Hull Balance

Discussion in 'Boat Design' started by Wardi, Nov 23, 2003.

1. coozemanGuest

Wardi,
The method is used by almost all designers I know. It provides a good baseline to determine balance. Other than building a model or the real thing, it is a tricky one to predict. A model, by the way, is another check if you have the time. Build an accurate scale model and then heel it by whatever angle you like....then just push it forward. Watching which direction the model goes (if not straight), and how quickly will give a good visual on balance

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WardiSenior Member

Agreed that a heeled model when pushed forward will give a good indication of hull balance.

It seems that the design methods mentioned by Henrikb and Coozman are rather subjective.

I am interested to know how each system works in detail, ie: how do you actually judge the amount of helm that will be induced by the hull from the drawing, without the need to build a model and test it.

Wouldn't it be useful to have a proper analysis available so that you can judge accurately the effects of your design changes?

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Stephen DitmoreSenior Member

I think the method referred to by Coozeman is the same as that discussed by designer Cyrus Hamlin in his book "Preliminary Design of Boats and Ships", pages 195-197. His method is to calculate heeled LCF (center of the waterplane) at 30 deg (25 deg for flat bottomed/shoal draft hulls) without allowing the hull to retrim from static upright trim. He then takes the distance between the upright and heeled LCF's as a percentage of waterline length. Hamlin recommends that the LCF should shift aft as the boat heels, but the shift should be less than 1% of DWL.

Cy Hamlin was a lecturer at The Landing School when I was there, and explained that his method was a simplified variation on the method referred to by Ian Ward involving wedges of sections. BTW thanks, Ian, for the reference concerning the term "metacentric shelf."

Many computer programs with hydrostatics will tell you how much a boat will trim by the bow as it heels, or will tell you the LCB shift and the moment to trim 1", making the change in trim easy to calculate. A parallel method could thus be easily developed based on change in trim, which would amount to the same thing.

Henrikb is referring to a different possible cause, the turning moment resulting from the assymetry of the heeled hull shape. The approach he suggests seems reasonable to me.

I'm a little surprised to see the J/24 taking as much flak as it is. If the weather helm is uncontrollable it may be that the mainsail is too full, and its draft too far aft. The optimum heel angle of a wide & shallow hull is lower, so in heavy winds a J/24's main should be trimmed pretty flat, resulting in less weather helm. Also, it sounds to me like the rudder is stalling and may need to be refaired, possibly with a rounder (less sharp) leading edge. I'm an advocate of larger rudders than that of the J/24 (or dual rudders) on boats of its type. Increased rudder area would make it more tollerant of aging sails.

When comparing boats of different sizes keep in mind that a boat scaled up or down will not have the same relative stability characteristics. A better equivilancy is obtained by scaling beam by the scale factor raised to the 2/3. When scaling a J/24 by a scale factor of 3 (to 72 feet) scale beam by 3^(2/3) = 2.08 Also note that if you were to cut the sheer lower (as you surely would at 72 feet length) the beam at the (new) deck would be less owing to the way the J/24s hull is flared.

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WardiSenior Member

Stephen,
Thanks for your comprehensive response and reference to Hamlin.
It seems to me that this method is used as a qualitative guide only, and that it does not apply to a boat which maintains a level waterline when heeled.

It is quite possible to have a badly misbalanced hull, which does not change CLB when heeled. This is mainly due to the assymetry of the heeled sections.

The heeled resistance of a hull can increase by around 40-60% compared with upright resistance and some part of this can prouce very significant steering moments, which need to be counteracted by constant rudder and rig adjustment as you have suggested for the J24.

This imbalance adds significantly to the resistance of the hull, perhaps 15% of the total heeled resistance. I am very surprised therefore that there is little or no real quantitative design analysis available to measure or reduce these errors.

Even more surprising is the seeming lack of recognition that hull balance plays a significant factor. It seems so well accepted that we have to constantly trim sails, use big balanced rudders aft etc in order to control modern boats.

These seem to be big compromises, when a balanced hull would offer the opportunity to have more efficient and simpler rigs, with far less need for adjustment, smaller rudders and reduced overall resistance heeled. In all, faster and simpler boats, much sweeter to sail.

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Stephen DitmoreSenior Member

Thanks for the reply, Ian, but I'm puzzled on a couple of minor points. I'm know I'm being picky, but when you say "it does not apply to a boat which maintains a level waterline when heeled" I'm not sure what you mean. Waterlines are level by definition.

Isn't Henrikb proposing a method that does exactly what you're asking? The problem I see is that you could have a boat with a fine bow and broad stern that has straight waterlines when heeled (though not parallel to the boat's centerline). While it would not necessarily trim by the bow as it heels, it might, depending on the subtlties of the hull shape. There's a lot of water pressure at the bow, so there's no question that a boat that trims down by the bow as it heels can develop a weather helm even if the waterlines are symetrical. A truly sound method would have to account for both factors, as well as those having to do with the sails and rig.

