Hull Balance

Perhaps SailDesign has not experienced sailing in a strong gusty breeze..... with boats rounding up uncontrollably all over the place! Don't get to weather of one or you risk getting hit! Some even tack involuntarily...

Sure you will not die from such incidents ...I hope... but they are certainly dangerous and have caused significant damage on impact, appart from that unnerving feeling of loosing control!

I agree that a balanced rudder works fine and you cannot feel the difference....until it is too late that is!.... when finally the load is too much, the rudder stalls and you loose control altogether..

The point I am making is that the root cause can be fatal at least in terms of control. Also, even before this happens, you are applying the brakes, even if all feels well and under control. I therefore disagree with the statement that "if you cannot feel the difference, what does it matter?" It does matter and it is slower.

I presume it is preferred to manage the symptoms because there is currently no known cure for the root cause! With so many factors tending to unbalance a boat, surely we should be looking to reduce these to a minimum.
This is why I have raised this discussion topic.

 

Wardy - Yes, I have sailed in those conditions, and with those boats (sometimes on them).
Your quote made it sound as though you felt a balanced rudder was all it needed to correct the symptoms, so I ran with what little I had ;-)
Yes, the root cause can be "fatal" - but what else do we have currently. You asked at the beginning of this thread idf it would be benficial to have a system to examine the directional stability of boats. I said "Yes", and asked what ytou could offer.
Still waiting... ;-)
Not trying ot be funny, or push any buttons, but quite seriously, do we have a system anywhere? Or are we stuck with the same old method of saying "well, that boat was good, so let's base the next one on the same kind of hull but tweak _this_ aspect and see what happens."
I don't see a viable alternative right now that brings in all the variables we have discussed here - leeway, flatter sterns to help with planing for those that wish to, canting keels and asymm daggerboards (or was that the other thread?). Anyhho, you get my drift?

Steve (long day)

 

I respect you for raising and grappling with this important topic, Wardi, but I'm still inclined to dissent on some of your points...

I recognise that a turning moment due to assymetric heeled waterlines can be distinguished from a shift in center of pressure due to a change in trim as the boat heels, but both are "purely due to the intrinsic shape of the hull itself" (assuming we are considering change in trim due to change in heel alone). [stipulation: by waterlines I mean all heeled waterlines from the water's surface to the lowest point of the hull, i.e. the hull's entire heeled underwater shape.]

If trim is measured at centerline I think the photo DOES show bow down trim. Consider how much the rudder has been lifted. I contend there is variation in how much boats trim as they heel, and that this IS significant.

Yes, and this is due to an assymetric heeled shape. It is not necessarily due to a transom.... it can occur with a double-ender as well.

By the way, I have also had the experience of paddling a canoe in a very stiff crosswind, and I can attest that the effect of fore-and-aft trim is VERY significant!

I'm afraid I find this empty assertion unconvincing. Why are you sure you'd have weather helm?

What Henrik is asserting is that a boat with heeled waterlines not parallel to the centerline will make less leeway, or none, or even creep to windward of its compass course. I think he's right only if it has a gybing daggarboard, bilgeboards with toe-in, a trimtab on the keel, or some other method of adjusting the keel's angle of incidence to match or compliment that of the heeled hull. But regardless, this IS ENTIRELY DIFFERENT from a turning moment. The rudder angle required to maintain a strait heading should be the same relative to the waterline, though it may be different relative to the centerline.

I contend that keeping the CP of the entirety of the underbody, including keel and rudder, aft is a good thing for directional strability. Even on a perfectly balanced hull I believe a large rudder has merit. The keel area can be reduced to compensate in some cases. The intention is not to mask, it's to control.

I'm happy to consider that, but not to exclude other important considerations.

Flat-bottomed canoes exhibit a turning moment when heeled in my experience. Doug Peterson's early IOR designs, particularly Ganbare and its decendents, were quite symetrical fore-and-aft. While they went to windward better than their competitors, these IOR types are now widely criticised for their poor overall handling characteristics.

