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Discussion in 'Boat Design' started by lewisboats, Aug 2, 2012.

  1. lewisboats
    Joined: Oct 2002
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    Location: Iowa

    lewisboats Obsessed Member

    Open. comfortable, laid back day sailors in the 15-17 foot range. How much freeboard do you like to have. No hiking out, just some relaxed sailing in rather than on the boat. Would a depth of 20 or so inches and a freeboard of about 16 inches be about right on a hull something like this. Use would be on small to medium sized lakes at first... more ambitious stuff perhaps later.

    LOA 15 ft 10 in
    Beam 4 ft 10 1/2 in
    Draft 5 1/4 inches.
    Depth 20 1/2 inches at lowest point of shear


    [​IMG]

    I have only ever sailed on stuff you have to hike out on (Butterfly or my Scow) or in small underpowered canoe styles or my daughter's Eider duck or in a large cruiser style. What makes you feel comfortable in as far as sitting comfort to shear height in this size boat.
     
  2. LP
    Joined: Jul 2005
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    Location: 26 36.9 N, 82 07.3 W

    LP Flying Boatman

    Steve,

    I think that on a boat of this nature, beam will play a larger roll than sheer height. Of course, you have the trade offs of going with the larger beam. You get two advantages with more beam. You get higher form stability and you get the "hiking" factor without the hike. In theory, you could design a hull to be sailed with X amount of heel and only the leaward portion of the hull is left in the water. An aggressive V to keep the BWL narrow for sailing flat, but quick to pick up form stability when heeled and working to windward.

    Nice looking hull. It looks like you have already worked a lot of the qualities I've mentioned into this hull. Going back to your sheer question though, What angle of heel do you plan to sail her at and how much margin do you want for before downflooding. Small side decks might help keep the sheer low, but that is more work, weight and limits weight shifting within the hull.
     
  3. jehardiman
    Joined: Aug 2004
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    Location: Port Orchard, Washington, USA

    jehardiman Senior Member

    An adult should be able to reach the water without having lean over rail. This will also make her easy to board from a beach/float. Make her self rescuing with proper floation tanks/underseat foam.
     
  4. lewisboats
    Joined: Oct 2002
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    Location: Iowa

    lewisboats Obsessed Member

    Not looking to ride it on its ear but some heel is plenty fine. According to DS the shear won't touch water until around 38 degrees of heel. I would feel comfortable up to about 20 degrees which would maybe make it up to the second chine level. I plan on putting in small (3-4") side decks/tanks, incorporating the backs of the longitudinal seats as the inner tank side. Not looking to carry a lot of people...2 or max 3 but 90% only me. Since the Rend lake messabout my knee has been on the fritz and Saturday it dislocated on me. I want something that I don't have to fling myself from side to side as I maneuver the boat, and something I can do a bit of camping in too. The cockpit will be around 6.5-7 ft long and around 4 ft wide with front deck/tank and after deck/tank. I can bump up the beam a little if need be I don't want to get too wide to make rowing it too difficult. I don't want to go below 3:1 L:B ratio. I also want to keep the weight down. I am considering a 120 sf (total) Gaff Yawl with Jib with 65 main, 37 jib and 18 mizzen. Power but low down and spread out to keep the lever arm down... lots of reefing options too.
     
  5. Wayne Grabow
    Joined: Aug 2003
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    Location: Colorado

    Wayne Grabow Senior Member

    My impression is that you have this pretty well thought out, Steve. Looks like a good plan. When we lived in Panama, I built a spritsail-rigged sailboat 4' by 15' 10" to use on Lake Gatun for fishing and exploring or we could take the whole family to picnic and snorkel on an ocean bay near Portobello. It was much simpler than what you propose, but lots of fun. Keep the beam somewhat narrow, it will row better and be more easily driven under sail.
     
