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Laser Foil Details sought

kmisegades

Swamp Goose
I recently developed new efficient daggerboard and rudder for my Sunfish. See aerosouth.net A friend in Florida has asked me to do the same for his Laser. I have the overall dimensions but need yet details on the foils used for both, as these are the baseline for my design. I used the same approach on the Sunfish and the results were quite good. I do not own a Laser (yet) but was hoping someone might know what foil is used on the current board and rudder. The drawing in the rules show foils that are much thicker than the actual shape, which should be around 10% thick based on the constant chord. Thanks in advance.
 

LaLi

Well-Known Member
Just measured my older centreboard which I have here at home, and it's 340 mm across (about 364 along the flow/parallel to the bottom edge) and just a bit over 31 mm thick. You can't change the cross-section much because the case opening is similarly foil-shaped, and there's no point really in trying to "improve" on it anyway. It's good enough the way it is.

The rudder is whole another matter. I am fundamentally against "improving" one-design boats against their class rules, but it's also a fact that Bruce Kirby has said that the rudder blade is the only part of the Laser that he'd re-design; elliptical planform and deeper, with 33 % more area. If you want something along those lines, you'd probably want to find a Laser rudder head and start from scratch: the only relevant measurement of the legal foil is the 20 mm thickness.

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kmisegades

Swamp Goose
Just measured my older centreboard which I have here at home, and it's 340 mm across (about 364 along the flow/parallel to the bottom edge) and just a bit over 31 mm thick. You can't change the cross-section much because the case opening is similarly foil-shaped, and there's no point really in trying to "improve" on it anyway. It's good enough the way it is.

The rudder is whole another matter. I am fundamentally against "improving" one-design boats against their class rules, but it's also a fact that Bruce Kirby has said that the rudder blade is the only part of the Laser that he'd re-design; elliptical planform and deeper, with 33 % more area. If you want something along those lines, you'd probably want to find a Laser rudder head and start from scratch: the only relevant measurement of the legal foil is the 20 mm thickness.

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Thank you sir, this is very helpful. The Laser has a daggerboard though, not a centerboard? Perhaps our terminology is a bit different.
The Sunfish had a similar limitation on what could be done to the daggerboard given the fixed dimensions of the trunk slot. Nevertheless I was able to improve the critical CL/CD ratio by 20%, through a slightly different foil and a better planform that produces a near-ideal elliptic force distribution. The rudder allowed more flexibility and here I am seeing a 30%-40% improvement in CL/CD, ie lower drag for the same side force. In both my new 'Sabre' daggerboard and rudder my 'stall' behavior is better/softer, ie the loss of lift/force with increased angle is less abrupt. I suspect that the existing Laser board and rudder fin use a symmetric NACA thickness distribution. Not having access to one at the moment, this is just conjecture. I am sure I can find one here on our lake though and figure it out. All my work is focused on recreational boaters, however I am designing the fins to fit existing hardware and dimensions to make a retrofit easier.
 

kmisegades

Swamp Goose
I guess it is how you first learned the terms. We use 'centerboard' for a device that pivots, and a 'daggerboard' that slides through a trunk. They both have the same purpose, of course, but some subtle differences that will affect usage and performance.
 

Horizon

Member
According to Ian Bruce's chapter on the Laser in "Small Boat Racing With the Champions (Edited by Bob Fisher 1976), the Laser centreboard is a NACA 0009 section.
 

kmisegades

Swamp Goose
According to Ian Bruce's chapter on the Laser in "Small Boat Racing With the Champions (Edited by Bob Fisher 1976), the Laser centreboard is a NACA 0009 section.
Thank you! That makes sense as it is a popular airfoil for airplane control surfaces.
 

Andy B

Member
I made one to the Bruce Kirby design and it sails like a dream. It changes the boat completely no more weather helm and the rig is so well balanced you can sail upwind and take your hand off the tiller. It is a bit like sailing some of the Lasers modern competitors. I wish ILCA would consider this again as part of the new rig package.

A word of warning, the foil must be strong, my first attempt snapped in a 5 with a 4.7 rig.

IMG_5575.JPG
 

kmisegades

Swamp Goose
Nice. You made a new rudder fin, correct? What materials did you use? Are there details on Mr. Kirby’s design? Thank you.
 

LaLi

Well-Known Member
I made one to the Bruce Kirby design

View attachment 35629
Interesting. My understanding is that a prototype that was tested in New Zealand around 1999 looked very much like that. Is that actually designed by BK himself? Do you have drawings?

The reason this is interesting is that ILCA was actually going to change this at the time. However, it got kind of buried under the control system changes, and then forgotten. Of course, it would be interesting to hear what Tracy (Usher, president) or Aileen Loo (chief measurer) would think of this as a future project now.

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kmisegades

Swamp Goose
Interesting. My understanding is that a prototype that was tested in New Zealand around 1999 looked very much like that. Is that actually designed by BK himself? Do you have drawings?

The reason this is interesting is that ILCA was actually going to change this at the time. However, it got kind of buried under the control system changes, and then forgotten. Of course, it would be interesting to hear what Tracy (Usher, president) or Aileen Loo (chief measurer) would think of this as a future project now.

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I will look at both designs in comparison to my clean-sheet Laser rudder fin, which will likely appear similar to the ‘Sabre’ rudder fin I’ve designed for the Sunfish. See some details at Aerosouth.net.
 

Andy B

Member
MK1 was made using ply which is only 18m thick. This did tell me the profile is critical and it hummed so loudly the other boats could hear me coming. I took the profile down to the aero 9 foil section and it was brilliant, but it made it a bit thin and it snapped. Admittedly the ply was only WBF and it might have been better with marine ply. MK2 was made using a laminated foil from a larger dinghy I picked up on ebay and resized and profiled. I decided this was a better option than 18mm marine ply as it is 20mm thick and stronger. As has been pointed out the vertical aspect removes some of the weather helm but the additional area gives a positive feel to the tiller and tiller movements are much more gentle. Using a basic see saw analysis I calculated the additional area on the rudder moved the two foils centre of effort aft by about 5 inches.

In terms of design I applied BK's statement in the following way. I decided that if you are using an aero 9 section and the stock takes a 20mm fin this defines the width of the blade and I then made the under water section 30% larger.

It does make the boat a real pleasure to sail.
 

Eyeper

Active Member
Here's what I did about 25 years ago. "Repurposed" a Hobie 16' rudder to fit into the stock Laser head. It's made the boat much more balanced and cut weather helm entirely. I don't race, but I do go fast with it.
 

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LaLi

Well-Known Member
"Repurposed" a Hobie 16' rudder to fit into the stock Laser head.
Yeah, we've talked about that adaptation on this forum before, and it's quite close to my impression of what Kirby had in mind at the time.

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kmisegades

Swamp Goose
Very nice. I did something similar on my Sunfish, which comes with a highly swept rudder fin guaranteed to create weather helm. The foam-and-fiberglass process from aircraft designer Burt Rutan was followed. I kept the overall area and draft approximately the same. It lowered weather helm and allowed more power in maneuvering as its thicker section had softer stall characteristics. My new ‘Sabre’ rudder design shown at aerosouth.net adds an efficient planform, better than the typical trapezoidal shape. These are harder to design and make, unless one has flow simulation software and access to a 5-axis mill, which I fortunately do. This image shows my trapezoidal Sunfish rudder fin, an interim design between the original (very similar to the Laser fin) and my Sabre fin.
 

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