Bulbous Bow Design

[Guest625101138]

I have been using Michlet/Godzilla to produce optimum hulls only constrained by displacement and the design speed. Having got these hull shapes I am able to reduce drag slightly by adding a bulbous bow section. It is a tediuos process by trial and error to design the bow.

Are there any basic starting points to begin a bow design that should provide an optimum.

If there is a switch in Godzilla that would force a bulbous bow I would like to know that as well.

For small displacement boats the benefit is probably even greater than for larger boats because it is one way to encourage laminar flow over a portion of a hull. See how smooth my submerged bow operates on V10 boat at 5.5 knots.

Rick W.

 

[frosh]

Hi Rick, many years ago, before the home PC, I designed and built a "Skate" class racing dinghy that was allowed to have some freedom in hull shape. The bow from memory might have been about 300mm in height, and was max. width around 70mm up from the lowest point. There was only an intuitive feeling that this could give a good drag result in displacement sailing as such a bow design had already appeared on large ships already. It was not possible to test this one feature alone against other Skates without the bulbous bow, but we were a fairly inexperienced crew, and our boat was nearly always an excellent performer, so that I believe the idea had merit. The other factor was the Skate sailed on the plane most of the time, with the bow clear of the water, so only occasionally was the bulb immersed, so you can't really know for sure in my case.

 

[yipster]

i can read that bow drawing and like it frosh
good site you have Rick and the V6 looks impressive
got michlet going but still have to start godzilla

at http://www.boatdesign.net/forums/showthread.php?t=930&highlight=holtrop-mennen the bulbbow wsa

Holtrop-Mennen formula : WSA=L*(B + 2*T)*Cm^(0.5)*(0.453 + 0.4425*Cb - 0.2862*Cm + 0.003467*B/T + 0.3696*Cw )+ 19.65*ABT/Cb

The last term, 19.65*ABT/Cb, is the bulbous bow wetted surface and it may be added to the WSA for any other formula.

 

[jehardiman]

What's your target froude number? Bulb size ( and it effect can be both positive or negative) is dependent on froude number. The benefits of a bulb bow don't begin until Fn ~ .21-.27 depending on shape.

BTW, designing for and encouraging laminar flow is counterproductive in all real sized vessels. Real water conditions limit laminar flow to Renyolds numbers were laminar flow results in higher skin friction than turbulent flow (the entropy conundrum). 5% less wetted surface will give much better performance.

 

[Guest625101138]

The hull I am working to improve is a product of Godzilla. It is 7.2m long 250mm beam and displaces 95kg. Design speed is 4m/s so Froude number is just below your nominated kick in range.

Godzilla typically ends up with a minimum drag solution where wave drag is about 10% of total drag. But I know I can play around with a bulbous bow and get a lower drag when analysed with Michlet. I have been able to get about 2% lower overall drag. The bulb is something that I don't think Godzilla will hunt out although I am not fully conversant with the hull control parameters in Godzilla.

Rick W.

 

[jehardiman]

With your L/B, B/T, and Cp I don't think a bulb is going to get you anywhere. The tradeoff is between surface area and pressure (wave) drag. To extend the bow to cause a higher pressure drag to get a slightly more favorable pressure drag further back on the hull is counter productive when that total pressure drag is only 10% of the total drag. You will never get the increase in wetted surface back (example... a wash would be to increase the wetted surface by 1.1% at the same time lowering pressure drag by 10%....ain't gonna happen). Bulb bows are used on hull forms with much lower L/B ratios and much high Cp and Cb where a significant reduction of pressure drag can be achieved.

BTW, do not confuse the ram bow you show or that is applied to modern high speed vessels with a bulb bow. they are fitted for totally different reasons and are optimized differently. A ram bow is to reduce pitching, lower wetted surface in a seaway, and increase WLL, a bulb bow is to manipulate pressure on the bow and forward quarter.

 

[Guest625101138]

John
Thank you for the input. I was able to show a slight reduction in drag with the bulbous nose but I understand the point you make regarding the pressure reduction and I am unlikely to get much if any benefit by trying to optimise it.

The reason I included the photo was to make the point that a submerged nose on a long boat enjoys very stable flow conditions even when there are small surface waves. I think a portion of the nose may actually maintain laminar flow. I do not think it possible for a hull on the surface to achieve laminar flow over any significant portion of the hull. Although I have read reports of kayaks in mirror smooth water maintaining laminar flow over the first metre or so.

My aim is to do as much as I can with my 120W. So I try to understand all possibilities.

Rick W.

 

[Frosty]

My boat has bulbous bows, over 20kts it gets a bit skittish to drive.

I read an article that said bulbous bows do this, and are not much good for vessels under 60feet. The article went on to say that a bulbous bow encourages bow steer.

I would like to remove them as the manufacturer Seawind cats do not fit them on the smaller 38foot power cat. However this would be a big job even though they are cosmetic

I have a supicion that they are more for static boancy-- keeps the boat floating to a nice level.

How could I work out a froude number

 

[PI Design]

Frosty,

Froude Number (Fn) = sqrt(boat speed/(9.81*waterline length))
boat speed in m/s and length in m
So, for example, 10m boat at 10kts:
10kts = 5.1m/s
Fn = (5.1/(9.81*10))^0.5
Fn = 0.228

My understanding is that bulbous bows have two effects. First, they reduce pitching. Second, they produce a bow wave of their own which interferes with the 'normal' hull's bow wave, cancelling it out. But this only works within a limited Fn range because otherwise the bow waves may not be out of phase and the resulting wave may be bigger than if there was no bulbous bow. That is why they are uncommon on sailing boats, because your speed varies so much. On ships which do a constant speed, they can be very effective.

