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Discussion Starter · #1 · (Edited)
This has not been one of my greatest months when it should have been a cracker... I got new, expensive dinghy davits for my boat...

So now I am trying to raise the F'ing dinghy like davits are meant to allow you to...

I wanted to keep my tackle small and light but have good lifting power so I went with triple blocks, one in each pair with a becket.

Well I have set it up but couldnt even winch the engine end of the dink up! (OK I could have but would take all day and kill me). Dink is 100 pounds and OB is 100 too... Thats 50kg and 50 kg.

So something is vastly wrong. :mad:

1) Because the line you pull on is at the top of the davits the becket is at the top, the anchor point, not the load.
2) the sheaves of each triple block are really quite small, about 1 inch in diameter, about 25mms.

So discussing 1) does the attachement of the line at the anchor end make such a big difference instead of the load end? See the diagram attached of the Triple and Triple reeving method I used, it shows the becket at the top too. Do I need to go to a quadruple! There are $150 each... :eek:

2) I spose ya gunna tell me instead of the cute little bocks I have to buy the expensive big ones?

Are there any other considerations, tricks etc? (Please remember I have a brain enough to make sure there was no twisted lines etc.)
 

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If my HS physics is still right, your dead lifting200#/3=66.6 pounds Perhaps evenly divided between two sets of blocks; but still,. Thassa lotta dead weight, regardless of how ya stack it. Those leetle block sheaves don't help much either. Y lose a lot of energy bending the line. that tight, too?
Mebbe some sorta ratchet winch (ala trailer winch?) or windlass to aid ? Mebbe a set ofdoubles on the workingnend to divide the load yet again?
And you're right. Afull set of quads in a big enuff size would be near what the dink is worth! :eek: I'mthinkin'a Maasdam power-pull untilya can belay it?off? Won't be pretty; but it'll work ;) and y. can alwaysuse the PP for elsewhat.e
 

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You've got six moving lines between the lower block and the upper, which means you're pulling about 15 pounds per side. That shouldn't be a problem. However, you can build up a hell of a lot of friction in that system if the line is too big for the pulleys and/or it's stretchy. You didn't say what line you're using but try running some thin dyneema if you've got some and see if it makes things better.
 

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The numbers
100 kg with uneven distribution
75 kg at the engine end and 25 kg at the bow.
Your tackle give you a gain of 1:6 minus friction..
At best the load is 75/6=12.5 kg (no friction) with friction load is more close to 20kg
Small sheaves give more friction than larger shaves.
Rope with a tight fit will increase friction more.

Can you use a cockpit winch to lift?
 

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Mark, you likely have the lines crossed somewhere, creating a drag instead of lift, or that the rope is too big and creates drag on it's own. Running a rope through 3 blocks is confusing. Make sure the dead (not pulling) end of the rope is tied to a becket on the rearmost block.
Try experimenting with just using 2 blocks, or not using all sheaves. Or use smaller line. You should be able to lift that dink even with a 2 block system.
 

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Discussion Starter · #8 ·
The numbers
100 kg with uneven distribution
75 kg at the engine end and 25 kg at the bow.
Your tackle give you a gain of 1:6 minus friction..
At best the load is 75/6=12.5 kg (no friction) with friction load is more close to 20kg
Small sheaves give more friction than larger shaves.
Rope with a tight fit will increase friction more.

Can you use a cockpit winch to lift?
That maths is aprox what I thought. So I shouldnt need to use the promary genoa winch... I should be able to pull 20 kg with one hand.

I need to re-read everyones comments.

The line is new 8mm double braid so there is streach in it... Dynema at $2.50 per foot is a bit out there.

Thanks for everyones advoce. All worthy stuff that's got me thinking :)
 

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I would not use dyneema line - cost aside, it will be way too thin to work well when pulling by hand. 8 mm line should work ok with small sheaves unless you see it twisting too much or be visibly too big for the blocks (would be hard to thread through the block). Check one more time how the rope runs, pulling on it with just a small small load to lift (like a bucket with water).
 

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What blocks are you using?

In my experience the normal thing to expect is for people to grossly oversize lines which means the friction builds up quickly. And don't trust manufacturers advertising copy. If a block is designed for up to 8mm it probably really needs <6mm to run efficiently.

do you have any pictures of the setup? I think better visually. And include part numbers if you have them.

