Unusual design for an extra-heavy-displacement 11ft long Atlantic Proa for circumnavigation

[CarlosK2]

I think the IOR was a lot worse than what Marchaj said, i.e. I think his critique fell short and was chaotic.

The list of problems of the IOR is a good list:

(1)

The bows were (1a) very thin, the (1b) sides of the bow very vertical and (1c) the stems quite straight.

Bows (a+b+c) that sunked (Pitch) and heeled (Roll) and yawed (Yaw) produced a monstrous Lateral Force

(2)

The curve of the stern produces suction and therefore more Lateral Force in the opposite direction to the Lateral Force of the Bow.

The Hull Yaw/Turning Moment due to Yaw, Heel/Roll and Pitch (Munk_1 + Munk_2 + Munk_3 =) ... is monstrous

Max Munk - Wikipedia https://en.m.wikipedia.org/wiki/Max_Munk

(3)

On top of that, the HydroDynamic Center of the centerboard/keel is forward of the center of gravity of the yacht, and to top it all off ...

(4)

The rudder is small

(5)

And everything is wrong from the beginning because the Center of Buoyancy is forward of the Center of Flotation ... both with the boat stopped in port and even worse with the stern wetted by a wave.

In other words, the sailboat is not well Balanced Longitudinally.

 

[CarlosK2]

This century old sailboat ...

thanks to its Longitudinal Balance and thanks to the fact that it keeps its Bow up and Stern down ...

Surfs as well as a modern Scow

 

[CarlosK2]

Since no one bothers to start right at the beginning:

LCB = LCF

... Well, let's throw in the towel

and Welcome the unstoppable trend towards the Scow

 

[wet feet]

Actually,those scow sections are not too far removed from an IOR hull shape,the principle difference being that there would have been a curved element instead of a sharp chine.I think it is an unfair generalisation to describe IOR rudders as small as I well remember transporting an example in a long wheelbase Land Rover and it was only just possible to close the back door.One of the less desirable traits was to design for a favourable result from the inclining test and it was a feature that they floated a little down by the bow to help achieve a helpful result when heeled.The shape that gave optimum results usually featured a spherical mid-section that faired into a lean and raked stem with a stern that indicated exactly where the measurement points were located.If there was a peak or a trough,it was there for a reason.It wasn't an era when all was terrible, as the size of fleets was very healthy and the competition quite intense.It also led to a healthier type of boat as the phase came to a close.I hope it will be possible to attach a few images to illustrate the type(s).

 

[CarlosK2]

1979, unbalanced modern sailing yacht

2025, modern sailing yacht balanced as a 1930's British Classic

 

[CarlosK2]

1979

(1) Being unbalanced longitudinally and (2) having little Area and Volume in the bow ...

the bow when the Wave arrives at the stern ... the sailboat sinks the bow easily, for example 2 degrees bow down, and then it has the worst possible bow with the Center of lateral Pressure (CP) very far forward and a huge Yaw/turning moment ("Munk Moment") due to the sum of (A) the shape of the leading edge, (C) the large angle of attack due to the beam, and (B) the quite vertical sides.

At the top of the drawing a bow with the center of pressure further back thanks to its delta wing shape, for example an old Dragon.

 

[CarlosK2]

2025

(A true Classic)

Longitudinal Balance

LCB = LCF

Longitudinal position of the Center of Buoyancy = Longitudinal position of the Center of Flotation

LCG

Longitudinal position of SailBoat Center of Gravity

HC

HidroDynamic Center of the Keel

We have the sailboat well balanced longitudinally, in addition the longitudinal metacentric height is gigantic (= 3 x LWL, in 1979 it was 1 x LWL)

So we started very well. Now the sailboat sails at 0.50 Froude, and we also have in our favor (A) the hydrodynamic lift in the bow (B) a pile of water and a true launch ramp and (C) the suction of the stern

The Wave arrives from the stern and the sailboat accelerates elegantly and overcomes the bow wave, Surfing as well as an old Dragon

 

[CarlosK2]

We sail downwind in a 150-300 D/L sailboat (out of the world of high competition) and blowing 20-30-40 knots of wind

The sailboat sails at a speed of 0.45-0.50 Froude with the bow +2 degrees bow up.

The Wave arrives

And here two paths arise

A sailboat sinks the bow say -3 degrees bow down, stumbles over itself, the helmsman or the autopilot suffers, and in the worst case with a young, steep, big Wave, the sailboat turns violently and capsizes.

In the other way the bow levels out (+/- 0 degrees) or sinks -1 degree, and in either case the sailboat quickly accelerates and quickly adopts a surfing bow-up attitude.

 

[CarlosK2]

T. Harrison Butler in memoriam