Outboard motor skeg mounted foil?

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

  1. Toolate
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    Toolate Junior Member

    Hello all,

    Was thinking of tinkering something like this up just to create a little lift and hopefully reduce planing speeds as well as add some efficiency to outboard boats NOT I repeat NOT to get the hull up out of the water.

    There are some ideas out there already (Patent US6162104 - Hydrofoil propeller guard https://www.google.com/patents/US6162104) and after googling around this (Patent US20070207684 - Inboard/outboard motor protector with underwater hydrofoil https://www.google.com/patents/US20070207684) is the closest thing I can find to what I have in mind. Would prefer a delta shape I think to ease manufacturing/size changes that will be required to adjust something like this to the boat power/size.

    Anything like this been discussed here yet or have some thoughts to share? I am envisioning a simple mechanism to adjust angle of attack using a wrench when engine is tilted up.

    Would think a couple hundred pounds of lift would help any boat with gains being better the smaller and smaller the boat gets. An inflatable suffering from soft floor/low power being maybe the best candidate for this but also could see boats up to maybe 17’ seeing gains. Of course expecting to have you all help me realize the shortcomings of this idea....

    Thanks,
     
  2. ondarvr
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    ondarvr Senior Member

    People use them all the time, incorrectly most of the time, but still use them.

    Some motors even had them mounted on the gearcase right from the manufacturer, drag is about the only thing that was increased when they're mounted and stay below the water line at speed.
     
    Last edited: Jan 1, 2018
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  3. Toolate
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    Toolate Junior Member

    Interesting- been boating all my life and never seen one that wasn’t mounted on the cavitation plate (and thus not submerged while planing). Can you give me some examples (names/links)? Thanks!

    Did some more searching around and the only thing I can find online anyway is this (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/) which is pretty much what I had in mind but would think a angle of attack adjustment would be necessary and a better form of attachment as well.

    If you watch the video, skip to 1:00 or so and try not to pay too much attention to the host...

    Also found this product (Rock Hopper Motor Guard : Cabela's http://www.cabelas.com/product/Rock-Hopper-Motor-Guard/737581.uts?productVariantId=1192013&WT.tsrc=PPC&WT.mc_id=GoogleProductAds&WT.z_mc_id1=02926910&rid=20&gclid=EAIaIQobChMIktuvxuO22AIVVrXACh0GEAGhEAQYAiABEgLU_PD_BwE&gclsrc=aw.ds) which might be a good starting point to tinker around with an adjustable wing unless anyone says its a dumb idea (waiting for a lecture on foil efficiency/drag/adjustability that crushes my dreams :)...
     

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    Last edited: Jan 1, 2018
  4. ondarvr
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    ondarvr Senior Member

    Putting one on the skeg or the AV plate is the same if they are both in the water at speed. The at-speed part is where negative affects come into play. All those you posted a pic of tend to reduce top speed due the drag. At least when used correctly the AV mounted models are out of the water at higher speeds.

    They are all adjustable if the motor has tilt and trim.
     
    Last edited: Jan 1, 2018
  5. Toolate
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    Toolate Junior Member

    Sounds like you are saying the end result is the same for cav plate types and a submerged foil? I would think a submerged foil would generate more lift than one on top of the water but again need a higher level of education to tell me. Submerged foil would have more drag but its not top speed I am after.

    Also, I would think there would be some advantage to being able to adjust the trim of the boat to choose the best running angle first THEN play with the foil angle of attach to generate the best lift. Understanding that wave action will change/negate much of this. Talking submerged foil here.
     
  6. ondarvr
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    ondarvr Senior Member

    Many people mount one on the AV plate and then leave the motor too low, so it's in the water the entire time. There are times when the positive results outweigh the potential negative ones when doing it this way, it's up to each boat owner to balance the pro's and con's and decide for themselves, but for best performance you typically want it out of the water. Trim tabs tend to do a far better job in this area in solving the problems boat owners want help with.

    I'm sure if it was infinitely adjustable independently, and you did enough research on size and shape, you could most likely make improvements at certain speeds and under certain conditions. But as soon as you put the setup on a different hull it may all be irrelevant, those old setting and results would be thrown out and testing would start all over again. This would be a very complicated and costly setup to develop, let alone using and maintaining it in normal use. What market do you plan to target that will pay for the possible benefits.
     
  7. Richard Woods
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    Richard Woods Woods Designs

    All the sailing foilers have a set displacement and cog position. Not the case with most outboard powered boats. So I agree I think you will struggle to get a simple working system. Even if you do turning at speed may be surprising

    Richard Woods
     
  8. ondarvr
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    ondarvr Senior Member

    I always chuckle when I listen to someone like on the video say that bolting something to the skeg is going to help protect it and help keep it from breaking. Skegs are designed to break off low enough that it doesn't damage the gearcase, making the lower section stronger increases the chances that you are going to break it off up higher and crack the gearcase itself. Also they do try to make the skeg's shape correct so there's less interference with water flow to the prop, adding a bolt on steel shroud that increase it's size isn't going to help in this area.

