Bilge Keels - to remove or not

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tillergirl

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By way of a serious Christmas debate, advice please on the following proposal.

TG is an East Cost 'design' built at Leigh on Sea by Seacraft who used amongst other things to build Bawleys. She was designed by the yard on a basic outline suppiled by the commissioning owner. I don't know everything about what was said but I know that the previous boat built in the yard was a Vertue and TG's owner wanted the Vertue bow and LG knuckle on a 32ft length. The rest is a bit Maurice Griffiths (the yard built some MG designs). Indeed some aspects are very Golden Hind like even though the build predated the design of the Golden Hind - three keels, shallow draft (3' 9") and the hind quarters are very 'bawley'.

The original bilge keels were quite massive iroko plates probably some 6' long but these were replaced in the 80's for two steel plates - flange top and bottom, the top flange bolting through into very substantial stringers.

The build of the boat is 1" iroko on CRE and everything is kinda over engineered - all the floorboards are 1" iroko as are the locker fronts etc!

This is a cruising design for the East Coast. Very easy to handle as a single hander but not the fastest thing out of the blocks but this kinda matches the owner. She is, though, a comfortable and reassuring boat to be on (tried one of those light GRP things on a Greek Flotilla - nice but boy did it bounce around in a blow; gee was it slippery on deck and B' H' all that slapping on the hind quarters in harbour - I digress).

The issue is that I have come to the conclusion that the bilge keels make little difference to leeway and in fact I think their benefit in that area is outweighed by their braking effect. There is certainly no doubt in my mind that the starboard keel is not properly aligned. I can tell when I get to 5.5 knots on the wind in a seaway as a whoomping and crummping start. Also I have a picture of her hard on the starboard tack with what looks like a second bow wave at the front of the bilge keel. This doesn't happen on the port tack! It is also clear that the seam 'below' the bilge keels work quite hard on a beat. I've probably partially re-caulked those seams 6 or 8 times in 18 years.

The centre keel is wide - so wide that she will sit upright on it on the hard, although occasionally she will rest on a bilge keel which are about 2" shorter. This width of the central keel mean that TG will never be a star to windward. I guess a Twister would be a Concorde to TG's Short Sunderland. They both fly and both have their uses!

Proposition is in the spring to put TG on the beach (from her winter mud berth), unbolt both bilge keels, leave the studs in place and fit and bolt up temporary 'runners' to keep her water tight. These would probably need to be a max of 3" deep so I can hide the nuts.

Downside? I may go even more sideways - 3' 9" draught on 24' 6" waterline What do people think about that? - but will that be mitigated by better speed. I guess I won't be able to casually stick her on the hard without making some legs if I want to be sensible. I guess in fact making the legs is the most difficult part of the process.

Upside? - I'll go faster? Removal of the strain on the stringer and under keel seams. Anything else?

Or should I just reflect that's she's been that way for 44 years and leave her alone? /forums/images/graemlins/confused.gif
 
For what they are worth, my thoughts are that as the current bilge keels do not extend below the base of the keel, the leeway resistance of the hull is not changed to any degree. The lateral resistance would remain the same but the turbulence created by them would be greatly diminished , particularly as it is practically impossible to align them perfectly without creating a braking effect and more slowing turbulence.
It is unlikely they have been designed with aerofoil section to provide lift to windward as more modern bilge keels are and if they were originally timber, they were not part of the ballast, however you might want to add the equivalent internal ballast if they are weighty.
There is a possibility the rudder control might be improved in a more reliable waterflow.
But I am not sure about relying on the width of the keel to stand upright under any circumstances, beaching legs are the obvious choice.
Might just have to fit heavy internal frame at the right position fore & aft with bolt through legs at mid height on topsides.
 
Thanks for the comments.

There is no question of any 'designed' aerofoil lift as the plates are just that - straight plate steel. Weight - good point. There is already some internal ballast so there is limited space to add more - well elegantly at least. I can just about pick up each individual keel by myself so I guess the weight isn't that much in relative terms.

No I definitely wouldn't trust the balance on the centre keel and I note a curious hole (inside) in the port beam shelf which looks like such a provision for some legs. However there is nothing on the starboard side!

Umm - heavy internal frame might be an issue given the position - just about removing the internal stuff of course but I can probably plan this and leave it for a while, drying out on the posts instead for a season while I try this.

I like the positive nature of your comments about the negative effect on leeward resistance. If that is right, there is no point to having the bilge keels.
 
Unless she dries out on a hard every tide, I'd do away with them.

You will probably go quite a bit faster with the loss of all that extra wetted surface, and I doubt if the leeway is going to increase much, if at all.

There are several ex-centreboarders around with about TG's dimesions which have had the centreplate removed; they have about your draft and get along fine. Alan Staley's Dallimore, "Tasia", comes to mind as one such.
 
Doesn't MG say something in one of his books about the bilge keels helping the boat to turn when tacking? Not sure - I might have made that up.
If the keels are angled outwards then wouldn't the leeward keel be more efficient than the centre keel when heeled to windward? You might gain a little speed and lose a degree or two to windward. But then again you might not and it would be interesting to hear the outcome!

Merry Christmas
 
It may be of interest that the American designer L. Francis Herreshoff designed the H-28 to have a draught of 3'3" on a waterline length of about 25ft. These yachts get along really well without bilge keels.
Peter.
 
Thanks to all of you and Merry Christmas.

The bilge keels have no appreciable angle. I'll have a scan/read of MGs books when the grandkids have gone. I know there were a couple of MG designs that had shoal draft and the same draught/waterline length/beam ratios. The problem is that I don't know if these were his better design. Thanks Peter for the Herreshoff comment. All v reassuring.

