Twin keel research

the yellow hull part 2

Sorendfk:

Think twice about what you said about the CE of the keel: Of course the geometrical CE appears to be where is belongs, namely about halfway of the waterline. However: The EFFECTIVE CE of the lift of these keels is somewhere around their forward quarter, assuming they are some kind of NACA foil, or similar high-lift section.

When lenghtening a correctly positioned moderate fin keel equally in in both directions you push the effective CE way forward. If you want a long keel, you can't employ a high-lift section at the same time, unless you push the keel back, or, alternatively or additionally, deepen it aft and cut away forward.

Moreover, due to the trmendous sideways resistance of the twin keel configuration so far forward, this hull will run like on rails and be very, very hard to put through the wind, if at all.

On a run, with waves of some size, this hull will trip over its forefoot and broach uncontrollably.

As a rule of thumb (according to Riverdale, Heinrichs etc.) twin keels should be pushed pack as far as ballast placement and aligning the keel root with the hull diagonals at the keel/hull intersection permits. For the above reasons as well as to achive the much-desired wave-cancellation effect on the windward side.

The keels on this hull look more like bilge keels (for roll damping) that have grown too large, than like a feasible sailboat keel ( I assume this is a sailboat hull?)

Ben

 

Well, I did say "CE of the sailplan", not "CE of the keel".
I believe that even with this keel arrangement, there must be some relation between the CE and the CLR, that is, there should be some amount of lead.
It's been a while since I read about Prof. Heinrichs keels in "Yacht Bau", so I don't remember if he says anything about the balance of helm.

 

Hi Mike, you may be interested in seeing a genuine "Bluebird of Thorne" here in Oz. I stumbled over her while searching for a liveaboard in August this year. Fell in love first sight even though I had no idea of her pedigree at the time. "Kallista II" was professionally constructed in Victoria in 1986 to the original "Bluebird" plans of 1963. She is authentic right down to the wishbone rig on Main and Mizzen. I have all the construction data but (sadly) not the tank test data collated by A. Robb and Lord Riverdale. I plan to voyage "Kallista" from Southport to Adelaide post Christmas. If you would like info I can be contacted on 0427 85 15 17.
Cheers, Warwick Williams

 

Warwick

Thanks, yes I'll definately be in touch. Perhaps you could post a few photos into your gallery.

I gave my email address earlier as mjeng@... it should have been:
mjeng1@supernerd.com.au sorry all.

 

Eyeing the Pump Effect (of twin keels)

Long ago I was, and remain today, very interested in the subject of twin keels (twin-keels). So I was glad to see this topic of discussion come up again on the forum, and with a core of similarly interested participants.

A search around the internet will turn up a few other interested participants, and some mention of the subject on their respective websites.
1) http://www.brayyachtdesign.bc.ca/
2) http://www.kastenmarine.com/index.htm
3) http://www.runningtideyachts.com/monohull/

Regrettably, it appears as though most of these participants, including myself, were very interested in the subject for some period of time in the past, but then we pushed it to a 'back burner', either for a lack of interest by potential clients, and/or a lack of funds to go further into the research.

So now I hope this forum discussion will become a new focal point to bring together the highly scattered information on this subject, as well as the participation of those many persons who have an interest in this subject. If anyone knows of any person(s) who would have an interest, please contact them, make them aware of these discussions, and encourage them to join in the discussions. Maybe together we can all move this concept back to the 'front burner' of modern cruising boat design.

Certainly there is some very interesting correlated work being carried on present with the multi-faceted foil/keel configurations appearing on a variety of ocean racing vessels such as the Open 60's, the Volvo vessels, and the Maxi's. This considerable experimentation with innovative underbodies with multiple foils could provide some filter-down data material for cruising vessel twin-keels as well. Hopefully this inquisitive experimentation does not get 'rated' out of existence and disappear so quickly as happened with Bruce King’s innovative "Terrorist" vessel in the early 70's hay-days of the SORC racing circuit.

Mention has been previously made of Lord Riverdale's presentation document to the Royal Institution of Naval Architects, and designer Arthur Robb's tank test data. I have a copy of Riverdale's paper that I intend on posting to this forum so all might have a look at it as reference to one of the more complete studies made of twin keels. I see no problem with this public posting, as all that was stipulated originally was that no publication be made prior to its presentation before the Institute on Dec 1967. I am currently working on a viable source for some of Arthur Robb's work that I also intend on posting since he has passed away some time ago. And there are referenced some other studies done by Southampton Univ, etc, that we should all seek to get access to if possible, and get it posted to this forum.

