sail-world.com
 
 
News Home Cruising Photo Gallery Video Gallery

 

Sail-World.com : A look at the Finn under the water

A look at the Finn under the water

'Underwater forces - Finn sail development cycle'    © Mikko Brummer/WB-Sails

International Finn Association .

In a recent Finn sail development cycle, WB-Sails needed a 3D Finn model, with hull, cockpit, crew and all, to be able to simulate the sail performance more realistically. With the hull model there, they thought why not add a centreboard and a rudder and take a look under the water as well. Mikko Brummer from WB-Sails describes what they discovered.


The model was taken out of Gilbert Lamboley’s and Richard Hart’s work on digitising the original Finn plans, 'Body lines definition and control', from 2003. We inserted a roughly shaped cockpit and centreboard case, with the centreboard exiting from the bottom. The under water flow model was simplified so that wave-making was ignored, representing the sea as a flat surface, because our aero-code does not support free-surface modelling. Wave-making drag cannot be modelled, but otherwise the free surface is not believed to have a major effect on the flow patternand pressures over the hull.

When sailing at steady speed, the underwater and above water forces and moments must be in balance. Care was taken that the simulated sail forces, drive and heel, and heeling moment match with the corresponding underwater forces, and the righting moment by the sailor hiking. The real righting moment is easy to estimate from sailing photos, considering that from trapezing dinghies we know that the centre of gravity of a human being lies more or less at the height of his navel.

The simulation was run for medium-heavy conditions of 18 knot true wind. With the sail forces solved, the attitude of the underwater hull was adjusted (leeway and rudder angle at the given boat speed of 4.9 knots) until the underwater forces matched the sail forces. The sail shape was recorded with a masthead video on Lake Garda, in a steady sea breeze of 18 knots but with rather a nasty chop, hence the GPS recorded boat speed less than five knots – on flat water the Finn can easily exceed five knots of speed.


Pressures on the bottom and the appendages. Blue/green is lower pressures (suction), orange/red indicates higher pressures. - Finn sail development cycle -  © Mikko Brummer/WB-Sails  

A comparison was also made to a Finn VPP (velocity prediction programme) run: All three simulations, aerodynamic above water, hydrodynamic underwater and the velocity prediction programme forces agreed surprisingly well. The total drive (equal to the total underwater drag) of the Finn in 18knots of wind is 20.5 kg force, or roughly 45 pounds – it’s not much, a weight that you easily lift with one hand.

Streamlines coloured with flow speed around the centreboard and the rudder. Note how the flow bends down on the windward side of the centreboard. - Finn sail development cycle -  © Mikko Brummer/WB-Sails  

A look at the results

The results of the analysis is best showcased by the accompanying computer screen dumps. The 'Finn drag balance' pie-chart contains most of the information. 'Drag' is here understood as a force in the direction opposing the motion of the boat. The largest portion of drag comes from the hull wave-making, predicted here by the VPP since it was not considered in the underwater CFD simulation. Nearly as big is the hull viscous drag, meaning the skin friction caused by water flowing along the hull surface.

This includes also the pressure drag by the pressures varying from bow to stern, and also a bit of transom drag - the transom was modelled to lie a few centimetres under the sea surface, dragging some water along. The viscous drag was predicted to be exactly similar in the VPP and the underwater flow simulation.

Shear stress is due to skin friction. It serves well to indicate where the flow is separated, in dark blue areas where shear stress is close to zero. Here the rudder is stalled at 2/3 of its depth. - Finn sail development cycle -  © Mikko Brummer/WB-Sails  

The centreboard proved to be more efficient than expected – the thin, triangular plate-like board is not what you would call a modern appendage, but it does its job quite well. The rudder, so refined on the real Finn, was not modelled very carefully but rather the profile was just drawn freehand. So maybe its drag component could be slightly less than in the simulation. The rudder angle was varied until the boat was longitudinally in balance with the sail yaw moment.

The yaw moment wants to turn the boat into the wind or away from it, and it has to be balanced by the rudder so the boat moves in a straight line. This resulted in a positive rudder angle of 1.5 degrees which is not too unrealistic. The simulation shows, too, how sensitive yaw balance is to the traveller position (sheeting angle). The underwater hull also contributes to the side force, to resist leeway – nearly 10% of the total side force is from the hull.

Finn drag balance sailing upwind in 18 knot of true wind and rather a nasty chop. The largest source of drag is the waves generatedby the hull moving through the water (28%, in green). The hull and crew air drag is the smallest at only 4% (in grey). - Finn sail development cycle -  © Mikko Brummer/WB-Sails  

The drag due to seaway comes from the VPP. It means the additional drag caused by the (wind generated) waves. Finally, the smallest drag component, hull + crew air drag represents the aerodynamic drag of above water boat hull and the crew hiking, in the direction opposing the motion of the boat. At approximately 0.5 kg force the drag of sailor is twice as big as the drag of the Finn hull itself – Rickard Sarby designed the Finn also very aerodynamic.

If you compare the hull air drag of about 250 grams to the total drag of 20,500 grams (20.5 kg), it is nil. Part of the reason for this is that close to the water surface the wind speed is much less, less than 10 knots, and this was allowed for in the aerodynamic simulation. So there is no reason for the sailor to dress in a downhill skier’s skin-tight suit, either.

