Hickman Sea Sled Information

Discussion in 'Boat Design' started by DogCavalry, Feb 12, 2021.

  1. DogCavalry
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    DogCavalry I aim to misbehave.

    An interesting exchange. The designer, Jacques Mertens, was a bit misinformed. But the torsion question is an interesting one. Do fast catamarans suffer from torsion problems?
     
  2. BlueBell
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    BlueBell "Whatever..."

    Not necessarily, but they certainly have higher torsional loading than monohulls.
     
  3. DogCavalry
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    DogCavalry I aim to misbehave.

    Sleds would be less than cats, but more than V hulls.
     
  4. DogCavalry
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    DogCavalry I aim to misbehave.

    Baekmo posted a sample of work a few weeks back to illustrate functionality of some software. But the image was of a sled hull. That makes him the only guy on the forum at the moment who has actually built one. Maybe if we ask nicely, he will tell us about it. (Gather round kids, and listen to the tale I tell ye)
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    Do not use round drains, only rectangular slots, the former are much less effective, with equal area.
     
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  6. BlueBell
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    BlueBell "Whatever..."

  7. DogCavalry
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    DogCavalry I aim to misbehave.

    More great stuff!
     
  8. DogCavalry
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    DogCavalry I aim to misbehave.

    From the generous hand of @baeckmo. I rather embarrassingly misunderstood some of what he had to say, so I'll insert his words here, and stop wasting your time with mine.
     
  9. DogCavalry
    Joined: Sep 2019
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    DogCavalry I aim to misbehave.

    Hi John

    I’ve only got a few pics of my W-hulls for you, but since there is a background story to the development, I’m afraid I have to contaminate the text with some “catamarania” despite your “anticat” declaration. You find the pics in a separate, compressed file; “Hickbilder”.

    In the late 1970:ies I built a 6.4 m open workboat cat for a local fishfarmer, based on the series 62 mother hull, split along the center to create an asymmetric cat. At the time there was practically no info available on small planing cat resistance, movements or strength requirements. Some preliminary data on symmetric hulls were available at the SSPA (Swedish State Towing Tank), but not suitable for my purpose. So using the series 62 provided a useful referrence base. The straight tunnel sides resulted in a reasonably well-defined wave system in the tunnel, allowing a single prop propulsion.

    It came out far better than expected, particularly in bad weather, so much that we frankly didn’t quite understand why. Later the boat was borrowed and tested by an ergonomy research team that looked into the health problems faced by coastal fishermen; nearly all of those suffered from orthopaedic trauma from overloads, particularly in the lumbar region. In the end I got involved in the development of better hull forms for the job.

    We were able to identify some major factors in hull response to incoming waves that introduced high loads and very tiring movents into the human body. It just happened that our first cat had the “right” proportions for avoidance (from what we now know, it might just as well have been a failure; there are sh*tloads of bad cats around, believe me). As a result, the guys worked 20 % faster on their rounds, yet with a 20 % reduction of oxygen consumption (a direct measure of work intensity). The first two pictures show a typical cat from this development, still going strong, now on her second diesel engine driving the original PP90 waterjet. She is no beauty, but I have been watching this guy coming home with good catches and a smile when no one else dared to go out.

    During the development, I also looked at the Seasleds as possible alternatives; the Coast Guard here had a number of sled barges for oil spill clean-up equipment, but none self-powered. In the SSPA library I found an old David Taylor research report, with “real” data on the Hickman sleds. You find the front page and the Seasled data page as pics 3 and 4. If you look at the drag/lift values as function of Froude volume number, you will see that it has very high resistance at speeds below about Fn=4. I added some data points for the corresponding ser. 62 hull (soft line between small circular marks). The difference is huge in favour of the series 62.

    Now, for workboats and fishing vessels, the ability to use the whole speed range is an absolute requirement, both from an earning aspect and from a safety view. This ruled out the original Hickman shape from further development at the time, but I had an itch that there was more to it than the towing tests showed. More to that later.

    The next two pics show examples from a couple of Fifi/Resque/Diving support boats we built, based on our cat experience. I just added them because they are about the size of your project, and might give some inspiration regarding deck layout and interior details. They are 8.5 and 9 m long, 3.2 m wide, diesel power 250 to 320 hp, speed 30 knot, separate diesel fifi pump, foam tank and “the full resque monty”. The deck house is Alu/Divinycell sandwich, rubber suspended for noise insulation. You can also see the size of emptying ports necessary for keeping the deck free of green water.

    All my boats have been built to the DNV “15-m rules, decked boat” or the “Nordic Boat Standard”. As none of them had stability rules suitable for catamarans (or W-hulls for that matter), the authorities accepted the use of “equivalent heeling energy” values instead of the prescribed shape of righting moment curve.

    As time passed by, the “W-hull itch” was still gnawing; there were some questions unanswered by the DTMB tests. There was no mentioning of, or compensation for the vertical forces developed by the SP propellers; they are strong enough to influence the hull trim and thus drag in the critical speed range. And the sled hull in full scale produced a lot of aerated foam along the bottom, which was not dealt with during the tests. The foaming qualities are a function of the relation between inertia forces and surface tension in the water/air interface, as expressed in the non-dimensional Weber number. It does not scale like the Froude number scaling, and is a serious headache when trying to find best shape and placement of chines and spray rails for planing hulls.

