Stability criteria of pond sailboats

[George S]

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!

[Rick Willoughby]

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

[rednev]

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

[rednev]

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

[ancient kayaker]

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.

[George S]

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".

[George S]

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

[Rick Willoughby]

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

[ancient kayaker]

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.

[George S]

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

[George S]

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

[George S]

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.

[Rick Willoughby]

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

How high above the waterline will the rig be?

Rick W

[ancient kayaker]

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.

[rednev]

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

[messabout]

George;
I doubt very much that you can have a 36" boat that will displace 25 to 30 pounds and still sail well. The boat will need to be either very wide or very deep or both to get that kind of displacement in 36" of length. If you adhere to some degree of authenticity or scale, the plot thickens because you may not be able to use suitable hull section dimensions without violating scale.

I suggest that you get a copy of the book; The Nature of Boats by author Dave Gerr. The book deals with basic design subjects more than adequately for your purpose. It does so with a minimum of mathematical acrobatics as well.

If you are a history buff then a boat such as Mayflower is perhaps appropriate. It will, however, carry with it some pesty problems. Not least of which is that it will be too complicated, in terms of sail control, to sail with any degree of satisfaction. I suggest that an an old time boat such as Joshua Slocum's Spray would be much easier to deal with while maintaining a degree of historical signifigance.

[ancient kayaker]

A half-submerged log 36" long and 10" dia will displace 51 lb. 30 lb is a bit on the light side for a 36" hull, I think. The ships of the Mayflower's day had very wide beams.

[waikikin]

George s, if you want a sailing model with some real cool history & performance, google up "balmain bugs" & check em out, might suit you! Regards from Jeff.

[George S]

Fellows, what you have said is very helpful. I bought plans for Mayflower II but loaned them out and have not gotten them back yet. They are for an 11 1/8th inch lwl static model hull. So currently I am "working" from the littlle 5 inch plans in the book "Mayflower and other colonial vessels."
I am thinking of a "look alike" model so what I need is for the hull above the waterline to "look like Mayflower II". Underwater I can adjust things to get a ready to sail weight of 25-30 lbs.
I am thinking of making the underwater sections semicircles so I can just calculate their areas rather than measuring them, and also adjust the waterlines aft so the transom just touches the water.
Several books say to measure the section areas using the trapazoidal rule then plot the section areas on a baseline and make the length of the vertical lines proportional to the section areas, then measure the plot using the trapazoidal rule to get the displacement.
Is there an alternative method of calculating the underwater volume? Is there a mathematical way to multiply each section area by the distance to the next section and sum these figures to get he underwater volume? Thanks again.

[waikikin]

George, you can use Simpsons multiplyers in tabular form for calcs of area under curves & for volumes too, although its about 1/4 century since I've done this, trapiziodal rule is ok for quick & dirty calcs or tanks or barges or even straitish sectioned hulls.Regards from Jeff.

[messabout]

George; There is a quick and dirty method for calculating displacement. As follows: Calculate the areas of the sections in square inches.. Sum them, divide by the number of sections. This will give you the average of the underwater areas. Use that average figure multiplied by the length of the waterline in inches. You now have a reasonable approximation of the immersed volume. Multiply that number, which will be cubic inches, by the constant 0.03611....That will give you the displacement in pounds. Simpsons rule or the trapezoidal rule may be a tad more accurate but the method described is simple and fast and will get you real close.

If the book tells you the prismatic coefficient, Cp, of the boat you can do the math even quicker . Find the area of the central or largest station...mutiply by the Cp. I would guess that the Cp of a tubby boat like the Mayflower would be 0.60 or more. Note that this is always a decimal number less than one. Take the product of the main section area times the Cp. Now multiply by the waterline length. Multiply by the constant 0.03611. You have the displacement in pounds. This is very easy with nothing more than a hand held calculator.

The bit about plotting the individual areas on a graph (curve of areas) is useful for determining where the center of bouyancy is likely to occur but also the manner in which the water will be displaced. I see no reason to bother with that as you are going to build a somewhat boxy boat and there is no need to make the design more involved than necessary or useful.

