Low HP Foils

Discussion in 'Hydrodynamics and Aerodynamics' started by jesdreamer, Dec 22, 2015.

  1. jesdreamer
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    jesdreamer Junior Member

    Foils came up in our thread on drag of a partially submerged transom at low speeds. It sounded intriguing but seemed off-subject so therefore a new thread -- Possible use of foils to help a low HP displacement or semi-disp lightweight hull?? We have a 12ft hull with transom somewhat smaller than the 24" max beam. Hull weight only 30# or so, but equipped with a 55 pound-thrust trolling motor sitting inboard in a well just behind helmsman's seat which in turn is near overall CG. Location of CB or CF can be varied by forward location of the 80# 12V battery. Software shows the loaded hull at 300-325# displacement to generate perhaps 10-12# total drag resistance at close to hull speed -- But the 55# trolling motor doesn't seem to get us out of displacement or barely into semi-disp mode. YouTube Flyak movies show a (muscular) paddler getting a kayak with foils out of the water at speeds beyond that of skulls --

    Hull has a lot of flat area submerged for aft 2/3 or so of wetted area. Could we get our 12ft hull at 300-325# total up to or near zero draft with foils?? If so, any idea of foil configurations, placement, etc?? -- Presently we pivot the trolling motor for steering, any thought per the idea of a rudder between submerged (somewhat lower) power head and hull with a foil attached horizontally at power head axis -- would the boat tumble over if turned with this kind of setup??
     
  2. daiquiri
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    daiquiri Engineering and Design

    Yes, foils will decrease the total drag of your boat above certain speeds. They will start paying-off in terms of drag indicatively above 1.2-1.4 times the so-called "hull speed". Below that, they are no match for the L/D ratio of the displacement hull. The exact value will depend on boat characteristics. If you wish, I could make a drag analysis with and without foils, but would need the lines plan of your boat. Could you post it please?
     
  3. Doug Lord
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    Doug Lord Flight Ready

    Here is an example of an electric powered hydrofoil: https://quadrofoil.com/
    Ray Vellinga wrote a book about small power and sail hydrofoils called:"Hydrofoils Design Build Fly" and it would be very helpful. It's available at Amazon. I would suggest that you get this book, study it and use it as a basis for your own design.
     
  4. philSweet
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    philSweet Senior Member

    The trouble lies in finding real thrust vs speed curves for these trolling motors. You will need to know the actual thrust at 4,5,6 knots, because it isn't 55 pounds.
     
  5. jesdreamer
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    jesdreamer Junior Member

    Output of "55# thrust" MinnKota

    I had run across something several months ago which indicated these really only put out around 1HP or so even though the "55# thrust" sounds more than enough to plane our 12 ft boat which the software shows only 10-12-15# total resistance up near hull speed
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    Common sense says you won't be planing that thing with a tiny electric outboard. You have 300-325 lbs in a little 12 foot boat with 24 " beam ? You are a lot further away from planing than you think. 6 hp might do it.
     
  7. PAR
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    PAR Yacht Designer/Builder

    A 55 pounds of thrust trolling motor is about 2/3's to 3/4 of a single HP. Using the usual heuristics (square root of [total shaft HP/ weight] x constant =), a 300 pound boat with a 3/4 HP motor, no prop slip and a modest qualifier, gets you to about 9 MPH on a perfectly glass smooth puddle. This assumes a shape that will promote low speed plane and considerable efficiency. Without hydro's and some lines to look at, a very generous guess at the speed.

    Assuming an 11' LWL, you'll be traveling at about a 2.36 S/L ratio which is just on the verge of full plane mode or more realistically, the higher end of semi plane. From practical experience I can tell you that the 55 pound trolling motor is propped all wrong for this speed and seeing 5 MPH would be lucky. It could be repropped and you might see some modest speed gains, but you'll have to address a number of other issues to get to 3.0 S/L.
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    He can theorize till he goes blue in the face, a simpler method would be tow the boat to various gps speed points, and check a weight gauge reading for each of them.
     
  9. HJS
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    HJS Member

    "55# thrust" is bollard pull

    "55# thrust" is bollard pull at zero speed with a propeller adapted to provide only this result. This is a measurement that has no value in these contexts.

    js
     
  10. PAR
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    PAR Yacht Designer/Builder

    As easy as it is to put a little trolling motor on a whatever he's trying to push, the results will be pretty much instantaneous. You can attempt to hypothesize all you want, but given many of us have already done this sort of thing and offered the appropriate advice . . .

