Statement of Requirements – Foveaux Strait Commuter

Discussion in 'Boat Design' started by FoveauxSailor, Sep 4, 2015.

  1. Mr Efficiency
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    Mr Efficiency Senior Member

    There's something you don't see too often, two big outboards with a little one for company !
     
  2. whitepointer23

    whitepointer23 Previous Member

    I don't think i have ever seen twins with a kicker before.
     
  3. FoveauxSailor
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    FoveauxSailor Junior Member

    Yep - the wonderful locally produced Stabicraft. Its a great concept for marketing boats, they really are unsinkable. But that is one of their few redeeming features for my purpose. I know the company owner and have discussed with him the objectives of the design. I have ridden on many versions of the type. I do not desire to own one as I am not tough enough to ride on right across Foveaux strait in anything except flat water, and neither could I afford the fuel bill !!
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    If there is a lot of confused water out there, a good cat is the go, and really it is not a long haul, so the fuel bill should be manageable.
     
  5. groper
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    groper Senior Member

    If the water is rough, anything that DOESNT plane is the go... If you want to decouple the boats movements to any worthwhile degree from the water surface, then don't use a boat that generates dynamic lift from th same said surface... Pretty simple IMHO...
     
  6. PAR
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    PAR Yacht Designer/Builder

    I disagree Groper, simply because, as the OP described in his adventure with a pure displacement ride in a moderate gale. You'll take a beating for a lot longer, even if you ultimately feel safer. Making passages in these conditions is subject to several design elements - length, weight, speed range available, etc. Ideally, you want to be able to make the best speed you can, to limit exposure to the conditions. The ride and comfort of the the passage will be dramatically affected by the length and weight of the vessel, with longer being better in most cases.

    With displacement speeds, you are literally at the mercy of the conditions and simply along for the ride. If you have more speed potential, you can throttle up or down to suit the conditions, thus make best possible time, limiting exposure to the elements. For example you can ride on the back of a swell, keeping pounding and rolling at bay, if you can power up or down to suit. You haven't this option at displacement speeds.
     
  7. nzboy
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    nzboy Senior Member

    Having speed potential is the moot point .Sure a Stabicraft will get you there in 45 minutes but you have burned 100 litres of fuel in a craft that is the average price of a house .The alternative is a 30 ft launch which is about the right size for 4-5 people plus provisions .Something with a beam of 8ft might be rule beater concerning displacement speed .If you can cruise at 13-15knots that is probably the best compromise .To be able to plane at 20knots with 5 people and their gear with only a 100hp motor is a bit of an ask .Tads boat wedge point 31 is similar to a number of boats launched in nz in the past few years that you don't need a fuel tanker dockside
    W31-3GA.jpg
     
  8. groper
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    groper Senior Member

    This comes back to the old displacement vs planing definition PAR. My definition of planing, be it right or wrong, is when the craft is supported by a considerable amount of hydrodynamic lift, regardless of the actual speed in knots.

    What i thought was appropriate for this purpose, was a high speed displacement craft, such as a long, low displacement to length ratio craft (LDL) .

    If the craft is not supported by any significant amount of dynamic lift, then surely it's response to uneven surfaces in a seaway are lower than that of a fully planing craft whereby the lift generated is proportional to the ever changing angle of attack to the sea surface.

    Furthermore, monohull planing craft of small length to beam ratio, tend to ride on a lesser portion of the "at rest" waterline length, which can increase pitching and slamming as the centre of lift moves fore and aft all over the place depending on where the wetted area and CoG are at various instants in time. In other words, they can produce violent accelerations as a product of the constantly varying amount of lift and moving center of lift. All of this goes away if you significantly remove the dynamic lift from the equation...

    And everybody already knows by experience that longer boats generally have a more friendly response to pitching and more favourable natural period with respect to the choppy seas found in shallow waters...
     
  9. PAR
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    PAR Yacht Designer/Builder

    I have some experience with the now called LDL vessel types. It's actually an old (very) way of getting speed, but is also quite dependent on length to get comfort in a nasty batch of wet. The original "inclined plane" bottoms used this very technique, but there are limitations.

    Yes, I agree a LDL design might be an answer, but his SOR was for a fairly small craft, meaning you're going to have one skinny asss boat (~6' beam on a 30' boat is wee bit tight). We also agree in a maximized LWL, if for no other reason than to help bridge the short confused stuff, let alone make best speed in displacement mode. The violent acceleration periods will be greatly diminished with the speeds he's interested in. Running in the mid to high teens will be fairly comfortable, if the boat has sufficient way on to stay on top and run with the crowd, given he'll likely not have the length to bridge the wave trains. Will he pop off, sure will, but it'll be fairly infrequent, if skippered properly. This means a design that can get up at relatively low speeds and remain up at these modest speeds or a boat that is long and narrow enough to cheat the typical planing parameters and semi plane along in the mid teens. And yes, there's a few ways to approach this issue, but extra light, small craft, tend to get the crap beat out of them in anything other than smooth water, unless the 4 -6 people on board are centrally located. In any case it would be a fun project to undertake, regardless of the direction the OP wanted to go. Personally, I wouldn't want to be in a beam sea and a rough slosh in a light narrow boat, but I can see the appeal for the LDL argument.
     
  10. groper
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    groper Senior Member

    Indeed, however the OP also stated that length is not such an issue for him as th boat will be moored on a swing mooring so marina fees are not an issue. Sounded like a perfect opportunity for a very long boat.

    As is happens, I used to own a 30ft x 6ft beam panga boat... It was wet as all hell because it was a center console set up for fishing tropical waters, but it was of the best boats I've owned I'd have to say... Certainly not an LDL type hull tho, it was definitely a planing hull and built like a tank which quite heavy for its size.... Powered by a single 200hp Yamaha...

    I don't think 30ft hull is long enough , I envisaged something around 40ft length, and quite narrow for this length. It would be an essentially empty boat, having nothing more than lots of empty space/ voids, a long rear deck and a center cabin for the passengers to keep the building costs under control as I don't think this type of thing could be bought off the second hand market. I would power it by a marinizsd car engine of an aluminium block type common rail turbo diesel of less than 2L displacement. These can be had for under $2500nzd from auto wreckers for a very late model unit with only a handful of kms on it - already checked into this myself...

    I'm quite interested in this whole idea as I'm nearing the end of my current build and already thinking about the next one :) hint hint :p
     
  11. buzzlymutt
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    buzzlymutt Junior Member

    I'm Surprised no one has mentioned a power trimaran! a bit like a scaled down Peter Bethune"earthrace/Ady Gill". Around 12m long I'm sure 100hp could be OK.
    Also Peter Sewell's "whio".Google both.Cheers Mikee
     
  12. FoveauxSailor
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    FoveauxSailor Junior Member

    Hello Guys - I am watching but in limited coverage with a crap phone - will reply in full as soon as I get to a keyboard. Thanks

    Foster
     
  13. FoveauxSailor
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    FoveauxSailor Junior Member

    I meant to add that Grpher is getting very close to the money I think.
     
  14. Mr Efficiency
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    Mr Efficiency Senior Member

    You are not going to get away with safely and comfortably traversing this waterway in a motorboat that runs on the smell of an oily rag.
     

  15. nzboy
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    nzboy Senior Member

    Some of Bruce Roberts designs fit like a 28-32 coastworker with an 8ft beam planing and trailerable .Once you have a boat thats to big to get out of the water and take home the costs of ownership start heading upwards. These boats are no nonsense and very similar concept to the newer cray boats being built in the south eg. deep vee and planing
    CW28-05.jpg

    CW28-07.jpg

    CW28-06.jpg
     
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