Daggerboard position: Maneuverability VS. Acceleration

Hi together,

I came accross an interview about the daggerboard placement with Martin Fischer on catsailingnews.com, see F18 Convertible: Daggerboard case placement explained by Martin Fischer.

He sais that ...

I guess the straight line speed is due to the fact that the rudder oparates in the downwash of the daggerboard. The further apart they are, the more both operate in an undisturbed flow -> less drag -> more speed, right?

But, I don't understant the part with the acceleration.
Why does a boat accelerate better after a tack when rudder and daggerboard are closer together, and what does "close" mean? Way behind the CE of the sails?


Furthermore he sais that ...

When considering force vectors from the sails and from the daggerboard the leverarm between these two vectors becomes greater for a wider boat - assuming a same position for the daggerboard. Thus, the board needs to move a bit further fwd in order to reduce this lever arm again. Am I correct in this assumption?


Perhaps someone can shed some light on this subject and explain why things are as described by Fischer.

Many thanks and best regards
Nicolas

 

Hi Nicolas,

Now this is an interesting thread. I must say that I'm pretty astonished that no answer has been made till now. Would it be possible that the Aura of Martin Fisher has some kind of internet wide influence ? Don't know...

Thanks for this article, that I've just read. And frankly speaking... I cannot explain the "vodoo magic" of this sentence :
"For a given boat speed the induced drag of an appendage is proportional to (Lift/span)^2. For our two appendage case we obtain the lowest induced drag for a given total lift for an equal lift distribution between daggerboard and rudder, hence the dagger case should be really far forward!"

I design rudders and daggerboard for fast sailing multihulls, on regular basis. It happened in some cases that I had the chance to cross Fisher's calculations with mine. And my opinion is that this statement is just wrong.

The definition that he gives is 100% inline with the standard definition of the induced drag. This induced drag is calculated for a given wing, without any reference to some spatial reference. In case of multiple lifting surface, you just make summations, but again, this summation do not take account into the interference drag, the only aerodynamic force that is dependent on the relative positions of the different wings (rudder, daggerboard) of the considererd body.
Induced Drag Coefficient https://www.grc.nasa.gov/www/k-12/VirtualAero/BottleRocket/airplane/induced.html
https://www.researchgate.net/figure/Induced-drag-definition_fig1_325517501

In the context, "an equal share of lift between daggerboard and rudder" makes some sense if the lift he is talking about has something to do with the lifting surfaces. (horizontal wing planes). Because, on the other side, the drift equilibrium is calculated taking the lift of the rudder equal to zero (balanced configuration where you do not touch the rudder to sail the boat), the daggerboard longitudinal position being choosed to be as close as possible as the one of the aerodynamic center of effort, while its surface is balanced with regards to the total surface area, at the expected boat speed.
So, in the context, the separation that will minimize the flow disturbance due to the lift produced by the daggerboard (vertical), and the lift produced by the rudder (vertical,again), shall be the vertical separation between the two.

My guess is that it has something to do with the relative position of the relative positions of the center of efforts of the horizontal planes of the rudder, of the daggerboard, with respect to the center of gravity of the boat. So I will drop the CE of sails in this reasonning.

I don't think so. Again, longitudinal positions of the CE of sails and CE of daggerboard are to be vertically aligned. While the leverarm that you refer to is the one due to the change of lateral position. To me, Martin Fisher was talking to Martin Fisher in this article, with obscure references of his own work in his own language ; which is, unfortunately not a scientific one.

A little retro-engineering of this article, to try to make sense of all of this, may I propose this :




In a second time, I hope I will come to an explanation for statements in relation with "acceleration". But frankly speaking, I find the explanations from M.Fisher very poor. His work on the foil is very basic, and every words that are reported in this article seem just big talk, hiding small (very little small) calculations that can be done by everyone, but embedded in some kind of awfull discourse, making everything appear like "black Vodoo Magic".

 

Dear Alan,
many thanks for your reply.

I guess the statement with the very far forward position was a hypothetical assumption, but not practical - as you also described in your drawing - in order to minimize the induced drag.
Since the article is about the daggerboard placement of F18 catamarans, he is definitively not talking about lifting surfaces - in the manner of horizontal wings / foils. This would be against the class rules.

M. Fischer also contributed to the development of the Hobie Wildcat. This boat and also some other designed after it, like the Nacra F18 Evolution or the Goodall C2, etc. - they all have the daggerboard pretty far forward, in relation to some older designes. I tried to replicate the LC of different sailplans and marked them as points at the daggerboards. At least to have a rough indication (assuming the graphics are more or less correct).

This is the Hobie Wildcat - The longitudinal center of the sailplan is estimated to be a bit forward of the daggerboard TE. (Image taken from - Wild Cat | Fiberglass Sailboats | Hobie https://www.hobie.com/br/en/sail/wild-cat/)



Here for the Nacra F18 Evolution - Also here the daggerboards are placed ahead of the estimated LC of the sailplan. (Image taken from - Nacra F18 Evolution – Nacra Sailing https://nacrasailing.com/boat-portal/racing/nacra-f18-evo/)



I mean this might be just for one sail trim condition, since the LCE of the sails will vary based on cunningham and main sheet tension, etc. But, in terms of Lead, there seeams to be nearly zero.

Maybe someone else has an opinion on how the position of the daggerboard affects maneuverability and / or acceleration after a tack.

