Sailfish Redux

Discussion in 'Sailboats' started by parrottdesign, Jun 26, 2011.

  1. parrottdesign
    Joined: Dec 2010
    Posts: 15
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Cincinnati, OH

    parrottdesign Junior Member

    I am trying to create a cartoppable, simple, inexpensive, light air sailboard, and I have no idea where to start. If you have a moment, any direction you could provide would be most appreciated!

    Here's the skinny: Basically, I want to make a new version of the old Sailfish but with a hull modified for light air, Midwestern sailing. I started with a couple of lines (attached below). It's 16ft long and 40" at the widest point. (I know that the drawing format is incorrect. My apologies. This is my first go.)

    As for the rig, again I need direction: I'm thinking of a tall (for the size), high aspect ratio, square top main with a genoa (small bowsprit?) and whisker pole.

    ____________________________________________________

    I would love to get some advice on the hull shape. In particular, what modifications should be made to improve low wind performance while balancing it against some attributes that will allow performance if the wind (ever) picks up.

    Thanks!

    David
     

    Attached Files:

  2. Doug Lord
    Joined: May 2009
    Posts: 16,653
    Likes: 322, Points: 93, Legacy Rep: 1362
    Location: Cocoa, Florida

    Doug Lord Flight Ready

    I think it looks very good except for the bow. I think the bow sections could be sharper which would help in penetrating short lake chop. Take a look at the unfinished balsa/carbon model below. Its an 18' planing main hull for a trimaran. Best reference I have at the moment for the sharper forward sections. Do you have any numbers?
    PS: you might try narrowing the boat up at about 25% aft of the bow and widening the transom

    Just for reference you might take a look at the Cherub designs here-they are a high performance dinghy but might give you some ideas: http://www.sailingsource.com/cherub/idx-chistory.php Their Length to Beam ratio overall is to high for your app-you might just look at the bows and waterline shape. Good luck-it looks like a fun project!

    Pictures: l to r 1-3 unfinished tri model, 4,5 6 & 7: the red hull started life as a foiler but the bow really impacted the short steep intercoastal chop and hurt performance. The model is based on this hull with the bow modified. Boat was used by a sailing school as a sort of Sunfish-much, much faster even with the little rig shown. All up weight of the unmodified hull about 110lb. (carbon and foam)
    click on image:
     

    Attached Files:

  3. parrottdesign
    Joined: Dec 2010
    Posts: 15
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Cincinnati, OH

    parrottdesign Junior Member

    Now that's a great looking project. Do you know how that hull performs in light air?
     
  4. Doug Lord
    Joined: May 2009
    Posts: 16,653
    Likes: 322, Points: 93, Legacy Rep: 1362
    Location: Cocoa, Florida

    Doug Lord Flight Ready

    ===================
    Thanks. When it was sailed at the sailing school, the guy sailing it told me that it was faster than a Sunfish-that is, the hull w/o the bow mod was faster than a sunfish in light air. With the bow mod it would be slightly faster due to the 2' added length. Modifying the bow would not hurt light air performance and would help a lot in any chop. Here, the intercoastal can get a nasty little chop in 6-8 knots of wind if it is from the south or north so the bow would help a lot in those conditions.

    PS-might want to try to keep the waterline entry angle at the bow around 22 degrees or less......
     
  5. parrottdesign
    Joined: Dec 2010
    Posts: 15
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Cincinnati, OH

    parrottdesign Junior Member

    Rev 2

    How is this? I've made the bow a bit sharper and rounded the chines on the back. It's still 16' in length and 40" beam. I don't know how to calculate the position of the waterline, however, or to take a stab at any of the more technical details.

    If I'm trying to optimize for light air, should I be narrowing the stern or something to make it slip along easier? What's a good compromise that might still allow it to get up on plane if the wind comes up?

    Thanks again for all your input!

