Bolbous Bow design theory

http://www.deltayachtsbrokerage.com/news/YWorld_gb62.pdf

"The Gunboat's hulls are cylindrical in section below the waterline for almost their entire length,-------. Combined with a gentle, uniform rocker along their length, this serves to dampen the pitching motion of the boat and with no vee forward, she is much easier to tack too."

I admit I should have written "gentle rocker" rather than "strong rocker" but D section does mean cylindrical to me.

As for the technical parameters for designing an optimum bulbous bow for a sailing catamaran, I guess you are on your own. Length is good. Cruising catamarans are being made longer.

http://www.boboramdesign.com.au/multihull-design-philosophy/

http://www.multiplast-yachts.com/

http://www.gunboat.info/gunboat_flash.html

Good luck,

Pericles

 

The original purpose of Lock's bulb bows were to reduce drag much as you state.

But he found that they were so effective at reducing pitching that performance was enhanced from that end rather than from any hull drag reduction.

So as time went the bulbs became more and more discreet as he worked out how to get the same effect with less volume and less complication.

So the reduction in wavemaking drag was taken to be insignificant - and maybe so variable as to be useless.

Once he discovered how that worked, his sterns started to evolve for the same purpose of damping pitching.

His son Brett, continues to develop that line of thinking but mostly is focussed on larger commercial projects.

 

I recall a research paper by the US Navy on bulbous bows (sorry I don't remember the citation) that stated the benefit was due to the reduced energy needed to break the surface tension of the 'stretched' water.
Yes, I can see how the increase in speed may be attributed to longer waterline or reduced wave making. If so, then why aren't bigger and longer bulbs being designed?
I believe that the Crowther's (both Lock & Brett) bulbs got smaller because they still got the energy-saving surface-tension benefit without the bulk or stress of a larger or longer bulb.
Larry Modes

 

Sailing Anarchy ran a story about bulbous bulbs and published a reply on 30.1.08 under the heading Fat Cat. Go to site for photos.

http://www.sailinganarchy.com/index_page1.php

"Lebelle started life as a Lock Crowther 44’ cruising catamaran. Lengthened three times finishing at 53’ consisting of rear steps and enlarging cockpit area. Lebelle weights 10 tons and is 28’ wide. Prone to hobby horsing, with narrow ends and generous rocker. Bulbous bows were lengthened 300mm to streamline shape, and increase forward buoyancy, as this was a small modification, 10 KLG buoyancy in total little reduction in hobby horsing resulted I attached horizontal hydrofoils under rudders to further reduce the hobby horsing, they act as double shock absorbers, progressively increasing the size from 200mm wide to 700mm, and 1 meter in length. Rudder hydrofoils have been on for five years and improved ride substantially. The latest addition is removable carbon fibre swimming steps, they will also improve motion. My interest in improving the performance of sailing boats started with A Class catamarans, resulting in winning the 1981 world titles against 70 competitors.

01/30/08"

 

Exactly.

 

There you go again. A 62' boat with a 2' draught has "strong rocker"????

 

Lock and bulbs

Hello all,

I was just reading an old magazine where Lock Crowther was talking about his just launched Wahoo - his 38ft Super shockwave. (I have lots of old mags I love to flick through)

In it Lock talks about a now historic matchup where 45ft Bagatelle (with bulbs) was racing a Spindrift 45. Same length and inshore there was no difference in speed. When they went offshore the newer bulb fitted Bags walked away leaving the pitching Spindrift far behind.

I was on a cat a couple of months ago and got to talking to the owner of the Spindrift in the story. He remembers it as Lock told it.

In a pretty good test the bulbs worked - sort of. Spindrifts are very narrow with very fine ends. Almost all other designers got the bulb effect by making the hulls fuller at the ends. The moral of the story for my money is that going for a low resistance hull has to be carefully weighed up with pitch resistance and pitch damping. Offshore a higher resistance hull with better damping could be far faster than the hull the computer says has lower resistance.

cheers

Phil Thompson

 

Hello again Freenacin,

I have managed to get right up your nose. Good!

"The empty vessel makes the most sound."

I'll translate Shakespeare's quote for you shall I, or can you work it out for yourself?

Pericles

 

That's a very amusing quote. Especially coming from someone who has posted 234 times, to someone who has posted 23 times.

Have you worked out how to spell your own name yet?

 

Freenacin,

Have you considered suing your brain for non-support?

That 234 is the reputation awarded as a gesture of goodwill from others on this forum. It'll be interesting to see which way they go now?

Pericles

 

Thanks for pointing that out. It reinforces my point nicely. 834 posts to achieve 234 points vs 22 points from 27 posts... well, at least it's clear who's making the most noise isn't it?

What's really funny is that it seems to be so important to you.

 

Thou art a very superficial, ignorant, unweighing fellow

Of course I should point it out. If I hadn't, you might have decided on a cheap shot raid on the rep points. It is about your standard of repartee.

I am disinterested in you now.

 

Bulb bows

The december edition of Catalyst the journal of The Amateur Yacht Research Society carries an account of a cat design and build project which might amuse you.In particular it uses the actual bulb bows of Lock Crowthers boat Glaucometer built for the Ostar race but in fact wrecked in a Round Britain Race with Lock on board. The cat was rammed by a cargo boat in fog off the south coast No one was hurt but the wreckage was dragged ashore and sawn up into 6ft lengths. I used the bits of kevlar glass and epoxy to build my 40 ft cruising cat Vamp of Savannah.
Prior to that I did access some tank testing research that suggested that bulb bows were advantageous and suprisingly so were bulb sterns! I did not find any mathematical statement about the ideal shape or size.
Crowther added bulbs to the amas of a racing tri Sanscara but they pounded when the elevated float hit the tops of waves.The bulb bows on Vamp are smaller and normally immersed.They are integrated with the hull shape and very elegant unlike the very large bulbs fitted to ships and some fishing boats.Lock wrote to me how pleased he was with the perfomance of the hulls on Glaucometer,and for the last ten years I have spent many happy hours watching those same bows on Vamp showing transparent flow over the bulb and little disturbance at the surface.This is my subjective assessment;

In flat water the sharp bow at the suface and the bulb below each minimise the resistance.If the bulb leads significantly there could be a phase effect otherwise it just results in a softer entry.

In small waves there is very little increase in displacement at the fine bows so that little energy is fed into the pitching motion to which multihulls are prone. Less pitching more comfort more speed. I think the bulbs have more influence in these light to moderate conditions


In larger waves the bulbs are often out of the water and the reduction in displacement acts as a restoring force.When the bows are deeply immersed there must be a rapid increase in bouyancy near the bow to compensate for the fine entry.That can take the form of forward overhang or lateral overhang [flare]or at the exteme ,a bridge deck.

In the end Crowther dropped bulb bows but I dont know why.Could be the extra cost or the appearance or the convenience---bulbs on narrow hulls tend to hit the jetty before the fenders!Maybe they are just fun to have like go-fast stripes.

Let me know if you would like more info pics etc perhaps by direct email

Walter schofield 02/02/08

 

A bulbous bow is an asset at a very narrow speed range, usually less than one between high and low. AT OTHER SPEEDS IT IS A LIABILITY. Sailboats do not sail at one chosen speed; neither do they need any more liabilities.

 

Sandy,

Thank you for your reply. While I fully appreciate the limitations of bulbous bows I also know they have other benefits (apart from having a very narrow speed range). You also did not ask why I was asking this question nor to what application I intend to put the vessel. So with respect you did not answer my question.

Do you know a mathematical formula for calculating the optimal size of a bulbous bow (for a given size and expected velocity)?