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  • Fins on the front or back

    Makes me wonder why we put our so called stabilizer fins on the front of our decks instead of the rear like jets,or darts, rockets, or even arrows?

    0b3428e2d96ec2e4744de57b9bf0e74e--cool-boats-tractors.jpg Lone-Star-exterior.jpg boats_in_the_belfry_Page_09_Image_0003-1024x768.jpg 1957Herters_big.jpg
    bill hoctor likes this.

  • #2

    Re: Fins on the front or back

    Assume you are talking about the fences on the deck between the front deck and the sponson deck? If so they are there to act like a stall fence on the wing of a swept wing jet. They attempt to stop the relatively high pressure under the hull from spilling up and interfering with the lower pressure over the center portion of the hulls deck which in theory reduces induced drag & increases lift. If you mean something else, forget what I said. Ha! Wingtip 'fins' on jet wings do the same.

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    • #3

      Re: Fins on the front or back

      But, on a wing, air flows outward, especially near the tip. This is what causes the vortices. Winglets trap that air and force it to stay perpendicular to reduce the vortices and increase efficiency.

      The ''wind fences'' on the hydros keep the airflow perpendicular to the hull and flowing over the airfoil shape of the main body in a similar manner.

      However, due to the dimensions and weight distribution of the hydro, those wind fences also have some effect like that of trying to fly an arrow backwards.
      modsquad and john527 like this.
      Dane Lance
      700-P
      CSH/500Mod

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      • modsquad
        modsquad commented
        Editing a comment
        most hydros today dont have the nice CLARK-Y airfoil shape like they use to, HEDLUND,BUTTS AERO WING to name a few. Most seam to be flat similar to a sheet of plywood, no aerodynamics which makes sense why we have to kick our motors under to keep from blowing over. Best way to explain would be hold a flat piece of plywood out the window going 60 mph,angle it slightly up or down and see how fast the wind wants to rip it out of your hands, now do the same thing with an airfoil (model airplane wing) and see how much more you can angle it without the wind ripping it out of your hands.

    • #4

      Re: Fins on the front or back

      I got 99% of my non-math aerodynamics flying early swept wing jets like the F-9 Cougar (which had a fence to stop spanwise flow because that's how engineers at the time thought to fix it) the Russians did the same with MiG 15's, 17's, and 19's. The F-4's, A-7's, and F-16's I flew for the next 30 years just overpowered the aerodynamic penalties of spanwise flow. Airlines have winglets for fuel efficiency and the sneaking of 'higher' pressure underwing air to the upperwing at the wingtip does create vortices which rob lift and create drag. I want my hydro 'fences' to keep flow over the deck from bow to transom for lift. Jack Stotts, help me out here. I fly better than I write!

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      • #5

        Re: Fins on the front or back

        Clark Y airfoil
        images.jpgimages.png

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        • Jack Stotts
          Jack Stotts commented
          Editing a comment
          Mike, it sounds good to me. Jack

      • #6

        Re: Fins on the front or back

        Actually, given that the airfoil shape on the hydro is limited to the main body section, but is surrounded at the leading edge and side by the sponsons (unless it's a drop sponson like a Hemp style), it would make more sense for the wind fences to be located further aft to maintain directional flow over the mid to rear section.

        Also, the airfoil portion of the hydro's body has a very low aspect ratio (long chord (distance from leading edge to rear) and a narrow width, or span), which in aerodynamics means high induced drag with poor lateral stability.

        In other words, they're muches betterer at being a boat than an airplane.
        Dane Lance
        700-P
        CSH/500Mod

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        • #7

          Re: Fins on the front or back

          The thickest portion of the 'airfoil' on my Hemp hydro is adjacent to the steering wheel, thus the lowest pressure of the airfoil. Sam Hemp designed this with that in mind and his fences are highest there. I'm glad....plenty of lift in my boat. As to longitudinal stability, I think it is affected most by not having enough of the bottom 'strakes' in the water because of too much trapped air and engine jacked too high to keep skeg from being effective.

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          • #8

            Re: Fins on the front or back

            Here's one for the guys who have been at this game steadily for a long time:

            My opinion, based on nothing more than what little I know about aerodynamics, is that modern hydros with picklefork bows are not inherently more "slippery" (low-aero-drag) for having that feature than the last of the old round-bow hydros. I see the cutaway bow as simply a means of making the boat less sensitive in the pitch attitude, thus less likely to blow over when the boat takes a bounce or catches a wind gust. (Of course they still DO blow over sometimes, just because they are very light boats going very fast).

            So ultimately what I want to hear about is whether you old guys think that (say in the smaller Stock classes) if a guy got the plans and built a new copy of one of the last good round-bow hydros, say a Craig-Craft, and built it with the lightweight woods, was on weight, and had a good engine and modern prop, could he be fully competitive with the newest-style boats? And if not, why not?

            Just to keep the discussion related to aero deck-fences, I believe that when Mark Demeray won the alky Nationals in 125ccH in '73 with a Jim Hallum Yamaha, his Craig-Craft had deck fences.



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            • #9

              Re: Fins on the front or back


              So ultimately what I want to hear about is whether you old guys think that (say in the smaller Stock classes) if a guy got the plans and built a new copy of one of the last good round-bow hydros, say a Craig-Craft, and built it with the lightweight woods, was on weight, and had a good engine and modern prop, could he be fully competitive with the newest-style boats? And if not, why not?
              Not a chance. The biggest advantage of contemporary hulls is turning ability - corner entry without needing to settle the boat resulting in higher speed in the turn and especially corner exit speed. Top speeds may not be much different but lap times are better.

              Comment


              • modsquad
                modsquad commented
                Editing a comment
                I don't think most boat builders of today would have the patience to build an old school hydro with a nice compound contour deck(airfoil) and beautiful wood finish. But it sure would be interesting to see a lite HEDLUND or CRAIG CRAFT with a good motor,prop and driver, I think a lot of people would be surprised!

            • #10

              Re: Fins on the front or back

              John Runne has the patience to build a boat with a beautiful wood finish
              Perfect IMHO
              Very interesting thread BTW- THX
              522-P



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              • #11

                Re: Fins on the front or back

                I've seen some pretty fast round nose boats in CSH, 300ssH, AMH, FAH, AXH, and JH...

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