Catamaran Hull Modification Question... bow change

Discussion in 'Boat Design' started by ryanonthebeach, Jun 5, 2016.

  1. ryanonthebeach
    Joined: Mar 2007
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    ryanonthebeach Junior Member

    My old cat has a severe under-bite, wasn't such a bad thing in her day, these days it's just not cool. She wants more than just cosmetic surgery, put a new smile on things.
    Someone with the same boat has extended the bows so that they drop straight down instead of an under-hang. He likes the performance improvements and is pleased with he change overall.

    Questions for the pro's :
    He modified the sail plan by moving the mast forward a few feet, added a seagull striker and put a trampoline in (it has a solid for-deck)

    That's a lot of modification... which of these do you think are actually necessary if I extend the bows? can I get away with just extending the bows?
    What advantages would the underhang actually bring?


    Original (my) hull
    [​IMG]

    Modified hull
    [​IMG]
     
  2. Richard Woods
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    Richard Woods Woods Designs

    On a HT you would be much better increasing buoyancy aft. The bow is very full. So Pat fined it down when he did the Ocean Twins 33. Too much, so a couple of owners have made new bows but not nearly as extreme as Grey Dove. I don't recommend doing that

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
  3. petereng
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    petereng Senior Member

    The original reason for such a curved bow is so when you hit something the bow rides up onto it vs a straight bow in which it will just stop suddenly. The curved bow will endure less damage then a straight bow in such a case. By using a straight bow you maximise waterline length and gain boat speed via the longer hull. If you have the rig to achieve that speed. You need to estimate your weight distribution and match that to your volume distribution. As RW says you may need to add volume aft to balance the additional vol fwd. cheers Peter S
     
  4. ryanonthebeach
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    ryanonthebeach Junior Member

    Thanks for the reply Richard & Peter. Your comments certainly make sense, and did cross my mind. Looking at the hull shape.. It more kayak shaped than anything you'd see these days. Looks like modern cats have flatter, shallower, broader transoms at the waterline.

    What is the reason for this as apposed to a kayak shape?

    [​IMG]

    I guess it would be a rather large project to do all 4.... was looking for excuses to practice GRP skills... am completely gutting refitting / refurbishing the old girl. So while she's all striped down and out of the water was considering improvements, Just thinking about it at this point.
     
  5. petereng
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    petereng Senior Member

    The kayak shape is an early shape based on canoes. Its also easy to shape with timber. But its has a low prismatic co-efficient. Around 0.5. You will have to look that up. If the bow and the stern are the same shape it has no pitch damping. So stern transoms are made flatter so as the boat pitches this flat damps its rocking. Plus motors and heavy things tend to be at the stern of a cruiser so you need more bouyancy aft. Also look up the sectional area curve. The boat has a distributed mass and the distributeded volume must equal the distributed mass for it to float flat. So depending where the boat designer put various things especially heavy things the hull needs to have the same volume to support that mass at that point. With this boat heavy bits need to be at the middle of the hull as his is where the volume is. Very modern shapes use axe bows and sterns to give increased volume at the ends to give high prismatics and by being deep at the ends they stay in the water reducing slamming and giving maximum WLL. If its a fast cat the bow is raised , called a spoon bow to allow the water to go under the bow rather than around it. Like the AC45s. Peter
     
  6. Richard Woods
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    Richard Woods Woods Designs

    Personally I wouldn't change the boat, it won't be cost effective for one thing.

    I knew Pat well, my office is in the Patterson owned Multihull Centre

    In front of my office is another Ht, one that has made 6 (or is it 8?) singlehanded transAtlantics. another has sailed round the world. Accept it for what it is. A small seaworthy, albeit slow, cruiser. Don't overload it

    Richard Woods

    www.sailingcatamarans.com
     
  7. petereng
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    petereng Senior Member

    Agreed better to go on a series weight reduction campaign vs changing the hull. Peter
     
  8. ryanonthebeach
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    ryanonthebeach Junior Member

    Richard & Peter

    I do really like the old girl, wouldn't be "investing" as much in rehab otherwise, she is slow but also very sturdy.
    Thanks for the advice, appreciate it.
    I'll skip hull modifications and do some of the other projects I had in mind.. like a wing mast and foils... just kidding... maybe something more mundane like ripping out the old galley counters and replacing it with a lighter brighter version.
    I had been on a weight reduction kick for a bit, replaced the old lead acid batteries with lifepo4 (from an old medical cart) and taking the heavy old charger/inverter out. I'll see what else I can trim..
     
  9. Stumble
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    Stumble Senior Member

    I would also consider doing a sail plan update. A square headed main and code zero would probably do a lot more than worrying about the hull shape.
     
  10. EL_LEQUE
    Joined: Jan 2019
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    EL_LEQUE New Member

    Thanks for sharing.
    I am also modifying my catamaran Endeavour/Hutchin/Com-Pac 30’.
    Similar to SYMONS 30 and AMERICAT 3014.

    OAL 30’
    LWL = 27’ ,
    Beam 14’
    Each hull beam 4.5’
    Under bridge beam 5’
    Draft is 3.5’ of which 0.5’ is flat keel,
    Keel overall lenght is 9’

    Submerged central hull is only 20’ x 2’ x 2’ (x 0.65 prismatic) = 40 Cu-ft = 1.5 ton x 2 hulls = 3 ton Approx as Displacement.

    I used 20’ instead of 27’ because fore and aft edges are barely submerged couple of inches. I Used 2’ median beam and draft due to prismatic ratio.

    My problem is having only 0.5’ clearance under the bridge, so much pounding, even at anchor, marine growth etc.

    My intended solution is to raise the bridge about 12”; by modifying hulls to gain buoyancy.
    The displacement of the top 12” of the underwater hull could reach: 3/4 ton each hulll, total 1.5 ton ( 50% of actual displacement)

    Enlarging the keels yo a square section of 1’x1’ ( made of foam laminated with GRF, distributed on both sides of each keel):

    Added buoyancy Aprox:
    9’ x 1’ x 1’ = 9 Cu Ft = 1/4 ton x 2 hulls = 1/2 ton .... not be enough ( that is the same weight of my two sweet water tanks)

    How else do you think I could I rise my bridge clearance?
    And....
    How will a square, or inverted triangle additional keel volume affect navigation?
    1. As keel are centered over the LW; with added center buoyancy, pitching will increase.
    2. What about side dragging when sailing?
    3. What about tracking stability? On sail and on engine?
    4. Of course water resistance and fronts wet area ( not the sides) will increase.
    5. How would that affect narrow rudders behind the keel?
    6. Water pressure against a non rounded hull will exert unequal forces on the existing structure
    7. What else could you foresee? Or correct from above?

    Your thoughts and help would be appreciated.
    Rod.
     
  11. bajansailor
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    bajansailor Marine Surveyor

    Eric Sponberg wrote an interesting article in Professional Boatbuilder re how he modified the bow of a Freedom 38 to give it extra buoyancy and waterline length.
    Scroll down a bit in the link below, past the article about the Wilbur.
    https://www.ericwsponberg.com/wp-content/uploads/case-studies-in-redesign.pdf
    A LOT of work went into doing this - it was not an easy modification!
    As Richard says, for a Heavenly Twins it really is not worth it to go to this extreme to get a bit more waterline length.
     

  12. Clarkey
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    Clarkey Junior Member

    Certainly seems like adding lightness and keeping weight out of the ends is the most worthwhile modification. Although it would seem a bit incongruous on an HT a carbon mast (and synthetic rigging?) might be especially beneficial to both overall weight and pitching moment?
     
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