How to Design a modern Yacht

Controlling the band of the three bandits (Yaw, Pitch, Roll) is the essential issue in the design of a comfortable and safe sailboat

Control of the three must be very strict because there are a good handful of feedback loops

Yaw

We have to do four things, all four very important

1) with the design of the stern eliminate the lateral component of the low hydrodynamic pressure, suction, lateral suction that in a sailboat could be a good part of the Hull Yaw Moment (Max Michael Munk Moment)

2) with the design of the bow reduce the lateral component of the high hydrodynamic pressure, that is a big contribution to the Max Michael Munk Moment

3) neutralize the centerboard, that is, place its hydrodynamic center below the center of gravity of the sailboat or very close

4) calculate a very prudent calculation of the rudders from a cloud of various estimates of the yaw moment of the hull in various conditions

Pitch

1) LCG > LCB > LCF that is: place the center of Gravity (CG) aft of the center of Buoyancy (CB) which goes aft of the center of Flotation (CF)

A solution known since 1880-1925, and which most designers do not apply, some out of ignorance and others for seeking more speed, which is the misfortune of yacht design: a tenth of a knot upwind and a couple of knots more downwind are large amounts in hands of professional or semi-professional crews

2) a large o very very large Longitudinal metacentric Height

3) hull shape and profile that creates the vertical components of hydroDynamic and hydroStatic pressures that trims the Hull in a good Hull Attitude for Surfing

 

Roll

One of the best solutions to control Roll is to combine (1) a favourable hull shape (for example a good beam with a low chine) with (2) heading and (3) speed

placing the wind right on the stern is a disgrace because you invite Roll to spoil your sailing

However, a sailboat that hoist with total comfort 25-30-40 squared meter per Ton and AWA (Aparent Wind Angle) 135° is a pleasure

 

When doing a Downwind zig-zag with 25-30 square meters per ton, Roll is controlled very well and on top of that the sailboat (Jib, StaySail, MainSail) sails alone or almost alone like Joshua Slocum thanks to its large bowsprit and in any case the stability of the balance is magnificent if you want to take the helm for a while

the 300K Euro 120 D/L sailboat now in fashion among the wealthy in Europe needs to hoist a huge spinnaker to reach 30 square meters per ton, it is not comfortable to sail alone, and on top of that the huge spinnaker does not help to stabilize the sailboat's course

Less is More

 

I would like you to listen to your own advice.

 

Below 6 meters the D/L ratio gets worse

and also the longitudinal metacentric height gets much worse, something that is forgotten, in fact it would be very reasonable to measure the size (and see who pees the furthest) by the longitudinal metacentric height, so a Dinosaur of 12 meters in length and longitudinal metacentric height 1 x LWL is a 12 meter sailboat, and a small boat of 6 meters in length and longitudinal metacentric height 3 x LWL is a tremendous 18 meter sailboat

on the other hand above 9 meters in length (and about 3 tons) the ratio "Hoisted Sail / Displacement" falls, and the joke of cheating is not worth it

Sail area-displacement ratio - Wikipedia https://en.m.wikipedia.org/wiki/Sail_area-displacement_ratio

but an honest and sincere hoisted surface divided by real displacement under sail

 

Saint Bayesian and Saint SA/D

Saint Bayesian and Saint SA/D are the patron saints of this very perplexing industry

very disconcerting industry seen from the aeronautical industry and even from the automobile industry

 

My estimate of the payload in ocean mode is between cautious and somewhat exaggerated because it assumes 100 kg of food and 100 kg of water

With this estimate a very comfortable 9 square meter Jib is 24 square meters per Ton which could well be 26 square meters per Ton

The sailboat goes wonderfully with AWA 135° at a speed of 5-6-7-8 knots

which is quite a lot for its length and for what we are looking for

1) Roll control, and
2) Stable Balance

With the Jib in the long bowsprit the sailboat sails practically alone like Joshua Slocum's barge in harmony with the wind

A small and efficient "mountain bike" to redefine cruising sailing

he enjoys a sport boat, she goes by train or plane, two or three days anchored in a nice place and a civilized Hotel

 

Let's suppose that the 9 square meter Jib seems too little to the "mountain bike" user

no problem

the bowsprit turns aft, the sailor gets up from his comfortable seat (a cruelty of the designer) and hoists a 30 square meter Gennaker and changes gear:

40 square meters per Ton

 

the last gears of the mountain bike Downwind

the penultimate

Three reefed MainSail: 4 sqm
StaySail with its reef: 3 sqm

the sail arrangement with the StaySail reefed and the CLR far aft ... the sailboat sails alone in calculations if Heel/Roll < 20°, and in the real world it means that the helmsman does not get tired or curse the bad behavior of the sailboat and a small autopilot can take over the steering of the sailboat

last gear of the mountain bike

only the StaySail reefed and centered, and rudders fixed, and the sailor sheltered in the cabin in the navigator's bunk which minimizes the effect of Roll because it is in crosswise direction: it is the large nautical chart table and kitchen counter that is stepped on when entering through one of the two hatches

the old sailor smiles

 

About a month ago a small trimaran anchored in front of my house, a young Frenchman got down onto the sand with an inflatable board, walked to the market, it was market day, it was Sunday, an open-air market like centuries ago, and from the bottom of the small bay (which he couldn't cross with the trimaran because there is a bridge) he came back with the board propelled by the wind and a beach umbrella (!)

The fuxxx boss and Mary Poppins at the same time

 

Approximate position of the Frenchman's trimaran and his way back after buying fresh goods

 

two superimposed images

On one side, the small, functional boat of an old sailor is anchored in a shallow area with the enormous daggerboard inside the cockpit, following one of my idols: Margaret Dye and her delightful little book Dinghy Cruising

On the other side, sailing, in the cabin a young ocean adventurer is sleeping, or the retired German doctor who in the Caribbean complained to me about being the "janitor" of a huge Dinosaur

 

The young or old ocean adventurer sleeps close to the centre of Floatation, and in a way that minimises the effect of Roll, which will be small anyway, these light boats with adequate speed and course behave as if they were a catamaran

He sleeps on top of the cargo box for food which is divided into four boxes

The bow is divided into 10 compartments filled with a very low density material

Passive safety is two things:

(A) impact against an object: resistance to sinking

(B) impact of a Rogue Wave, resistance to overturning, overturning, sailor's ribs, flight of frozen chickens (no joke) and tin cans, fast and devastating flight of batteries, whether lead or lithium (perhaps the impact on the skull of a lithium battery is preferable) and righting

And let's not forget that many dinosaurs right themselves because they are flooded, a small or not so small horror film with a happy end