# About inclined underwater hull form

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

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### miragejeffNew Member

Chines

What I cannot "get" is the talk of hulls etc producing lift.

I don't believe that.

I think that some hulls/foils resist leeward drift better than others!.

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### fredschmidtNaval Architect

I think that in some cases it improves the hull shape.

Shape. Good shape.

It's not what we pursue?

If we think in a hull and keel to resist the sail side force, we need think in foils, that is a better shape to creating lift. I think that a rectangle is not so good to generate lift.

But the Star waterlines are approximatively a rectangle when the hull is not heeled, but heeled the rectangles take the form of foils. So if we think in sailboats we need think in the hull heeled.

Thinking in a body traveling submerged and with the necessity to develop a side force against the wind direction I think that thinking in a wing form (with foils in the right position) to do this is no contradiction.

But in some cases the foils ( the inclined hull waterlines ) are not in the correct position. They form inverted foil and here the chine correct this.

The chine ensures a correct position to the foils (waterlines), because it cut the waterlines in the immersed side.

We need not have a sharp chine it can bee rounded. Yesterday I was having fun with the subject doing some hulls and at a given moment I see that if the chine are rounded the hull would be better, see the figures attached.

I eliminate totally the sharp chine and with the side of hull vertical the effect is much better.

However all that needs to be studied and tested, are assumptions.

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### miragejeffNew Member

Chines

My first post was prompted by the postings which seem to suggest that boats lift to windward.

I think that is not probable.
Unless you have foils and/or hull shapes that can change on each tack so that the foil has a lifting surface on the windward side, I don't see how a boat can "lift".
In reality the foils and hull allow an easier passage through the water by maintaining flow on their surfaces without turbulence etc..Obviously there are other factors in play such as minimizing pitching, trim etc., but as far as generating lift,well?????.

Anyway that is my take on it!

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### fredschmidtNaval Architect

The hull generate lift. May be to downwind or upwind, depends on the shape of the hull. When the boats with chine appears and have a performance betterer than the boats without chine, My curiosity appeared.

"Unless you have foils and/or hull shapes that can change on each tack so that the foil has a lifting surface on the windward side, I don't see how a boat can "lift"."

This is exactly what we're talking. The boat is symmetric and when heeled the chine put the waterlines as foils in the right direction, starboard inclined as much as in port.

The lift appears because the water runs in different velocities in one side of the hull and in the other side of the hull. This different velocities generated different pressures that introduces a force in the direction of the bigger pressure to lower pressure, the lift.

What I am saying is that we need study better the underwater hull form in view this.

And try to do as a way to do it.

If the hull generate lift, why not try to do a hull with a form more appropriate to optimize this lift?

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### DCockeySenior Member

Perhaps it depends on what's meant by "lift". A hull generating a hydrodnamic force perpendicular to the direction of travel would be viewed as producing "lift" by many definitions of lift.

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### fredschmidtNaval Architect

DCockey, normally this force can be decompose by one in the direction we want, in our case in the wind direction (what I named lift) and the other in opposite to movement direction (drag). This force is not necessary perpendicular to the movement., depend on the hull form.

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### Earl BoebertSenior Member

Which leads me to think it's not the amount of lift that matters but the lift to drag ratio.

Cheers,

Earl

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### Eric SponbergSenior Member

Lift, by definition, is always perpendicular to the direction of the body's movement. Drag, by definition, is always coincident with the body's direction of movement. All other components or combinations of these two forces into other directions require different names other than lift and drag, such as driving force and side force, the resultants of the hydrodynamic and aerodynamic forces on a sailing boat.

So far in this thread, there is a lot of supposition as to what truly happens. We have a lot of "I believe such and such...." and I am the first to admit that. I simply don't know for sure. So, in order to find out what really happens, one would have to set up two sailing models in a towing tank--same length, beam, displacement and roughly the same shape, except that one has chines and one doesn't. Tow these boats in the towing tank at various angles of heel and leeway, and you measure the lift, drag, and heave forces on the vessel. Lift will be perpendicular to the towed model's motion, drag directly in line with the boat's motion, and heave is the force in the vertical direction. It may take double sensors at two locations for each force in order to determine the moments involved, or perhaps use torsional sensors in addition to linear sensors to get all the data. From those results, one could compare the two hull forms and determine how much lift, if any, is generated by the hull forms, and the differences between the two. Easier said than done, especially with regard to cost. One would easily spend US\$20-\$30,000 for such testing in order to obtain the accuracy required. Any takers?

Eric

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### fredschmidtNaval Architect

Earl

Yes and is about this that we are talking, do a better underwater hull to have an ameliorated lift. If we have a initial hull and we can do a better form (and the chine do this) to ameliorate this, let's do.

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### fredschmidtNaval Architect

Eric

Dynamic and concise man.

But I think that wee need spends more.

