# Net Drag on a fishing trawler

Discussion in 'Boat Design' started by harisankar, Mar 21, 2009.

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### harisankarJunior Member

can any body suggest a method to calculate the Drag force on a net while trawling?

The boat dimensions are
L = 20 m
B=3 m
T 1.7 m
Trawling velocity 5 knots

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### tom28571Senior Member

I don't know but from watching local shrimp trawlers for years, I will say that the drag is very high and 5 knots sounds very fast. Asking this question to the staff of National Fisherman Magazine would be in order. Perhaps they have a website.

Just googled it and the website is: http://www.nationalfisherman.com/month-content.asp

Last edited: Mar 21, 2009
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### marshmatSenior Member

Hi Hari,

If you're just after a first approximation, I suspect you could get reasonably close by calculating the drag per unit length at your target speed on a cylinder of the same diameter as the lines making up the net, and multiplying by the total length of line used to weave the net.

This would probably be reasonably close to the actual drag on an empty net, but would underestimate the drag encountered once there are some fish in it. I suppose you could then estimate the drag on a sideways fish, to get an idea of how much the drag increases as the net fills.

Or, if you're feeling more practical and less theoretical, just go with the breaking strength of the line holding the net to the boat. Any load higher than that and you'll lose the net anyway.

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you can contact FOOD & AGRICULTURE ORGANISATION & FISHING NEWS BOOKS

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### PrevaricatJunior Member

Hi Hari,

This is a very well researched area, and one I was involved with 30 odd years ago - it would be interesting for me to revisit...

For the net itself you will need to know twine size(s), mesh size(s), panel layout/sizes and some idea of the geometry of the net as set.

For a pelagic trawl you will also need to calculate the drag from: warps, doors (up to 40% of the total gear drag), flotation, and weights.

For a demersal trawl you will need to calculate the additional drag of other bottom gear and the effect of the nature of the ground. These together can be very significant (in the order of 20% of the total gear drag for the case of heavy gear on soft or rough ground).

If you could provide more information on the type of gear you anticipate using I will help out if I can

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### harisankarJunior Member

Dear All
Thanks for the replies.The boat is already built and I am redesigning the propeller.

The drag force on the Net is estimated at 4.5 knots to be 36.45 KN according to marsh matt suggested method
1300 m twine rope length
2.5 mm wire rope dia.a
ssumed Cd=1.3
assuming net is a cone 60 feet base 100 feet height varying mesh sizes from 800x800 mm to 25x25 mm).

Breaking strength of wire rope is 50 KN.

Do you think the calculated drag resistance is good for moving forward?

I have found a paper "a net drag formula for pelagic nets" a paper published by dept of agriculture and fisheries for scotland. Drag force on net is derived from twine sizes,mesh sizes paney layouts.But i do not have much idea abt the trawl net as well I did not under stand what they mean by panel.I suppose prevari cat has good know how about that.

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### ned LJunior Member

I believe "panel" is refering to the individual panels of twine that are cut out and then sewn together to form the net. Trawl nets are created much like articles of clothing in that individual flat panels of premade twine (or fabric in the case of clothing) are cut out and then sewn together to form the net.

I too studied net design and construction about 30 years ago, but I haven't had the enjoyment of working with it much since then. I used to enjoy it, it is part science & part art, being able to hang a net that pulls well.

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### PrevaricatJunior Member

Ned L is spot on in his description of 'panel' - they are just the sections of premade netting that are put together to form the shape of the trawl. The reason that I mentioned 'geometry as set', by which I meant the shape of the net as it is being towed, is to enable you to have some idea of the angle of attack of each of these various panels for your drag calculations.

A cone shape is a reasonable first approximation I expect, but 25 by 25 mesh is very small, and most probably illegal, for any part of any trawl. However if your figure of 36.45 kN is close to correct (to which you will have to add the drag of the other gear I mentioned) then you have very little safety margin with 2 x 50 kN towing warps. You did say 'breaking strength' and not working load.

I can rough out a spread sheet for you by the end of the week if that would help. I can add the drag from the other elements I mentioned too but so I don't add anything irrelevant can you tell me what sort of gear is used by other fisherman in the area (if any) and/or give me a reasonably precise location, or description, of the fishing grounds and species targeted.

I would love to see a picture of your boat too...

Another thought occurs... I have no idea what facilities you have available but if there are other similar boats in the area and you are on good terms with them why not arrange a trip out and a load cell on one of their towing warps (what you have called 'wire rope' in your description)? Maybe a hassle but you will have real data to base your prop calculations on...

Andy

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### PrevaricatJunior Member

After promising to help you with a spread sheet I have just been brushing up my knowledge on the web - it was 30 years ago remember - and found exactly what you need.

Google 'Calculations for Fishing Gear Designs By A. L. Fridman' the first link is a google book. It is an incomplete 'review copy' but very little is missing and certainly everything you need is there - and a lot more besides. It costs nothing.

I'm still happy to do the spread sheet for you as it will be easier for me since I already have a handle on the terminology.

Andy

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