Entry Angle vs Hollow Waterlines

[Mat-C]

So... it's my understanding that hollow waterlines lead to an increase in wetted surface.... bad
But, wouldn't they also lead to reduced entry angle... good....

Which is of more importance?

 

[Leo Lazauskas]

So... it's my understanding that hollow waterlines lead to an increase in wetted surface.... bad
But, wouldn't they also lead to reduced entry angle... good....

Which is of more importance?

It is impossible to answer that question in general. The answer depends on
the size and type of vessel.

 

[CT 249]

So... it's my understanding that hollow waterlines lead to an increase in wetted surface.... bad
But, wouldn't they also lead to reduced entry angle... good....

Which is of more importance?

I'm no expert, but surely the entry angle doesn't just "go away" simply because there's a hollow just aft of the stem?

If you put a hollow in the waterline, you get a finer entry angle where the hollow is, but a wider entry angle further aft....the overall angle from the stem to BMAX is just the same whether there is a hollow in the first bit, or not.

Does (broadly speaking) the resistance of a waterline rise in linear proportion of the entry of the bow angle, or is it one of those things that rises at a higher rate?

I noticed on a couple of older IRC maxis with bow extensions that the waterlines were so hollow that there was effectively no wavemaking going on. I asked a designer who agreed that if there was no wavemaking in the first couple of metres, then effectively the waterline length was shortened by that distance. Neither boat seemed to go anywhere near as well as a boat with straight waterlines, but of course many other factors were involved.

 

[PAR]

Hollow can be useful, so can a fine entry angle. Both carried to extremes can rob preformance considerably. As to how much of what and when to use it, well this is dependent on the preformance envelop you envision for the yacht.

Hollow is a bit of black magic, used by designers and also a way to beat measurement rules. The target speed also has a good bit to do with where and how much hollow. A cruising boat will do well with some hollow at the LWL forward, but this should convex out as the boat heels, which picks up displacement so the boat doesn't nose dive on some points of sail. Higher preformance craft generally take a different tack with their entry, preferring to get the bow up and with the ability to pierce the wave train. The hull shape in the forward sections of these types of hulls is quite a different shape, generally being flatter athwartship, but finer in plan, compared to a typically fuller cruiser.

Naturally, these comments are very broad strokes at a complex subject and a very unrefined question. In short it depends on what you want to do and the preformance SOR.

 

[idkfa]

Anyone have pics/seen hollows on multihulls??

 

[DCockey]

.....

I noticed on a couple of older IRC maxis with bow extensions that the waterlines were so hollow that there was effectively no wavemaking going on. I asked a designer who agreed that if there was no wavemaking in the first couple of metres, then effectively the waterline length was shortened by that distance. Neither boat seemed to go anywhere near as well as a boat with straight waterlines, but of course many other factors were involved.

Good comment. There is a natural tendency to try to simplify complex subjects to a few simple concepts, and it usually works. But then the assumptions needed for the simple concept to be valid get lost. It's not waterline length by itself which is important. Rather the length and distribution of the immersed volume is.

 

[daiquiri]

It's not waterline length by itself which is important. Rather the length and distribution of the immersed volume is.

That's how I see it too. The waterline shape is a minor player and the curve of volumes is the primary, because the immersed volume moving through the fluid is the culprit for the wave train and for the relative drag.

 

[DCockey]

That's how I see it too. The waterline shape is a minor player and the curve of volumes is the primary, because the immersed volume moving through the fluid is the culprit for the wave train and for the relative drag.

Though not all volume has equal effect. The influence of volume on surface waves in deep water decays exponentially with depth with a coefficient of gz/U^2 where g is the depth under the surface. At slower speeds the decay is more rapid. So the volume of a bulb keel has less effect then the same amount of volume just below the surface.

 

[daiquiri]

Though not all volume has equal effect. The influence of volume on surface waves in deep water decays exponentially with depth with a coefficient of gz/U^2 where g is the depth under the surface. At slower speeds the decay is more rapid. So the volume of a bulb keel has less effect then the same amount of volume just below the surface.

The bulb keel is usually one of those things which fall into "everything else being equal" category when the influence of the hull shape (waterline shape in this case) is being studied. So my above reasoning was for the canoe body hull of a limited depth, like (in particular, here) the CatBuilder's case.

 

[Leo Lazauskas]

A case can be made that the boundary layer displacement thickness "fills in" the
hollow at the bow returning the effective shape to a convex one, rather than concave.
I have tried this sort of thing with laminar layers, fully-turbulent and even with
transition, but it was just an exercise. In reality, boundary layer growth and stability
is very sensitive to external forces and ambient conditions.

 

[daiquiri]

A case can be made that the boundary layer displacement thickness "fills in" the hollow at the bow returning the effective shape to a convex one, rather than concave.

Mmmh... How much can the BL displacement thickness fill the hollow placed immediately after the bow, when the BL is still in it's infant stage in that area of the hull? I could see it play some role in the aft sections, where it is grown and "beefy" enough to make some influence on the flowlines...

 

[michael pierzga]

New 60 footer ACCIONA hauled at the shipyard... bow profile

 

[DCockey]

I'm familar with boundary layer behavior on automotive type shapes. On the front portion of an automobile like shape I've never seen a boundary layer cause a concave are to become effectively convex, ie the streamlines outside the boundary layer reversing curvature. What does happen is the boundary layer may thicken faster than the usual "flat plate" growth rate due to an adverse pressure gradiant. If the concave area has a combination of sufficiently tight radius and large enough angle change then there may be a separation bubble form. While the separation bubble may add very little to the drag the flow immediately outside the separation bubble will still have concave streamlines. Note that these comments do not include free surface effects.

 

[DCockey]

New 60 footer ACCIONA hauled at the shipyard... bow profile

Thanks! I don't see any indication of hollow in the bow surface.

 

[michael pierzga]

The bow is full...no hollow....bouyancy. Most of the high performance offshore boats, optimized for reaching , I see are the same.