# Designing a symmetrical hull under heel

Discussion in 'Sailboats' started by Justinet, Dec 22, 2018.

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### JustinetJunior Member

Hi,
Are they any tips to design a hull to remain symmetric (as much as possible) when heeling at a certain angle, say 15 degrees ?

Design tips, hints or even literature references would be helpful (i am learning !)

Below are some water lines of hull A (better despite the chines) and hull B (the design i am trying to improve), both heeled at 15 deg doesn't seem obvious with a beamy hull and a bulky stern.

Thanks much for your help !

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### TANSLSenior Member

The only way is for the hull to be a circular cylinder. The more it resembles that, the more similar will be the various floats at heeling.

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### BlueknarrSenior Member

Radial symmetry yes. Cylindrical not necessarily. A square hull will have symmetric waterlines at 0* and 45* heeling.

Look into scowl. Symmetric heeling waterlines was one ot their design criteria.

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### Mr EfficiencySenior Member

I'm interested to know what the incentive involved is, why give priority to a symmetrical heeled waterline ?

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### TANSLSenior Member

As the water line is not symmetric, its centroid separates from the centerline, with which the ship's turning axis changes and a certain trim occurs, which in turn changes the ship's drag. Perhaps this effect is what the OP wants to avoid.

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### Mr EfficiencySenior Member

OK, I understand a turning effect, and/or a "crabbing" movement can be set-up by asymmetry, but what is lost there may be gained in other ways, so the nett effect of symmetry may be less desirable. And obviously is the case, as most boats don't have symmetrical heeled waterlines.

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### JamesG123Senior Member

But at points of heel in between?

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### Mr EfficiencySenior Member

Depends on immersion of the hull. Wide and shallow will quickly lose symmetry with heel, as the square edge leaves the water.

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What a fun subject we have here. I have been messing around with designs for small boats for an eon or two. SO far I have not been able to discover a design that has what the OP has asked for. It is entirely possible to draw up a simple little boat like a flattie or sharpie that has perfectly straight centroid lines at some discreet heel angle. Alas that straight line is an elusive one because it will curve all over the lot with any heel angle that is not the one that the hull form likes.

There are some small boat designs out there that have attempted to address this notion. The Inland lakes scow class M20 has tried, but not successfully, to mitigate the presumed energy consumption of a curved line of section centroids. The M20 has a tunnel and when heeled there are some degrees of heel that yield rather pleasing results. Too bad that it does not work as well at other than ideal angles. Back to square one.......

Having fiddled with this concept for a long time, call it a "hang up" if you like, I confess that my small boats have shown but little measurable difference in performance. In light air with artificial heel it matters. Probably less than centroid alignment but more because the wetted surface area is substantially reduced at certain angles of heel. In general the wetted surface area is reduced more and more with increasing angles of heel.

I think that Tansl is right. If the boat is cylindrical, heel angles will not change the geometry of the immersed sections much if any. Pitching motions may change the whole deal. ...............I will go to my room now.

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### JustinetJunior Member

Hi all,
A scow design, as far as i understand, has many advantages including an almost parallel heeled axis plus more RM; hence the trend towards "bulky" round bows on some race (ultra light)
boats such as some mini 650 e.g. 747 or Formula 40.

However in my project case, a rather "heavy" light displacement blue water cruising boat with strong scantling, adding too much volume to the bow or forward sections
would lead to "push too much water" ... and perhaps less confort in waves or chop.

As i need much hull stability (less efficient ballast due to shallow keel in project specifications) i cannot use something close to a circular shape.
With my beamy design including flat stern sections, heeled water lines tend to be quickly and significantly asymmetric.

However i am just trying to be as symmetric as possible at a given angle : using 15 degrees here.
This would be the most common (or desired) heeled angle given the sail plan and boat RM (hull design and ballast).

My goal is to diminish the natural tendency of the boat to luff up (too much, in addition to some relative speed increase given the reduced wetted surface) when heeling, even if some design lead between CE and CLR would yield some compensation.

Would like the boat to be well balanced and go straight at this 15 heel angle without using the rudder(s) i.e. without too much weather helm.
I am assuming (could be wrong) for most common shapes, should be more or less OK at smaller heel angles when OK at 15*. No problem to use the rudder(s) at higher angles.

Hope this clarifies my purpose and question.
Thanks

Last edited: Dec 23, 2018
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### Earl BoebertSenior Member

Here's a paper I wrote some time ago on the subject of hull balance. If I wrote it again today I would put greater emphasis on having the LCF coincide with the LCB.

Cheers,

Earl

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### JamesG123Senior Member

While this isn't a answer to your theoretical technical question, the obvious solution to the above stated SOR here is a cat or tri configuration. Double/triple hull or a mono with outriggers (Proa?), foiling or not. Heel, and all its consequences, is removed (mostly) from the design equation.

Last edited: Dec 23, 2018
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### JustinetJunior Member

On this monohull design (my hull B project, multihulls are another challenge) LCF and LCB do not coincide, but are not so far apart.
LCF is some 2.5% of LWL aft of LCB which is also a few % aft of midship
Believe this is not uncommon for these types of design ?
Have been looking at the heeled 15* LCB to prevent too much trim and e.g. a noise down attitude.

Last edited: Dec 23, 2018
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### TANSLSenior Member

What is the advantage of LCB and LCF matching ?: Does not LCG intervene in that "equation"?

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### Earl BoebertSenior Member

Just observational data from a lot of model sailing yachts over the years. I assumed LCB and LCG coincide. I have no clue as to what the mechanism may be.

Cheers,

Earl

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