View Full Version : An engineering query

04-10-2005, 06:13 AM
I'm about to start building sets of sawn frames for my mythical boat .They will weigh arounnd 100 pound per set,ie a port and starboard frame joined.I have to move then around 40 feet (at most )from the place of construction to the backbone and place them gently on the backbone.

I had thought to run a 1/4" galv rigging wire from strong point to strong point passing over the backbone and run the frames along the wire on a sheave and handy billy.

Will the 1/4" wire be adequate seeing that the load will be applied from the side,essentially "sweating " it ?

I know some will comment that I should just carry them over and bolt 'em up ;) but the weight is just a little outside my comfort zone.

Thanks for any advice.

Ian McColgin
04-10-2005, 07:03 AM
I much approve of using the brain first. A lot of 100# frames is a drag and could be a slipped disc waiting to happen.

That's a lot of weight for a tight wire, more likely to pull out whatever you use for uprights. Even if you allow a 1/2' sag at mid point, you're talking about a ton pull on the wire.

I'd be inclined to join them closer to the site but perhaps you don't want to build an outdoor flat surface.

I take it the wire idea is to keep the frame verticle and thus easily fit out a normal width door and that laying them flat on a dolly is not an option.

How about two sets of a couple of wheel barrow (or similar moderatly large pneumatic wheels - maybe salvaged dirt bike wheels from the town dump) on the simplest of axels. One set under each upper end, midpoint joint in the air. You have a sholder against the inside of the forward half providing stability, steering and locomotion without having to bear more weight than keeps the unit upright.

If you make the frames a tad long (always a good idea to mark and cut the shere later anyway) then you could have a handy notch in each to recieve the axels.


Jack Heinlen
04-10-2005, 07:12 AM
Seat of the pants I think you've got plenty of oomph in 1/4 inch wire. But you're still going to have to brace them in some manner, move them to the wire etc.

If they are too heavy to move them alone I don't see a wire as being much help.

I'm always intrigued by the stories of building on some strand, out in the open. Axes, saws, adzes. There was a early, failed, English colony just down the coast from here. They built a small ship in the open that was worthy enough to get some of them back to England. The first European vessel built up this way. Can't remember the name. 1607. No wire, hands.

Good luck Peter. I probably don't have a clear picture of what you propose, but I'd make the frames and then have a party to raise them.

04-10-2005, 07:23 AM
Peter, would it be feasable to set up a movable table ( two saw horses and a piece of plywood) straddling the back bone and assemble the frames close to their stations and then stand them up?

A taut cable pulls the ends by a ratio of the length divided by the deflection. So if you set up a forty foot long cable pulled bar taut and load it to sag one foot, the pull on the ends will be forty times the weight of the load.

Even after you get the frames in place I think you will need three hands to hold them in place and secure them.

I think that you are starting on the right track, a whole lot of thinking before the doing.

Ross in Bel Air

04-10-2005, 07:35 AM
Ian,I should have mentioned that I can secure the wire to the trusses overhead with rope to take localised sag every 5' or so..This operation is to take place inside my shed.It a large solidly constructed building, the main posts being 12" diameter posts sunk in the ground 4'.The trolley idea would be good but the floor is in 2 levels ,the upper ,the current loft floor and soon to be frame floor is 2 ' above the lower earth "floor".Being able to slide the frames along a wire directly to their place on the backbone would be very pleasant.

That's a lot of weight for a tight wire, more likely to pull out whatever you use for uprights. Even if you allow a 1/2' sag at mid point, you're talking about a ton pull on the wire.

Thats exactly the calculation that I don't know how to make.What's the formula ? Then I wouldn't have to trouble people . smile.gif

Thanks Jack, you're invited to the party smile.gif

[ 04-10-2005, 08:36 AM: Message edited by: PeterSibley ]

04-10-2005, 07:59 AM
Think levers. If you had a bar forty inches long and placed a fulcrum in the center then the load on one end would equal the force on the other end. if you move the fulcrum to a point four inches from the end the you can use the ratio of lever lengths to determine the force ratio. Now if you made a hinged lever with the short section at 90 degrees to the long section and connected a cable between the ends to apply the tension component. the force applied to the rigid parts will be tension in the cable. Think of a nail pulling bar. If the bar were hinged then a cable could apply the force to pull the nail.

Ian McColgin
04-10-2005, 08:04 AM
I hope Michael or one of our many engineers on this forum will correct any errors I make here. It's an application of the 'parallelogram of force' that I've used in rigging and mountain rescue.

