Twin keel research

Rudder Configuration

Mike, you can see the reason I asked this question about the rudder configuration. From all my reading thus far I would really question the use of, or the need for, twin rudders on a cruising vessel, and particularly if a skeg-rudder is utilized

 

Brian
re Riverdale's dicussions with Arthur Robb, wonderful bit of information that, you did well to find it. I am waiting for Kallista's delivery voyage summary with interest.

Yes I tend to agree, I would want the prop wash over the rudder anyway, I would prefer to see a skeg on the centreline I am a little concerned about the keels channeling debri into an exposed prop rudder and a skeg fwd of the prop would protect it and the rudder.

 

First Hand Experience

John Keeley has just posted some interesting 'first hand experience' discussion on this subject at http://www.boatdesign.net/forums/showpost.php?p=49095&postcount=7

 

I love the twin-keel approach, but how do you design it?

Hi Everyone,

I must say that I have found this thread extremely interesting! And topical for me as well as I have been toying with trying to design and build my own “dream yacht” and the twin-keel approach seems to be exactly what I want. Let me give you all an overview of the sort of boat I am thinking about so you can comment.

My first requirement is that this yacht be seaworthy and tough. Come to think of it, that would be my second and third requirement as well. I live in Florida, and have an absolute talent for finding storms to sail in. Sooner or later I will probably find myself trying to survive in a category 5 hurricane. I don’t want to, obviously, but with global warming and the gulf stream possibly stopping (thus leaving a bunch of heat down here) I think I can expect things to get periodically very wild, so I need to build tough.

I was thinking of roughly a 12 meter aluminum boat with a gaff-schooner rig. Heavy construction, retractable, individually controlled props (port and starboard) powered electrically for harbors, and no wood at all topsides (I really hate sandpaper). Ugly is fine with me, and considering some of the places I am thinking about going — is probably an advantage. I will be living on this boat for the foreseeable future.

The paper by Nils Lucander (Eying the Pump Effect) kindly supplied by Brian Eiland is most interesting, and I take from that the idea that I should have my largest hull sections at about 60% of the waterline length, and should make my entry very fine if possible.

I have also read the paper by Ian Wright (Proper Twin Keels) kindly supplied by Mike Johns, and of course the paper by Lord Riverdale also supplied by Brian Eiland. From these I surmise that I need to place my keels well aft for a variety of reasons — but they must also balance the CE from the sails so they can’t be too far aft.

All of which is not a lot to go on!

So, is there a systematic approach to this problem? Making and testing models is a real pain as you all know, and to be avoided if at all possible. Or at least put off till the last possible part of the design process!

In aircraft design there are computer programs for visualizing air flow over wings and fuselages which would seem to be very helpful for boat design if modified to represent the properties of flowing water. Is there an equivalent program for yacht design that could be used for hull-shape development? I would think that being able to study virtual wave-making under various conditions would be very helpful!

Will design programs like Maxsurf, et al. handle the analysis part of designing a twin-keel hull? Or is that asking WAY too much?

I apologize for my dumb questions, I’m starting from scratch and trying to get oriented and greatly appreciate any insights into this problem. If all else fails, I will go the model route to sort out the parameters.

Regards,

Bill Cushman

 

Fs...

Hello...

See http://www.upl.cs.wisc.edu/~aselle/fluid/ ...

I have always wanted to take some time and build some twin keeled dinghies to test some ideas - two asymetrical keels hung 15 degrees off the center line in section - with a 2 degree toe in to same in plan...

I sail my boat fast at 15 degrees heel - thus one keel would be vertical in the water under heavy winds and would be 'lifting' me to windward - the high keel would be chopping up the bow wave and would also be trying to right the boat - sounds good to me...

Would be great to build two hulls - one with a symetrical centerline keel and one with the above - both with same wetted surface - and go sailing...

SH.

 

I have a 50' steel hull that, at present, has a large centreboard " slot " right through to the deck. I'm not happy with the room it takes up as it makes a reasonable boat into a lot of small cabins ! [ efectively ]. So I've been looking to rip it out --- but what's the alternative !?.
I've found a sistership [ albeit 60' ] with a brave owner who's welded heavy wall pipes @ what I assume is the correct position, through the port & stbd sides of the hull and tells me he'll be having " foils, shaped like aircraft wings " [ sortof ] controlled by rams on the inside [ on a small "arm" off the 300mm shaft] .
Our vessels are flat bottomed [ misleading ] with a small - ish aft keel [ formed as the hull begins to rise toward the rear ] with a central prop & rudder well suported by the 20mm plate bottom as it tapers aft.
I'd love to show anyone some "lines" as the designers an " unknown" & or a pic of the hull --- but not sure how to post it !

