Unusual design for an extra-heavy-displacement 11ft long Atlantic Proa for circumnavigation

Discussion in 'Boat Design' started by artis, Nov 24, 2023.

  1. artis
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    artis Junior Member

    Isn’t that only because these boys are towed at a ship’s speed which far exceeds the hull speed of the buoy travelling through the water? If yes, then you would never get those issues when trying to sail the buoy, and therefore never even reaching its full hull-speed, so you wouldn’t get the delamination of the water flow and the resulting turbulence, which causes all those things which you mentioned. That being said, I maintain that my intended main hull is more akin to a shape of a thick asymmetrical hydrofoil, rather than to a round blunt buoy.
     
  2. artis
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    artis Junior Member

    That’s exactly why I wrote all those things in my long post about the intended rig. The sails will be forcefully steering the boat and keeping it on the same course. It is a very different picture from the one where the boat is getting towed by its centrally located mast and allowed to wobble and gyrate and all kinds of directions…
     
  3. artis
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    artis Junior Member

    No need to list all of them, would you mind pointing out only the three biggest ones?
    I agree. I intend to start with smaller models for testing, then move to 1/2 size model which I could sail myself, and only then (if the idea still looks promising and all the details are already fine-tuned) I intend to build the actual full-size boat.
     
  4. artis
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    artis Junior Member

    Please see my earlier post where I describe the intended ventilation system.
     
  5. artis
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    artis Junior Member

    Yes, and that’s why my intended project is also radically minimalistic and it is as over-built as Roger Taylor’s ferrocement yacht with his massively over-build coach roof, that he built in New Zealand after the big shipwreck.
     
  6. artis
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    artis Junior Member

    Isn’t it exactly what the bell buoys are designed to do? As far as I have noticed, they have rather shallow (compared to the above water part) and almost round underwater part. They are meant to wobble and sway around to ring that bell. That’s why their movement is so different from the long-underwater-body spar buoys, which stay relatively calm even in big waves.
     
  7. artis
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    artis Junior Member

    Please see my earlier post where I explain my attitude towards the currents and how I intend to deal with them. I will be thankful if you spot any problems with that.
     
  8. artis
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    artis Junior Member

    True. That’s why I said, I will be happy if I can achieve halfway between a typical production boat and the FLIP.
     
  9. Rumars
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    Rumars Senior Member

    I'll give it a shot, maybe you'll hear me. Many of the things have already been said, but apparently you don't understand why those posters are right, I'll do my best to explain.

    Motion sickness has nothing to do with rolling, it's generated in our brains by conflicting sensory information. That's why people get sick in cars, trains, planes and even elevators. There is a whole body of medical literature on the subject if you want more info. If roll would be worst then pitch, all you had to do to alleviate the suffering would be to orient the patient differently in the boat, so that from his point of view he isn't swinging left to right but forward and back.

    The FLIP isn't moving because it's supported by still water (the deep layers). Your boat won't have sufficient draft to benefit from that effect in waves over 0.5-1m, while 3-4m waves aren't exactly rare in a storm. To visualize get a big soda bottle, ballast it so it floats vertical in your bathtub, then make different height waves with your hand. Observe the motion of the bottle, your boat will behave similarly.

    You are building the perfect pitching machine, and roll behavior will not be far behind. Let me explain why:
    There are several factors that affect pitch. First is waterline lenght, this has to do with basic form stability, longer is better. Second is underwater shape, or better said the buoyancy distribution. Symmetric is bad, asymmetric is good. Third is mass distribution in relation to buoyancy. Weight in fine ends bad, think levers.

    Your boat is short so no form stability. Since it is about as long as other yachts are wide, it will pitch as much as those yachts will roll.
    You have chosen a proa, wich by definition has to have symmetrical ends. Even if you have no rocker at all this doesn't change the fact that there is no asymmetry to dampen and stop the oscillation once it begins. This is why virtually all modern boats, regardless of how double ended they are above the waterline, have pronounced assymetry in the underwater shape. Nobody goes symmetric unless forced, and proas are the only boats where it's kind of mandatory.
    On top of the above you have chosen to not only put weight on a fine end, you actually cantilever the masts beyond the end of the boat. Without any buoyancy underneath the only thing that resists the combined gravity pull and downward force from the sail is the weight of the hull. As soon as the system is out of equilibrium (meaning it moves) a perpetual fight for balance will begin. The only time it is in equilibrium is in a flat calm.

