# About inclined underwater hull form

Discussion in 'Hydrodynamics and Aerodynamics' started by fredschmidt, Feb 18, 2012.

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### Boat Design Net ModeratorModerator

As I write this, .csv has now been added as an allowed attachment format on the forum as well.

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Not sure if either of you are aware, but there is an excellent paper that addresses both these issues.

"Hydro- and aerodynamic analysis for the design of a sailing yacht", by Kim W-J., Yoo J., Chen Z., JMT Vol 15 No.3 2010.

They show the differences between waterlines, and hence the waterplane area of a static (ie a simple cut through) and actual free surface (including near field waves). The 2 are very different and varying angles of heel, more noticeable at 20 and 30 degree heel. At 20 degree, is exactly as Eric describes in the static case, yet the free-surface results indicate the opposite.

Also the side force to total drag varies considerably with varying heel and leeway angles. From what you noted Mikko, yet they found that at 20 degree the side force exceeds the drag (but not all angles of heel of course). They show both experimental and actual measured at each heel and leeway angle. At 20 degrees the difference between experimental of drag and measured for example is 175 to 120 N respectively.

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### DCockeySenior Member

Thanks for the reference.

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### Eric SponbergSenior Member

I am not familiar with this reference: Can you post a link to this paper?

Eric

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I can do better. I'm a member, so here is a copy, just downloaded from website:

View attachment Kim-JMS&T-Hydro-aero sailing.pdf

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### daiquiriEngineering and Design

Excellent, thanks A.H.!

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### fredschmidtNaval Architect

Riptide 3D:

Could be improved in view of improvement in the forward part of the waterlines? I think so.

With the chine could be better?

Earl

The immersed volume was 8.00 m^3? I think that for we see in 3d the form is similar to Riptide.

The vertical side act as a chine, compressing the helled waterlines making a good botton foil.

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### fredschmidtNaval Architect

I was not able to see the emmersed part of the waterlines, but looking at the hull in 3D wireframe surprise was great. The triangular sections of the stations provides an reasonable form to the top of the foil. I think that if the botton of the stations are more rounded will be better.

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### Earl BoebertSenior Member

Great stuff. It's a pity that Ted Houk isn't alive to see this, he'd be fascinated. (BTW he was a cardiologist, not a naval architect.)

Remember that Rip Tide was designed as a free sailing boat so hull balance and tracking were paramount. I have verified with Ted's son that the boat was designed using the Turner metacentric shelf approach.

FYI, now that the moderator has kindly added them to the allowed file list, here are the offsets in csv format.

Cheers,

Earl

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### Eric SponbergSenior Member

Great, thanks!

Eric

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### Eric SponbergSenior Member

OK, I read the paper, and I find it a bit disappointing in some respects, intriguing in others. First, by and large it does not tell us much more than we already know. The authors go to great pains to point out the fact that the geometric center of the sail plan has little to do with the actual center of lift of the sailplan. We already knew that. And the only reason we designers rely on the geometric center of the sail plan and underwater appendages is that they are easy to calculate and we make adjustments as we discern performance. Unfortunately, we don't all have access to reliable CFD codes, towing tanks, and wind tunnels (it's expensive to use these facilities.)

Second, in Figure 18 the authors show differences in the heeled waterlines both with a static waterplane and with a free-surface waterplane. They do not define what a free surface waterplane is, precisely, but we can guess that it is the submerge outline of the hull where the waveform intersects the hull rather than a static waterplane. The authors draw no conclusions from this, and there is nothing in here that Fred Schmidt can use to investigate the questions that he asks. What we do see is that the free-surface waterlines actually accentuate aft-shift of area of the static waterlines at heel, and so both the waterplane area and the center of waterplane area deform further aft. I was half expecting that they would go the other way--that is, if the static waterline shows an adverse shift aft of the LCF, then would the free-surface LCF correct itself by not moving aft so much? Not so--the area and LCF move even further after.

Finally, there is no overall performance result in the paper. I was hoping that they authors would be able to show me what they would predict for speed polars or speed made good. Not so, they did not go that far. They talk about lift and drag results, but they do not show any result for driving force and side force for the boat as a whole. What was the optimum sailing condition of the yacht, that is what I wanted to know, and how does that compare to real life? It's not there.

The one intriguing factor that I saw was their statements that the jib angle to the main has a profound effect on performance, to wit: "It was found that the jib angle is a key parameter, since the flow interaction between the jib and the main sail depends on it." We have known this for a long time. And we also know that if you move the tack of the jib to leeward when sailing on the wind, that you will alter the jib-mainsail interaction and greatly improve the pointing ability of the yacht. This passed over the author's head completely, and they made no comment that was worthy of further investigation. Personally, I think is it most worthy of investigation and is one of the unsung tactical advantages available in sailing, one that is unfortunately prohibited by racing design and handicap rules, and has been for over 80 years. But I digress from the current thread relating to chined hull shape.

