Crest of the Royal National Institution of Naval Architects - Click to return to the homepage

The Royal Institute of Naval Architects

Jotun 09/11/2016

Bentley Systems 10/11/16


Paper Title: Reef and Flat or Twist and Ease: An Improved Trim Parameter Model for Yacht Sails



H Hansen, FutureShip GmbH, Germany
P J Richards, Yacht Research Unit, University of Auckland, New Zealand
P S Jackson, College of Engineering, University of Canterbury, New Zealand


Velocity prediction programs (VPPs) are commonly used to assess the performance of a yacht during the design process. The ‘reef and flat’ trim parameter model has been used to describe the changes in aerodynamic forces acting on the yacht during de-powering of the sails since the first VPP was developed about 30 years ago. Based on the concept of the ‘reef and flat’ model this paper introduces the extended ‘twist and ease’ model. Wind tunnel tests with the Real-Time VPP have been conducted to obtain realistically de-powered sail shapes and the results are compared to the trim parameter models. The comparison shows that the ‘twist and ease’ model describes the changes in lift, drag and centre of effort height due to de-powering more accurately and the predicted boat speed is within ± 1% compared to the under prediction of up to 4% of the ‘reef and flat’ model.


Transactions RINA, Vol 152, Part B1, International Journal of Small Craft Technology, Jan-Jun 2010 

DOI number

DOI No: 10.3940/rina.ijsct.2010.b1.91

Search and Order IJSCT papers

All International Journal of Small Craft Technology (IJSCT) papers are listed in the RINA Publications Database. You can search the database by author name and paper title. Publications may be ordered and paid for online. Other publications are also listed, some of which may be downloaded free of charge. Follow the link to Search Publications & Order.

Alfa Laval 260x120

Stone Marine 260x120

Rolls Royce Marine 2016

JETS VAC July 2015


Drew Safety