    On top of that we had seen that in certain loading conditions with cat hulls, there would be a strong transverse wave “rolling” in the tunnel entrance, just like what you see in the Hickmans. There were also a couple of “copy-cats” built by competitors, trying to mimic our successful hulls, and some of them were “swimming on their bellies”. Both events were associated with huge increases of drag. We ran some tests with a simple Seasled-shaped transparent polycarbonate panel to see if there was any indication to what was going on. Again, the transverse wave was obvious, but it was also clear that the foaming was not correctly scaled. So, to get rid of the “roller”, the bottom rise was kept roughly constant from midship>aft.

    Although we did not measure drag in these tests, the improvement was immediately noted as a reduction of the hump trim and wavemaking. Hickmans flat bottom aft is simply “à la mode” of its time including the drawbacks. But the questions about the foaming were still unsolved; the full scale Weber number was between 25 and 50 times the model number. Finally we used a small pump (and a small amount of dish-washing fluid) to “trig” an increased foaming in front of the model by spraying the surface. That made a difference. By varying the bottom rise and using spray rails we could shape the foam layer to some extent.

    Now don’t get me wrong here, there is no such thing as “air-lubrication” along the Hickman bottoms, and no “ram pressure” of any significance; the foam has a completely different function. It is a “slamming damper”. The maximum pressure spike has an amplitude that depends on vertical speed times speed of sound in water ( pmax = Vz * Vcrit/g;) when hitting the surface (same principle as “water hammer”). Now Vcrit in water is about 1200 m/s, but in a homogenous water/air mixture it can be as low as 35 m/s, which makes a huge difference for slamming pressures.

    Anyway, now I just had to build a “seat-of-the-pants” test hull to get an idea of the ride qualities of the “revised” inverted V hull. I made it 5.8 m long x 2 m beam in order to compare with previous cat workboats (we have a legal limit of less than 6 meters for workboats operated commercially with non-certified crew). The prototype had slightly too high deadrise for the non-tripping chines (~45 degrees), but it had very pleasant movements in a seaway, and the planing hump was practically absent. With a Mercury 40 hp outboard (actually too small…), it has a max speed of 23 knots with two pax and some tools.

    The pictures show this prototype. We just finished the first test run, when a previous customer (local harbour authority) needed an extra workboat, and off it went…… Later we built a couple of similar (with 30 degree chines) open workboats, same size, but ~30 degrees non-tripping chines. Engines 60 to 80 hp, speeds 30 to 35 knots. The “claws” on the prototype are used for barge pushing and “hovering” with nose to a pole when crew is working on moorings and harbour structures.

    Then came a call from a collegue boatbuilder who had offered commuter catamarans for a canal authority, but he got cold feet when he got the job and found out that his cat design lacked carrying capacity. So I drew the lines of an inverted V hull for him, based on our developments. It measures ~7.5 m x 2.5 m and is carrying bike tourists across a lake system along a cross-country canal. Engine 150 hp Mercury ob, speed light 36 kn, load ~1000 kg. The pictures show some of those boats.

    From my experience (although somewhat limited) with the W-hulls I’d say that the shape deserves a bit more serious treatment. If you look at the DTMB test of the Sea-sled, you find the results from one model with two loadings; that’s all there is in terms of published scientific tests. The modernized W-hull should be tested in a series of L/B/bottom rise/load proportions, both in flat water and in waves to find its plus and minus. When we ran the tests with the cats in the old days, we had wheelbarrows of measuring equipment (accelerometers, gyros, tape recorders, accumulators and inverters) in order to find out what made the difference. One of my neighbours was skipper on the commuter ferryboat in the archipelago, and he was happy to supply a well defined and repeatable wave system for testing any time we needed it (“just gimme a call”)! Today you have all the necessary instrumentation in your phone, its just amazing.

    I think the next-to-religious statements made about Hickman and his hulls, without real scientific or engineering evidence, has created sort of a “don’t touch” attitude among boatbuilders and designers in the west. Funny enough, some of the hulls used in the big Russian fleet of river buses built from about 1960 up to 1980-ish use a hull shape that reminds of the inverted vee, but they did their job from the ground with thorough tank testing to find the sweet spot.

    Now this story became longer than planned, so I’ll stop here, and hope it gives you some inspiration for the future. If you have questions and comments, you’re welcome.


    Best regards,

    /Bodo Baeckmo/
     
    Last edited: Jul 18, 2021
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  10. DogCavalry
    Joined: Sep 2019
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    Location: Vancouver bc

    DogCavalry I aim to misbehave.

  11. DogCavalry
    Joined: Sep 2019
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    Location: Vancouver bc

    DogCavalry I aim to misbehave.

    The last image is the 1955 towing tank stuff. Probably didn't help Sea Sleds any.
     
  12. BlueBell
    Joined: May 2017
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    BlueBell "Whatever..."

    Have we seen this video here yet?
     
  13. DogCavalry
    Joined: Sep 2019
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    DogCavalry I aim to misbehave.

    We have not, surprisingly.
     
  14. kenfyoozed
    Joined: Mar 2005
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    kenfyoozed Junior Member

    Are there links to the aluminum designs shown above, anywhere?
     

  15. cracked_ribs
    Joined: Nov 2018
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    cracked_ribs Senior Member

    Yeah those boats of Baekmo's are totally awesome looking, I think. Maybe it's my bias for a boat I can load up with gear and get onto the beach at my cabin, then go fish out of...but those look like they'd handle local conditions perfectly and haul a TON of stuff.
     
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