toe in and keel changes through Centaur production run

dylanwinter

dylanwinter

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Be gentle with me chaps for I am but a poorly trained agricultural engineering graduate and I should not stray too far into fluid dynamics for fear on incurring some sort of brain jam

I am writing something about Centaurs for Classic Sailor

http://classicsailor.com/

so I had a look at the yachtsnet pages

http://www.yachtsnet.co.uk/archives/westerly-centaur/westerly-centaur.htm

where it says that the keels are shaped like foils with a toe in of two degrees

does anyone know what that ends up in inches?

did westerly use the same keel castings and toe in right through the production run?

Yachtsnet also says

"The sailing performance is helped by carefully designed bilge keels which are handed port and starboard with a two degree toe-in, so that once the boat heels under sail the more vertical leeward keel is developing lift to windward."

that may be true but it is also attempting to trip the boat up while the toed in windward keel is attempting to bury itself. If you let a Centaur heel too much that windward keel comes too close to the surface and starts to cavitate and gurgle turning poor old LGs tank tested physics to go to hell . On the helm it feels as though you have caught a mooring bouy and then the boat starts to try to stall out. Upwind Centaurs are fine as long as you keep them flat - that means either playing the mainsheet of stuffing her into the gusts just like an overpowered dinghy. My son is a laser sailer and had her going to windward beautifully by playing the mainsail. I am an old bloke so I tend to roll away a little bit of genoa or main and sail her flatter but slower.

However, I have noticed that when a Centaur is going down hill with the wind up her chuff that toe in makes her sit down in the water - the opposite of a foiling cat or moth.

That does give them a feeling of stability as the hull settles deep in the water at four knots.

I assume she is moving more water than a boat with toe out - or is she?

. I am not sure exactly what toe in brings to the table and do non westerlies have them?

I believe the macwesters have no toe in. Has anyone saled both a 26 and a centaur.

The seawych has almost vertical keels.

incidentally

I am still missing the mark 2 rudder. Centaur 1 was good to sail - Centaur 2 is rather more pudding like in the feel on the tiller so if you are in the market for a second hand centaur and you care at all for the feeling on the tiller - get one with the stub and more balanced rudder. Just an opinion from an old Sonata and E boat sailor who now sails clonky old Centaurs.

D
 
For all practical purposes, 2o toe-in is going to be a difference of 1/30 of the length of the keel between the front and back separation. So, for example, if the keels are 30" long, the leading edges would be 1" closer together than the trailing edges.

Note for non-agrics: based on 1 radian ~ 60o and sin x ~ tan x ~ x for small x.

Reason? I'd hazard a guess that it's so that the windward keel produces some downward lift (if you see what I mean) when heeled, but there are probably people around who know for sure. It might also be to do with yaw stability, like toe-in on a car, but I haven't convinced myself that that would work, except perhaps in a rather crude way by increasing the drag difference between the "front" keel and the "rear" keel when the boat is yawed a bit.

I believe that the Seawych has completely vertical keels to allow them to be moulded with the hull and then lifted out of a one piece mould ... hydrodynamics a very minor consideration.
 
JumbleDuck said:
For all practical purposes, 2o toe-in is going to be a difference of 1/30 of the length of the keel between the front and back separation. So, for example, if the keels are 30" long, the leading edges would be 1" closer together than the trailing edges.

Note for non-agrics: based on 1 radian ~ 60o and sin x ~ tan x ~ x for small x.

Reason? I'd hazard a guess that it's so that the windward keel produces some downward lift (if you see what I mean) when heeled, but there are probably people around who know for sure. It might also be to do with yaw stability, like toe-in on a car, but I haven't convinced myself that that would work, except perhaps in a rather crude way by increasing the drag difference between the "front" keel and the "rear" keel when the boat is yawed a bit.

I believe that the Seawych has completely vertical keels to allow them to be moulded with the hull and then lifted out of a one piece mould ... hydrodynamics a very minor consideration.
Click to expand...

thanks JD

that helps

D
 
The main purpose of the toe-in and assymmetical shape (not unique to Centaurs) is that the leeward keel is intended to generate a small but significant amount of windward lift as the boat makes forward way through the water hence reducing leeway. As a probably unintended consequence the windward keel generates a downwards lift as well as drag, possibly slightly increasing dynamic stability to windward or on a reach. Downwind the toe-in probably just slows the boat a bit.
 
the keels do appear to settle the boat lower in the water - I was wondering how much of that suction effect is also pulling the boat to windward until the boat heels to the point where the suction between the keels starts to cause cavitation. Until that happens they are surprisingly close winded.

D
 
JumbleDuck said:
I believe that the Seawych has completely vertical keels to allow them to be moulded with the hull and then lifted out of a one piece mould ... hydrodynamics a very minor consideration.
Click to expand...

No it is so that if the boat is kept on a mooring that dries to deep soft mud the keels pull straight out of the mud every time the tide rises instead of being wrenched off as tends to happen to the splayed keels on a Centaur ;)
 
Mine are handed as well on the tomohawk. One side is flatter than the other like an aircraft wing so would assume it works in a similar manor as providing a lifting type force.
 
I'd be very surprised if they changed the Centaur keels at all during production.

