Sea Sled madness. It’s in my brain.

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

  1. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    Looking good - although is it absolutely necessary (from a rules point of view) to fill that space under the cockpit with foam?

    Especially as it looks like the bulkheads on the fore and aft sides are substantial, and water tight?

    If it is allowable (or do the Crats think that she will be like a Boston Whaler skiff?) to not have foam there, I think I would be inclined to leave it empty.

    And if you have another watertight compartment at the stern, and a 'double bottom' compartment under the sole, with bulkheads, then you should have good watertight sub-division?
     
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  2. DogCavalry
    Joined: Sep 2019
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    DogCavalry I aim to misbehave.

    Good questions! All the bulkheads around there are watertight, to about 22 inches high, or higher. Perhaps the foam is entirely unneccessary. It seemed like a good shortcut to avoid more framing under the cockpit sole. I may have as much as 3000kg, 6600# in that cockpit. Don't want the floor to collapse.
     
  3. DogCavalry
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    DogCavalry I aim to misbehave.

    What do you think of the drainage gutter running across the hull? I'm trying to avoid a tripping hazard at the door, while maintaining a functional weather sill.
     
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  4. bajansailor
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    bajansailor Marine Surveyor

    I think it is a good idea really - what sort of depth and width (re the cross section area) are you thinking of?
    I presume that it will drain overboard port and starboard?

    Re a grating, you can buy ready made composite gratings (complete with rough non skid on them) - I saw some on a new dock built here recently.
    They are fairly expensive though.
    McMaster-Carr https://www.mcmaster.com/fiberglass-grating

    Re the load in the cockpit, you can simply add extra longitudinal stiffeners under the sole. I think that I would prefer to do this, rtaher than filling the space with foam. And maybe have a wee access / inspection hatch in the aft bulkhead ?
     
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  5. DogCavalry
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    DogCavalry I aim to misbehave.

    And that is why I ask! The guys who know bettr than I do.

    Yes, drains port and starboard. The drawing is to scale. 1" or 2.5cm squares.
     
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  6. BlueBell
    Joined: May 2017
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    BlueBell "Whatever..."

    Side view, sideways. I tried rotating it but no can do.
    Is that 30 cubic feet of foam at 6 pounds per cubic foot?
    180 pounds of foam?

    Is that a 1" weather sill?
    That's a worse tripping hazard that an six inch water sill, no?

    Will the foam offset a 6 000 pound load or do you have to jettison it when sinking?
    Will it then hold up the boat? Positive buoyancy?

    Have you considered bladders instead?
    Or ping-pong balls?
    Empty, sealed water bottles?
    It's a good idea to be able to access all of the hull from inside.

    Is this one of the steps or does it run all the way across?
    How does the inverted V affect volume.
    A three view drawing would really help.

    What is the objective here, buoyancy?
     
  7. DogCavalry
    Joined: Sep 2019
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    DogCavalry I aim to misbehave.

    Good questions. I'm circling around this detail right now, so challenges are great to clarify my thinking.

    Yes, that's a scale section, from the starboard side, looking to port. It's not quite right,because the sction is through a chine log, but also shows the location of the door.

    Curving areas intersecting, my best calculation is 33 cubic feet. 6 pound foam gives 196 pounds of foam. Immersed that gives 2100 pounds of flotation. That certainly wouldn't float 6000 pounds of bricks. If we're carrying that and we get holes through multiple compartments, she's going down.

    The drainage well is a gutter that runs full width to big scuppers. I wanted the effect of a 7" weather sill without making it super difficult to step through the door. The sole inside, on the left side of the image, is 7" below the sole outside. I'm thinking water taxi for old folks.

    The chief reason for using that dense foam is structural. I hope to put more cargo in that forward cockpit thst the weight of the rest of the boat, fuel and engines included. I might build sufficient framing under the cockpit sole to transfer such loads to the hull sides, but I'd definitely need some help from a real NA for those details. I'm not that good an engineer.

    J
     
  8. bajansailor
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    bajansailor Marine Surveyor

    Or (being pedantic) about 1,900 lbs of floatation if you subtract the weight of the foam?
    Whereas the empty compartment will provide 2,100 lbs of floatation if it is not breached.
    But surely the odds of breaching this forward compartment are very slight, never mind multiple compartments?

    That is an awful lot of added weight up in the bow of the boat - have you done any estimates as to how much she will trim down by the bow as a result of all this cargo up forward?
    What sort of speed would you be looking at when transporting this cargo?

    I am thinking that if you provide all the details required on here, you will get some calculations flung back at you fairly quickly!
    No need to consult a NA specifically. :)
    You could treat the structure in the same way as a metal framework, but using the properties of wood instead.
    I suppose it would effectively be a grillage, comprising a plate with longitudinal and transverse stiffeners underneath it.
    All the ends of these stiffeners will be 'built in'. re end fixing moments.
    The 6,000 lb load can be assumed to be uniformly distributed over the the floor / sole.
     
  9. DogCavalry
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    DogCavalry I aim to misbehave.

    Thanks Martin. Like I said, the hardest questions give the most rigorous answers.

