How high up do you raise your dock??

[IslandRadio]

This subject has always been on interest to me, and I'm curious what other folks here have to say about it.

The scenario:

We have a tilt-up wooden dock, about 30 feet long. The dock is made of 2x10s, tripled up on the sides, and of course there are cross pieces. Add in the 6 4x4 x 12 feet P.T. legs. All of this stays together as one unit. The only thing removed is the decking itself, which comes out in sections.

There is a tree back about 40 feet from the shore, and in this tree is a pulley up at no more than 20 feet. The base of the tree is about 4 feet higher than the shore. I'm guessing the dock weighs 700 pounds? It could be more.

A cable runs from the winch at the base of the tree, up to the pulley, and attached to other cables which are attached to each side of the dock, at about 25 feet (not right at the end).

Doing some quick calculations, the angle formed by the cable to the horizontal is about 20 degrees. Assuming you only have to lift half of the weight (350 pounds - the other half is supported by the pivot point), it will take somewhat over 1000 pounds of force on the cable to lift the dock. I figured this by first getting the sine of 20 degrees and dividing this into weight of the dock at the free end (1/2 the total).

I see a lot of docks only raised a small distance above the water. Much of that 1000 pounds of force would still be required (in the case of our dock) if that were the case. You could calculate it, but just turn the crank on the winch and you notice that it doesn't start to get easy until the dock starts to get close to a 45 degree angle with respect to horizontal (0 degrees).

Furthermore, more of the weight is borne by the pivot point (and less by the cables, etc.) as the angle increases. You have (1/sine) x weight getting smaller as the angle increases (at 0 degrees it would be infinity) and of course the actual weight that the cable system is required to support is also going down because, as the angle increases above 45 degrees, the majority of the weight is now being borne by the pivot point and the ground.

Anyway, I generally pull the dock up to over 45 degrees (but not to 60 degrees), and at that point the winch is getting fairly easy to pull, and the force on the cable, winch, pulley and attachment points at the dock is CONSIDERABLY less.

No one else (at least around us) does this (all of the docks are only raised a little as opposed to really pointing up). Am I missing something here? It just appears to make sense to me that lowering the force as much as possible puts less stress on the whole system. Yes? No? Maybe :rolleye2: :)

[Slickcraft]

Being the engineer, I always raise the seasonal dock to about 45 degrees balancing the load between the hinge and the winch cable attached to a tree. We also remove the decking. Also I tie ropes from the outer corners of the dock to secure shore points and tightened to limit sway in the wind.

[DickR]

It's useful to note how aluminum frame docks typically handle this. There is an arm, hinged at the shore end, that in the down position helps support the decking. For lifting, that arm is raised considerably above deck height by the lifting cable, and there is another cable (or perhaps an extension of the main cable) going from the end of that arm out to the end of the dock frame. This creates a triangle between the two cable sections and dock frame. The main cable is thus pulling at quite an angle above the center of gravity of the dock, approximating the effort of your raising the wood dock frame to 45 degrees or so while the dock frame itself is raised a lot less.

The "right" angle to use when there is no lifting arm depends a lot on how far above the dock level you have the shore end of the cable placed. Imagine if the attachment were on a huge tree at 40 feet above ground. Imagine also if the attachment were only a foot above the dock; the initial cable tension might well crumple the dock before lifting it.

[LIforrelaxin]

There is now right and wrong answer here, that covers all cases. Every case is different. As I increasing got tired of cranking my dock up far enough to keep the inner legs clear of the ice, I know take the inner legs off the dock, and raise the dock to about 30 degrees or so... it puts some what equal stress on both the cable and pivot point...I have often though of discontinuing the winching almost totally by removing all the legs, and just pivot the dock 5 to 10 degrees....

no aside from that, cable diameter, winch rating, etc all come into play here when thinking about what is necessary....

[IslandRadio]

It's useful to note how aluminum frame docks typically handle this. There is an arm, hinged at the shore end, that in the down position helps support the decking. For lifting, that arm is raised considerably above deck height by the lifting cable, and there is another cable (or perhaps an extension of the main cable) going from the end of that arm out to the end of the dock frame. This creates a triangle between the two cable sections and dock frame. The main cable is thus pulling at quite an angle above the center of gravity of the dock, approximating the effort of your raising the wood dock frame to 45 degrees or so while the dock frame itself is raised a lot less.

The "right" angle to use when there is no lifting arm depends a lot on how far above the dock level you have the shore end of the cable placed. Imagine if the attachment were on a huge tree at 40 feet above ground. Imagine also if the attachment were only a foot above the dock; the initial cable tension might well crumple the dock before lifting it.

I have seen some aluminum docks with a very high triangle at the pivot point at the shore. The cable angle is increased considerably, along with the mechanical advantage because the cable comes from the top of the shore triangle high above the dock and often hangs over it a bit, and attaches near to the end of the dock. The cable angle is at least 45 degrees - and higher - to the horizontal and therefore the amount of force required to lift and hold the dock is quite a bit less.

Maybe this is similar (or identical) to what you are describing. I have never looked closely to see how such an arrangement is constructed. Perhaps the triangle lays into the dock frame somehow when the dock is down. In any event, it's a very good system and when I have to rebuild the dock some day, this sort of arrangement - whether wooden or aluminum will be in play!!

[Rattlesnake Guy]

Couple other factors:

Our wood dock has very long legs. As the dock comes out of the water the weight of the dock get heavier. As you say, the angle gets more favorable to help cancel this out.

As the cable on the spool builds in diameter, it takes more force on the crank to pull the same weight. Also in a favorable direction.

Lower angle gives the wind more leverage to twist the dock.

The water can get higher during the winter and ice can be pushed above the water level.

I have a pulley on mine so that I put two feet on the winch for every foot the dock cable moves. (Saves my back and allows me to use a drill for some of the cranking) But, the pulley reaches the winch which limits how far I can crank the dock.

Depending on the geometry of the pivot point and the legs, the angle to get above the ice can vary.

I usually stop when the close legs are about a foot above the water and balance my fatigue with the effort to repair the dock in the spring.:) So far that has been fine. Probably about 35-45 degrees depending on which dock.

For some reason I much prefer lowering the docks in the spring at the beginning of the season.