weight v speed

Discussion in 'Powerboats' started by gtflash, Nov 18, 2013.

  1. gtflash
    Joined: Jun 2006
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    Location: uk

    gtflash Senior Member

    I am planning new deck arrangement for my boat, and wondered if there is a way to work out how much speed will be lost by the extra weight of new deck arrangement.

    Is there a calculation, someone simple like myself can use. Boat is 8m long planning hull with twin stepped hull. It is an open boat with centre console, and the new deck will add around 150-200kg as a closed bow. Speed as is is 75 ish mph with 250hp outboard.

    Thanks GT
  2. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Weight is critical on high speed craft, especially those with bottom configurations that permit high speed running, like yours.

    What is the current weight of your boat? What is the make, model and year? How is it presently equipped?

    I'll assume your boat is quite light, say 2,700 pounds or so. An increase of 200kg will knock off a few MPH at the top end, lets say 3 or 4. If this is going to be at the very front of the boat, she'll also come up on plane a little slower and at top speed, she might have some handling differences (she'll feel "muddy" at times) and you might have to change trim to keep from porpoising. The very bow is the worst place to add weight to a high speed boat.
  3. philSweet
    Joined: May 2008
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    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    Yes. Get a barrel and a hose and put 200Kg of water in it and go for a spin.
  4. gtflash
    Joined: Jun 2006
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    Location: uk

    gtflash Senior Member

    approx weight with engine is 1200-1500kg, but not sure. she is considerably lighter than the previous smaller version of that boat i owned. it is a scorpion rib. http://www.scorpionribs.com/

    deadrsie aft is 24deg, and fore is 75degrees. early rib racers in uk discovered an extra 4-10mph by closing the bow, via covers or decks, inlcuding my own boat. one report noted increased bow steer, but on a different model.

    Current centre console is 3/4 aft atleast, with 2 seats and a 250hp engine. I want more seats so have 2 choices, move centre console forward 18" and then fit another pair of bucket seats, or fit a traditional closed bow boat deck, which would add the 150kg but balanced overall.

    I have removed over 100kg of very poor stringer repairs and replaced with light weight balsa and epoxy skins.

    I was intending to load her with bags of sand, but thought id check on the theory 1st, the closed deck is not cheap but is ashetically pleasing

  5. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    Good advice that, and a cheap as chips way to get the query answered too ! Just make sure the 200 litre drum is firmly anchored in place, otherwise could get a bit hectic if it breaks loose.
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