Hollow water lines?

[ancient kayaker]

Messabout: true to the small boat tendency to break the rules of physics, the best gliding of my boats, the rotomolded one, has a hideously scarred hull as a result of assaulting many a stony beach. This appears to have no detectable effect on its performance. When I stop paddling it sheds about 1/2 its speed quickly then seems to glide on and on. This could be subjective as I have never checked it with the GPS. The same boat, as Brent notes, loses speed abruptly if it runs into a wave: on the other hand it is quite willing to surf in a following sea. It has a hard life: it spends winter outside while the other boats bask in my insulated garage. I remember experimenting with it to see if it would function as an ice breaker so I could extend the paddling season; not my finest moment.

I used to have a long sea kayak, yards of it, which augured clean through a wave if I really pushed it.

[sailing canoe]

Brent - I always thought that clipper bows on small boats looked pretentious - Now you have given me a cause to back my prejudice up. Does your new boat Have overhang ? Or is the improvement solely the result of out curving water lines; i.e. not hollow.

[PAR]

Clipper bows don't tend to plunge or any other bad habit. What a foolish and ridiculously generic set of statements. Any bow profile can be designed with a poor plan form. The profile selected has very little to do with the function of a yacht's entry, particularly with the clipper, which is generally little more then a raked straight stem and a knee mounted on top to support a sprit or other on deck structure. Does this mean all straight stemmed yachts with similar rakes of clippers are "plungers" too? Please . . .

[ancient kayaker]

I have to agree with Par. There isn't much connection between clipper bows and hollow water lines. All my kayaks have hollow waterlines, none have clipper bows, they're all plumb. A clipper bow is concave viewed in profile; water lines are viewed in plan by definition. If hollow water lines are continued above the surface they result in a concave stem but they are no longer really water lines. A concave stem can also be generated by hollow stations near the bow. I suspect the main historical reason for the clipper bow was to support the bowsprit.

Getting back to the topic: intuitively it seems that hollow water lines at the bow and stern should smooth water flow around the hull by spreading out the lateral fluid acceleration thereby reducing drag at low speeds but anything can be overdone, or done badly.

[PAR]

Bingo, hollow waterlines are a function of displacement distribution in plan view. At low S/L's they can keep flow attached across the forward sections of an immersed body. At S/L's over 1.4 they begin to retard the boat's potential with wave making resistance, which increases exponentially with speed.

[Paul Scott]

Most sledish uldb's here on the left coast have hollow bows, and they are definitely going past 1.4 s/l's. Mine, for example. Combined with some v back to the keel, very soft in the bumpy stuff, upwind and down. Going down the mine and stuffing have not been a problem, in Puget Sound or Straights of Georgia (upwind against the tide!). .53ish prismatic, 40'loa, 36' wl, 8.5' wb, 98d/l, almost vertical bow. We'll do 8+k upwind, 15-20k downwind. Windspeed at least in anything under 7+k. Lightwind performance was the intent with her, but medium and heavy air performance are up to snuff. It seems some are arguing that she'd go faster with no concaves? Or are planing hulls outside of this discussion?

Paul

[PAR]

Planning hulls are different animals, The forward sections of your boat will have punched through the bow wave at 1.4 S/L (or is just about to), so these sections are no longer in "play". Also the dynamics are quite different between the two types, primarily because your boat carries ample power to drive through LWL limited speeds and the hull form to take advantage of it.

In fact, with most cases you'd want good "penetration" in the forward sections, from a boat that will plane off. This decreases frictional resistance and weight, both necessary for most planning hulls, particularly in the bow. The net result of these function derived shapes are plumb stems (though not necessarily so), with low entry half angles and little displacement carried in these sections. If the bow sections have convex shapes employed, then you're also carrying displacement further forward then you need, increasing drag with a fuller and heavier bow.