    A picture of the hull or better yet lines and the prerequisite hydro's would be a more definitive way to look at things, don't you think, as the rest is just guessing or empirical observation.
     
  11. Joakim
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    Joakim Senior Member

    And test show that even at bollard pull the 55 lbs model only provide 30-40 lbs. They use 500-600 W electrical power thus 2/3-3/4 hp, but probably have only 1/2 hp at the propeller shaft.

    At higher speeds they use less power and provide much less thrust.
     
  12. jesdreamer
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    jesdreamer Junior Member

    Drag Analysis

    Daiquiri, I appreciate the analytical help and CFD analysis you have provided in our Transom Drag thread and am most anxious to take advantage of your continued willingness to help in our quest for help on this power and planing thread. Of the few hull design programs I have used, I did this design using Ross Leidy's Kayak Foundry software for ease of design fairing and convenient output of station patterns.

    His output is copyrighted so I hesitate to publicly post it on a forum -- but will try to attach an output sheet to a PM direct to you -- and hope the lack of actual station configuration XY data will not hamper your analysis. If it does, I will try to convert station patterns into actual XY measurements --
     
  13. jesdreamer
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    jesdreamer Junior Member

    On the Verge of Planing??

    Par, your post is encouraging -- What would you recommend for a better prop and might you know of any aftermarket suppliers of anything appropriate?? We are now at nearly 12 ft OAL and 24" beam (nearly vertical sides from the flat bottom beginning 1/3 of the way back from bow with zero rocker until very slight 2" upsweep last 1 or 2ft toward transom & transom width a good 66% of max beam -- Would it help to narrow max Beam to around 20" (approaching 80% of current, but 20" is absolute min to consider) -- We perhaps have more flexibility to lengthen LWL from current 11.5 ft range to 12 ft via straight vertical bow -- as well as lengthen overall length to maybe 13ft (but I prefer not going any longer than this). Unrealistic to lighten hull beyond the current 30# range and with MinnKota and battery with any usable range we have around 100# minimum -- with Operator at 170# we can't really go below 300# displacement. However I could get more aft hydrodstatics as well as hydrodynamics by using a transom closer to max beam -- I could even exceed "max" beam to some slight extent (more delta shape wetted area -- I could add small x section skegs, parallel & out toward edge of the flat keel if that might help prevent "leakage" of Hytrodynamic support out to the sides --

    Par says we might be close but have a lot of things to get right in order to get more % planing -- I have mentioned a few and would like your thoughts on these as well as any other possibilities. We would have a major battery weight problem if going to 24 or 36Volts so I realistically feel that route to be not really suitable --
     
  14. Doug Lord
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    Doug Lord Flight Ready

    If you want to use foils you might consider that a relatively skinny hull can foil before it planes. That would change your whole approach, though.....
     

  15. jesdreamer
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    jesdreamer Junior Member

    Skinny hull can foil before planing??

    Thanks Doug, great point -- I started this thread to explore foiling our small electric launch and would not mind at all if we could get it to foil before it might plane w/o the foils -- I just read through the thread on J/Foils and found some interesting reference to elevator foils on rudders in America's Cup and other international sailing. Earlier in this thread I had mentioned that we are steering by rotating the vertical shaft supporting the MinnKota or similar electric power head -- I have been planning to drop the head axis a little more below hull (tops of prop now only 1" or so below, so with perhaps 2-3" draft, the axis of prop might be 6" or so below hull -- Might this be too shallow??) -- We would drop power head axis to perhaps 4" lower and add a rudder integral to the vertical support shaft (this would help steering when docking w/o power). And we could easily mount a horizontal "Elevator Foil" at the rudder (would 3 or 4" below hull be too shallow for the Elevator foil??) -- I had posted the idea of adding a foil to a rudder but was worried about the boat tumbling over under too sharp a turn at speed -- Might such an add-on rear rudder Elevator Foil work w/o some kind of foil up front?? (Perhaps to overcome sinkage trim w/o resorting to broader beam aft to get more hydrostatics but with the downside of added friction, with friction the major problem below hull speed)

    From the YouTube video of the "Flyack" which foils faster than a multi-man scull, I had always assumed that during acceleration the paddler was going through planing before foiling, but I admit seeing no evidence of this in the video -- Quite interesting --
     
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