Bolbous Bow design theory

[designz]

I have spent a considerable amount of time searching (Library and the interenet) for the technical theory etc for the design of bulbous bows for sailing catamarans, - with little real success.

I am currently designing a 14.7 metre (48 feet) sailing catamaran for ocean cruising. My extensive research suggests that bulbous bows have many advantages, including;

1) Increases the Prismatic Coefficient
2) Assists in avoiding the depression of the leeward hull
3) Provides a crash barrier, thus protecting the integrity of the hull.

I know Lock Crowther used bulbous bows extensively, particularly in his earlier designs. In his later designs he tended to terminate the bulbous bow shape at the stem, - but theoretically they still worked much like a bulbous bows.

Can any one give me the details of how to optimise the design of a bulbous bow for a sailing catamaran? One theory that I came across suggested that the cross sectional area of the bulbous bow is a percentage of the midship section, - however I can not find the percentage - leaving me feeling just as frustrated, or why it is so. Other theories would also be most welcome.

Regards

Designz

there is a man I used work with called Albert Sedlemeyer, a qualified navel arch, up on Gold coast works for asdmar or seatransport I can not let you have his email add, but you should google seatransport and ask for him, you may have to pay a consultation(not him, the company) but it will be worth it

[Trevlyns]

Hi Designz and welcome to the forum

I'm not qualified to comment, but if you scroll to "similar threads" at the bottom of this page, you'll find some intelligent discussion on the subject.

Good luck!

[designz]

Originally Posted by Trevlyns

Hi Designz and welcome to the forum

I'm not qualified to comment, but if you scroll to "similar threads" at the bottom of this page, you'll find some intelligent discussion on the subject.

Good luck!

Trevlyns,

Thank you for your response. I have already read these discussions, - very interesting too, but they don't relly answer my question.

Designz

[Pericles]

http://www.brayyachtdesign.bc.ca/

Pericles

[designz]

Perciles

Thank you for your reply, I have read your article on your website. It is interesting but it does not give all of the details that I seek. I am particularly interested in the design of bulbous bows pertaining to sailing catamarans (i.e. slender hull vessels). If you have any further technical information pertaining to this area I would be most grateful if you could share it.

Designz
Naval architect.

[designz]

Perciles

Thank you for your reply, I have read your article on your website. It is interesting but it does not quite give all of the details that I seek. I am particularly interested in the design of bulbous bows pertaining to sailing catamarans (i.e. slender hull vessels). If you have any further technical information pertaining to this area I would be most grateful if you could share it.

Designz
Naval Architect.

[Pericles]

Bulbous bows on sailing catamarans would need to be under the surface of the water. Catmaran hulls require rocker at both ends to enable tacking through the wind at speed, hence the bulbous bow will cause the catamaran to be caught in irons. Look at the Gunboat 62. Vertical stem, D section hulls, strong rocker, knuckle above waterline.

http://www.sailingmagazine.net/perry_gunboat62.html

http://www.deltayachtsbrokerage.com/...unboat-en.html

The longer the hulls are, the beamier and taller they can be and much more space and accommodation are available. Therefore, it makes sense to construct the vertical stem to incorporate a collision zone above the waterline, just remember to make it tougher than the bow of Banque Populaire.

http://www.jacques-vabre.com/ml/imag...laire1_250.jpg

I think you will find the general consensus amongst designers is that sailing catamarans do not benefit from bulbous bows, unless others know differently?

Pericles

[PI Design]

The main purpose of bulbous bows is to create a bow wave that is out of phase with the bow wave from the 'normal' boat. In other words, the trough from the bulbous bow bow wave co-incides with the peak from the boat's bow, cancelling it out. They therefore reduce wave making resistance. But as cats have relatively small wave making resistance anyway (due to the slender hulls and light displacement) I would have thought the benefits of bulbous bows on a cat were slight. Also, BBs are problematic on sailing craft because the wave systems are only out of phase at particular speeds (Froude numbers). At other speeds, the wave systems may actually combine to give bigger peaks. Obviously, with sailing vessels, speed is reliant on wind strength (and angle) so it is not possible to optimise the BB for all conditions.

[designz]

Originally Posted by Pericles

Bulbous bows on sailing catamarans would need to be under the surface of the water. Catmaran hulls require rocker at both ends to enable tacking through the wind at speed, hence the bulbous bow will cause the catamaran to be caught in irons. Look at the Gunboat 62. Vertical stem, D section hulls, strong rocker, knuckle above waterline.

http://www.sailingmagazine.net/perry_gunboat62.html

http://www.deltayachtsbrokerage.com/...unboat-en.html

The longer the hulls are, the beamier and taller they can be and much more space and accommodation are available. Therefore, it makes sense to construct the vertical stem to incorporate a collision zone above the waterline, just remember to make it tougher than the bow of Banque Populaire.

http://www.jacques-vabre.com/ml/imag...laire1_250.jpg

I think you will find the general consensus amongst designers is that sailing catamarans do not benefit from bulbous bows, unless others know differently?