Flow field around the sail coloured with relative pressure. Inset in the lower left corner is about 1 m above the deck: Note how strong the suction on the leeward side of the mast is (the purple area). This gives the mast plenty of positive drive in the lower part. Close to the top of the sail (inset upper right), the low pressure area is shifted on the backside of the mast, creating drag insteadof drive. - Finn sail development cycle -  © Mikko Brummer/WB-Sails  

Not too much can be revealed of the aerodynamic sail simulation here, but there is one interesting notion we want to share. It has been general thinking that mast twist is desirable: You would want the mast top to twist in the same direction as the sail, to better align the top part wing profile with the luff of the sail. This simulation contradicts that belief. In heavy winds, the upper part of the sail is more or less ‘feathered’, or twisted away, and does not carry much load.

The wind hits the mast on its leeward side and the topmast is effectively drag. Now, the more you twist the wing-shaped mast top, the more it presents itself sideways to the local airflow and the more drag there is. Negative twist, if achievable, might actually be better.

The Finn differs from regular, sloop rigged boats with its rotating mast – the mast is actually already ‘twisted’ about 12 degrees when you sheet the boom down to the gun whale. It must be added that we have little knowledge about how much and in which way the mast top twists in reality – it’s difficult to measure and hard to judge even from masthead video.

International Finn Association




by Mikko Brummer/WB-Sails

  

Click on the FB Like link to post this story to your FB wall

http://www.sail-world.com/index.cfm?nid=82645

9:42 PM Tue 19 Apr 2011 GMT






Click here for printer friendly version
Click here to send us feedback or comments about this story.


News - USA and the World







470 Men and Women Worlds - Champions decided in Santander by International 470 Class Association,










































Big Boat Series - It's not the size, it's the length by Erik Simonson, Pressure-Drop.US,


















2014 Chicago Match Cup - Four go through to quarter finals
470 Men and Women Worlds - Champs take over leaderboard
ISAF Sailing World Champ - Buckingham finishes with career-best result
America's Cup: Oracle's Larry Ellison to step down as CEO
Classic Yacht Regatta - Sonny takes top classic prize
ISAF Sailing Worlds, Santander - Buckingham finishes with career best
ISAF Sailing Worlds, Santander - Rio 2016 RS:X spots claimed
ISAF Sailing Worlds - Leaderboards shaping up in Santander + Video
ISAF Sailing Worlds, Santander - Giles Scott continues to extend lead
ISAF Sailing Worlds, Santander - Day 7 images by Sailing Energy
ISAF Sailing Worlds - Laser and Radial winner images by Jesus Renedo
ISAF Sailing Worlds, Santander - Medal Race images by Sail-World.com
ISAF Sailing Worlds - Dutch dominate Laser and Radial in Santander
Royal Cup Marina Ibiza - Quantum Racing more than doubles overall lead
J/24 World Championship - Excitement builds for Newport racing
Volvo Ocean Race - Seb Marsset joins Team Alvimedica
Fall Seamanship training opportunities at New York Yacht Club
Rolex Big Boat Series: More images from San Francisco - Final Day
AWT Hatteras Wave Jam action commences on day 3
Rolex Big Boat Series: Images from San Francisco - Final Day
ISAF Sailing World Championships - Watch medal races live here   
Hobie 16 Open North American Championships and Pan Am Games qualifier   
ISAF Sailing World Championship - Videos from Santander   
ISAF Sailing Worlds: Buckingham into Medal Race, 470's stay consistent   
470 Men and Women Worlds - Vadlau and Ogar into pole position   
2014 Chicago Match Cup - Defending champion on top   
ISAF Sailing Worlds, Santander - Strong competition for RS:X fleets   
ISAF Sailing Worlds, Santander - Big breeze on day 6 + Video   
ISAF Sailing Worlds, Santander - Buckingham into Laser Medal Race   
U.S. Multihull Championship - Hobie Alter Trophy to be decided   
ISAF Sailing Worlds - The kind of form most sailors only dream about   
Royal Cup Marina Ibiza - Quantum Racing ahead of target   
ISAF Worlds Santander - 19 Rio 2016 Laser Radial spots awarded   
Santander 2014 ISAF Worlds - 21 broadcasters to take live coverage   
Rolex Swan Cup - Looking forward to the next 30 years   
America’s Cup: The Future is foiling – AC45s to be modified   
Volvo Ocean Race: Win a stopover trip by designing an ECsix T-Shirt   
Red Bull 49erFX: On the eve of the Worlds in Santander   
America's Cup: Ben Ainslie Racing launches partnership with Yamaha   
2014 Chicago Match Cup - Starts tomorrow   


For this week's complete news stories select    Last 7 Days
   Search All News
For last month's complete news stories select    Last 30 Days
   Archive News







Sail-World.com  


















Switch Default Region to:

Social Media

Asia

Australia

Canada

Europe

New Zealand

United Kingdom


http://www.sail-world.com/event_images/image/Twitter_logo_small.png http://www.sail-world.com/event_images/image/FaceBook-icon.png  http://www.sail-world.com/event_images/image/RSS-Icon.png

United States

Cruising Northern

Cruising Southern

MarineBusiness World

PowerBoat World

FishingBoating World

 

Contact

Commercial

News

Search

Contact Us

Advertisers Information

Submit news/events

Search Stories/Text

Feedback

Advertisers Directory

Newsletter Archive

Photo Gallery

 

Banner Advertising Details

Newsletter Subscribe

Video Gallery

Policies

 

 

 

Privacy Policy

 

 


Cookie Policy

 

 



This site and its contents are © Copyright TetraMedia and/or the original author, photographer etc. All Rights Reserved.  Photographs are copyright by law.  If you wish to use or buy a photograph contact the photographer directly.
XLXL NEW US
LocalAds   DE  ES  FR  IT