Catamaran Hull Squat

Discussion in 'Hydrodynamics and Aerodynamics' started by francotripepi, Feb 28, 2019.

  1. francotripepi
    Joined: Feb 2019
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    francotripepi Junior Member

    Good Day to you all,

    I am interested in understanding the why of something that I observe and perhaps discuss remedies (if warranted)

    Firstly, my premise: I boat in South Florida, Biscayne bay and the Keys, the waters around here are full of shallow water areas we call Flats, as in...really shallow, maybe 12 to 20 inches and are everywhere.....
    Sea conditions are generally around 1 to 2 foot of constant short waves, anything bigger and I personally do not see the reason to navigate, as I do it for pleasure and gunkholing, I do not seek the big blue waves, nor I find it pleasurable taking my girlfriend and my 11 years old son into such sea conditions for the sake of it.

    I am enamored with the agility and motion of the catamaran used by Tour operators in the Caribbean and the Keys, majority of them seem to be of the displacing type.

    See below for some examples.....pictures taken from Google images, (no intent in advertising any particular tour operator)

    Displacement Catamaran 4.jpg


    Displacing Catamaran 1.jpg


    Displacement catamaran 3.jpg

    As we can see, these are thin long hulls, and need beam to spread their loads so that the hull itself does not draft too much...I am guessing so, please correct me if I have any principle wrong

    The wider the beam, the shallower can the hull be, even thou it's a Displacement/canoe hull...but most importantly, they stay true to horizon at rest and in motion.....almost no bow raise

    This concept cannot be scaled down into a trailable 24 footer, since I can get away only with a 9 foot beam on these roads...(I know it's only 8 ft, but you can still get away with 9'), hence I would probably have to make up by using volume in the hulls, and thus increasing depth too

    Speed is not my concern, I prefer to glide away at 15 to 18 knots all day long and use less power/weight/consumption...etc

    There is another alternative, which is why I am writing to you all.....a planning catamaran...like the BeachCat pictured below.....

    BrachCat 1.jpg

    BeachCat 3.JPG


    BeachCat 4.jpg

    As you can see, when at rest, it squats sensibly at transom, I also have to admit, that Fuel Tank (small), Batteries and engine are all located at transom.......also notice a small and short Central Pod where they mount the engine...more to prevent prop cavitation than buoyancy.

    Hull height is constant, so you can get an idea....but it does not rest true to horizon

    How can this hull should be made in order not to squat so much at rest?

    A - Redistribute weight?
    B - Have it wider at transom than at bow?
    C - Make it end flatter?
    D - Fill rear half with Floating Foam?
    E - Any other suggestions???

    These type of hulls could navigate at shallow depths and could run (around 20 mph) in even shallower waters.....opening a wider percentage of waters to be navigated and enjoyed....if you ever go Flats Fishing or just see a program on it, you'll understand why I wish to reach such waters...

    If you consider that i could achieve same with a Displacement type of hull....then please let me know how or which things should I consider

    I am talking conceptually, so no specific weight, load, etc has been defined yet

    My interest stems from seeing Rowing Umpires/Coaches catamaran glide throu lakes.....boy what a beautiful sight, Google Win Tech 24 launch to see what I mean....problem is that they are waaaaaaay too low of a profile for sea conditions and carry almost no load

    Thanks in advance for any and all responses and useful/helpful guidance
     
  2. fallguy
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    fallguy Senior Member

    There was recently a discussion about displacement vessels in shallow water. They are limited in speed based on water depth due to the way shallow water behaves.

    As for stern squatting...

    A - Redistribute weight? YES, some degree of help there
    B - Have it wider at transom than at bow? Does nothing unless the weight changes unless you mean change the hull.
    C - Make it end flatter? A change to the hull shape will change the squatting, if this is what you mean.
    D - Fill rear half with Floating Foam?will have no effect other than to make it squat more from weight of foam
    E - Any other suggestions??? A better designed boat!

    There is a great catamaran you can build or commission for build in the 24' size that is a nice enough boat to make the crossing to the Bahamas. And it is trailerable. But it is a custom boat. The Skoota 24 powercat. It is beachable, great fuel economy, low horsepower needs. The hulls are on scissors, sort of, and the hull is wider on water, but narrow enough to trailer.

    Sailing Catamarans - Skoota 24 trailable centre cockpit weekender http://sailingcatamarans.com/index.php/designs-2/6-powercats/263-skoota-24

    It can also be made without the aft cabin and a larger forward cabin I believe.
     
  3. Mr Efficiency
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    Mr Efficiency Senior Member

    Boats like that Beachcat are a bit sensitive to weight distribution fore and aft, and with people moving around in smallish boats, changes of trim are unavoidable. But taking fixed weights further forward will help, if it is sitting down too much for your liking. Underway at speed, however, the planing type cat will have a nose up attitude, which you can lessen by engine trim, but it will be different to the displacement cat's more level attitude. That Beachcat seems a reasonable solution for shallows, too, with the central mounted engine saving a fair bit of draft, I'd say it could manage water 18" deep pretty well. It is unusual for people to worry about fore and aft trim on boats from the comfort angle, it is usually tippiness side to side, that bugs people more, and in cats, that problem is minimized.
     
