Tug Weight estimation

[Hisham]

Could any one tell me how to estimate the light weight of a towing and oil recovery tug (principle dimension are known)

I know a several methods based on a basis or sister ships, but I have a shortage in available data so I can’t use these methods

I need fast formula (not accurate but almost near)

I found this formula:

Light Weight = 0.2 (L.B.D) tons

Is this formula true?

[jehardiman]

If in meters, more like 0.5*L*B*T = tonnes Where L = LWL, B=max beam waterline, and T = midships keel draft.

How you measure D (depth of hold) or T (draft) is the key for these "rule of thumb" "calculations".

[Hisham]

Your formula can be used to determine weight of an existing tug (has a known draft)
This equation based on Cb and Cp that they vary from (4.5 – 5.5) for tugs

What I need is to estimate the light weight in first design stage, then I can make a prediction of draft on witch the coming steps of design will depend on

[jehardiman]

You have it backwards, in the classic design spirial...
1) select L, B, and T first from shape/tonnage laws/mission considerations
2) draw the lines and curves of form
3) do the structural weight/strength
4) add the propulsion, outfit, and stores/cargo
5) check against the L, B, and T, and around the design spirial again.

If what you want is a quick and dirty structural weight then here is a way to get close.

shell weight = surface area of the hull * plating weight
deck weight = surface area of the decks * plating weight
plating weight = shell weight +deck weight
structure weight = 50-70% of plating weight
light ship weight = plating weight + structure weight+ machinery weight + 7% of the total other weights for paint and welds/fastenings.

[Guillermo]

According to my notes on tugs, a first approach to displacement in light ship condition (WR), when in the first stages of design, is:
0.24*L*B*D < WR < 0.29*L*B*D (Metric units) (L =Lpp)
For ice-going heavy reinforced tugs, superior limit may go up to 0.34*L*B*H

[Guillermo]

A second way of estimating WR, is:

WR = k1*L*B*D + k2*PB (being PB the propulsive power in kW)

k1 varies between 0.18 and 0.26
k2 varies between 0.08 and 0.1