Sea Sled madness. It’s in my brain.

Discussion in 'Fiberglass and Composite Boat Building' started by DogCavalry, Nov 11, 2019.

  1. DogCavalry
    Joined: Sep 2019
    Posts: 3,078
    Likes: 1,572, Points: 113
    Location: Vancouver bc

    DogCavalry Senior Member

    Fallguy, I must echo Bajansailor's sentiment. Your boat looks great!
     
    Lloyd Too and fallguy like this.
  2. DogCavalry
    Joined: Sep 2019
    Posts: 3,078
    Likes: 1,572, Points: 113
    Location: Vancouver bc

    DogCavalry Senior Member

    My pair of 6x6 freeing ports, as the literature calls them, are too small by half. I have 32" high bulwarks, 5' long port and starboard. That's a total of 25 square feet. That calls for 1 square foot, by total councidence. The cockpit lies between bulkhead 1 and bulkhead 3. Bulkhead 2 is 2' aft of 1, 3' further to B3. Again, the numbers are just coincidental. If Serenity takes a huge wave into the cockpit she'll be down by the head. I think I'll forget the big trough, and cut a 3" rectangle between B2 and B3. A heavy screen to keep wallets and toddlers on board, and some sort of flap or hinged cover to keep spray outside, and that should do. Big hole to cut through the hull, but Bluebell has promised to bury her bow. It's for science!
     
    BlueBell and bajansailor like this.
  3. fallguy
    Joined: Dec 2016
    Posts: 7,598
    Likes: 1,674, Points: 123, Legacy Rep: 10
    Location: usa

    fallguy Senior Member

    This bulwark calculation...is it square feet or cubic feet?

    I calculated the area of my deck in square feet area. You are using height.

    Do you have a dimensioned tops down drawing of the front?
     
  4. fallguy
    Joined: Dec 2016
    Posts: 7,598
    Likes: 1,674, Points: 123, Legacy Rep: 10
    Location: usa

    fallguy Senior Member

    It looks to me where the lovely lady is standing is about 7'x7'.

    If you make the forward side storage boxes angled out, they are not really part of the calc anymore I'd say, which would help. Then raise the entire platform and and inside landing, door coaming say 125mm and make the scuppers 6x12" wide. No trough, shallower if you do take a wave. Not 4% by my calcs, but a lot of water can leave in a square foot and close to 4%, unless doing it wrong.
     
  5. DogCavalry
    Joined: Sep 2019
    Posts: 3,078
    Likes: 1,572, Points: 113
    Location: Vancouver bc

    DogCavalry Senior Member

    In this context bulwark refers to the hull walls that will retain water, rather than the sole it will lie on.
     
  6. fallguy
    Joined: Dec 2016
    Posts: 7,598
    Likes: 1,674, Points: 123, Legacy Rep: 10
    Location: usa

    fallguy Senior Member

    Well, something seems amiss in the logic. If you are 2 feet high and 5' for and aft and 8' athwart, that is a hell of a lot different than 2' high, 2' for and aft and 8' athwart. The latter is 40% the volume of the former. Maybe when that fellow in Japan wakes up; he can correct me.

    Also, the gutter is then in the calc. If you raise up the gutter and put scuppers on the deck, it reduces the volume as well.
     
  7. DogCavalry
    Joined: Sep 2019
    Posts: 3,078
    Likes: 1,572, Points: 113
    Location: Vancouver bc

    DogCavalry Senior Member

    Volume is not part of the freeing port calculations. Consider a long ship, where there is a well amidships. It can only be filled by a wave breaking over the side. If you increase the beam of that ship, the water that comes in doesn't change, and if you increased freeing ports with beam, very quickly there would be no bulwarks, just a gap between the structures for and aft.

    So the calculation is percentage of bulwark area.
     
    BlueBell likes this.
  8. DogCavalry
    Joined: Sep 2019
    Posts: 3,078
    Likes: 1,572, Points: 113
    Location: Vancouver bc

    DogCavalry Senior Member

    And it's for big ships. I bet none of those bow riders in the haulover inlet videos have freeing ports 3" by 24".
     
