About inclined underwater hull form

[fredschmidt]

I see the Volvo boats as big Laser. Buttocks running near horizontal, great stern and not so good ability to go windward.

But I do not know in what Fn they travel.

But the thread subject is about conventional sailboats, with different approach.

The powerboats are very beautiful with "certain je ne sais pas quoi" but certainly with a lot of power.

 

[Eric Sponberg]

Eric, I just love the look of the boat in the first picture!
Very nice hull shape.

Daiquiri, thanks, and it runs very nicely.

Fred, sorry, I do not mean to distract the thread. I merely wanted to elaborate on my bottom shaping for fast sailboats as they related to chines, and how my powerboat designs are related with similar design considerations.

For a typical Volvo 70 with a Loa = 71', Lwl = 64', and displacement = 31,000 lbs. will readily hit 25 knots and higher. A volume Froude number, then, at that speed is 2.65. Fn based on length on the waterline would be 0.93. Length Froude number is a little meaningless, however, since much of the hull is out of the water, so volume Fn is a more accurate means of comparison.

Eric

 

[fredschmidt]

Eric

No problem, but in the moment I want some opinion about the chine above DWL turned for improve performance in traditional hulls. Thanks by your opinions that always increase my knowledge.
But the subject seems that is not very developed yet and I am thinking now that we do not have any serious study about.

 

[Eric Sponberg]

Eric

No problem, but in the moment I want some opinion about the chine above DWL turned for improve performance in traditional hulls. Thanks by your opinions that always increase my knowledge.
But the subject seems that is not very developed yet and I am thinking now that we do not have any serious study about.

I am sure some of the designers who have designed chined hull forms and tested them in model tanks could tell you more accurately what, if any, performance gains, or not, come with chined hullforms with chines above the DWL. A lot depends on what you are comparing it to. For example, if the beam is restricted in the design at hand, then a chined hullform will have slightly more form stability than a non-chined hull form. More stability of any sort (form or weight) is always a speed and performance enhancer. Any increase in total hull area of a chined form over a non-chined form will result in negligable increased wetted area, so that is not important. Other than that, that's about all the difference there is. Too many other factors in the boat design and the skill of the sailor will likely overpower any differences due to whether the hull has a chine or not.

Eric

 

[fredschmidt]

In some aspects we have a gain in use the chine above DWL:
- we can use a larger bottom station aft without the deleterious effect in have a large side shell aft that induces the bow down. According the bottom width we can do a reverse side shell (turned to inward). The vertical or turned inward side shell improve the final LCB and LCF positions decreasing the trim necessary for restore the LCB to its initial position.
This is the principal and more visible effect of the chine.
-despite Daikiri claim that the shell does not contribute to the lateral force ( poking the lion) something may occur both in the creation of this force as in the size of yawl angle. Seeing the hull bottom shape of a non chined hull I see a wing inverted to use. With a chined hull the straight cut introduced by the chine creates a foil in the right direction to generate a counter force to the direction of the wind.
Here we will need more studies and tests to prove the real influence.

On the other hand we have a reduction in the width of the hull, slightly decreasing the stability and the wetted area.

 

[quequen]

What about the scale issue?. IOM run in a very low Rn, water has different behavior. I've read that there is almost no turbulent sate (unless induced) and separation occurs directly from laminar situation. Maybe a submerged chine acts different at that little scale?

 

[fredschmidt]

Rn IOM = 1.13*10^6

Rn Boat LWL 10.0 m = 3,74*10^7

Transition for plate = 5.0*10^5

I do not know, but is not so different. The problem is: after successive good performance in several world championships, the boats with chine (above DWL) are really better than the boats without chine? Why?
If they are better, the improvement of a immersed chine is greater than the worse by the chine being immersed. Why?

 

[fredschmidt]

More about the chine:

http://www.seattleradiosailing.org/wp-content/uploads/pages/Seattle IOM Update - MAR_2012.pdf

 

[Eric Sponberg]

I read through the newsletter section about chined hullforms, and a few things are apparent to me from the discussion and the photographs. The picture of the IOM chined boat on page 14, compared to the pictures of other boats on later pages, shows the hull with comparatively full sections forward, a lot of rocker, and very slight sections aft. The other boats, such as the SKAs, have finer sections forward, less rocker, and fuller sections aft.

