Are guide blades better than a bulbous bow?

[Leo Lazauskas]

Originally Posted by ivor Bittle

When I was exploring the physics of the bow wave and of the bulbous bow I came across the possibility that active guide blades fitted to a conventional bow might just be better than a passive bulb.

Another way to make an active bulb is to squirt water forward through a nozzle at the bow of the ship. This, in effect, makes a bulb-like form without skin-friction. Furthermore, the flow rate can be varied to adapt to changes in trim and sinkage.

The question now is, does the energy used to create the bulb-like structure at the bow result in an overall gain in peformance?

All the best,
Leo.

[Guest625101138]

Ivor
Determining the energy lost to wave making is not a matter of speculation it is one of science and engineering. There are accurate methods to determine the losses.

rwatson's observation on my yellow boat is correct and I am happy you agree with him. That said, my developable hull with the 2.8 degree transition costs 3% in drag over a perfectly faired hull. I might have been able to get it less if I had fiddled about more with the maximum beam. BUT I knew the cost of the compromise I was making - roughly 1% in speed.

You are right about practical boats and the hull shape. What you see though now is much greater use of multi-hulled craft to get finer hulls while maintaining stability. Such vessels are finding increasing use:
http://www.gizmag.com/go/4753/picture/16242/
So the effort is aimed at reducing wave drag by a better distribution of the buoyancy using longer narrower hulls. They simply provide a better trade off between wave making and viscous drag.

A tug is not intended as an efficient means of transport in its own right. It is a water tractor. Its design objective is to get the most traction in the water using the installed power. Hence they swing large props and require very deep hulls so the prop does not ventillate. Adding a bulb could reduce the bow wave but to what point on a harbour tug. They have power to burn to reach hull speed and the hull and gearing will simply not allow them to get beyond hull speed. They are normally required to limit speed to reduce wash for other craft.

There is energy lost in a bow wave particularly if it is breaking but you need to look at the stern to see where the majority of the energy goes. Bow waves are manageable with bulbs and this is their purpose. There are no doubt other ways to control a bow wave but are they as practical as a "simple" bulb.

There are accurate means to determine wave drag so you can determine what role each part plays. Your original premise of significant energy being lost to bow waves is not based in fact - it is your interpretation of an observation.

I am giving you the benefit of what I have learnt through extensive trial and error before I leant how to accurately calculate wave losses and work to reduce them by good hull design.

I have attached two photos of the same boat. One is looking forward and the other is looking backwards doing the same speed - just over 4 knots. As you can see I did manage to eliminate the bow wave. Did I reduce drag? - NO!! It is looking backwards that counts as that is where most of the wave energy can be seen. In this example that is where ALL the wave energy can be seen. The bow was perfectly calm. In fact it is hard to determine that the boat is actually moving from the photo.

I have done a large amount of experimenting using the same engine, rated at 120W continuous duty, to compare performance of different craft with the object of going as fast as possible with that power. I used to focus a lot on bow waves - now I know it is not the place to look. My great appreciation goes to Leo for providing me with the tools to apply the physics effectively and extremely efficiently.

Rick W.

[ivor Bittle]

I have written a summary of the outcome of this thread. It is an attachment as I cannot find a way to paste into this window. Please read it at your leisure.

Ivor Bittle

[ivor Bittle]

Leo Lazauskas
The forward facing jet has the potential advantage of being active but only a test could settle whether it would work. This is my problem with the guide blades.
My thanks Ivor Bittle

[ivor Bittle]

Rick Willoughby,

Please open attachment.

Ivor Bittle

[Guest625101138]

Ivor
I have done iterative analysis of lifting foils in conjunction with displacement hulls. I could not find any point where there was a reduction in energy until the hull was clear of the water.

There is a hull form where you combine submerged buoyancy with lifting foils that can get foil borne control at low speed and this has lower drag than a displacement boat of similar length, planing hull or full foiler. SWATH ships use this principle in the extreme being essentially displacement craft:
http://www.swath.com/concept.htm
I played around with this idea but found difficulty with static stability. I was also limited with depth of operation so could not get the hull deep enough to negate wave drag.

One problem with "adjustable" forward vanes that are locked in position is the risk of an inadvertent dive coming down a wave. They would provide downward force when the boat pitches bow down that causes accelerating downward attitude such that the boat just dives completely and pitchpoles.

So you should not conclude here that no one has analysed bow foils. I have and concluded they offer no merit over a bulb unless they are used to correct for a trim problem caused by incorrect weight distribution.

Rick W.

Rick W.

[ivor Bittle]

Rick,
I do not have the temperament for this sort of dialogue. I want to close it and wipe Boat Forums off my computer and put it all behind me. I should have known.

My regards

Ivor Bittle