Center of gravity
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Center of gravity

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    Default Center of gravity

    Once the center of gravity of your boat is found what do you do with that info? I guess What I am looking for is a detailed description of why my C/G is important and how to place weight in your boat according to it. Thanks SJ

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    B1 Racing cs19's Avatar
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    Knowing where your CG is at is a great thing to know. Its interesting to see how moving things around changes the CG. Another big factor is where the thrust line is in relation to the CG. Did you find CG with you in the boat?

    Tmott did some cool drawings of how the CG and thrust line work together back on the hotboat message board a long time ago. If anyone still has his drawings it would be cool to see them again.. His whole point was that when your thrust line is under or below the CG, things can get interesting and hes right. He was wrong in thinking ALL boats will crash if you run it this way though, there are dozens of fast boats I can think off the top of my head that run that way.

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    Senior Member kdub's Avatar
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    What is the preferred method for finding the CG with a fully rigged boat and driver?

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    Senior Member SlowRyd's Avatar
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    Damn really I thought for sure someone would have jumped off in here with some kinda backyard rocket science by now.

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    Pro Gas Jet RedPearl's Avatar
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    Default cg

    Quote Originally Posted by SlowRyd View Post
    Damn really I thought for sure someone would have jumped off in here with some kinda backyard rocket science by now.

    Serious !! I thought this topic would of been Handled !!!! I know Ross
    Wilder would hang boats by there front engine plate and would look for 150 pounds on the nose to set a boat up right. but that was then. Im curious of the new formula now a days.
    Don't be afraid to try something new. Remember, amateurs built the Ark...Professionals built the Titanic!

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    No I have not found the c/g yet. I Plan to weigh the hull and find c/g this weekend. Thanks for the response Chris I would really like to know more on this subject any body have those drawings?

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    Senior Member kdub's Avatar
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    Quote Originally Posted by RedPearl View Post
    Serious !! I thought this topic would of been Handled !!!! I know Ross
    Wilder would hang boats by there front engine plate and would look for 150 pounds on the nose to set a boat up right. but that was then. Im curious of the new formula now a days.
    Was that with the drivers weight?
    Last edited by kdub; 07-22-2010 at 09:55 PM.

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    Senior Member kdub's Avatar
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    Still no takers? thought somebody would be all over this one

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    Quote Originally Posted by cs19 View Post
    Knowing where your CG is at is a great thing to know. Its interesting to see how moving things around changes the CG. Another big factor is where the thrust line is in relation to the CG. Did you find CG with you in the boat?

    Tmott did some cool drawings of how the CG and thrust line work together back on the hotboat message board a long time ago. If anyone still has his drawings it would be cool to see them again.. His whole point was that when your thrust line is under or below the CG, things can get interesting and hes right. He was wrong in thinking ALL boats will crash if you run it this way though, there are dozens of fast boats I can think off the top of my head that run that way.
    I did some searching for the drawings but have been through a couple of computers since then and they seem lost. If I stated "ALL boats will crash" I was in error or didn't state the case appropriately. All thrust vectors on planing boats are below the CG - I think the only exception are air boats, displacement boats, or boats that can trim way down. The critical balance is the thrust vector relative to the CG on all three axis and hull friction from the water. Note also that as the boat changes attitude (call it pitch) the CG changes further complicating the evaluation. This is where the pad / ride plate really come into play and the reason CG must be evaluated at the boat's pitch on the pad running. The desire is to obtain the highest possible thrust forward yet maintain the least wetted surface yet still maintain control of the boat. This is why a droop snoot is viable for heaver boats even though velocity losses are taken because of the change in flow direction a better trade is made in un-wetting the hull. The secondary desire is to not have to compensate (read that as vector thrust off of forward) for a Yaw condition such as would be caused for a CG not in line with the point of thrust. Eagle really have this subject nailed down to a science in their white water and sprint boats.

    To find the CG for axis X and Z are easy, obviously, but Y becomes more of an analysis / best guess exercise. There four main contributors (engine, hull, fuel, and passengers) can be evaluated individually then put on a composit plan to find the common Y axis Center. If you needed more exact determinations this link is a start:
    http://www.grc.nasa.gov/WWW/K-12/airplane/cg.html
    Last edited by ttmott; 07-26-2010 at 01:07 PM.

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    Senior Member kdub's Avatar
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    That's what I'm talkin about!! Thanks for the info!

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    Most teams will want to rig their own boat or have oversight in the rigging so the intake, suction, bowl and nozzle angle can be set to their needs (engine, driver, fuel, etc). Production boats are a different story needless to say, consequently wedges, shoes and what-not are the corrective means. Diverters should be used only for getting out of the hole and fine trim to compensate for the varing water conditions. I'm not sure if the top teams are using diverters or not???

    As a final note the hull bottom is a critical factor on the setting of the thrust angle relative to CG. A hook in the bottom, for example, will require even more of a below CG offset resulting in not only the drag from the hook but also a nozzle flow velosity loss due to the additional downward angle; a compounding issue.

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    Senior Member wizard612's Avatar
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    One use of CG is if you are going to circle race the boat. We set the turn fin on the CG. Too far forward the boat would try to pick it's own line, too far back and it wouldn't turn well. If you haven't noticed Eliminator type tunnels that run well (read F-bomb) have moved their CG forward and have great and may I say safe attitudes. We started putting long drive lines in our Daytona Circle boats back in the late 70's.

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    Pro Gas Jet RedPearl's Avatar
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    Quote Originally Posted by kdub View Post
    Was that with the drivers weight?
    I asked dallas this question, and we dont remember Ross doing this with the driver in the seat.
    Don't be afraid to try something new. Remember, amateurs built the Ark...Professionals built the Titanic!

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    Thrust Whisperer Doc Nozzle's Avatar
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    If you want to look at equations and free body diagrams then here is a link to a lot of the basic principles of racing boat static and dynamic balance theory. Yes the articles are all about tunnel hulls so you will have to adjust your thinking to compensate. But if you want a starter on how boat static cg interplays with a bunch of other forces then this might be a good portal. At least it is better than "normal" wetted hull boat equations that aren't concerned with high speed.

    http://www.boatdesign.net/articles/t...sign/index.htm

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