Low power semi-hydrofoil?

Discussion in 'Boat Design' started by Jeremy Harris, Jun 10, 2012.

  1. Jeremy Harris
    Joined: Jun 2009
    Posts: 978
    Likes: 59, Points: 28, Legacy Rep: 711
    Location: Salisbury, UK

    Jeremy Harris Senior Member

    As some will know (from this thread: http://www.boatdesign.net/forums/boat-design/duck-punt-twist-42466.html) I've just come back from a race event for boats powered only by cordless power tools.

    This year my boat, and a couple of others, were going fast enough to be well up on their own bow wave, with the commensurate use of a lot of power (I pretty much drained a 54 Wh lithium Makita battery in less than three minutes, which is around 1 kW.........). Despite me being sat right forward, apparently the front 1/4 of the hull was out of the water, not really surprising as I was around 50% faster than hull displacement speed.

    Clearly there is some potential for getting on the plane using just cordless tools, but efficient power delivery is hampered a bit by the large props needed, both because power tool rpm tends to be quite modest, but also because acceleration, and hence good low speed thrust, is important in getting a good start and making a tight 180 degree turn around one of the marks on the course.

    Clearly my hull (photos below of the hull underside - the outboard well hole gets filled with a flush plate when afloat) isn't an ideal planing hull, as it hasn't got much planing area aft. It does have a pretty low resistance at speeds up to maximum displacement speed though, and accelerates pretty quickly to above that speed.

    I'm toying with the idea of adding a single high aspect ratio horizontal foil at the stern, not much below the bottom of the transom, to provide some lift and hopefully get the hull planing. I don't really fancy my chances of making it into a complete hydrofoil, as there would be all sorts of complications in terms of launching, retracting the foils, providing control etc.

    I need to be able to lift perhaps half the boat's total displacement, starting at a speed of around 4 or 5 kts, and have just enough lift to allow the flatter sections forward to get over the bow wave and up on to the plane. I'm reasonably sure that once on the plane the power consumption would come down a bit when compared with the current situation where the boat is running half way up its own bow wave.

    Anyone think this idea has some merit? I'd like to keep things simple, so am currently thinking along the lines of an inverted T foil fitted to the transom at a fixed depth.
     

    Attached Files:

  2. Doug Lord
    Joined: May 2009
    Posts: 16,679
    Likes: 344, Points: 93, Legacy Rep: 1362
    Location: Cocoa, Florida

    Doug Lord Flight Ready

    I think it has merit and so do a lot of International 14 sailors. Read this article about rudder t-foils and the recovery of some energy from the stern wave. Good luck Jeremy! It's simple enough to experiment with:
     

    Attached Files:

  3. Jeremy Harris
    Joined: Jun 2009
    Posts: 978
    Likes: 59, Points: 28, Legacy Rep: 711
    Location: Salisbury, UK

    Jeremy Harris Senior Member

    Brilliant! Thanks for that, Doug, it was just what I was after. As you say, it's simple enough to experiment with, as I should be able to knock up a jury rig that would clamp on to the transom for testing.
     
  4. daiquiri
    Joined: May 2004
    Posts: 5,373
    Likes: 254, Points: 93, Legacy Rep: 3380
    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    It would be so nice if you had a pic of your boat at max speed, to see how it trims and how does the water flow around the hull. Do you have any?
     
  5. Doug Lord
    Joined: May 2009
    Posts: 16,679
    Likes: 344, Points: 93, Legacy Rep: 1362
    Location: Cocoa, Florida

    Doug Lord Flight Ready

    Jeremy, you're welcome. The guy that wrote that is a brilliant experimenter who knows technical foil design inside and out-and he lives in the UK. A guy like him might collaborate with you just for the fun of it.
    Jeremy, note that the rudder foil works for energy recovery upwind on an I-14.
     
