drag differences between daggerboards and centerboards

[PAR]

Here we go again, look race course configurations, even within the confines of this class, will vary dramatically and the most important factor isn't considered, which is how well the boats were sailed. You can say what ever you like about race course results, but there are are way to many variables, for it to be an effective comparison of this single issue.

[Paul B]

Originally Posted by PAR

the most important factor isn't considered, which is how well the boats were sailed.

What hilarious statement!

If two sisterships sailed for a couple of years int eh fleet and traded places pretty evenly, then one boat was modified from CB to daggerboard and suddenly was far superior, well that seems to take the "most important factor" into consideration.


Let's see, I owned a pre-'75 Rule boat, did development work in the class, and followed the rule changes pretty closely for almost 20 years. Maybe, just maybe, I might know something about the subject.

I never noticed your name in Planing On in those days. Maybe I missed the accomplishments that provided your expertise.

[PAR]

Once again, you've found a way. I didn't bring up the race course, you did, I in fact agreed with you initially, then you changed the parameters and refined you statement to include just two boats, now with a long history. Is it you sole intention on this sight to specifically find ways to butcher discussions into your personal diatribe on whatever subject you care to flip flop on? You start rants, then you bait and switch, this thread a classic example with the seemingly innocuous comment about a whole class, then once the hook is set, redefine the debate to a couple of boats with a history against each other. More threads have been locked to your rants than I can imagine and this is one of you classic methods. So, what skipper decisions are you going to give to the drag of the board slot and what drag in the board slot preformance degradation, are you going to blame on the skipper's lack of skill? Your point about many races on the face of it seems reasonably, but unless the crew and skippers swapped boats every race, the races only prove which boat generally runs at a high S/L average, per race.

[jehardiman]

Originally Posted by cardsinplay

Truth is, the factory centerboard version is slower than the daggerboard boat. It becomes readily apparent when they are sailed together.

Originally Posted by CutOnce

Thanks for the answers. I was kind of expecting the real world differential to be minimal with a well-implemented gasket.

There won't be autobailers (self bailing cockpit through transom) so that isn't an issue.

There are distinct on-the-water advantages to a centerboard, other than damage mitigation - being able to adjust the CLR aft is nice, as well as adjusting the area of lateral resistance. Also helps when you rake back the rig in wind.

Daggerboards are generally left down all the time, except when reducing area in heavy winds by pulling up the board a couple inches.

It's really worthwhile to have all of you folks to bounce the issues off - thanks!

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CutOnce

Originally Posted by Paul B

If two sisterships sailed for a couple of years int eh fleet and traded places pretty evenly, then one boat was modified from CB to daggerboard and suddenly was far superior, well that seems to take the "most important factor" into consideration.

I think here may be the issue. How the boats are sailed and how the two different systems are affected at different amounts of board out.

Cutonce, while in crusing you may put the board in or lower it and leave it down, in racing, the board is being constantly adjusted not just for CLR (which both types of boards will change), but to remove wetted surface. Because a CB swings and a dagger rises, the daggerboard is a more efficient lifting surface (maintains higher AR) as well as having less wetted surface when half retracted. Of course, board weight is also going to effect this, larger heavier boards being moved less. Additionally, in very small boats the very size of the trunk may play a significant role in performance just due to weight, CG, and mass moment.

While the actual drag of the two board systems would be insignificantly different when both boards are exactly the same shape and both fully out, there would be real differences in performance in how they are used in practice.

[Paul B]

Originally Posted by PAR

Once again, you've found a way.. blah blah..crazy talk..blah blah.. high S/L average, per race.

If getting schooled on an internet forum caused me to be twisted into a ball of anger, confusion, and frustration until the wee hours I would seek professional help.

[CutOnce]

Mr. Hardiman

I sail in a high performance skiff fleet - predominantly I-14s. As a rule, people don't play with their daggerboards once inserted unless the wind starts to blow dogs off chains. These are all trapeze boats with asymmetric kites and there really isn't a lot of time available for board adjustments as you are either well ahead of the boat or you are swimming. Typical daggerboards are pretty small and high aspect ratio, so reducing LR area isn't a huge factor.

There isn't a cruising mode possible - either you are on the gas, or righting the boat. The boats are only really stable when at speed.

I used to sail 5o5s, and they seem rock stable like the Queen Mary to me now. That's part of the appeal in this boat design - it should be possible to sail in a "cruise" mode and the centerboard will make it a little more flexible. I'd like this boat to be able to stand up at a dock unattended if possible.

My real concern is that there isn't too much of a penalty to the centerboard - more than 5% slower would be unacceptable.