I will comment on that. I think Fred is looking for some identifiable advantage of the chined waterline shape being able to provide reasonable lift to windward because of its similarity to an airfoil shape--that is the hypothesis as I read it. At present, I disagree, mostly--there may be some effect, but it may be small because of the free-surface of the sea around the hull. I think any advantages of a chined shape over a round bottom shape still lie mostly with the differences in form stability between the two types of hullforms. But I have an open mind to be convinced otherwise. I just haven't seen that information presented yet.

Eric

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Indeed, it is the Koreans playing catch-up with yacht design. And doing it the Asian way, prove it to yourself rather than accept someone else’s word (or endless text books) for it.

Defined on page 237.

They are academics, not designers. The old adage that scientists will tell you hundreds of ways why your “design” is wrong or lacking or improvements, but have never designed one themselves.

This is not my field, so I can’t comment too much on it, especially since I just glanced at the paper when it was published a few years ago. I only read in-depth when it is more relevant to my field or self interest.

The Koreans have published several papers like this in the past few years. Being Korean, there is a lot of technical data, experiments and computerised CFD whizzardy to support their findings. But, as you point out Eric, most of it already known anyway.

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### fredschmidtNaval Architect

Eric

"identifiable advantage of the chined waterline shape being able to provide reasonable lift to windward because of its similarity to an airfoil shape--that is the hypothesis as I read it."

More or less.

The chine cut the heeled waterlines, making the hull more efficient to generate an adequate lift.

Is not to provide a reasonable lift. I think this affirmation is not mine and I think that these two statements are different.

Thinking in a hull like a wing, in some boats (not Star) the form of this wing is a foil inverted, not in right direction. The chine ameliorate this.

If the wing (the hull) has not a foil in the right direction more the leeway angle, angle of attack, more the resistance, drag.

The chine immersed, cutting the waterlines in the flooded side make the wing (hull) foil in correct position, ever.

Naturally that if we have several hulls with chine, the better will be that has the better foil.

I think that Star has a very good waterlines shape doing a hull (wing) very able to have a little leeway angle and a very good performance to windward.

I think that we need see the set of waterlines in 3D. I see the Star plan view as a perfect wing, beautiful in all angles of heel. Not so with the Riptide.

But a sailboat is much more complex than this. This is verification necessary when we do a design, I think, but not sufficient.

Unquestionably the chine brings all the advantages that you know and use in yours designs, this is perhaps more an advantage that could be studied further to see if it actually improves in these new aspects

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### miragejeffNew Member

Chines

Hi,
This is my first posting on this site.

I was made aware of it by a colleague.

Interesting take/s on the effect of chines.

My first chined boat is the one in the photo in the Seattle news letter which Eric has referred to.I was asked to write a few lines re the effect of chines on an IOM R/C yacht. In brief I stated that what I saw was a reduced hull size, especially on the windward side.Reduced mass and what I regard as a useful function in providing "grip" on the leeward side.
This last function drew a contrary opinion from an unnamed source who stated that it served no such purpose.

I do not have letters after my name relating to boat design, but having sailed for longer than I like to remember and gathering information and designing what could be referred to as successful R/C boats, I think I have some idea of what happens to a hull when sailing.
After seeing chined R/C hulls and how they handled, especially in stronger winds, I decided to try my hand at designing one.

The result was a boat named Cheinz that performed very well "straight out of the box" at the 2011 IOM Worlds.
I still say that chines have an effect on minimizing leeward drift.Don't have to be a scientist. Take a piece of plywood and drag it sideways through the water at say 45 deg to the direction of drag. Now do it at 90 deg. Which produces the most resistance?.The chines angle the topsides to give a less angled "bite" to the water.
All the discussion about "lift" to windward from foils does not gel with me. I think they minimize slip to leeward.After all they are not like plane wings. They are uniform on both sides!. Some are better than others!.

If you can add "grip" from another source,(chines), it can only be beneficial as long as the chines do not have detrimental effects to cancel out the benefits.
Have yet to find out if there are detrimental effects so far and have tried the chines out on a 10 rater and a just launched Marblehead (Won 29 of 34 races in it's first event).This boat also has a reverse bow. On first tryout this also seemed to do the job. In one race (over rigged), it powered downwind with the bow depressed but not right under and still steered dead straight. Very encouraging!.

Still have things to try, but for me chines work. Don't fully understand why and don't have the training etc to try and explain. Sometimes ignorance is bliss. Ask a hummingbird!.

By the way, have discussed this subject with another R/C boat designer. First of the modern era to use chines and also another who designs big boats and both agree that chines provide resistance to leeward drift.

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### Remmlingerengineer

The following explanation from wing theory could help (see "Low-Speed Aerodynamics" from Katz/Plotkin page 191):
If the canoe body has a deep V keel or a sharp edge chine that creates vorticity it can be regarded as a slender wing with a leading edge and Jones' theory applies. In this case the lift force is:
L = Pi/4 * rho * V^2 * alpha * d^2*2
V = boat speed
alpha = angle of attac in radians
d = maximum draft of the sharp edge chine or keel

A round bottom hull will exhibit a smooth flow and no vorticity will be created and hence no lift.

Uli

Last edited: Mar 12, 2012
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