Balancing Dinghy 1.0

Discussion in 'Boat Design' started by Tony_C, May 17, 2011.

  1. Tony_C
    Joined: May 2011
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    Tony_C No idea

    After seeing the patient guidance you all gave to Fshngrandslam I feel better about asking the dumb questions ;)
    Some time I just have to stop tweeking the lines of my first design so I'm calling it version 1.0.

    Now for balancing the rig. I need to do enough to confirm that the seating arrangement I'm planning is viable with the mast step position etc. Here's my thinking - please tell me if I'm barking up the wrong tree:

    Linesplan is here: http://public.sn2.livefilestore.com/y1pAVyEpk6bUlIi_MypPpSS9hhOnOoU0b-pukUESWcsvsj7OfnFuftPZ-uBdYeE9yRsNV5cgf0d5JesDrUzTceYsw/Linesplan.png?psid=1
    I put some section lines in where I'd like to have the thwarts.
    Sailplan is here: http://cid-5492f908e7d2782c.office.live.com/self.aspx/Public/sailcalc1.0.pdf

    The sailplan is shown with zero rake on the mast so I figure that gives me room to shorten the lead with rake once I'm up to sea trials. And the CLR is calulated with the CB vertical so I can lift it to increase the lead as conditions require. Together these factors give me the following extremes in terms of lead (all in mm, LWL=3700):

    Max lead: No mast rake, CB at 45 degrees: CE=2400, CLR=1715, Lead=18.5%
    Min lead: 5 degrees mast rake, CB down: CE=2166, CLR=1795, Lead=10.0%

    For those calcs I've included all of the rudder as its all or nothing with the software I'm using (Delftship)

    I'm hoping with that range of tuning available I'm good to go for building the hull and I can tune the balance on the water?

    One other thing is that the centre of buoyancy is at 1806mm. I think that means the centre thwart should really go forward a bit more. But I really want to have the CB case contained between the front 2 thwarts and not stick out past the middle one. Not worried at all about rowing but I'm hoping having the three thwarts and sailing generally with 2adults+2kids I have enough options for ballast under sail.

    All this based purely on googling and these excellent forums so please point out where I'm missing something.

    Thanks
    Tony
     
    Last edited: May 20, 2011
  2. Tim B
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    Tim B Senior Member

    If you draw the rig above the hull you'll be able to see if it looks right. The other thing to do is have a look at other similar boats and find out how much lead they're using. Using lead is a somewhat empirical method which is not based on particularly good (if not just completely wrong) theory. Don't get too hung up on it as long as you're in the right ball-park. Testing on the water will give you the only real metric. The mast design and rig tuning will also have a huge effect, particularly in gusty conditions. Have a look at dinghy mast manufacturers to see how they design the mast, and also from dinghies which controls are present, and how they're run. Typically you will need halyard, cunningham, outhaul and vang.

    Also consider your mast design, particularly around the mast step. How will you mount it? A round mast in a hole? a track mounted fore and aft with pins? there are lots of solutions, some more flexible than others.

    For the location of thwarts, you need to do at least a static stability analysis to work out the anticipated running trim with crew and sails. The hull will sink and trim slightly under-way, so this analysis allows you to ensure that the resultant trim is reasonable. The sails can be modeled using a rolling and pitching moment.

    Was there a particular reason for using a Gaff rig? Does it give more room in the cockpit? I ask because the top spar could put quite a bit of weight high up which will do you no favours in stability.

    Hope some of this helps, remember there are no definitive answers.

    Tim B.
     
  3. Tony_C
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    Tony_C No idea

    Thanks Tim

    The gaff choice is largely for the traditional look - I really like any quadilateral sail. And also to get maximum sial with a mast that fits in the boat. And now you mention it it does give a forward mast step and more room in the boat.

    The mast step will be pretty traditional too. A hole in the fore deck and the step itself a removable wooden plate so I can swap it for a different plate with a different rake.

    Tony
     
  4. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Here we go again . . .
    Unless you are running a full keel or a substantial skeg, you shouldn't include the rudder area in the CLP calculation. This is precisely what I've been trying to point out. Software doesn't design anything, but faithfully regurgitates what you input, hydrodynamic understanding or not. Even though your model 1.0 is considerably better then most in your position, it still suffers from the same problem.