  6. Jeremy Harris
    Joined: Jun 2009
    Posts: 978
    Likes: 59, Points: 28, Legacy Rep: 711
    Location: Salisbury, UK

    Jeremy Harris Senior Member

    I've been promised some photos that I should get some time this week, with luck. I believe that the attitude of my boat at speed was similar to that in the photo of the winner, Velociraptor, I posted on the Duck Punt thread (copied below), even though I was laying down in the boat with my weight much further forward (I laid down to reduce windage!). Velociraptor clearly has her bow wave back around 1/4 of the distance from the bow, and is suffering a bit from trying to plane on the curved section of the hull. All I could hear on the way around the course was the banging of ripples/waves under the flat section of the hull forward, which also supports the observation by the spectator that the bow was up in the air.

    [​IMG]
     
  7. Doug Lord
    Joined: May 2009
    Posts: 16,679
    Likes: 344, Points: 93, Legacy Rep: 1362
    Location: Cocoa, Florida

    Doug Lord Flight Ready

    Jeremy, I have a very big file from Paul Bieker that goes into some detail on the energy recovery aspect of the rudder T-foil on International 14's. Might be of some assistance. Click on my name and e-mail me and I'll send you the pdf-too big for here.....(15mb vs the allowed 10)

    Jeremy, I tried e-mailing you with this file and it wouldn't go. I sent you the notice I got from your ISP. Sorry.....
     
  8. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 489, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    You'd be much better off just my adding some extensions to the bottom aft, much like what the Bartender has done, to provide the plane area necessary and help stabilize the hull once on plane. Picture the bottom panel widened in the after half of the hull, leaving the topside panels where they are, just some plywood squaring off the after half of the bottom. This doesn't need to be foil shaped, just stiff enough to support the boat on plane. I can't see how much rocker this boat has, but it doesn't look like much, so once she squats, approaching semi plane speeds, she'll have enough incidence angle to effectively offer considerable assistance in pane mode. A small amount of fore to aft taper, on the extensions, will help maneuverability on such a narrow hull form too.
     
  9. Jeremy Harris
    Joined: Jun 2009
    Posts: 978
    Likes: 59, Points: 28, Legacy Rep: 711
    Location: Salisbury, UK

    Jeremy Harris Senior Member

    But wouldn't doing this add a fair bit of extra wetted area?

    Power is limited, as is low speed thrust to an extent, as the prop is designed to give me best efficiency at around 8kts (almost double hull speed). One advantage of the canoe-shaped hull is that its resistance is fairly low when in displacement mode, and this, combined with the low overall displacement (about 100 kg, 220 lbs) seems to give pretty good acceleration.

    There are two key points on the course where this really helps, the dash from a standing start to the first turn (whoever rounds the first turn first has the inside line) and the acceleration out of the 180 degree second turn. At the moment the boat is pretty good at this, but is perhaps a knot or two slower than the fastest boats when running straight between the marks. It was frustrating to get a good start, get around the first turn nice and sharply, then get gradually overhauled on the straight, only to regain the lead on the next turn. If I can just get a little more straight line speed, but without having a significant impact when at full displacement speed, then that would be ideal.

    My speed at the moment is about 6 1/2 kts, the fastest boats are doing maybe 7 to 7 1/2kts. The fastest boat this year was probably less than a kt faster than the fastest boat last year, so I'm pretty sure we're all up at limit for semi-displacement operation right now, as even adding a lot of power (the four drill powered winner, for example) doesn't seem to have made a lot of difference. The competition rules mean we can't increase waterline length, either.
     
  10. Ad Hoc
    Joined: Oct 2008
    Posts: 7,115
    Likes: 1,057, Points: 113, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    You need to compare a canoe hull against "other shapes" to fully qualify that Jeremy. Here is an extract from a hydrodynamics papers on a series of different hull forms, from a canoe to a min WSA (semi-circular) and others. The parent hull form is a series 64 hull. The L/D ratios are 8:

    Family of Hulls.jpg

    What is the Lwl of your hull and max full load? If the L/D ratio is on the high side, then as you can see from the above, the shape plays a very minor role.
     
