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Old 09-12-2012, 12:04 PM   #1
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Bulbous Bows: Split from Previous thread

The old iron fantail is a beauty.
Would like to know the history on "Fleetwood".
The Nordy w the long bulbous bow makes me wonder if they ever come off in really rough head seas. Seems the forces must be extremely high. I'm sure my Willy would benefit from one but I'd rather extend the stern. Why get the effects of a longer boat when you can have a longer boat. Must just be cheaper.
Thanks for the good pics.
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Old 09-12-2012, 12:25 PM   #2
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I saw a show on TV about the Queen Mary II or queen something.
They said that to get the speed out of the boat that they wanted they added on to the length of the bulbous bow.

I have done some reading on this and as I understand it . It will not work on smaller boats. I don't understand why.

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Old 09-12-2012, 01:45 PM   #3
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Bray Yacht Design & Research Ltd - Bulbous Bows

The benefit of a modern-day bulbous bow will reduce your fuel consumption 12% to 15%,

Vessels as small as the 46 foot Nordhavn have had bulbs fitted. Below this size the bulb's effectiveness seems to be lost, however. Over 45 feet the results are adequate and closer to the 60 foot size real gains are being achieved.

At low speed (around 6 knots and lower) the bulb will even cause an increase in drag because of it's greater wetted surface area.

Probably not in anyones cards.
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Old 09-12-2012, 01:55 PM   #4
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Skip,
I'm sure it will work hydrodynamically and have seen numerous small boats like ours w them.
One of the reasons I think the BB would work well on Willy is that she pitches quite a lot or hobbyhorses. The BB would dampen that a lot to be sure and directional stability would increase a lot too. But sometimes a higher level of control and less stability is preferable. Perhaps on a short boat like mine the BB would dig in and over control the bow quartering seas in either direction. Perhaps not too. I'm quite sure you could find a good discussion about it on BoatDesign.net. If you take the trouble .... share.
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Old 09-12-2012, 01:56 PM   #5
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I Read through Ramblers link and it would seem that it works but no one knows why.

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Old 09-12-2012, 02:11 PM   #6
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Rambler,
Yes I've read that but fail how size would have anything to do w it. Kinda like Marin and his big anchors and big boats are better than small boats and small anchors concerning the Bruce. They sell very small Claw anchors. Reynolds numbers and such seem to get involved. But never seen a rowboat or sailboat w a BB.
Most cruise ships have a highly racked bow and a BB. I'd love to compare one to an identical ship w a vertical stem and a keel that had a forefoot that would go fwd to the same extent as the BB .. rather like the Titanic. Seems to me the deep forefoot should be as fast and have less drag. I'm sure I'm wrong ... just don't see the reason why. I think it has something to do w a principle that if explained could use the expression "shock wave".

If there's any more to this we prolly should start a new thread.
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Old 09-12-2012, 03:48 PM   #7
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They look like they are slightly angled upward, and would then create a pre-wake that would maybe create some amount of drag on the actual bow of the boat, hopefully in a greater amount than the power necessary to push the BB through the water at speed?

Maybe there is some principle of physics that I slept through in class.
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Old 09-12-2012, 05:38 PM   #8
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Quote:
Originally Posted by skipperdude View Post
I saw a show on TV about the Queen Mary II or queen something.
They said that to get the speed out of the boat that they wanted they added on to the length of the bulbous bow.

I have done some reading on this and as I understand it . It will not work on smaller boats. I don't understand why.

Sd
Have heard the same thing and that about 90 percent of even the NA designed ones for smaller vessels either don't work or only work in a narrow range of normal operating parameters.

There have been numerous ones added to larger vessels that have caused more problems than they were worth. Days lost to adding, fixing, modifying, etc...etc all cost way more than the added benefit.
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Old 09-12-2012, 08:13 PM   #9
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I am sure someone could find the reason for the Hydrodynamics ? and the reason they will not work on smaller craft. I am sure it has something to do with L X W issues, I read about narrow hulls in the past a ship that was 96 ft long would make 10 knots empty then with 129,000 lbs of clams aboard it would make 9 knots. it was very narrow Not like my Honey Badger whos beam is 10Ft and the boat is only 24 ft long at the water line. Fat and stubby
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Old 09-12-2012, 10:41 PM   #10
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I know they work, I know they are a pain in the butt around crab/lobster traps, I know they are designed for a constant speed to move the pressure wave caused by the hull moving through the water, they dont work on sail boats because of inconstant speed and the healing of said boats.

