Stability criteria of pond sailboats

Discussion in 'Stability' started by George S, Oct 25, 2008.

  1. George S
    Joined: Oct 2008
    Posts: 32
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Washington DC

    George S The old fellow

    I recently bought two copies of a pond sailor intending to sail them with the grandchildren. Both looked like "Mayflower" type hulls and had masts, sails and lead "sailing keels." Now I am told that they are not sailboats but are power boats. So now I am working on designing and building my own "historic" looking sailing ship, but I have no stability criteria in mind.
    Of course I can go through some cut and try builds but would prefer to have some stability criteria to reduce the number of failures. I am interested in pond sailors from 2 to 5 feet lwl.
    How do "toy boat" designers design pond boats, try and try again? Or do they use computer assisted design and what are the stability criteria?
    Puzzled!
     
  2. Guest625101138

    Guest625101138 Previous Member

    Problem with model boats is that they do not sail as well as purpose designed RC sailing boat.

    If you build an old style square rig sailing vessel perfectly to scale it will sail like they did. They will not point so you will need a small pond where you can wade in or retrieve from the lee side or have a rescue boat.

    The RC boats typically cary very tall rigs and deep keels. They are self-righting. I would not go for anything less than self-righting for a kids toy. This is easy to check by setting the mast on the water and see if the boat readily returns upright.

    So the exercise gets down to what is the purpose. To learn how to sail and have fun at the pond an off-the-shelf RC yacht for about USD150 to 200 would provide real value. This site shows the typical proportions of good RC sailing boats.
    http://www.sailrc.com/
    The prices shown are more than I would expect to pay if I shopped around. Maybe Ebay.

    If the purpose is to build a model boat of an old style square rigger then what you have is likely to be as good as it gets from a sailing perspective. Old square riggers do not sail particularly well compared with modern high aspect sloop rigs with deep high aspect keel. The hull is effectively a means of carrying the sail and the keel (the working bits) in the correct position relative to the wind.

    The boats around 40" with 6ft tall rig sail incredibly well. Now talking more like USD300 but they are exciting in any sort of breeze. They respond as well as a nicely balanced sailing dinghy but do not capsize. Need a decent pond to get a good course but will provide hours of fun - at least for me.

    There are a few others here into RC models who might contribute more specific ideas.

    Rick W
     
  3. rednev
    Joined: Jun 2008
    Posts: 6
    Likes: 0, Points: 1, Legacy Rep: 23
    Location: australia

    rednev Junior Member

    the biggest issue with scale sailing vessels is not so much stability as sailing performance . if you want true scale out of the water you will need a false keel for in the water. if you just want a salty looking vessel in the water you
    can make the under water profile to give more lateral resistance.
    if you go the false keel route you can play around with shape to bring your center of effort and center of lateral resistance to give you a balanced helm
     
  4. rednev
    Joined: Jun 2008
    Posts: 6
    Likes: 0, Points: 1, Legacy Rep: 23
    Location: australia

    rednev Junior Member

    on second thoughts if you only want sailboats to have fun with the grandkids there where lots of designs over the years published in model boats ect by people like vic smeed .simply chine hulls balsa construction [ the grandies could help with the build] modest but adequate perfomance. personal recomendation dont go smaller than 18 inches 24 to 36 inches is a nice size
     
  5. ancient kayaker
    Joined: Aug 2006
    Posts: 3,497
    Likes: 146, Points: 0, Legacy Rep: 2291
    Location: Alliston, Ontario, Canada

    ancient kayaker aka Terry Haines

    In a scale model you'll need lots of weight way down on a deep keel if you want to carry anything like the sail of the original. The reason is partly the model is dealing with a full sized wind, but also because of the scaling factor.

    The righting moment of the hull scales to the 4th power, but the heeling moment of the sail scales to the 3rd power.

    For example, if you have a 1/10 model you'll get 1/1000 of the heeling moment and 1/10,000 of the righting moment.

    A real (human) sailor will appreciate the validity of a model with reduced sail, after all, the great sailing vessels of the past didn't spend all their time under full sail. However, the grandkids are gonna want to see all the washing hung out.
     
