Any fuel savings possible by towing another boat?

Discussion in 'Diesel Engines' started by Ryan McEachern, Jun 14, 2008.

  1. Ryan McEachern
    Joined: Jun 2008
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    Ryan McEachern New Member

    Hi,

    I am trying to find any information on what the total effect on fuel consumption would be for two vessels if one was to tow the other instead of both traveling under their own power.

    Obviously it would depend on the relative efficiencies of the different engines, the speed of the tow, etc. but are there any hard and fast rules to consider?

    The setting is a family commercial fishing fleet of 6 gillnet boats, all semi-planning hull design, 35-40 feet in length. Fuel prices and limited fishing oppurtunities have led to us traveling to and from the fishing grounds at around 7 knots, which has resulted in substantial fuel savings versus the 9 knots we used to travel at.

    Is there any further savings to be had by one vessel taking another under tow? It seems counter-intuitive to me, as the force pulling back on the tow rope of the tug vessel would be equal to the force pushing forward from the propeller of the towed vessel if it wasn't being towed, plus the towed vessel must be pulled through the prop wash of the tug vessel, which would still be pushed slightly astern wouldn't it?

    Anyway, some have started to play around with this concept lately, and I am hoping you learned folks could add some science to the speculation.

    -Ryan McEachern
     
  2. Ryan McEachern
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    Ryan McEachern New Member

    Other information source?

    If no one has any suggestions right now, is there any recommended reading I could do? I have exhausted all of my seamanship, sailing, and oceanography books without seeing anything mentioned.

    -Ryan McEachern
     
  3. masalai
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    masalai masalai

    Only for the guy being towed.... There is a risk of doing substantial damage to either/both boats as neither are built or designed for such. That is why small "sea rescue craft are fitted with towing bollards during initial construction & designed for it, and the tow usually progresses very slowly so as not to rib the bow of the towed boat to pieces - Those parts are designed for light weather anchoring not heavy towing loads... Try it if you must having fitted an accurate fuel use measuring system coupled to an accurate log - measure through the water and only go slowly - then travel at the same speed without the towed boat to get the fuel use difference..... You will find NO ADVANTAGE and NO BENEFIT and a high probability of damaged boat/s...
     
  4. TollyWally
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    TollyWally Senior Member

    LOL, we're talking about fishermen here not the Coast Guard. I'd imagine a family with 6 gillnet boats can probably be trusted with the details of successfully rigging for a tow.
     
  5. masalai
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    masalai masalai

    Didn't seem that way in the first post? - gillnets are illegal here (I think?) and the "weekend leisure folk" here in their production glass boats with big outboards, would be ill-advised such a stunt - even if their mate ran out of fuel....The "coastguard" here is a FULL VOLUNTEER SERVICE, and usually rescues idiots who run out of fuel...
     
  6. Ryan McEachern
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    Ryan McEachern New Member

    Thanks for the come-backs.

    No problem with safely towing or being towed. It is a pretty common event for us, I was just wondering if there was any wisdom or experience around the fuel consumption issue.

    In the 1930's before a reliable engine was developed for the small boat fleet, it was common place for the cannery tug to tow dozens of little double-ender gillnetters up the coast on two strings behind the camp barge. More recently, when reliable high-speed diesels began to replace the chysler 440's and other large gas engines, it was commonplace to see a diesel powered boat towing a gas powered boat up to the fishing grounds because of the significant fuel savings. Now days, most everyone has a diesel engine of similair efficiency, and any towing we do in our group is usually due to break-downs, or just to take a break and socialize in the tow vessel for a while.

    I have been thinking the issue through for a few days, and I think those who are claiming fuel savings by towing are probably wrong, and haven't taken the time or applied enough science to see it.

    The biggest problem I can see is that when one vessel is idle, and the other is towing, the second boat is going to be trimmed more bow down that it would if it was under it's own power, due to the fact that the propeller isn't pushing the nose up, and that the tow rope is pulling down on the bow bridle.
    The way the hull is designed on our boats, I would guess a more bow-down attitude is less efficient.

    The possible fuel savings could come from the fact that the towed boat has its engine off, and would be saving the friction from the engine turning over at a given RPM, which I guess could be calculated to be the same fuel burnt as the engine at cruising RPM, but out of gear. Of course, wear and tear would be saved as well, but the added cost of maintaining the towing gear and loss of time in travelling slower would probably outweigh that.

    I definately wasn't advocating for mariners to take up towing if it wasn't already part of their operation. I was just looking to see if there was any science around to disprove the "fuel saving" theory, or whether I would have to get out the fuel burn analysis kit I use on sea-trials again and prove it to be false myself.

    -Ryan McEachern

    p.s. Masalai - you might be confusing different types of gillnetting. You are probably thinking of deep-sea driftnetting which is quite illegal most anywhere, except for the high-seas pirates, and the type of terminal fishery gillnetting we do which is a very enviromentally sustainable and selective fishery conducted close to shore is various inlets and mostly protected areas.
     

  7. masalai
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    masalai masalai

    OK , Thanks for the understanding - I as usual spot an ignored thread and hopefully draw attention to it. Just another dumbarse service. :D:D:D:D:D
     
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