Outboard motor skeg mounted foil?

Discussion in 'Hydrodynamics and Aerodynamics' started by Toolate, Dec 31, 2017.

  1. PAR
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    PAR Yacht Designer/Builder

    Lets get the terminology right, it's not a cavitation plate, in fact there's no "anti cavitation plate" at all on outboards or outdrives. It's an anti ventilation plate that exists on all outboards and outdrives and this is what the foil's are attached to. These don't prevent cavitation, but do help ventilation and to some degree improve how some hulls get up on plane and hold lower minimum plane speeds. The plan form is mostly irrelevant at lower plane speeds, though at higher speeds the delta form is less draggy. I've experimented with higher aspect wings at lower speeds and there's some improvement, though sections seem to make more of an impact, but not so much at high speed. At high speed a simple plate will be nearly as effective, as a carefully crafted section. Additionally at these high speeds, the AV foil hold the top speed back at WOT, so becomes esentally self-defeating.

    What type of information are you looking for, as data is available for the basics? Most of the newer "foils" are just chunks off the old idea, to make a viable product with sponsored testing, to confirm what their advertising department suggest is happening. IMO, the latest ones are way too draggy or attempting to address other issues, than the originals seem to contend with, such as low speed handling. As to lifting the butt of a boat, well you'll need some longass (technical term) wings to hold up the 2 tons or so, of a fulling planing 20' fully loaded bowrider and now engine position (as previously mentioned) and ventilation become ever increasing concerns, which the whole idea was supposed to prevent. Additionally, why would you want to after the boat's trim by lifting it's butt off the contact patch? It's just going to get squirrelly and you'll likely dry out the chines, making for a much worse situation.
     
  2. Toolate
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    Toolate Junior Member

    First paragraph is all discusssing standard AV plates and the store bought fins/foils/plates you can bolt to the correct? When you say “higher aspect wings” what do you mean exactly?

    2nd paragraph:
    Just wondering if there is a study or thread I might read on the use of a submerged foil on the aft end of a smaller boat (<20’ or so).

    The newer foils you mention are just AV plate add ons and I think the intent is just to have thebottom surface touching the water only (not full submerged) when planing (right?) so gains are only to be had when coming up on a plane. -OR- are most people now running these submerged at all speeds?

    Mostly I think of my idea like trying to find a way to provide some lift when planing. For my 17’er, maybe 200-400lbs of lift to hopefully let me get the bow down a little and maybe gain a couple kts of cruise speed. Would be like running the boat without the weight of the motor and me in it. NOT to get up and “foil” or even lift the chines although I think they might be up and out at planing speed anyway but the point is that I could adjust the trim of the engine and angle of attack of the foil independently until I found the best compromise and then hopefully achieve some of what I am talking about (slower plane, faster cruise and if so I assume better fuel economy).

    I really do appreciate the response! Lots to learn.
     
  3. Toolate
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    Toolate Junior Member

    Something like this but with the ability to adjust the angle of attack (not just in a fixed position on the skeg).

    One Product To Bolster Performance, Protect Props, Preserve Marine Life http://www.greatlakesscuttlebutt.com/news/featured-news/one-product-to-bolster-performance-protect-props-preserve-marine-life/

    Would expect to have to play with the size of the wing, cross sectional shape and angle of attack until happy . Would also expect to lose some speed at full throttle but would hope to gain some cruise (maybe take my cruise from 20kts to 23 or something like that).

    Just reposting this to make sure you know what I have in mind.
     
    Last edited: Jan 3, 2018
  4. ondarvr
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    ondarvr Senior Member

    You seem a bit confused on the this, whether you mount the foil on the skeg or on the AV plate it will do the same thing if it's dragging in the water under the surface, how deep you place it under the surface doesn't make that much of a difference as far as lift goes.

    And yes, many people have the AV plate mounted foil under water at speed, and it can create huge handling issues on some boats, this is not the recommended way to use them, but it happens often. The lower your top speed the less of an issue it may be though.
     
  5. Toolate
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    Toolate Junior Member

    I just haven’t seen it with my eyes and having owned boats from 7’ to 36’ I am surprised that people are doing that- sorry I will accept it from now on. I understand that a foil if submerged will act as its going to act regardless of whether its above the prop or below. Sorry to be so thick!
     
  6. ondarvr
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    ondarvr Senior Member

    Most boats equipped with I/O's that use an AV mounted foil have all or most of it submerged at speed, this can be a major handling issue, and why some people hate them. Probably far more than half of the foils mounted on outboards are in the water too, people have no clue as to how they're supposed to be used.

    The easy thing for you to do is get an AV mounted foil, or just a flat plate, it will do about the same thing, and just see what it does for you. Raise and lower the motor on the transom and see if it helps.
     
  7. Toolate
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    Toolate Junior Member

    Interesting- I had no idea but that is fact now in my head. Always assumed because they were on the AV plate that they would be planing at planing speeds.

    So, maybe you have said this 3x but I have to confirm- wouldn’t the same foil (given a nice wing shaped cross section) provide more lift if fully submerged rather than just skimming the top of the water? I forget the terminology from engineering but a foil when submerged produces actual lift from the low pressure above it and higher pressure below while the same exact foil would produce less lift if just the bottom were in the water (if the foil was planing on the surface). Is this correct?

