Anchoring a Swim Platform: Calculating Wind Force on Raft at 6000 ft Elevation

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Discussion Overview

The discussion centers on calculating the required weight of anchors to secure a swim platform (raft) against wind forces at an elevation of 6000 ft. Participants explore factors affecting anchor effectiveness, including buoyancy, wind speed, and the nature of the lake bottom, while considering practical anchoring techniques and materials.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested, Experimental/applied

Main Points Raised

  • One participant notes the need to account for the buoyancy of concrete anchors in water, suggesting that the effective weight of the anchor is significantly reduced by buoyant forces.
  • Another participant emphasizes the importance of the angle of the anchor line, indicating that a steep angle could lead to dragging due to wave action, while a shallower angle might reduce this risk.
  • Several participants discuss the density of materials, comparing concrete and steel, and suggest that adding weight to the concrete anchors could improve their holding capacity.
  • There are references to anchor ratios, with suggestions of using a 4:1 or 7:1 scope depending on conditions, and the importance of allowing concrete anchors to settle in muddy bottoms before use.
  • One participant proposes using stone or concrete doughnuts as anchors, suggesting multiple smaller weights might be more effective than one large block.
  • Concerns are raised about the need for sufficient chain and the potential requirement for heavy equipment to place anchors in the water.

Areas of Agreement / Disagreement

Participants express a range of views on the best methods and materials for anchoring the raft, with no clear consensus on a single approach. There are differing opinions on the effectiveness of various anchor designs and weights, as well as the importance of specific conditions like lake bottom type and wind exposure.

Contextual Notes

Participants mention various assumptions regarding the raft's design and environmental conditions, such as wind speed and lake characteristics, which may affect the calculations and recommendations provided.

Who May Find This Useful

This discussion may be of interest to individuals involved in recreational boating, DIY projects related to floating platforms, or those seeking practical advice on anchoring techniques in varying environmental conditions.

Dadman58
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My son and I built a swim platform (raft) on a half dozen 55 gallon drums. I then made some 1 cf concrete anchors, which do nothing to hold the raft in a moderate wind. I have since learned that I need to account for the buoyancy of the concrete in water, but now I need to calculate the required weight of anchor to hold the raft in place in a 35 mph wind on an open lake at 6000 ft elevation.

What is the force that the wind exerts on a surface? I'll just assume a flat-sided raft and not worry about all the spaces between drums under the platform unless that would significantly change the results.

Simple units would be preferred (instead of things like Newtons, joules, pascals, etc.) because I'm old and my brain doesn't take to new units too well. Note: I'm still good with math, just not new units that I don't know how to convert.
 
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While it is possible to make a model of this system, getting a proper estimate would be a lot of work (and a much more detailed description of the raft and the lake). Testing it is much easier.

The SI units are much simpler than the US-units. All conversion factors are 1, 10, 1000 or similar. They are also older than the current US-units.
 
What kind of angle are you using for the anchor line? If it is a steep angle and a short rope then wave action can exert a great deal of force and could cause the anchor to "drag". A shallower angle would reduce the vertical component of that force and would allow for the cable to sag so that it would not then snap taut as easily.

One reference I found for this mentions three to one or seven to one ratios. http://www.ftkltd.com/How%20much%20Anchor%20Rode%20do%20I%20need.htm

That said, six fifty-five gallon drums riding high in the water in a 35 mph wind -- sounds like a handful.
 
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Dadman58 said:
My son and I built a swim platform (raft) on a half dozen 55 gallon drums. I then made some 1 cf concrete anchors, which do nothing to hold the raft in a moderate wind. I have since learned that I need to account for the buoyancy of the concrete in water, but now I need to calculate the required weight of anchor to hold the raft in place in a 35 mph wind on an open lake at 6000 ft elevation.

What is the force that the wind exerts on a surface? I'll just assume a flat-sided raft and not worry about all the spaces between drums under the platform unless that would significantly change the results.

Simple units would be preferred (instead of things like Newtons, joules, pascals, etc.) because I'm old and my brain doesn't take to new units too well. Note: I'm still good with math, just not new units that I don't know how to convert.

An anchor is more than just a heavy weight. An anchor has flukes and whatnot to help it set into the bottom, which increases the amount of force it takes to drag once it is set. You haven't said what shape your concrete anchors are, but if they are smooth on the outside, they won't be able to hold very well.

Concrete is a fairly light material, unless you add something to it, like scrap steel, to make the anchor heavier.

Concrete - density 150 lbs. per cubic foot
Iron or steel - density 490 lbs. per cubic foot
fresh water - density 62.4 lbs. per cubic foot

As you have found, the buoyant force acting on a block of concrete affects its net weight quite significantly. About 40% of the weight of the concrete anchor in air is negated by the buoyant force of the water it displaces.

This article discusses some important points to consider when choosing an anchor:

http://1source.basspro.com/index.ph...-how-to-choose-the-right-anchor-for-your-boat

You should find out what sort of lake bottom in which you are trying to anchor (muddy, rocky, or something else) and how deep the water is. Even on a lake at altitude, sustained winds of 35 mph are going to generate some waves, so the amount of anchor line you use will have to take this into account. If you expect a storm or rough conditions might develop which will last for an extended period, it might be prudent to move your platform to a location which offers some shelter from any wave action.

Vessels, whether a simple swim platform or an ocean liner, have to have their moorings tended to periodically to keep them in position. You can't just heave an anchor over the side and forget about it.
 
If you have a muddy bottom, give the concrete anchors a month or so to sink in the mud before attaching the raft.

See How to install a permanent mooring.

You will need sufficient chain, and about 200 pounds for a sheltered location, 800 pounds for an exposed location. A single large block is better than multiple small ones. You may need a barge and a crane to get it out there. A mushroom anchor is lighter and better than any block. All the alternatives and tables listing sizes needed are in the linked article.
 
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Use what the ancients used for thousands of years. Stone doughnuts.
Or concrete ones with metal for weight. Make several maybe 20kg each. If your need is a platform then tie it to a rope and throw it some distance away from a corner. Do the same at the other corners. Let them settle a bit.
If you still require more then slide new rings into the ropes.
Caution: this is a heavy job and never underestimate the power of water