If you have ideas about how to design a better boat, find a development class and have at it. But don't go in thinking the people who've been racing and tinkering for years are idiots. They're not.

Stephen

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gonzoSenior Member

Wardi:
Boats move in the water, and the wind shifts and gusts. Adjusting sails in always necessary. Big balanced rudders have nothing to do with it. The old " fin keels" where terrible in gusts. They would get the end of the boom in the water making it imposible to ease out the main. The earliest literature in the subject mentions hull balance as an important issue. Read Skene, Herreshoff among others. What makes you think it is "well accepted"?

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Stephen DitmoreSenior Member

Is there something wrong with big balanced rudders? They're a beautiful thing in my book. Even with one a fin keel boat will have less wetted surface and more stability than a full keel boat of the same displacement, other things being equal.

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gonzoSenior Member

With increased speed, which causes more effort on the rudder, there are two options: heavier, bigger steering gear or a balnced rudder.

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WardiSenior Member

Stephen,
You are quite correct, Henrikb proposes a method that does provide some indication of the hull imbalance, but it seems to be entirely qualitative as he has not elaborated on how it is used in practice to design a balanced hull.

I agree entirely that "a truly sound method would have to account for both factors, as well as those having to do with the sails and rig."
This is exactly what I am after, but so far it seems that no quantative technique is currently in use or available, even from commercial designers.

I also agree with Stephen and Gonzo that balanced rudders are a very effective solution to a difficult problem.
What seems to be missing in your argument is the recognition that they are only necessary to provide control over boats which do not behave themselves, ie: misbalanced hulls in the first place.

It is truly hoped that I have not "given the impression that people who've been racing and tinkering for years are idiots". To the contrary I have every respect for what has been achieved to date.

What I would like you to consider, is that if it were possible to have a truly sound method for designing a properly balanced hull with light helm at all angles of heel and hull speed, would this not provide significant benefits?.

Would these benefits be of use or interest?

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henrikbSenior Member

I do not understand what you have against balanced rudders! A balanced rudder is not a solution of a problem, a non balanced rudder will require alot of more effort when steering the boat. I like to be very active on the steering, especially when sailing in choppy sea condition. If the rudder is not balanced, I'd have to wrestle the tiller instead of steering with the fingertips!

The method of looking at the center of areas will give an indication on how or if the hullshape is taking a turn in some direction when heeled, this is only one of the parts of the equation....

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WardiSenior Member

Hello Henrik,
I have nothing at all against balanced rudders, and agree with what you have said.

My comments about balanced rudders relates to the fact that they are so successful at steering boats with quite large weather helm and poor balance, that these problems appear to disappear. The result is that we tend to accept the poor balance instead of addressing the root cause.

With regard to the curve of heeled centres, how do yo use this to calculate the misbalance of the hull. What does a balanced boat look like as opposed to an unbalanced boat?

I agree that a balanced hull alone is no guarantee of good performance against rating, but it seems to me to be a consistent factor, which is is generally overlooked. This is why I have raised this issue.

My question to you, is if it were possible to design a balanced hull, would this be beneficial?

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SailDesignOld Phart! Stay upwind..

Wardy, you say "My question to you, is if it were possible to design a balanced hull, would this be beneficial?"

The answer is "A - Yes".
Now what?

Steve

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henrikbSenior Member

First, I am really not an expert on this subject, just a newbie taking my first steps.
This is how my design No 1 looks when heeled 20 degrees, wich will make it trim 0.75 degrees on the nose. The curve of area centerpoints looks straight, the waterline also look quite symetric. This would indicate, I belive, that the hullshape is in good balance when heeled.
Please correct me if I am wrong!:!:

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WardiSenior Member

Hello Henrik,
Thanks for making this contribution.

The way I see it is that if you now draw a new centreline through the heeled CB, parallel to original upright centreline, you will then see that the centres of the bow sections are to windward of this line and the stern sections are to leeward.

Assuming the hull pivots about its centre of bouyancy, then both for and aft sections are contributing to weather helm. If you were to make the bow fuller and stern finer, this would result in better balance. Hence it is a complex compromise to get good balance and good performance.

The ultimate test is to make a model of this design, heel it to 20 degrees and push it along in the water. A balanced hull will go straight ahead. Any misbalance will show as the boat veering off course, which of course would need constant correction by steering with a rudder...ie: putting the brakes on permanently when heeled!

Imagine now compounding this intrinsic hull misbalance with the steering moment of a rig to leeward...it is no wonder some boats can round up uncontrollably

If you get in a conventional stem dinghy, you can actually steer it by heeling alone. They tend to be very badly balanced.

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gonzoSenior Member

That looks like an IOR hull. They nose dive. Also, because the aft sections are so wide and the forward narrow, it will pull the rudder out of the water and lose control. Balanced hull have rather symetrical shape.

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