Basicly, imagine your 40' span with the 100# weight suspended in the middle and enough tension that the line dips 1/2' below straight. That's half the parallelogram. Make the other half above it. Now you have a parallelogram with a 1' spread the narrow axis and 40' spread the long way. The arms are of course now a tad longer than 40' but the fraction is too small for me to worry. The porportion of stress is just 1:20.

I was hoping to give a proper explanation rather than just demonstrate but my computer is so old that it crashes on the sites that explain it. If you google "parallelogram of force" you will find it.


04-10-2005, 08:04 AM
If you want to consider a light permanent trolley system sliding barn door track is capable.

Ian McColgin
04-10-2005, 08:11 AM
Or combine my thought of wheels underneath with your tight wire to keep it upright - just a little steadying hand needed then.

Supporting the cable every 5' helps but then you have to transfer the load seven times.

The forces can be amazing. I was recanvassing a canoe once. We had a great system for each end and a web that led to a single rope at each end to lollycolums in the basement. Got the canoe started in and I got inside to give it all a bit of push. We were very lucky to observe the lolly's begining to shift before we collapsed the house.

04-10-2005, 08:17 AM
How about a big plywood dolly kinda thing, but with cross country tires on it instead of little hard rubber wheels? Sure it's not as much fun as Rube Goldberging a contraption, but it might get the frames in place anyway... :D

Wild Dingo
04-10-2005, 10:00 AM
aaahhh the legendary Peter Sibleys mythical boat in the bush shed trick eh!! ;)

So Peter me ol china what about... now Ive got no flamin idea if this would work in practice but...

okay so if you recall in factories they have these over head gantry cranes right?... NO!! Im not suggesting you go buy one and set it up... but what about if you set up a length of channel along the beams right over the centre line of the boat then with small say the phunamatic tyre from a wheelbarrow on an axle sittin in the channel... so the wheel axle just rides the channel and so stays in situ... attached to a lead rope which is taken to the shed in the bush doorway and tied off on a made to order tie down thingymajig...

Then attach a hook to the D shackle youve welded to the wheel axle before hand add a length of chain through it which is then attached to the 1/4in wire which you would then attach by way of TWO connection points to the frame... {there being two D shackles one either side of the axle} which would then distribute the weight easier...

You would then lift by way of a small pulley attached to the chain before the wire... thus you can lift her which Im imaginin here not too great a height... and by way of the attached rope led to the doorway you can move the wheel along the channel to the appropriate spot on the keel

I can see this thing far better than I appear to have explained it... I know Ive used a similar thing at various jobs Ive had over the years

OR!!... what about a davit thingy such as the old time miners used or the backyard mechanics use? Have one set up over your lofting floor and another at the stem end of the mythical boat... or the stern depending on which way your facing Im simply assuming the lofting floor is behind the boat which would be facing the shed in the bush's door... You could then attach the said channel to both davits a wheel with attachments lift on the davit above the lofting floor then attach a connecting wire to the frame and undo davit connection leaving frame attached to the wheel attachments and simply walk it along the channel till above the appropriate place thus you could do away with the lead rope idea in 1 above...

This of course means you will have to attach a lowering method to enable the said frame to be gently lowered in place... but mate I will leave that to you to design now Ive created not 1 but 2 alternative methods of creating an overhead gantry crane for your shed in the bush for the mythical boat your building somewhere in the Aussie scrub :D :cool:

Oh and Im not an engineers earhole but were a flamin imaginative and innovative mob us Aussies! ;)

[ 04-10-2005, 11:01 AM: Message edited by: Wild Dingo ]

04-10-2005, 04:22 PM
Ian and Ssor,thanks for the explanations .I knew there was a problem, I just didn't know what it was !I think that I can work something out now and not do any damage ! :D

Shane,I didn't know you had it in you :D :D I thought you were a councellor..well OK that was counselling, thanks mate.I guess the reason for wanting to go the wire cable path is that its cheap and easy ,quick to put up, easy to reroute .That kind of thing.
I actually have an overhead gantry in another shed, my foundry .Its only small ,capacity around 500 lb,I built it to carry heavy flasks of sand and crucibles of hot metal.Its a beam of 6mm wall 70x40 RHS with a homemade winch on it...a cable drum ,worm and wheel reduction box all powered by my battery drill.Works very nicely too.
I just didn't want to build another track in the big shed, steel is expensive etc.I'd rather spend my small pile of $ on wood for the boat . smile.gif

Thanks all,

04-10-2005, 06:58 PM
The concept is good, the devil is in the details….