  11. portacruise
    Joined: Jun 2009
    Posts: 1,247
    Likes: 83, Points: 48, Legacy Rep: 218
    Location: USA

    portacruise Senior Member

    Jeremy:

    Thanks for the interesting postings. Here's a fellow that seems to think planing at 2.5 hp is possible with a very light version of his hull:

    http://rosslillistonewoodenboat.blogspot.com/2012/04/update-on-low-powered-planing-hull.html

    Also have read that a ducted propeller is better suited to electric power than an open one:

    http://www.qis.net/~jmgraham/boatspd.htm

    Why don't 4 cordless drills have considerably more power/potential speed than 1 in this racing application?



    Hope this helps.

    Porta
     
  12. kerosene
    Joined: Jul 2006
    Posts: 1,210
    Likes: 153, Points: 63, Legacy Rep: 358
    Location: finland

    kerosene Senior Member

  13. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 489, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Jeremy, the size and placement of the bottom extensions certainly could move. The picture above shows a boat in semi plane mode and working more like an inclined bottom than a more typical full plane craft. This was the approach very early in power craft design, at the turn of the 19th to 20th century. Judging by her wave train, she's at 2.2 to 2.3 S/L, which is cooking pretty good, considering the hull form limitations and available power.

    Picture two triangular wings, attached above the bottom plane, say a few inches. In plan view they would square off the aft sections. In static profile (at rest) they'd be just above the LWL, sort of like big splash rails. Underway at low speed they wouldn't harm a thing, but as the stern squatted, they offer some bearing area. Of course you'd have to play with the heights a bit to get them right, but if interested in just all out, I'd just place them paralleling the static LWL on the bottom. Hell have some fun with it and make them a fat foil section, maybe an asymmetric NACA 00 series. Yep a bit of drag, but lots of lift. What is the length of the canoe pictured? What is the length of your flat bottomed boat?
     
  14. Jeremy Harris
    Joined: Jun 2009
    Posts: 978
    Likes: 59, Points: 28, Legacy Rep: 711
    Location: Salisbury, UK

    Jeremy Harris Senior Member

    Thanks for that graph, although I think that realistically I'll be off the bottom of it in terms of speed; I'd guess that the most I could hope for is maybe 9 or 10 kts on the limited power that's available.
    LWL is about 4.5m (14ft 9ins), total displacement is between 100 -105kg (220 - 230 lbs), so I make D/L about 30 to 34.

    One slight complication may be that the hull has a total wetted area when stationary and loaded of about 2.9 m², but about 2.4m² of that is the virtually flat bottom of the hull (it has very little rocker). I suspect this makes it tend to behave more like a flat plate on the surface than a deeply immersed hull - it only draws about 50 mm or so of water at full displacement. In many ways it probably behaves more like a crude sail board than a canoe!
     

  15. Jeremy Harris
    Joined: Jun 2009
    Posts: 978
    Likes: 59, Points: 28, Legacy Rep: 711
    Location: Salisbury, UK

    Jeremy Harris Senior Member

    Thanks for those links. I think the chap building the light boat has the right idea for economical cruising at speed. My boat is a lot lighter than his, and less than ideal for a speed boat, but it was easy to build, plus there's an element of fun in doing something unusual with a historic boat hull shape!

    I have a ducted prop on the big electric boat now, and it works well at low speed (below about 5 kts). The drag from the duct has a significant impact as speed increases, though, so overall propulsive efficiency drops with speed. I'm happy that the big 13" prop I've settled on is OK for another three or four knots, as it's over-pitched (deliberately) at the moment and was seriously over-loading the drill. I plan on having a couple of drills driving it next year, if I can and also want to do some testing of the propulsion unit using one of my brushless motor drives, so I can get some accurate power and rpm data versus boat speed.

    I think the four drill boat was simply not putting that much more power into the relatively small, fine pitch, model aircraft props. The props were turning at, or close to, drill rpm, so around 1100 to 1400 at a guess. You can't get model aircraft props in the 12 to 15 " diameter range that are much coarser than square (pitch = diameter), so I strongly suspect the boat wasn't able to use the available power. On the other hand, I was bogging my drill down from its no-load rpm of about 1500, to around 700 at a guess, just from the high torque that my relatively coarse pitch prop was loading it with. My hope was that I'd go a kt or two faster, and allow the drill to run at a faster, and more efficient, rpm.
     
Loading...
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.