  6. Doug Lord
    Joined: May 2009
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    Doug Lord Flight Ready

    Steve, I like the looks of your hull! My new boat has a freeboard of 17" with the bottom of the single seat 8.5" above the waterline. Waterline beam is 3.75' and overall beam (not incl DSS foils) is 4.75'. Check this thread out when you can: http://www.boatdesign.net/forums/sailboats/crossbow-fl-43615.html The boat is designed as a singlehanded "sit-in" sailboat with a short rig(to sail first) and a Turbo rig. The short rig version uses a little ballast on the end of the daggerboard and DSS foils for additional stability at speed.
    click-
     

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  7. Milehog
    Joined: Aug 2006
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    Location: NW

    Milehog Clever Quip

    Our Skerry is about 18 inches deep midships though it's sections are more like a swampscott dory.
     
  8. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    In small craft, seat rise and back heights are the determining factors most of the time. You can cheat a bit with a tall combing, but these are hard to sit on if you do have to hike out.

    If you want to sit comfortably, the seat rise should be in the 16" range, which includes a cushion and the seat back would need to be several inches at least to be comfortable and make the crew feel "in" the boat, rather than on it.

    Working around ergonomic realities, can be a pain in the *** at times.
     
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  9. Petros
    Joined: Oct 2007
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    Petros Senior Member

    I have built some 18 or more small boats including a number of sailboats, I think you have way more free board than you need, but it depends on what size surf you intend to use it on. Many of my sailing dingys have only 9 or 10 inches of freeboard and I never felt I needed more. The flare of the hull is what keeps you dry from wave splash, not the depth of hull. I would think you do not need anything more than about 12 inches at most. I have paddled sea kayaks in open ocean with as little as 3-4 inches of free board and felt perfectly comfortable.

    I would also make the hull a bit flatter (less V to make it heel less), and reduce the free board. It will make the hull lighter and allow you to reduce the gunwale height, climbing in off a beach with that much free board will be troublesome.

    Unless you intend this to be a cargo hauler, you do not need to have that much hull I think. All of my boats I like lightweight and easy to move around on land (from car top to shore and back preferably by one person), or taking it out becomes too much of a chore.

    Good luck.
     
  10. philSweet
    Joined: May 2008
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    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    One of the things I look at is how the righting moment changes at the downflooding angle as the boat's displacement changes. You said the angle is 38 at your design load. What is the RM and what is the angle and RM if you are 100 pounds heavier, ie when you've got a bit of water in the bilge? You want to make sure the boat has a good feel over a wide range of displacements. I'd like it to stiffen up a bit if 5 gallons of water get thrown into it.

    You also really need to pick a target heel angle if you plan on staying inboard. I think a flatter bottom and fairly heavy build will be needed. My dad picked up an old 16' Xboat thirty years ago. Miniature Lightning basically. Double diagonal planked and weighed about 500 pounds. But you could sail it inboard most days. I think you need about another foot of beam. Look at catboat proportions. You can sail those inboard. Either that or you will have a really tiny rig that won't work in light air or heavy air, just midlin' stuff.

    What you've designed looks like a row boat more than a sailboat. You need a big fat transom that will bury its corner about six inches when you heel. Without it you can't generate any power. Ok, six inches might be a bit of an exageration.
     
  11. lewisboats
    Joined: Oct 2002
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    lewisboats Obsessed Member

    I bumped up the beam to my (arbitrary...I know) 3:1 ratio so now it is sitting at 5.25 ft, I also hardened up the bilge chine (thus reducing the deadrise a bit) and added another 3/4" of freeboard...without changing the overall design excessively. I did narrow the transom a tad after scaling the beam... it...much like mine...was getting too big. I also tucked up the aft run of the chine a bit to ease the angle at the transom. This will induce a bit of twist to the panel but not enough to make FS see red. KM went from 4.8 ft to 5.54 ft and displacement went up to 360lbs at the shown waterline.

    New
    [​IMG]

    Old
    [​IMG]

    ETA: sitting here and looking at both together I see that I will have to shave the shear on the new one. 3% of the height at the bow is more than 3% at the lowest point of the shear. A few minutes of work...but not now. Off to bed.
     
