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Skid Fin Location

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  • Skid Fin Location

    I’m working on a small unmanned boat that has a length of 66 inches overall and a beam of 20 inches. It is a padded V hull with a small waterjet propulsion system in it that should be capable of speeds in the 25kts range. The hull is a 20 degree v and the pad is 4.25 inches wide and the weight is between 70 and 84 pounds depending on fuel load.

    In recent testing the boat planes off nicely and zips along just fine up to about 18 mph. At that point it is riding pretty high on the pad. If we go to steer it at high speed just spins out. The boat does roll a very small amount, but it’s not a digging in and rolling up like a proper runabout should by any means. Since this is a waterjet there isn’t anything like an outboard fin in the water, nothing at all, and that’s obviously why it’s spinning out. The customer has some lower speed experience, but the plan is to go 25kts at which point this thing is going to barely touching the water based on what it’s doing now at 15 knots.

    Obviously we need some sort of fin setup to keep this thing in line.

    As we are getting to higher speeds the waterjet is also getting ventilated on small bumps, so I was thinking I’d configure a set of fins similar to my roll up “D” runabout (center fin a bit forward of the waterjet and a rear chine fin on each side in the back since I have to turn both ways). I was thinking that a fairly shallow fin (1" deep) and maybe 6" long forward of the waterjet but behind the CG location and a couple of 1" deep and maybe 4 inches long chine fins would do it.

    Does anyone have any experience with fins on V or padded V hulls that can help steer (pun intended) me in the right direction? Remember there isn’t anyone on board, so you can’t “help” the boat take a set by shifting weight forward and to the inside of the turn, it’s just got to turn in and roll up by itself, which isn’t the way a classic roll up works.

    I figure if anyone knows how to make a boat turn without spinning out it's the guys here, so let me know what you think.




  • #2
    Only one place to put it and that's on the transom on both outer edges and perpendicular to the bottom. Fin needs to be 2-3" long. Any RC boat forum and you'll see examples. Center fin like a flat bottom won't work!
    Chris Fabbro 2-F CSR / Carson Fabbro 95-F 25ssr, CSR, CSH, 300SSH / Greg Fabbro 63-F CSR, 25SSR

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    • #3
      Originally posted by Chris 75F View Post
      Only one place to put it and that's on the transom on both outer edges and perpendicular to the bottom. Fin needs to be 2-3" long. Any RC boat forum and you'll see examples. Center fin like a flat bottom won't work!
      Ok, that sounds like a plan. We'll give that a try.



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      • #4
        In looking at the R/C stuff, most of those monohulls are deep V's all the way to the transom. I can see a fin setup like Chris noted working with that. With that setup you have a splitter surface running the length of the hull and the hull will always roll when you turn. Our hull however has a pretty good sized pad, and at speed we are way up on the pad. In fact, when it corners it is cornering flat, with almost no roll to the inside. Not sure, but this thing is more like a flat bottom boat in that there is very little tendency for it to roll into a corner. At lower speeds it does roll and handles nicely when it does, but once it gets high up on the pad it's acting like a flat bottom hull, in that it corners flat and then spins out when we try to steer very much at all.



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