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Old 05-03-2016, 16:54   #1
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Hull design?

Will this hull design ever hit smaller boats? Like a 30' sailboat or fishing boat. I never undestood why the bulb helps vs the sharpest 'V'.



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Old 05-03-2016, 16:58   #2
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Re: Hull design?

Some smaller boats have been built with bulbs. Long story short: they don't work.
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Old 05-03-2016, 17:29   #3
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Re: Hull design?

The 60' fishing boats have them in the port we are in.

I've seen a couple of catamarans with them also.
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Old 05-03-2016, 20:46   #4
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Re: Hull design?

to quote Wiki:

Bulbous bows have been found to be most effective when used on vessels that meet the following conditions:

the waterline length is longer than about 15 metres (49.2 ft)

the vessel will operate most of the time at or near its maximum speed

Thus, large vessels that cross large bodies of water near their best speed will benefit from a bulbous bow. This would include naval vessels, cargo ships, passenger ships, tankers and supertankers. All of these ships tend to be large and usually operate within a small range of speeds close to their top speed.

Bulbous bows are less beneficial in smaller craft and may actually be detrimental to their performance and economy. Thus, they are rarely used on recreational craft like powerboats, sailing vessels, tug boats and yachts.

https://en.wikipedia.org/wiki/Bulbous_bow
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Old 05-03-2016, 21:06   #5
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Re: Hull design?

A little more detail...

Bulbs are only effective within a very narrow speed range. Say Xkn +\- .2kn. At any speed faster or slower than this they add substantial amounts of drag. However at these speeds the bulbs creates an inverse wave to the bow wave, meaning they cancel each other out.

Bulbs are also very tricky to get right, typically requiring a good bit of preliminary engineering, as well as tank testing and then sea trials to dial in. To offset these large initial costs the bulbs have to generate enough fuel economy savings to justify the initial outlay. Obviously doable, but far more important on large vessels moving at high constant speeds.

For a sailboat there would be some speed where it helps a lot, but the rest of the time it would just add drag.
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Old 05-03-2016, 22:59   #6
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Re: Hull design?

I believe they are put on multihulls to add buoyancy at the bow to compensate for the very fine entry, lack of buoyancy forward and consequent pitching some catamaran's exhibit rather than for a drag reduction.


On big ships it appears that they are willing to suffer the slight increase in skin area and consequently skin friction and drag for a substantial reduction in form drag?
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Old 06-03-2016, 07:35   #7
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Re: Hull design?

Quote:
Originally Posted by RaymondR View Post
On big ships it appears that they are willing to suffer the slight increase in skin area and consequently skin friction and drag for a substantial reduction in form drag?
The reduction is actually in wave making resistance. Bulbous bows effect the same wave drag that defines hull speed. In effect they act as noise canceling headphones for the bow wave.
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Old 06-03-2016, 12:20   #8
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Re: Hull design?

Yes.

I have seen two or tree boats with such bulbs. One was a cat and two were monos.

One of the monos was (erghhhh) methinks an Elvstrom.

Sailing boats are not as stable in the seaway as cargoes hence the idea is fine mostly in flattish seas, under engine, etc.

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Old 06-03-2016, 12:26   #9
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Re: Hull design?

CORONET ELVSTROM 38 MS sailboat specifications and details on sailboatdata.com

Here we go. We have one of them here. Fantastic ship BUT I would not like this setup where many lobster pots or pelagic nets are set ;-(

When she powers on the harbour there is hardly any wake. I think the idea works well then.

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Old 06-03-2016, 14:16   #10
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Re: Hull design?

All very interesting. Even though I do not understand it completely apparently it works for the large cruise, cargo, navel ships. It looks like it would not work at all, that a sharp 'V' would work the best but guess I am wrong...yet again.

Cool sailboat with one to catch the lobster pots. An added use for the bulb?

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Old 07-03-2016, 04:49   #11
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Hull design?

One of the British12 metre America's Cup yachts tried out the idea, perhaps in the 80s, but it wasn't used in anger, I don't think.


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Old 07-03-2016, 11:16   #12
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Re: Hull design?

The yacht in question was designed by David Hollom, called White Crusader ll. It was not selected for racing.


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Old 07-03-2016, 13:34   #13
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Re: Hull design?

As Stumble writes, a bulbous bow reduces the wave-making resistance in a narrow range of speeds but it increases the wetted area and the friction resistance at all speeds. On small boats, the friction resistance is a larger part of total resistance (lower Reynolds number), so the gain in wave-making resistance often doesn't compensate the loss.

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Old 07-03-2016, 13:47   #14
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Re: Hull design?

I've heard of a few powerboats adding bulbs to reduce pitching. A useful side effect.

Also they make a nice spot for the bow thruster to sit.

But they can be a problem with the anchor fouling it. We always had to walk the anchor back carefully under power past the bulb before releasing the brake.
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Old 07-03-2016, 13:52   #15
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Re: Hull design?

some megas have the bulbous bow. over 100 ft. i have seen here in barra--often a location of incredible wealth
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