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Old 23-01-2019, 16:41   #1
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Odd hull protrusions. Boat ID?

We saw this cat in Ft. Lauderdale at the end of December. Didnít notice the odd bow section until the sun clouded over for a moment. Reminded me of a bulbous bow on a freighter.

Any ideas what kind of design this is?

Thanks!
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Old 23-01-2019, 16:53   #2
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Re: Odd hull protrusions. Boat ID?

Island Spirit with custom bow bulbs, cut down on pitching I imagine.
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Old 23-01-2019, 17:33   #3
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Re: Odd hull protrusions. Boat ID?

Reference article regarding fitting with bulbous bows.

https://www.passagemaker.com/web-ext...xs-bulbous-bow

Snipet:

"Nordhavn saw the benefits of a bulbous bow first hand during seven exhaustive test sessions at the University of British Columbia Ocean Engineering Center within its test tank facility. I went with Jeff Leishman, my brother, who is Nordhavnís in-house naval architect,

The results were favorable, to say the least. The bulbous bow achieved reductions in resistance on all boats tested. On the 40 we noted a 6.5 percent reduction in power required to run at cruising speed, with 8.6 percent reduction on the 50, 10.2 percent on the 57 and an impressive 12 percent on the 62.

We did not test every model for sea-keeping, but when we did, we noted reductions of up to 20 percent in pitch amplitude while running into head seas. It was interesting to see the resistance increase on the bulb fitted hull as the sea state increased which illustrates that power is being consumed to reduce the pitch motion. In other words while running into head seas the resistance benefit of the bulb fitted hull diminishes and is replaced by pitch reduction.
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Old 23-01-2019, 17:47   #4
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Re: Odd hull protrusions. Boat ID?

Quote:
Originally Posted by Montanan View Post
Reference article regarding fitting with bulbous bows.

https://www.passagemaker.com/web-ext...xs-bulbous-bow

Snipet:

"Nordhavn saw the benefits of a bulbous bow first hand during seven exhaustive test sessions at the University of British Columbia Ocean Engineering Center within its test tank facility. I went with Jeff Leishman, my brother, who is Nordhavn’s in-house naval architect,

The results were favorable, to say the least. The bulbous bow achieved reductions in resistance on all boats tested. On the 40 we noted a 6.5 percent reduction in power required to run at cruising speed, with 8.6 percent reduction on the 50, 10.2 percent on the 57 and an impressive 12 percent on the 62.

We did not test every model for sea-keeping, but when we did, we noted reductions of up to 20 percent in pitch amplitude while running into head seas. It was interesting to see the resistance increase on the bulb fitted hull as the sea state increased which illustrates that power is being consumed to reduce the pitch motion. In other words while running into head seas the resistance benefit of the bulb fitted hull diminishes and is replaced by pitch reduction.
No issue with anything in your post however it is extremely doubtfull if any of it is relevant to those tiny bulbs on a small cat.
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Old 23-01-2019, 18:28   #5
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Re: Odd hull protrusions. Boat ID?

Another reference:

Shape Shifter - Power and motoryacht magazine, June 2015. Two 44' motorcats, one with and one without bulbous bow modifications

http://http://www.aquilaboats.com/-/...0pdf.pdf?la=en
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Old 23-01-2019, 18:34   #6
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Re: Odd hull protrusions. Boat ID?

Quote:
Originally Posted by smj View Post
Island Spirit with custom bow bulbs, cut down on pitching I imagine.
Now THAT caught me off guard! Iíve seen several I.Spirits and went along for a week when my wife took her 4 ASA courses and none of them had bulbs. It was probably an option, or limited to specific years maybe. Interesting.
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Old 23-01-2019, 18:37   #7
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Re: Odd hull protrusions. Boat ID?

Quote:
Originally Posted by Montanan View Post
Another reference:

Shape Shifter - Power and motoryacht magazine, June 2015. Two 44' motorcats, one with and one without bulbous bow modifications

http://http://www.aquilaboats.com/-/...0pdf.pdf?la=en
Iím familiar with the concept; Iíve just never seen it on a cat, and was especially surprised that they were on that small of a cat.
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Old 23-01-2019, 18:37   #8
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Re: Odd hull protrusions. Boat ID?

