stern trawlers engine

why do most stern trawlers have been designed by one engine?

 

Why you need more than one engine ?. Maybe that answers your question.

 

because in my country 1 engine with more than 2500hp is prohibited.

 

Well there's your answer: because in other countries there is no such prohibition.
Two engines means doubling the spare parts, duplicate maintenance, etc.

 

What size trawler are they?

 

Two engines means you have redundancy to make it back home safely and gives you 1,000% better slow speed/docking maneuverability.

 

Two engines increases the probability of the nets fouling the prop.

 

size of trawler:L=47 B=10
is there any problem with stability during net pulling?

 

During trawling operation, the propeller should operate at greatest possible depth in order to give as steady speed forward as possible in rough conditions. Constant speed through the water is a requisite for the trawl itself, and one big propeller at maximum depth is the best way to achieve this.

When this operational requirement is combined with the others, as mentioned above, the most rational solution is one single, low speed engine that is designed "to run forever, once started".

 

Thanks A Lot. Very Good Guidance.

 

A typical load condition to be studied is the "last hoisted of the net" (sorry for my tranlation), ie, the boat almost completely filled with fishing and the net, full of fish, hanging from the rigging. Usually there is no stability problems but this loading condition must be considered.

 

One of the dangers in trawlers is that when hauling the nets the bow turns downwind and the stern faces the wind and waves. In rough weather, if the nets hungs up on something, it can bring the stern underwater and ship waves. Also, 2500HP seem adequate for a trawler of the size you mention. They are low speed vessels.

 

All fishermen know that "In rough weather" (moments before) should stop working, lifting the net, close all openings which can lead to flooding, stowing and securing cargo well, and try to weather the storm. Any other action may be dangerous, not because of the net but because of the incompetence of the master.
No naval architecture calculations or actions to take on board that can anticipate and solve such unwise.
It is normal for the bow or stern is underwater (not when the boat is fishing), waves normally sweep the poop deck. It may be that a very large wave attack boat unexpectedly but that is a situation that can not be foreseen. It is the experience of the pilot that should solve it.
But I fear that all this has nothing to do with what the OP asked. We are taking the flower out of the vase.
Another important thing: these boats do not need much power to reach high speeds, need lots bollard pull, which results in needing of powerful engines.

 

I may not be farmilar with your trawl or trawling process. But. Been trawling since I was a child......and have built several.....and worked several different kind of trawler's built by others. I grew up in a commercial fishing industry and my family was boabuilders. I have never seen the need for 2500 HP in a trawler of that size. As mentioned n this thread...the efficiency of the trawler is best when the prop diameter is the largest possible and at the greatest depth achievable.... but we get the power to produce bollard pull thru reduction ... I'm not sure if you are meaning engine horsepower... or shaft horsepower. I ran a boat that was powered by two 3412 cats producing 715 HP each coupled to twin disc 522 gears with a 7.5 to 1 reduction. The propeller size was 78x78 in nozzles. It was Wes me... I pulled four 80 foot shrimp trawls with 2 sets of otter trawl doors measuring 12 foot x 4 ft. I pulled the trawls efficiently at 3 knots with a English he rpm speed of only 1175 rpm. This boat have unbelievable bollard pull.... and that's where its at when it comes to trawling. But keep n mind that this boat was documented at 108 ft. . Two engines are better than one when it comes to trawling.. even more so when it is rough. . Power is calculated as bollard pull. . And in the trawling industry.... more power = more performance = more profit...

 

And one thing for sure.....different captains have different ways of doing things. But you will never find me puck n up my rigs good ng with the sea.... I'm always head up into the sea..... when going with the sea The trawl bags are subject to be washed under the boat . I always g head into it so my bags stay tight behind me until lifted out of the water.... then again I am not famlar with all trawls and trawling processes.