Ball in front of bow

[leong]

Brain Teaser:

Why don’t kayaks have bulbous bows like this?

[Paul Sylvester]

If I tried to answer this I would be above my pay grade......but....A 16-18 foot kayak is close to the length for max hull speed for the size of the engine? Where as a 400' ship can have larger engines that can push it beyond hull speed....But not planing...So give it more length underwater and more efficiency at greater speeds.

Let the laughing begin???

[Pintail]

No laughing: this might warrant some thought...

Paul, are you sure about the extension of waterline by these means? Why not extend it with a sharper, cutting edge? (Less drag? No, I'm wrong there, aren't I?)

You know those "birds" that are lowered from twin booms on deep sea fishing boats to keep them on an even keel (What do they call them? Chickens or pigeons or something avian like that?) Well, perhaps that massive bulb is rather to keep the bows steady, thus reducing longitudinal forces along the length of the vessel -- important when it is a <long> or super-vessel (There is enormous torque applied longitudinally to one of those hulls when in big seas -- tankers have meters on the bridge to watch this very phenomenon)

I'm still thinking about it...

[jason]

http://en.wikipedia.org/wiki/Bulbous_bow

[Pintail]

Oh, Jason: you clever sausage, you! Thanks for thinking of Wiki -- now why didn't I think of that? Doh!

Of course: now it is quite clear -- and very interesting. I especially like the bit about Mr. William Armstrong, of course...! (Did you know, for example, that the swing-wing concept, as used in the old F1-11 was also a British invention? And vectored thrust, as in the Harrier? Not to mention the good old steam engine for locomotion...) ;^)

[jason]

Can also google and find a number of good articles:

http://www.marineinsight.com/marine/marine-news/headline/why-do-ships-have-bulbous-bow/

http://www.dieselduck.info/library/01%20articles/bulbous_bows.htm

[leong]

Okay, I’m neither a naval architect nor an expert in kayak hydrodynamics. Nevertheless, here’s my take on bulbous bulbs for kayaks:

In layman’s terms the purpose of the bulb is to create a wave ahead of the regular bow wave, which partially cancels out the bow wave, thus reducing wave-making resistance.

Most ships will not carry enough power to get anywhere near their "hull speed". At these relatively slow speeds (relative to hull speed) the bow wave is short and, accordingly, the bulb will be very short compared to the length of the ship. You can see that in the pictures.

However, a kayak usually moves at speed ranges close to hull speed. At these speeds the wavelength of the bow wave is close to he kayaks length. Accordingly, for it to cancel out the bow wave, the bulb for a kayak would need to be long compared to the kayak’s length (perhaps a 5-foot bulb). That long a bulb would ruin a kayak’s ability to be paddled in even mildly rough wager. In addition, the extra weight and the increased wetted water surface area due to such a long bulb would more than negate any reduction in a kayak’s wave making resistance.

-Leon (who could be wrong)

Edited May 9, 2015 by leong