Fin/Rudder for a racing shell

[huzenhagen]

Being the tinkering sort and recently seeing a very fancy rudder made by carl douglas on an eight at my boathouse i was wondering if some people could help guide me in the right direction for the design of the fin and rudder for a racing shell.

My reading so far has made it pretty clear that the tiny flat fins that come standard on shells are doing absolutely nothing other then add drag once they are put past just a couple of degrees. With easter and exam revision coming up i've always found a small project helps relax me between bouts of work so i'd like to make a prototype fin and rudder based on a proper foil.

I've downloaded xflr5 and had a look at a few pieces on foil design and played about with foils and adding flaps to them as rudders stuck at various angles. I've hit a couple of snags however in how to proceed with the actual design of a 3 dimensional fin.

For those of you unfamiliar with a rowing shell they are long (~18m) and thin (~60cm) and in a 2k race can be expected to average around 5m/s. Shells generally weigh around 100kg with 8 rowers (say 80kg each) and a 55kg cox so a total weight of around 800kg.

First wall i've hit which i think i have got the hang of but want to check is how big to make it. Obviously i could just copy one off a boat but i think it they are probably made more to look like a fin rather then any calculations actually being done.

http://www.onemetre.net/design/FinArea/FinArea.htm
The equation here gives the formula as
Fin area =

but is using an eyeball figue or 90% of the lift coming from the fin which im not sure applies to an eight

The heel angle is also something that is confusing me. An eight is supposed to be as level as possible while racing (giving an angle of 0) but that ruins the equation so i guess it would be best to use the greatest angle an eight could achieve. So because of the riggers my rusty trigonometry has that at about 10 degrees. Does anyone disagree with this? could it make the fin too big?

how ce to clr would work for this situation has me stumped at the moment so suggestions please

http://www.btinternet.com/~furnivall.sc/fscbrb.htm

this source gives the metacentric height as 19.8cm so that's that sorted


Once this bit is sorted it would give me the overall area for the finn/rudder combo and i can start properly playing with the shape. Foil shape should be simple enough, plug in various shapes with various angles of rudder and look at drag vs lift and choose.

Planeform is the other thing that is troubling me. I haven't found many solid resources on it. I get the deeper parts of the fin create more lift so i'm guessing using a fairly simple equation the fin can be tapered down in proportion to the extra moment it can generate. With a vertical trailing edge this would make the forward edge swept, problems? Would a bottom plate be helpful?

Thank you very much in advance for any help you give me. I promise to do my upmost to get a picture of the finished article once the design is nailed

 

[DCockey]

Initial reactions after a quick look at your reference for the formula:

1) The formula is for the area of a fin keel on a model sailboat. Unless you are planing to sail the eight I don't think it's relevant.

2) The equation is very suspect for a sailboat fin keel, even on a model. The area of a fin keel needed to sail to windward doesn't vanish as the heel angle goes to zero. I'd expect the fin keel area to be proportional to something like the inverse of the cos of the heel angle. I suspect there are fundamental flaws in the assumptions and/or derivation.

3) Back to your project. Probably the simplest way to determine the required fin area is by observation of exisiting fins and then experimentation.

 

[Leo Lazauskas]

I'd start by estimating the boundary layer thickness and displacement thickness. You will be surprised at how little the fins protrude through that turbulent mess, especially near the stern of an eight.

 

[DCockey]

Good point Leo, but the portion of the fin which is within the "boundary layer" still functions, although not as well as the portion outside of the boundary layer.

One question: what is the function of the fin? Presumably it's for directional stability. Is it to lessen the effects of ambient wind, to dampen oscillations due to uneven or unequal rowing, or something else?

 

[Leo Lazauskas]

Would a bottom plate be helpful?

It might be a little helpful so long as it didn't catch weed. You need to check if it contravenes any FISA rules.

 

[DCockey]

Are there limits on depth of the fin? If not then a deeper fin will be more effective than one with a bottom plate. The added depth needed to reduce (trailing vorticity) induced drag by the same amount as a bottom plate will have less surface area then the bottom plate.

 

[DCockey]

Suggestion for a starting point - a fin with the same planform as what's currently used but with an airfoil section rather than a flat plat. Then the question becomes what airfoil section to use.

 

[huzenhagen]

Initial reactions after a quick look at your reference for the formula:

1) The formula is for the area of a fin keel on a model sailboat. Unless you are planing to sail the eight I don't think it's relevant.

2) The equation is very suspect for a sailboat fin keel, even on a model. The area of a fin keel needed to sail to windward doesn't vanish as the heel angle goes to zero. I'd expect the fin keel area to be proportional to something like the inverse of the cos of the heel angle. I suspect there are fundamental flaws in the assumptions and/or derivation.

3) Back to your project. Probably the simplest way to determine the required fin area is by observation of exisiting fins and then experimentation.

Cheers this is exactly the sort of thing i need guidance on. Will carry on looking

One question: what is the function of the fin? Presumably it's for directional stability. Is it to lessen the effects of ambient wind, to dampen oscillations due to uneven or unequal rowing, or something else?

Pretty much all 3. Major competitions take place on straight multilaned courses but the vast majority of time for the vast majority of clubs is spent on windy rivers so with such a long boat a good rudder would be an advantage. Wind can also be problematic for balance so a good fin would help with that and yes as a rowing boat is very unstable due to the center of gravity being so high the fin helps there too.

Are there limits on depth of the fin? If not then a deeper fin will be more effective than one with a bottom plate. The added depth needed to reduce (trailing vorticity) induced drag by the same amount as a bottom plate will have less surface area then the bottom plate.

none other then practical that i know of but as the tidal thames can be quite shallow where we row it couldn't be massively deep.

I'd start by estimating the boundary layer thickness and displacement thickness. You will be surprised at how little the fins protrude through that turbulent mess, especially near the stern of an eight.

Interesting. As i'm very new to this any programs or formula to have a look at this? Obviously they won't be as effective but does this mean foils are of little use?

It might be a little helpful so long as it didn't catch weed. You need to check if it contravenes any FISA rules.

No weeds on the tidal thames to catch. I'll have a look at FISA



Thank you very much for responses so far. Guess i have a lot more thinking to do!

 

[johnhazel]

On something like this you can use the approach ofr finding a weakness and then designing an improvement.

This specific situation: 5 m/s, 10cm ave chord, turbulent flow, flat plate (adjust as needed)

first check your reynolds number (Re).
830,000 * 5 * 0.1 = 415k

The turbulence in the river and from the hull, plus the relatively low Re indicates that the "laminar flow" airfoils would be a mistake. You would lose any chance of laminar flow because the flow is already turbulent before reaching the fin. You would not however lose the stall hysterisis. So laminar flow foils here would result in a fin with draggy mushy control of heading.

Strangely enough this 400k-800k Re range has a very good set of foils to use. The old NACA 4 digit, specifically the 009 to 0012, is the one to choose. These foils can be made 25% smaller than any laminar flow airfoil because of thier comparably high lift capability.

This same analysis indicates that the Drela HT foils would work well for model sailboats at thier ~100k Re.

Also a swept (to shed weeds) constant chord (to avoid interference drag at the root and low re at the tip) fin is probably going to be your best choice.

I have applied this approach to windsurfer fins with good results. Typical windsurfer "experts" use the laminar foils on thier fins and the result is poor handling and reduced upwind performance. Using my NACA 0009 fin I get on plane quicker because I can load the fin sooner (a result f the higher lift foil) and point higher (because the NACA 0009 has lower drag than the laminar foils below Re = 1 million and keeps the flow attached better).

....Yes I am implying that the vast majority of boat design "experts" don't know what they are doing in this area......