Many thanks and best regards
Nicolas

 

My guess is that the slower acceleration, due to the dagger board being further forward, is because, when changing tacks, the stern has to scoot upwind. And that takes some energy out of the forward acceleration, afterward.

I once got first hand experience with this when I sailed an inflatable raft that I had converted to a sailboat. I had the lee board too far forward, and I was using a canoe paddle to steer it. The paddle was inserted through a hole in the transom board. With two people aboard, this worked fine. I actually amazed onlookers on how well this contraption sailed, especially upwind. But it had no sheet line. It had just a line that went from the end of the boom to the base of the mast, which I used as a sort of a handle. At less than 40 sf of sail, it was exhausting to sail. But it came about reliably (with the help of a few strokes of the canoe paddle to scoot the stern upwind).

But, with only one person aboard, it became an unmanageable brute. I was driven all the way to the downwind end of the lake, as I couldn't get the thing to sail. I'd paddle the stern upwind only to have it blown down wind, before the raft started moving. I had two theories of why this was:

1.) That the lee boards were not deep enough with one person aboard. One of the reasons I thought this might be the case is that I sat in the aft end of the raft, and the lee boards were forward. The aft pitch of the raft probably decreased the amount of blade area in the water. This, up until recently, was my favorite theory of why I had so much trouble. I had planned to make deeper lee boards and add a real rudder, but I never did because I got my hands on a real dinghy for a very low price. Looking back over fifty years, I wish I had stuck with that project.

2.) That the added momentum, given by having another person on board, maintained steerage way during the time the stern was scooting to windward. Once the boat was moving, the rudder would have the authority to keep the stern to windward. Had I not abandoned the project, I would have still never found out if this second theory was true, because, with the revised design, I moved the lee boards aft (the rudder took the place of the canoe paddle, so no more paddling the stern to windward).

I can imagine that a light catamaran might have this problem unless it is sailing very fast.

 

I think the difficulty to reaccelerate noted by Martin Fisher has little to do with induced drag. Imho the main reason is the boat with daggerboard CE forward of sail CE need lift from the rudders to sail an equilibrium course to windward at constant velocity (the CE resulting of lift on both appendices is in line with sail's CE).
In this case the boat has a tendency to develop a strong "weather helm" during the tack and to stay into irons because at low velocity the rudders don't develop sufficient lift to help the boat to turn and achieve its new equilibrium course to windward.

 

Hi everyone,

Thank you for your answers. I think I'm beginning to understand the subject.

Let's assume that the boat is travelling in a circular arc - in other words, any change of course. The longitudinal axis of the boat is not tangential to the circular track, as the boat is still drifting. The bow therefore points into the circular path.

(Sorry for the bad drawing)


The rudder and daggerboard generate forces in opposite directions and a resulting moment around the pivot point of the hull.

If the daggerboard is now far forward, possibly even in front of the pivot point, the resulting torque is the greatest and the boat turns the fastest / easiest. The boat is therefore very manoeuvrable.

However, if the daggerboard is positioned too far forward, in front of the CE of the sails, the boat tends to be turned into the wind by the sails when travelling slowly. The further the daggerboard is in front of the CE of the sails, the greater this effect. The boat does not pick up speed well or does not get through the wind at all.

If the daggerboard is further back, i.e. closer to the rudder and behind the CE of the sails, the boat is pushed to leeward when travelling slowly, for example after a tack, and the boat can pick up speed again more quickly.

The further the rudder and daggerboard are together, the lower the resulting moment around the pivot point -> manoeuvrability decreases.

In practice, as always, it's a case of weighing up the pros and cons, taking into account the balance and individual requirements of the boat.

 

"For our two appendage case we obtain the lowest induced drag for a given total lift for an equal lift distribution between daggerboard and rudder, hence the dagger case should be really far forward!

In real life we cannot achieve an equal share of lift between daggerboard and rudder because it would be very difficult to re-accelerate the boat after a tack or at the start"

Yes ... Yes ... Yes

If the rudder has the same Aspect Ratio as the centerboard then the load can be shared 50-50. (About 40 years ago the easiest way to fool a VPP (Velocity Prediction Program) was to tilt the mast aft to load the rudder more and install a longer rudder.)

And to achieve a 50-50 load sharing you have to load the rudder more by moving the daggerboard forward.

And that can lead to less ease of accelerating out of a tack.

 

The effect of the Beam, the difference between Monohull and Catamaran, can be understood by thinking with the help of this drawing by Pierre Gutelle

 

Here you can see it very well

We move the daggerboard far forward to load more a high Aspect Ratio (AR) rudder and distribute the load 50%-50% which minimizes induced drag.

(The appendage that grips the ballast, the lead torpedo, is moved to windward and contributes little to the lateral force.)

This solution is not so simple in the case of a catamaran, and it is also complicated to try to explain and understand it.

 

In the case of the Monohull there is another detail: how the pressures in the Hull and the Daggerboard fit together, as it is typical that there is an interference when the high pressure zone of the daggerboard coincides with a low pressure zone in the Hull. And moving the daggerboard forward is one way to get rid of this entanglement.

 

Hi Carlos,
Thank you for your reply.

I'll have a look at the information you provided and get back to you later.

 

Hi

"If the rudder has the same Aspect Ratio as the centerboard then the load can be shared 50-50"

I think this is not well formulated. It is better to say:

if the rudder is of high Aspect Ratio then the load can be split 50%-50% and that minimizes the induced drag as in his time a century ago in another context argued Max Michael Munk.