pretty much stuck with on the upstream side.

    I think as per your original advice I'll opt for large radius rounded corners without going for the whole "streamlined" shape.

    Thanks,
    George
     

  15. Bruce Woods
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    Bruce Woods Senior Member

    Dangerous?? for you maybe. Having spent years on the QLD coast in an enginless Adams I can't disagree more.

    I'm pretty sure most of Lucus's boats drew more than 700 mm, and there isn't two many creeks he hasn't been into.

    For those at the Tasmanian wooden boat festival, earlier this year would have been treated to a converted meter boat (engineless) sailing/ sculling polling etc in out and around constitution dock. Those on a tight schedule, no patience or no skill and deep pockets, generally can't do without a large engine.

    As for hull stream lining, It always cracks me up the lengths people go to ie Schionning, to minimize windage and then carry the spare sails aloft on furlers. Oh well , each to their own.
    Regards
     
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