Major u turns in sailboat design?

"... Bulbed keels are less efficient (i.e. have more drag for a given lift) than a plain keel. In order to pay for this drag you need a bigger rig.... which needs more righting moment....which needs a lower CG (weight being fixed)...which means a longer keel...or a bigger bulb....."

I have two questions about this statement:

1) A properly designed bulb can help control tip vortices off a fin keel. Isn't it possible that the drag associated with those vortices can be greater than the drag associated with the bulb? Furthermore, tip vorticies rob a keel of lift, with increased pressure on the low pressure side of the foil. The winglets on airplanes wings are designed to improve the efficiency of the wing and use less fuel. That is why jet airliners have them. If a bulb helps control tip vorticies, then it improves efficiency.

2 I believe a bulb keel can be more efficient than a fin keel through the increased righting moment. If the keel stays more perpendicular to horizontal, it requires less planform area to provide the same lift, as compared to a fin keel. Less planform area translates into less drag. The high aspect ratio keels are usually less than 2% of sail area. Most fin keels are 3 to 3.5% of sail area.

Very interested in your thoughts on these issues.

 

Dear Mr.VEGA,

this kind of bulbe is made for three reasons!

-facility to stock the keel for the keel builder and the boat builder.that stand up alone .

- Resistance of a crash with a rock under the water. It also not crush it.
- You can still dock the boat on the keel ( if the structure is also made for)

That's why you can see this form family on cruising boats.

Concerning a real bulb keel . the lead is only in the bulbe. that's the most important thing .the foil is in fiber with a steel structure inside.

keep in touch!!

 

Square tops

A U turn occurred with square top mains. The Moth class had hockey stick masts in the sixties and early seventies and then got rid of them. They have made their way back in over the last decade or so as materials have gotten better. I think Manfred Curry did something with hockey stick masts in the 30's CT249 would be better than me at this.

U turns will happen when new developments allow a reappraisal of a previous idea that has fallen out of favour. I am about to put a carbon tip gunter rig on my 19ft trailer sailer cat as new slides and good tips allow the gunter to work well (I hope). Frank Bethwaite thinks it is a fine idea but I wouldn't do it if better track slides weren't available and carbon tubes were getting pretty common - mine comes from broken surf boat oars.

Centreboards have come and gone, and so have ketch rigs. In the 80s Peter Blake and Grant Dalton went back to the ketch rig when the rating rule and the course showed an advantage. Modern developments swung the pendulum back to sloops.

cheers

Phil Thompson

 

All sailboat designs are influenced by trends and fashions. Every single one. That is the whole point of the game. Sailing is not a necessary activity (although many may argue semantics that they must sail, including myself).

There is no single purpose to a sailboat other than emotional fulfillment. And as such, different people seek different paths to happiness. Different strokes for different folks.

 

1) No, tip vortex drag can alway be made less by increasing span and taper. What you are thinking of is that for a given span, increasing lift increases tip roll over. Aircraft use winglets not because they increase efficency (lift/drag), but because they increase maximum allowable gross weight for a given span. This allows more fuel or passangers without having to have larger wingspans which allows them to use existing gate spacing and runway widths at airports, and is a HUGE economic concern. From the drag point of view, Voyger, the Fosset Round-the-World-Unfueled aircraft actually got better fuel consumption (i.e. lift/drag) after she lost her winglets than she ever did in testing and while they took-off thinking that they would n ot make it around, the better drag that occured after the winglets broke off during take-off allowed them to get around with fuel to spare. For a complete discussion of winglets, see Heorner FDL. Soooooo... you ask, if long span and taper are good, why don't we see more such things in sailboats? The answer is that while a powered aircrafts fwd speed is large compared to it's yaw, a sailboats fwd speed can be small compared to it's pitch. Too long and too thin is constantly stalling is a seaway due to pitch effects.

2) You may be correct for a perfect world here, but you would be sadly mistaken in the real world. First, the interference drag between a modern canoe hull and a fin keel accounts for ~15% of the total drag. Adding a tip plate would increase drag by about 7-10%, a well designed bulb increrases drag by about 3-4%. This is why you only see plates on draft limited shoal keels or high aspect keels that cannot be increased any deeper due to pitch effects. Again, these can be looked up in any good hydrodynamic text. Secondly, as I pointed out above, boats roll, pitch, and yaw. As these rates change with the distance from the instant center, so does the delta V over the foil. That is why you often see a less "high performance" crusier slogging to weather better that a racer in gusty confused conditions. Lower aspect sails and keels perform better variable conditions and angle of attacks than high aspect ones. Look at bird and aircraft wings, or fish fins. There are practical limits for effectiveness depending on the operational conditions.

 

Jehardiman, regarding this:

That means that the same hull, having all of its ballast on a bulb has slightly more drag (compared with a fin) but the boat (same ballast) is also able to carry considerable more sail (due to an increased RM due to a lower CG). If this extra power is capable of more than compensate the loss of speed due to the added 3% drag than we will have a more efficient combination. Or not?

