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Old 18-10-2016, 14:45   #1
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Design

Hello All:

I notice that on the larger sea going vessels they have a bulbous protuberance below the waterline. I understand that these "things" help the vessel go faster.

Why don't cruising yachts have these if they improve performance?
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Old 18-10-2016, 14:58   #2
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Re: Design

Quote:
Originally Posted by NorthSea View Post
Hello All:

I notice that on the larger sea going vessels they have a bulbous protuberance below the waterline. I understand that these "things" help the vessel go faster.

Why don't cruising yachts have these if they improve performance?
They don't improve performance in all conditions. You need to be operating at 20 knots or above and be constantly at that speed like a carrier, freighter/cruise ship.

Notice they're never on small destroyers. Won't help a sailing vessel.
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Old 18-10-2016, 15:00   #3
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Re: Design

Thanks for the quick response. Do you know what these underwater bulges are called?
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Old 18-10-2016, 15:02   #4
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Re: Design

Nordhavn put bulbous bows on a few of their earlier full displacement cruising yachts but then they pretty much discontinued the use of them. There wasn't much if any benefit apparently although some owners liked them. Maybe the testosterone effect?
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Old 18-10-2016, 15:06   #5
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Re: Design

They are called "bulbous bow".

By the way few of them where used for a different purpose, they added buoyancy forward on some catamarans.
Catana still does it to my knowledge, Crowther added them on few designs.

www.ladyrover.com
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Old 18-10-2016, 15:07   #6
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Re: Design

Bulbous bow.

They have to be calibrated for expected payload, cruising speed and engine/prop specs. Some years ago a large cruising company built a new extreme large vessel and the bulbous was incorrectly designed. Very expensive error that hurt performance.

Shouldn't be on sailing vessels
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Old 18-10-2016, 15:50   #7
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Re: Design

Thank you all.
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Old 18-10-2016, 16:10   #8
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Re: Design

They did try them on a few monohulls, Paul elvstrom designed a motorsailer with a proper bulbous bow, and they experimented with them in the 6 meter class. Problem (as mentioned before)is the very narrow band where they are effective, outside that band they just add frictional drag. Some say they help reduce pitching, but then you can get more bulb slap/pounding under them on smaller boats.

Interesting that the old Ram bows on the Mediterranean galleys may also have functioned like a bulb bow and reduced drag. Another interesting varient is the Alutian Baidarka kayak with its bifurcated bow. Them old timers were pretty smart!
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Old 18-10-2016, 16:29   #9
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Re: Design

Not a cheap thing to make. May have good benefits but cost is likely the issue.
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Old 18-10-2016, 16:48   #10
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Re: Design

Here's some interesting info :

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

Note that it works by balancing out the bow wave, with an opposite phase wave generated by the bulb. The bulb creates drag in the process, but this is more than compensated for by the reduction in drag along the hull, resulting from the smoother flow.

Therefore I think they would work best when :

You are generating a large bow wave, so that the bulb can cancel it out.

You have a long hull, so that the effect of reduced drag along the hull is significant.

Neither of these things is true on a small sailboat.
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Old 18-10-2016, 17:36   #11
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Re: Design

From Pierre Gutelle's book 'the design of sailing yachts' 1984
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Old 18-10-2016, 18:40   #12
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Re: Design

For the bulb to do its job it must be below the surface. When it is at the surface it is a horrible fat bow with a 180 degree entrance angle. Ships that have bulbous bows are large, freighters and tankers, and their size renders them comparatively stable re pitching. Small boats have their bows coming out of the water when sailing to windward on pretty much every wave. Ditto destroyers when they are running fast is a good sized sea. I've seen them with at least the forward 1/4 of the hull out of the water. For that type of thing a sharp bow works better.
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Old 18-10-2016, 19:25   #13
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Re: Design

Q: Good name for a fishing boat with a bulbous bow?j

A: "Cod Piece"
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