Towing vessel to tow ratio

[MoonlightShadow]

Really hurting my brain trying to suss this one out (it is of limited capacity), but...

We need a boat to handle emergency towage of vessels up to 5,000 tonnes grt, 6m draft, 85m loa, probably when the wind is blowing hard, and most likely against the tide. (and probably just as I was about to have dinner).

But, this requirement is only "in an emergency" such as a vessel losing power when it comes off the berth.

Could we get away with an existing vessel such as a high speed (22kts) semi-diaplacement Pilot Boat? Or would we need a full displacement tug?

Are there any tables or advice anywhere to give indicative information?

[jehardiman]

If you have the prop and engine curves for the pilot boat, you could work out her capability. In all honesty though, my first impression would be that her screws would be too small to be of any use even if she did have the requsite hp, and with no proper deck gear, the chance of girding would be high.

A swag would be that the just for berthing you would need HP equal to 5% or so of the vessels deadweight tonnage, (not GRT). To hold off a lee shore in a full gale would require hp equal to 30-40% of the deadweight tonnage. This is why ocean salvage tugs are so large. See this draft report for Puget Sound on the necessary size for escorting tugs. http://www.environmental-research.co..._report_6A.pdf

Edit: Here is another requirements document for LA/Long Beach http://www.mxsocal.org/pdffiles/Chap...%20Vessels.pdf which lists a minimum bollard pull of 25 short tons.

Also read this report http://www.maib.gov.uk/cms_resources...oom_Report.pdf

[MoonlightShadow]

Wow. Thanks jehardiman. Excellent. That's exactly what I was looking for. I will now spend the next few hours reading and trying to understand.

Very much appreciated.

One cannot use a semidispl. boat for a serious tow job!

And it will be serious in case of emergency. As clearly stated.

You need sufficient displacement in the towing vessel, not just power. Otherwise the "tug" gets turtled.

Regards
Richard