Dangers of shallow draft for sailing cat?

Discussion in 'Multihulls' started by Markusik, Oct 2, 2018.

  1. Markusik
    Joined: Jun 2017
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    Location: Lake Michigan

    Markusik Junior Member

    My thoughts tend towards shallow draft sail cats intended for liveaboard at shallow anchorages. I’ve always thought of low draft as a safety feature, more likely to ride over or avoid submerged objects. A naval architect (name escapes me at the moment) on YouTube was talking about the risks of having too shallow a draft as it pertains to heel angle and rotational force imparted by waves. It was implied he was talking about motor cats, but many of the same factors would apply. Taking him at face value has complicated my vision.

    The cat I’m playing with is intended to be 12.9m LOA, 7m BOA, 0.79 bridgedeck clearance, <0.46m draft with boards up, 11-1 LWL/BWL, ~5500kg at DWL. Mostly semicircular hull sections, or multiple chines, depending whether FRP or aluminum. The sail plan is conservative, the weight is mostly low and lateral in the hulls despite a large high-windage bridgedeck saloon. Weight saving strategies include desiccating (aka composting) heads, watermaker to reduce storage needs, ample solar with reduced battery capacity (say 300Ah@24V, probably LFP), finish to a light inexpensive standard, and petrol outboard auxiliary propulsion.

    It’s unlikely this cat will ever fly a hull by wind forces alone. I understand the concept of rotational force being imparted by waves and winds, but I don’t really grasp how a shallow draft will put this hypothetical cat more at risk. Could someone please take a moment to tell me, or tell me that I shouldn’t place too much weight on what I hear on YouTube?

    Thanks in advance,
     
  2. JosephT
    Joined: Jun 2009
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    Location: Roaring Forties

    JosephT Senior Member

  3. Markusik
    Joined: Jun 2017
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    Location: Lake Michigan

    Markusik Junior Member

    Thanks JosephT. I’ve read that article, and many others. My cat (besides the furry one, and the Hobie) is purely hypothetical for now. I did manage to track down the video that prompted this;



    Simplistic, but it was enough to make me wonder about the wisdom of a shallow draft. It’s true that everything is a compromise with boats, I’m just hoping for a brief explanation of how (in general, on a daggerboard cruising cat) having a shallow draft increases risk.
     
  4. JosephT
    Joined: Jun 2009
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    Location: Roaring Forties

    JosephT Senior Member

    Hi Markusik, that video is nebulous. He basically says "go find a naval architect" to resolve various items without explaining anything. The rolling behavior he describes really boils down to the center of gravity (CoG). Other factors play a role (e.g. center of buoyancy, righting moment, etc.), but the CoG is the main concern. With a shallow draft boat the CoG is higher so they would be more prone to capsize. Since you have a retractable daggerboard, if a storm is approaching, for example, you would be smart to leave it down to keep your CoG as low as possible. Conversely, the boat should be loaded so the gear & suppliers are as low as possible (keeping the CoG low). Being stuck in the shallows during a storm is not good any way you slice it. Deeper, protected harbors are best.
     
  5. Markusik
    Joined: Jun 2017
    Posts: 22
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    Location: Lake Michigan

    Markusik Junior Member

    Thanks JosephT, I appreciate the concise explanation.

    I agree, in rough weather deep harbors are best. In fair weather I still prefer getting right close to shore.

    Fair winds!
     

  6. BigCat1950
    Joined: Nov 2018
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    Location: Seattle

    BigCat1950 Junior Member

    JosephT, none of this information is correct. Calculations for sailing multihull stability take into account only hull center to hull center distance, and displacement. The Coast Guard also adds in the height of the center of the sail plan and sail area for passenger carrying sailing catamarans to calculate stability under sail. As for harbor depth, being sheltered from waves is the main issue, if the water is deep enough to keep you from going aground at low tide. (I designed a Coast Guard approved passenger carrying sailing catamaran, and sailed across the Pacific via the scenic route in a monohull.)
     
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