Will air injection in a concave sailboat or catamaran hull make it faster

Discussion in 'Boat Design' started by jon haig, Nov 19, 2019.

  1. jon haig
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    jon haig Junior Member

    Will air injection in a concave sailboat or catamaran hull make it faster.

    I read about project AMAZON, and that sailboat had a flatish concavish hull with air injection or air suction ,The builder writes that it was very fast ,,easy 20 knots plus.
    I wanna do this to my catamaran hulls by adding as shown in the diagram.

    Will i get more speed?? CAT HULL REDO.png
     
  2. portacruise
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    portacruise Senior Member

    Yes, if is done right. Moving through an air bubble takes a lot less resistance than through water. Also used in high-speed torpedoes.
     
  3. BlueBell
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    BlueBell Ahhhhh...

    Maybe.
    How much air over what surface area?
    You will also lose some buoyancy...
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    I might be reading your drawing wrong, but you seem to have added wetted area, and thereby, drag.
     
  5. portacruise
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    portacruise Senior Member

    My guess is that Cat design might be an issue, as the two hulls might have to be closely aerated to avoid potential control issues.
     
  6. Heimfried
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    Heimfried Senior Member

    I don't know, but I guess no.
    Some simple thoughts:
    - The air will only move along this narrow concave bottom channel if the velocity direction of the surrounding water (in that depth) is perfectly aligned with hull velocity. That means absolute calm water and straight course, far more towing tank conditions than real sailing conditions. In reality air will only be in a small surrounding of each air nozzle in the channel and then by the water stream wiped towards one semi-hull side and go up.
    - The air bubbles can help to decrease the friction between hull skin and water only at that parts of semi-hulls which are "thight covered" by air bubbles.
    - At about 20 kt hull speed is 10 meter per second. The rate of climb of an air bubble depends on its diameter. Supposed an average bubble will need 3 seconds to get up from bottom channel to water surface, a bubble which is ejected near the stem and starts immediately to rise will passing the stern when it is at 2/3 on its way to the water surface. That means, that most of the semi hull will not be covered.
     
  7. Doug Halsey
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    Doug Halsey Senior Member

    Could you please provide a good link to project AMAZON? My attempts to Google it brought up a bunch of things that may not be relevant.

    My concern is that injecting air may require a powered pump that would offset the possible drag reduction.
     
  8. Doug Lord
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    Doug Lord Flight Ready

    -------------------------------
    I talked to Eric Sponberg about this a few years ago-he said the air system on Project Amazon worked.......
     
  9. Ilan Voyager
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    Ilan Voyager Senior Member

    Injecting air or using air suction has been done several times since a very, very, very long time. There has been the concave bottom hulls with big air pumps made by the Russian, there has been the hulls with a myriad of calibrated air injecteurs and always the big air pump and a very complex plumbing also by the Russian. There are also the stepped planing hulls with air suction like the air step by Beneteau AIR STEP® | Beneteau https://www.beneteau.com/us/page-innovation/air-stepr. I know one of the conceptors so I got some details. The tuning of the system has been a hard task but that pays. The illustration is very artistic and pretty far from the reality...
    [​IMG]
    But they are all pretty fast motor boats, yachts or warships, no sail boat.
    The reasons are evident; the system is complex and has a major drawback at low speed by increasing the drag. At my knowledge the system is interesting above 20 knots of steady speed, not a ordinary situation on a cruising catamaran. And the system of Beneteau works only on a planing hull.
    If we examine the multihulls sailing racers of today, those able of 45-50 knots on open sea and of making 900 NM a day (more than 37 knots of mean speed) as the maxi trimarans, none has air injection nor air cushion as it's useless (yes this solution has been examined on computer by hydrodynamicians on the account of NA like VPLP, for racers all the solutions even the craziest has been explored).
    Foils pay far more. So now a lot of oceanic big tris can fly on foils and are looking for the 55+ knots and more than 1200 NM/day.
    The system shown on your drawing does not generate enough "lubrication" and will be a nuisance on a sailing catamaran, as it's a permanent drag generator. A possible system would be a stepped hull, not the best for sailing in light winds nor the structural lightness, which after a certain speed would maybe suck air.
    On a cat for going fast the recipe is simple. Very light, as long as possible with thin hulls, lots of sail surface, minimal wetted surface and a determined crew. Eventually the expensive foils, which are not a simple matter.
     
  10. wet feet
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    wet feet Senior Member

    An interesting idea but it looks like a lot of work for no certain gain.In fact,it Archimedes was right,the boat is likely to run a bit lower due to being supported by a fluid that has had it's density reduced by the addition of air and as was noted in an earlier post there will be a gain of wetted surface.A situation that won't be helped by the weight of any necessary hardware.
     
  11. messabout
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    messabout Senior Member

    ^ No

    That has been extensively tried for small planing boats with less than astoundingly favorable result. To generate enough air entrainement to influence a sail boat will use more energy than the sails can generate while still propelling the boat. Very well, do not use the sails to generate required air pressure. So where will this energy come from but from the forward motion of the beat. Will that cause a significant amount of dynamic drag?

    There ain't no free lunch in physics, I will argue.
     
  12. portacruise
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    portacruise Senior Member

  13. Heimfried
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    Heimfried Senior Member

    Thank you for the link. The linked Sponberg paper contains another link to a Sponberg article in "Marine Technology" (Vol. 37 No. 2 pp 65-78). Described is a "passive" system which works only when the boat is planing. She had a "low deadrise hull" so a completely different hull shape th the given (OP) kat hull.
     
  14. Pammie
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    Pammie Senior Member

    Didn't clinker-built viking ships use this as a passive principle? Once heard a story of a viking ship being faster than a modern monohull?
     

  15. Ilan Voyager
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    Ilan Voyager Senior Member

    No. The strakes are not passive systems for hull ventilation. At the max speed of a Viking ship with the power obtainable from its square sail there is no need of ventilations tricks.
    A Viking ship being faster than a modern monohull is total falsehood, or urban legend if you prefer. The viking ships, at least the long snakes or warships were rather fast considering the technology and materials of the 9th century. The merchant ships sailed well, were rather seaworthy and also rather "fast" always considering the technology and materials of the 9th century.
    A 20 meters long ship was able of somewhere 10-12 knots in strong wind, and have long trips around the 5-6 knots if enough wind. That's impressive on such an open boat with no true deck and just one hard to manoeuver square sail.
    But modern sailing monohulls are in another league. When I say nowadays it's from 1990, I do not consider a 1950 boat as modern up to date.
    A racing 2018 Mini Transat 6.5 meters long is able to sail at 17.5 knots mean speed during 10 hours crossing the English Channel and to cross the Atlantic at 7.5 knots mean speed. But it's a racer.
    A good modern cruising dériveur lesté "ballasted dinghy" of around 15 meters is able to maintain a steady 7 knots fully charged. The max speed is not very high but the interest resides in good mean speeds. In the hands of an able crew the 240 NM/day are feasible in the right conditions. We are talking of a cruising sailboat with comfortable amenities, and plenty of reserves of food, water and fuel and not huge racing sails, a boat far more cozy that the Viking ship.
     
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