40 ft cruising cat design. First steps.

Discussion in 'Multihulls' started by bscatam, Mar 28, 2016.

  1. bscatam
    Joined: Mar 2016
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    bscatam Junior Member

    Last renderings. I have done some changes and corrections but work is still in progress. Final material will be 80 kg PVC core and epoxy vacuum infused.
     

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  2. Squidly-Diddly
    Joined: Sep 2007
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    Squidly-Diddly Senior Member

    Why is the boom so high off the roof?

    Shouldn't it be skimming the roof for best power under sail?

    How about a wave breaker?

    Dual pods should be good for docking.

    I'd go with dagger boards and a single kick up stern drive in a wave breaker for extreme shallows.
     
  3. catsketcher
    Joined: Mar 2006
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    Location: Australia

    catsketcher Senior Member

    Not really my style of cat but looks nice in its own way. A couple of questions

    - Why is the boom so high? You don't need headroom above the cabin for the boom. Why not just enough height to safely clear a seated person in the chair. I would have to try hard to put a reef in or get the boom bag zipper going. I have seen some oldish people getting really unsafe on the end of a cabin doing the boom bag thing - about 4 metres up and using boat hooks and the like.
    - How do you sheet your extras? The idea with this type of helm and sheet arrangement is that it gets it all to one place and stops the cockpit having to have messy lines. I get it but its dumb.

    Real time use will have this thing flying a kite or Code 0 lots of the time. How do you sheet a line from the aft sterns? It will be very tricky. You will want to use a Code 0, or as I call it a reacher. They are fab sails, I have one up MOST of the time when coasting (up to 18 knots). Get the ergonomics of this right, put the winches (at least the secondaries) down in the cockpit.

    I want my ropes and controls in the cockpit. This means I can be having a rest or reading and then 2 seconds later be easing or trimming sheet. How long will it take to get to the helm from inside or in the cockpit. This is where you will be when the boat is under autopilot, which is a HUGE amount of the time. You won't be steering, you will be down getting out of the sun and wind.

    Last one - what are you going to do on a broad reach? Easing the traveller down will reduce twist until the wind is 80 degrees but what about bearing away more? I see lots of cats with twisted mains looking terrible. With the boom so high you will find it hard to barber haul the boom to the deck. A broad reach and run will have the main chafing itself to bits on the sidestays. Get the boom lower, put 4 extra winches on the deck for the extras and the barber haulers, so the main can have twist reduced.

    Most cat designers just leave it up to the owners to deal with twist. You need a large track and barber haulers (or a wishbone). I saw one bloke have to have his main restitched after only 1000 miles because of twist in his mainsail.

    cheers

    Phil
     
  4. UpOnStands
    Joined: Nov 2015
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    UpOnStands Senior Member

    not sure I like the idea of reaching over the wheel to access the winches and there is a space conflict between helm/nav instruments and rope tails?
    I would want to keep all lines clear of the wheel to prevent jamming.
    Is it possible to move the winches to the inboard side of the roof cutout and keep the space in front of the wheel completely clear?
     
  5. groper
    Joined: Jun 2011
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    groper Senior Member

    wow, this thing looks very much like my old Oram 39C custom... i cant beleive the similarities!

    Note:- This pic has some distortion due to the vertical panorama i made in order to capture it all in 1 pic.

    [​IMG]

    [​IMG]

    Couple things ive noticed on a quick read of this thread.

    Thicknesses - hulls were 16mm balsa core. Roof was 25mm and bridgedeck floor 20mm. Most bulkheads not inc the main beams were 12mm or 16mm. The main beams were 20 and 25mm.

    My roof had compound curve - yours doesnt seem to have any so stiffness will be a real problem for you - think about how your roof will get stiffness. My floor was also too springy, adding more structural furniture solved it, the stringers alone couldnt do it.