However one reason I can think of for any variations might be the October' 87 Hurricane.

It was carnage around here, hundreds of boats blown ashore off moorings, quite a few sunk at their moorings, and loads more toppled like domino's at yards.

lots of 1-2 man band ' Grp Restorers ' sprang up overnight, and after a while quite a few stories circulated of ' keels stuck back on at the wrong angle ' - not Centaurs, all boats.

So a good reason to thoroughly check any pre-87 boat one may be considering, starting with a casual ' where was she in October 87 ? ' ...
 
VicS said:
No it is so that if the boat is kept on a mooring that dries to deep soft mud the keels pull straight out of the mud every time the tide rises instead of being wrenched off as tends to happen to the splayed keels on a Centaur ;)
Click to expand...

I think that's a side-effect of the design being made for an easy one-piece moulding. Colvic 26s and a few others are the same - if a hull has angled moulded keels or much tumblehome on the topsides the only way to mould the whole thing is to have separate half moulds or keel moulds and then join them together. That is done - lots of 1970/80s boats such as She 36s had tumblehone, and the hulls were moulded in two halves.
 
JumbleDuck said:
It might also be to do with yaw stability, like toe-in on a car .....
Click to expand...

I'm not sure what that means ...... but I tend to think about toe-in as simply a way to ensure that the wheels run parallel when the car is under power.

Rear wheel drive = front wheels tending to be splayed out = toe-in to correct splay.

Front wheel drive = front wheels tending to be pulled in = toe-out to correct pull.

I know that there are other more esoteric second order effects but the above is the primary consideration. :)

Richard
 
Am I totally confused here but doesn't toe in mean that the keel is at a minus angle to the water flow when boat is heeled like when you push the stick forward on an aircraft for a dive so I don't see how it contributes to lift to windward, more like less drag more speed maybe.
 
Billows said:
Am I totally confused here but doesn't toe in mean that the keel is at a minus angle to the water flow when boat is heeled like when you push the stick forward on an aircraft for a dive so I don't see how it contributes to lift to windward, more like less drag more speed maybe.
Click to expand...

I confess that I am a bit confused as to how it works

there is no doubt that the Centaur behaves slightly differently to every other boat I have sailed
 
RichardS said:
There are two keels .... so when the boat is heeled one will be at the opposite angle to the water surface compared to the other. You presumably need to consider the relative effect of both keels.
Click to expand...

I agree. The interesting thing is not the angle of attack; it's having two slightly different angles of attack.
 
Seajet said:
I'd be very surprised if they changed the Centaur keels at all during production.

However one reason I can think of for any variations might be the October' 87 Hurricane.

It was carnage around here, hundreds of boats blown ashore off moorings, quite a few sunk at their moorings, and loads more toppled like domino's at yards.

lots of 1-2 man band ' Grp Restorers ' sprang up overnight, and after a while quite a few stories circulated of ' keels stuck back on at the wrong angle ' - not Centaurs, all boats.

So a good reason to thoroughly check any pre-87 boat one may be considering, starting with a casual ' where was she in October 87 ? ' ...
Click to expand...

Pre '74-75 the keels were handed, post 74-75 the keels were re-designed with regard profile, bolt pattern and unhanded design.
 
guardian said:
Pre '74-75 the keels were handed, post 74-75 the keels were re-designed with regard profile, bolt pattern and unhanded design.
Click to expand...


Interesting; I've sailed a couple of late model Centaurs quite a lot, and an earlier one a bit.

in that somewhat unfair comparison I admit, the later boats sailed significantly better; I always put this down to the rumour that later boats were lighter built; and of course probably had more recent sails.

However the earlier model I tried really DID feel and appear to act ' heavier ' - not just neutral on the helm, which is the main drawback of an otherwise great design - whichever rudder.
 
From memory, the front edges of the keels are 4.5" closer than the trailing edge, that is each front edge is 2.25" closer to the centre-line than the aft edge. I do not recall the length of the keels so I cannot give you a figure for the angle.
Mine was built in 1976, had asymmetric keels and a spade rudder.
 
PuffTheMagicDragon said:
From memory, the front edges of the keels are 4.5" closer than the trailing edge, that is each front edge is 2.25" closer to the centre-line than the aft edge. I do not recall the length of the keels so I cannot give you a figure for the angle.
Mine was built in 1976, had asymmetric keels and a spade rudder.
Click to expand...

excellent stuff....

so how does she feel on the helm and does anyone know when the spade disappeared and turned into the skeg and balanced rudder

I am going to forget all the stuff about what the windward and leeward keels are doing and just imagine the boat sucking herself down into the water.

Just as she is sucking herself down when going downwind so she is also sucking herself to windward and downwards when heeled a few degrees. The more the heel the better the windward sucking until that top keel gets so close to the surface that it starts sucking air and then it all goes to pot

As a result of this discussion I may be on the lookout for a late centaur with heating, a new engine and an anchor winch

maybe I should sell this one at the boat show and buy my third Centaur


D
 
I had that Centaur for fourteen years and I do not recall ever experiencing any problems of the windward keel coming too near the surface and sucking air. That would entail severe heeling with the leeward rail being well under water; a sure sign of reefing being well overdue.
Perhaps others had a different experience... or have a better memory than I.
 
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