I'm going to begin my contribution to this forum subject with an article I had saved from the Dec/Jan issue of Professional BoatBuilder entitled "Eyeing the Pump Effect", by Nils Lucander. I've attached a scanned copy of this article for everyone's review.
To summarize:
"When a boat moves through water the fluid is displaced, and in accordance with physical laws, it goes in the direction of least resistance, which is up. The resulting vertical motion of the water is what I call "the pump effect", and what others refer to as wave-making. Any hydraulics engineer can tell us that the faster water is 'pumped' up, the more power is required."
"I began to realize the significance of the water's vertical motion in the mid-50s and sought to use this knowledge to reduce wave-making in subsequent designs."

He then conducted a number of model test that were towed in pairs such that a very visible selection could be made as to the model with the least resistance.
"Three models proved to be nearly equal in wave-making performance. They all had fine entries, and their largest sections were located about 60% aft along the waterline."… "This appeared to demonstrate that by moving the underwater hull geometry further aft, the initial upward motion of the water at the bow was vastly delayed, reducing the power-robbing pump effect."

He then had the occasion to practice these wave-making tests in several full-sized vessels of his design, one sail, and one power. He observed a difference in the traditional wave form generated by these 'aft displacement' designs that appeared to produce superior hull speeds to those that would be predicted by the very widely accepted Froude's Law.

Next he produced a number of "so-called '3-point' designs for trawler yachts, fishing trawlers, and tugs. In these, I eliminated the" aft portion of the center keel aft, substituting twin-keels ….

"When running at a bit more than 13 knots, or 10% higher than the 'hull speed' predicted by the formula (Froude), this vessel displays no bow or stern wave. And, instead of the customary deep midships trough, it shows a shallow wave that actually adds to the propulsive efficiency. Again, the key is shifting the underwater hull form geometry further aft, delaying the initial upward motion of the water and minimizing the pump effect."



This article first caught my attention as a result of the illustration that included multiple keels, and I was expecting to see some discussion of the constructive or destructive interference between these multiple keels. Instead I found a rather interesting discussion on the placement of these keels, and particularly as their displacements interaction with that of the hull itself, and the overall effect on displacement generated waveforms.

This in turn led me back to discussions I had seen on twin keel sailing vessels:
a)http://www.kastenmarine.com/roll_attenuation.htm .....a few points emerge from Lord Riverdale's research that are more subtle, and that may be of some interest to those experimenting with Twin Keels, as defined by Lord Riverdale, on power vessels and motor sailors... The most successful Twin Keels were located approximately amidships, with the root of the keel beginning roughly 45% of the DWL aft, and the largest section being perhaps 60% or more aft. Forward twin keel locations were not favorable, as that location tended to provide poor steering characteristics and a poor wave form at speed.
b)http://www.kastenmarine.com/bilge_keels.htm .......The twin keels on Boojum are absolutely huge in proportion to the size of the boat, and also by comparison to the usual concept of "bilge keels." In addition, tank tests on other vessels having twin keels have shown some "wave-cancellation" effects provided by twin keels. In other words, there is a tendency for the displacement of the twin keels to help "fill" the 'midship wave trough’ at speed. The relative benefits of this "wave cancellation effect" seem to depend on placement of the keels and their size.
c)http://www.brayyachtdesign.bc.ca/ ....under "The Advantages of Twin Keels" .......The wave pattern reshapes to reduce the fore and aft crests. At hull speed a hollow forms amidship, but the bilge keels cause a wave to form in this hollow, canceling out the stern wave and giving a flatter wake. This increases the maximum speed of the hull, as much as 15 - 20%, in the same way that a bulbous bow or stern bustle works; by reducing wave making resistance at hull speed where it constitutes 85 - 90% of total resistance. To ensure the desired effect is at cruising speed the correct fore and aft placement of the keels and proper proportions must be checked by model testing. This placement is critical, as the model data shows. Too far forward or too far aft and the resistance will dramatically increase. .....We tested a model of a 37'-9" motorsailer, first with a deep fin keel (6'-4") and then with shallow draft twin keels (3'-9"). The vessel was to have a top speed under power of 14 knots. The difference in speed between the single and twin keel version varied from 3/4 of a knot at 6 knots and 14 knots to 1/4 of a knot at 10 knots for the same power applied. Still we felt we could do better. After re-reading the previous test results and information on other twin keelers we decided to place the keels 2'-5" farther forward. This was as much for convenience as anything else as the forward keel bolts now became the aft ones, a new set drilled forward, and the now useless after most ones plugged. With all our hopes raised we again ran the tests only to find that the resistance had increased from 2% more at 6 knots to 93% more at 14 knots, for this twin keel placement over the previous twin keel position. It was at this point that our project ended unfortunately….