You reply about it (in a very interesting post):

And I can understand that, but you talk about “gusty confused conditions…and say: "Lower aspect sails and keels perform better in variable conditions”..

But what about “normal conditions”, I mean a steady wind?

If bulbed keels don’t perform overall better (even if they add drag) I have difficulty in understanding why all pure racing boats and most of the cruiser-racers have them.

 

Funny, I NEVER see this. An equally well sailed "racer" is always better upwind than a "cruiser", regardless of condition.

If your hypothesis was correct then "racers" in areas with "gusty, confused conditions" would have low aspect keels and rigs. Where do you see this?

 

Vega, I read lemstevens comment as saying that the increased weight and righting moment causes less heel which alows greater side force even though the lift is lower; and this is not the case. Making an "average" sized 10m racer 0.3m (1 foot) wider in the beam will add far more righting moment than any bulb. Which bring up your comments

It is the wind (heel) and the seaway that causes pitch response. Steady or gusty really doesn't matter, other than add additional Y velocity which causes even more variance in AOA which is less well tolerated by high aspect foils. For lightweight hulls (i.e. as I said in my first post those that have weight available for bulbs), there is a need to get weight distributed in the hull so as to manage for a good Iyy in pitch response. And a bulb is a good way to tune that response. Others wise, for the second part of the comment, I don't see "all" boats getting them over here and it may be a "band-aid" function of the artifical requirments of making a certian STIX in order to get a sellable class rating over there.

As for Paul B's comment:

PPFFfffffftttttt!!!! You never see it because those "racers" aren't out in those conditions! One of the major complaints even before I left the Bay area in '94 was that the "big boat" IMS and IRC racers were taking over the "little boat" courses inside the Bay. Too bumpy outside.

No, I've often sailed and seen a well balanced older CCA "cruiser" roll a modern lightweight high aspect "racer" in conditions inside and just outside the Bay. Bumpy? Yes! Wet? Yes! 2 reefs and a blade? Yes!

I think a bigger question is: "Are modern racers measuring themselves against the right stick?" Here is an recent article http://sailmagazine.blogspot.com/2007/09/986.html
that I found ineresting....one of the comments was the need to have wind and smooth water.

 

Pull the other one.

First, assuming you are talking about SF Bay, in 1994 there was no IRC class. There wasn't one anywhere. IMS was almost non-existent in the USA, only used in a few big events like BBS. I know, I did race IMS there during those years.

Next, what "outside" race courses are you talking about? What clubs ever had start lines and marks set outside the gate? There have always been races like the Farallons, but those races start and finish inside. BBS was always inside, and it can get very "bumpy" there, given a big breeze day and a big ebb. On those types of days we would be out there racing and would not see too many of your beloved cruisers bashing up the city front with us. Make that none.

Please tell us what boats were involved where a CCA type "rolled" a modern boat of the same size in bay racing? You take whatever CCA design you want, we'll stack up a same sized modern bulb keel racer next to it in any condition from drifter to buster, and you know who is going to sail away. At least you should know.

An old S&S design won the Hobart race last year, but they sure didn't sail boat for boat with the modern designs of the same size, regardless of how big the weather got.


Of course you misrepresent what the referenced article says. It merely says the Chesapeake has the condition of wind and flat water. It does not say the boats "need" to have that.

 

There is a trade off there, but normal yachts are nowhere near it. Real world constraints for making longer span are draft and more complex/expensive engineering.

Have you looked at model yachts? Very long span. They also have ultra thin foils which stall very easily. They pitch like a rocking horse, but still goes to windward like there's no tomorrow.

 

Modern Gaff / Wing Tip Rig™

----------------------
K76, the cute picture of the Optimist rig in no way whatsoever depicts the "modern gaff" I described earlier except in the fact that it has a peaked up planform. That planform is superior to a normal "squaretop" provided it is part of a relatively high aspect sail and that it uses the gaff system I described earlier with an UPPER OUTHAUL.
On model yachts the upper outhaul/modern gaff(wing tip rig™) has been proven in extensive two boat testing to be superior to a fully roached main of the same area. The other advantage of the modern gaff (wing tip rig™) in model yachts is that it allows
a more rectangular planform w/o the need for full battens.
To summarize the advantages are:
1) superior planform in the "peaked up" version,
2) superior sail shape control with the upper outhaul,
3) the advantage ,particularly in model yachts, to have a more rectangular planform w/o full battens.
4) the advantage, shared with a "normal" square top of having automatic gust responce w/o a complicated rig.
a) twist range control via a more powerfull than normal vang or vang/mainsheet combination.
Wing Tip Rig™

 

I had thought... that is to say it was explained to me... that a large part of the reason for bulbed keels is that they help get weight down low when draft is constrained by a rule.

From there... if the "name" racers have it, then it becomes a marketing point.

And, of course, even when not constrained by a rule, you become constrained by the bottom. That is to say (and this is especially true in the Chesapeake) it is always easier to find a dock with a five foot draft than it is an eight foot draft.

