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Thread: Sport C tunnel boat

  1. #21
    Team Member DeanFHobart's Avatar
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    Quote Originally Posted by zul8tr View Post
    Jim Russell's TBDP Tunnel Boat Design Program explained here, left click on the detail pages on the right for further insight:

    http://www.aeromarineresearch.com/tbdp6.html

    Also lots of good articles here as well
    Thank you, I will check it out.
    Dean Hobart

  2. #22
    ExperiMental BYOBoat's Avatar
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    So I've been busy racing and tuning, haven't posted for a while. The boat and setup really likes rough water on medium length courses. I'm still maxing out around 54mph on the straights, just not enough for the larger courses. I did see 60.1mph with a 20 pitch signature prop I've been tinkering with. Definitely need to make some changes though because it took 2 miles and a headwind to wind up to that speed. I hope to test a couple props Ron sent up this weekend.

    I also want to add a step to the bottom. Other sport c's around here have one. My corner speed is good, but acceleration out of the corner seems a little slow. I'm thinking 20-24" ahead of the sponson end. I want to target the 40-45mph range, which seems to be just over 2' of wetted length. I haven't found much info on swept forward vs swept back step shapes. Anyone know which is better, or which is the most common?
    mini tunnel boats for sport, now an experimental Sport C added to the fleet.

  3. #23
    ExperiMental BYOBoat's Avatar
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    Quote Originally Posted by Hounddog View Post
    Nice to see a picture of Sammy Miller's #7 SST 60. Sam passed away and we miss him.

    We started in Sport C in 1992 when the class first appeared in Ontario Canada. We were totally all new to the class and there was no one that had Tohatsu engine knowledge, boat set up or prop info. We only had a APBA rule book and the class record sheets. We purchased a Mirage TOM CAT 2 from Wayne Worthy [his sons]
    Here is what we did have:
    - a 14 year old boy who could flat out drive a boat on the edge
    - a boat builder Rob Scythes who was a superior craftsman
    - a prop guy named John Beer who was an artist
    - a dad who had been racing since 1964
    Circle racing is about Lap ET time not straight line speed. Acceleration, cornering speed and especially exit speed out of the turn are MOST important.
    We never had the opportunity to have a Mike W. motor or any professional engine build.
    We focused on the boat set up and the prop selection.
    You drive a Sport C WOT all the time. Never back off because it takes too long for the boat to get back up to speed. No need to trim in on the turns either.
    Properly set up the boat will spill enough air on entry into the turn. Just trust the boat.
    Many add 4 to 6 inches of set back to carry the nose. Problem is the nose flies up on entering the turn. We never added set back.
    You DRIVE the boat into the turn and thru the turn.
    Here is what we did to the boat:
    - we straighten and corrected all the flaws in the hull. There were many!
    - we reworked the tunnel with a wedge panel to provide more lift to the stern Big improvement!
    - we added a series of 3 steps on the sponsons to help it thru the turns. Boat was QUICKER thru the turn.
    We tested again and again making small changes in the boat set up and reworked MANY props. We took a year.
    We ended up with a very competitive boat that was easy and predicable to drive.
    The wedge panel to lift the stern, was this in the tunnel? Did it compress the air, or extend the tunnel to carry it farther aft? I've seen vertical extension flap kind of additions on the tunnel sides to hold air in all the way to the transom, been thinking about adding some so the bow doesn't rise so much with every wind gust.
    mini tunnel boats for sport, now an experimental Sport C added to the fleet.

  4. #24
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    Quote Originally Posted by BYOBoat View Post
    The wedge panel to lift the stern, was this in the tunnel? Did it compress the air, or extend the tunnel to carry it farther aft? I've seen vertical extension flap kind of additions on the tunnel sides to hold air in all the way to the transom, been thinking about adding some so the bow doesn't rise so much with every wind gust.
    Rob Scythes reworked the last two feet of the tunnel with the panel to compress the air. It did not extend outside the boat. It was done so clean that you could not tell it was added.

  5. #25
    ExperiMental BYOBoat's Avatar
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    Right now my tunnel/wing section has 1 deg of angle built in, so it's 1.5-2" height difference between the transom and front of the tunnel, depending on where I measure to. I'm wondering if I need more compression. Maybe wedging down the last couple feet of tunnel more than my 1deg? Or could I widen up the center pod aft to compress the volume, or bring in the inside of the sponsons? My tunnel is on the tall side, but it handles well in rough water.
    mini tunnel boats for sport, now an experimental Sport C added to the fleet.

  6. #26
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    Default Tunnel forward shape

    Most of the tunnel boats I've seen, and stock outboard hydros for that matter, use the lower shape. The upper shape is more of a NACA foil. TBDP and a couple other places suggest it. Both are flat on the bottom for most of the length. Airfoil camber, S tunnels, angle of attach/compression are all other variables, but I'm really curious about the forward area. The upper should stall later, blow overs more likely? Does one shape make more lift than the other, or change where the center of lift is? Any other benefits to one over the other?
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    mini tunnel boats for sport, now an experimental Sport C added to the fleet.

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