  6. George S
    Joined: Oct 2008
    Posts: 32
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Washington DC

    George S The old fellow

    Hooked on square rig

    Thanks fellows, I agree with all you have said. But...Take a look at Steel, Chapman and Hutchinson Ltd (SCH) of Palo Alto California at <modelsailingships>. I was lucky to sail their brig twice. The first time was in a DC pond only 36 inches deep, luckily, because we blew a fuze that we did not know existed and the ship went "dead"; so one member of our three man team got his boots and went wading to get the boat back. The second time I had the model alone in a river like bay and had a short sail on a lee shore with lots of bushes threatening to screw up the rigging so I aborted after a few tacks. (The boat was a loan and was insured for $6,000.)
    That was some few years ago, since then I have reduced the weight limit on heavy things so as to protect my "trick" back. The current limit is 30 Lbs.
    I know that square rig may only sail a point or so to windward. I am prepared to reduce sail at any time. What I am wondering is how do I design a deep keel with lead low down using some sort of calculation to "defeat" the scale factor. If I can not figure that method of calculation I will have to resort to trial and error, (the old fashioned system). So any mathematical assistance will be appreciated.
    I have just started preliminary studies to try and counter the scale effect on a 36 inch lwl model I call "Mayflower III".
     
  7. George S
    Joined: Oct 2008
    Posts: 32
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Washington DC

    George S The old fellow

    Continued. I know folks must design all those sloops on sale for cash. How do they design them? "Calculations", software or lots of prototypes?
    George
     
  8. Guest625101138

    Guest625101138 Previous Member

    Doing a stability calculation is complex and you do not need to do this.

    To get it to be self-righting (my criteria) assume the centre of buoyancy is at the waterline. Estimate the weight times distance of everything above the waterline including masts rigging and sails. You need a lead weight on a keel of certain length to counter the calculated tipping moment.

    Without calculation you can do it by trial and error without even placing the boat in the water. Tip the boat on its side resting the hull on a dowel at roughly where you expect the waterline to be. You now add weight to the keel until it balances keel down. If the weight is too large for the required displacement then make the keel deeper so the righting moment is the same for less weight.

    This gets the limiting condition for stability, which is all you are really interested in. If it can come up after a knock down it will be safe in any hands apart from the obvious of ending up on a lee shore.

    I can imagine you trying to explain to the grandkids why your boat sails like crap when all the others shoot past it and do not get stuck on lee shores. You are lucky if you have a grandchild with the attention span and interest to appreciate the merit of the historical significance.

    Rick W
     
  9. ancient kayaker
    Joined: Aug 2006
    Posts: 3,497
    Likes: 146, Points: 0, Legacy Rep: 2291
    Location: Alliston, Ontario, Canada

    ancient kayaker aka Terry Haines

    Practically speaking it is unlikely that a model sailboat would not be self-righting provided it cannot fill with water, if it is ballasted properly. One has an advantage when modelling a sailling ship, that one does not need to reproduce holds and other below-deck volume, and "guns" can be wood instead of iron, all of which keeps bouyancy high. The amount of lead needed to load the model to the waterline should, if carried low on the outside of the hull, provide enough righting moment to raise even a full set of soaked sails from the water.

    Build it, sail it and see; nobody is going to get drowned. If an external, not to scale keel is necessary to improve sailing performance it an always be added later.

    With a small wood boat it is usually easy to make changes. Heck, I ripped out and replaced the entire bottom twice on one of my canoes that I wasn't happy with. Not a model one, either.
     
  10. George S
    Joined: Oct 2008
    Posts: 32
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Washington DC

    George S The old fellow

    Thanks fellows, I will get cracking on my "design" of a 36" lwl Mayflower III and let you know how many changes I have to make to the sailing keel.
    George
     
  11. George S
    Joined: Oct 2008
    Posts: 32
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Washington DC

    George S The old fellow

    My first calculation will be to determine the dimentions of the underwater part of the hull. I aim to have sufficient volumn to support 25-30 lbs of hull, sails etc and lead. I expect the next step will be to design a fin so the draft is between 12-14 inches.
    George
     
  12. George S
    Joined: Oct 2008
    Posts: 32
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Washington DC

    George S The old fellow

    Well fellows I know of the build and try method , but how about some data? If you have a good heavy weather sailor can you tell me the weight of the lead bulb, the depth of the bulb below the water (upright), the sail area of the rig, the height of the geometric center of the sails above the waterline, and the total weight of the boat ready to sail? Aquacraft sold a square rigger for $300 but did not do their stability calculations and had to discontinue sales.
     
