Barge Tow Calculator

Barge

Load a barge from the workbook

Length m Breadth m Draft m Block coefficient Freeboard m Cargo height above deck m Cargo width % of beam Cargo length % of length Hull form

Areas

Bow windage area m²

Stern windage aream²

Starboard windage aream²

Port windage aream²

Immersed section m²

Wetted area m²

Windage is taken face by face. Wind from ahead acts on the bow area, from astern on the stern area, and from either beam on that side's area, so a deckhouse at the stern or a crane on one side can be entered where it is. Type each face as the area the wind actually meets, allowing for items hidden behind others. Zero is allowed, for a face with nothing to catch the wind. Leave the bow or starboard area blank and it is estimated from the dimensions. Leave the stern blank and it takes the bow area, and leave the port side blank and it takes the starboard area. Tracing a drawing fills in the bow windage, section and wetted areas, and you can type over any of them.

Coefficients you can change

Bow drag coefficientApplied to breadth times draft in the tow speed model only. Set by hull form. Wave reflection coefficient Rc0 to 1. The PLB Eclipse workbook uses 0.67. Waves on the beam use 1. Wind coefficientShape times height coefficient on windage area. Resistance margin %Towline drag, yawing and fouling.

Barge sketch

Drawn to scale from the Barge inputs and the weather. Each elevation shows the immersed part in blue below the waterline, and the windage above it: the hull freeboard in grey and the cargo in pink. The cargo is drawn at the height that gives the windage area the calculation uses for that face, so a typed area changes the drawing. The gold dashed line is the wetted surface: the bottom and the sides below the waterline. The rake and bilge shapes are illustrative, not a hull drawing. The end views are drawn to a smaller scale when the barge is very wide.

Weather

Wind speed kn Wind from ° Significant wave height m Waves from ° Current speed kn Current from °

Directions are where the weather comes from, in degrees clockwise from the bow: 0 is from ahead, 90 is on the starboard beam, 180 is from astern, 270 is on the port beam. A current from 0 is against the tow and slows it. The presets are the DNVGL-ST-N001 Table 11-17 conditions: open ocean is 20 m/s wind, 0.5 m/s current and Hs 5 m; benign is 15 m/s, 0.5 m/s and Hs 2 m; under 24 h is 15 m/s, 0.5 m/s and Hs 3 m. Picking a preset sets all three directions back to 0.

Tug

Load a tug from the workbook

Continuous bollard pull t Brake horsepower BHP Propulsion Bollard pull per 100 BHP tA rule of thumb. Use the certificate when you have it. Free-running speed kn Engine load Tug length overall mUsed for the efficiency formula, capped at 45 m. Tug efficiency, override %

Target speed

Target speed over ground knThe weather limits below are for holding this speed.

Tow speed in this weather

-kn

Calm sea, same current

Zero-speed check

Required bollard pull
Tug available
Margin

Weather limits for the target speed

Largest wave height
Largest current
Largest wind

Drawings

Load a drawing of the barge and the load on deck, set the scale, then trace the outlines. What you trace is written into the Barge inputs above, and you can still type over it. Accepted files: PNG, JPG, PDF, and ASCII DXF. A DWG must be converted to DXF first.

Drop a drawing here

PNG, JPG, PDF or ASCII DXF

Choose a drawing

Trace as

Read from the drawing

These values are written into the Barge inputs above.

Force breakdown

Component	0 kn	-

Forces in tonnes-force. 1 t is 9.81 kN. Sideways force is the signed sum for the wind, waves and current. The towline pull needed is the vector sum of the along-tow and sideways loads. At 0 kn the along-tow current is the guideline simplification, breadth times draft. At the operating speed it is hull resistance through the water.

How it works

Zero-speed check. DNVGL-ST-N001 [11.12.2.8] requires bollard pull × Teff / 100 to be at least the towline pull required (TPR). TPR is the wind, wave drift and current at zero speed: wind is ½ρ_airV²A with ρ_air 1.226 kg/m³, wave drift is ρgRc²BHs²/8, and the current is ½ρ_wV² on breadth times draft. These follow the PLB Eclipse bollard pull workbook. Forces are resolved along and across the tow, and the towline has to balance both, so TPR is the vector sum. Wind or current from astern is not credited. Teff comes from [11.12.2.10]: 80 − (18 − 0.0417 × LOA × √(BP − 20)) × (Hs − 1) %, with tug LOA capped at 45 m, BP between 20 and 100 t and Hs from 1 m. You can override it.

Tow speed. Total resistance is found at each speed. Wind uses the tow speed plus the wind speed. The effective tug pull is bollard pull × Teff × engine load, falling in a straight line to zero at free-running speed. Teff is the DNV value for the wave height set, and the calm-sea speed uses 80 %. Tow speed is where the two lines meet. A current against the tow raises the speed through the water and slows the tow over ground. A current with it does the reverse. Weather at an angle is resolved on the relative wind: the bow or stern area takes the along-tow part and the starboard or port area takes the sideways part, whichever face the wind meets. Waves use the projected width, with the reflection coefficient rising to 1 on the beam. A current on the beam acts on length times draft. At speed the tug pull is compared with the vector sum of the along-tow and sideways loads, which is a screening way to cover drift and yaw correction.

What is from DNV and what is not. The weather conditions in the presets (Table 11-17), the pass check and the tug efficiency formula are from DNVGL-ST-N001. The force formulas and Rc 0.67 are from the PLB Eclipse workbook. DNVGL-RP-N103 section 7, which ST-N001 [11.12.2.1] refers to for guidance, was not available to check them. The tow speed model, the bow drag coefficient, the wind coefficient of 1.0 and the beam reflection of 1.0 are screening values that DNV does not set.

Barge sketch. Plan view with the weather, then the bow, stern, starboard and port elevations. Below the waterline is the immersed area, above it the windage. The cargo is drawn at the height that gives the windage area the calculation uses for that face, so typed areas show. The gold dashed line is the wetted surface.

Drawings. Tracing writes into the Barge inputs: the waterplane sets length and breadth and the wetted area, the immersed section sets the draft and the section area, and the windage shapes set the bow windage area, each with its own coefficient. Anything you type over stays as typed. Tracing accuracy depends on the scale you set and how closely you follow the outline.

Not modelled. Tug thrust loss beyond the Teff derating, drift and yaw drag beyond the vector sum, towline length and catenary, wind on the tug, and wave drift changing with speed. Hull resistance is a simple friction plus bow-drag model, not Holtrop-Mennen.