# Trawler stability

Discussion in 'Stability' started by cyro, Sep 30, 2012.

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### cyroJunior Member

I am from Myanmar(Burma).i m on project named STABILIY OF TRAWLER.Trawler type is side trawler.LOA is 127.8 ft.I have little resource for stability during trawling process.I want to know how will it affect GZ curve.Concept i now have is that the ship will list due to trawling.It will affect GZ curve.Ballast tank will need to fill water to counter the list.To know the amount of water,force that make the ship list will need to know.i don't know how to find that force. (My english skill is poor,not concern with my country....personal defect...) WITH RESPECT..

Last edited: Sep 30, 2012
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Location: Flattop Islands

You can think about the load acting on your vessel in (at least) two ways. The net drag is being transmitted via a wire to a towing hook or winch base. (breaking strength) And the load is imposed by the vessel (Bollard Pull).

Once you have a load and towing location and angle, you can calculate the heeling arm.

From a study I did a while back......I think y may be a yaw angle of the tow.

In working through the Draft Paper “Application of ISO 12217 for small vessels involved in towing, trawling or lifting operations” my findings are thus:

Section 3 – Trawling

Heeling Moment

Mt = ( Pb * sin (angle of trawl tow)) y

Pb = least of either bollard pull or breaking load on gear, in this case rated breaking load is 1000 pounds or 454kg. Bollard pull is approximately 2200kg.

Angle is 15 degrees

y = 0

Mt = 117kg

Heeling Arm

HA = ( 117 / (loaded mass + heeling mom) cos angle

HA = (117 / 2616 + 117) cos angle

HA = .0428m * cos angle

Criteria that shall be met when the heeling arm is superimposed on the righting arm curve (see previous page).

The residual righting area above the heeling arm curve up to an angle of 40 degrees or the downflooding angle shall be at least 40% of the residual righting area below the heeling arm curve between 0 and 40 degrees.

This criteria is met and the boat satisfies this Draft Standard in operation with a towing load up to a maximum of 454kg, but only under the load condition described (under Net Towing) above.

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### cyroJunior Member

Thank you sir.But LOA of trawler is 127.8ft.I forget to describe. Can above calculation still apply?

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The principals of stability remain the same no matter the size.

When towing a net there are (possibly) two angles, one vertical as the net is under water to some depth, and the other horizontal (not parallel to the ship's centerline) as when the ship is turning.

You will have to establish the operating parameters.

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### gonzoSenior Member

One of the dangers trawlers face is sinking when a net gets caught. The stern sinks and specially in rough weather, the vessel will weathercock. That is, the boat will swing downwind exposing the stern to the waves. I have been in that situation and it gets pretty hairy really fast.

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### cyroJunior Member

What is the meaning of (y=0) in above formula??.Which angle should i take.Horizontal or Vertical?GZ is for transverse stability.I like to to take horizontal angle.Could i?

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### cyroJunior Member

THANK U SIR ....My concept is far from stability in rough weather.All i know for such condition is WIND LOAD.I still trying to know stability in rough weather. I now have is static and little of dynamic.

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### Stephen DitmoreSenior Member

If you're modifying this existing ship with additional heavy equipment on deck, you need to adjust the center of gravity used to calculate the GZ curve. This can be done by adding in the new moments, but once the changes are made it should be confirmed with a new inclining test.

A problem I've seen in fishing boats when either equipment or payload (hold) capacity is added is that stability decreases. The easiest remedy is to add fixed ballast, but that diminishes freeboard, which may affect GM required or cause your payload to your load line to be further limited.

If building a new design or modifying the hull, the solution is to add beam near the waterline, increasing the beam to fairbody draft ratio. You see such sponsons added to ships sometimes, including older cruise ships. I've written a spreadsheet that can help get the beam to fairbody draft ratio (among other things) right for a new design. I wrote it for a longline fishing boat, so in its current form I have not calculated in the forces you and Tad are discussing. Private message me if interested in my spreadsheet.

If I'm stating something obvious to you I apologize.

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### cyroJunior Member

senior,Thank for ur attention and ideas.I cannot make inclining test.It does not exist.I have only6.5 feet model ship.

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### Stephen DitmoreSenior Member

I thought 127.8 ft ...?

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### cyroJunior Member

so close:!130....how did u get the scale.scale is 1:20

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