Anyway, my point is to ask opinions / thoughts as to possible pro/cons of the, as yet untryed ??, design.
I'm told that the foil , aprox 2.2m deep & .6 --.8m wide , will virtually want to "fly" [ isn't water the same medium as air, only more dence [ like me ha ha ] ] to windward as it's assemetrical , broad at the top & narrow at the bottom .
The foils [lee boards] can and would be raised / pioveted up, horizontal when taking the ground [ 850mm draft in my case ] or in shoal ground.
It's felt the single chine will suffice to give sufficent control at low speeds [ when perhaps the "wing" will/cannot give lift !?. I intend to have a bow thruster also for docking & tight areas as I realise she'll be "slippery" then .
Go, chew me up & spit me out !
Hey " BILLY DOC " you sound like my kinda guy [ hey !, not like that !! ] I'm all for tough & to hell with looks, if I had to choose . Oh & I intend to make her a diesel electric for all the reasons " solomon technologys" promote . I won't go into the bipod rig here as I fist must sort out the keel/foil thing.
Oh, I'm told & I'd concede , that when heeled, the foil - now pushing against a teflon rubbing block on the chine, would lift not only windward, but upward tward the chine, as she's angled over. The " balance" of the boat, from the chine over to the windward side, would then efectively be a counter weight to the piovet point [ chine] to ??? what degree ?, shurely some degree !?.
All balast is designed to be in the " bilge" [ 4 ton lead in my case ]
It seems nothing is "new " & everything / design is a compramise depending on what you want, & like us all , I want my cake & scoff it now .
Pete Metcalfe.

 

BillyDoc
Fluid dynamics prediction definately not up to it. Design will be either hit and miss, copy similar hulled solutions, or model test.


Globaldude

There's a lot more to this than we can cover here. You really need some local and knowledgable help otherwise you are going to end up with a disaster in the balance , drag, lift and reliability department.

It is probably better to simply redsign the centre-board arrangement. To properly implement twin keels is going to be a long haul.

You can eamil me some drawings if you want and I'll have a quick look at what you propose, and suggest alternatives.

 

Model Testing

Excerpted a quote of Mike's back at item #9

I feel this approach might be more productive than the Computer Flow Analysis one, as there are so many assumptions necessary for the computer anaysis to proceed. And then the interpetations of the results can be misleading. I made some similar observations with respect to sail flow analysis, http://www.boatdesign.net/forums/showpost.php?p=47744&postcount=19 . Besides this computer analysis is generally VERY expensive.

I think Mike's other observation is very true as well, "non-paying projects tend to be at the bottom of the pile"

 

Heres the recent delivery voyage account of a "Bluebird" kindly forewarded to me by Mr Warwick Williams of Adelaide Australia. Posted here with his permission.


Friday, 3 June 2005

Hi Mike

In summary, "Kallista" now resides in Adelaide after a nineteen day
voyage from Southport. It was not without event, but, our "bluebird"
performed to the expectations of her pedigree.
On the first evening out of Southport on 27th Dec. we encountered a
white line squall which arrived unheralded literally out of the blue. We
were making good time at 11 knots over ground in the 4 knot southern
current. The wind was NNE and sea conditions fair. As we were running
square before the wind we ran a preventer from the stbd main wishbone to
an unused sheet winch to avoid accidental crash jybing, ( we had some
inexperienced folk who were going to be helming overnight), and in
hindsight this proved to be a major error of judgement. While we were
preparing a meal the helmsman called us up and said the atmosphere was
strange and he could see a dark cloud coming in from the east. We
decided to reduce sail and just as I was zipping the mizzen into its bag
we were knocked down by a ferocious gust of wind that hit us on the port
side. Luckily the genoa was already afly but the main was unable to jibe
to leeward because of the preventer and there we were lying on our side
with our world a cacophony of noise and water. Just as I thought that
the mainmast must go by the board the cast aluminium brackets at the
mast end of the wishbones gave way, closely followed by the one joining
the two at the clew and the main was free. This allowed Kallista to come
upright and we were able to start the diesel and push her into the wind.
All this time the anemometer needle was stuck on its maximum reading of
65 knots. Two of our experienced guys fought their way to the mast and
managed to drop the main and secure the wishbones. Ten minutes later the
wind had died to a breeze and we stood shivering with shock in the
cockpit as the Coastguard started broadcasting a major storm warning.
Following this little episode we motored into Coff's and took some R.&
R.
By huge good fortune we encountered a sailmaker who was on holiday and
he sewed a new leech into our genoa and we four,(the crew), sewed up the
damaged batten pockets in the main. The wishbones were beyond salvage so
we jury rigged the main to be flown loose footed and this worked
reasonably well for the rest of the voyage. In a testament to Kallista's
good design the two least experienced crew decided to stay with us for
the remainder of the voyage because the yacht had stood up so well in
the appalling conditions.
We had lousy weather from Sydney to Eden. Wind on the nose most of the
way as one low depression followed in the heels on another. We used the
steel main for much of this leg and I was delighted that we maintained
an average speed of eight knots for about 5.25 litres/hour diesel
consumption. Engine is an 80 HP Perkins and it performed admirably.
After another stopover in Eden for four days when we took another two
crew on board for the Bass Strait leg we headed off towards Gabo with
the wind on our nose again. However, after rounding Gabo the wind moved
to the South east and we enjoyed a magic two days sail across to
Portland. With the main working with a less than ideal profile we
managed to make 8 to 9 knots and the yacht was very comfortable. Her
wake is eerie since it is so flat. I spent years in warships that would
tear up huge rooster tails and my old long keeled steel yacht builds a
following wave when pushed but one word describes Kallista's path
through the water - elegant.
We sat out another blow in Portland for three days as another front
blasted its way through. (this one gave rise to the awful Bushfires on
Eyre Peninsula). We got under way again with the wind coming in from the
SW and a huge swell in attendance. As we were on a NNW heading this
proved to be the most uncomfortable part of the voyage for me. We rolled
significantly and the wind meandered towards the south and was flukey.
Kallista's rate of progress fell to 4.5 knots and so we ran the engine
again. Her motion eased a little with the increase in speed and I feel
that the keel design probably contributed to this. When we came under
the lee of Kangaroo Island the swell died away and we had a very
pleasant sail up Gulf St.Vincent to the Outer Harbour of Adelaide.
Since the trip we have had the wishbones replaced through insurance and
Kallista is looking sveldt again. I am trying to make contact with the
owners of the original Bluebird up in Seattle so that we can compare
notes but to date my email enquiries have not borne fruit. I believe
that Bluebird has been significantly modified in accommodation and
perhaps sail plan so I'm interested in learning of how that has gone.