Pericles

Percles,

Thank you for your reply.


I understand that bulbous bows need to be under the water (not much point in having them if they are not!) - pretty obvious I would have thought!!.

I would suggest that the modern trend is to avoid rocker, as it makes the catamaran hobbie horse, especially in a short chop. Tacking can be achieved by superior sailing skills and other chartacterisitics of the overall design (eg sail plan). Many modern catramarans have very little rocker (I know of some that have almost none and they sail exceptionally well).

"The longer the hulls are, the beamier and taller they can be and much more space and accommodation are available. Therefore, it makes sense to construct the vertical stem to incorporate a collision zone above the waterline, just remember to make it tougher than the bow of Banque Populaire."

Vertical stems are not included in the design to provide more accommodation etc as you suggest (see quoted above). They are included to provide maximum waterline length but at the expense of flare in the forward sections. I would agree that a collision bulkhead is a good idea.


In your reference to the "Gunboat 62" you suggested D section hulls.
I could not find any reference to the hull shape, but I would be surprised if the hull section shape has a "D" shape. This would sugggest that the hull section shapes are assymetrical, which would preclude the need for such a large centreboard see

http://www.sailingmagazine.net/perry_gunboat62.html


"I think you will find the general consensus amongst designers is that sailing catamarans do not benefit from bulbous bows, unless others know differently?"

I would suggest that there is a strong case for some sailing catamarans to have a bulbous bow, especially if the vessel is going to be used for long ocean passages. In this case the benefits are several and well worth the investigation effort.


My original question still remains unanswered,- what are the technical parameters for designing an optimumal bulbous bow for a sailing catamaran? While the other discussion is interesting it has not addressed this original question.

Designz
Naval Architect

[designz]

Originally Posted by PI Design

The main purpose of bulbous bows is to create a bow wave that is out of phase with the bow wave from the 'normal' boat. In other words, the trough from the bulbous bow bow wave co-incides with the peak from the boat's bow, cancelling it out. They therefore reduce wave making resistance. But as cats have relatively small wave making resistance anyway (due to the slender hulls and light displacement) I would have thought the benefits of bulbous bows on a cat were slight. Also, BBs are problematic on sailing craft because the wave systems are only out of phase at particular speeds (Froude numbers). At other speeds, the wave systems may actually combine to give bigger peaks. Obviously, with sailing vessels, speed is reliant on wind strength (and angle) so it is not possible to optimise the BB for all conditions.

Thank you for your reply.

I agree with much of what you say, except to add that the bulbous bow only works for a certain velocity no matter which vessel it is fitted to (catamaran, fishing boat, or ship).

My original question still remains unanswered,- what are the technical parameters for designing an optimumal bulbous bow for a sailing catamaran? While the other discussion is interesting it has not addressed this original question.


Designz
Naval Architect

[masalai]

My experience/observations in this are very limited but I must subscribe to more than idle curiosity.

I have only had a "close-up" on two occasions, one the owner glorified the increased w/l length effect giving added top hull speed, but by the stains on hull-sides, I guess he is a bower-bird, (continuously adding this & that inside & was still grossly overloaded)

The other was a very amateurish add-on to an otherwise pretty hull. The conversion added a plumb bow. If the designer ever saw it I am sure he would have cried.

Don't mess with a professionals careful work & effort as an after market add-on. Now I will go and read the rest of the thread carefully.

[masalai]

From my limited playing around with cats (I only sailed the occasional mono skiff in other hull forms) To add a bulb on the bow beggars the question "Why not increase the hull length?" I reckon that would be more advantageous.

For a bb to work it has been suggested in this thread that it is effective in a narrow speed range. If the LOA is restricted then this gives an extra LWL "cheat" and extra load capacity? Far better option is up the LOA & LWL as Lock Crowther went to.

[designz]

Originally Posted by masalai

From my limited playing around with cats (I only sailed the occasional mono skiff in other hull forms) To add a bulb on the bow beggars the question "Why not increase the hull length?" I reckon that would be more advantageous.

For a bb to work it has been suggested in this thread that it is effective in a narrow speed range. If the LOA is restricted then this gives an extra LWL "cheat" and extra load capacity? Far better option is up the LOA & LWL as Lock Crowther went to.

Thank you for the reply. If we took your idea to its logical conclusion we would all be building or sailing vessels the size of ships. At some point you have to say that this is going to be the length of the vessel. Then it becomes a question of how to optimise the vessel for that length, thus bulbous bows provide one possible solution (for secific applications, as suggested in previous theads, see above).

Lock Crowther was one of the main proponents of catamaran bulbous bow design, - he even included a modernised version in his later designs. So I do not agree that he would say add length rather than a bulb, - otherwise there would never be an end to the size of the vessel, (as suggested above).

Designz
Naval Architect

[masalai]

Precicely Des,
The bulb was at a dead end as far as development & consumer interest etc. so refinements in form, not adding a bulb.