    He also talked about his foil helping to stop cavitation in turns, the prop is ventilating in the turn, not cavitating, they are two very different things. And ventilation comes from above, not below the prop. A foil mounted on the AV plate can help with ventilation, and is their real purpose.
     
  9. ondarvr
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    ondarvr Senior Member

    That's a huge issue with the AV mounted versions that are mounted low and underwater, the handling of some boats can become downright scary when turning at speed.
     
    Last edited: Jan 1, 2018
  10. Toolate
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    Toolate Junior Member

    Either my inflatable or my 17’er :).

    I hear what you guys are saying re:steering and I have read a great deal about foiling power boats and the inherent instability (another thread going on now with heavy discussion on this) but isn’t this different in that we are just talking about maybe developing some lift at planing speeds and not trying to lift the boat hull out of the water (and thus lose the stability)?

    Def not the expert here but I have been on a large cat with a fixed foil between the hulls and it makes me think that there might be a happy medium out there with speed/efficiency gains to be enjoyed even if there are some ill effects. For example I could see my 17 being faster at cruise but slower at WOT and my inflatable planing at slower speeds even if its slower at WOT. For the 17 which has power trim on the outboard (60 etec) I could see being limited to a much smaller window of trim adjustment with a fully submerged foil on it.

    Most of what I have read tells me that this is something that will require lots of experimenting and possibly no success. I am prepared for that. Lets say I wanted to try anyway. Was thinking of maybe purchasing the stainless prop protector and setting it up to accept a home made wing (cut off the wing that it comes with) on a pivot so I could adjust the angle of attack with a threaded mount of some sort. I have a bunch of 3/8 al plate around and good woodworking skills/medium fiberglass skills. Seems to me I would be out the cost of that thing and my time. It only requires a couple holes in the cav plate on the engine too so no big deal.

    Has this been done and documented on an outboard or I/O setup that anyone knows of ?

    Also, are there other threads on this type of thing you can direct me to? Happy to read on my time rather than have you explain it all to me if its written somewhere.
     
  11. Toolate
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    Toolate Junior Member

    I cautioned you about him lol... I agree and intuitively that wing seems like it would need some level of adjustment to me- for example, if the engine its on is on a hull that always wants some negative trim then that foil might always just be stalling or if the hull required some positive trim then it might be pulling the stern down all the time....
     
  12. Toolate
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    Toolate Junior Member

    What is cog position? Angle of attack as I am calling it? (Foils are set at fixed angles relative to the waterline). Like airplane flaps with no adjustment.
     
  13. ondarvr
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    ondarvr Senior Member

    People do what you're talking about doing all the time, they use an AV mounted foil and it stays in the water at all times, this is common on small under powered boats. It's a cheap way to get on plane, or stay on plane a little easier, not the best way to do it, but common. This is where the boat owner needs to sort out the pros and cons, sometimes the cons don't bother them at all, other times they're a real problem.

    Trim tabs tend to do a better job of helping a boat get on plane, or stay on plane at lower a speed, and have almost no negative effects when used correctly. But there isn't always room for them to be mounted.

    I will say that most people don't do much fine tuning to get the boat to perform better, most bolt some gadget on and if it works as mounted the first time great, if not they claim the product is no good. To use an AV mounted foil correctly you need to mount it and try it with motor mounted at different heights and log the results, then you may need to change the prop and do it all over again. And the foil shape is really of no value on the AV model, a flat plate would do the same thing.

    Mounting the foil low, as on the gearcase, may complicate the handling at-speed issue even more, it will have more leverage than when mounted on the AV plate, so when turning it could amplify the bad tendencies.
     
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  14. Toolate
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    Toolate Junior Member

    The lack of interest or response from anyone else makes me think this horse has been beaten to death and its a waste of time haha.

    I have to say I have never seen or read about anyone running a cav plate fin/foil submerged either. Lots to learn. Been searching around the site but nothing really relative to this comes up. Anyone give me a coupe leads ?
     

  15. ondarvr
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    ondarvr Senior Member

    If you do a search on most boating sites about fins you'll see an intense distain for them, mostly because nobody does any of the work to get it dialed in. Most say something like " I tried one, my boat handled terrible, so I took it off, they're junk and don't work". These people typically have the motor mounted low, then bolt on the fin, it stay's in the water at speed and the boat handles bad. They never raised the motor to get the fin out of the water, which is how it's supposed to work.

    Another group uses them on boats like you have, they tend to like them better because there was more of problem that needed to be fixed, plus the speed is much lower.
     
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