I am of two minds about the wishbone rig at this time. I think it is
wonderful on the mizzen. Simple to raise and put to bed, no fuss or
bother, but, the main is another matter. Simple, yes, but is it too
simple? Perhaps with some "quality" time with it I might get over my
misgivings. Reefing, (which we never did get to play with) appears
awkward to set up at the clew, and, the inability to set up controls for
uncontrolled jybes when running - ie a boombrake - puts it at a
disadvantage in my present opinion. (For sure I'll never be inclined to
put a preventer on a wishbone again!)
Well I have rambled on enough for the moment.
As a final word, I believe that "Kallista" is a class act and deserves
all the plaudits that have come her, "Bluebird's," way. I would be
delighted to show her to you at some stage.

Cheers
Warwick

 

Would you like to talk to the builder?

 

Twin Keel builder

Hello Doug. I understand you were the builder. We look forward to your participation in this forum subject.

 

Morning (well here anway) Brian, I am in the process of organizing all the 'photos I have to post a few of the more interesting ones. Spoke to Warwick Williams yesterday by land line he is off the planet that he has located me. Bluebird/Kallista were quite fantastic (overused word that). Lord Riverdale visited to check my progress most likely to make sure I didn't bastardise it, she was quite a challenge to build with the keel and skeg angles involved. Kallista was a faithfull copy in all respects apart from fairness of the hull, I spent many months on that detail and Deck etc. deck & upperworks were aluminium, all welded. Not many people know Bluebird was built with overlaping plates and riveted and a ply deck.

 

Mikejohns,
I was thinking just this afternoon of a method of grounding a keel sailboat.
I live in an area with 8-10 ft tides. This is not very original, just some improvements from something I read somewhere. Pretty simple. 4 2x whatever, with a large say 2'x2'x4" plywood foot for each. Cast enough lead into each foot to make sure the feet and pad sink and stay on the bottom. The legs need to be about the same length as the draft up to the rub rail. Hinge the legs to the feet with a removeable hinge for stowage. Put 4 stout thrubolts at appropriate locations thru the rub rail. A cleat on each of the legs. Maybe a copper lip on the top of each leg. As the boat begins to ground, put the legs out with hinged feet and tops onder rubrails, lash to an onboard cleat and let the keel do what it was meant to do...support the weight...while the legs merely balance the weight of the topsides.

 

FWIW, here are some model test results of a twin keel vs single keel yacht rolling at anchor: http://www.rina.org.uk/rfiles/IJSCT/Discuss/Klaka.pdf. I don't know how long the paper will be up, so download it if you want it for reference.

Bottom line is the twin keel rolled about 20% more than single keel of the same depth.

 

Chandler's thoughts on 'balancing' a single keel at low tide has been pretty standard practice aroud the West Coast of Scotland and the Hebridean Islands where few 'marinas' or jetties exist - but extreme tidal variations do.
'Sea legs' as they are called really do work. However, when in place heavy physical activities should be performed with caution (no plump ladies or ceilidhs aboard - unless accommodated on the centre line).
The legs can either be removed (not those of plump ladies) and stored on the deckhouse roof - or swung up and lashed against the rubbing strake.