I’m not gonna go thru all the calculations, but rely on my experience (pre-naval architecture) constructing aerial CATV pole-line systems. I have strung a couple of thousand miles of wire between telephone poles and abused it mightily hanging equipment and cables on it. Here’s my thoughts on how you should do this:

To string up a 6 x 7 galvanized plow wire between two supports and have it not sag more than about a foot with 150 lbs (frame, tackle, pulley, plus “oops” factor) hanging off it in mid-span, you will need to tension it to about 500-750 lbs with a small “come-along” jack. This will mean that your end supports will have to withstand about a 1000 lb pull (tension plus suspended weight). Your average 2x6 building wall studs will not support this, and ¼” wire is too light. Steel H-beam columns in an industrial building will be strong enough, or down-guys on both ends will help a weaker structure. For wire, use 5/16” galvanized 6 x 7 plow wire that utilities hang telephone or cable wire on. It is readily available and cheap.

Assuming you have or can rig a building mount that is strong enough, go see your local utility construction foreman (power, phone, CATV, etc.). Ask him to sell you the required length of 5/16 aerial support wire and required number – plus one - preformed eyes. You will need two pre-forms for the zip wire and two each for the down-guys. Let’s leave the down-guy anchors alone for the moment – if you need to go forward on this, get back to me and I’ll do “lesson #2”.. The extra pre-form is to use as an attachment point for the come-along. Buy two of the ½” or 5/8” eyebolts they use on telephone poles – these are often discarded when they replace poles, so you may be able to get them second hand or as freebies. If he is a GOB (good ol’ boy) and you are good at schmoozing, you may be able to get him to make these up for you in exchange for a cold six-pack for the crew (crews are usually two or three linesmen), thereby giving you a crash course in how to put on and take off the pre-forms. The guys will be able to rig up your two guylines, the zip line, and give you the assembly crash course in less time than it will take to drink one of the beers each. The pre-forms are not complicated, but there is a trick to getting them off without destroying them if they have been twisted on completely. They are much better than two- or three-bolt in-line clamps for holding wire in tension.

Mount the eyebolts in the structures at each end of the building area. Fasten one end of the wire to the eyebolt with a pre-form. Thread on your snatch block or captured pulley, then reeve the standing end of the wire through the other eyebolt. Have an assistant haul the wire taut by hand and temporarily fasten it with another pre-form, leaving the pre-form ends a few wraps from complete. Hang the come-along on the eyebolt and partially fasten the third pre-form to the wire at a point almost at the full extension of the come-along. Put the hook of the come-along in the outer pre-form and take a strain. When the tension is through the jack, unwind the eyebolt pre-form and continue to tension the wire to desired tautness. Pull the slack wire tight, twist on the pre-form, and slowly release the jack. Remove the jack and the outer pre-form, and you’re done. If you need down-guys, use the same procedure.

Use heavy work gloves – the ends are sharp and the pre-forms pinch. You’ll nee beefy bolt cutters, too.

Here’s the gear, from left to right

5/16 wire
pre-formed eye used as tensioning attachment point
1-ton cable come-along
pre-formed eye, terminus
square washer
guy anchor block
square nut


[ 04-10-2005, 08:01 PM: Message edited by: mmd ]

Cullen T.M. McGough
04-10-2005, 08:24 PM
How about social engineering?

You can hire high school kids for surprisingly little money.

Probably quicker and cheaper than running guide wires.


04-11-2005, 12:09 AM
If you want to consider a light permanent trolley system sliding barn door track is capable.

Thank you Ross...the simplest and the best.My local steel supplier has a variety in stock capable of handling 100 to 500 kg .I'll go for the 200 kg version. smile.gif Track is $58 per 20' length,trolleys $26 .So easy.I'll support the track with a couple of lengths of 5"x2" hardwood beneath the trusses and suspend a chainblock from the trolley for lift.It should work beautifully.

mmd....thanks for your input, while I have most of that gear around the shed the track will prove more flexible.

04-11-2005, 01:28 AM
Get some innertubes and fill them with helium. Attach them to the frames and they will only weigh a few lbs. smile.gif

04-11-2005, 05:45 AM
I think that you will be safe with that system. 200 kgs. is a big assembly. :cool:
Ross in Bel Air

04-12-2005, 12:37 PM
You could set up a line of ropes with hooks from the overhead and use two block and tackles to hop the frame along.

Hook Block/tackle #1 to rope/hook #1

B/T #2 is setup on rope/hook #2

Hoist frame with #1

lower #2

Pull frame over to #2

Hook on, tension, loosen #1 til slack, release #1


Yeah, it's crazy. But its another method! :)