Welds length estimation

Is there a rule of thumb how many meters of welds are per metric ton of boat? For example if steel motor boat displaces 5 tons how many meters of welds are done when building?

 

Simple answer no.

Weld lengths depend upon many factors. The thickness of the attached and parent metal, the location in the vessel and the type of metal and lastly if you are building to Class....there are different allowable lengths for each class society too.

 

In addition to the above, surely the developed plates will give all the edge lengths? Then add any ribs, stringers, other framing and gusseting etc etc, it is all in the drawings whether CAD or paper.

 

Can you at least give me an example in 7-9 meters range?

 

What is the purpose of this calculation? If it is for preliminary estimate of costs, then you can roughly assume that welds shall be on average approx. 3% of the overall structural weight of the boat (steel structure). That's the number which works pretty fine for us, as a general-purpose estimate. From there (with some allowance) you can work out to the required quantity of wire and rods and the man-hours required for the welding job.
Of course, if there are many very small parts and particulars to be welded together, then both the weight percentage and the man-hour numbers will be corrected as necessary.
This is in addition to the considerations given by Ad Hoc and SukiSolo, which remain perfectly valid and true.