[George S]

This is all very helpful. Baker did NOT give a prismatic coefficient. He spends a little time on how he drew the sections using circles and straight lines. Then he gives the tonnage rule as follows:

keel length x breadth x depth divided by 100 should equal about 180 (for Mayflower)
Baker's results for Mayflower were length of keel 58 ft, breadth 25 ft. and depth 12.5 ft. gave a burden of 181.
He selected a rake aft of the stern post of 4 ft, the rake of the two circle stem was 21 feet.
He located the midship section 21 ft. aft of the forward end of the keel.
Then he fiddled with the floor rising line etc. All of which is interesting but my objective is only to design and build a model that LOOKS LIKE Mayflower ABOVE THE WATER and design an underbody that will support between 25 and 30 pounds of boat and lead (as much lead as possible in a fin) all with a draft limitation of about 14 inches (due to the limitations of our local pond.)
And to get away from the Mayflower design I think I should call the boat "Sally of Salem"!!! So I think that I will use Messabout's method of calculations.
(I still have an itch to move the "date of construction" forward so I can get the transom out of the water, add topgallant masts and sails AND add staysails for really poor sailing conditions.)
The first model will be free sailed. Later I will try and figure out radio control of rudder and sails. (Steel, Chapman & Hutchinson Ltd uses an invisible stainless steel bentinck boom in some of their courses. This eliminates sheets and tacks and their braces go from the tops to about 3 inches from the CENTER of the controlled yard. The sails and yards above just "follow along."
A spritsail and sprit topsail WOULD ADD COMPLICATIONS!

[messabout]

George;
Ancient Kayaker has some good advice. Make a really simple, unfinished model first. Tinker with the stability by using a basket or whatever you can dream up for finding the right ballast weight. Really racey models have ballast ratios on the order of 3 pounds of ballast to one pound of boat. I dont think that applies here because Mayflower (or Sally) is very wide. Models such as the meter classes (one meter 39.375 inches long) have beams on the order of 8 to 10 inches on deck and less at the waterline. They have 60" tall masts and perhaps 600 sq. inches of sail. Your 58' x 25' x 12.5' boat will scale to 0.62 inches to the foot of full sized boat. Might as well call it five eigths inch to the foot. So your model will be 15.625 in beam. That will give it a lot of initial stability and lessen the need for such a big ballast ratio. You do need enough ballast to right the boat in case it is knocked down. And remember that the sails and rigging will be wet.

For a quicky underwater platform, or merely an example of something that would hold the 30 pounds or so, you could use a pirogue or canoe shape with square chines and flat bottom. That type would be an easy build. A double ended box with a midsection beam if 15 inches more or less and a draft of 4 inches will give you somewhere around 37 pounds of bouyancy. I'd give the box about 3 inches of rocker. Rocker is needed or the boat will be difficult to turn. That may not be what you are after because ships of the subject type have rather full ends. So a box that loosely resembles the shape of the planform would carry its' width pretty far fore and aft. In such a case you would neeed less draft because there will be more interior volume in the shape. Less draft might be good because you can have a deeper fin and bulb while still keeping maximum depth within the 14 inches you suggest. In any case, a square chine type underbody will give decent results while being simple to construct and perhaps easier to replace when/if the spirit moves you.

[ancient kayaker]

Speaking of radio control of sails, I am not up on square rigging, but is it practical to control the sails by mast rotation?

[George S]

1/22/09 Thanks fellows. I have Baker's plans from Plimoth for $25. They are for a 11 inch model. I have stretched them to 36 inches lwl, and calculated the displacement at about 7-8 lbs. I decided to increase the depth of the hull 1 1/2 inches to get more displacement and will add a false keel so the draft is 12-14 inches. I have cut semicircular frames (bulkheads) to glue under the lower deck and will next draw the upper frames (bulkheads ) etc. shortly.
George

[George S]

Traditional Model Yacht Design by Thomas Moore's Text and Diagrams.
A caution about scale models in "Elements of Yacht Design" by N. L. Skene 1904 from Sheridan House ISBN 1 57409 134 4 P. 243 "Scalle models of larger vessels do not perform well. Their stability is insufficient for the wind velocity is not scaled down and stability varies as the fourth power of a lineal dimention, whereas heeling effect varies as the cube. Therefore scale models have relatively much less stability." etc. It is good on pre-computer design. THE STORY OF SAIL Illustrated with 1000 scale drawings" ISBN1 55750 896 8 will give you lots of ideas. "An introduction to Radio Controlled Scale Sailing Models" by P. V. Williams ISBN 1 900471 20 0 is very helpful, but the figures on page 20 are wrong. The center of gravity does not move. THE GALLEON ... ISBN 1 55750 300 1 Gets you into these great ships. MERCHANT SAILING SHIPS ...1775-1815, ISBN 0-87021-418-7 is good. SCALE MODEL SAILING SHIPS ISBN 0 8317 7700 1 is very helpful in counteracting the scale effect. YACHT DESIGN EXPLAINED ... ISBN 0 393 04646 X has great details about the scaling effect. ISBN 1 55750 098 3 Details a model ship build.

