Cold molding framing requirement & weight Issue

Discussion in 'Boat Design' started by Woodboat33, May 29, 2013.

  1. Woodboat33
    Joined: May 2013
    Posts: 5
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Miami

    Woodboat33 Junior Member

    Hello Everyone,

    Been reading many of the posts but posting myself for the first time!
    I want to build a 33' mahogany boat 10ft beam similar in shape and size to a Aquariva.
    The people I would be working with recommend a structure as attached to this email: there are three full plywood bulkheads, 16 ring frames (every 2feet) that are made of 5 layers of 10mm thick and 60mm wide strips of mahogany (height would be 50mm), a keel and chines and stringers (not sure how many stringers it doesn't show on this drawing) that will be made of 15 layers of 10mm thick and 100mm wide strips of mahogany (height would therefore be 150mm). The planking would be two marine plywood 6mm thick two layers of fiberglass (not sure the weight or type) and one finish layer of mahogany 3mm thick.
    They estimate the weight of the boat (before engines etc...)wood work only (hull, deck, structure, cockpit and cabin furniture) to be 6.5 tons.
    I wonder if this boat could not be built much lighter than that maybe 4 tons instead? Without compromising safety or strength. The boat will receive two 300 HP engines (with bravo drives) and run at top speed of 40mph.

    What are your thoughts on that?

    Thanks for the input ?
     

    Attached Files:

  2. TeddyDiver
    Joined: Dec 2007
    Posts: 2,570
    Likes: 119, Points: 73, Legacy Rep: 1650
    Location: Finland/Norway

    TeddyDiver Gollywobbler

    Heavy.. way too heavy. 2.5 to 3t might be ok IMHO. Use veneers instead of plywood..
     
  3. Woodboat33
    Joined: May 2013
    Posts: 5
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Miami

    Woodboat33 Junior Member

    Thanks a lot for pitching in.

    How many layers of veneers and what thickness would you recommend to save weight?
    How about the framing I think a lot of the weight is in those frames?
    Should they be less high or less wide?
     
  4. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 473, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Welcome to the forum Woodboat.

    Developing scantlings for high speed craft isn't something to guess at. Who ever prepared those plans has made a monolithic structure, well over convention, likely quite unnecessarily so. This suggests less than desirable engineering skills and dynamic understanding of the forces and physics involved. Also, looking at the chine, there appears to be a hollow, right at the worst spot, suggesting it's not fair and again possibly representative of a less then skilled designer.

    If you've drawn up this boat, I apologize for being so blunt. No insult is intended, but I see and have read several things that toss up red flags right away.

    For example this is the Hankinson "Miss Chris" and a 28' triple cockpit.

    [​IMG]

    The raw hull weight, sans engine, controls and equipment is a half a ton, so as you can see, even with everything you can think of, tossed into your boat, the weight estimate is grotesque to say the least.

    Plans for this type of craft are fairly plentiful. Find a well founded set and adhere to them faithfully, for best results. If you need twin V8's just to hit 40 MPH, someone's screwed up somewhere. I designed a 30' racer last fall with similar HP, though from a single V8 and it's running over 90 MPH, of course at a fraction of the weight this baby is suggesting.

    Who's designed this boat? What model, etc.?
     
  5. TeddyDiver
    Joined: Dec 2007
    Posts: 2,570
    Likes: 119, Points: 73, Legacy Rep: 1650
    Location: Finland/Norway

    TeddyDiver Gollywobbler

  6. Woodboat33
    Joined: May 2013
    Posts: 5
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Miami

    Woodboat33 Junior Member

    Yes Teddydiver, the Riva aquariva would be representative but I want it all covered with mahogany (hull etc...) and of course I don't want to reproduce the same boat I will have my own styling. But proportion and type of boat would be very similar.
     
  7. Tad
    Joined: Mar 2002
    Posts: 2,303
    Likes: 185, Points: 73, Legacy Rep: 2281
    Location: Flattop Islands

    Tad Boat Designer

    The boat below is 31' by 8', cold-molded, with twin 454's and surface drives she did 65+ knots. Full load weight was 7500 pounds. Bottom is 4 diagonal layers, 1.25" total thickness. Topsides are 1", six plywood longitudinal bottom girders...massivly overbuilt.

    Rooster01.jpg

    Rooster03.jpg
     
  8. Woodboat33
    Joined: May 2013
    Posts: 5
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Miami

    Woodboat33 Junior Member

    Thank you very much PAR, I have the feeling this is too much which is why I am eager to seek advice.
    The shipyard who gave me this drawing may have the woodworking skills to build the boat but maybe not to engineer it.

    I just clicked on your web link. Very interesting.
     
  9. Woodboat33
    Joined: May 2013
    Posts: 5
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Miami

    Woodboat33 Junior Member

    The boat I am thinking is beamier with much higher freeboard than other boats it is being compared to. See attached pics of the size and volume of what I intend to have built.
    Still even taking the beam and height in account the weight seems way off compared to Par's or your example Tad. Although from the pictures I can see double the volume of your boats maybe?
     

    Attached Files:

  10. Hampus
    Joined: Jan 2010
    Posts: 49
    Likes: 3, Points: 0, Legacy Rep: 34
    Location: Sweden

    Hampus Junior Member

    Hello!

    That does seem very heavy. I couldn't hope to add anything with more value than you already got from PAR and TeddyDriver but I'll chip in none the less.

    As a comparison to what you got here. A friend of mine is building a Didi 40, a 40 ft sailboat with a beam of 11' 2". The finished but empty hull weighs in at 600-700 kilos. Wight of finished boat (displacement floating at LWL) will be 4.75 tons according to the data from the designer http://www.dixdesign.com/didi40cr.htm.

    I attach two JPG's. They show a preliminary design that I did to finish the assignment for a lesson. She's 28 ft on deck and estimated weight is 2.18 metric tons with engine and all. Engine would be a 220Hp, 5 cylinder turbo diesel and estimated top speed is 35 knots.

    For what it's worth with errors and all, take it for what it is...
     

    Attached Files:

    Last edited: May 30, 2013
    1 person likes this.

  11. philSweet
    Joined: May 2008
    Posts: 2,258
    Likes: 150, Points: 63, Legacy Rep: 1082
    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    If you scaled Hinckley Picnic Boat to 33' and 10' beam, she'd come in about 5.5 tons all up when adjusted for your engines.

    I don't see the weight you stated in the structure, and I don't think I see it in the lwl on the picture, either. You should at least see if the displacement is correctly drawn.

    One other question, if those frames are on 2' centers, the the boat is bigger than 33' LOA. If you are talking about a 33' waterline and 10' waterline beam, then the whole thing makes a lot more sense.
     
Loading...
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.