Funny.... if a canoe-like planform is what you advocate I'd have thought the trim issue, not the heeled waterlines issue, would be your focus in support of your position.

 

Finally I can see that this topic is stirring up some decent discussion! Thank you to Steve and Stephen and also Henrikb.
It is not so easy to answer all points raised at once, perhaps each deserves its own thread, but at least I will try.

I am glad that you are prepared to admit that hull misbalance resulting in weather helm is common, can be fatal and that we currently have no theoretical or practical design solution.

a) I agree it is not so easy to distinguish turning moment due to the hull & keel and that of moving centres of resistance. All are of course interlinked, but I have felt it necessary to try to distinguish the misbalance of the hull due to its assymetry when heeled, as opposed to moved centres. The reason, is that it contributes a significant resistance and change in handling when the boat heels and yet is hardly mentioned in the general texts on yacht design, I presume because it is best left alone if it is not understood and no satisfactory design technique exists.

b) In the case of the chined hull example given by Stephen, you have challenged that "my assertion is empty". Well not quite, I have given a practical example of a simple skiff dinghy of similar propoerties, heel it and push it along and you will see the effect for yourself. I therefore respectfully submit now that the proposal is put in theory, it is up to Stephen to show that it actually produces no turning moment.

c) I agree completely that a heeled flat bottom canoe and double ended yachts you describe can all have significant misbalance. It is certainly not only to do with the transom. All immersed sections, including the keel and rudder contribute to the assymetry of the hull and the resultant turning moment. It is however certainly easier to obtained a balanced hull with a canoe afterbody than a wide transom, but this by no means precludes a wider stern if the rest of the hull and keel are designed appropriately.

I also think we should be careful not to confuse the inherent longitudinal instability of a canoe, which will veer off course even when upright. Moving your weight forward not only buries the bow, but also moves the relative positions of CLP and CB, which further enhances longitudinal instability. While this is a significant pain in the ***, it is not due to the assymetry of heeled sections.

d) I have not asked you to drop all considerations other than balance alone, nor dorp the idea of large rudders. It is simply that when I raised the topic of hull balance in another thread, it was basically dismissed as not being important, so I am simply taking the opportunity here to place an emphasis on this in detail in this thread.

e) Thank you for clearing up the position of Henrikb. What I think you are saying is that as a boat heels, the effective centreline will alter and the rudder will need to follow this new centreline. I agree that it is little use if the keel does not also alter incidence to this new centreline, but do not agree that this will not result in a truning moment if the keel is kept straight.

f) I have asked inferred previously and now confirm that it is possible to design a balanced hull. The big question is if it is really worth anything! Which is why I have raised this discussion.

My feeling is that your reactions so far to my proposals are quite common, and that while ever it is not thought possible to produce a balanced hull, then there will be little interest in this. On racing boats you have plenty of crew to trim sheets as the boat heels, many people actually like to do this!....In fact it may only be cruising and pleasure sailors who would really benefit, and they are perhaps generally more interested in the creature comforts of the galley and bunks than easy handling habits of the boat under sail.

I am therefore firstly seeking if there is any accepted method for designing balanced hulls and secondly if there is any value in being able to do this. You thoughts are welcomed!.

 

Stephen D says:

"Why are you sure you'd have weather helm?"

I wondered that, too. If we are considering a boat heeled without sails, and examining only directional stability as an issue, then it is perfectly possible to have a hull that wants to veer to "leeward" when heeled and pushed. The rig, if not reasonably-placed, will most likely have a tendency the other way, thus cancelling out the hull's inherent imbalance.
Would that satisfy your lust for balance, Wardy? <evil grin>

In many cases, the rounding up is noty a hull error, but an operator error (and I know I'm gonna get flamed for that...) I co-designed a boat once whose owner called after a month of sailing and said "It rounds up - a lot, and uncontrollably" This boat had twin rudders, and a high-roach main.

