Bilge keels are good.

[incognito]

I have been doing some sums on moments of inertia and stuff, and have come up with a surprising result. This may be because I am doing something silly with the numbers, but it seems that ALL THINGS BEING EQUAL, a twin-keeler should be better than a single keeler. It is important that things must be equal, as usually, the twin-keeler, referred to as a bilge-keeler, has shorter keels, and the weight is not put right down at the tip, like a single-keeler. If you take a keel of x depth, with the keel cofg at y depth, and split it into two keels, both slightly longer than x, so that the boat would ground at the same depth, and made them so taht the cofg is at y -then by the time you get round to where the avs would be, you still have one lagging keel, giving positive righting moment.

Anyone authoritative out there who will straighten me out, or better still, say Yes, that is right?

 

[Strathglass]

I think the fly in the ointmentagainst your argument is the fact that the draft of a bilge keel yacht is usually a lot less than her fin keeled sister.

Iain

 

[Stork_III]

It also depends on the centre of buoyancy when heeled and the increase in lever arm achieved from the c of b to the c of g of the half weight windward keel.

 

[Twister_Ken]

Yes, you are right!

(please put cheque in post)

 

[Twister_Ken]

Wassmore

If you 'toe-in' BKs by a degree of two it helps you climb up to windward (though this works better with retractable bilgeboards.)

 

[bedouin]

It's not so much about moments of inertia as righting moments at various angles of heal.

I'm not an expert on BKs but two problems occur to me

(a) The ballast is usually carried much nearer the waterline so has a much shorter righting arm.

(b) If the keels themselves are weighed then it is possible that at small angles of heel one actually counteracts the other, reducing the righting moment still further.

 

[Bergman]

Re: ?

All things being equal???

How do you get 1 to equal 2?

 

[Rowana]

Re: ?

1 = 2?

Inflation, dear boy!

It gets everywhere, you know !

Jim

 

[oldharry]

a 'twin-keeler should be better than a single keeler'

At what? At the risk of nit-picking what constitutes a 'better' boat.?

At the end of the day it all depends on the design, the hull/keel configuration, the position of the ballast inboard or do the keels carry the ballast? ARE they the ballast?

So what constitutes a 'good' boat. One that will win races? If so the B/keeler is unlikely to be competitive because of the big increase in wetted surface area. But an ex works Sadler 25 fin and a bilge keeler were pitted against each other. The fin keeler pointed a little higher and faster, but on a reach and off the wind the bilge keeler was the faster of the two.

A good sea boat: one that is going to look after her crew in a blow? Again that will depend entirely on the design of the boat. Arguably bilge keelers are just as good sea boats as their fin keel counterparts.

Windward performance: some would say this is essential in crusing as well as racing giving the ability to claw off a lee shore. And Bilge keelers are renowned for having inferior windward peformance - usually because they have shallower draft.

Most long distance boats are fin keelers because of their deeper draft, but a number of long distancers are quite happy with their Bilge keels.

So its not so much which is the 'better boat', but which configuratoin is best for your kind of boat. Many pretty drying harbours and creeks are largely closed to fin keeled cruisers, while the B keeled counterparts enjoy the seclusion!

Horses for courses!

 

[jimi]

I've checked your calculations as well and they are absolutely spot on. Well done! I did take it a stage further and calculated that a triple keeler is even better, would you concur?

 

[incognito]

I did mention all things being equal, so for a flat bottomed barge (like certain unmentionable types of boat) this would mean we compare bilge keels of same length, weight and position of cofg.

As to one counteracting the other, at low angles, and the number of keels ( jimi), I would have thought by the principle of superposition that they would have the same nett effect.

Any other sailing mathematicians out there? (I know you are, Bedouin, but as a software freak you don't really count, you have sold your soul to the devil).

 

[Gordon]

Take any fin keeler and a similar bilge keeler. Point them upwind to
a common target. After one hour you will be saying bilge keelers are no good.
Einstein said if the theory doesn't fit the observation the theory is wrong.

Gordon

 

[Guest]

Definitely lost interest now !

nm

Nigel ...
Bilge Keelers get up further ! I only came - cos they said there was FREE Guinness !

 

[bedouin]

On the contrary, I'm a mathematician (of the purest kind), not an arithmetician. It's a well known fact that real mathematicians can't even add up, (as well as being constitutionally lazy).

If you want quantitive results then either find a Physicist or use a spreadsheet!

 

[bigmart]

Of course Bilge Keels are good.

Have you seen the price of deep water moorings on the South Coast.

 

[Guest]

Water Density ?

Having owned both I would agree that the bilge keeler is a poorer performer to winward. But like yourself, I think this is due to the lesser draft.

I have often wondered though how much water density has to do with it. A deeper keel should get more grip on the water. I do know that divers start taking depth seriously at around only six feet.

Does having a deeper bodied hull and keel retard progress ?

 

[chippie]

Re: Water Density ?

As water is incompressible I dont think there would be an increase in density. There would be an increase in pressure as you went deeper due to the weight of the water above.
I wonder whether if bilge keels were as deep as a normal fin keeler,they would point as well. There would be more drag though.

 

[simonn]

Re: Definitely lost interest now !

Having just purcashed my 1st bilge keel yacht after 3 fins, I Found beating upwind depends not just on keel type but mainly on good design.My new bilge keeler sails far better than my last 3 fins in all but light airs.There are plenty of fin keel dogs about .

 

[Ohdrat]

Some of the early smaller sadlers can out up wind modern (Bav) types of similar length .. and a supposedly pretty sea worthy too..

I would say that your equation is only part of the whole picture tho.. as pointed out in other's posts.

So no I personally don't right off bilge keelers per se but there is a tendency to build bilge keelers for shallow sheltered creek exploring rather than ocean crossing as that is one of their great strengths .... the ability to dry out without assistance safely. It's a case for horses for courses and should I also say horses for riders too as the right boat for one person is not for another..

 

[peterb]

No one so far seems to have mentioned the hydrodynamic effects. The keel is a wing, in the same way as an aircraft wing, though the force it provides is sideways rather than upwards. In aircraft parlance, the fin keel is a monoplane, the bilge keeler a biplane. Early aircraft were mainly biplanes, not for aerodynamic reasons but because the use of cross bracing between the wings could make the wings lighter and stronger.

As structures improved the monoplane could be built just as strongly as the biplane, and aerodynamic effects became more significant than structural strength. It was found that the bottom of the upper wing interfered with the top of the lower wing, increasing the drag and reducing the lift. Nowadays few biplanes are made except for special purposes such as acrobatics.

Exactly the same arguments apply to bilge keelers. The inner surfaces of the keels will interfere with each other, resulting in increased drag and reduced lift. The reduced lift is countered by an increased angle of incidence (or, as yachtsmen call it, leeway angle). You can reduce the interference by moving the keels further apart, or by splaying them, but you can't eliminate it.

In stability terms you should not consider the two keels separately. What is important is the position of the centre of gravity of the pair taken together, and because of the lower draught this usually gives a reduced restoring moment.

But for many people the availability of a boat which can sit upright in a drying harbour will override all the minor reductions in sailing efficiency. Horses for courses; boats for coasts.