[George S]

MESSAGE TO MANUFACTURERS OF MODEL SAILBOATS: Please include the weight of the lead bulb at the base of the fin on your boats. With that we can estimate the righting moment at 30 degrees angle of heel and estimate sail area and get an idea of stability.

[George S]

I bought a copy of Baker's Mayflower II plans for a static 11 inch model and expanded them to about 36 inches LWL. I have the model about 5% built and will add 1 1/2 inches to the depth of the hull and add a fin keel so the draft will be between 12-14 inches (the depth of "our" pond) and then I will add lead to the bottom of the fin and see what sort of stability I get.
I asked the same question in a thread at <rcgroups.com> under sailboats, hoping for some numerical data but, while we had an interesting discussion, I did not get numbers.
While I am "at it" I plan to add a sprit topmast and staysails so I have changed the name to "Cape Ann" and the date of the design to about 1675.

[George S]

Feb 18, 2009 The model is about 5% done. I have taken Baker's Mayflower II lines for an 11 inch static model and expandeed them so the water line is about 36 inches long. I also altered the lines a little so the transom will not "drag" in the water. I also converted the sections to semicircles so I could calculate the areas rather than measure them. I leveled the lower deck so it is level fore and aft. Displacement to this waterline is calculated at about 9 lbs. The half angle of entrance of the LWL at the bow is about 60 degrees. That is all for now

George

[ancient kayaker]

I haven't been able to find a picture of the Myflower's lines. However, noting how short and wide she is compared to modern designs, and how high the masts are, it occurs to me that she may lean forward under sail, thereby lifting the transom. Thus it may not be necessary to modify the underwater lines.

[George S]

Dear Ancient Kayaker you may be completely right BUT I have cut the wood- so the lower deck is double ended (and the lower frames added.).
The next departure from Baker's lines will be to increase the depth of the hull by an inch and a half to increase displacement so I can increase the amount of lead on the bottom of the to be added fin keel. I am hoping that my calculations will be adequate for the job and that displacement will be about 25 pounds so I will have a stiff boat.
Baker's book "Mayflower & other Colonial Vessels" has a small sent of lines in it. To get a bigger set you need to buy them from <Plimoth plantation> for $25 + s&h. Still that boat is only 11 inches long.

[George S]

The CR914 is 36 inches long rigged as a sloop. The weight is said to be 6.25 lbs., the sail area 658 Sq. inches, the ballast bulb slightly over 4 lbs., I estimate the length of the fin at 9.5 inches, thus the righting moment at 90 degrees heel is approximately 10 inches x 4 lbs or 40 inch pounds.
Similarly, the Tommahawk, a 10 rater, has a mast ht. of 85", a fin of 20", and a lead bulb of 8 lbs. 9 oz. (irrelevantly the loa is 60" and DWL 63", sail area is 1,250 sq. inches) and I estimate the max. righting moment as
20" x 8.5 lbs = 170 inch pounds.
Any similar data would be helpful in designing your own model.

[George S]

Gerr's book was helpful and reassuring. The bottom third of the hull is maybe half done. I am waitinng for better weather to finish fiberglassing the bottom as I do that outside. Then I will add the keel, lower part of the stem and the deadwood and sternpost. I expect to freesail the boat in our local pond so I will not have to worry about RC for now. Next I will add the frames and decks for the upper 2/3rds of the hull. etc.

[George S]

I am looking at Model Yachting (a US mag.) issue 155 and at p. 21 Russell Potts speaks about vintage British models. He says the 1730 tonnage rule produced very narrow, deep hulls which were heavily ballasted and came in 5 ton,10 ton and 20 and 40 tonners with model weights from 80 pounds down to 20 for 10 tonners. but I cannot tell from the photos how long the hulls are.
Shifting to stability criteria, it is hard to find hard data on many boats. The Laser is discussed in issue 154 where it notes that the fin is 16 inches long and has a four pound lead bulb. Thus I deduce that the maximum righting moment may be about four pound feet. The boat's tall narrow una rigs range from 949, to 710 to 600 square inches.