"F3 Floats": Monohull sailboat with small floats

Discussion in 'Sailboats' started by Dolfiman, Jul 27, 2020.

  1. Dolfiman
    Joined: Aug 2017
    Posts: 1,123
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    Location: France

    Dolfiman Senior Member

    I did a previous investigation of the Figaro III with retractable foils which showed that the foils were a priori oriented to hit and help sustain more high speed from beam reaching by breeze, when the boat speed can be > 12 Knots. But upwind, when the boat speed is around 7 knots at the best, or even downwind when the speed is around 10 Knots (which is already high for a 30' boat), the foils contribution seems low and may be not worth the investment for the average sailor. In quote # 43 of this thread :
    New Figaro 3 launch and first sail https://www.boatdesign.net/threads/new-figaro-3-launch-and-first-sail.59086/page-3

    On the other hand, the foils shape + the retractable systems are designed to reduce at the best the beam overall when in a marina, which is a good point towards a more broader use of such concept.

    So I started from these two observations to imagine an alternative version, more cruising oriented, with replacing the foils with streamlined 3D bodies, i.e. replacing the dynamic lift by an archimedean lift of the order of 300 kg ± 100, with the objective to be rewarding when upwind at 6-7 Knots, or downwind at 10-12 knots. In these speed ranges, a submerged streamlined 3D body can provide an archimedean lift significantly contibuting to the righting moment with a good Lift/Drag ratio. It is not yet a trimaran concept, but just a fraction of the archimedean lift (~ 10% of the displacement) extended leeward thanks to a submerged streamlined body.

    From this exploratory study here attached, made by comparison with the same hull without floats and at same displacement (the floats system estimated mass being then put in the bulb of the keel), it is possible to identify and quantify the advantages and some disadvantages or at least uncertainties :

    Advantages :
    • a significant reduction of 4 ° to 6 ° of the heel angle (with the 400 liter float option), i.e. a maximum heel of around 20° before thinking of reducing sail to the first reef,
    • a higher speed : ~ + 0,4 Knots from wind force 3 upwind, + 0,5 to 1,0 Knots from wind force 4 downwind (with the 400 liter float option).
    • with light winds, it is possible to sail with the leeward float retracted with little or no contact with the sea surface.
    • in a marina, floats retracted, the overall width remains that of a beamy monohull, in the case studied Boa 4.07 m for a Loa of 9.75 m. The floats are then clearly above the water and not affected by the bio-fouling which develops while not sailing for a long time.
    • in case of dangerous wind / sea, it is always possible to sail with retracted floats so to not expose them to significantly high efforts, and the knock down eventual case is then the one of a monohull without the capsize risk inherent to a multihull.
    • The sliding structure, common to the two floats is situated on the deck in front of the mast: it does not have to be watertight, it simply has to be strong and simple and can be kept open for an easy access to the mechanism. The volume under deck is not reduced by the system. The risk of water ingress through this sliding structure is null due to the absence of any communication with the interior of the boat.
    Disadvantages or Uncertainties :
    • we cannot avoid a slight speed deficit when close-hauled at around 8 knots of wind speed, of the order of ~ 0.1 knots, when in the transition between the retracted and deployed modes
    • the flexural / torsional strength of the support arm have to be checked
    • the comparison is made at equal boat weight, the mass of the float system (180 Kg for the F300 liters, 240 kg for the F400 liters) being added to the mass of the bulb-keel in the version without floats. It is not an economic comparison which should compare the two versions at equal price (only a manufacturer can make this comparison), i.e. the version without floats could be a little longer and / or wider at equal cost. But that would only affect the gain in performance and habitability, not the heel angle reduction.
    To summarize the concept, it can be seen as a heel limiter (-4° to -6° less heel angle) together with a gain in speed of + 5% to + 8%.

    By hoping this exploratory study can be of some interest and as usual many thanks in advance for your comments.
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