  13. Guest625101138

    Guest625101138 Previous Member

    What is the estimated weight of all the rigging and sails?

    How high above the waterline will the rig be?

    Rick W
     
  14. ancient kayaker
    Joined: Aug 2006
    Posts: 3,497
    Likes: 146, Points: 0, Legacy Rep: 2291
    Location: Alliston, Ontario, Canada

    ancient kayaker aka Terry Haines

    I'll give you a start on your research if you need it. The best known expert on the Mayflower and similar ships is probably William A. Baker 1911-1981, who wrote several books and was the designer of the Mayflower II which crosssed the Atlantic in 1957. Here are some of his publications that are likely to have the hull lines, rigging and build details that you will need for an accurate model, unless you are working freehand from pictures. I don't know which if any are still in print, but a library or museum may be able to locate a copy.

    -The new Mayflower, her design and construction, by her designer. Illustrations by R.S. & W.A. Baker. Barre, Mass., Barre Gazette, 1958.
    -Colonial vessels; some seventeenth-century sailing craft. Illustrated by the author. Barre, Mass., Barre Pub. Co., 1962.
    -The Mayflower and other colonial vessels. Annapolis, Md.: Naval Institute Press, 1983.

    You're asking for help, specifically design inputs. If we have any specialists on this topic they aren't leaping into the fray too much so I will give it a shot. I'd guess a 36" LWL model with a hull shape typical of the period will displace about 30 to 35 lb. The main stability criterium from the hull shape is the metacentric height would be about 9" for a draft of 4.5" which isn't very much. The metacentric height is effectively the pivot of a pendulum representing the ship as she heels and is a measure of primary stability. Of course as she heels all the freeboard will provide a lot of secondary stability, but you must get the CoG below the metacentric height or she will float on her side at best. Keeping everything reasonably light I would guess that you can reserve more than 50% of the total weight for the keel, but I would use the good old test and cut method somewhat as follows:

    Get some drawings that show the hull, rigging, waterline etc. Build, finish and rig the sucker, weigh it, temporarily fasten a wire basket under it and insert the rest of the estimated displacement in the form of lead pellets, ammo, scarp iron, cutlery and float her. Add or deduct whatever it takes to get her to float to the waterline. While it's floating there give it a few sideways shoves and see if she seems to have enough stability.

    Stability is a gut call; adjust the basket depth as required and try again. You can't have too much stability for sailing purposes but if she snaps back upright too quickly she will look toylike so let her have some dignity.

    Take out the ballast and weight it, bearing in mind the fore-and-aft location of the ballast is as important as the total amount, cut a sheet metal keel to the required depth and cast the lead keel weight. Temprarily fit the keel and do a float check before finally attaching it. Then sail her! Just don't expect sparkling, zesty performance; there's good reasons why this type of hull was abandoned as soon as building techniques and new materials allowed. In the days of the original launching would have been accompanied by a lot of prayers; there were good reasons for that as well.

    One more hint; you don't have to get everything right first time; I would build a crappy poorly-finished model first to get the dynamics right before investing hours in a perfect job.
     

  15. rednev
    Joined: Jun 2008
    Posts: 6
    Likes: 0, Points: 1, Legacy Rep: 23
    Location: australia

    rednev Junior Member

    the september 2008 issue of marine modelling international about the four mastered barquentine catherine louise. based on two vessels built in 1904.
    model appears to be closer to 5ft than 3 carries 7.1kgs of lead on its false keel. whilst the article is long on techical detail might give you some ideas
    if you can get a copy of the mag.
    one point he mentions is the use of an oversize rudder prudent if you are sailing in traffic
     
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.