    Roll: it may surprise you but atlantic proas roll. They behave exactly like a trimaran with a lot of dihedral, that is like a bicycle with training wheels. The outrigger will only stay firmly planted in the water if you have enough force on the sails to keep it there. You might say that the boat doesn't actually have enough beam to behave like a typical proa, but that's just a question of wave height.
    Why you have chosen the atlantic proa or indeed any multihull concept is beyond my understanding. The main hull is so heavy and deep that it will override the float in any conditions. Its weight is also more then enough to whitstand the force of the stated sail area alone.

    Steering: Steering with the sails is only possible if you have enough sail area for the given wind conditions. Short of a gale the boat simply doesn't have enough sail for its weight and surface area. Proas also like to have the CoE moved laterally to balance without rudders, but that won't be a concern.

    Here is how I imagine your boats behavior:
    Light winds, you don't have enough sail area to move and steer in a directed way. It will slowly drift in the general direction the wind is blowing and the waves are moving. Motion will depend on actual surface conditions, residual swell will be unpleasant.
    Moderate winds you will have a modicum of control over where you are going as long as it is in the right direction. No upwind sailing whatsoever, motion as above. Any sort of wind against wave, tide or current will overpower you, motion will be like beeing in a washing machine.
    Heavy winds to full gale, you finally have the power to steer and move, only you won't be able to. The waves will be high enough that they will periodically blanket the sails, you loose drive and move on momentum alone. With such a short boat directional stability will be poor, the thing pitches and yaws, if you don't broach immediately you do when you catch the wind again because now it's from the wrong angle.

    Get your wife to take a tour on the biggest catamaran you can find. If she can stand it in moderate conditions that's your best ticket for a boat. You don't actually have to spend a fortune to sail a 20m cat as long as you are willing to make some compromises on comfort and forget the no rudders, no engine approach. To give you an ideea of what's possible, search for "Ontong Java 2" and "prao-planche a voile". Both concepts would need some tweaking (especially the proa) for your purposes, but it's a start.
     
  10. artis
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    artis Junior Member

    Just a quick correction of my error before I properly read the good, long Rumar’s post: I was referring to my previous post regarding the ventilation system, but I only now realised that the post was not here, but on Cruisersforum. Sorry about that, I will now paste that post below here:

    Thanks for the comment. Yes, I have been thinking about that. My plan is to have the ventilation done through the unstayed masts. On top of the masts would be dorades, facing the opposite directions on each mast so that one of the masts funnel the air in and the other one sucks it out. Then the air would flow through the mast itself as through a pipe. At the mast base would be a siphon with a drain back to sea. From the siphon the air would go through the ventilation pipes to the lowest underwater room, which is at the highest risk of accumulating too much CO2 without ventilation. Just before the air exits in the room I would install a heavy duty valve on the pipe (like the ones made for the extra thick water-pipes), so that I can slow down the airflow during heavy weather, or close it completely in an emergency. The pipes can be made as thick as the masts, and the dorade funnels can be made big enough to have more than enough airflow through the saloon. Plus during calm whether our hatch will be open too, equipped with the Hassler hatch cover, which actively sucks the air out from the saloon. All of that combined, I hope should be more than enough to keep the boat well ventilated.
     
  11. artis
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    artis Junior Member

    Thank you so much Rumars for taking your time and writing this long and amazingly valuable post. Let me think, re-read it again for a few times and rethink and reconsider all my assumptions again, before I say anything. The text which you are writing is literally full of pearls… Thank you again, I appreciate it so much! I will be back.
     
  12. CarlosK2
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    CarlosK2 Senior Member



    Here we see the rudder in the hands of the jib sheet.

    You can be sailing like this for days and days.

    "Fast surfing is fun for a single afternoon"

    One thing is a racing machine Planing at high speed and another thing is to sail a small sailboat at 7 knots of speed but able to go in harmony with waves reaching 14-18 knots for a few seconds.
     
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  13. skaraborgcraft
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    skaraborgcraft Senior Member

    I find it strange you ask this question, when you have said you have owned engineless sailboats. As already pointed out, your "hull" is always going to be in the water rotation zone and not in the deeper less disturbed area. I have experienced less wave induced roll on a flattish bottom lightweight boat with a skinny fin keel and rudder, than a full bodied Colin Archer type. The flip side to full bodied keels and their "roll damping" is often just a slower induced roll, but one that can be larger and possibly more violent, as they offer more resistance/ surface area. Most of my offshore sailing has been done in what would be described as Ultra Light Displacement Boats, but equally i have been sea sick on a Colin Archer type. I have yet to try a catamaran, still reckon I would be sick, as Rumars has pointed out, its accelerations that matter. The body adapts over periods.
     
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  14. gonzo
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    gonzo Senior Member

    How will you survive in the doldrums?
     

  15. skaraborgcraft
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    skaraborgcraft Senior Member

    My guess is a lot of time cooling off in the "shower room".
     
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