I’m not biting: I am not an unrelieved admirer of Turner, partly for most of the usual reasons but partly because a great many well-balanced designs that were not metacentroids have been produced, particuarly in offshore racing. I regard Turner as an excerise in how to avoid rather than how to achieve: balance is almost guarnteed but non-metacentroids can also be well balanced. At best we are talking about a sub-set.
Might I recommend:
Ship Dynamics for Mariners
Clark, Ian C.
ISBN: 1 870077 68 7
I most admit now that the author is a good friend of mine, but try it. [Lester: your university library really should have a copy]
I think of Turner’s work as more of a puzzle than an answer. The historical evidence quite clearly shows the method is only consistent at the extremes:
Hulls with “very good” curves are consistently well-balanced.
Hulls with “very bad” curves are consistently unbalanced.
Nothing consistent can be inferred from curves in the middle.
I would be interested in learning of any design with a “very bad” (crossed) curve that was well balanced in practice.
I do think that his explanations are nonsense, but I also think that the curves (again, at the extremes) correlate with some other physical phenomenon that affects a heeled hull. Which is why I find the LCF/LCB relationship intriguing.
I’ve also read many discussions where it appears to be assumed that the hydrodynamic forces trump the hydrostatic ones in the generation of yaw under heel. Is anybody aware of a treatment that includes numbers, either measured or computed?
Don’t have any numbers, or even any “treatments” to refer to, but hydrostatic forces cannot produce yaw, or any force other than one vertically upward (in a floating vessel, that is). This can probably be shown by calculus, but the easiest way to see it is to realize that a yaw produced by hydrostatic forces would produce continuous turning, and that could be used to provide power, so would be a perpetual motion machine (of the first kind, I think, but in any event, contrary to the First Law of Thermodynamics).
I suspect that the most that can be said for Turner’s theories is that a hull badly balanced according to those theories presents a shape that causes the hydrodynamic forces to produce yaw, or at least that most such hulls do so. If that is in fact the case, then Turner’s theories might still be useful in design.
One of the few times I suffered from seasickness, was abord a 32 foot older boat (don’t recall make) and sailing in moderate chop (3-5 feet), the boat took on a corkscrew type of movement going to windward. Standing near the helm holding the lifeline, and watching the bow, the thing made circular motions as it sailed forward through the waves.
Very disconcerting to say the least. :yuck: I would venture a guess that this was NOT a well balanced yacht.
Hi Earl,
I may know something soon,I am having a plug built that has a linear relationship of lcb/lcf on heeling.
I am not sure that the LCB and LCF have to be in the same place,I think they can be in different places but stay the same distance apart on heeling.
Of course this is just my take and not proven fact.
Re: hydrostatic vs hydrodynamic. I was (as often) insufficiently clear. The observed phenomenon is obviously hydrodynamic. External force (heel) is converted by an unknown (to me anyway :-)) mechanism into yaw while the boat is underway. The Turner calculations involve hydrostatic factors only. My question was whether anybody knew of numbers that quantified the situation.
As an aside, WJ Daniels explanation of why the CB shift under heel caused imbalance involved coupling roll to pitch to yaw. Roll (heel) caused CB shift which caused pitch, moving the sailplan and changing the CE/CLR relationship. Given how our little boats bounce around on the water, I find this explanation about as dicey as Turner’s.
I’m eager to hear the result of Brett’s latest effort
Since I belabored the probably obvious point that hydrostatic forces on a hull cannot cause yaw, perhaps I should at least concede that in the presence of waves and chop, there are forces that can act on a hull that are different from the usually considered dynamic forces of skin friction, form drag, and wave drag, and there may be some high correlation between any yawing-type of imbalance in these forces and the “imbalance” in Turner’s hydrostatic computation.
Even in smooth water, wind variations producing momentary changes in heeling angle could lead to “funny” behavior that might be attributed to, shall we say, an awkward distribution of buoyancy.
My original impression of what Admiral Turner’s criteria led to are double-ended vessels. The imbalance in his calculations seemed likely to come from a wide, chopped-off stern. I’m not at all sure that wide, chopped-off sterns perform badly, but I can see that double-ended vessels might have a general tendency to track well, even when heeled (so long as the heeling isn’t extreme - bets are off when gunwhales start going under).
Wide, chopped off sterns do, indeed perform well - but only at small angles of heel. I have had quite my share of wide-sterned boats that becaime dramatically unbalanced at 20 degrees of heel or less. For obvious reasons, such boats almost always have moveable ballast (people, water, canting keel).
It should be noted that some double enders can be viciosiously hard mouthed. One of the first boats Iremember sailing was a copy of Albert Strange’s own boat, Cherub III. This apparently innocuous double ender could tke charge of things in a way that was positvely lethal!
Well, the narrow, mechanistic definition is that the boat does not yaw when heeling. This is crucial to a free sailing boat because it keeps it from wandering all over the pond as a result of encountering puffs. The broader definition is that the boat is sweet to sail and does not require fighting the helm. The attached picture is a good example of that: Charles Francis Adams at the helm of the J boat “Yankee,” taking 150 tons of hull and 7500 square feet on sail area in (one) hand. It’s not a posed picture; he always steered like that. It is likely, but not certain, that this was taken during the prestart sequence of an America’s Cup defender’s trial race against Mike Vanderbilt and “Rainbow” in June of 1934.
