A tiny push-tug ?

Discussion in 'Boat Design' started by SAE140, Sep 22, 2006.

  1. SAE140

    SAE140 Guest

    I've got a 21 ft sailing yacht that I'd like to take many hundreds of miles through the French canal system over a several month period, but don't really want to wear out my outboard engine in the process.

    I've just acquired a 3.5HP horizontal shaft low-revving diesel engine from a cement mixer which is designed to run all day on just a couple of pints of diesel. It's a beaut runner, but the yacht is really too small to install such a beast, and life on board during the several months in question would be pretty unpleasant due to hull vibration. There's also cooling and the exhaust to think about. Also - I really wouldn't want this as a permanent fixture.

    As I only want to use the diesel engine on inland waters, I've come up with the concept of a push-tug. This would be nothing more than the diesel engine fitted into a half-sized 'dinghy' coupled to the stern of the yacht by something similar to a tractor's 3-point linkage. Vents would be needed as the engine is air-cooled; exhaust could either be direct to the atmosphere, or directed underwater (preferred). Otherwise the push-tug would be covered to keep noise down to a minimum. Basic controls to the engine and battery charging cables would link the 2 vessels. No clutch, no reverse. I'd be relying on killing the diesel and firing-up the yacht's outboard if I really wanted to stop in a hurry.

    Has anyone tried this sort of arrangement at this scale before ? Can anyone see any obvious snags I've overlooked ?

  2. Poida
    Joined: Apr 2006
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    Poida Senior Member

    During my sales career I have sold air cooled engines.
    Quite often the question would come from someone wanting to mount one of these engines in a boat.

    The answer was always, "It is an air cooled engine, designed to work in open air, and not suitable to be enclosed at all."

    They are designed for air to circulate upwards from all around the engine upwards. As the hot air rises it sucks in fresh cool air to virtually take the place of the vacuum left by the rising warmed air.

    Vents are not recommended as they channel cool air onto small areas of the engine creating uneven heating that can cause expansion or contraction cracks.

    I also think you would have problems with your steering with that configeration.
  3. SAE140

    SAE140 Guest

    Thanks for the reply - appreciated. This isn't your average air-cooled engine - the air-cooling fins of the engine are already enclosed within a steel shroud. There are fan blades attached to the shaft which pull or push (can't remember which) thus circulating the air inside this steel shrouding, and thus around the engine's fins. In a cement mixer, this engine would have further been operated within a steel 'cabinet' fitted to the mixer's frame which would have had vents to coincide with those of the engine's shrouding. Hope my description is reasonably clear.

    Hopefully steering shouldn't be too much of a problem on a canal - but thanks for mentioning it. Something to consider, certainly.

  4. safewalrus
    Joined: Feb 2005
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    safewalrus Ancient Marriner

    SAE140 (named after an extremly viscous grease I see - trying to tell us something?) I wouldn't worry too much about the air cooled side of the equation lots of ships lifeboats and naval seaboats where (and as far as I'm aware still are - apart from the fact the ubictious outboard appears to have taken over most things) such beasts! Prevented them from seizing up when run at the davit head! :rolleyes: Just remember to ensure that the air can get to the motor in a plentiful supply:rolleyes:

    I would however be concerned with the steering problem - please remember that a lot of the French canals are still working canals and bloody big barges crashing through can be somewhat interesting - especially at 21 foot! And the average bargee don't care! :p especially the French;:eek: Red Ensign to a bull if you get my drift (no pun intended - well not much anyway!) :eek: I'd be more inclined to obtain a small second outboard as a trolling motor (second hand maybe with a view to ditching it after the trip) it would use the same fuel as the other outboard which cuts down on the different amounts of fuel you will need to carry amongst others - unless of course you intend to make your own anchors enroute? can't beat a good block of concrete as a throw away anchor can 'eee!:D
  5. JPC
    Joined: Jun 2005
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    Location: Hong Kong

    JPC Junior Member

    Sounds like a fun idea for one inclined to tinker a bit. I'd want a good "Plan B" with several hundred miles of canal in front of me.

    I might not be sufficiently appreciating the voiced concerns over steering, but if the worry is that the arrangement will not have the advantages of either (i) prop wash directed over the rudder, or (ii) the ability to direct the thrust vector as in an outboard or stern-drive, I don't think you will have a problem. Many small sailboats using outboards leave them locked in position and rely on the rudder (forward of the prop) to steer. Granted, if you've got a slow-steering vessel with keel-hung rudder, this might not be so easy -that I haven't tried.

    I'd wonder about docking maneuvers (stern-to?) with that extra projection off the stern that won't be convenient to leap on to for fending off or passing a line.

    I also wonder about the stopping/emergency-maneuver plan. Abandoning the helm to hang over the transom and tug an outboard start-cord is a lousy activity to have to interject into you pre-collision sequence...

  6. Poida
    Joined: Apr 2006
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    Poida Senior Member

    Great, never seen an engine like that but sounds good. Made me realise that an air cooled diesel engine inside an enclosed aircompressor cabinet would be OK.

    Steering - I had a problem steering a surfcat where I removed the existing rudders and tried to use an outboard attached to a transom I attached to the back of the trampoline frame.

    As the back of the frame was forward of the stern when I turned the motor the surfcat wouldn't turn but started to drift sideways.

