My improved dagger board and rudder

Thread starter #1
Dear friends, I recently designed a new dagger board that promises a 20% improvement in performance, based on the ratio of side force to drag compared to the composite/FRP board I am now using. I am currently making a composite prototype to test the design on my boat. When this is done I will share more details. I am turning my design attention now to the rudder. At 60% of the area of the dagger board and with a rather poor profile and planform, there should be much room for improvement. Is anyone aware of a more detailed drawing of the current rudder design other than that attached? My primary interest is on the hydrofoil's cross-sectional shape. I have reviewed a thread from 2017 concerning the need for more area (or not), the sweep of the rudder (weeds) and to reduce weather helm. I am not really concerned whether my board and rudder meet ISCA specifications, but both should fit existing hardware just fine. While my focus is mainly on improving L/D (ratio of lift/side force to drag) I am eager to know what other concerns people have. Many thanks. Kent Misegades, Seven Lakes, NC
 

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mixmkr

Well-Known Member
#2
I'll be eager to hear more about the db. I extended a Barrington board by about 14 inches, sanded it more into a foil shape and glassed the final result.
A little more effort yanking it out when beaching, and I could find the bottom easier. Didn't really notice an improvement.
 

beldar boathead

Well-Known Member
#3
The current rudder is slab-sided and not foil shaped. I am sure there is improvement to be had with the rudder. a

How will you measure the performance difference between a Sunfish with the racing board and current rudder vs your version? If one is night and day better than the other it should be apparent, but if that isn’t the case how will you tell if there is a difference? I’d be curious about the difference also with just your new rudder and with just your new board vs a boat with the racing board and normal rudder.
 
Thread starter #4
I'll be eager to hear more about the db. I extended a Barrington board by about 14 inches, sanded it more into a foil shape and glassed the final result.
A little more effort yanking it out when beaching, and I could find the bottom easier. Didn't really notice an improvement.
Thank you for the good comments - expert input is very important to me. A small difference in foil cross section and fin planform can make a big difference in performance, especially at for low speed flow in water. Lengthening a board will help if other aspects are considered. This is of course undesirable from a handling standpoint in shallow water. But everything is a compromise. Proof is in the pudding - I look forward to testing my board and expect to need to make multiple ones before knowing if they are better.
 
Thread starter #5
The current rudder is slab-sided and not foil shaped. I am sure there is improvement to be had with the rudder. a

How will you measure the performance difference between a Sunfish with the racing board and current rudder vs your version? If one is night and day better than the other it should be apparent, but if that isn’t the case how will you tell if there is a difference? I’d be curious about the difference also with just your new rudder and with just your new board vs a boat with the racing board and normal rudder.
Great question! It is the same for aircraft design, but planes mostly fly straight and level in calm air which makes testing much easier. Prediction methods (CFD codes) are so good these days for basic foils and wing-like shapes that such things are rarely tested anymore in wind tunnels or tow tanks. I am confident that the performance my work is showing will pan out - assuming the fabricated board and rudder are accurately made and I sail in ideal conditions - which of course never exist. I have a timed course on my lake between buoys spread a mile apart and will gather statistics from these myself. I have contacted my local rep for ISCA racing and hope they will test by parts in actual conditions similar to racing. Ultimately the only reliable source of performance data will be from experts like you.
 

LaLi

Well-Known Member
#6
Ahem... devil's advocate here again :rolleyes:

What is the whole point of "improvement" here? You can't race with "improved" foils, and going a few per cent faster by yourself is pretty meaningless. I understand it if the goal is to make the boat more comfortable or easier to handle, but just "performance" for the sake of... what? Is going across the lake a few seconds faster enough of a motive?

_
 
Thread starter #7
Why do people climb mountains? It is an engineering challenge, and I like that. The current foils also have a lot of drag during maneuvering and I hope to quicken tacks. I doubt many people would want to sail with the original dagger board - the FRP ones are nice, but could be better. 2/3rds of all Sunfish owners are recreational boaters, I understand, but we too like going fast.
 

L&VW

Well-Known Member
#8
What is the whole point of "improvement" here? You can't race with "improved" foils, and going a few per cent faster by yourself is pretty meaningless. I understand it if the goal is to make the boat more comfortable or easier to handle, but just "performance" for the sake of... what? Is going across the lake a few seconds faster enough of a motive?
It is, if the other boat is a Laser. ;)

.
 

beldar boathead

Well-Known Member
#9
Ahem... devil's advocate here again :rolleyes:

What is the whole point of "improvement" here? You can't race with "improved" foils, and going a few per cent faster by yourself is pretty meaningless. I understand it if the goal is to make the boat more comfortable or easier to handle, but just "performance" for the sake of... what? Is going across the lake a few seconds faster enough of a motive?

_
If it’s fun to do, why not? On the other hand if the objective is to commercialize the product and make money, then it’s not a good idea.
 
#10
Why do people climb mountains? It is an engineering challenge, and I like that. The current foils also have a lot of drag during maneuvering and I hope to quicken tacks. I doubt many people would want to sail with the original dagger board - the FRP ones are nice, but could be better. 2/3rds of all Sunfish owners are recreational boaters, I understand, but we too like going fast.
Speed is of the essence, even for recreational boaters... whenever two boats are traveling in the same direction, the race is on, LOL. :rolleyes:

It is, if the other boat is a Laser. ;)
Haha, L&VW, you funneh... :cool:
 

mixmkr

Well-Known Member
#11
I made my daggerboard longer in hopes of being able to point higher and less sideways slipage. My launch is often at the end of a cove, with the prevailing winds usually blowing staight out the mouth. Tacking angles of 45+ degrees getting home, make one late for delicious suppers. I usually try getting better angles along the shore line, but wind speeds are typically less. Downwind runs back to launch sites are usually a factor, determining where to put in.
 

wjejr

Active Member
#12
Hi Kent. You may be interested in my efforts to build an improved rudder for my 1971 Sunfish. The thread is here: Making new style rudder for old style boat .

It was a fun project, but I've never been able to determine if it's made a measurable performance difference. I think it's better, but I don't race the boat, and there are no other Sunfish where I usually sail. Sure looks cool though. :)

I hope you find my thread helpful. Good luck!
 

Webfoot1

Active Member
#13
You'll eventually hit the problem of the daggerboard slot
being too narrow for the NACA airfoil most commonly used
for keels. Find a way around this and you may have a shot
at something better. Remember that the optimum L/D ratio
varies with the hydrodynamic speed so it's always a compromise.
X-Plane uses Blade Analysis so perhaps components could be
modeled using that program.
 
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