Having said that... most of my knowledge/research revolves around the international rule. As I remember the explanation behind, say, Australia II (KA-6), it was found that the swept angle was important, but by "reversing" it (i.e. giving the bottom of the keel more length than where it attaches to the boat), and adding a bulb, more weight could be kept low. Again, the International Rule penalizes draft. The problem with this "reverse/bulb" keel was that not only were stiffness and lift increased, but drag was increased as well. The wings were added in order to "manage" the tip vortices of that huge bulb.

Not often considered when discussing the '83 race... the "other" design advantage of KA-6 had to do with the LWL/SA trade off. That is to say, in general terms, under the rule, greater LWL will mean less SA. If you compare KA-6 to Liberty (US-40), or for that matter any of the other Newport 12s, KA-6 is a bit shorter, but has more sail area. September (or for that matter, most of the summer) in Newport tends to light air. While the longer LWL tends to make a faster boat, trading that off a little bit for more sail area meant "more horsepower" in the conditions: a major factor in her improved acceleration out of a tack... of course to use that, you needed a stiffer hull...

Then... compare the "Newport 12s" to those used off Perth. Stars & Stripes (US-55) was a bit longer and heavier, but had less sail area... In the high(er) winds off the Indian Ocean, this was a better answer. The days where US-55 did poorly tended to be lighter air days. It was said that US-55 did not really get moving in less than 15 knots.

As an example of the reverse sweep/wing that I'm speaking of, consider this photo of the 8mR Octavia (USA-37/ex KC-30). This is actually her third keel, but I think it shows the design points well.

 

I believe they will bring the weight down if the draft is constrained by a rule or not....but draft are always a limiting factor. Even Ocean racers need to enter into the Marina .

I believe that one of the tendencies to come is variable draft bulb keels. We will see it on the next America Cup boats, you can see them already in some very fast ocean cruisers. They make sense and their are a lot easier and less expensive to build than canting keels.

 

bulb keels

I see two reasons for the resurgence of bulb keels.

1.) The need for lighter ballast on trailerable boats that had to be pulled by under powered cars during the 1980's', and

2.) The canting ballast keel.

Originally, in the 1980's, the bulb was only intended to improve the range of stability on an otherwise wide and shallow hull with great form stability. So, in essence, the bulb added little if anything to the sail carrying ability of the boat. This is because, when the boat heeled over enough for the bulb to start adding significantly to the righting moment, the wide shallow hull would be heeled past its most efficient underwater form.

Around the same period, canting ballast keels were first being played with.

Meanwhile, back at the extreme ocean racing circuit, narrow hulls were being used with deep bulb keels. This made a lot of sense because, when the boat heeled over enough for the bulb to significantly add to the righting moment, about 30 degrees or so, the very symmetrical under water shape was effected hardly at all. This made a relatively fast, easy to control, hull that had plenty of reserve stability. And, out in the ocean, you could make the keel as deep as you like.

Allowing transverse, movable ballast in these races, with the stipulation that the movable ballast not heel the boat over more than 10 degrees in static conditions, encouraged beamier hulls.

Adding a deep bulb canting keel to this hull type did not take a great stroke of genius. It was all but inevitable. Now you got the advantages of both high form stability and transverse movable ballast compounding each other. This is because the keel can be canted to windward, making the ballast effective before the hull even heels at all. This, as we all well know, allows for some extraordinarily fast sailing yachts.

Not only that, but this system is arguably safer than the old shifting water ballast it replaced. This is because the shifting water ballast usually had to be above the water line in order to be far enough from the centerline to have much effect.

But these new boats made the narrow ones, with deep, fixed bulbs (arguably a safer system yet) instantly obsolete.

The big advantage I see to deep bulb keels is that they allow a lot of righting moment for a minimum amount of ballast.

So, if you want to blame anyone for the resurgence of keel bulbs, blame the cruising community.

They started it.

Bob

 

Many years before the Open 60s had a canting- keel (or canting keels were common) they had already a deep bulb.
The deep bulb was essential to give the boat a decent final stability (and AVS) and to diminish the inverted stability.

I agree that regarding cruising, only light modern cruisers have a substantial benefice with a deep bulb. Regarding Racing, I am quite sure that the gain in weight more than compensates the drag loss.

For instance, on a Pogo 40, the difference in weight on the bulb for the same RM curve, between a 3m draft bulb and a 2.2m is about 400kg. That’s huge for a 40ft racing boat that only weights 4.800 kg.

Even for a moderately fast production cruising boat, like the Jeanneau 42i (with a keel that has not properly a bulb but that has more weight on the bottom), the difference between the weight of the swallow draft version (5’3’’) and the deep draft (7’7’’) is 576 lbs. in a boat that weights, in its lighter version 17 995 lbs.

Not a big difference, but we are only talking about a 2’ 4’’ difference in draft on a partially bulbed keel. If we were talking about a full bulb and 5’ difference (lifting bulb) that difference would be more than double and we would be talking of more than 1000 pounds on a 17 000 lbs boat.

If we were talking about a boat like the 47’Azzuro ( lifting bulb keel) that displaces only about 22 000 lbs, the proportion of the saved weight versus the boat weight (1.5 to 3m draft) would be much more significant.