I think the answer is that I have a go at it. If it doesn't work then I can always put them back with not much more expense than the two pieces of iroko for the 'runners'. I am thinking about whether to add some internal ballast to compensate.

I suppose Peter's going sailing bt t the rest of you, Happy Rest of Christmas Day
 
[ QUOTE ]
For what they are worth, my thoughts are that as the current bilge keels do not extend below the base of the keel, the leeway resistance of the hull is not changed to any degree.

[/ QUOTE ]

I wouldn't think that this is right. Otherwise the second wing on a biplane or adding a jib to a mainsail would have no effect because the span is the same.

I suspect the bilge keels will be making a reasonable contribution.

You can always take them off and see how the boat goes. The time you will notice is when the drag goes up and the sailing speed is reduces such as in rough water and gusty wind.

The smoother and the more moderate the wind the less difference it will make.

Properly designed, shaped, aligned and finished bilge keels would add quite a bit to windward performance. If you take them off and performance improves it probably shows that the existing set fell down in one of these essential areas.

Michael Storer
 
Thanks. I think I'll try it and see what happens. I suspect though I ought to have taken some accurate leeway readings in various conditions for comparison purposes.
 
For what it's worth, I am of the opinion that flat plate bilge keels will do very little for the boat in terms of performance, and from what I have read, and discussed with some Eventide owners, the flat plate type are just there to stop the boat falling over when drying out.

The long straight keel will do more to prevent leeway, if it is unhindered by spurious bit of ironmongery, spoiling the flow past it.
 
This is all quite interesting, I am in the process of building a MG steel riptide and I am also thinking of ommiting the steel bige keels. the under water profile of the riptide is very similer to that of a bruce roberts spray I built and the spray has excellent sailing qualities (cruising I hasten to add...)
I think that I will put in all the structual support for the bilge keels but leave them off as I'm not convinced that they are really necessary. this gives me the option to add them at a later date if needed.
More input in to this thread would be interesting ( all you designers out there)
Hope your all having a great xmas,
regards Rachel. /forums/images/graemlins/smile.gif
 
IMHO, bilge keels need to be designed for the job required of them, ie. Hydronamically Designed? Have a look at a bilge keeled Pandora, they are probably about the most effective bilge keels as ever was, the boat points well, and is pretty fast. The other design I think that might be useful to look at, is the Achilles 24 tripple keeler, this is something akin to TG's boats layout? but again very effective, with properly designed keels that do the job. my own boat, although not very quick (Westerly Nomad) is also a tripple keeler, and the bilge keels on that are worthy of more than a quick glance.

All the above boats have one thing in common, their bilge keels were properly designed to do the job.

Having said all that, flat plate bilge keels have their place in the scheme of things, and there are plenty of boats that use them successfully to some degree or another, but I would generalise and say that the keels in this case are part of the overall design, not an add on.

It's all a bit complicated really, and the performance of any given boat depends on so many variables, that the choice of keels has to be part of the whole design package.
 
I recently changed from a bilge keeled Golden Hind 31 to a bilge keeled Barbican 33. (Maurice Griffiths approved of this configuration on the Barbican, and was apparently unhappy with the alternative of a rectangular centreplate with no stop.)

I have no experience of twin keelers, but my understanding as regards Griffith's long-keelers with bilge plates is that the bilge plates are entirely "optional". The Hind's keel was encapsulated (yes, sorry, tupperware hull) but the bilge plates were bolted with easy (external) access to the bolts.

As I understand they were there soley to support the boat when drying out, but did offer the additional side benefit of reduced downwind rolling.

Most of the disadvantages have already been covered in this thread, but one not yet mentioned is restricted access to repair the hull at sea if it receives whale damage! Maurice & Marilyn Bailey could not get a collision mat in place because the bilge keel was in the way, and consequently abandoned ship, setting their 117 day record in a liferaft.

I rarely disagree with Mirelle, because he would have forgotten more than I've learned if he didn't have a photographic memory. I have a deep water mooring and don't dry out there on any, let alone every, tide. I do like the comfort though of knowing that if and when I get it wrong the boat will sit upright until the next tide - nice to know exploring the East Coast. It’s also nice to have access to the most secluded anchorages a lot of which dry out. Less comforting though when we ran aground in a non-tidal bit of the Netherlands - if the horsepower hadn't got us back off the way we came we might still be there! 2ins difference in keel depth doesn't allow for much careening!

I heard that a few years ago a marine ply Golden Hind grounded on the Deben Bar and in the subsequent pounding had a bilge keel driven through the hull, despite the amazingly strong reinforcement. It was towed in to the river along the bottom before being re-floated.

Happy New Year

Little Knot
 
The bilge keel through the bilge has always been a bit of a concern. I've never fancied hitting the Sunk or Long Sands at speed (or what passes for speed) well heeled. The idea of seven and half tons being stopped by the front of the lee bilge keel digging in is quite sobering - not that its happenjed to date. As best it would crack some frames.

the mooring I'm on now doesn't dry -incidentally I've never like the pressure forcing outwards on the bilge keel when she first settles in the winter mud berth until she wiggles her wallow. I have seen very small dribbles from the bottom seam so you know what is going on.

Well it all seems clear. Lets have a go. Expect a post in May!
 
My experience with flat bilge keels is different to this.

I would suggest that the actual configuration of the boat might make a lot of difference.

Certainly bilge keels with a hydrodynamic profile will work better than a simple plate - and deeper is better - to the limits of draft - but there may be a substantial loss of performance in rough water when trying to get up to windward.

So leaving the studs in place is a good idea.

MIK
 
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