I went out for a sail to see what the problem was. Having only ever sailed boats with "regular" (IOR max) roaches, the owner had not hoisted in that "loose" is the word with a deep roach. He had about 6000 lbs on the backstay, and the and the mainsheet was like a steel rod.

Once everything was loosened up a bit, and the sail allowed to do what it does best, we gained a 1/2-knot, and the boat was as gentle as a kitten. Set-up is key, and with so many folks learning to sail in J24s, we have a lot of folks out there who feel that every boat must be set up like a J24.

I know I'm maligning a lot of people here, and I hope they will forgive me, since it is really just for illustrative purposes.

Steve

 

In response to your question concerning accepted methods, Wardi, I quote from my previous post.

In his lectures at The Landing School Cy Hamlin went into more detail than he does in his book. He claims his method has served him well, and his insistance that this is a really critical issue reminds me.... of YOU! A Landing School graduate joined Cy as his partner in the mid 1990s. I don't know if Cy has since retired, or how he's doing, but you could find out with a call or e-mail to The Landing School.

If someone were to improve on Cy's method I'd be interested, but some sort of analysis or data should back up any recommendation associated with it. It seems to me rocker, or smooth diagonals, might be helpful, but the latter might be hard to quantify.

One thing that interests me is the relationship between LCF and LCB. I think New Haven, Connecticut designer Brendan Dobroth has spent some time investigating this. Dobroth may be best known as the designer of the Canada's Cup winner Coog.

 

I agree that setting up a boat is as important as designing it. Operator errors, both in sail and powerboats, make for many complaints. Sailing conditions matter too. The balance changes from flat water to heavy seas. Also, going upwind and downwind.
henrikb: To explain my earlier post, I wasn't reffering to the boat's centerline, but to the direction of advance. A boat's crab angle makes the static and dynamic centerlines different.

 

To continue the point just mentioned, let's define the term "leeway" as referring to the angle between the boat's centerline and the boat's direction of advance through the water. I'll leave it to Gonzo to define his term "crab angle," but whatever the definition I think we'd agree it does not necessarily imply a turning moment.

The critical difference between the sharpie in my example and a typical skiff is that the chines on the one in my example have more curvature in profile than they do in planform. If it were to round up (when heeled to the angle where the chine lies in a vertical plane) I'd suspect a trim issue, which might be solved by giving the bottom a gently rounded or V shaped section aft to move the upright CB, and with it the CG, aft (without changing the chine and while leaving the bottom flat forward of midships).

 

Crab angle is leeway.

 

Related issues seem to be figuring into this discussion. Any thoughts?
http://boatdesign.net/forums/showthread.php?threadid=2648

 

There's a paper posted for comment on the Royal Institute of Naval Architects site that might shed some light on this topic:

"The Yaw Balance of Sailing Yachts Upright and Heeled", http://www.rina.org.uk/rfiles/IJSCT/Discuss/keuning.pdf

 

unfortunately several of the links refer to the same pdf, I have sent them a msg about it.

 

Well actually only one was wrong but one appeared on two places on the page... anyways. This is the correct one for Investigation of Slamming Loads...

 

helm balance

The best way I've heard of to balance on all points is to build a multihull. I was on an F27 a few years ago beating at 10-14 kts. with two finger steering all the time. My 32' cat likewise had great balance, and could sail several minutes at a time with just a lashed helm.

 

I do not understand how you are considering the design of a hull that is balanced.

When a yacht is designed it may only be designed for optimum performance in one aspect of sailing. Unless if you plan to produce a hull design that represents a cylinder, the yachts waterplane area will change it heels throught all angles of degrees.

A yacht is not produced by designing a hull that is completely balanced, it is done by designing balance into the design taking into consideration the effects of the rig and the appendages.

If the hull is neglected, then so is the centre of lateral resistance of the hull and then no lead exists at all, there is only the centre of effort of the rig which would provide an inbalance in the design.

Confused??