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Thread: Sport C tunnel boat

  1. #11
    ExperiMental BYOBoat's Avatar
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    Since the expert's here, I have another question. Any ideas on how long to keep the sponsons straight, compared to what TBDP predicts for wetted length? Right now I've taken my lowest corner exit speed, added a little to that wet length, and then started the bow curve. I suspect longer sponsons will help in rougher conditions. I'm also trying to keep from dragging too much over the water when the boat comes out of the corner with little positive trim. I'm sure handling is affected but prop height and setup should keep it manageable.
    mini tunnel boats for sport, now an experimental Sport C added to the fleet.

  2. #12
    AeroMarine Research Jimboat's Avatar
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    Quote Originally Posted by BYOBoat View Post
    Since the expert's here, I have another question. Any ideas on how long to keep the sponsons straight, compared to what TBDP predicts for wetted length? Right now I've taken my lowest corner exit speed, added a little to that wet length, and then started the bow curve. I suspect longer sponsons will help in rougher conditions. I'm also trying to keep from dragging too much over the water when the boat comes out of the corner with little positive trim. I'm sure handling is affected but prop height and setup should keep it manageable.
    BYOBoat - it sounds like you've got a pretty good handle on using TBDP to optimize your performance. Good job! I think you're on the right track with the ideas you're following in your explanations above. I presume that when you say "straight" sponsons, you mean the "flat" aftward section, prior to where the sponson bottoms must curve up to meet tunnel roof and deck height? TBDP will tell you what the sponson wetted lengths will be throughout the velocity range. Water conditions can wet more area inadvertently, however. Longer sponsons will certainly be more forgiving in rougher water, but are exposed to unwanted wetted area (drag) in the lighter water. It's a compromise based on what you think your most frequent operating conditions are likely to be. If you utilize longer "flat" sponson lengths, another alternative is to increase deadrise further forward.

    If you want to discuss, just give me a call anytime.

  3. #13
    ExperiMental BYOBoat's Avatar
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    TBDP is an awesome tool. So far it's predictions and actual boat response have been really similar. The program is so much easier/quieter/cheaper than grinding and sanding!

    The boat runs much better now. It isn't stuck to the water at corner exits any more. Top speed is a little shy, but corners and accelerations are better.

    Currently my center pod (it's sticks down about 1" from tunnel roof) matches the tunnel angle inc of 1deg. Tunnel height is 6". I'm wondering if that is contributing to my current porpoise challenges from about 39-48mph? I don't really want to cut it down and expose more of the gearcase to spray. My prop height is good for straight and corner rpm, I still have cooling, and handling is just enough to prevent spinouts. The program shows porpoise should reduce consistently from 33mph on. It's difficult to tell but I may also have a very slight hook in the aft end of the pod. I forgot the GoPro so I didn't get footage of the transom today. I assume the pod is well above the water but may be seeing a lot of spray?


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    mini tunnel boats for sport, now an experimental Sport C added to the fleet.
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  4. #14
    ExperiMental BYOBoat's Avatar
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    Another question, does anyone know how much HP a good Sport C motor really makes? Or even better a typical OEM motor at racing rpm's, and full HP when race prepared?
    mini tunnel boats for sport, now an experimental Sport C added to the fleet.

  5. #15
    ExperiMental BYOBoat's Avatar
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    Fixed a little sag in the tunnel aft end from initial build issues. Also adjusted the center pod bottom to be parallel with keel line instead of tunnel 1* angle. Boat is running better. Still have some porpoise at 40-47mph range. Moving weight aft calmed it down but still an issue. Down wind is worse than up. I don't have access to a prop with more bow lift at the moment, but I'm gonna try changing setback next day it's not raining and blowing.


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    mini tunnel boats for sport, now an experimental Sport C added to the fleet.

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