Database


These pages are for we have a more detailed database.

The menu database have sub menus for boats that we have a great more informations about his design.

The Restricted Database is a page that contains some few informations about the boats.

Below we will provide some definitions of TERMS used in the database

LOA – Maximum length – length over all

BOA or Bmax – Maximum beam – beam over all (in transverse direction)

LWL – Maximum design  waterline length

The intercession of  the water surface plan with the hull surface generates a figure we call waterline and we represents it by the boundary line. The waterline on the boat will sail is called the design waterline (DWL). The others waterlines are parallel to the DWL and serves to represent the geometric shape of the hull. To designate these waterlines, normally, we associate it with the respective draft – waterline at o.o3 m or WL 0.03 or 0.03 m WL or 8′ WL.

The waterlines are drawn in plan view:

BWL – maximum breadth in DWL

Draught (T) – is the measure of the maximum boat immersed point.

Displaced Volume – Is the volume of water displaced by the boat, naturally is equal the immersed boat volume. The importance in know this volume comes from Archimedes ( http://en.wikipedia.org/wiki/Archimedes ) that do the principal scientific discovery base for the naval architecture: ” a body immersed in a fluid experiences a buoyant force equal to the weight of the fluid it displaces ”

This means that if we have an IOM weighting 4 kg it need displaces a volume of water such that this volume weighs 4 kg.

So when we draw a IOM boat with a weight prevision for 4 kg we need design a underwater volume whose displace 4 kg water. What will be this volume?

Depends, if this volume is in salt water we have:

salt water density = 1.025 t/m³ = 1025 kg/m³ –>  4 kg in salt water have a volume = (4 kg) / (1025 kg/m³) = 0.0039024 m³ = 3.9024E-3 cu m (E-3 = 10⁻³)

if this volume is in fresh water we have:

fresh water density = 1.000 t/m³ = 1000 kg/m³ –> 4 kg in fresh water have a volume = (4 kg)/(1000 kg/m³) = 0.004 m³ = 4.000 E-3 cu m

Well, if I want the boat floating in a specific waterline we need have a hull underwater volume (that is equal the displaced volume) =

in salt water –> 0.0039024 m³

in fresh water –> 0.004000 m³

What  underwater volume I need have for a IOM boat?

The IOM is measured in fresh water. In fresh water we need more volume underwater  so we need design the IOM boat for fresh water. The IOM boat need a minimum underwater volume = 0.00400 m³

Well, here it is a warning:

the maximum canoe draught (boat without keel and rudder or any appendages) by IOM rules is 0.06 m. If I do a IOM design with 0.06 m draught with 0.004 m³ displaced volume, I do not have any margin for complete hull weight. In this case (dangerous) the complete boat need weight exactly 4.000 kg, no gram is more or less. What I do? For my designs I use a maximum for draught o.o57 m with 4.000 kg displacement, so I have a margin both for the weight of the prototype or for future changes in the hull or equipment as well as the difference of measuring equipment at the time of the race.

Wetted Surface (WS) –  Is the hull underwater surface in contact with water. See his importance in naval architecture page.

Waterplane area (WA) – normally is the design waterplane area.

Whole hull surface area – Is the total hull area (immersed area + emerged area )

Whole surface LCG – LCG is Longitudinal Center of Gravity or Mass so, this data is the LCG for all boat surface. We use this value to calculate the boat Center of Gravity if the hull has a uniform thickness (never).

Whole surface VCG – VCG is Vertical Center of Gravity or Mass, is the same above only that is in vertical position.

Midsection freeboard – Is the hull height in midsection above water.

Stem freeboard – Is the hull height in stem above water

Stern freeboard – Is the hull height in stern above water.

Profile area above waterline – We can use to calculate the wind force actuating on the surface above waterline. Only used in real boats.

Profile area below waterline – Very important to calculate, with his LCG, together with the keel and rudder, the balance (sail) of the boat, to put the sails in position to give a little weather helm.

Longitudinal center of buoyancy (LCB) – Is the longitudinal position of immersed volume center of gravity. Is the point where acts the resulting from the forces of water pressure on the hull. Buoyancy force. Very important to determine where the boat center of mass should be ( the same vertical). Knowing where is the LCB we can place the bulb, keel weights and all weights on board so that the final resultant is in the same vertical, same longitudinal position.

Vertical Center of Buoyancy (VCB) – Is the vertical position center of Buoyancy.

Longitudinal Center of Flotation (LCF) – Is the design waterline longitudinal center of gravity, is the point where pass the trim transverse axis. Where the boats rotates along. The relation LCB/LCF  indicate the  hull distortion fore and aft.

WL transverse moment of inertia ( It )– Moment of inertia in our case is a  property of area figure, also known as second moment of area. All plan figures has his moment of inertia and the waterlines also. For us this area property is used for determine the transverse metacenter position in little heel angles – Initial stability. As curiosity, the moment of inertia of the beam’s cross sections are fundamental to calculate structures. The moment of inertia is always referred to a axis, and in our case longitudinal axis,  passing by LCF,  (the waterplane’s center of gravity) .

Wl longitudinal moment of inertia ( Il )–  is the same above but referred to the transverse axis  passing by LCF .  We use Il for calculate the longitudinal metacenter.

Metacenter – Is a point in station center line passing by the boat center of gravity, where the direction of buoyancy force crosses it. See figure:

Up to 7 degrees heel, known as initial stability,  we can calculate the transverse metacenter position by the formula:

BMt = It / Displaced volume

and the longitudinal metacenter position:

BMl = Il / Displaced Volume

Righting Moment – When in heel or in trim the center of buoyancy change his position because the immersed volume change his form. The force of Buoyancy departs from its initial position under G and with this creates a moment with the Weight force called Righting Moment. The formula for the Transverse Righting Moment is:

RMt = W * GZ = B * GZ

GZ is called Righting Arm and is the lesser distance between the forces.

The principal importance of stability in RC boats is to assess the ability to withstand the lateral force exerted by the wind. A boat with better stability heel less and therefore sail will be more efficient.

Transverse metacentric height (GM) As you see in figure above is the distance between the boat center of gravity and the point M, the metacenter. This distance is a stability measure. If M is above G GM is positive and if M is under G, GM is negative. For real boats we have minimums that we need have obtain in design to assure that the boat is stable. For our RC boat we do not have problem in stability due to weights because the bulb, the greater weight is in the lowest position possible.

X center of mass (LCG) – Is the boat longitudinal position  center of gravity G . The Center of gravity position is the point where is the resultant of all weights forces on board.

Z  center of mass (VCG) –  Is the boat center of gravity G vertical position .

The coefficients

Prismatic coefficient – is the principal sailboat coefficient since it is associated with wave resistance. See the text for prismatic coefficient in Naval Architecture page.

Midsection coefficient – Cm

Cm = Am / B * H

Waterplane Coefficient – Cwl

Cwl = Awl / LWL * BWL

Block Coefficient – Cb

Cb = Displaced Volume / LWL * BWL * H

Continue…..

2 Responses to Database

  1. Rob says:

    Like it – good explanation in relatively simple terms of some of the language used to describe some aspects of design.

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