  9. fallguy
    Joined: Dec 2016
    Posts: 7,598
    Likes: 1,674, Points: 123, Legacy Rep: 10
    Location: usa

    fallguy Senior Member

    Most of those bow filling videos are in boats with bilge pumps only. I took about 15 gallons over the bow once.

    In your case, the only consideration is volume. A wave over the top fills the boat with x amount of water and it can leave in y amount of time with z discharge. Reducing the volume by raising the scuppers to the deck level reduces the maximum amount of water, increases the height of clearance above water, and speeds the rate of discharge (less water).

    As the skipper, you can also know that your boat is not designed to take a sh..ton of water over the bow and operate it accordingly.

    And, if you decide the freeing ports you installed are insufficient; you can add more. The best place is aft-ish, for obvious reasons.
     
    DogCavalry likes this.
  10. DogCavalry
    Joined: Sep 2019
    Posts: 3,078
    Likes: 1,572, Points: 113
    Location: Vancouver bc

    DogCavalry Senior Member

    I tried, this morning, to calculate rate of water discharge and I found I don't have the math anymore.
     
  11. DogCavalry
    Joined: Sep 2019
    Posts: 3,078
    Likes: 1,572, Points: 113
    Location: Vancouver bc

    DogCavalry Senior Member

    As an aside: a couple professionals from the big outfits in the boatyard came by. They say my glasswork is better than anyone else's in the entire yard. There's at least a dozen companies here. I was dismayed. The local skill is so low my sad efforts look good.
     
  12. DogCavalry
    Joined: Sep 2019
    Posts: 3,078
    Likes: 1,572, Points: 113
    Location: Vancouver bc

    DogCavalry Senior Member

    Wrong again, *******. Bernoulli's equations. PE at the top equals KE at the freeing port. Empirical tests is bests, but on the close order of 20 seconds to drain from overflowing to empty, through Ad Hoc's 4% ports. Sounds good enough for me.
     
  13. Ad Hoc
    Joined: Oct 2008
    Posts: 7,774
    Likes: 1,679, Points: 113, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    An alternative method...from my post HERE to the freeing port calculations and the 4% rule, you could just use basic flow through a notch.

    With a given depth of notch, H and breadth, B, we can calculate the flow.

    The total discharge is = 2/3.Cd.B(2g)^0.5 x H^(3/2)

    Where Cd is the coeff of discharge, which would be a nominal 0.6

    Does that help?
     
    DogCavalry and BlueBell like this.
  14. BlueBell
    Joined: May 2017
    Posts: 2,685
    Likes: 961, Points: 113
    Location: Victoria BC Canada

    BlueBell . . . _ _ _ . . . _ _ _

    DC,
    What is the weight of the forward cockpit full of water?
    Estimated volume would yield that answer.
    I'm thinking in the 5 cubic meter zone.
    5 tonnes...

    I don't think it would even drain, there wouldn't be enough buoyancy in the rest of the boat with it down in the head like that.
    At what percentage full would it drain from? No need for the scuppers to be any larger than to accommodate that.
     
    Lloyd Too likes this.

  15. BlueBell
    Joined: May 2017
    Posts: 2,685
    Likes: 961, Points: 113
    Location: Victoria BC Canada

    BlueBell . . . _ _ _ . . . _ _ _

    6x6=36 x2 = 72 square inches
    8.5 x 8.5 = 72 x2 144 square inches
     
Loading...
Similar Threads
  1. Darkzillicon
    Replies:
    145
    Views:
    15,025
  2. Dillusion
    Replies:
    7
    Views:
    1,030
  3. Tungsten
    Replies:
    26
    Views:
    2,670
  4. valvebounce
    Replies:
    3
    Views:
    1,648
  5. Runhammar
    Replies:
    17
    Views:
    1,469
  6. fallguy
    Replies:
    4
    Views:
    901
  7. massandspace
    Replies:
    5
    Views:
    1,968
  8. fredrosse
    Replies:
    5
    Views:
    1,340
  9. Tiger51
    Replies:
    15
    Views:
    2,996
  10. cy fishburn
    Replies:
    5
    Views:
    1,883
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.