Bulbous Bow applications

[juiceclark]

As you know I've been asking a few brainstorming questions related to the 46' sportfishing boat project I started recently. It's a very wide boat (17.5' at the transom) with little deadrise. Obviously, this boat is meant to move economically and sit stable while on the hook or moving slowly.

This design relies heavily on a sharp entry to break the waves. If the hull starts pounding, you're just going too fast and must back-off a bit. But I can't stop thinking about the advantages of a bulbous bow to this design. If you take some of the energy out of the wave even before it hits the sharp entry, that would be ideal.

I've never seen a bulbous bow design which would come out of the water, and be out of the way, when a boat is planing...say above 13k or so. Recall reading somewhere the bb design has not proven useful on boats smaller than 80' in length.

If the wing on the back of a porsche comes out above a certain speed, why can't a bb come out below 10mph?

I would love your input on this...the more imaginative the better. We're building 5 of these and the 5th is mine. So, I plan to have great fun with this design as I wait for the boat.

Here's the stuff I've read so far:
http://www.brayyachtdesign.bc.ca/article_bbows.html

http://www.boatdesign.net/forums/showthread.php?t=18043

http://en.wikipedia.org/wiki/Wave_making_resistance#Bulbous_bow

http://www.nordhavn.com/50/overview.php4

http://www.globalsecurity.org/military/systems/ship/beginner.htm

http://www.sponbergyachtdesign.com/Molokai72.htm

Thanks for your thoughts, Tony in Sw Florida

 

[kach22i]

Interesing concept, I'm doing a Google search on it now.

So far found this:
http://www.aviadesign.com/powerkeel/index.htm

Basic Hull
9.4 Knots

Note Bow wave,
stern turbulence

PowerKeel
9.4 Knots

Note flat bow wake,
reduced stern turbulence.

Question: Is it the weight or the shape which makes it work?

If it's the shape, then inflating the bow like an RIB or Hovercraft and deflating it when you want to plane-out might work. Would need a strong compressor and heavy duty rubber material to make it work because of the water pressure involved I would think.

 

[juiceclark]

very cool

Yeah...look at that! I was looking at all the different bolt-on keels different commercial boats would have for different uses. Wonder if a keel could slide forward somehow to break the wave then fall back to work normally as a keel when planing??

 

[kach22i]

Good idea, a telescoping keel.............is there a patent on this already?

 

[juiceclark]

I dunno

I don't know. But if this boat will have a secondary, hydraulic motor so the generator can push the boat at trawler speeds, you'd think the same hydraulics could slide the keel on a rail to and fro.

 

[brian eiland]

Bulbous Bow

A properly designed bulb bow basically cuts the size of the bow wave down very considerably by drawing the surface water in the front of the bow down. This decreases the size of the bow wave of the vessel as it approches its 'hull speed', and thus decreases the vessel's resistance thru the water. Note that each bulb bow is usually 'tuned' to the displacement speed of the vessel, or in other words the length of the vessel that in turn is directly related its 'hull displacement speed'.

Be careful when you try to take this 'bulb bow' pass the hull displacement speed into a planning situation, and particularly as you meet a 'disturbed seaway'. In this case the bottom shape of the bulb must be considerably altered to not cause a great amount of pounding as it first rises and then re-enters the sea. (now waves, verses smooth sea or towing tank)

You will probably find that bulbous bow just will not cover this great variety of conditions unless, or maybe if, you employ a hull with a higher displacement speed capability.

 

[Alan Mikkelsen]

My father has a 48' trawler that he (himself) built. Wonderful boat. He has been going from Port Townsend to SE Alaska for many years in this boat. From Port Townsend to Ketchican originally took 390-410 gallons of diesel. He made two signficant modifications. The first was to fair the stern area up and out, the second was to add a bulbous bow. It now takes him 195 gallons, Port Townsend to Ketchican. He attributes about 90% of this to the stern work, maybe 10% to the bulbous bow, although it looks neat. Breaking the suction on the stern really made the difference.

 

[Squidly-Diddly]

Alan, very interesting...any before or after pics, or any

links to boats that look anything like you are talking about?

How about an example of plans from one of those 'home builder' sites of the various sterns.

Did he use any plans and was the boat LOADED as designed?

Maybe it was loaded oddly without noticing it was causing a problem in handling but just fuel consumption.

Was it any faster after? Much faster?