    David
     

    Attached Files:

  6. Doug Lord
    Joined: May 2009
    Posts: 16,653
    Likes: 322, Points: 93, Legacy Rep: 1362
    Location: Cocoa, Florida

    Doug Lord Flight Ready

    ==================
    I think it is much improved. Unfortunately, to do it right, you need to start looking closely at the numbers-displacement calculations, prismatic etc. That gets very involved and is a bit much for someone to be able to tell you in a post. Do you have Skenes Elements of Yacht Design or Principles of Yacht design by Larsson and Eliasson?
    I've got a paper done by another member that has detailed numbers for several different boats around this length -see below. And another stats paper as well. Also, see the Design Ratios paper by Eric Sponberg-written in terms that are understandable, clear and concise(below).
    Another way to proceed is to enlist the help of a pro if that would fit your budget-some na's charge by the hour and can be very helpful in a short time.
    Eric Sponberg is an na and marine engineer and can consult with you and give you an estimate of how much time he thinks it would take. He would help only to the degree you'd want him to for an hourly fee.

    Note: At the very least the stats below can give you an idea of the SA and weight of other boats. To the extent you don't understand some of the comparitive ratios presented in either stats paper, use Erics design ratios(commonly accepted) and calculate them for the boats closest to yours- esp. D/L and SA/D .
     

    Attached Files:

  7. CT 249
    Joined: Dec 2004
    Posts: 1,701
    Likes: 78, Points: 48, Legacy Rep: 467
    Location: Sydney Australia

    CT 249 Senior Member

    It sounds and looks as if you're going for a scow type. Interestingly, in several scow classes (As, Fireballs, old Moths) and even scow-style Raceboard windsurfers, the wider bows perform better.

    The International Fireball is fairly close to your design in some ways and in that class it's generally accepted that the "wide bow" style made the older fine-bow boats obsolete. In 'balls it's consider that the flatter rocker created by the wide bow is the advantage, which fits with the original scow theory I believe. Personally I also wonder if the wider bow offers straighter waterlines and possibly greater lift.

    There's plenty of information about wide-bow Fireballs; see for example

    http://www.fireball-international.com/html/wide_bow.html

    http://members.boardhost.com/fireball/msg/1249679990.html

    http://www.fireballsailing.org.uk/forum/forum_view.php?message_id=178

    http://www.fireball-international.com/html/jhs_intro.html

    Since the A Scow "Victory by Design" (IIRC) team also went for a wide bow as did later scow Moths, it's likely that the extra speed of wide bows is not just a Fireball matter.

    This does go against design trends in other classes, which have gone to finer bows as Doug pointed out. However, the theory is proven in practice, and it's not surprising that scows work differently to conventional hulls.
     
  8. Doug Lord
    Joined: May 2009
    Posts: 16,653
    Likes: 322, Points: 93, Legacy Rep: 1362
    Location: Cocoa, Florida

    Doug Lord Flight Ready

    A scow shape can be a good hull but you have to sail it a fairly substantial angle of heel in light air to reduce wetted surface. Before attempting a scow hull you should probably have some experience with the type. They are all over the midwest. Here are some images and specs on the MC scow-16'-note the 68" beam and sail area:
    http://www.melges.com/boat.php?p=pages/boats/MC-scow

    For your purposes,as I understand them, I think your last sketch is right on the money. But now you have to learn a lot about design--good luck!
     
  9. JRD
    Joined: May 2010
    Posts: 224
    Likes: 17, Points: 18, Legacy Rep: 192
    Location: New Zealand

    JRD Senior Member

    David, I like the look of what you are trying to do. I assume its just for your own use at this stage?
    I have Larsson and Eliasson, and recomend it as being easy to apply provided that you have reasonable maths ability and preferably a little engineering background and time to work through the examples they give. Even though it seems to focus on keel boat design, my interest lies in centreboard yachts and most if not all of their workings can be scaled back to smaller boats with some thought where the theoretical design is required.

    Unless you add some kind of wings there is not alot of righting moment so you will be limited in the height of rig you can balance with more than a little wind. You can get more area and keep the COG down with a jib and main, (I wouldnt go to an overlapping genoa) but there is more cost for extra sail and extra running rigging.
    At 3.3' beam for the main hull you may find its more comfortable to sail if you flare out to give more width at deck height (look at NS14 photos), or add some short wings. You need to keep in mind the length from your ankles to your thighs/knees to give a distance from the hiking straps and the gunwhale where you will be sitting out.