“Yankee” is an interesting case. Her hull form is perfectly balanced by the Turner system (which also provides another counterexample to the commonly held belief that the system only produces double-enders). This may not have been a coincidence. She was designed in 1929 by Frank Paine, whose partner was Norman Skene. Skene was a model yachtsman (he was the measurer for the Marblehead Model Yacht Club) and quite possibly read Turner’s first series of articles, which were published in The Model Engineer in 1927 and 1928, a time when there was heavy interaction between the model and full-scale yachting communities in the U.S.
>>Her hull form is perfectly balanced by the Turner system (which also provides another counterexample to the commonly held belief that the system only produces double-enders). <<
I’ve mentioned several times on this forum that it appears to me that Turner’s system leads to double-enders. While there may well have been others in other places who made similar statements, I haven’t noticed them. I’d hate to have something I said be taken as a common belief, just because I repeated it several times. :approve: :ziplip:
I’m wondering if perhaps problems with model sailboats being unbalanced isn’t often a case of a deep fin not being well placed. When heeling angles change, the resistance to leeway of the fin changes, and unless this is accompanied by offsetting changes in the hull, a yawing couple will arise. Is “balance” maybe a matter of designing a hull such that the Center of Lateral Resistance of the submerged portion of the hull remains in about the same position for a range of heeling angles, with the fin placed on (or a tad forward or aft of) that position?
Once upon a time I did the Fastnet on a David Boyd 8 Metre CR called Sunburst. She was not a very good design, to put it mildly, but after a bit we decided that in moderate conditions to windward in smooth water we went faster if we let her sail herself. This included our hotshot helmsman, who was objectively pretty damn good.
But how you translate that into measurable physical quantities is beyond me.
I’m including a a DXF file, plan views, a Redering, and Text files with the hydrographic data. I’ll appreciate if anyone can take a look and comment on this. I was able to keep the center of balance with in a mm as the hull heels. that also goes for the VCB. The only thing that changes alot is the LCF.
On virtually any theory of balance, beware of big changes of LCF relative to LCB. At the very least big movements of LCF can lead to some VERY unfortunate pitch characteristics (I know: I done it, seen it, worn the T-short and suffered the consequences). However, this is a very common feature of competitive IOMs.
If it was my design, I would probably move the maximum canoe body depth and LCB a little further aft. However, given the characterstics of a skiff-type hull with no very clearly defined centreline over most of its length, that is probably over-caution on my part. Probably build as is.
Please note that this opinion is given purely by eyeball. I have not done any calculations except mentally converting some of the figures you produce into percentages.
Hope this helps. Now waiting for the 15 dissenting views!
Looks great. I note from the hydostatics that the displacement is a tad over 4kg - and that is presumably the hull alone. You may want to see how it floats (in the software) if you go for a displacement of around 3.8kg. You need to allow for the volume of the keel and rudder.
Apart from that, and based on what I can see - I’d build it.
If it was my design, I would probably move the maximum canoe body depth and LCB a little further aft.
This is one aspect (of many) with regard to IOM design that continually perplexes me. All other things being equal, moving the LCB further aft means reducing the volume forward, which depending on how that is dealt with might mean fining up the lines forward. That gets me thinking about tendancy to bury the bow (which IOMs do) - but then with the volume further aft, the rig will come aft too - which helps reduce burying the bow.
Then if have the LCB around the middle, then the volume is a little further forward, giving us some bouyancy, which may help at the top end of each rig downwind. It should also help to windward. But then fuller sections equals a less easily driven hull.
But then Cp is moderate at .54, and volume is fairly well spread.
Oh, I give up!
For what its worth, there are successful IOM designs with both the LCB further aft and further forward then it appears to be in this design. Compromises, compromises!
First of all, thank you for your comments. I do have one question, When you refer to 3.8kg for the displacement of the hull. I thought that the displacement of the hull was to be 4kg not including the rudder, fin, and bulb? I was under the impression that those would be on top of the the hull being 4kg. Can someone comment on that.
The class rules require that the weight of the boat in dry condition be not less that 4000 gr.
There is a difference between the weight of different components of the boat having a density greater than water, and the weight of the volume of water displaced by those components when not floated as part of the complete boat.
So the keel bulb, which will weigh about 2,350 - 2,400 grams, does not displace anything like its weight in water. It displaces a volume of water equal too the volume of the keel bulb. Water being less dense than lead, that water will weigh only a few grams. We have to factor that into the design, when fixing a displacement for the hull without appendages.
So as a rule of thumb, I assume the water displaced by all appendages to be about 200 grams. I set displacement at various points between 3,700 grams and 4,000 grams to see where my boat is likely to float and how sensitive it is to being over weight.
Maybe I didn’t explain that too well. I’m sure some smart alec will chime in to correct my use of terminology and point at the difference between weight and mass, the moon and the earth, temperature and altitude :rolleyes:
I went ahead and changed the it to 4 different loads. 3.8, 4.0, 4.2, 4.5.
I’m attacking the results. I think the rusults came out consistent. I did not see any big jumps anywhere.
With hull displacement set at 3,800 grams your waterline length does not exceed 971mm at any angle of heel. There is of course nothing wrong with that, but you probably have a little room to drag that out slightly to get a fractionally longer sailing length if you want to. These boats do create a wave when sailing fast so you do get some extra sailing length aft from the quarterwave if the transom does not touch the water when trimmed without heel in smooth water.
Some people design their IOMs to have a 1000mm waterline, even when upright in flat water. Others try to have the boat reach its full waterline length only when travelling fast enough to generate a wave (on the basis that in very light conditions, when travelling well below hull speed, the shorter waterline length is better).
There seem to be plenty of quick boats in both categories.