    Extensive conversations with Marcys Lee reveal that the hull form generates so much lift in this region under way that in his experience, he was never able to load a boat too much. He built his with drop bows, like a landing craft, and he talked about loading the forward area so heavily that there was water coming in, but as soon as he got under way, the bow popped up like it was empty.

    Based on George Crouch's extensive experiments, the sled can plane with up to 100 pounds of weight per horsepower. Empirically determined. So with a pair of 115s, 23000 pounds all up. My target of 5000 pounds of vessel and 7000 pounds of paying cargo is conservative. One of Winninghoff's boats, a 55 footer, planed with 70000 pounds of harvest, every day.

    So all this is reassuring, but still just anecdotes to me, until, as someone said recently, I get her bottom wet. I'd rather not hand-baum 3 tons of bricks inside my accommodations, so the would be craned into the forward cockpit. I'm picturing distributing that load out to each hull side, shear forces, point loads etc. I'll take a bunch of photographs today, in response to direction from anyone on here who cares to ask. My phone number is 604-442-6625, in Canada.

    J-Dog
     
    Last edited: Mar 6, 2021
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  10. BlueBell
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    Location: Victoria BC Canada

    BlueBell "Whatever..."

    John and Martin,

    This is helpful, thank you.

    I understand the trough now, interesting idea.

    Dunnage can dissipate point loading on the foredeck.

    I'd like to see compartmentalizing of this void space with individual inspection ports.
    When the boat is near completion foam filling could be reviewed.
    It's easy to put it in but really hard to take it out...

    Really interesting about the bow loading testimonials.

    Off to enjoy the sunny day.
    I'll be watching for HMCS VICTORIA (-one of few ships designed to sink... and resurface)
     
  11. bajansailor
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    bajansailor Marine Surveyor

    That is impressive, re the quantities of cargo being carried by the previous Sea Sled you mention above.
    So she should still be able to get up on the plane when carrying all these bricks.
    What would be the typical passage distance to your building site, and what would be the typical sea states you would encounter?
    I suppose the important thing to consider now, once you are happily up on the plane, is coming back down slowly, without stuffing the bow under.
    And how much will she trim down by the bow when stationary.
    If you do a 'crash stop' while on the plane then there is also the momentum of these 3 tonnes of bricks to take into account - they will all be trying to break down the forward bulkhead and escape.

    Edit - Re Bluebell's HMCS Victoria sinking and then re-surfacing, at first I was thinking that maybe she is one of the floating dock ships, like what Dockwise operate -
    Dockwise https://boskalis.com/about-us/dockwise.html
    If you click on this link, be warned, there is lots of very enjoyable distracting video on here of heavy lift projects successfully accomplished.

    But Google told me who she is -
    HMCS <i>Victoria</i> (SSK 876) https://en.wikipedia.org/wiki/HMCS_Victoria_(SSK_876)
     
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  12. DogCavalry
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    DogCavalry I aim to misbehave.

    A picture is worth a thousand words. 20210306_150455.jpg 20210306_150245.jpg 20210306_150227.jpg 20210306_150216.jpg 20210306_150114.jpg
     
  13. DogCavalry
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    DogCavalry I aim to misbehave.

    Bluebell, can you let Bajansailor know about local sea states? As far as distance to jobs, not generally further than 40nm. But if the client shows us the money...
     
  14. BlueBell
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    BlueBell "Whatever..."

    The usual, from flat calm to diabolical stay home.
    It really depends where we get work.
    The entire west coast of Canada plus another 100 nm into Alaska and down into Washington State.
    It boils down to whatever the boat, crew and load can handle given the circumstances.
    We get everything here including current.
    Tides can be 3m in the south and up to 6m north.
    Lots of inland coastline, huge inlets.
    More logs in the water than you can believe some days (and some nights).
    Hugely nicer in the summer months and cruelty meaner in the winter.
    They call it the Graveyard of the Pacific for a reason.
    The Coast Guard here does more recoveries than rescues.
    The rugged remoteness increases one's exposure considerably.
    1' - 3' wind-against-current waves, sometimes with a bit of a swell on the outside (open ocean).
    Travel distances grow as one moves north and it becomes more desolate.
    40nm is probably a good place to start.
    From the video I've seen, they don't appear to be up on plane completely.
    They still sit a bit low in the water but they are riding on the compressed air trapped under the hull.

    John,
    Nice pictures from today.
    Those are some crazy hull lines forward.

    A beautiful day by the sea but alas, no submarines... that I saw.

    Cheers, BB
     
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  15. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    Okay, if you are going to barge 6000 pounds of bricks; tow it behind the Seasled on a barge. No wise captain risks life for load. Not to mention the simple fact a barge can be loaded a hell of a lot faster. As a guy dealing with a literal clunk in the middle of my back, why in the name of God would anyone want to plysocally load 6000 pounds of bricks in a scuppered boat that may put the scuppers closer to water?

    There is a difference between what one can and ought.

    You can buy 2 or 4 pound foams as well.
     
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