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CutOnce

[Paul B]

Originally Posted by jehardiman

While the actual drag of the two board systems would be insignificantly different when both boards are exactly the same shape and both fully out, there would be real differences in performance in how they are used in practice.

I think you are correct in stating that there are many reasons the daggerboard is more efficient than a CB.

I disagree with your statement above (in red). Even the best mylar gaskets move and flutter and allow surging in and out of the CB slot. This is draggy compared to a solid hull material, and the knock on is more than the previously claimed "1/20 of a knot".

[gggGuest]

There are various ways you can mimimise damage on impact with the ground, most notably having the board in a cassette and having something like a styrofoam crash box behind that can be replaced without having to disturb the main structure. Better of course not to hit the bottom.

Dagger board boats have of course far less clutter, and rubbsih in the cockpit which is good. The inconvenience, hassle and maintenance overhead of slot gaskets is a clincher for me. Never again.

[cardsinplay]

Returning, again, to the original post of this thread, I note that there is no concern at all for what happens should the boat be raced. He mentions no such intention. Please see below

Originally Posted by CutOnce

I'm in the process of refining my design brief for building a low cost cartoppable home build...

Given this will be a car-topper, and a boat friendly to spur of the moment sailing anywhere, I'm wondering about the real world drag differences between a well build daggerboard slot and a well built centerboard trunk aperture. If using a centerboard, I'd certainly install mylar or whatever overlapping gaskets to seal the slot.

Some of the places I want to use this boat have rock shores, reefs and beaches, so a daggerboard will require serious attention to avoid major hull damage.

My concern is how much of a performance penalty will a kick up pivoting centerboard place on the design? Is there any studies published? Any design concepts worth hearing about on how to minimize drag?

Clearly, CutOnce is looking to construct a daysailing car-topper for pure pleasure sailing in a varied and potentially obstruction filled environment. This fact, mentioned quite strongly in the second paragraph truly reduces the value of a daggerboard, even if it does have a crash box.

The indicated maintenance factors of a centerboard are still a quite minor process compared to the ongoing business of raising and lowering the daggerboard and the repair issues associated should it take a hard grounding.

I prefer that he design and build a centerboard for this intended boat, get it out on the water where he can evaluate the convenience issues when weighed against the somewhat lesser degree of pointing ability. Then, he can either remove the centerboard and put in a daggerboard trunk, or simply proceed to enjoy his boat for its intended use.

Sailing in the waters around Seattle, I frequently have to watch for submerged rocks, sub-surface logs and forest debris and sometimes unpredictably shallow water when approaching the rocky beaches in the area. I have a centerboard equipped boat and for the gunkholing style of sailing/camping I do, it's the absolute best answer.

[jehardiman]

Originally Posted by Paul B

I think you are correct in stating that there are many reasons the daggerboard is more efficient than a CB.

I disagree with your statement above (in red). Even the best mylar gaskets move and flutter and allow surging in and out of the CB slot. This is draggy compared to a solid hull material, and the knock on is more than the previously claimed "1/20 of a knot".

If you are seeing a lot of difference, then the gaskets and/or opening are designed wrong. I have designed slot opening in fairly fast vessels and have measured insignificant drag changes. Do you have nice sharp corners on all sides?

[ancient kayaker]

For a car-topper weight is crucial unless the boat is to languish unused after only a few trips out. A centerboard trunk is bigger therefore heavier and the centerboard cannot in general be quickly removed. If the centerboard is unweighted it requires a pull-down as well as a pull-up unless the trunk is open-topped. Also a car-topper is - by definition - a small boat so the smaller daggerboard trunk conserves scarce space.

The self-retracting feature of the centerboard is nice, but it might be counter-productive in a blow if you get shoved sideways further into a rock-filled area; in that situation the daggerboard boat will usually stop and wait for you to lower the sail. Which design is preferable probably depends on the location; with isolated rocks near the surface you can slide over with a CB.

Although there are exceptions center-boarders are typically at the lower end of the performance range although there is nothing wrong with that.

My sailboat’s daggerboard has it’s upper forward edge cut back at an angle as Par suggests so it pivots forward at the moment of impact. It is held down with a wedge between board and the aft end of the trunk so if it hits something hard enough it pops the wedge loose. The board is fairly light weight and floats up. Maybe at speed it would work loose but that has not been a problem so far - I have provision for a bungy cord hold-down which I haven’t used yet. The leading edge of the board is protected by brass stem band at its lower end.

As an added convenience, when the board is removed the daggerboard trunk slot accepts a small wheel mounted on a short board, converting the boat into a wheelbarrow, with the oars as handles. On the water the wheel resides in the boat, a location where I prefer not to go in such a small boat!

Of course, there is the other option that nobody has mentioned as far as I know, i.e., the infamous leeboard ...