Print

Please login or register.
1 Hour 1 Day 1 Week 1 Month Forever
Login with username, password and session length.

[roycv]

Hi all, I am getting a part built model of the Billings kit Zwarte Zee finished.  Although in need of a lot of rubbing down of the plank on frame hull the bilge keels are in place.

They have been mounted at a constant angle from the hull and curve with the hull lines.

But, if you look head down the hull on, they are curved and look like they would cause a resistance.
 
However I was thinking that if you look at the flow of water over the hull they would follow the flow.

Would appreciate comment from a forum member that knows the way they should be.
I am quite prepared to relocate them if necessary.

(Original cost of kit with fittings was £13.95!!!)
kind regards Roy

[Colin Bishop]

Leave them as they are, they won't cause appreciable hull resistance but they will help stop the model rolling.

Colin

[Bryan Young]

Hi. Bilge keels curve into the line of the hull. They are not (in my experience) directly fore and aft.
The uses of these things are, and will always be, a subject of debate.
Fitted to a ship of perhaps 650ft long, you would be looking at bilge keels of about 400ft in length...and with modern construction they are hollow...and when leaking can hold about 100 tons of water. Gives a bit of a list,does that. BY.

[bluebird]

hi

I wonder if this is any help to you, its from the instruction booklet which goes with the Zwart Zee Model from Billings - got to bear in mind - it is over 40 years since I built this model and I have the plans somewhere too - which show exactly where the bilge keels are located.   As you can see the bilge keels do curve with the hull towards the stern and a little bit to the bow.     I think the plans are up in the loft, and if I get time tomorrow I will dig them out for you and scan that in for you.

aye
john

[Colin Bishop]

When they were introduced to Victorian battleships, bilge keels in the form of simple plates were enormously effective in reducing rolling and thus giving a more stable gun platform. The main practical limitation was that they should not protrude too far from the surface of the hull beyond either the lowest part of it or the widest part of it as this caused obvious problems with dry docking.

Colin

[tobyker]

Like they should go with the flow, man.

[roycv]

Hello and thanks to all for your replies, I am not sure that the original builder used the right part for the bilge keels as I have found 2 items not cut out yet from the wood that look suspiciously like bilge keels.
 I have the plans, thanks very much Bluebird for your offer, and have also found the web site for Billings kit old instruction books.
Amazingly as far as fittings go I think all are in the parts boxes including a lot of flat type stanchions.

The first job I did was to remove the old prop shaft and insert one I had made myself some years ago with a stuffing box at the motor end.  The original shaft was 1 mm dia screw thread and there was no way I could get a large propeller to fit.

I have also taken out 56 port holes all covered with thick paint, I have  set myself the task of seeing how much I can do in 3 weeks.
Thanks again,
regards to all, Roy

[deadwood]

Of course, the bilge keels do have to be applied in accordance with the stream lines of the flow along the bilge without violating the flow.
Even on a bare hull without bilge keels often a chine vortex is created starting from the forward shoulder.
That's why the ship hydrodynamicist should try to avoid or reduce it, as it can impair the effective wake in the propeller plane and result in separation and eddies which usually increase the resistance and decrease the propeller efficiency (in behind condition) and sometimes can even induce nasty vibrations.

I think that the positioning of bilge keels these days mostly is already found in the preliminary design stage of the hull lines at the shipyard through computational fluid dynamics (CFD),
often satisfactorily with potential flow analysis alone (i.e. neglecting viscosity but taking into account the free surface and gravity waves).
However, most hull designs that are to be built still undergo model testing in a ship model basin/towing tank, even today.
In suite of these model tests often an ink test is carried out where some sort of ink is applied along the section curves of a taped grid on the model hull's wetted surface.
When the model is towed the ink disperses in patterns which indicate the stream lines across the hull.
Then photos are taken from these smeared ink lines which are submitted together with the power prediction and test results in a report handed out to the customer (i.e. usually the shipyard).

So a poorly fitted bilge keel can indeed cost efficiency.
But this seems to be more true for the real ship than the model as in general our model ships are grossly overpowered (compared to the scaled down power requirements) and because the scale effects of viscosity are exaggerated on the model anyway.

[deadbeat]

I thought you might be interested in the entry in my copy of Practical Construction of Warships by R.N. Newton (2nd edition) which was my bible through my apprenticeship in HM Dockyard Portsmouth. Apologies for the poor scanning, but hopefully you'll be able to read it.

[irishcarguy]

Thank you Deadbeat, it is always nice to receive more & accurate information on building. When I joined the forum in March of 2011 I honestly did not know what a bilge keel was, a true admission, Mick B.

[roycv]

Thanks deadbeat for the pages on bilge keels, all very logical, I shall now go and inspect the ones on my model.
I can see now how to work out if they are going to be helpful or otherwise.
Off to the shed now,
Roy

[RAAArtyGunner]

Deadbeat,

Brilliant 

[roycv]

HI all, update:-
The line of the bilge keels if projected into the hull come out at around deck level.  So I have filled in the underside of the keels to form a triangle and the angle ( maybe 30 degrees) through the centre of the triangle now projects lower and nearer to the centre of gravity of the hull.  It also looks less of a drag when viewed from the bow.

Fingers crossed, should be able to do bath tests within the week.  Considering it has taken the hull 40 years to get to this point, progress is now very fast.

thanks for all your contributions,
regards Roy

[CF-FZG]

Thank you Deadbeat, it is always nice to receive more & accurate information on building. When I joined the forum in March of 2011 I honestly did not know what a bilge keel was, a true admission, Mick B.

Totally agree

Although I knew what they were, I didn't fully understand how they worked or what they were for


Mark.

[deadwood]

As an addendum to what I had written about determining the correct position of bilge keels and its importance above
 I would like to refer to section 5.2 Bilge Keels of this article about Improving Fuel Efficiency in Ro-Pax Design presented at the RORO 2004.