1/4 scale model

Discussion in 'Sailboats' started by Howlandwoodworks, Jun 11, 2019.

  1. messabout
    Joined: Jan 2006
    Posts: 2,966
    Likes: 181, Points: 63, Legacy Rep: 1279
    Location: Lakeland Fl USA

    messabout Senior Member

    The location of the lead with respect to the Center of buoyancy of the hull is a deciding factor. Think in terms of a lever arm. If the ballast is internal, as on the deadwood, more weight will be needed than if it could be placed lower. If the hull will float on its lines with internal ballast that is good.

    It might be a shrewd move to apply some epoxy or other coating to the hull for the preliminary purpose of protecting the wood from moisture, take it to the lake. Load weights into the boat to see how much total weight you can tolerate for the boat to float on its designed lines. At this point righting moment is a secondary proposition. You could, of course, do some inclining experiments while you determine the weight and weight locations that she can ideally carry.

    You will be wading around near the shore line and it is possible that some knot head observer will suggest that you are in your second childhood with your "toy" boat That has happened to me while I was messing with my models in a local lake. Take no offense. In such a case the knot head has no idea that you are doing serious research and development work and he/she would not understand the concepts of Archimedes even if you explained it to them. The first instinct is punch them in the nose but that is a counterproductive reaction.

    Keep up the good work John. The boat is a beauty.
     
    alan craig likes this.
  2. gggGuest
    Joined: Feb 2005
    Posts: 806
    Likes: 17, Points: 18, Legacy Rep: 76
    Location: UK

    gggGuest ...

    A wide shoe/bulb on the bottom of the keel would probably be the easiest way of adding ballast and put it where it would do most good - and protect the wood from grounding. One cold also approximate the Scheel keel effect. It could perhaps bolt on with long bolts right through the keel in order to be removable for transit and/or display. Bags of sand are of course a convenient way of checking ballasting calculations before making stuff.
     
  3. Howlandwoodworks
    Joined: Sep 2018
    Posts: 54
    Likes: 12, Points: 18
    Location: Columbia MO

    Howlandwoodworks John Howland

    Thanks,
    I was going to use some leftover madrone burl or birds maple above the water line and will mill some mahogany for below. I have a vacuum pump, bag, and tape and I will probably just use some powered urea formaldehyde glue and Water Lox Marina transparent finish that I have around the shop.
     
  4. BlueBell
    Joined: May 2017
    Posts: 749
    Likes: 76, Points: 28
    Location: Victoria BC Canada

    BlueBell Ahhhhh...

    I like the removable idea for display or modification/adjustment of lead ballast.
    Getting the centre of gravity aligned will be important.
     
  5. Howlandwoodworks
    Joined: Sep 2018
    Posts: 54
    Likes: 12, Points: 18
    Location: Columbia MO

    Howlandwoodworks John Howland

    gggGuest,
    At first I thought that it was odd to have the keels leading edge angling downwards towards the aft section, the drag would out weigh any lift. But not all attribute on a sailboat have do with speed or the conservation of energy. If I were to hit something with the near vertical angle of the leading edge on a keel, say on my Star One design Class it would most likely rip the 900 lb. bulbed keel right off. The angle on the keel is about kissing a rock or the bottom. The boat will be made to sail in inland bays or lakes, not open ocean.
    I have done a lot of calculations on center of buoyancy and resistance but I plan on having the keel somewhat adjustable for trials, hence the model. Moving the the keel fore or aft will also change the center of resistance so I am hoping that my numbers a correct. I plan on having the opening to the deck mast slot framing long enough to change the center of effect. I am also hoping that thous number are also close.
    I have thought about changing the keel to a shoe/bulb if all else fails and I need to adjust the center of effect and not move the center of resistance much.
    From what I understand of a 1/4 scale model and with my limited knowledge, the viscous sublayer between the hull and the turbulent boundary layer could have a large potential difference between the model and full scale boats. I suppose the sail has a similar effect.

    John
     
    Last edited: Jun 19, 2019
  6. Howlandwoodworks
    Joined: Sep 2018
    Posts: 54
    Likes: 12, Points: 18
    Location: Columbia MO

    Howlandwoodworks John Howland

    messabout,
    Yes you have the correct way to calculate the weight conversion. I was confusing weight with the area.
    I started working of my spreadsheet again and had done my original calculation for all the components that I could think of as (weight /64) but all of a sudden I was doing area in my head. Must have been a short between the headphones.
     
  7. Earl Boebert
    Joined: Dec 2005
    Posts: 373
    Likes: 45, Points: 28, Legacy Rep: 302
    Location: Albuquerque NM USA

    Earl Boebert Senior Member

    The closest AMYA class to what you are building is the Canterbury J. They are very successful boats. Dimensions can be found in the class rules:

    https://www.theamya.org/boats/canterburyj/CJRules.pdf

    They displace 14 lbs, which is about the minimum experience shows you can go with a 48" full keel boat. (Scaling down is tough.) Their dimensions will give you an idea of what size rig such a boat can be expected to carry.

    If I were you I'd do a removable fin and bulb to get the righting moment you need. This would retain the integrity of your original design, and give you the ability to experiment with cg placement and draft/RM. The alternative would be a much wider keel to get the displacement up to what will work. As a rule, form stability is a minor factor in the RM of model yachts below 5 ft LOA or so.

    Hope this helps.

