Balancing Act

Discussion in 'Boat Design' started by Capt. Chris, Sep 12, 2006.

  1. Capt. Chris
    Joined: Feb 2005
    Posts: 67
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: north atlantic

    Capt. Chris Junior Member

    The Topaz 28 project is finally at a stage where it will see some water! Many of you were nice enough to reply to my posts over the last 3 years I have be working on this hull. To recap: I took a 28' Topaz twin inboard, gutted it, stringers and all, added more bow flare, changed the shear line and built up the transom to accept the bracket we built for the twin Yamaha 225's. The hull came out great and really looks like a Carolina custom express. The big question is: should I try to maintain the original water line? I've had 2 other Topazs before this one and they sit in the water just about level in reference to the chine. They also ran nose heavy underway, so alittle weight shifted aft wouldn't hurt. My plan was to sandbag where the 200 gal. fuel , water and waste tanks go, in case some weights have to be shifted around. I'm no engineer but I did assume the tanks should be moved foward to compensate for the 30" offset transom bracket and the outboards at around 550# each. Any thoughts or input is always greatly appreciated!
    Thanks, Chris

    Attached Files:

  2. Willallison
    Joined: Oct 2001
    Posts: 3,590
    Likes: 130, Points: 0, Legacy Rep: 2369
    Location: Australia

    Willallison Senior Member

    Without doing a proper detailed weight study it's impossible to tell where the LCG (and hence LCB) will lie. So in turn there's no way of really knowing where the at-rest waterline will be.
    But there are a few points worth making.
    1st up - fuel and water tanks are a poor means of balancing weights - obviously as the tank contents vary, so too do their mass. It's generally best to locate them as close to the LCB as possible. Which of course is a bit of a catch 22 here as we don't know where that is!

    Do you know where the LCB of the boat as it was originally designed to be rigged is?
    If so, you can do a simple weights/moments table to establish where the new one will be, by subtrating the moents of the old motor location and adding in the new one(s).
    If not, it's pretty safe to assume that you've shifted the LCG aft by a bit less than your 30 inches. I'd whack the boat in the water, move a few weights around 'till you're happy with how she sits, then take it from there. You have to be careful not to move the CG too far though - fwd could induce dynamic instability, too far aft and you might start porpoising at high speed....
  3. Capt. Chris
    Joined: Feb 2005
    Posts: 67
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: north atlantic

    Capt. Chris Junior Member

    Will -
    I don't have the original LCB and the folks that manufacted this hull have long been out of business, so there's no one to call. Can I find the LCB by jacking and blocking the hull along the keel until I find the balance point? Once the LCB is found what's the best way to lay out the weights? My main concern is the fuel tank as it will be about 200 gal. Thanks for your input.

Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.