DSS

Discussion in 'Stability' started by terrnz, May 31, 2017.

  1. terrnz
    Joined: Mar 2014
    Posts: 65
    Likes: 0, Points: 6, Legacy Rep: 10
    Location: NZ

    terrnz Junior Member

    Does any one have some numbers regarding righting moment and performance increase, plus optimum foil shape and placement?
    I find the concept exciting as it does not require stored power and presume it works, just how well is my question?
     
  2. Doug Lord
    Joined: May 2009
    Posts: 14,025
    Likes: 121, Points: 73, Legacy Rep: 1362
    Location: Cocoa Beach, Florida

    Doug Lord Flight Ready

    There are multiple threads on DSS in the "Sailboats" forum. There is DSS-1 where the foil doesn't fly the boat but adds lots of RM and DSS-2 where the foil does what DSS-1 does and also flies the whole boat. DSS-2 requires a rudder T-foil.
     
  3. terrnz
    Joined: Mar 2014
    Posts: 65
    Likes: 0, Points: 6, Legacy Rep: 10
    Location: NZ

    terrnz Junior Member

    thanks for your reply and I have looked at multiple threads concerning DSS and most are full of news and pictures but not numbers. The closest thing I get tp a performance increase quantified is that IMORCA 60 expect to be 2 days faster round the world about 77 days so say 5% in most reaching conditions.

    I was specific in my areas of interest.

    I have seen you are undoubtedly knowledgeable on foiling so if there is already a thread giving information on performance increase, righting moment effect I would be grateful.

    I am thinking DSS-1 configuration. straight foil about 2 metre protruding from hull. no canting keel, single rudder, possible trim tab on keel.
     
  4. Doug Lord
    Joined: May 2009
    Posts: 14,025
    Likes: 121, Points: 73, Legacy Rep: 1362
    Location: Cocoa Beach, Florida

    Doug Lord Flight Ready

    =======================
    If you're seriously considering using DSS, contact Hugh Welbourn through the website. You won't get accurate info specific to your application from any other source.
    For a rough idea you can calculate the lift on a foil of the span , section and chord you're thinking of using which would give you a very rough idea of the RM the foil will add at a specific speed or speeds. A book that is available reasonably is "Hydrofoils Design Build Fly"* by Ray Vellinga which will help you to calculate lift.
    But the smartest thing you can do is contact Hugh.
    *Amazon
     
  5. terrnz
    Joined: Mar 2014
    Posts: 65
    Likes: 0, Points: 6, Legacy Rep: 10
    Location: NZ

    terrnz Junior Member

    Thank you for your suggestion. I am very much at the exploratory stage..what if? How much? I am not trying to fly. Just reduce the lead(Pb) needed and increase stability with some damping motion thrown in. That is without the power consumption of say a canting keel which could be said to make a motor boat out of a sail boat.
    Any pointers as to where I might find appropriate formulae for calculation of lift?
     
  6. Doug Lord
    Joined: May 2009
    Posts: 14,025
    Likes: 121, Points: 73, Legacy Rep: 1362
    Location: Cocoa Beach, Florida

    Doug Lord Flight Ready

  7. terrnz
    Joined: Mar 2014
    Posts: 65
    Likes: 0, Points: 6, Legacy Rep: 10
    Location: NZ

    terrnz Junior Member

    thank you for that I will do my best to digest.
     
  8. terrnz
    Joined: Mar 2014
    Posts: 65
    Likes: 0, Points: 6, Legacy Rep: 10
    Location: NZ

    terrnz Junior Member

    I found the attached spreadsheet on the net. As it was on the net I assume there are no intellectual property issues. It seems to be a spread sheet to calculate hydrostatic lift of a foil through water.
     

    Attached Files:

  9. Doug Lord
    Joined: May 2009
    Posts: 14,025
    Likes: 121, Points: 73, Legacy Rep: 1362
    Location: Cocoa Beach, Florida

    Doug Lord Flight Ready

    Thanks.....
     
  10. terrnz
    Joined: Mar 2014
    Posts: 65
    Likes: 0, Points: 6, Legacy Rep: 10
    Location: NZ

    terrnz Junior Member

    a question you may be able to answer for me. The aspect ratio...should this be doubled as the effective ratio according to plane theory the end plate effect of the hull?
     
  11. Doug Lord
    Joined: May 2009
    Posts: 14,025
    Likes: 121, Points: 73, Legacy Rep: 1362
    Location: Cocoa Beach, Florida

    Doug Lord Flight Ready

    Keep in mind that these DSS foils operate relatively close to the surface-within one to two chords and also are effective as planing surfaces. I wouldn't count on the endplate effect of the hull but you could ask Hugh......
     
  12. terrnz
    Joined: Mar 2014
    Posts: 65
    Likes: 0, Points: 6, Legacy Rep: 10
    Location: NZ

    terrnz Junior Member

    they are of the same order as keel or rudder in relation to the surface yet do we not count the endplate effect when considering keel and rudder effective aspect ratios?
     
  13. Skyak
    Joined: Jul 2012
    Posts: 1,158
    Likes: 18, Points: 38, Legacy Rep: 152
    Location: United States

    Skyak Senior Member

    That spreadsheet you posted has the most complete handling of the shallow depth effect that I have seen ...thanks for posting your find. It includes the effective attack angle due to AR and depth effects. What it does not include is the effect of wave generation. DSS operates in shallow water at speeds slower than wave propagation sometimes, hydrofoils never do so they did not include any calculation. I would model it as a loss of any lift from the top of the foil at any depth less than half a cord, at speeds less than wave propagation. Somewhere between 1.5 and 2.5 times the wave speed I would expect the spreadsheet above to become accurate.

    For aspect ratio, if your DSS were attached to a flat wall at a 90 deg and sealed off the AR would be 2 instead of 1. I am pretty sure none of those 3 conditions apply to your design so you are left to make an educated guess. I would say closer to 2 than 1, but I would be more concerned about getting the angles right. The flow of water along the side of your boat might be at a significantly different angle than it is 2 feet away -so the lift distribution will be distorted. If you have a round bottom and you try to place your DSS low and horizontal you could have a negative attack angle at the base -which would make the AR less than 1 times the length.
     
  14. terrnz
    Joined: Mar 2014
    Posts: 65
    Likes: 0, Points: 6, Legacy Rep: 10
    Location: NZ

    terrnz Junior Member

    thanks for the reply, yet more thought needed.
     

  15. Skyak
    Joined: Jul 2012
    Posts: 1,158
    Likes: 18, Points: 38, Legacy Rep: 152
    Location: United States

    Skyak Senior Member

    More thoughts:

    -I am regretting my last remark about the angle of flow around the boat. The DSS foil will most likely be positioned near or at the maximum beam so the flow should be close to the free flow direction. Thinking how it would differ is thinking too much for too little benefit.
    -If you use AR ~2l in that spreadsheet then assume the force in the center of the foil you will overstate the righting moment because the lift is greater closer to the hull where the moment arm is smaller. If you just use the 1l AR it will lead you to a longer foil that at worst will outperform your calculations. DSS foils are unlike keels and rudders in that they have much greater reason to increase span. There is more value in the leverage than in increased AR.
     
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.