Bow bulb

[catalac08]

As most large commercial ships have a bow bulb that reduces power requirements and presumably this implies a speed increase why do pleasure craft not have this design feature or is there some aspect of scale involved that renders this ineffective on small boats?

 

[DaveS]

I have seen a post with pictures showing a DIY bow bulb someone fitted to his boat. No idea where, and I can't work the search "facility", but maybe someone else with greater skill / knowledge can assist?

 

[earlybird]

For my own interest, your post prompted me to google "bulbous bow". There is some information in Wikipedia. web page

 

[Mandarin331]

Ther was an article in PBO and few months ago about somone who fitted one to a 45' displacement mobo. Improved speed and fuel consumption.

 

[charles_reed]

I suspect it has to do with scale and size. The additional benefit that a small hull would gain is offset by the additional drag.

I have seen a bulb fitted by builders on some of the larger Amels, but those were made before their take-over by Beneteau and were probably fitted to reduce pitching moment rather than increase hull speed.

Here's a web-site explanation, with none of the (somewhat bewildering) maths.

Displacement hulls push water aside as they move. The water that is pushed aside forms a wave that begins near the bow. The length of this wave is proportional to the speed of the hull through the water—the faster the hull moves, the longer the bow wave. At some speed, the length of the bow wave increases to the point that it matches the length of the hull, and the hull operates in the trough of the wave, with a peak near the bow and another peak near the stern. This speed is called the "hull speed" and it is approximately 1.34 times the square root of the waterline length of the hull. (Expressed as knots and feet)

The purpose of bulbous bow is to change the nature of this bow wave to reduce the drag it induces on the hull The bulbous bow creates it's own wave that is farther forward and "out of phase" with the natural bow wave created by the hull, effectively subtracting from the normal bow wave and reducing it's drag-inducing effect.

Many sailboats improve on this 1.34 factor.

 

[DaveS]

Ah, maybe that's the one I remembered - but a print article and not a post at all! /forums/images/graemlins/blush.gif

 

[LittleSister]

I did read about this some time ago. As I recall one of the problems is that you have to design your bulb for a given operating speed - it's less use, and possibly a drawback, at other speeds. That's fine for a powered commercial vessel spending most of its working life at a pre-set speed, but not really practical for leisure craft, especially sail.

 

[charles_reed]

Not speed but LWL, so for a 100' LWL it's about 15' long but about 10' for a 50' LWL.

Below 65' LWL it's so big and so ungainly that it's a positive danger in a following sea.

Some put them on smaller boats in an attempt to reduce pitching but most naval architects seem to disapprove strongly of the idea.

 

[misterg]

There's a (30-35'??) sailing cat moored in Conwy that has has them (one on each hull). Each bulb also has a little wing either side.

Andy

 

[Robin]

There did used to be a motor sailer abot 34ft I think, maybe an 'Elvstrom' something? I remember it being in the Sea Ventures charter fleet about 25 or more years ago when my then company ran a sailing weekend.