Dimensions in cm
126.0 - LOA - Length overall
  86.5 - LOD - Length on deck
  73.4 - LWL - Waterline length
  73.4 - DWL - Datum waterline length (designed LWL)
  24.0 - Beam max - Greatest width
  23.5 - Beam WL - Waterline beam
  14.5 - Draft - Depth of hull below waterline
  8.2kg - Weight - Measured weight of boat including sails
  6.0kg - Displacement - Weight of water equivalent to the immersed volume of the hull.
Lateral dimensions measured along the waterline from the bow
51% - Centre of Gravity including sails (0.7cm below WL)
51% - Centre of Bouyancy
50% - Centre of Effort of full sail plan (47cm above WL)
56% - Center of Lateral Resistance (6cm below WL)
54% - Centre of Floatation.


Displacement and Lateral Centre of Bouyancy

Centre of Bouyancy

The outline of each section/frame below the waterline was copied onto graph paper and the area calculated. The volume is found by adding up the area of all the sections and multiplying it by the distance between one set of sections. Displacement is the volume x density.
The outline of the curved graph was transfered onto card and cut out. The position of LCB (BL) was found by balancing the card on the edge of a ruler.


Centre of Floatation

Centre of Floatation

The boat pitches (rotates) about the lateral centre of the water line plane. The Centre of Floatation was found using the same card method. It is at 54% LWL.
The outer line show the waterline plane with the boat sitting 1.5cm lower. The addition is at the stern. However, as a percentage there is only 0.3% difference in the position.


Centre of Lateral Resistance CLR

Centre of Lateral Resistance

The side view of the hull below the waterline was copied onto card and then cut out. The position of the CLR was found by balancing the card on a pin.


Centre of Effort of Sail Plan

Centre of Effort of sail plan

Centre of effort of the full sail plan is 7 cm rear of the mast at section 4. This is puts it 50% or 36.3 cm from the bow end of the waterline. It is 47 cm above the waterline. Total sail ares = 5313 cm sq.

Centre of Effort of sail plan

The centre of effort moved rearwards as the sail area was decreased to suit the increasing wind speed.



Sailing boats should be set up to carry "Weather Helm". The wind pressure will swing the boat into the direction of the wind, bringing it to a halt.
Think of a weather vane. The large area is forced round until it is trailling behind its pivot point. So you would think that the sail's Centre of Effort would be behind the Centre of Lateral Resistance.
The problem is that the hull is curved and the shape changes shape when heeled so in all sailing boats the Centre of Effort is placed ahead of the Center of Lateral Resistance. The distance between the two centres is called the "Lead" Pronounced "Leed" this amout varies depending which book you read.
The Nature of Boats by Dave Gere pages 297 to 303
Us Vintage Model Yacht Group
Jolise Brise's mast is at 40%, The CLR is at 56% including rudder. The CE is 50% (lead +6%) with all 5 sails set and 53% (lead +3%) with Jib, Staysail and Main.


Ballast Layout

Ballast layout

The distribution of the ballast is not exactly as indicated as the side view does not show the shape of the hull see the photos
On completing the boat, a little more ballast (300g) was added through the rear hatch between frames 8/9 to adjust the fore/aft level. It settled below the waterline.


Tank Tests


The tank was filled up to the top then allowed to drain out the plastic pipe at the top left of the tank. When the level reached the pipe outlet, it stopped.
The boat was gently lowered in and the displaced water collected, measured and the displacement calculated. (1cc = 1g)
Model Displacement = 7.33kg at 0 deg and 7.39kg at 30 deg (+1%)
Design Displacement = 6.0kg


Tilt Test

The jig was fitted around the hull at the mast at frame 4. It was immediately noticable that the hull was floating lower in the water than the designed Water Level by 1.5cm.

Tilt Test

The top of the mast was pulled over to different angles of heel from 0 to 30 deg using the Inclinometer on the mast. The Bulwark Rail submerged at 22 deg.
The water levels were measured on both sides using the jig. The boat was lifted out and the jig removed. The jig was reassembled
The under water section was drawn for each angle of tilt and cut out. The centre of bouyancy was found by hanging the section on a pin through a corner and drawing the vertical. Repeating it at about 90 degrees. The two lines crossed at B.

Tilt Jig



The measure of stability of a vessel is shown by the metacentric height MG = 4.4cm.
Centre of Gravity is 2.2cm below the actual waterline. Metacentric MG is 4.4cm above G. Centre of Bouyancy is 2.1cm below G at 0 degree tilt.
To find M, a verticle line was drawn through B, M is where this line crosses the boat's centreline. The lines for each tilt angle all passed through the same point except 10 deg. So the theory does work.

B G M diagrams

The weight of the boat acts downwards through the centre of gravity G. The bouyancy acts upwards through B. The horizontal distance RM between the lines of action of G and B multiplied by the boat's weight is the boat's Righting Moment in kgcm. Boats weight is 8.2kg.


Righting Moment

To calculate the Righting moment, At 10 degs tilt the weight is 8.2kg and RM is 5.5mm so Turning Moment = 8.2 x 5.5/10 = 4.5kgcm
By experiment, the top of the mast was tilted over and the force is found using a spring balance. At 25 deg, force is 8N or 80g, Distance from point of rotation G is 1.05m, so Turning Moment = 80/1000 x 1.05 x 100 = 8.6kgcm.
The calculated Righting Moments kgcm were consistantly higher than thoses found by experimentation. I can see no obvious explination. Both used the same actual LWL not the designed DWL.

Righting Moments

The righting moments caused by the weight of the boat, equipment and ballast balances the heeling moments caused by the wind pressure on the sails. The sections at different angles of heel have been aligned on their Centre of Gravity G so it has been assumed that the centre of gravity is the point of rotation. It's like balancing a see-saw.


Heeling Moments

One of the problems with model sailing boats is the wind speed does not alter for different sizes of boats.
Heeling moments are caused by the wind pressure on the sails. This has been calculated for different wind speeds and for different sail set-ups.

Heeling Moments

The target heeling moment needs to be about 50kgcm to cater for wind speeds upto a Force 4 (13 - 18 mph)
This Heeling Moment must be balanced by the same Righting Moment = 50kgcm.
In the present situation, one remedy would be to replace the lead gel battery (0.9kg) with a lighter Ni-cad and try and remove some of the ballast. To obtain the 50kgcm righting moment, a new lead ballast weight could be fitted externally below the keel.


External Keel

The ballast weight and position must float the boat to its waterline. The depth of the ballast weight can then be lowered until the correct Righting Moment is found.


Design Sites

Thames Barges - Ivor Brittle
Bouyancy - Ivor Brittle
Metacentre - Shaggy K
Yacht Design - Ted Brewer
Elements of Boat Strength
Gaff Rig - Design and history
Nature of Boats
Gaffs, their saddles and other things
Photos of a Cutter being built
Notes on Scale - South Manchester Model Boat Club
Vintage model sails
Nautical Glossary
Glossary - Seatalk
Bristol Channel Cutter models
Free boat design resources from around the world