Weight Distribution In A Small Boat

Discussion in 'Sailboats' started by SuperPiper, Jan 26, 2014.

  1. SuperPiper
    Joined: Jan 2003
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    SuperPiper Men With Little Boats . .

    Take a look at my small boat in the attached pic. The literature claims that this boat weighs 1200 lbs. A sister-ship was weighed on a highway scale and her weight was calculated to be 1710 lbs without outboard and gas tank, cruising gear, anchors, etc. The keel ballast is 300 lbs of lead.

    Now add an outboard motor on the transom, a gas tank and battery in the lazarette, and 2 adults in the cockpit. The boat's centre of gravity is nowhere near the keel. It's probably somewhere in the cockpit.

    The Bible tells us to pack additional gear close to the keel to minimize the gyradius and to prevent excessive hobby-horsing. For this little boat, are we better to store gear in the quarter berths? Under the cockpit? In the lazarette? Or as far forward as possible to correct the trim of the boat?

    It's hard to keep the transom from dragging in the stern wave.

    What are your thoughts?
     

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  2. Doug Lord
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    Doug Lord Flight Ready

    Just looking at the picture I'd guess that storing gear under the window and keeping the crew as far forward in the cockpit as possible might go a long ways toward fixing the trim. I'd try not to separate(fore and aft) the gear and the crew any more than absolutely necessary.
     
  3. gonzo
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    gonzo Senior Member

    Many of these small boats squat when loaded with an outboard, tank and batteries aft. You could load as much gear and supplies close to the bow and sit forward in the cockpit.
     
  4. SuperPiper
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    SuperPiper Men With Little Boats . .

    Hobby Horsing

    Here's a photo of one of these little boats sailing on Lake Erie near Pelee Island. You can see the leading edge of the keel coming out of the water. The chop is only 3 - 4 feet (1.0 - 1.5m).

    The boat has its vee-berth loaded with camping gear, has 2 athletic adults in the cockpit, plus an outboard on the transom, and gear in the lazarette. The load is spread from anchor locker to transom.

    I think the angled bow does a lot to promote hobby-horsing, too.

    It is this photo that prompted the original question: place weight near the keel, or place weight near the cockpit?
     

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  5. gonzo
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    gonzo Senior Member

    If the boat is squatting, the load should be as far forward as possible. To minimize pitching, the load should be as spread out fore and aft as possible. If you concentrate the load in the center, the ends will be light and tend to move up and down more.
     
  6. gggGuest
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    gggGuest ...

    Its always a compromise with boats, but I'd sort out the trim first and put the weight distribution strictly second. If you're heavy in the ends you won't go upwind through waves as quickly and you'll hit the waves a bit harder, but to mind better that than dragging a fat stern through the water all day.
     
  7. Skyak
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    Skyak Senior Member

    Just the other day some rummy on another thread was harping about mass located away from the center causing uncontrollable pitching. He wasn't worth correcting.

    Locating mass close to the center INCREASES a boats ability to raise it's bow over waves. It is something you should do if you don't like how your boat is plowing through waves. You appear to have the opposite problem and that area frequently has a sharp chop. I sailed many times from cedar point to put in bay. It gets terrible on weekends. By all means, move some mass forward. It will change the trim bow down and increase the rotational inertia. Note -it is not certain that it will reduce pitching, only that it will reduce the natural frequency of your boat. If the wave frequency is closer to this reduced natural frequency (depends on waves, heading, and boat speed) it can increase amplitude. You can test it by sending one crew to the v berth while maintaining a heading, better or worse? If it's better pack some weight under the v berth.
     
  8. SuperPiper
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    SuperPiper Men With Little Boats . .

    Good replies. Excellent dialogue.

    Here's a brain teaser. There are 300lbs of lead billets encapsulated in the boat's dagger board. If the ballast was put in a bulb on the bottom of the keel, the weight could be reduced for the same righting moment.

    Reducing the keel weight would further shift the centre of gravity aft and require MORE weight distributed forward.

    Is it possible that the weight reduction in the keel could result in a net increase in boat weight to correct the trim?
     
  9. boatbuilder41
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    boatbuilder41 Senior Member

    I could say much about a sailboat, honestly. don't build them and never had one, I'm just more comfortable sitting on even or near even keel, and sailboats are not sitting like that unless its in the slip. but I'm following to learn myself
     
  10. Skyak
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    Skyak Senior Member

    Maybe possible in a super stupid constrained case. Realistically you have supplies/cargo of varying density that can and should be moved.