The new IOM chined hull seems to have better pointing ability because as the boat heels, the volume distribution and particularly the waterplane shape at heel promote lifting the bow. As the French designer in the article indicates, the chine is just a consequence of how he wanted to shape the heeled waterplane--he wanted slack sections aft, not full sections, so that as the boat heeled, the narrower waterplane aft allows the stern to sink a touch more and, correspondingly, the bow to rise, pointing a little more to weather, which is the direction you want to go anyway. And as the boat heels this way, it also increases the angle of attack on the keel and rudder, which increases lift, and with more lift, there is more power pulling the boat to windward. the boat balances better with less increase in weather helm.

This effect has been known for well over a hundred years, and as I have stated before elsewhere in this forum, most latter-twentieth-century designer types, collectively, seemed to have unlearned this lesson. This has more to do with heeled waterplane shape and controlling the center of that shape, the longitudinal center of flotation (LCF) than it does with the chine. Therefore, I agree with the French designer.

Capt. Nat Herreshoff, the winningest designer of the latter 19th and early 20th centuries, promoted this feature in his hull shapes--slack stern sections compared to fuller bow sections to control the LCF. As the boat heels, LCF should either stay in the same fore/aft position, or move forward slightly. As the boat heels, the bow lifts, the stern sinks, angle of attack is increased, the boat sails to weather better.

In modern times, over the last few decades particularly, we have seen hulls getting wider and wider and with fuller sections aft. Of course, you can get more stuff into such hulls, but at the same time, the builders and designers of such craft have been touting "powerful stern sections" as an indicator of superior performance. Nothing can be further from the truth, and in my opinion, "power stern sections" is a hoax. Such boats do not sail well, they balance poorly to more bow-down attitudes and pick up weather helm as a result. They're cranky. You'd see this in their heeled waterlines, they are fuller aft; the LCF moves aft as the boat heelsl. So, heeling over, they pick up more buoyancy aft, which raises the stern, depresses the bow casting it to leeward, and reducing the angle of attack on the keel and rudder--all just the opposite of what you are seeing on your new chined IOM models.

So, the better performance does not come strictly from the chine--rather, it is all about area distribution of the waterplane which is shaped by consequence from the chine to produce the desired bow-lifting effect. You can do the same thing without a chine, but the chine, in this case, seems to accentuate the effect.

That's my take.

Eric

 

[fredschmidt]

Eric
I agree with you. However besides the facts cited by you I would like know the influence of the waterline form in the angle of attack, curious as I am. I think that we have an improvement here that do a noticeable difference.
Undoubtedly we have an improvement in LCF and LCB position, this is very ease to prove.

 

[daiquiri]

I have read the article, and agree with the "Anonymous" reader who has technically explained why a hull is not a good lift-maker. It uses very similar aero/hydroynamic arguments l have used in my previous posts. If you want more "grip" on water then you work on foils (keel, rudder), you don't rely on the hull as lifting surface. It is ABC of aero and hydrodynamics, though I understand that might sound counterintuitive at first glance.
A person not trained in fluid dynamics tend to reason in terms of things he knows from everyday experience - like how sharp skates bite the ice more efficiently than blunt ones, or how a showel easily digs into the earth while a tennis ball doesn't, and so on. Fluids behave differently and sometimes in a less obvious way.

And I also fully agree with Eric's arguments in the post #24, regarding enhanced volume distribution of a hull with chines in a heeled condition. Very neat post by Mr. Sponberg.

 

[Eric Sponberg]

Eric
I agree with you. However besides the facts cited by you I would like know the influence of the waterline form in the angle of attack, curious as I am. I think that we have an improvement here that do a noticeable difference.
Undoubtedly we have an improvement in LCF and LCB position, this is very ease to prove.

"...I would like to know the influence of the waterline form in the angle of attack...."