But most of all I know this one looks way cool!

HOLLYWOOD
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Old 09-12-2012, 11:00 PM   #11
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It's all about phallicism, leastwise for boats.
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Old 09-13-2012, 12:39 AM   #12
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I haven't researched it, but my understanding of the bulb bow is much more to do with reducing drag by sort of parting the waters ahead, than it has to do with waterline length. After all, most ships never actually do the speed their waterline length would allow under normal physics anyway, especially ones like the Queen Mary.
This may help - haven't even had time to read it fully myself yet.
What is a Bulbous Bow? - Cruise Radio
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Old 09-13-2012, 06:10 AM   #13
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Bulbous bows are most efficient at one specific speed.

On many cargo ships the BB is being modified (about a million bucks) to work at the slower speeds more common today.

Be sure you want to travel at the "ideal" speed the BB was designed for.

On smaller boats the BB seems to help dampen some heavy weather pitching .
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Old 09-13-2012, 06:37 AM   #14
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It's all about phallicism, leastwise for boats.

Hmmmmm, what do you make of this one?
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Old 09-13-2012, 07:38 AM   #15
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It's all about phallicism, leastwise for boats.
Thank you.I was thinking it but didn't know how to put it delicately.
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Old 09-13-2012, 10:26 AM   #16
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"Phallicism" Hmmmmm
Had to look that one up. With Rick's B I can see the relationship.
That's the longest BB I've ever seen. In that length I can see how the BB would be considerably cheaper than lengthening the whole boat.
Perhaps I'm a clutz w the search function on BD.net but couldn't find much BB stuff. More on X bow and Axe bow and I think they are the same thing.
And what would be wrong w being locked into a certain speed Fred?. We w disp hulls are anyway. But most here have SD so your comment would apply but who would even think of a BB on a SD hull?
Ben wrote "Thank you.I was thinking it but didn't know how to put it delicately." I think Mark already did that.
As to how they work ... they hydrodynamically lengthen the wave the disp boat rides in thereby reducing the drag the boat has at hull speed or increasing the potential speed. It takes more power for a Willard 30 to go 7 knots (hull speed) than it takes for a W40 to go seven knots. The W40 is huge by comparison but more importantly the W40 has a longer water line length. It's because the drag of a FD hull drops of quickly at less than hull speed. Notice that disp hulls don't travel at hull speed .. too much drag. But just below ... pay dirt .. much reduced drag. I think the BB is more about speed than efficiency. It makes them more efficient at a slightly higher speed so it effectively increases their speed. That is .. the speed it's practical to go. It makes the boat/ship more efficient at a slightly higher speed. But not more efficient that if the boat/ship slowed down. You can put a spoiler on the back of a race car and reduce the drag and make it more efficient but the Honda Civic will still get to market on less gas.
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Old 09-13-2012, 10:34 AM   #17
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I also think the BB are used to weight the bow, especial if it has a large flare to it so the bow does not lift out of the water. Dampen the pitch? Jim, retired commercial tug, that owns a sister 58, in the Queen Charlottes BC, said the 58’s bow should have more weight as the bow rises up out of the water to fast/much. However he was glad the bow was flared which push most of the water to the side.


Many of the boats from Alaska have dented/damaged bows/BB as many run 24/7. Most BB seem to be added on to the bow/hull, so a BB may also protect the bow/hull?

Mnay of the new mega yatchs have the bow slanded forward or streaight down, ax bow.
I would add to fine tune the pitch/bow and to protect the bow/hull and not so much for fuel efficiency.
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Old 09-13-2012, 10:48 AM   #18
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I also think the BB are used to weight the bow, especial if it has a large flare to it so the bow does not lift out of the water. Dampen the pitch? Jim, retired commercial tug, that owns a sister 58, in the Queen Charlottes BC, said the 58ís bow should have more weight as the bow rises up out of the water to fast/much. However he was glad the bow was flared which push most of the water to the side.

Many of the boats from Alaska have dented/damaged bows/BB as many run 24/7. Most BB seem to be added on to the bow/hull, so a BB may also protect the bow/hull? I continue having a feeling/dream that I am going to ground the Eagle, so more interest in safety and protection So if I was going to added a BB it would be to fine tune the bow/pitch and protect the bow/hull. Not as much for fuel economy.

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