Ground tackle load calculations

[dmarine]

I'm trying to find a calculation for ground tackle loads on a 70' 50.00 LT aluminum yacht with wind speeds around 70-75 miles per hour. I need a calculation and not a chart, can anyone help?

[PAR]

Without using a chart, the calculation for a specific yacht, can be quite extensive. In hurricane force winds, you'll want to literally be up a creek, with every line available tied to something solid. I rode out Hugo in a Swan 44, had 5 anchors out with maximum scope, everything removed from the weather deck (sails, dink, bags, covers, life lines, etc.) 9 dock lines tied to mangroves and trees and received very little damage, though there was some. Riding a hook or two in force 12 and building winds is asking to be washed ashore or out to sea. If you can't out pace it or out maneuvered it, you have to run up a river as far as you can or find other suitable sheltered water. I've been in many hurricanes, have lost a few boats to them and you have to respect the power of that kind of low pressure system. Why doesn't a chart suit you? If you don't trust it, pick the next size up.

[dmarine]

Thank you for the reply; however, I need the information to determine if the vessel was adequately anchored during a hurricane. Yes I agree with you, but this is a done deal and now I am trying to determine what should have been. So I still need the calculations regardless how invoved they may be.

BJ Dunson

[mgpedersen]

First order approximation of wind force is

F (lbs) = .004 * A * V^2

A is Area in square feet, V is wind speed in feet per second.

Area used should be more than frontal area due to swinging - a more conservative number to use for area is the full side/profile area divided by 2.

A 75 knot wind gives a load of approximately 30 pounds per square foot.

Matt

[FAST FRED]

The problem of loasd is also how the loads are accepted from the anchor.

As the wind will frequently gust 50% higher than the steady state the anchor loads will vary coinsiderably.

Where it becomes tricky is

IF the anchor rode is too thick it will not stretch 10% - 15% or so to buffer the loads and you will pull out.

Hurricane anchoring is usually donr with multiple anchors to share the loads and give a backup.

Van Dorn wrote a great book "Oceanography & Seamanship" that covers hurricane and deep water anchoring.

FAST FRED

[jehardiman]

Analyzing environmental risk is one of the things I do for a living (a NA for USN). NO vessel can be “adequately moored” during a hurricane, or anything over Force 7 for that matter. There is always a small (sometimes not so small) but real risk no matter what the material quality of the vessel or ground tackle that the environment will overwhelm the vessel. Look up NAVFAC Design Manuals DM 26.4 Fixed Moorings, DM 26.5 Fleet Moorings, and DM 26.6 Mooring Design Physical and Empirical Data for everything you’ll need to know.

If this is for some litigation or insurance issue, the real issue is “was everything reasonable and prudent done to prevent the loss”, not the size of the ground tackle accept as it relates to the former. If you had ground tackle sized per the rules, then the size of the tackle was “reasonable and prudent”. This however will not address other issues relating to the loss.

[Guillermo]

Originally Posted by dmarine

I'm trying to find a calculation for ground tackle loads on a 70' 50.00 LT aluminum yacht with wind speeds around 70-75 miles per hour. I need a calculation and not a chart, can anyone help?

Go to:
http://alain.fraysse.free.fr/sail/rode/rode.htm
Very interesting reading.
At the 'Synthesis' page you'll find a most useful calculator.

You'll find more info on ground tackles (for smaller boats than yours, but with some useful info) within the pages I edit and post for the Banjer 37 Motorsailer Club:
http://banjer37msclub.tripod.com/workshop.htm

Cheers.

[safewalrus]

Personnally can't see the problem 'dmarine' - if it was still there in the morning it was if it wasn't it wern't!

[MikeJohns]

Originally Posted by dmarine

I'm trying to find a calculation for ground tackle loads on a 70' 50.00 LT aluminum yacht with wind speeds around 70-75 miles per hour. I need a calculation and not a chart, can anyone help?

Do you want just the static wind loads ? or the dynamic loads from the surge waves etc.

I suspect you are being very hopeful to try and produce a definitve number for the loads. You will have to fall back on tried and tested rules of thumb.

Forget the wind load, Consider your vessel surging back at 3 knots and coming to a jerking halt on the end of the tether , what is the deceleration ? depends on how well the anchor is holding , if we presume the anchor to be a fixed point and we have a catenary in the chain then we also have to factor in the pitching and riolling moment of the vessel and its transfered load....... start to get the idea we can come up with a number but its a dynamics problem.

[safewalrus]

Your using that word "presume" probably one of the most dangerous words in the English Language!

Presumtion is adangerous concept and can lead to a lot of dangerous positions!

[MikeJohns]

The problem with many calculations in a marine environment is that we have to make a presumption before we can calculate a load. In this case the presumption is the worst case ie that the anchor is firmly fixed. This is not a dangerous presumption but in fact the safest approach in this case.

[safewalrus]

Mike I grant you the fact that a certain amount of presumtion has to be made at times, however it is still a dangerous activity and it should be noted that this presumption is being made, and why, as it is very easy to fall into the trap of taking your last presumption for granted!

Whilst normally this is of an adequate standard, the minute you forget, that you are presuming certain actions and activities! problems may occur!

Probably one of the most famous pieces of 'presumption' must surely be the fact that one Captain Smith presumed that the route to New York was clear of ice at the time of year he was going that way back in 1912! OOps!

[MikeJohns]

I think you miss the point that in engineering you have to presume the worst case loads before you calculate the sizes. This is not dangerous, quite the opposite.

[safewalrus]

Worst case loads are what I'm talking about! In this country it's called Risk Assessment i.e. find the risk! as in WHAT can hurt you, if that's not about Danger what is?

Looks like we will have to disagree about this, or as in most cases, you'll ignore me and I'll slag you to the point of nausea

[MikeJohns]

Risk assesment sorts the probability of an event into categories eg low moderate severe and has nothing to do with the above. You misunderstand this too.

This is completely different to worst case load calculation which I am talking about where you "PRESUME" the worst case; ie you use the greatest possible load.

No insult intended but I think the problem here is that you do not understand engineering parlance.