It would appear as though the fore-to-aft placement of twin keels would definitely favor the more aft placement.

 

Hi Everyone, just a quick note of information re "Bluebird of Thorne". I have started digging thru the plans/ papers that I received with "Bluebird's' doppleganger "Kallista" and I am excited by the prospect that I have the plans that have come from Lord Riverdale's personal papers. The big news is that all the plans state that the design and tank testing data remain the property of Lord Riverdale. Although now deceased seven years it may be possible to track down the tank testing data thru his estate instead of the late Arthur Robb.

 

Mr Williams was kind enough to forward some under waterline photos to me so I thought I would post them

 

Canting Ballast Twin Foils

Just as a cross reference there are some very interesting discussions at CBTF (Canting Ballast Twin Foils)

 

Hi Everyone

Regardless of different views and theories on twin keel research, I believe, as a cruising sailor, it is the best way to go.

Everything in live is a compromise. Even our beloved sailing boats.

I for one wants the following in a blue water cruiser;

1. Minimum leeway
2. Damped rolling motion
3. Able to beach my boat upright at low tide for under hull repairs, cleaning or painting with no fuzz
4. Shallow draft

All this attribrutes can only be obtained from a twin keeler.

Additional induced drag of twin keels is a small price to pay for the above benefits. Remember, it is a cruising boat, open another beer and enjoy the ride.

Fair winds

Wynand Nortje

 

I for one wants the following in a blue water cruiser;

1. Minimum leeway

Great for racing , but the wetted surface to do this will create a slow boat in light airs , which is a real pain on any voyage.

2. Damped rolling motion

All keels do this to some extent , bilge keels have not been shown to do it better.

3. Able to beach my boat upright at low tide for under hull repairs, cleaning or painting with no fuzz

Any boat can be beached with sheer legs set to the chain plates .
These can work on less than level bottoms with the pull of a line.

4. Shallow draft

The Bilge keels will have a far higher draft than a boat either with a long low aspect ratio keel , or a centerboarder. There have been many southern ocean worthy boats that use boards , for the lower wetted surface running and shallow draft and EZ beachability.


Lots of ways to get your desires , twin keels have compromises like all the rest.

FAST FRED

 

Just another question

Hi, What will happen when the root cord of one of the keels leaves the water? Is the keel no longer a foil with end plate (or a very reduced foil) and just an plate carrying a ballast? If so will it be a dramatic drop in lift.....

Could this be a problem on a shallow draft quite flat bottom hull....

Rgds
Ola

 

Mike, was that Melbourne designer called Wright or something and was one of his boats Warlock, about 36-38'? Was the mag Cruising Skipper? He also had a design of a 45-ish boat published in Oz Sailing IIRC.

I worked for the CS group in those days and I sailed a Crossfire 20 (IIRC) against Warlock in Middle Harbour twilighters. I think I recall articles saying that the boat was a flyer. I can recall that in fact it never seemed anything special whatsoever as regards racing performance.

I'm not saying it was a nasty boat and racing in upper MH is not a great test-tank, but there was never any evidence that it had a huge amount of speed.

PS Wynand - all those attributed can be obtained in a cat, or a centreboarder, can't they? Have you see Andrew ('Aussie') Bray's design by Max someoneorother (Risely????) or various French centreboarders? They're not lightweights.

 

Dear 249 thanks for your post

Yes you have the gentleman and the article.I'll dig it up again and post the full details and his article here.

You wouldn't have an approximate date or issue number for this would you? I really want to know what research he did to back his design work.

Could you please email me your contact details mjeng1@supernerd.com.au

Cheers
Mike

 

What about steering behaviour when rolling?

If twin keels are canted like a Scow's boards to provide lift to windward, how might this affect steering behaviour on a roll-ey run? Could it make the boat squirrely? Also, what about drag downwind?

 

Another comment , while the bilge keels will allow standing upright in a tidal harbor , they also are impossable to heel over to get unstuck WHEN you run aground.

With a fast dropping tide this could lead to many complications.

FAST FRED