  4. fallguy
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    fallguy Senior Member

    The Skoota 24 has a 13' beam on the water and an 8'6" beam on the road. fyi
     
  5. francotripepi
    Joined: Feb 2019
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    francotripepi Junior Member


    Fallguy, I was already aware of the Skoota, and I agree in its efficiency, but the scissor system is not what I would like, and overall lines are cartoonish, even thou ultimately is the wetted part of the hull and the overall dimensions that make it perform, the rest can be redesigned.

    What I mean with wider at transom...I am referring to the sponsoon shape/width, narrower at bow and gradually wider up to the transom, kind of like an arrowhead shape.

    Regarding the “make a better hull” , even thou I am not the manufacturer, that is exactly what I am trying to “do” with this discussion.....find ways to improve

    Thou I must admit, I liked that comment, lol
     
  6. francotripepi
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    francotripepi Junior Member



    Me Efficiency, you are right on, these types of boats are great around here, and the squatting is inherent in this particular hull, one of the picture shows it without passengers, and still squats.....at low speeds/idle it will draft more than desired
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    Squatting, which all planing boats will do to some extent, is different to static fore and aft trim, which is all about weight distribution, and hull buoyancy characteristics, you can't do much about the buoyancy on the day, but you can move some weights around to help. It would be possible to make a trailerable displacement cat, but the hulls would need to be so slender, that problems would arise with fore and aft weight sensitivity, and general weight carrying ability, that it would be a rather limited boat.
     
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  8. fallguy
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    fallguy Senior Member

    The lines of the Skoota 24 are cartoonish?

    Do you mean the cabins setup? I would not consider those lines. The hull lines are made for ply on frame construction, so they are limited to developable lines. I am not sure, but imagine the l/b ratio is probably around 11. So this is a very nice boat overall. The only possible downside is the scissoring, but it fit your wishes for trailering. And it would be a fast build.

    Richard Woods is something of a minimalist, so his tendency is to build things like cabins and such rather small. This comes from his knowledge about the importance of wind on sailing vessels. The cabin could be changed easily and I am not a fan of the cabin layout either.
     
  9. fallguy
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    fallguy Senior Member

    What about the Skoota 18? It is a displacement power cat. Very inexpensive to build and rig. If nothing else, you can get an idea about the hulls that are sort of the staple of the Skoota series.
     
  10. fallguy
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    fallguy Senior Member

    ...by the way, the beam does nothing to 'spread the load'...

    the displacement of the two hulls does all the work, the wider the cat becomes, theoretically, the larger and the deeper draft (generally) you would need … of course, if you want to float into the world of exceptions, this is possible for a shallow draft wide boat, but not for a boat that can manage any sort of rough seas without imparting lots of misery on the riders

    the narrower the hulls, the more the wave action of one hull will impart into the wave of the other hull and create a drag force, so a very narrow catamaran hull also has a problem, and the water for the engines can become turbulent which is also a potential issue
     
  11. Ad Hoc
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    Ad Hoc Naval Architect

    Design it correctly from the outset.

    The LCG in lightship and full load should match the change (if any) of the LCB in lightship/full load.

    That's it...no magic.
     
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  12. fallguy
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    fallguy Senior Member

    Can this be determined from typical hydrostatics?
     
  13. Ad Hoc
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    Ad Hoc Naval Architect

    Yes, very easily.

    Here is typ output from one of our previous cat designs as an example:

    upload_2019-3-1_11-20-20.png
     

    Attached Files:

  14. Mr Efficiency
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    Mr Efficiency Senior Member

    Fallguy, if he wants a boat that is "stiff" on the long axis, a small displacement cat would be a poor choice. When I used to go round to the bow of an 18' planing cat to weigh anchor in a bit of chop, I'd get a bath, on occasion, and the ends were rather fuller than a displacement cat of that size, would need to be ! But it is/was just a superb boat underway at speed. Nothing that a sane man would want to be out in, would slow it to the point of being off-plane. But at displacement speed, in rough conditions, even with no-one at the bow, it would occasionally scoop water.
     

  15. francotripepi
    Joined: Feb 2019
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    francotripepi Junior Member

    77471C12-F1AB-4D81-9F24-228A612DF0C9.jpeg 8E6AB14A-1E7A-4690-9DA0-36C0B1819AE9.jpeg




    Fallguy, Skootas are indeed nice boats, there are two part of the boat if I may, the wetted lines (bellow waterline) and Design (above waterline) the first is what give the sailing qualities (or lack of) the later only gives a pleasure to the eye (or not)....my “cartoonish” look is referred to the latter and is highly personal, as all “beauty” judgements

    I personally love simple lines....look at this cat designed in Australia, it’s called Casacat....I personally do not care for cabins in small boats, specially in South Florida heat and humidity......but I would love a center/side console with two long boxes running to the sides where you can sit and or lay to sleep the night over, camping.....
     

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