The waterplane (correct term) itself does not create or experience lift, and it has little direct effect on the boat's windward performance. The trick of increased performance is not about the waterplane per se, it is about what the waterplane does to the flow around the keel blade. The hull and its waterplane do have an angle of attack, namely the leeway angle of the boat, but there is no lift generated by the waterplane; there is some lift generated by the hull, but its magnitude is very small compared to that generated by the keel blade which provides the lion's share of the lift.

The waterline form controls the boat's trim as it heels. If the boat trims down by the bow as it heels, it is because the waterplane shape at the aft end is more full than the waterplane shape forward. Bow down trim reduces the angle of attack on the keel blade. With smaller angle of attack, the less lift. The less lift, the poorer the windward ability.

If the boat trims up by the bow as it heels, it is because the waterplane shape at the forward end is more full than the waterplane shape aft. Bow up trim increases the angle of attack on the keel blade. With higher angle of attack, the more lift. The more lift, the better the windward ability.

Eric

 

[Earl Boebert]

You can see the phenomenon Eric describes if you've ever watched one of Ted Houk's "Rip Tide" designs sail. When a gust hits going to windward the boat comes up out of the water like a dolphin and just takes off.

Cheers,

Earl

 

[fredschmidt]

When God scattered men changing their languages ​​fully achieved their goals, so let's define our understandings:

Waterplane - is only a plan parallel to water surface. A plane is infinite.

Waterlines - a line that is the intersection of the hull form with a water plan

The shape of the hull can be represented by series of parallel' waterlines, such as a mountain is represented in topography by level curves.

Waterlines make sense for us, because they represents the hull form, we need see them in 3D.

Being the waterlines the hull form representative and a chine a device that changes the shape of the hull we speak about waterlines, the curves that represents the hull form being affected by chine.

Lift has severals, rudder, keel and hull even if small. The total lift is the vector sum of all.

I think that the chine immersed change the shape of the hull, so the waterlines, appropriately.

The symmetric hull with chine above DWL when heeled becomes asymmetric like a Hobie Cat hull (adequately asymmetric), improving all hydrodynamically.

Without chine symmetrical hull when heeled become asymmetric but in not so good form.

We see that in this figure. One side of the hull, flat, vertical, and the other shaped, like a hobie cat:

 

[Mikko Brummer]

I have read the article, and agree with the "Anonymous" reader who has technically explained why a hull is not a good lift-maker. It uses very similar aero/hydroynamic arguments l have used in my previous posts. If you want more "grip" on water then you work on foils (keel, rudder), you don't rely on the hull as lifting surface. It is ABC of aero and hydrodynamics, though I understand that might sound counterintuitive at first glance.
A person not trained in fluid dynamics tend to reason in terms of things he knows from everyday experience - like how sharp skates bite the ice more efficiently than blunt ones, or how a showel easily digs into the earth while a tennis ball doesn't, and so on. Fluids behave differently and sometimes in a less obvious way.

And I also fully agree with Eric's arguments in the post #24, regarding enhanced volume distribution of a hull with chines in a heeled condition. Very neat post by Mr. Sponberg.

I tend to disagree - the hull is not insignificant as a lift producer. Look at my post #43 in the wing keel-bulb keel thread http://www.boatdesign.net/forums/sailboats/winged-keel-bulb-keel-40596-3.html. In the simulation, the hull produces one third of the side force, or half that of the keel (rudder lift excluded). It's mostly due to the keel/hull lift transfer, as can be seen from the numbers when the keel fin is omitted. As for the drag, look at the numbers with and without the fin, it only rises about 15% while the lift is doubled when the fin is under the hull. But of course, this is just a simulation, and not even very complete as such, since the free surface effects are neglected.

Including the free surface effects could make the hull even more signicant, or at least more efficient... As the boat is heeled, the keel root gets close to the surface and makes a wave. So there is a lift induced wavemaking drag, both for the keel fin and the hull. Here, the hull is at a much better position than the keel fin: For wavemaking, the Froude number gets significant, and the long hull has a much, much smaller Fn than (especially a narrow) keel fin. Thus, the hull may not be at all that inefficient in its lift production. You may also want to read the post #76 in the same thread.