Floating dock which way ? Mast derrick ?

What difficulties do you come across when launching and recovering your vessel?

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Refueler

Refueler

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Under pressure from 'peers' I am now going to construct a floating dock to go with my new jetty construction at home. I have 8 x 200 litre plastic drums on their way to my place for this.

The question is which orientation for the drums ... in a line along length of dock in two rows or spaced across at intervals ... Dock is expected to be between 6 and 8m long and about 1.5m wide.

Refer this sketch for ideas ...

pontoondrums.jpg


Second question is the mast derrick I want to add to jetty.

I have some real beefy timbers of about 8 - 10 ins square and up to 7m long. I mean these are hefty !! They were originally vertical supports of the house.
To get one vertical and set-up for king-post of a derrick ? That's one hell of an operation.
I have loads of 6.1m 3 x 6ins sawn timbers - new.
I can get steel piping galv or un-galv ... at local builders from small diam up to about 4ins diam. and in long lengths up to about 6m.

Now question is what to use ?

Big section timber will be difficult to handle ... and also how to drive into river bed ?

Sawn timber can be driven in easily and no problem to bolt / band two sections together to give extra strength.

Steel pipe of maybe 3ins diam ? but then harder to pivot boom on ... but can be u-bolted to any other verticals as well to add support.

Come on you lot ? What would you do ? Check the polls and give me a hand to decide ....
 
Excuse me if I'm wrong, but I assume by "floating dock" you don't mean a dock which you sink below water level, float your boat in, then pump out buoyancy tanks so the whole thing lifts up enable underwater mainyenance.

If you are talking about a pontoon, then I would have the tanks arranged as in sketch A so that failure of any tank would have less of a de-stabilising effect than in the other arrangement.

Regarding the mast crane: I would say "none of the above" and have sheerlegs. I have a book somewhere entitled "The Young Sea Officer's Sheet Anchor" which shows how masts were stepped aboard HM's sailing warships when no dockyard facilities were available.

Anyway, I hope we see regular reports and pics on your project. It's good to have some "practical" boatowning on here!
 
longitudinal drums will support weight on the edge of the jetty better, as the centre of upthrust is nearer the edge and there's is more mass on the opposite side to act as a counterweight.


What about a "tripastos" for the crane. Good old simple Greek technology. It could be used to help drive in the dock piles, and also act as an engine lift, mast derrick, etc. ? Easy to dismatle and move if needed.


[image]
crane.jpg
[/image]

Wiki has got the info under "crane".


EDIT. Sheerlegs. Excellent idea, TO !
 
Floating dock - yep floating pontoon.

I have thought about a lifting dock as I have large containers that could be rigged for it ... but that's a project for much further down the road. Idea being to create one for underhull work etc. on a commercial basis ....

Sheelegs were considered and not discounted. The idea was to create a king post so that I didn't have stays / guys stretching over the jetty to duck under etc. Also the position of the post is at least 2 - 3m from the lift point, making thne overall leg length quite long to be able to work a mast at approx. 7m above w/line.

But the sheerleg and others Greek idea will be seriously looked at and measured to see if it will work. Jetty is secure enough for it ... mast with furling gear cannot be more than 200kgs ... Only thing missing is sheerlegs don't allow a swinging boom to move gear ... mast would be lifted and then swung into position ... mmmmmm

Great - good to see practical stuff coming to the front again !!
 
The longitudinal drums will be more stable as sarabande says, also less likely to have overall stability compromised if ever one of the drums leaks - less of a 'free surface' to create a list. Sofar as the crane is concerned if it's just for un/stepping masts on modest yachts it doesn't need to be very elaborate, sheerlegs would probably be fine if it's possible to arrange a suitable foundation and anchorage for the guys.
 
I also have s small mobos and a weekender that 6 guys can lift each onto trucks / trailers - so they are not too heavy for lifting. It was planned that I could lift those as well for winter etc.

Lets say max. weight to lift ~ 500kgs. Not an impossible figure.

It's why initially I was looking at a King Post and derrick boom. That way I can swing the boom from land to water. But main idea of course is the mast stepping. I could rig the sawn timbers on the deck of the boat of course - but then it's on / off boat with the gear all the time. I was looking to make something a bit more permanent to do many other jobs as well.

But I'm game to measure up and try !!

I'm enjoying this - reminds me of all the days I made monkey walks, sheerlegs etc. for scouts days / fetes etc.
 
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longitudinal drums will support weight on the edge of the jetty better, as the centre of upthrust is nearer the edge and there's is more mass on the opposite side to act as a counterweight.

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I disagree. Don't be misled by the rectangular cross-section on the sketch - the drums are cylindrical. As you apply weight to the pontoon the transverse arrangement will generate more flotation to the edge as the drums are pushed further underwater.
 
[ QUOTE ]
[ QUOTE ]
longitudinal drums will support weight on the edge of the jetty better, as the centre of upthrust is nearer the edge and there's is more mass on the opposite side to act as a counterweight.

[/ QUOTE ]
I disagree. Don't be misled by the rectangular cross-section on the sketch - the drums are cylindrical. As you apply weight to the pontoon the transverse arrangement will generate more flotation to the edge as the drums are pushed further underwater.

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I think this question cries out for an experimental test!
 
I am thinking of the pontoon cross section with the longitudinal druims being more like a catamaran, whilst the transverse ones are more like a flat bottomed barge.

The centre of upthrust on a long. barrel is further out from the CofG of the pontoon, than a trans. one.

To do the maths, we need to know the width of the pontoon, and the dimensions of the barrels. I think we can discount barrel failure because the engineering of lids etc is so simple, and the QA team will obviously do ultrasonic testing and x-ray analysis of the barrels before use. That leaves catastrophic failure due to Refueller ramming the pontoon at speed, though ... /forums/images/graemlins/ooo.gif

I am willing to make a little model and see how a long. and trans. pontoon work out. It will beat the yellow plastic duck in a bath anytime !
 