    It seemed the motor was trying to push the back of the hull sideways through the water, which took too much force so the boat slewed sideways.

    That's why I invisaged a problem in your configeration as your rudder would have to swing the small motor carrying barge sideways restricting the acuteness of your turns.

    However may I make a suggestion. Please realise I am not an angineer or a naval architect just a natural born genius.

    As you were going to attach the motor barge via a 3 point linkage system, what about a 2 point linkage. One in the centre of the barge and the other, a standard boat hydraulic steering system.

    This means you would be steering using your motor in an outboard configeration enabling sharper turns.

    I 'spose you have thought of legal things since you will really be changing the size of your boat.

    I would if it was me, and unfortunately it's not I would be thinking of making up a small barge to use the motor in as my thoughts are a barge would be nicer to tour canals in and just as much work as trying to attach the motor to the back of a boat for which it was not designed.
  7. SAE140

    SAE140 Guest

    Some great replies there - many thanks for the words of wisdom ....

    I'll just run through a few of the points raised.

    My real name is colin powell, but during the first Gulf War I received so much hate mail (many hundreds - from people who really believed that the American General with *my* name would be openly chatting on boating forums) that I decided to change me moniker. Stuck for a new 'handle', I'd just topped-up me trusty ol' Seagull's gearbox with that dreaded mixture, and not being a person of much creative imagination - thick oil - that'll do ...

    Love the 'concrete lump on a string' method of braking - sometimes we forget the obvious !

    Although I've got an inboard-outboard (5HP Mariner 4-stroke in a well, inside a cockpit locker), you're right - I wouldn't be able to produce reverse thrust in less than a couple of minutes (minimum) - which might not be quick enough to avoid a mega-ton Frenchie bearing down on me toy yacht bent on revenge for how we treated Joan of Arc ...

    Legal stuff - now you've put your finger on a problem. Not so much the extended length I'm thinking, but the nature of the resulting craft itself. By adding a tug, what effectively results is an articulated vessel (2 rigid lower attachment points, with a flexible upper) - with the motive power being generated by an unmanned vessel operated by remote control. Or at least I can forsee the lawyers arguing that if anything were to go pear-shaped. And maybe remote control of another 'vessel' ain't legal.

    So it would appear that there are two separate issues here - 1) can it physically be made to work, and 2) will I be allowed to use such an arrangement. Hmmmm - I'm starting to get cold feet.

    Poida - I've managed to track down a couple of graphics from Ebay.co.uk.
    120028870748 shows the actual model I've got, although mine has a slightly diferent front end/ cowling.
    220025157513 is a slightly larger HP than mine, but has more-or-less the same cowling design.

    Hope you find these of interest.

    Many thanks for your input guys - I think all-in-all I need to find another way of solving the problem of touring the canals with an economical diesel engine - I've got an idea in mind, but it means starting from scratch. I'll introduce this in a separate post.

    So - back to the drawing board ....

  8. safewalrus
    Joined: Feb 2005
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    safewalrus Ancient Marriner

    'Oily' hope we haven't put you of too much, that wasn't the idea, I do think your method is feasable in general just needs some tweaking! As I said the main problem really is our French cousins who would take great delight in squashing the 'English roas bef' (there again I believe they try to do the same to any other yottie as a matter of principle anyway!) Best of luck with adventure look forward to the next phase

    the Walrus
  9. Crag Cay
    Joined: May 2006
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    Location: UK

    Crag Cay Senior Member

    Hi Colin,

    I love the idea of a little pusher tug. It resonates with the engined tender the Thames barges sometimes used in the later years to help them into port.

    But I think people have hightlighted some of the shortcomings in the idea, especially at the 'scale' you have. I would like to add another thought: Cost. I know you have the motor, but that's still some way from being a viable tug. Are you sure all this will not cost more than a replacement outboard, should you wear out your current one (which is not a certainty)? Remember in France you don't have 'red diesel' so the fuel price differential is much less. (In France, diesel is about 70 percent the cost of petrol).

    I would suggest that as the speed limit in the canals is limited, you might even get away with using a 3.5hp outboard.

  10. SAE140

    SAE140 Guest

    Thanks again for *all* the posts on this one.

    The points you've raised are valid, some have highlighted bigger problems than others. It's the legal acceptance (seeing as I'll be on inland waterways where I'll be at the mercy of French law) which is my BIG concern, 'cause that is completely out of my control. Bugger spending a lot of time and effort only to arrive on French soil and be told I can't use the tug. And as Crag has just suggested - it may prove false economy anyway.

    Thinking back - the steering issue could probably be sorted by fitting a rudder to the tug aft of it's prop, and leaving the yacht's rudder to 'float' without it's tiller attached.

    But seeing as this tug would be both unusual and visible - i.e. would probably attract some attention - all it would take is one bureaucratic official bent on stopping a precedent being created, and that would scupper the enterprise. I think there's a need to play safe when abroad. And I haven't even thought about the insurance position ...

    Having people to bounce ideas off is valuable - it's very easy to get wrapped-up with one's own armchair designs to the point of self-delusion !! If you've ever watched 'The Dragon's Den' on TV, you'll know exactly what I mean by this ....

    I'm just firming up some figures for my alternative idea - will be entitled "A 33ft Sumner ?".

    "Coming soon to a screen near you ..."

    Colin Heavy Oil.
    (Chief of the Duckham's tribe)
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