Hull Balance

[Stephen Ditmore]

I'd like to note that Rosebud (same designer as Pyewacket) won its class in the San Francisco Big Boat series. Braveheart finished well down. Do any of you believe a difference in balance / tendancy to trim had anything to do with that result?

Doug, I think another admonition would be more appropriate than "When designing the hull ALWAYS allow the hull to trim freely as it heels." How about "ALWAYS be consistent in your methodology." I assure you I'm correct that the boat should not be allowed to trim freely when applying Cy Hamlin's LCF shift test. There might be a better test made possible by computers. Cy would tell you, however, that a test that doesn't require a computer comes in handy sometimes, and it may be that the simpler it is, the easier it can be to see what corrective action is called for.

 

[dougfrolich]

Stephen, I agree with your correction. I should have chosen my words more carefully. But you get my point, a designer needs to quantify trimming moments and track the migration of hydro. centers as part of line development and optimization.

 

[gonzo]

Static tests like that do not represent actual sailing conditions. There are significant differences. For example, the bow wave lifts the forward end of the boat and increases heeling because there is a hollow in the water amidships. Also, the sheets pull from the aft end of the boat lifting the stern. Dynamic tank testing is an inexpensive way to find these problems.

 

[Wardi]

Just a few comments on the prior discussions.
1) I do not quite see what bow down attitude when heeled has to do with the intrisic hull balance under discussion. If you design correctly, then the boat should maintain its waterline at all angles of heel. If it does set the bow down a little, this would make little difference to the intrinsic heeled hull balance.
Downwind burying the bow deep into a wave is of course another matter, and there I agree with previous comments that this can cause significant problems.

2) The evidence that symmetrical waterlines not parallel to the centreline induces weather helm is easily seen if you heel the hull and push it along. Make a model, do this and you will immediately see what I mean!

3) The wave along the hull makes quite a difference to the hull balance and in general induces some lee helm when the boat is heeled at speed. This effect can be sufficient to counter the weather helm induced by the rign moving to leeward as the boat heels. The net effect is that a balanced hull can retain neutral helm at all angles of heel. I speak from 30 years experience in sailing such boats, not just some theoretical discussion.

4) I do agree that very slight weather helm is advantagous when pressed hard in a breeze, both for safety, and performance, as it transfers some load to the rudder, increasing the effective lateral plane. If well set up, such a boat can respond to lifts and knocks by itself, without any hand on the helm! The benefits of this flow to many areas of rigging and handling the boat. For example, there is no need to trim sheets in a gust, therefore no need for fancy travellers and the like. No need for balanced rudders and big wheels with massive mechanical advantage. A small tiller is therefore all that is required, mine is a vertical tiller, that takes up almost no space in the cockpit. More room for entertaining and basic comforts!

5) Mention of open 60's as examples of good boats are does not gel with me. Sure they can be very fast downwind, but lets see how they perform around a course with a decent work involved. I am sure a Volvo 60 style boat of similar displacement and sail area would outperform these downwind skimming dishes.
I have seen proof of this in the Sydney-Hobart race.

6) Stephen has made some very good observations and points.
The discussion about balance affecting both upwind and downwind behaviour is important and I agree with his description.

I would also like to raise the topic of longitudinal stability here, because even upright, most modern designs seem to be quite directionally unstable. In fact I note that some steer much better under power in reverse than forwards! This means that when pressed hard downwind, if the boat shears off course, it takes a lot of work to bring it back on course, you need to be ever vigilant, this is only the beginning of the process of a broach described by Stephen.
It is even a pain in the neck having to man the helm at all times while under power, which is a disaster when sailing short handed as I do.

I do not really understand why it is necessary to design boats to be directionally unstable ...perhaps you could enlighten me.?!!

 

[gonzo]

I think a serious omission is the assumption boats sail in the direction of their centerline. This discussion hasn't taken into consideration that boats sail to leeward in different degrees depending on design and conditions. It affects balance because the lateral symetry must be measured not from the centerline but from the boat's course.

 

[SailDesign]

Gonzo - you couldn't have brought that up on page 1?!?

Nice observation - I wish I could honestly say that i was wondering when someone would mention that, but I can't

;-)

 

[Wardi]

Gonzo,
Surely leeway will only accentuate any weather helm due to poor balance of a hull. Even more reason to have a balanced hull in the first place!.

 

[SailDesign]

Wardy,
Yes, but .... not necessarily. If your hull is perfectly balanced for "dead-ahead" motion, there is absolutely no guarantee that it will be any use at all with a few degrees of leeway.
It may add to weather helm, otr it may produce lee helm. Depends on a lot of things.
You may now officially start the "second string" discussion.
;-)
Steve

 

[Wardi]

Steve,
Many things are a matter of degree. I think this is a case where practical experience comes in.

While my 36ft yacht was designed for balance at all heel angles, without taking into account leeway angle, this makes no noticeable difference to the final result. The boat remains balanced, despite changes in leeway. What seems to have more effect is the wave form at higher speeds, and this effect is beneficial.

Any thoughts on directional stability?