    I couldn't open your PDF, but am assuming a gaff rig sloop of usual proportions. How did you calculate sail area? What is your sail area/wetted surface ratio and SA/D? How did you calculate the area of the centerboard and rudder?
     
  5. Tony_C
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    Tony_C No idea

    Don't panic - I'm well aware of my ignorance, which is what I'm doing here in the first place. If I thought computers could design anything I'd have started building my future disaster by now.

    As far as I'm concerned a computer based tool is just an expensive version of pencil and paper. And I don't think anyone suggests that owning a drafting board means one knows how to design a boat either.

    I am here because I reckon learning some fundamentals of design is great fun. Even if it means just trusting those who do know what they are talking about.

    I'll get back to you with answers on your questions when I have a minute to think about it. (Supposed to be working right now ;) ) I included the comment about the rudder as I am aware there is some variable opinion about whether any or perhaps a percentage of its area should be included in the CLR calcs.

    PS: G.I.G.O.

    Tony
     
  6. PAR
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    PAR Yacht Designer/Builder

    Going from your previous profile, you're employing a spade rudder, which wouldn't be included in the CLP calculation. A discussion forum is a terrible way to learn yacht design.
     
  7. Tony_C
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    Tony_C No idea

    Some answers, numbers and more questions:

    The linesplan above shows the rudder hung on the transom (the drawing in the previous thread was a bit misleading).

    Area of the rudder and CB I'm using is the area of the under water profile.

    Fixed the link to the sail plan. Its a standard gaff rig with a decent bowsprit and an optional tops'l. The main might be considered a bit short and stubby as I'm trying to fit the mast within the length of the boat for storage.

    Some numbers according to http://www.sailingusa.info/cal__sad_ratio.htm
    SA/D fully loaded without tops'l: 18.3
    SA/D fully loaded with tops'l: 21.8
    SA/D half weight crew with tops'l: 27
    Interested to see what people think those numbers mean.
    My interpretation is that its fun to sail with the tops'l up and still a nippy day sailer without it.

    Some more lead calcs (No mast rake, CB down):
    Including rudder in the CLR: CE=2400, CLR=1795, Lead=16.3%
    Including 50% of the rudder area: CE=2400, CLR=1924, Lead=12.8%
    No rudder included: CE=2400, CLR=2072, Lead=8.9%

    Seeing the rudder is so influential in the lead calcs I'd love it if someone could turn all those numbers into words. Does it mean the deisign has an unbalanced helm? or not?
     
  8. Tony_C
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    Tony_C No idea

    Finally got the sail plan drawn along with the hull:

    [​IMG]
     
  9. PAR
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    PAR Yacht Designer/Builder

    Why do you have the mast in the eyes of the boat? This location is about the least desirable place for it and it's only a few inches aft of where a catboat's stick would be. Let the boom hang over the transom and see where the jib falls and place the board as required. The staying base will be more appropriate (if stayed), the sprit shorter and therefore considerably stiffer, plus you'll probably be able to handle it without a crance or other arrangement.
     
  10. Tony_C
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    Tony_C No idea

    Yes I'm conscious the mast is well forward.
    A large driver is my perhaps warped idea of asthetics for the boat. I'm looking for a tradional look, not quite a real traditional look - perhaps more of a caricature of the traditional look.

    On that basis I like the exaggerated bowsprit and a nice bronze cranse is just an excuse for more bling! I'm hoping to have the mast unstayed though. On the other hand the boom hanging aft of the transom also looks great according to me.

    Based on your comments I tried moving both the mast and centreboard back a little. But the effect on the CLR of moving the centreboard is interesting. Basically in the case of this particular profile, moving the centreboard aft only moves the CLR by about half as much. So to move the mast (and therfore the CE) back say 12 inches I'd have to move the centreboard about 24 inches to maintain the same balance. I've already got a pretty chunky skeg so not sure what else I could do to pull the CLR aft.

    I can't quite see a way out of that without changing the sailplan completely and it ticks quite a few boxes for me personally at the moment.

    A side benefit is that the forward mast give more room in the boat. Currently got three thwarts fitting in easily.
     
  11. NoEyeDeer
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    NoEyeDeer Senior Member

    If you use an unstayed mast with a jib, then the jib will be useless unless you are going downwind (broad reach or run).
     