    The others have already made comments about the sailing practicality- all valid points.
    On my boat, when working with the sails whilst standing on the roof in a seaway, it could get pretty nasty up there with a risk of falling down onto the deck. Most of the sail handling should be done from down below in the cockpit to reduce this risk when things get nasty. Your boom is way too high, your wasting sail area....

    Your upwind genoa sheeting point will be on your roof, whereas downwind it will be on the deck. When its on the roof, its going to be right where all your lines are led back to your winches = keep than it mind when routing your other ropes...

    I agree with catsketcher- you will hardly ever sit up on top there, you will be down in the cockpit in the shade letting the auto pilot steer the boat. I put my winches up there to make routing the ropes easier from the mast and genoa... and i left the helm down below... but for every tack, i had to go up teh steps onto the deck and release the clutch, then run down, across the cockpit and back up the other side to sheet it back on-which was a PITA instead of leading the ropes back to the cockpit like i should have...
     
  6. bscatam
    Joined: Mar 2016
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    Location: Black Sea

    bscatam Junior Member

    Running rigging plan.
     

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  7. bscatam
    Joined: Mar 2016
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    Location: Black Sea

    bscatam Junior Member

    Steering.

    Just finished steering design. Conduit 5 mm cable.
     

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  8. UpOnStands
    Joined: Nov 2015
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    UpOnStands Senior Member

    nicely done but
    not finished until all support bulkheads are set and check how the cross bar can be removed/installed
     
  9. bscatam
    Joined: Mar 2016
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    bscatam Junior Member

    Crossbar will be devided in two parts and connected after instalation. Croossbar tunel cover will be laminated after that.
     

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  10. UpOnStands
    Joined: Nov 2015
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    UpOnStands Senior Member

    cross bar -- in or out?

    I have been wrestling with the bar inside the body but access to parts is tough.
    SIG 45 has semi covered
    Wonder if this is viable. It allows unlimited access for emergencies.
     

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  11. bscatam
    Joined: Mar 2016
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    bscatam Junior Member

    It will be covered. And it's an additional structural beam laminated to aft bulkhead.
     

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  12. bscatam
    Joined: Mar 2016
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    bscatam Junior Member

    Last renders.

    Last renders.
     

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  13. groper
    Joined: Jun 2011
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    groper Senior Member

    Looks like a lagoon copy- was that the intention?
     
  14. bscatam
    Joined: Mar 2016
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    bscatam Junior Member

    Any bridge deck configuration will be "looks like". Vertical cabin windows will always looks like Lagoon. The main difference should be in weight 5,5 t empty instead of Lagoons 9.:)
     

  15. DennisRB
    Joined: Sep 2004
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    DennisRB Senior Member

    Didn't you say it looked like your old Oram a few posts back? :p

    There is no mistaking the practicality of vertical windows. Anything to avoid the greenhouse effect in the tropics. Aesthetics well?? Beauty is in the eye of the beholder...

    As for aerodynamics? Well, it would appear obvious that raked windows would present less drag. But IMO the cruising catamaran form in general as drawn by bscatam, CNO, lagoon or anyone really punches a huge, unclean hole into the air. I would not be so sure that just adding raked windows will make that much of a difference. The structure is going to create a lot of drag anyway. Turbulence from wind blowing over the hulls at an angle as it funnels between them is going to be bad. Wind coming up over the front of the bridgedeck etc will create rotor and raked windows will not reduce the frontal area. I have never seen any attempt to quantify the actual drag in anyway other than the assumption it must be "much" better to have raked windows. I would really like to know how much it matters as a percentage of the total drag. I could stomach the look (oh maybe some rake) providing the drag penalty would be small. Seems like many modern designs are now going will much less rake then before. Look at HH etc.

    Great work bscatam by the way. You cad skills are amazing. If I built your design I would extend the hulls forward and back a little to get some sailing length without adding extra accommodations, add maybe a bit of window rake and go for the dagger board option. Its a nice design. Congrats.
     
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