Bulbous Bow Design

David Gerr (author of many boat design books and director of the Westlawn Institute) has developed a 'hull speed' calculator that is more accurate than the 1.34*sqrt L (he has verified it against lots of models etc). It uses the displacement length ratio to change the 1.34 factor.
The formula is: V = 1.24*L^1.433 / D^0.311
where v is kts, L is feet and D is displacement in pounds.
So long, light boats can reach a higher 'hull' speed.

 

I have a 23 foot, 1976, cruising, sailing catamaran. It hobby horses badly. Is there a type of bow bulb that will help without creating too much drag in flat water?

 

'Hobby Horsing" is much better addressed in narrow hulls by modification of the stern quarters than by adding to the bow. Adding flare to the waterplanes aft will move the Center of Area away from the CG and the add to the variation in Iyy from bow up to bow down which will help the problem. The only problem with this is you need to keep the buoyancy up in the bow above the design waterling to prevent pitch-poling when running down the back of large waves.

 

Adding to the stern is not an option because of the rudders. This is a little boat with a lot of accomidation that seldom sees 7 knots, so pitch poling is not a problem.

 

seldom have I cut and pasted into my personal notes as much as I did in these few 19 posts
thanks folks
very informative
B

brings my desired hull speed to precisely were I wanted it ~15 knots
assuming Ive remembered how to use my old scientific calculator properly

for the 57' flat top chine form with a canoe bottom

V = 1.24*L^1.433 / D^0.311

V= 1.24 x 57^1.433 / 42000^0.311

V=1.24 x 328.22 / 27.4

V= 416.84 / 27.4 = 15.2 knots fully provisioned

at 34000 D = 25.66 or

V= 16.24 knots empty

 

Question about the notation: What do ^ and * stand for?

 

^ stands for "x to the power of y" (ie 'D^0.311' D to the power of 0.311.... sounds strange in english )
* means multiplication

 

Its what I thought and then it makes sense. Thanks

 

Some people have used Michlet to assess the drag of hulls with bulbous bows but I am not completely in favour of this unless care is taken to assess the results.

Note that a very rounded, bluffish bulbous bow contravenes the small longitudinal slope assumption in Michell's theory.

If you run Michlet with a bluff spheroid shape for a large number of speeds so that the Froude numbers vary from about 0.1 to 0.5, you will notice that the curve of the wave resistance is very jagged for low Fr, typically less than about 0.3. That is a sign the theory is not working! At higher Fr the wave resistance curve settles down and becomes quite smooth.

The lesson to be learned is that as long as the (transverse) wavelength is large compared to the amount of hull contravening the theory, errors will not be significant. If the wavelength is small and comparable to the extent of large slope on the hull, the program will produce garbage. That's why it is always a good idea to produce a curve of resistance. Don't rely on one or two Froude numbers!

All the best,
Leo.

 

To make the drama short:
at 23 foot a bulb is not sensible, just leave it.

 

The only thing a bulb would be fitted to 23 foot for would be for static boancy where the designer has fked up and the boat floats bow down.

Bulbs would very likely make it bow steer ( bit like driving a fork lift truck) and will be dangerous in a following sea.

 

Sounds like the hull has a good deal of rocker. If this is so then reducing the rocker toward the bow will improve pitch damping.

If the hull shape lends itself to a hard chine below the waterline at the bow so you get a flat surface with a slight upward curve to ensure a bit of lift then you will get damping from this. The flat surfaces offer a bit of resistance to the pitching action. Moving into slop they actually making a slapping sound as the flat hits the water surface.

A change like this can be tested reasonably economically by gluing layers of blue EPS foam to the hull and shaping it. A triangular section about 1m long and 0.3m wide will slow the pitching up. It would need to be blended sweetly to the rest of the hull and will not suit a rounded section particularly well.

The other method that is used for pitch damping is an inverted "T" rudder. Basically a standard rudder with a horizontal foil attached to the bottom. It does not add much drag but will generate quite large opposing forces to any pitching. A bit more design involved than simply carving up some foam to stick on the bow but reportedly quite effective.

Rick W

 

Rick --I have found the opposite, can you offer some link that can confirm bulbous bows on a 23 footer would result in benefits as you have posted "quite effective" other than static bouancy.

If I could find it, I can certainly supply you with links that poo poo any benefit on a boat this size.

 

Frosty
I am not suggesting a bulbous bow here. I have suggested a flat surface with a bit of lift. I have personal experience with this. It works much the same as a flat aft section.

However there are claims of bulbous bows improving pitching stability. I am certain it has been discussed on the forum before - have a search for it. I think anything that extends the waterline will help pitch damping. Any of my hulls with no rocker and low buoyancy in the ends do not pitch. They just go through waves. Results in a wet ride.

Rick W

 

Frosty
I cannot find the thread here that talks about sailing cats and bulbs. I can recall some discussion about it becoming a fad after a well known sailor did well in a boat with bulbs.

You can find plenty of references like this by googling:
http://www.nordhavn.com/constr_con/bulbous.php4
But what does 20% mean. You have to wonder what conditions.

I have played with little bulbs on the narrow hulls and I can get the low pressure zone in the right place but they are only useful for calm water in my little hulls. I could not measure any reduction in drag.

I have found the hard chine and upward curving flat bow sections offer real benefits in a range of conditions. I get a little lift above 5kts and at slower speeds, where pitching could occur in the right waves, the bow slaps the surface and kills the pitching energy. It is more effective than a "U" section in damping pitching.

I expect the simple hard chine cat hulls must exhibit the same behavior. Maybe someone with one will comment.

Rick W