  12. messabout
    Joined: Jan 2006
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    Location: Lakeland Fl USA

    messabout Senior Member

    Sounds like you may be looking for comfort first, performance a secondary consideration. Your boat need not have excess freeboard to accomplish that aim. Let us assume that you are a sufficiently attentive sailor. In that case 30 degrees of heel before the boat is rail down is plenty. You will have eased the sheets and/or come up long before 30 degrees of tilt.

    Sail size and height is a determinant of heeling force. Keep the rig modest and you can sail comfortably in a brisk breeze. As you will sail in lakes, you will not need a lot of sailpower to overcome opposing tides or currents. A small rig costs less too.

    The problem with the small rig is that sailors seem to be afflicted with the need to go fast, then even faster. So now you need more sail....or a lighter, skinnier, boat that'll force you to install, and make use of, hiking straps. Whatta dilemma!
     
  13. philSweet
    Joined: May 2008
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    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    Ok, 360 pounds is a possible hull build weight, but you'll need a lot of ballast. 7mm okoume, 10 oz glass. Some mahogany trim. It ought to be able to carry about 900 pounds of payload above the build weight. So run stability checks (cross curves in FS) from 800 to 1500 pounds and see what you have. Change the displacement in the linesplan and check hydrostatics for each displacement. She will still be very tender at light displacements, but that's not necessarily bad if you are ok with it. I'd reduce deadrise at the transom to 5 degrees and have the transom CL just touch the water when no one is aboard but all usual junk is aboard, including kicker and fuel.

    below are the specs of the Marshall Sandpiper taken from their web site. It's what you need if you want to sit inside the boat.

     
  14. lewisboats
    Joined: Oct 2002
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    Location: Iowa

    lewisboats Obsessed Member

    I'm not looking to sail this on the briny blue...just the local lakes and my usual messabout haunts. I would say that half a ton is a bit excessive for what I need. There are many boats out there that are less than a quarter of that, light ship, and sail just fine without the need to hang your 'glutes over the edge to tempt the Pike. I have two other boats that require that much effort so this one is definitely a comfort first with eye watering speed as an option that is further down the list. 120 sf spread between 3 sails is probably sufficient to push 400-700 lbs of boat and people around in relatively small circles at reasonable speeds. If Puddle Ducks can run the Texas 200 this should be able to handle a reasonably sized lake if sailed properly. If not...then I get wet I guess. Been there before a few times.
     

  15. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    I have quite a few designs in that general size range, running the gamut of SOR's. The light dayboats run less then 250 pounds with preformance dinghies as much as 75 pounds less than that. A comfortable camp cruiser will be about 100 pounds more.

    The line between modest preformance (semi plane and occasional full plane mode) and optimized full plane mode, is a difficult thing to straddle. I tend to agree that she's got too much deadrise for her WLB, so she'll be initially tender, but should firm up once the flats of the garboards start to "bear", probably at around 10 - 12 degrees. The deadrise also seems nearly constant from midship aft, which isn't the way I'd approach a dayboat, but I'm not "in tune" with the SOR as well as you. For a "relaxed . . . sit in" kind of boat, I'd go for more initial stability, to keep her upright and you in the seats. Beam is the easiest way to get this. I wouldn't ballast a boat like this, though heavy garboards wouldn't be a bad idea along several points (something I do frequently). Also, another way of gaining some sail carrying power, yet having some shape at the transom is to widen the garboards aft, but leave the topside strakes where they are (vertically). This will push the lowest of these strakes out a little, but you can tumble home for style points.

    I don't think she'll row as well as you want, but she will not be a slug either. You'll want to tuck up the run a bit more for better rowing and of course narrow the whole boat, but now you in the classic design conundrum. Generally speaking you'll want a B/L ratio of ~2.7 for a nice, comfortable, stable (initially) design in that size. You will not hike out, but you'll also be able to keep it flat easily. If you want faster and/or better rowing, then get the B/L over 3 as much as practical. About 100 to 120 sq. ft. of area would be reasonable with 100 being calm and 120 offering modest preformance (semi plane in 10+ knots). 750 pounds of displacement would be a target for a crew of two and a cooler full of beer. Hull weight would be about 300 - 350 pounds.
     
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