    Is this for single handed or crewed sailing? This has a big impact on how much weight you need to carry, therefore how much displacement to design for, also how much sail area you can carry. Also if you are single handed the jib would be better with self tacking track, as tacking will be pretty slow if you have to set the jib on each side every time. If you want it to be a racer, have a look at moth and international conoe rigs.

    Any thoughts on what materials you will use?

    At the stage you are at I would tend to develop the rest of the layout, rigging plan, centreboard location etc. This can all be sketches, dont be afraid to post them here for comments. Work out the all up sailing weight, then continue to develop the hull shape based on that.

    Jeff
     
  10. parrottdesign
    Joined: Dec 2010
    Posts: 15
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Cincinnati, OH

    parrottdesign Junior Member

    To answer the first question, this thing is certainly for personal use at this point. I have been waffling between building this or a skin-on-frame, slightly larger version without integral flotation. I am still undecided, but I am leaning toward this version because if it actually works, I can make molds easily. (This is likely flawed reasoning as this is unlikely to be a good boat my first time out of the gate.)

    As for resources, I have been reading Larsson and the Bethwaite book and have been referencing a number of designs of similar size. I have thinking a lot about the International Canoe as reference for some of the basic proportions. As my main area of concern is low wind performance, a thin, slippery hull (not unlike a paddled canoe/kayak) is starting to seem more like a good place to start. I am hoping that a slightly wider hull with round chines will give it a bit more stability, room for a flat section in the middle and the ability to heel a bit to lift that flat section out of the water in really light air (like the scow suggestions above). I will look into the rig and centerboard placements this week and will probably opt for that self-tacking jib you mentioned. At this point, the project is intended for a single person.

    I haven't been mentioning my construction technique because I didn't want to distract from the light air theme of the thread. However...

    Since you asked, I plan to make the prototype by: dividing the hull into sections in CAD (as shown in the drawings), CNC cutting these sections in cardboard at a friend's display firm, sandwiching polystyrene foam between the sections and sanding (hot wiring) the foam down to the sections to make a male blank. Then I will lay the whole thing up like a surfboard with the added dimension of using woven flax and a UV-cure bio-resin that I have left over from an earlier strip-planked project. This stuff cures beautifully under a tanning bed (purchased for the purpose) or out in the sun. It will be heavier than necessary with all that styrene, but it will provide an ok test platform and mold plug.
    If I eventually make copies of the hull, fiberglass molds will be taken from this prototype and "production" models (by that, I mean copies for friends) cast in the same hand layup, UV-cured flax/bioresin composite.

    That said, let's just ignore the whole construction question for the time being so that the thread doesn't get sucked into an argument about the relative merits of flax vs. s-glass or the ecological impact of different construction methodologies. (I find that if I talk about these types of construction methods, all anyone wants to do is needle around for little ways to pick the methods apart, find evidence of "eco-hypocrisy", etc.) I really appreciate all the advice I've gotten so far and would hate to have this thread digress into that kind of negativity.

    Thanks again, to everyone, for all the amazing input!
     
  11. sharpii2
    Joined: May 2004
    Posts: 1,944
    Likes: 133, Points: 63, Legacy Rep: 611
    Location: Michigan, USA

    sharpii2 Senior Member

    Looks like you went for sail carrying capacity over whetted surface.

    I would be tempted to draw arc like sections to reduce the whetted area a bit. Even a deeper 'V' will do that.

    The real secret is probably going with a bigger rig that can be reefed when it breezes up.

    A number of people have made reefable boomed lateen rigs. My favorite is a fellow by the name of Jason Nieghbors.

    The yard on his lateen starts out damn near vertical with a jib in front to balance it. When reefing, the jib is struck, the reef is tied in the lateen sail and the boom down hold is shifted aft on the boom, allowing more of the sail to end up in front of the mast to make up for the missing jib.

    He has used this rig on a series of 'puddle ducks' he has built and used to sail the Texas 200 raid cruise. www.pdracer.com

    The main idea is that, in light air, you want a butt load of sail. Little else is going to be much help.

