flat bottom bridgedeck vs other shapes in heavier sea states

Discussion in 'Multihulls' started by big_dreamin, Oct 22, 2016.

  1. mydauphin
    Joined: Apr 2007
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    mydauphin Senior Member

    My first day in engineering school the teacher said to us the following....Anyone can build a 200 mph car, anyone can build a car that can do 50 mph, anyone can build cheap car. But no one can build all three. The first thing to learn you need define the mission, and if you add too many missions, you end up with something that does nothing very well like the F35 airplane or a Bayliner.

    In Boating KISS is king...
  2. big_dreamin
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    big_dreamin Junior Member

    Well my solution for that is twin engines. Yes it costs more than one but i'd consider it a must have without a full time engineer on board.

    Likewise i'm trying to figure out my own answers to other "what ifs" - what do I consider the best compromise for what i'm wanting to explore. Some of those have more than one answer because the design is not pinned down yet - but it makes me aware of "you have to make a choice between X, Y, and Z".

    Perhaps I misspoke a bit... my concern over the safety I suppose is more precisely defined as "what happens if I end up in a sea state worse than my intended 'normal' usage?" and is there a shape change, for the same weight, that handles that better than a flat underside of the bridgedeck?

    I think I get what youre aiming at though - proper seamanship is staying out of those situations anyways. So perhaps the question is "just how fast can the weather change on the Great Lakes", how likely am I to take off in a sea state 4 predicted to stay that which lets say the boat is designed for, and two hours into the journey in i'm a sea state 6 worsening to a 7. If that does happen, does it mean I should have designed for sea state 6 in the first place? Does it make for an uncomfortable ride or is the boat at risk? Do I have to slow down and does the longer journey open the door to it getting even worse before I get home? Etc etc. These are the thoughts going through my mind I guess.

    For right now i'm very focused on the Great Lakes because that's where the boat will spend it's first decade - my work is in Minnesota and that's not changing. Everyone tells me weather can change FAST later in the fall there. In the future I want to run down the coasts and eventually go transoceanic (possibly both pacific and atlantic) and maybe even do a circumnavigation in retirement - possibly impossible if my cat design will be destroyed by Drake's Passage, or expensive if I just run thru the Panama Canal. Whether rates of weather worsening is even equal to the Great Lakes on the open ocean i'm not sure - I have no experience or knowledge, but i'm all ears for advice on what I might expect.
  3. mydauphin
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    mydauphin Senior Member

    I don't know much about the Great Lakes except what I have heard. It is like a small ocean, and the waves are actually worse, and the weather depending on month can change on you rather quickly. That said you can get to a port rather quickly if you stay closer to shore. Sort of island hopping. Ask people that have been sailing in the region. Also be wary of the cold. A boat in 40 degree weather is not fun. There is usually very little insulation, expect very low cabin temperature. And it is not easy to light a fire to stay warm in an emergency. Which I did a couple of times in desperation.

    Also a boat of the lake may have to be stored for severtal months, add that to your cost equation.
  4. Richard Woods
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    Richard Woods Woods Designs

    I'm still not clear if you want a powercat or a sailcat with twin engines.

    The price of motion is accidents.

    The Great Lakes have short steep waves, like all lakes. I think because the water cannot escape but always has a shore to hit so cannot dissipate

    Deciding on the seaworthiness of a design is always tricky. Should it be according to distance (time) from a safe haven? Often used by commercial rules. Or dependent on sea state (as in RCD for example). Portland Bill is usually rougher than anywhere else in the English Channel, yet the worst waves are only 1/2mile from shore.

    In a powerboat it is the range under power that is the usual determining factor

    But this is a long way from your original question. As always, don't get too bogged down with one factor of the design. Look at the broader picture, as Adhoc is saying.

    Having said that, I don't like flat bottomed bridgedecks. I prefer them to be slightly Veed. Eve a small amount makes a huge difference to the amount of slamming. I like a 45deg angle at the bridegdeck edge as it spreads the loads and, unlike a rounded section, is easy to make and steps down into the hulls are generally safer

    My structures lecturer said in my first lecture as a yacht design student. "Static objects (buildings, bridges etc) are easiest to design. Moving objects in one fluid are also relatively easy (cars, spacecraft, planes). The hard things to design are "interface vehicles" things that move in two mediums at the same time, like a boat in air and water. "

    Think about it - few planes break up in the sky, it's landing, when they become an interface vehicle, that is challenging. Even more so with space craft which, when in space, have no loads and only one atmosphere pressure to resist.

    That's why I wrote any article entitled "Rocket science isn't rocket science but yacht design is"

    Richard Woods of Woods Designs

  5. mydauphin
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    mydauphin Senior Member

    I know this don't make sense intially, but one engine is more reliable than two. Research it, there are many many post about it here.

  6. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    Well, you should ask the sailors on board WingNuts:



    Like anywhere in the world, always plan for the unexpected. Procedures are far more important and understanding them, not ignoring them. @Design' is just one input.
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