    Thanks for keeping up with my stubbornness
     
  8. ondarvr
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    ondarvr Senior Member

    upload_2018-1-3_18-13-44.png

    This is a stock lower unit from an early 80's Evinrude 40HP
    People frequently cut them off.
     
  9. ondarvr
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    ondarvr Senior Member

    As to the question.

    Yes, sort of. As par eluded to, at low speeds the foil shape can help, but as you go faster the angle of the fin has far more affect on lift. Anything more than a flat plate can slow you down.

    And being on the surface can work well, if not better, normal trim tabs typically do a much better job than a foil at keeping the boat on plane at lower speeds.
     
    Last edited: Jan 3, 2018
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  10. PAR
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    PAR Yacht Designer/Builder

    As has been mentioned a few times, the AV plate additions work on some applications, not so well on others and can even screw the pooch on others and the best part, is there's no guideline as to which hull form can greatly, modestly or not at all benefit from one. Some generalities can be assumed with these, like an under powered pontoon boat, that just needs a little help from it's outboard or I/O, to get up and stay up, for example. I install these fairly regularly with good success, reported from owners. Of course, I give the boat a check ride before and after to check level off speed, WOT, cruise and minimum plane speeds, so I don't look like a fool.

    The skeg mounts I've seen, don't do anything more than the AV foils, except as low speed handling improvements (displacement). These can be identified with fences or curved tips to offer some lateral area, so the leg can have a more positive control as a rudder. In this regard they are marginal at best, but do make some modest improvements on some hull forms. Naturally, these skeg mount have a lot more drag associated with them, so you have to accept the trade off.

    Simply put, regardless of the type, sectional shape or plan form employed (delta, short or higher aspect wing, etc.), speed at WOT is going to be slower, unless you have trimmed the engine properly (jack plate or remounting). Tilt and trim controls will not do this, as the leg has to come straight up (mostly). Cutting the skeg off will offer a knot, maybe a 1/2 knot improvement at WOT on some boats, that have the potential to take advantage of this extremely modest decrease in drag. Low speed handling will not be very wildly affected, though you will notice a difference.

    If you want to play with horizontal foils for lift, leave the skeg and lower leg out of the equation. I've done this with success and a patent, but a fixed foil has a very limited operational range, before drag and/or angle of incidence comes to spoil the fun. Now, if you can adjust angle of incidence, with very fast, computer controlled actuators (or whatever) on a trim tab, well then you'd have something. This is currently patented, though I doubt you'd have issues, unless you started selling them by the dozen.
     
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  11. Toolate
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    Toolate Junior Member

    OK its finally sunk in- you just have to tell me 3 ways lol. THank you.

    I have to say the outboard options still hold more appeal (as experimental solutions to small problems) to me on both boats since I dont have to modify my hull (Tabs = holes in hull) plus a plate wouldn’t require electricity. Maybe I will play with one and see what happens. I have put standard tabs and Volvo QL tabs on boats so I am familiar with the results. Ondarvr I am 43 and owned several 80’s Evinrude and never seen a lower unit like that! I think I am living a sheltered life.

    Might just find that they dont work though :).

    PAR- would you be willing to share the patent # you mention in your last paragraph? Would like to check it out.
     
  12. PAR
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    PAR Yacht Designer/Builder

    Nope, currently involved in a suit over it, for the last 4 years.

    Rather than tabs and such, just try some "wedges". In the old days we used cedar shingles for this, though on plastic boats, you might just use a hot glue gun and something inert as a temporary wedge, that can be easily removed or added to. I'd start with an 8" deep (fore and aft) wedges by as long as you have room for the leg to swing on the transom, without obstructing flow (maybe 24" or so). Make it 1/2" thick at the trailing edge, tapering to a feather edge forward. Try it out and adjust height (first), depth (second) and width, until it works pretty good, at which point you'll make a good bonded on version. Yeah, a fixed tab, but a hellof a lot easier and given your relatively narrow operational range, possibly effective from 8 to 15 MPH.
     
  13. Toolate
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    Toolate Junior Member

    Understood- sorry to hear...

    Another trick to test the effect of tabs/wedges is to take a rope (1/4” poly) and tie it under your stern from cleat to cleat and take a test run. I did this on my 30’ lobster boat and it made a world of difference at 15kts (bow down). Old lobster boat trick for getting the bow down when you are running empty and your boat has been designed to run with a heavy load.
     
  14. Dale MacDonald
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    Dale MacDonald Junior Member

    Toolate, I saw your thread and wanted to respond regarding the Hydro-Shield, the skeg mounted hydro-foil you had a picture of. We have more information on our website, which is www.hydro-shield.com. The site includes testimonials from dealers, publications that have trialed the Hydro-Shield, as well as customers. Please let us know if we can provide you any information - we can be contacted at info@hydro-shield.com.
     

  15. Mr Efficiency
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    Mr Efficiency Senior Member

    The type that seems to have the most acceptance amongst experienced boaters is little more than a flat plate that widens the cav plate, so that when the motor is trimmed in, it acts like a trim tab. Seen quite a few of those on power cats where there is nowhere to place trim tabs.
     
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