Bulbous bow on a sailboat

mholguin · Mar 14, 2005

Does it make any sense?

I can imagine, it would add tons of drag, but wouldn´t it benefit from the "breaking avility" of the BB?

mholguin · Mar 14, 2005

I found this on 48º North, I think that's the answer:

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Vacanti Prediction Ringing True!

I am having a ball watching the AC turn out. As I predicted, Alinghi is fast in the turns and NZ82 is slower in the turns. NZ82 is fast in drag races and Alinghi is faster in the tacking duels. That long keel bulb is killing NZL. It takes much longer for the low drag flow to re-establish itself after a tack / gybe on the very long chord length versus a shorter bulb.
The Kiwi catch up when drag racing in clear air and no tacks or gybes. When they get into close action gybes and tacks Alinghi pulls away.
With respect to the HULA - there have been two previous attempts to cheat wave drag using hull changes. 1) In 1976 there was a Six meter designed with a bulbous extension under the bow - much like the ones you see on ships tracking through the Sound. The bulbous bow on ships reduces wave drag. On a small sailboat it reduced wave drag by artificially extending the waterline length.
The Six meter was fast in a straight line but "she took an age to turn" was the description of her performance. (Page 210 AeroHydrodynamics of Sailing) As a result she failed to win her races.
2) In 1974 the 12 meter Mariner had a "bustle" hull extension that is a more extreme example of what the HULA is today. The hull extension designed by Britton Chance was actually chopped off just aft of the rudder location (Page 225 AeroHydrodynamics of Sailing).
So the concept of hull modifications to achieve lower wave drag have been tried in forms somewhat similar to the HULA. NZL has encountered the same problems that the Six meter did.
Both 1970's examples above show that if you design for the steady state condition (drag racing in a straight line) you can do that well - but you have compromised the dynamic performance of the boat - tacking and gybing and responding to waves. The long keel bulb is a detriment in conditions that are constantly changing - as they do in tacking duels. That's because of the much longer time it takes for the water flow to settle down into a low drag flow versus the shorter bulb on Alinghi. I'm not sure that the HULA is not encountering a similar problem. It is designed to make the hull look longer for greater speed potential - but in doing so that also means it takes longer to tack and longer for the low drag condition to re-establish after a tack or gybe. In straight line drag racing the NZL82 is very fast. Just too bad they have to turn from time to time.

Best Regards,
David Vacanti
Vacanti Yacht Design LLC
http://vacantisw.com

It does look like they've traded maneuverability for straight line speed. At the time of this writing, after postponement after postponement, I'd just like to see them race.

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water addict · Mar 14, 2005

Bulbous bows reduce wave drag only for a VERY narrow range of speeds- and if you get in a situation where you are doing a lot of pitching, the bulb usually hurts resistance.

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Bulbous bow on a sailboat

mholguin Junior Member

mholguin Junior Member

water addict Naval Architect

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