sailboat bows

Ac

Do anyone know the story behind the shape of the AC bows

http://www.victorychallenge.com/show_pic.phtml?pic=gallery_big__582.jpg

 

Rules.

 

Yes but anyhow they seems woork pretty OK. Even not being to sharp in shape.

 

It looks to me like it's designed to promote planing. I'd imagine that with all the sail these boats are carrying they'd need a very bouyant bow like that to keep from stuffing it under too often

 

Alinghi won the last America's cup supposedly because their bow had more volume in it than TNZ. (TNZ falling apart also helped).

 

weight distribution

I've come late to this thread, but would like to contibute my thoughts and receive comment from others as they may...

Two quotes given above:

I believe you, absolutely. Weight in the ends increases moment of inertia because of increasing distance from the transverse axis of rotation.... and this decreases pitching by increasing force necessary to create the pitching motion. The transverse axis of rotation usually corresponds closely, but not exactly, to CG; (not exactly CG in a boat because of additional contribution to the axis of rotation due to CF and CB).

And ...

I agree with most of this as well (and said almost the same stuff above). Comfort and continued forward drive absolutely result. The only point I would add is that "bobbing gayly over it" (increased pitching amplitude, beginning earlier, in boats with less weight in the ends) must ALSO (and does) decrease forward drive as would the drag of "added resistance as it goes through the wave" ... while providing a far less comfortable - more fatiguing - motion for the crew. "Bobbing gaily" does require that the hull travel uphill on the wave ahead, decreasing forward speed. (I guess the size of the wave, shape of the bow, forward underbody, and beam must all come into early play here.)

On balance, however, I should think that in most conditions ... the forward motion retarded by drag going through the wave, is less detrimental to foward speed than the quickly rising bow of a sailboat that must sail uphill over each wave.

Exceptions to my argument above are easy to imagine. But for most conditions ....... well, like Steve said, "Compromises, compromises..."

Robert

 

Hard to add anything to this thread, and calculations are quite useless, when you see the variety of waves. But I believe the idea of going through waves is valid for small seas and maxi boats mainly. More weight near the ends is not a good idea to increase seaworthyness. Then there is something about comparing load to curve areas (volumes), with less weight in the ends you can afford sharper lines so better performances.

So back to banging anchors and plumb bows. MY question is, what's better, adding a prominent structure (increased weight and LOA) or keeping cans of paint onboard ?

 

"On balance, however, I should think that in most conditions ... the forward motion retarded by drag going through the wave, is less detrimental to foward speed than the quickly rising bow of a sailboat that must sail uphill over each wave."

I would find that very hard to believe. What goes up must come down -- the emphasis on going up the wave ignores the energy returned to the boat on the way down the other face. Plowing a hole in the wave, on the other hand, will generate lots of wave energy that does not return. Ignoring comfort for the moment, the only advantage I can see of not pitching is that the sails may flow more smoothly through the air.

Variety of waves is right. A small boat in big seas also doesn't have to worry much about pitching, as long the swells aren't breaking a lot. Going through a huge wave is out of the question, the little boat will have to "bob gayly". The most important situation is where the wavelength and boatlength are comparable.

 

weighted ends and wave punching

I should think it's a threshold question with both matters (and that's my seat of the pants sense, but let me know what I'm missing):

(this ignores a lot of other factors like the particulars of the hull shape, etc.)

1) Pitching: if the wave is small enough that the applicable inertia is still retarding the rise of the bow by the time the crest is reached (that is, the bow is still burying and hasn't 'caught up' with the rising wave yet), then the higher inertia produces the advantage and has reduced the total pitching;

however, if the wave is large enough that the bow has been accelerated, then the high inertia boat contributes to more pitching because the bow will keep rising after passing the crest.

(and I do mean acceleration, and not just that the boat as a whole has reached the angle of the wave face; in the latter case, it's a question of the balance point of the boat, not that it has developed rotational inertia)

2) Punching: hullform matter, which in conjuction with the pitching issue might dictate a 'sweet spot' for how much any boat should punch versus pitch for maximum efficiency in a given wave size. My experience has been that the fine bow'ed boats profit from punching (and they rise out of a buried bow situation more readily as well; as a general matter, the bow is not vertically yanked around as much by waves, and there's a bit of a wave-piercing value). But this obviously has a limit depending on hull/topsides shape (wet spreaders are too much )

Kind of a lousy comment, I suppose, since what I'm saying is 'it depends' - but I think that's the case -depends on the wave amplitude and wavelength, along with the speed of the boat through x-degrees of the given waveform.

Sharp, light bows with a high knuckle take more driving attention in bigger waves, but the driving is very easy.

-JPC

 

It is worth noting, that Adrian Thompson's work with wave-piercing hulls was highly sucessful on racing catamarans (especially on Team Phillips (until it broke)). On racing monohulls, the plum bow has the advantage that no matter how much wetted surface area is added going into a wave, it's nothing in comparison to the load (both slamming and wave drag) of a swept bow. If anybody wants a good example of this, race a LARK and a 420 in waves in a decent breeze.

Also, did someone mention plum bows giving wet boats? That's not a theory I agree with I'm afraid. Dryness has a lot more to do with the shape of the section near the deck-joint, and how the deck joint itself is put together. It is also dependant on the freeboard, heel angle and 101 other variables.

Tim B.

 

Has anyone used the bulbous bow to change the pitch motions?

Seems there would be a big difference if it was dry or intentionally flooded.

Pumps are cheap too.

FAST FRED

 

bulbous bow??? ouch, only works at one speed and think of the wetted surface area! hardly likely to be a gain there!!!

Tim B

 

Can I give some first hand sightings in North Atlantic storms? Clear, sunny, smooth sailing day for all the USN destroyers in our unit. The aircraft carrier was getting the crap pounded out of her at the same time. Reason, at 900' she was on course in 1/4 to 1/2 mile long swells that would slowly lift her bow clear for 1/4 of her length then drop it into the base of the next wave. It would roll and cover 1/3 of her deck for about 30 seconds. the stern, screws and rudders were all visable and spinning. That long, wide, flaring bow and deck were a handicap.

 

I just realized that is one of the reasons why Super Tankers have vertical bows.

 

Yes definately longer water lines make faster boats when approaching hull speeds. But faster boats are also penalised by the rating rules to give all boats an equal chance on the race cource. New rating rules like IRC are kept confidential in order not to produce rule optimised boats. Therefore are we any longer convinced and certain that for the same overall length a boat with a plumb bow maximising its waterline length is more advantageous than one which has an overhang and therefore a shorter waterline.
We simply do not know how the rule makers treat these parameters and favour one type over the other.
Probably the only way to find this out would be to create identical boats in every measurement and ratios except the bow profile and waterline length and race side by side enough number of times, switching crew as well and see whether one has a distinct advantage after rating corrections.
My point therefore is, it is probably no longer true to say that a longer waterline length is a sure recipe for success in race course any longer.
if there are any wiews to the contrary, i would be very interested to know.