Marine Performance Technology Exchange
May 2005
Copyright © 2005 HydroComp, Inc.  All rights reserved.  - 
topics products

Predicting performance by scaling from another boat

I am developing a new design that is derived from an existing boat. Can I use the known performance of my existing design to predict how the new boat will perform?

Froude scaling is a well-known technique that you can use to correlate the performance of an existing vessel to a new vessel. The methodology is rooted in the notion that at a common speed coefficient, there will be a common performance ratio, both of which will be equal between the two vessels. If both coefficients are "non-dimensional" in nature, we can then predict resistance or power at a corresponding speed for one vessel using the known performance of another vessel. 

Speed coefficients - The basis of any speed coefficient (SC) is the ratio of speed to the square root of length, or SC = V / L1/2.

For non-planing vessels (i.e., displacement hulls), the length is the length on waterline, leading to the most traditional speed coefficient - the speed-length ratio (SLR). This is defined as the speed (V) in knots divided by the square root of the length on waterline (LWL) in feet, or SC = V / LWL1/2. (A truly non-dimensional version of SLR is the length-based Froude number).

The waterline length on a planing boat, however, changes with speed and trim, so we need to use a more stable speed coefficient for these hulls. For this we use a "length" equal to the cube root of boat weight (W) or displaced volume, which leads to SC = V / W1/6. (This relates to the non-dimensional volumetric Froude number). Another technique is to use the chine beam for the length component, or V / BCH1/2.

Performance ratios - The performance ratio (PR) typically correlates drag or power to boat weight. Drag-to-weight ratio is how resistance (R) can be related between boats, or PR = R/W. Of course, you'll need to know the boat's drag to use this parameter. This can be found from model testing, of course, or by using a sea-trial analysis program like SwiftTrial.

Correlating performance by power requires a different PR based on weight times speed divided by power (P). This ratio also goes by the names transport efficiency or transport effectiveness, and is of the form PR = W * V / P. Remember that P is the total developed engine power actually used to make the speed, not the rated power. (Using rated power, however, will give you a conservative PR.) If your boat happens to be running overloaded (meaning it cannot make full engine rated RPM), then you'll be able to find the true developed power from the engine's power curve at that RPM. Otherwise, you can look to sea-trial analysis software to help find the actual developed power.   

Example - Let's say that our existing vessel is a twin-screw planing hull of 12400 kg, and it runs 19.2 kts. It has a pair of 202 hp engine that is slightly overloaded (50 RPM less than rated). Our new design is a scaled-up version weighing 16000 kg.

1. SC = 19.2 / 124001/6 = 3.99
2. V at 16000 kg = 3.99 * 160001/6 = 20.0 kts  (This is the corresponding speed.)

3. PR = 12400 * 19.2 / 404 = 589
4. P at 16000 kg = 16000 * 20.0 / 589 = 543 hp  (This is the corresponding power.)

Caveats - It is important to remember that the accuracy of these approaches will depend to a large extent on the similarity of the boats. If one is a true geometric "parent" of the other with similar propulsion system types, then the scaling will be quite reliable. Any difference in shape or propulsion system, however, will reduce reliability.

Fine-tuning pitch on a propeller

To fine-tune the pitch on a propeller, you can

  • Add or remove cup. Cupping changes a propeller's "effective pitch", so that it performs like a propeller with more or less pitch.
  • Cut back the propeller. Cutting down on diameter is a well-known way to relieve load and increase RPM. If you are using a propeller with progressive pitch (camber), you can also reduce effective pitch by cutting back the trailing edge.
  • Repitch (twist) the blade. For larger changes in performance you may need to repitch or twist the blade. The limit of a pitch change is typically limited to about 10% of the diameter.

SNAME presentation - Inboard Propeller Cavitation

For those of you in the Boston and New England area, mark your calendars for Thursday May 19th. HydroComp's Technical Director, Don MacPherson, will be delivering a presentation and paper entitled "Inboard Propeller Cavitation: A Practical Guide and New Performance Model" for the May meeting of the SNAME New England Section.