Perhaps this reference will help:
Prout Catamarans - bow bulbs
I was thinking that the bulbs helped lay down a more laminar flow and thus reducing resistance, but I could certainly be wrong. The ones that cat has, I doubt, were meant to reduce pitching.
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Old 23-01-2019, 19:14   #9
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Re: Odd hull protrusions. Boat ID?

Anti-jetski defences?
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Old 23-01-2019, 19:15   #10
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Re: Odd hull protrusions. Boat ID?

Quote:
Originally Posted by LeeV View Post
Now THAT caught me off guard! Iíve seen several I.Spirits and went along for a week when my wife took her 4 ASA courses and none of them had bulbs. It was probably an option, or limited to specific years maybe. Interesting.


Pretty sure it was done after the fact, custom job after boat was purchased.
Hereís a picture of a Crowther Spindrift 38 that we looked at quite a few years ago. Itís now on Youtube as SV Zingaro
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Old 23-01-2019, 20:19   #11
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Re: Odd hull protrusions. Boat ID?

Yup, I remember the Crowther bulbs, too. Isn't that 4th pic in Lees post a different boat???
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Old 23-01-2019, 20:23   #12
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Re: Odd hull protrusions. Boat ID?

Quote:
Originally Posted by Sand crab View Post
Yup, I remember the Crowther bulbs, too. Isn't that 4th pic in Lees post a different boat???
Oops! Youíre right SCrab! There were 2 other cats, one of which looked to be all carbon (not a GB); wish Iíd taken more pics.

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Old 24-01-2019, 12:41   #13
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Re: Odd hull protrusions. Boat ID?

Quote:
Originally Posted by Don C L View Post
Perhaps this reference will help:
Prout Catamarans - bow bulbs
I was thinking that the bulbs helped lay down a more laminar flow and thus reducing resistance, but I could certainly be wrong. The ones that cat has, I doubt, were meant to reduce pitching.

Those are very narrow hull extensions. I could be wrong, but they look more like a wave piercing design than a bulbous bow design. They probably serve a little of both purposes and support a reduction in pitching.
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Old 24-01-2019, 13:22   #14
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Re: Odd hull protrusions. Boat ID?

If I recall the hydrodynamics of a bow bulb are closely aligned to speed through the water. They work brilliantly on a big ship that travels at set speeds, but not so well on a yacht with a range of speeds.
There was a time when the 12m yachts were playing wit them, but again, these have fairly close tolerance on boat speed and wave conditions they race under.
All relatively similar to the wing keel, it worked because of a narrow range of boat speed and would be far less effective at the extremes

Wavepiercing is a whole another level of design and you had better have a good computer and bank balance for the necessary design programs, but they are more adept at the pitch management as well as waterline length deception.
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Old 24-01-2019, 16:41   #15
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Re: Odd hull protrusions. Boat ID?

I believe that Djarraluda has got it right. Hull speed is a function of waterline length and the bulbous bow forms the bow wave a bit further forward. This increases the wavelength generated by the hull form in displacement mode. The hull speed is a function of the square root of this wavelength (very approximately). Consequently for a large displacement cargo carrying vessel where fuel consumption is very important, this feature is essentially mandatory. For smaller vessels particularly those that can exceed normal displacement hull speed (as catamarans can), the benefits are presumably less. Stability may also be a factor. One disadvantage is that these bulbs can "interfere" with anchor or mooring lines. If you look at large displacement ships you can often see the abrasion on the bulb caused by the anchor chain. But this seems to a reasonable price to pay for the improvement in fuel consumption as the ship moves up to its normal cruising speed. These ships use a lot of fuel.
If a catamaran has these bulbs it would be be inadvisable to remove them without the designer's agreement as they also contribute to buoyancy and distribution.
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