    Cheers,

    Earl
     
  8. Earl Boebert
    Joined: Dec 2005
    Posts: 373
    Likes: 45, Points: 28, Legacy Rep: 302
    Location: Albuquerque NM USA

    Earl Boebert Senior Member

    Forgot to add (happens a lot these days ;) ) another option used by model yachtsmen in the 1890s: the "lead centerboard." These are useful if you are close to the desired RM with internal ballast.

    (Illustration by Franklyn Bassford, who wrote the most comprehensive instructions we have found of design and building techniques of that period)

    Screen Shot 2019-06-22 at 9.26.59 AM.png

    Cheers,

    Earl
     
  9. Howlandwoodworks
    Joined: Sep 2018
    Posts: 54
    Likes: 12, Points: 18
    Location: Columbia MO

    Howlandwoodworks John Howland

    Earl,
    That looks to be a excellent way of connecting a keel on a model. Leak prof with that tube, it's like a tiller tub.
    Thanks,
    John
     
  10. Howlandwoodworks
    Joined: Sep 2018
    Posts: 54
    Likes: 12, Points: 18
    Location: Columbia MO

    Howlandwoodworks John Howland

    Earl,
    The lead that I have started to shape is at 22.5 lb. I have a 1,300 lb. of lead that would be in the full size boat and I need 20.33 lb. for the 1/4 scale model. I still have some fitting and shaping to do. I plan on reducing the front of the foils width, moving the maximum width back to around to 40% to 45% from the leading edge. I have been reading a lot on this??? and am becoming more confused, I will sleep on it for a while and look for answers in other proven fast lead sleds.
    To make this a more accurate model I will need to reduce the deck framing weight and rebuild the keels dead wood out of White Oak. Like what I have in my displacement calculates. I think I have already complained about that early.
    I have access to Osage Orange lumber but large timber sizes are harder to come by. It's trunk has a lot of folds and are short, most sawyer don't like to cut them, unlike the mature white, swamp, and burl oaks that I have growing out back.
    The water department put in a new 12" line through a grove of oaks on our property. The ones that they cut 1/2 the roots system off will only make it for 10 years or so, that is hard on a tree. I need to get them down and milled before the bugs and diseases a get to them, but that is young mans work and labor intensive. They would be great ribs for a full size boat because I get flitches that way or sisters boards. I have been looking at two particular white oak trees in that group for quite a while with a treacherous gleam in my eye. They were perfect before the trenching for the water line, 35 ft. to the first branch, straight as a arrows, their leaves are huge, trunk 32" and 36" in diameter 5 feet up from the ground. Sounds cruel I know but I would turn them in to the most marvel things I can. I knew the walnut tree in the picture below and put them in my office.
    upload_2019-6-23_19-31-10.png
    upload_2019-6-23_20-37-12.png
     
  11. alan craig
    Joined: Jul 2012
    Posts: 221
    Likes: 29, Points: 38, Legacy Rep: 14
    Location: s.e. england

    alan craig Senior Member

    I like Messabout's comments and would advise (for the model) not to spend too much time calculating, just make a quick'n'dirty sailing rig and go sailing, you will learn wether to move the centre of effort forwards or back, and wether you can use more or less sail area. And it will be fun.

    I had lots of fun with a restored 20" toy yacht with a steel hull; the first sail showed me that I needed a bigger jib on a bowsprit to balance. I took it to the pond one work lunchtime and amazingly another grown up turned up with the same vintage model so we raced. Running round the pond to catch the model on the other side is fun, nothing wrong with a second childhood!
     
    Doug Halsey likes this.
  12. Howlandwoodworks
    Joined: Sep 2018
    Posts: 54
    Likes: 12, Points: 18
    Location: Columbia MO

    Howlandwoodworks John Howland

    Is a models sail to displacement number different than a full size boat?
    Is that because a hull weight is converted down in a cubed number?
    =SUM(2782lb./4^3) or 2782/64=43.47 lb.
    and the sail area is converted down in squared number
    =sum(350sf./4^2) or 350/16=21.8 sq. ft.
    Thanks,
    John
     
  13. Howlandwoodworks
    Joined: Sep 2018
    Posts: 54
    Likes: 12, Points: 18
    Location: Columbia MO

    Howlandwoodworks John Howland

    I am building a model so that if I get the chance to build a full size boat, I could avoid some of pit falls of a first time builder during the construction process and save some time and trees.
    I think if I want to put it on the water I will just cut the dead wood just above the lead, add a couple of bars or some more dead wood between the two sections, that will extend the leaded lower section too what ever I need to keep it from being knock down. I can still take that extension section out if I wanted to make the model look like the boat.
    That should keep me out of the rabbit hole on the model for now.
    As for the full size boats the calculation are daunting, Alice.
    John
     
  14. Earl Boebert
    Joined: Dec 2005
    Posts: 373
    Likes: 45, Points: 28, Legacy Rep: 302
    Location: Albuquerque NM USA

    Earl Boebert Senior Member

    What you describe, AKA "scale effect," is what makes designing a scale sailing model so tricky. The attached chapter from my Yankee III book discusses the issue.

    Cheers,

    Earl
     

    Attached Files:

    Doug Halsey and Howlandwoodworks like this.

  15. Howlandwoodworks
    Joined: Sep 2018
    Posts: 54
    Likes: 12, Points: 18
    Location: Columbia MO

    Howlandwoodworks John Howland

    Earl,
    Thanks, very clever indeed. Robbing Peter to pay Paul.
    John
     
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.