I think you're right - we should model it, but we don't know how it will flex if Refueller constructs it from bits left over from his house? I think the flexing will be key to a sound decision here.
 
I have a pontoon constructed as per sketch 'B' although only with 4 barrels under.

It works well but suffers when a passing vessel creates a lot of wash which rolls under the barrels and eventually up ends the pontoon... too close to the waters edge....

I would suggest that they barrels be as per sketch 'A' if you have any wave action in your area.

By the way I have now replaced the outer two barrels with a long 4 M section of tube which I hope will improve the stability problem.

The o ther problem you may need to consider is that if the barrels are 'launched' and fitted full of air on a warm or hot day this will contract on hitting the water and allow the barrels to 'deflate'. More of a problem if they dry out at low water when the additional weight exascerbates the problem.

Also another tip, ensure the screw in caps are well sealed otherwise the air will seep out gradually with the same problem as above.

As to cranes. Does it not depend on what weight you are lifting?

I doubt whether you would have sufficient bouyancy with the 8 barrels you are suggesting to lift very much. Our four barrels with admittedly a heavy pontoon and passarelle are really only good enough for two people to stand on at a time.

sounds a fun project though, where is it?
 
[ QUOTE ]
I have a pontoon constructed as per sketch 'B' although only with 4 barrels under.

It works well but suffers when a passing vessel creates a lot of wash which rolls under the barrels and eventually up ends the pontoon... too close to the waters edge....

I would suggest that they barrels be as per sketch 'A' if you have any wave action in your area.

By the way I have now replaced the outer two barrels with a long 4 M section of tube which I hope will improve the stability problem.

The o ther problem you may need to consider is that if the barrels are 'launched' and fitted full of air on a warm or hot day this will contract on hitting the water and allow the barrels to 'deflate'. More of a problem if they dry out at low water when the additional weight exascerbates the problem.

Also another tip, ensure the screw in caps are well sealed otherwise the air will seep out gradually with the same problem as above.

As to cranes. Does it not depend on what weight you are lifting?

I doubt whether you would have sufficient bouyancy with the 8 barrels you are suggesting to lift very much. Our four barrels with admittedly a heavy pontoon and passarelle are really only good enough for two people to stand on at a time.

sounds a fun project though, where is it?

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The whole project is underway upriver at my house on River Venta in Latvia. I have a private channel of the river for my boats. See other thread - Practical Boat Owners answer to Marinas ...

Most people out here use closed, welded end large diameter steel pipe to make a catamaran style pontoon. This was discounted due to the weight and lengths needed. The pontoon is to be approx. 8m long by 1.5m wide. It is with 8 barrels, plastic versions - not steel to avoid corrosion and also lighter. Buoyancy of these would be 8 x 200kg = 1.6 MT, less pontoon construction of possibly 300 - 400 kg = balance of just over 1 MT less 50% for freeboard factor. So I reckon a working weight on the pontoon of 500 kg +.

As to anothers comment about the wood from the house repairs - we over ordered timber for reconstruction with this project in mind. Verticals are the ex roof / main trusses of the house - substantial section. The rest is new sawn timber and strong.

The weight to lift by derrick is average 200 kg or slightly more but max. probably not exceeding 500kg.

So .... ideas ... comments ..... all really appreciated .... there will be a BBQ party for it's opening and hopefully "Coming alongside Ceremony" ----- with the OFFICIAL PBO flag flown - Yes - OFFICIAL PBO flag given me many years ago by one kind Mr. Kim Hollamby ..... plus of course own personal stuff !!
 
The barrels transversly fitted along the length of the pontoon will provide better protection against the inevitable leakage and concomitant loss of buoyancy. Second Moments of Area and all that. One fails and the effect is much smaller in the pitching rather than rolling direction.

In using the derrick, presumably with the king post on the pontoon remeber the centre of gravity of the lift goes to the top of the derrick. You could have severe stability problems.

You should have no problems doing the sums given your previous employment /forums/images/graemlins/smile.gif
 
Wouldn't this depend on width of the structure on top of the drums. If it fits drum size exact or with little overhang I agree. But if I want the deck to be wider bu a reasonable margin - wouldn't the drum be too narrow with overhanging deck then being a lever and unstable ?

At moment we haven't fixed the width of the deck yet ... initial thoughts are from 1.2 - 1.5m but may be more depending on depth of water at the intended spot. We may need to extend out a touch for boat to be afloat.
 
I'll just stick to the pontoon, what would do for mast stepping is about half as robust as what you want for lifting in and out small craft.

If you are getting these Drums the ones with 2x2" bungs are OK. They are only 980mm tall so I would use them in two rows as close to the edge of your proposed 1.5m decking as posible. Or you could use them as diagram A but staggered so alternate ones reach the edge.

If you keep the bungs east/west instead of north/south I don't think you are going to get them wet, ever, so they won't let water in unless you try really hard and have a party on the pontoon..
 
They are the ones with the two white screw bungs. and I agree that E-W is best.

We also are devising a design that allows us to remove any barrel that is suspect or needs draining without need to lift whole deck out.

The Mast derrick - well that's near as d*** it decided to be sheerlegs as others have suggested - as you say is only lightly loaded for masts. The Small Mobo's I have are still relatively light - they are in aluminium sheet construction. It would be engines off separately before lift, so major weight is removed.
 
Yes, you're probably correct. I just looked at the way you had drawn the examples and made my judgement based on those proportions.

Sorry, if I assumed that you had posted something that was accurate /forums/images/graemlins/grin.gif
 
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