    A well cut sail with a deeper camber will help too, as long as you can flatten it out when the wind builds.

    As for myself, I would go with a flat cut boomed lateen with about 100 sf of sail and keep the hull you have drawn. Flat cut sails are not as efficient as fuller cut ones, but they can be feathered in gusts without fluttering themselves to pieces.

    I once sailed a Super Snark(r) that had flat sails in gusts of over 30 mph, without capsizing once. And that included up wind sailing. It was slow getting upwind, as the sail had to be feathered a lot and I had to bail water that slipped over the bow, but the down wind ride was a sleigh ride I will never forget.

    I don't know how long the hull you drew is supposed to be, but I wouldn't be surprised if the boom and the yard ended up being longer than the boat.

    The boom and yard would be about 14.5 ft each. And I would be tempted to go with 15 footers to get about 109 sf. That would be at least 30% more sail than a stock Sunfish(r) has, IIRC.
     
  12. sharpii2
    Joined: May 2004
    Posts: 1,944
    Likes: 133, Points: 63, Legacy Rep: 611
    Location: Michigan, USA

    sharpii2 Senior Member

    I looked at your second drawing.

    You have a pretty pronounced pinched bow.

    That and lack of free board up front can equal the bow shunting under.

    To keep this shape, you may have to move the rig aft considerably and that means the boom extending way past the transom as well as the dagger board ending up where you have to sit. A taller rig with a shorter boom, to cure the second problem may well make the first problem worse.

    Such a hull works best with a big jib where the jib provides a lifting component as well as a driving one. A classic sloop/cutter arrangement. I doubt that fits in with your simple, car topper concept. You'll need a fore stay and at least two shrouds and your beam may be too narrow for those shrouds to get a good angle.

    If you look at just about all cat boats, you'll find that the Beam is pretty far forward.

    The Sunfish(r) hull may not be the most efficient hull, but it is likely the most efficient hull for that rig. Little wonder it is considered a classic by many.

    For your hull, I would move the Beam forward of half the hull's length, increase the Beam by about 4.0 inches, and shorten the hull by 17 inches. When it comes to sail carrying capacity, Beam is king. Shortening the hull insures the same whetted area, and, more important, the same deck/hull surface area (weight).

    I would also add an inspection port somewhere up front for some provision for dry storage. Tie down strap eyes for a sport bag would also impress future buyers.
     
  13. parrottdesign
    Joined: Dec 2010
    Posts: 15
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Cincinnati, OH

    parrottdesign Junior Member

    Thanks for all your advice! I'm working on another iteration of this hull that will incorporate a few of the features you mentioned, including a more rounded and slightly beamier hull. I'll take a look at Mr. Neighbors' rig as well.

    Are there, in your experience, ways to get a narrower, more rounded hull like this to plane if the wind ever does pick up? How much of the hull must be flat to accomplish this?

    Thanks again!
     

  14. sharpii2
    Joined: May 2004
    Posts: 1,944
    Likes: 133, Points: 63, Legacy Rep: 611
    Location: Michigan, USA

    sharpii2 Senior Member

    Rounded cross section surfaces can plane too, but not as well as flat ones. Surprisingly, a fuller bow facilitates planing. It does this in two ways:

    1.) It pushes the water aside rather rudely, down as well. This causes the bow to lift and
    2.) by having more curvature up front, in profile, you can have less curvature aft. In fact it is possible to have nearly straight buttock lines aft (or forward, if you are designing a non planing sailboat).

    The the longer and straighter your aft buttock lines are aft, the more willing the boat will be to plane.

    The thing is not to over do it. An extreme planing hull also has a wide, immersed transom. A very bad idea for a non planing boat. So, in light winds, its performance will suck. Watch a planing power boat in non planing mode and you'll get the idea. It's best to angle your aft buttocks up to keep the transom just touching the water when at rest, for this reason.

    You probably want good performance in a wide range of conditions, with the capability of planing. Taking my suggestions from my previous posts will go a long way towards getting you there.
     
    Last edited: Jul 6, 2011
Loading...
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.