 

[Squidly-Diddly]

PS Alan, what material was the ketch?

and how did he change the shape of the boat? Seems like hull shape is something you are mostly stuck with.

 

[GuestR01312011]

Bulbous bows are designed very specifically for a set 'service speed' when you drop below/above this speed the disadvantages are quite large in terms of effective power. If you were planning on trying to plane this hull shape while incorporating a bulbous bow the ammount of power needed to pass the transition to semi-plane with the bulb out of the water dur to trim angle would be rediculous as when you are planing the power reqd. would drop drastically as the viscous drag/wetted area dropped.
One appreoach might be to install a system of trim tabs/wedges along with a sterm ballast tank to try and promote a planning condition by altering the trim angle drastically. Once planning you would then have to stabalise the trim at substantially less than taht needed to initiate planning or else you would set up an unstalbe trim angle which would oscillate and increase resulting in dangerous slamming added to by the moment of the bulbous weight at the bow.
Hope it goes well, tank test it if you can. I have see a 70m motor yacht with a smallish bulb on the bow to reduce bow wave cause the owner was getting 'green water' on the deck but this boat was not intended to plane at all and the drag penalty below service speed was managable.

 

[Alan Mikkelsen]

The boat was a fish boat trawler hull, not a ketch. It was loaded and balanced perfectly. He did the modifications over a period of two years, doing the stern first. That's where the big improvement came. He then tackled the bow. He did the bow by 'eyeball'. He came up with some sketches of what he wanted to do and contacted a marine university in BC. It would have cost about $40,000 to have a model built and tank testing done. He went to k-mart and bought a basketball, stuck it in the end of a PVC pipe, got a sawzall and went to work. He is truly an artist and highly experienced, so it's not quite as corny as I make it sound. He picked up about another 10-15% efficiency with the bulb.

The efficiency came because the boat was faster, so he just throttled back to maintain a cruising speed of about 7 knots. The boat has a 150 hp Lugger, by John Deere, and probably weighs about 20-22 ton. I've attached a couple pictures. One is in Deer Harbor, Chichagof, and the other is Hawk Inlet, Admiralty Island, AK

 

[Alan Mikkelsen]

PS, don't be fooled by the twin dry stacks. This boat is single engine, wet exhaust. He wanted a place for a chest freezer, so he built what looks like a stack topside, with twin 'pipes'. The pipes are actually black PVC, but they serve to ventilate the compartment that holds a chest freezer! He will run a small Honda generator a couple hours a day and it stays cold with no problem. He has a large Onan generator below, in the engine room, but it sees little use.

 

[kach22i]

If you were planning on trying to plane this hull shape while incorporating a bulbous bow the ammount of power needed to pass the transition to semi-plane with the bulb out of the water dur to trim angle would be rediculous as when you are planing the power reqd. would drop drastically as the viscous drag/wetted area dropped.

Imagine swing down foils attached to the bulbous bow............you could plane on the foils.

EDIT: like this..............

This sketch assumes that it takes less power to raise the hull up on foils than to "plane-out" upon the hull bottom.

 

[kach22i]

GREAT SCOTT!

Click on R&D.............
http://www.seseu.com/

An open bow Surface Effect Ship (SES) or ASV Catamaran with bulbous bow rising up out of the bow wake (second to bottom image - scroll down).

Are you up to using lift fans in lieu of foils?

OOPS..............

Not a bulbous bow, just a bulbous bow wake, sorry.

Still a dandy idea, right?

 

[GuestR01312011]

If your going to go to the bother of sticking on 'Retractable' Foils you might as well just go and design a foiler, all the mechanical systems adn control systems for these retractable foils would be fairly power consumtive and weighty.
In general you dont need a bulbous bow, a VLBC will use one because it's designed to do its journey at a certain speed and therefore the wavelength produced by the buld will destructively interfere with the hulls bow wave to overall reduce the wavemaking resistance of the ship.
A different approach and I know there's reasearch goin on in the Open 60's about this is introducing small high pressure openings in the bow/amidships sections of the hull to exhaust high pressure small bubbles/wake in order to reduce the viscous resistance by introducing a layer of air, in effect increasing the boundry layer growth drastically. Once again the preoblem is the auxillary machinery to produce the pressure, perhaps if on a motor yacht the exhaust gasses could be re-routed, not sure what adverse affects it would have on engine performance etc...
Good Luck.
Ro