Bulbous bows - do they work?

snowleopard

snowleopard

Well-Known Member
Joined
16 May 2001
Messages
33,645
Location
Oxford
Visit site
I think we can probably agree that they improve fuel economy on a ship laden so that the bulb is below water but what about other circumstance?

Do they make things better or worse when the ship is lightly laden and the bulb is on the surface as in the 'right of way' thread? And how about smaller boats. I noticed the Sea Cadet boat in my captions post had one - she is around 70 ft. I've even seen a pair of them on a Wharram cat. Are they any use on a yacht in anything but flat water?
 
There is a 30 foot work cat in our marina, Blyth I think the make is, the hulls actually look very similar to Prouts with these bulbs stitched on, the owner seems to think they make a difference.
 
I can't remember the theory, but they should make a difference to a displacement hull when submerged, however I don't know what, if any improvement would be made if they were breaking the surface. I guess that you would effectively be increasing a boats waterline length, so maybe you'd get an increase in speed?
By the by, a new generation of merchant ships, the ones I've read about being mainly Platform Support Vessels, are being built with forward raked bows in the pursuit of higher efficiencies...

http://www.ulsteingroup.com/kunder/...nfl=flash#shipdesign/frontpage/shipdesign.itm
 
bigwow said:
There is a 30 foot work cat in our marina, Blyth I think the make is, the hulls actually look very similar to Prouts with these bulbs stitched on, the owner seems to think they make a difference.
Click to expand...

Famous?

I have seen one of these, the antifoul only comes 3/4 of the way up the bulb.

Stuart
 
If memory serves I think I remember an experimental hull(6 Metre) in Australia being built with the bulbous bow, possibly as a trial horse for the America's Cup 12 metres.
As it never was used in these races I guess the trials were inconclusive. This would be around Alan Bond's time who used plenty of innovations on the Australian contenders.

ianat182
 
My hazy recollection is that bulbous bows reduce wave-making resistance at a speed of roughly 0.9 x the square root of the waterline in feet.

As that's the speed at which many ships operate it makes sense for them, but yachts generally need to be optimised for faster speeds (hull speed is approx 1.33 x the sq root of waterline length).

Edit: However, I've no idea how this relates to a non-displacement multihull.
 
Last edited:
bulbous bows work by creating a wave 180 degrees out of phase with the 'conventional' bow wave, thereby canceling it out. as you're shifting less water around on a net basis, you're expending less energy.

Becase the lenght of your conventional bow wave varies with speed, and the wavelength of the bulbous bow wave is (I'm into educated guessing teritory here) mainly dependent on it's shape/length you have to pick a speed at which wyou want it to work best - usually a smidge under theoretical hull speed.

In answer to the original question, I would say no they don't work if not submerged to the optimal depth. presumeably if going any significant distance unladen a ship could take on ballast in the form of a nearby, plentiful, dense liquid.

edit: wikipedia has some info, and suggets that they are not suited to small yachts:

"Bulbous bows have been found to be most effective under the following conditions:

* when used on hulls with waterline lengths of more than about 15 m (50 ft)
* when used on long, narrow hulls
* when used at speeds close to the vessel's maximum speed"

On that basis, I would suggest that a typical yacht is going to be pitching and rolling too much for it to be useful. I suppose it's also a question of scale - big boat, big (amplitude and length) bow wave compared to the sea's movement; Small yacht, bow wave often pales into insignificance compared to what the sea is doing. Cats will do much better in terms of rolling, but still pitch and the scale issue still pplies.
 
Last edited:
Forget the theory. Tried it on a Prout 33CS and all it did was very marginally reduced slamming due to reserve bouyancy. No faster under sail or engine. Prouts promoted them in the early 90s as wonderful additions but soon dropped them when they didn't do anything!
 
snowleopard said:
Are they any use on a yacht in anything but flat water?
Click to expand...

No idea whether it works or not but this one is on an Ericsson 34. Regardless of its efficiency it would appear to be a first class trap for floating debris.

P1020419.jpg
 
bigwow said:
There is a 30 foot work cat in our marina, Blyth I think the make is, the hulls actually look very similar to Prouts with these bulbs stitched on, the owner seems to think they make a difference.
Click to expand...

Yes, it was the Blyth Workcats which I'd seen, actually down at their factory on Canvey Island. They had a stack of the bulb mouldings ready to be glassed on to hulls.

Incidentally, they have a stainless steel workshop, making everything for the Workcats. I got them to do a little modification job on my liferaft cradle the other day. They were very helpful and reasonably priced - see their stainless steel website for examples of their work.
 
IIRR, the Erikson was designed by Elvstom (or with his help) in the early 80's, and was a motor sailer. Perhaps the bulb was aimed at the motor bit. As it is a rare beast, prob self explainatry. Since subs work much better underwater, one would assume that the problem is at the air/water interface. A pitching small boat (sub 60ft? or bigger?) is not going to gain much, and stands to lose much as a debris catcher.
This was the subject of a thread a while back, some big ship guys explained that it is not very simple, but I can't remember the details.
A
 
You must log in or register to reply here.

Share this page

Top