Large merchant ship propellers are designed with the intent of minimizing or eliminating significant cavitation and its associated thrust breakdown. Commercial inboard propellers found on the majority of motor yachts and service craft, however, often operate with significant levels of cavitation where thrust breakdown is a frequent occurrence. As engine power density continues to rise, we should expect inboard propellers to risk greater levels of cavitation. Designers of these craft must understand the effect of cavitation on inboard propeller performance, and include the analysis of cavitation as part of their speed-power prediction and propeller selection.

Existing models for the prediction of thrust breakdown, however, only offer rough measures of the effect of excess cavitation on thrust and torque. They are generally inadequate to insure reliable prediction of performance in the critical “trans-cavitating” regime where contemporary heavily loaded propellers often function.

This paper will provide a practical guide to the nature and effects of cavitation on commercial inboard propellers, as well as introduce a new performance model for the prediction of thrust and torque breakdown. 

The paper is co-authored with former HydroComp engineer, Chad Turmelle. You can register for the meeting and presentation by contacting Gerrit van Dissel, New England Section Secretary, at

Propeller design? Review the new About PropCad presentation

Are you involved with propeller design? Learn about the propeller CAD tool used by over 60 companies worldwide with the new About PropCad presentation.

Visit the PropCad page from our web site to download the presentation, and you will discover...

  • Animated rotation of the 3D View
  • Supplemental Builder options for high-performance propellers, such as new sections, camber and cup
  • New ABS, BV/RINA and Korean Registry thickness rules
  • Wetted inertia calculation (i.e., entrained water)

Welcome to recent PropCad users Kahlenberg Brothers (USA), Clememts Engineering (UK), Evercraft Boats (Australia), Delta Marine Engineering (Turkey), BALINO (Spain), Tero Marine (Singapore), and HELICIA (France).

Trade shows  

You can discuss product capabilities with HydroComp staff at these upcoming trade shows:

  • Nor-Shipping, with Design Systems & Technologies, Oslo [Jun 7-10]
  • IBEX, Miami [Oct 19-21]
  • SNAME, Houston [Oct 19-22]
  • NMPA, Las Vegas [Nov 4-6]
  • METS, with Design Systems & Technologies, Amsterdam [Nov 15-17]
  • Workboat, New Orleans [Nov 30-Dec 2]

Check your version  

The following is a list of current program versions and dates. If you have a current MSU subscription, you can click on the appropriate link below to go to the update download page. (Note: users of SwiftTrial and SwiftCraft are on a perpetual subscription.)
NavCad 2005 [5.11.0099, Mar 2005]
PropCad 2005 [4.40.0138, Mar 2005]
PropExpert 2005 [5.12.0067, Apr 2005]
SwiftCraft 2004 [1.20.0045, Feb 2004]
SwiftTrial 2003 [1.02.0019, Oct 2003] 

It is never to late to update your MSU subscription!

PropExpert seminar at NMPA Convention

We are pleased to announce that our next PropExpert training seminar will be held in conjunction with the annual convention of the National Marine Propeller Association ( in Las Vegas on November 4, 2005. This one-day seminar is suitable for new and experienced users of PropExpert, as well as for anyone interested in learning propeller sizing.

You do not need to be a PropExpert user or a member of the NMPA to attend the seminar or the convention. For more information about the seminar, please contact HydroComp.

HydroComp seminar earns accreditation from SNAME

The Society of Naval Architects and Marine Engineers (SNAME) has accredited HydroComp seminars for the Continuing Education requirements of Professional Engineer license renewal. Designed to suit the needs of a specific audience, HydroComp's Propeller Seminars dissect both theory and practical application. For example, previous seminars have focused on a range of issues including the fundamentals of propeller sizing, engine performance, speed prediction, and cavitation.

Seminar participants enjoy an open dialogue with the instructor, and have an opportunity to discuss current projects and participate in round-table discussions. For more information, please link to the HydroComp training web page.

HydroComp, Inc. is the leading supplier of software and services for marine performance prediction, propulsion analysis, and propeller design. For more information, visit or one of the pages listed below.
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