 

[Guest]

Can you tell us more about your 36 footer.

designer
drawings
photographs

 

[Stephen Ditmore]

On the symmetric heeled waterline not parallel to centerline issue: I don't have it in me to make a model at the moment, so let's do our best with a thought experiment. Imagine a sharpie with a flat bottom, a broad stern, and topsides that flare at a 45 deg angle. As on a Star, the gripe (or knuckle, or whatever you call the intersection of stem with bottom) is carried clear of the surface at all heel angles. The curvature of the topsides is matched to that of the bottom, so the chine is drawn as a straight line when viewed from bow or stern. When heeled at 22.5 degrees the heeled waterline is symmetrical, but not parallel to the boat's centerline. The water "sees" a bunch of V shaped sections with a deadrise of 22.5 degrees, rockered, but all in a row. Let's say the boat is making 3 degrees of leeway but the chine is making 10 degrees of leeway. Even so, how would you attribute a turning moment to the hull shape? If I observed turning behavior the first thing I'd do is look for some other possible cause. [Note: the boat might trim by the bow as it heels which might cause a shift in the center of lateral resistance. I'm coming to that.]

Gonzo - Where do you see anyone assuming a boat is sailing in the direction of its centerline (is it where Wardy claims that trim doesn't matter downwind)? Upwind or down, the issue is lateral forces. They're lateral with respect to the centerline of the boat because forces along the centerline impart no moment. Centerline is the zero axis with respect to turning, so we break each force down into its longitudinal and its transverse component.

Wardy - In light of the above I don't understand your claim that trim doesn't matter. I take the point of the person who posted the photo of Pyewacket heeled: measuring trim at the centerline may not provide a complete picture. But if the center of lateral resistance shifts forward this will, coupled with the force on the rig, produce a turning moment. The fact that there's more pressure on the bow than on the stern will exaggerate the effect.

 

[gonzo]

I assumed the discussion took for granted that the hull was moving dead ahead because there is no mention of leeward movement. It changes many things. For example, hulls with a deep forefoot get a hollow to windward and water sometimes flows backwards and around at the bow due to the suction. It is about the same as when a boat heels enough for the keel to start coming out of the water. I think that it changes balance enough to make trim an important influence. I guess it would've been smarter for me to bring it up earlier in the thread, but it didn't hit me until now

 

[henrikb]

How could a hull where the stern is wider than the stem possibly have a heeled WL that is paralell to the original center line?
My picture posted on p2 of this thread shows that the heeled WL is not paralell to center line, I then discussed that this will make the boat sail to windward (the cource of the hull), NOT turn to windward.
The un-paralellnes (is that correct english?? ), I think, is is not alone producing the unbalance or weather helm. If a hull without apendages is heeled and pushed forward, it will sail in direction according to the "new" centerline (if the canoe body when heeled is not turning or being (to)asymetric).

 

[Wardi]

Gentlemen,
I think you are confusing several effects which is making it difficult for us to see the wood for the trees!
Sure a very significant bow down attitude when heeled can move the CLR forward, giving some windward helm. So too can the rig when heeled to leeward.

I have purposely, right from the outset of this discussion tried to separate the effects of misbalance due to shifts in CLP and CE longitudinally and laterally, directional instability and that purely due to the intrinsic shape of the hull itself which I refer to as hull balance.

Typically most boats are designed with no trim or a very slight bow down trim when heeled. Your photos confirm this. This is why I have said it does not have a great practical influence. If you get a very big bow down attitude when heeled, then there can be many more things wrong with the design.

In order to separate out any effect of the CE, then remove the sails, heel the boat and see what happens. OK, so you cannot build a model so easily and quickly, but I would like to ask if any of you have actually rowed a simple skiff shaped dinghy, stopped rowing and then heeled it over while it is still moving. The effect is dramatic! In fact you can easily steer a dinghy simply by moving your weight from side to side!

In the case of the chined hull example Stephen has raised, then I am sure you will have significant weather helm despite the symmetrical waterlines.

I do not see the difference between a hull which is moving to windward and one turning to windward as discussed by Henrik B. In both cases, you get a turning moment to windward which needs to be countered by applying a load to the rudder to keep the boat on course, or trimming the sails to move the centre of effort forward to maintain balance, usually it is better to compromise and do a bit of both.

Because this turning effect changes with heel angle, you need to constantly make adjustments, and in the case of applying the rudder, you increase drag.
These days it is common to design with a large balanced rudder well aft, which works well to mask the effect of weather helm, but this does not get rid of the problem, you just cannot feel it.
What I am asking you to consider is a hull which requires no such adjustments as it heels.

This does not necessarily mean a straight symetrical heeled centreline, offset but parallel to the upright centreline is the only design option available. This could only really happen with scows and catamarans.
A canoe shaped underbody does go close to being balanced and the impossibly long splinters proposed in another thread would also have very little helm. In both cases, the heeled line of centres deviates from a straight line and is not parallel to the centreline, but produces a balanced hull! Surely therein lies the essence of the solution.

The problem is that we are seeking a compromise with decent accommodation, form stability from the hull, displacement to carry the modern necessities of life, good sea keeping properties etc etc.

 

[SailDesign]

WArdy says:
"These days it is common to design with a large balanced rudder well aft, which works well to mask the effect of weather helm, but this does not get rid of the problem, you just cannot feel it. "

Are you a doctor, Wardy? Sounds like a stupid question, I know, but you have stated that curing the symptoms is not enough. In most cases, you are right, but if you cannot feel the difference, what does it matter? The boat will not die from the root cause ;-)

As far as the leeway vs no leeway discussion, gonzo has a point in that the CP of any foil (read "waterline") will shift with increasing alpha (anglke of attack). This will automatically make it unbalanced within Wardy's definition.

Steve