Hull shape addressing breaking effect from pitching

Discussion in 'Multihulls' started by David L. Huber, Jan 6, 2023.

  1. David L. Huber
    Joined: Jan 2023
    Posts: 2
    Likes: 1, Points: 3
    Location: USVI

    David L. Huber New Member

    I’ll say on the outset that my goal is to design a mid sized blue water cruiser (read motor-sailer) that keeps the build as simple as possible. I’d like some thoughts on my proposal and reasoning behind it. So here’s my conundrum:

    I have noticed reading many of the discussions on hull shape, there is little exploration of what I would term “surface reentry drag.” Modern cats all have very fine plumb, axe, or reverse bows with very low half angles at and just above the waterline. But they also have round bilges and some degree of rocker to minimize wetted surface. The only problem with that is when the boat pitches over a wave and comes back down, the plane presented to the surface is not the razor sharp bow, but the extremely blunt bilge. The resultant breaking effect is quite noticeable especially the soft “thunk” and spray of water.

    The only way around this is seemingly to add wetted surface from midship forward. Wharram designs accomplish this with their deep V hulls which kill two birds with one stone. Since most of the wetted surface is closer to vertical, the entire hull provides lateral resistance, relegating the need for daggerboards. Daggerboards add significant cost, build time, more moving parts to break, and take up valuable hull space. Eliminating them is quite appealing.

    Racing cats and tri’s eliminate rocker forward of midship, but those are explicitly planing hulls. Another approach is to add fullness and thus buoyancy fore and/or aft to reduce pitching but high Cp is really only beneficial to large powered displacement ships and planing designs.

    My first question is what is the trade off between wetted surface drag and bow wave making drag?

    Second question is what is the trade off between reduced pitching and wider/slower turning radius with low to zero rocker hulls.

    I’ve seen several monohulls with zero-rocker, teardrop shaped full keels, but they still had very blunt vertical bows or long raked bows. Has anyone ever tried a cat with the same premise but with a razor sharp reverse bow?

    The hulls of Balance cats and Schionning designs are really excellent at negotiating these trade offs but those are performance cruisers that can easily approach planing speeds.
    fallguy likes this.
  2. bajansailor
    Joined: Oct 2007
    Posts: 3,287
    Likes: 1,314, Points: 113, Legacy Rep: 37
    Location: Barbados

    bajansailor Marine Surveyor

    You mention Wharram cats, and their so called 'advantages' above - yet any more modern sailing cat will still usually run rings around any Wharram.
    I am thinking in particular of the cats designed by @Richard Woods
    Richard Woods
    I sailed with Richard once on his cat Eclipse, and I was very impressed, especially as she was in full cruising mode, and had just arrived here after a long transatlantic passage.
    Sailing Catamarans - Eclipse - 9.9m performance cruiser
  3. David L. Huber
    Joined: Jan 2023
    Posts: 2
    Likes: 1, Points: 3
    Location: USVI

    David L. Huber New Member

  4. oldmulti
    Joined: May 2019
    Posts: 2,195
    Likes: 1,333, Points: 113
    Location: australia

    oldmulti Senior Member

    David. Multihull design is a compromise. I have sailed on many cats and tris from 15 to 84 foot. They all pitch in any sort of seaway. But there was one consistency the fuller the ends and more asymmetric fore and aft the less they pitched and often were faster hull shapes. General statement. Now a specific example. Spindrift 37 catamarans designed by Lock Crowther have relatively fine ends on there 12 to 1 length to beam hull shapes. The 37 can sail well upwind but pitched a fair bit. Some owners added bulb bows and fuller stern sections by adding foam glass additions. The waterline length was not changed. Result was the fuller ended 37 pitched less and sailed faster upwind in rising wind speeds. We are talking 10% plus better speed.

    Now we will talk about sea sickness etc. Some people do not suffer from seasickness, but most do. On the fine ended 37 I was seasick in a seaway but on the same boat after the full end modification I was less sea sick in a seaway. It may have been a slightly rougher ride but overall it was more comfortable. There are ways to minimize pitching and increase comfort, have a big boat (the bigger the better) with rounded bilge sections or have a cat with really fine hulls, that is length to beam on the hulls of 16 to 1 or greater. In both cases the boat is less prone to reacting to waves etc.

    in the real world of boats you and I can afford it is a trade off between ease of build, simple construction and hull shape to minimize pitching. Richard Woods designs can give a good guide, for a given size he has hard chine easy to build options to round bilge more difficult to build (EG Sango cat). each will have there advantages vs downside but a designer like Richard has had a lot of experience at getting the balance correct.

    The underwater hull shape is one of the easier things to construct in a multihull, if some one has designed a shape that suits your need then use it as the model. The design and build of a multihull takes a lot of time try not to compromise on EG hull shape to get the a really good reentry at the cost of other features. PS I recently learnt that the Rapido 60 trimaran that is built in full moulds etc in a factory takes 40,000 hours to manufacture.

    I wish you success and have fun.
    bajansailor likes this.
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.