Is not only do a hull with chine and the same (or approximated) without chine. Is a little more. Is assess what type of procedure we should do when designing a hull form to obtain a hull with a better leeway angle, better lift, less drag, etc..

What I love in the RC sailboats Is that I can do my studies without spent much.
Thanks to the RC sailboat family we are here discussing this.

But to accelerate the process I think that the towing tank will do this. (Time is money)

"Lift, by definition, is always perpendicular to the direction of the body's movement. Drag, by definition, is always coincident with the body's direction of movement. All other components or combinations of these two forces into other directions require different names other than lift and drag, such as driving force and side force, the resultants of the hydrodynamic and aerodynamic forces on a sailing boat."

I want to say:

Some peoples also call the hydrodynamic force that appears in a foil, the total force, the resultant force as lift. As a foil originally have his studies for aeroplanes, the focus was on the support of aircraft, perpendicular to the movement, but for our naval applications usually speaking, the lift is referred as the component in wind direction when we talk about hull and the direction of motion when we talk about sail. So we associate the word lift as the force in the direction we need. Since the drag is always in the opposite direction to the movement.

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### DCockeySenior Member

Lift can be defined as the component of force normal to the oncoming fluid stream. The hydrodynamic lift would be would be the component normal to the direction of motion of the boat relative to the water. The aerodynamic lift would be the component normal to the direction of motion of the boat relative to the air/wind.

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### daiquiriEngineering and Design

How about vertical lift and lateral lift?

Ah, damn... Just checked it out - the (non-technical) dictionary says that "to lift" means "to direct or carry from a lower to a higher position; raise".
So no lateral lift, if we want to be linguistically pedantic, apart technically precise...

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### Eric SponbergSenior Member

We have to be absolutely clear about terminology within the field, and it is better to be consistent with the conventions that are already in place. Suggested reading: "Aero-Hydrodynamics of Sailing" by C.A. Marchaj (pronounced MAR-ki). See attached Fig. 1.2 from the book.

On a sailboat, Fig. 1.2A, on the sails there are forces that operate in the direction perpendicular to the wind (lift) and along the wind (drag). These resolve into the Driving Force in the direction of motion, and the Heeling Force perpendicular to the direction of motion.

On the hull, Fig. 1.2B, the forces are already resolved to the direction of motion, the Hydrodynamic Side Force that operates perpendicular to the direction of motion, and Resistance that operates along the direction of motion.

The total, Fig. 1.2C, shows the resultants and how they combine. Driving Force is equal and opposite to Resistance, and Hydrodynamic Side Force is equal and opposite to Heeling Force. The angle Beta, which is the angle to the true wind, is equal to the sum of the drag angles EpsilonA and EpsilonH. The whole point of sailing is to make Beta as small as possible, that is, to reduce EpsilonA and/or EpsilonB.

Step back a minute to Fig. 1.2B: If we break this figure into component parts, we would have lift and drag coming individually off the hull, the keel, and the rudder that can all sum vectorially along the hull centerline, and these all resolve, like the wind forces, along the direction of motion. The question at hand is, how important is the contribution of the hull? What are the magnitudes of the forces it generates due to the fact that it is heeled over and operating at a leeway angle? And how do these forces vary with hull shape--chined or no?

If we eliminate the keel and the rudder and test just the hulls with different leeway and heel angles, can should be able to see differences in Hydrodynamic Side Force and Resistance in the direction of motion? But thinking about this, in most boats the keel and rudder most likely have resultant forces that are quite a bit greater than the contribution of the hull--that is why we have keels and rudders. So to test the magnitudes and differences due just to hull shape, we have to have good-sized models and sensitive measuring equipment. To be really thorough, we should also test the same models with at least a keel--same design for each model, to see if any interference effects between the hull and the keel make a difference to the forces generated by the hull only. That is, you would likely see disproportionate effects of the keel and its effect on the hull. In fact, to be really thorough, you should test the keels separately, and these are usually done reliably in a wind tunnel, with mating mirror images built into a single model. You could test each separately, and then mathematically combine the components, and then compare that to the actual tested combination.

Yup, sounds like a lot of expensive testing. But that is what it would take if you want an objective and definitive answer. Sounds like a good Masters Degree study.

Eric

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### fredschmidtNaval Architect

The gender of the angels.

I think that in this forum most part of the participants do not have Masters Degree study in Naval Architecture and we can omit certain details, omit the florid language of the wise gods. Details that are not necessary.

Marchaj no. He wisely set the name and defined all the forces that he would treat.

To the matter that is the thread my definition of lift is explained above. An everyday language.

Eric

I think that the study of the subject bring new knowledge to naval architecture.

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### daiquiriEngineering and Design

So, where do we find an enthusiastic youngster who would like to get a good title for his/her (pardon me, I meant her/his ) NA thesis research? Anybody with relevant contacts in the academic world here?

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