 

[Frosty]

Oooh thats interesting. So the froude number will change with speed obviously.

Therefore (as you say ) the bows will be more efficient at a certain speed!

So what froude value should I have?

It is a power cat 13 meters and full out is 24Kts.

Do you double the froude for a cat or half it?

However I dont understand why there is no bulbous bow calulation or measurement involved. Your formulea will be the same froude with bulbous bows or not.

 

[PI Design]

Frosty,
Froude numbers are a way of comparing speeds of different sized boats. When you do model tests on an oil tanker, you tow the model down the tow tank at the same Froude number (rather than actual speed) as you expect the full sized tanker to sail at. A 100m tanker at 13kts will have a Fn of 0.213, so a 2m model would be towed at about 1.8kts to get the same froude number. Ignoring viscous effects, the resistance of the model can be scaled to the full size ship, for the same Froude number. You may have heard of 'hull speed', which is often quoted as 1.34*sqrt(length). This is the speed of a boat (in knots) at which the bow wave is the same length as the boat. Going quicker than this increases the length of the bow wave so that the stern is sitting in a trough. The result is that lots more power is needed to go just a little quicker, so 'hull speed' is a practical limit to boat speed. Well, hull speed is closely related to Froude number and in fact a Froude number of 0.4 is the same thing. That is, conventional boats rarely travel at speeds above Fn = 0.4.
Froude number is not related to bulbous bows, per se. Nor is it related to the number of hulls, so catamarns have the same Fn as monohulls. The only difference is that catamarans are not constrained by hull speed becasue the hulls are so slender that the bow wave does not cause much drag. So cats can go above Fn = 0.4.
Bulbous bows act as little boats in front of the big boat, and produce their own bow wave formation. As noted above, bow wave length is related to Froude number, so by carefully selecting a size and shape of bulb, the bulb's bow wave can be made to have troughs where the main hulls bow wave peaks are, cancelling them it out. By removing the bow wave the ship is able to go quicker. However, when travelling at the 'wrong' speed (and hence wrong Froude number) the two bow wave systems will not be exactly out of phase (cancelling each other out), and may instead co-incide, leading to an even bigger bow wave. So bulbs are generally used on ships which travel at a constant, known, speed (Fn), rather than sailing boats whose speed obviously varies with wind strength.
Because catamarans don't produce a particularly large bow wave, using bulbs would not have much effect on speed. Maybe they are on your boat to reduce pitching, there are other ways of doing that besides bulbs.
At 24kts, your boat has a Fn = 1.09, which is pretty high. A non-planing monohull of the same length has a hull speed of less than 9kts (Fn = 0.4).
Cheers,
PI

 

[Aramas]

The purpose of bow bulbs on ship hulls is to create an interference wave in the hull's wave system (ie make it smaller). They're designed to operate at a specific speed and partially cancel the wave system. They also create a finer entry on hulls that are effectively pretty close to shoe boxes.

Bulbs have also been used on sailing catamarans with some success, but they work differently. Their effect on the wave system is irrelevent, and their only contribution is increased pitch damping in waves at the expense of additional drag.

On a small conventional hull intended for use in flat water, bulbs would serve pretty much no purpose at all, regardless of what mathematical modelling says. The higher the L/B, the less effect it would have (ie when surface resistance becomes more important than wave resistance). You would probably be better off playing with displacement distribution and angle of entry - in fact, that's probably where your reduced drag is coming from in the modelling. You might want to look at the competition rowing shells/sculls for inspiration.

 

[ancient kayaker]

PI's remarks on hull speed caused me to think of the following.

Kayaks and baidarkas are claimed to routinely exceed hull speed by significant amounts even though only 2 AP (arm power) is available; these boats show almost no bow wave and very little wake.

Charles Parsons demonstration ship Turbinia achieved in excess of 30 kts with a displacement type hull only 100 feet long; lots of power and bow wave admittedly but definitely did not plane.

Are there different numbers for long skinny hulls or is the theory inapplicable?

 

[yipster]

http://www.engin.umich.edu/team/canoe/drag.html

 

[Guest625101138]

.........Are there different numbers for long skinny hulls or is the theory inapplicable?

Hull speed is a concept derived from the phase velocity of a wave in deep water that has wave length equal to the WL length of the hull. The most efficient hull (lowest power required for a given speed and displacement) is typically achieved when wave drag is around 10% of the total drag if there is no constraint on hull shape. This favours long skinny hulls.

Long skinny hulls do not generate as much wave resistance as wider hulls but "hull speed" is still a defining parameter if you want the most efficient hull. The wave drag will be just kicking in at the design speed.

If you do the numbers on rowing sculls you will see that their race speeds is not a great margin above their hull speed. It is possible to push them faster but this is exceeding their most efficient regime.

Racing kayaks are compromised in the sense that they have a length constraint so they are sub-optimal for the speed they achieve in a race.

It is possible to improve the flow onto the nose of a long skinny hull with a bulb but the benefit depends on the shape of the entry and from what I have been able to derive it only saves 1-2% at most. A similar benefit can be achieved by extending the overall length and improve the taper into the water.

Michlet is the best means I have found for determining hull performance.

Rick W.