  12. Tony_C
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    Tony_C No idea

    With my inclination toward disestablishmentarianism I do tend to look for the minority opinon on such matters. Seems I'm not the only one:
    http://www.boatdesign.net/forums/sailboats/unstayed-mast-jibs-14732.html

    Here's a couple of my favourite and very sucessful boats with unstayed masts and jibs set flying:
    http://www.duckworksbbs.com/plans/lillistone/phoenix/index.htm
    http://www.swallowboats.co.uk/content/view/69/87/

    Being a low aspect gaff rig my mast will be short (13ft), heavy(over 3" diameter) and straight(no taper up to the hounds) so not much bend at all.
     
  13. PAR
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    PAR Yacht Designer/Builder

    Don't kid yourself, even your large diameter mast will bend. You could put some balance in the jib to make it stand, but generally the flying jib is inferior to a hanked one of the same size. Even with the sag attended to by the sail maker, it's only good in a narrow wind range, the more it blows the worse they act. A big heavy mast in the eyes of the boat will cause handling issues, particularly in heavy air or a serious chop.

    Typically on this style of boat and rig, you'd employ a "slice 'o pie" style of board, which will have sufficient area aft to permit the rig to be placed in the boat, rather then in the eyes. No this board isn't as efficient as the high aspect appendages you've drawn, by your rig choices pretty much negate any advantages these appendages might bring to the table anyway, so why employ them. You boat is screaming for a barn door and a slice 'o pie, if for no other reason then to present the caricature of an old style.

    The examples of flying jib boats aren't "successful" unless competing amongst themselves. If you take any one of the examples above and stay the rig with a hanked jib, it'll crush the flying jib boats, in every race.
     
  14. GTO
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    GTO Senior Member

    I have a flying jib on my Payson/Bolger Windsprint and in winds less than maybe 10 knots, it is great. In particular, in light winds it seems to be the primary driver when working upwind. This is lightly loaded with only me in it.

    With 3 people in it, we were sailing in much higher winds (T-storm cells surrounded the lake), the mast was really bending as both main and jib were sheeted tight, working something less than 90 degrees off wind, but we made good time, keeping the boat on its flat bottom.

    The mast is an unstayed 19 foot stick (3 lams of 1x4 spruce), triangle 76 sq foot main and a 36 sq foot wire luff storm jib, of all things.

    My opinion of dinghy 1.0 is that you will have lee-helm issues of some kind. But thats just an eyeball feeling. I suggest moving the mast aft to the CB trunk. If you end up with weather helm you can always rotate the CB aft a bit for balance.
     

  15. Tony_C
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    Tony_C No idea

    Thanks guys. You've had me thinking about my own design brief and what I'm actually trying to achieve. While I've been thinking about performance its only within constraints related to function. And when I say performance I'm not talking about racing. The performance level I'm after is probably measured as "fun". By virtue of being a custom design it will be the fastest in its own class anyway! And "fun" also brings the character element into play.

    Functionally I'm looking for an open boat with decent passenger space (for its size ;)). In that regard a forward mast is almost a requirement. I even considered a catboat earlier on. I settled on the jib and bowsprit partly for style reasons.

    The other relevant part of the brief is the environment and usage pattern. Its certainly not a cruiser. Barely even a day boat. Its more of an "afternoon boat". Its playground will be the beautiful deep water harbour in the Derwent river in Hobart, Tasmania. Never more than a short sail away from one of half a dozen yacht clubs and marinas - all with attached watering holes ;). It has a wide range of wind conditions but needs to work well in light winds. If it gets uncomfortably heavy I'm more likely to head for one of the said waterholes.

    For the rig this means to me quick to set up is desirable. But ease of sailing is not a paramount concern or I'd have chosen a cat. A little bit of complexity gives me something to do (fun) and hopefully it goes at least a tiny bit quicker for the effort.

    I moved the mast back 8 inches which is all I could manage with the space requirements. As long as there's no clear safety issue I can live with whatever performance hit that brings.

    All I've been able to do to try to push it along is to choose a decent sail area albeit with efficiently compromises. Its 108 sq ft to give a SA/D of 22 and D/L is 155. Whatever that means.
     
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