The other option is to go to two line lifting instead of using one. This cuts the load on each side in half and allows for less purchase.
 

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Triple block is fine 6 to 1 purchase. I use them with my 11 ft. Zodiac its about 130 lbs and can lift it easily with one hand.. I suspect you don't have the reeving right or the block is too small. Yes, good blocks are around 75 to 100 US. I made the mistake of buying blocks that were to small.
 

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I should be able to pull 20 kg with one hand.
20 Kilos is roughly equal to a sack of cement - sure a guy should be able to do it with one hand but it ain't exactly a featherweight.

also, there's an old sailors curse that seems apropos - "Large lines and small blocks to you."
 
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check out Garhaurer fiddle blocks about $45. they will run a lot easier then the triple block and you need at least the 2" block for 8mm line. with a 1" block you could try some 5 or 6mm spider line. some double braid is to soft for a 1" sheave and causes a lot of friction. Garhauer Marine Hardware -9658605
 

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Friction builds up faster than you would expect. My guess is with pretty small sheaves and lots of turns you are well past the point of diminishing returns.
You've got six moving lines between the lower block and the upper, which means you're pulling about 15 pounds per side. That shouldn't be a problem. However, you can build up a hell of a lot of friction in that system if the line is too big for the pulleys and/or it's stretchy. You didn't say what line you're using but try running some thin dyneema if you've got some and see if it makes things better.
Friction comes from lots of places. In tension the line develops friction between the cover and core. Every time a line goes around a corner (including a sheave) more friction is generated - the inner fibers get compressed and the outer fibers are extended. The cover and core respond differently and more friction is generated between them.

The elements noted above (liked crossed lines and oversized line for the sheave) but friction can get pretty high even with perfectly fair leads.

Don't lose sight of friction between the sheave and the block. You get what you pay for. Get good blocks with large diameter sheaves. Most davit/dinghy arrangements are not short on space, even when two-blocked.

The numbers
100 kg with uneven distribution
75 kg at the engine end and 25 kg at the bow.
Your tackle give you a gain of 1:6 minus friction..
At best the load is 75/6=12.5 kg (no friction) with friction load is more close to 20kg
Small sheaves give more friction than larger shaves.
Rope with a tight fit will increase friction more.
I'm with you on the numbers right up to the adjustment for friction. Where did you get that number?
 

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Mark,

I have real tiny blocks on mine. Triples on the engine side and doubles on the bow. No problem at all hoisting the dink. The line must match the sheaves. Going down to the boat today, will check the size. I pull on the engine end and my wife does the bow. No need for a winch. 10'06" AB RIB and 15hp Yamaha 2 stroke.
 

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Mark,

You could put a cam becket and a cam cleat on your blocks and then you don't have to hold the weight of the 2 lines all the time. Haul a little on one, then the other. Also, I find these types of blocks (pictured below) seeem to get tangled and crossed much less than your standard double and triple blocks. If you've got the height, and have to do things over, I'd suggest these, from Garhauer.



If you don't have to re-do it, you can buy the cam beckets and cam cleats by themselves and perhaps salvage your project.

MedSailor
 

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also, there's an old sailors curse that seems apropos - "Large lines and small blocks to you."
I like that, short, snappy and makes one stop and think.

Also, does not axle diameter play a big role in friction? Seems like a small diameter sheeve with a small diameter axle and oversized lines you are doomed to a high friction lift.
John
 

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Mark,

I checked my blocks. The sheaves are a tad over an inch and the line is 3/8. They are Barton blocks (the cheapest I could find at the time). Never had a problem except once or twice picking up in a hurry and the lines are twisted.
http://www.bartonmarine.com/products-size-0-blocks.asp
 

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Discussion Starter · #20 ·
Mark,

I have real tiny blocks on mine. Triples on the engine side and doubles on the bow. No problem at all hoisting the dink. The line must match the sheaves. Going down to the boat today, will check the size. I pull on the engine end and my wife does the bow. No need for a winch. 10'06" AB RIB and 15hp Yamaha 2 stroke.
Thats what I want (wife too!).

Please check the diameter of the sheaves.

I am using 8mm line so the line size is right for my kit.

Just took a pic but the sun is wrong, etc but you see the size, why I dont need a cleat etc.

The set up should work as you describe.

So bigger blocks with bigger sheaves...
 

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