    I think internal ballast shifting would be a superior performance improvement to a lighter bulb.
     
  11. JCherubini2
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    JCherubini2 DianaOfBurlington

    Don't add weight to the ends.

    What should appear the most logical solution is that all added weight in a small boat should be located, longitudinally, as close to directly over the keel as possible. Any weight added to the ends will upset trim and promote hobbyhorsing. That's just a fact of physics.

    Further, all weight added to a boat should be as low as possible. I added two relatively large flexible water tanks and two Group-24 batteries to my Hunter 25, one of each under each main-cabin settee berth. Depending on how I load the water tanks, this amounts to 300-odd pounds right in the very best possible place-- and, though substantial, this goes far to stabilize the boat in the water and minimize the effect of (much less) weight in the ends. (In effect the tanks are water ballast-- but let's not get into the myth that having only water ballast is in any way better than having positive stability in a proper weighted keel.)

    The size of the boat means less than its design-inherent susceptibility to an out-of-trim condition. For example all my dad's designs are vulnerable to poor added-weight distribution. I caution all Cherubini 44 owners to not add any weight aft of Station 8 (on this boat, the mizzenmast step); yet they all do and as a result most people's 44s have the rudder fiddle underwater (when it should be fully within the bootstripe). The same goes for adding two all-chain rodes and a bow-thruster battery forward, which cause the bobstay to be underwater (when it too should be fully within the bootstripe). Seems too many of us can't resist filling up all that extra space. It's not a basement, friends! Keep all the weight out of the ends that you can. ;)

    * * *
     
  12. JCherubini2
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    JCherubini2 DianaOfBurlington

    You should probably not seek to either lessen or increase a small (daggerboard) boat's weight. Lowering the board would indeed increase the righting moment, the more so for having more weight in its very bottom. But I do not know what you are asking about lightening it and moving the CG aft. The boat's displacement remains the same so long as it is in the water, board up or down; only the righting moment (and sailing characteristics) will be altered. If the dagger board truly moves vertically (or close enough), then the center of gravity isn't (markedly) changed longitudinally. The boat just becomes tipsier because the center of gravity has been raised.

    A boat with a swing keel is another animal altogether, because as the keel is raised or lowered it moves in an arc. Some designers will balance the boat with the keel up; though they should do so with the keel down for better sailing performance. As you adjust the depth of the swing keel, the CG, along with the center of lateral resistance (CLR), will change fore-and-aft. Given that most swing-keel boats are small and otherwise very light in weight, this may requires the shifting of crew and gear to minimize weather helm and to keep the bow from lifting or stern from squatting as the keel comes up (the bulb moving aft).

    JC2
     
  13. Baltic Bandit

    Baltic Bandit Previous Member

    All small boats hobby horse in chop. Hobby horsing is a function of the resonant oscillation frequency of the hull being close to that of the waves you are driving through. A finer bow entry is likely to exacerbate this a bit but that's not the cause. the cause is that the boat is light and has weight in the hull and not the board.

    That said, I would not go rejiggering the centerboard unless you really know what you are doing. because the CB trunk is designed for the loads that CB is going to put on it. lets say the CB is 3' long so by adding 100# of lead to its bottom, you would be able to get rid of the 300# in the bilge. BUT you would also have more than tripled the lateral loads on the board and the CB trunk. so unless you are going to re-engineer and rebuild the whole CB trunk stay away from that change.

    what I would do is
    a) move the battery forward into the cabin. and re--run your running lights to it there. If you are using the battery to start the motor - stop and get a different motor
    b) get the smallest motor possible. The Tohatsu 2.5hp should drive this thing at pretty much hull speed anyway. and that weighs less than 40#. Then I would store the remote tank forward in the cabin.
    c) when sailing, unmounts the outboard and put it down below (easy with the Tohatsu)
    d) put nothing. Not one thing. Not even a water bottle, in the lazarette.

    But basically, its a small boat, with a somewhat pinched waist. and no weight in the keel, She's going to hobbyhorse not much you can do about it. there's a reason more modern daysailer designs like the Viper use a vertical cb with a torpedo of weight at the bottom.
    1) its lighter
    2) its more RM for the same weight
    3) it increases fore/aft angular momentum dramatically and thus reduces pitching.
     
  14. SuperPiper
    Joined: Jan 2003
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    SuperPiper Men With Little Boats . .

    What about a spray deflector? Will it reduce hobby-horsing?
     

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  15. gggGuest
    Joined: Feb 2005
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    gggGuest ...

    Not so as you'll notice...
     
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