Ok guys, anchors question

[Izikalvo]

Most boats usually have a weight/length ratio that is common,

what is your way of choosing anchor size when your boat is beyond "normal" say, you have a 40Ft boat that weighs 20 tons, or the opposite, you have an 80 Ft long boat that weighs only 20 tons.
Obviously, when considering only length your are doing something wrong.

usually, anchor charts are related to boat length, and some have also weight, but where does the weight factor come from?

Any thoughts?

[Mike OReilly]

My 37-footer rings in at 15 tons, so you bet I consider weight as well as length when choosing an anchor. But the guides offered by anchor makers are, as you say, focused on length, with a few including a weight measure.

But these are only guides anyway. There's no way they can capture the range of circumstances I face on anchor. This is why my approach is to go for a well-designed and constructed anchor that is the largest anchor my boat and crew can reasonably manage.

[rslifkin]

I'd use the more demanding of the 2 numbers from the chart. A larger boat will have more windage, so more load on the anchor. And a heavier boat will be carry more momentum, so if it sails at anchor it will pull harder on the rode.



Unfortunately I don't think anyone has come up with a perfect way to describe a boat for anchor sizing purposes.

[Anders]

I'm more interested in weight and windage than length but the rocna sizing guide cover both scenarios.

[michaeld]

I concur with Mike O'Reilly. Get the biggest anchor that fits. You'll never go wrong with more than enough.

[noelex 77]

Originally Posted by Mike OReilly

This is why my approach is to go for a well-designed and constructed anchor that is the largest anchor my boat and crew can reasonably manage.

[MichaelK]

I would also go for the bigger anchor for the two choices weight and length...and then pick the next heavier one.

Heavier boats carry more momentum when sailing around at anchor, that is true, but the momentum works in both ways. They also need heavier and longer gust to start moving around whereas a lighter boat accelerates easier but carrys over less force on the anchor when stopped by the chain.

A longer boat has a larger projected frontal area while swinging around than a shorter one, thus exposing a larger area to the wind. As both area and weight are linear factors in the equation for the force to which the boat is exposed, I cannot see any preference for choosing the anchor, thus taking the bigger one seems wise.

[thinwater]

The answer to the OP's question is that weight matters far less than windage, and windage correlates better to length than weight. But yes, heavier boats and multihulls are wise to go up one size because their windage and wave cross section are both greater than their length along suggests. Other factors include type of bottom, cruising waters, and how conservative the maker is with his charts (this varies a LOT).



The "biggest you can manage" saw makes no sense to me, because I have seen many boats that don't have rollers, rode, or stowage suitable for what they need. You get the size anchors you need, and then figure out how to manage them. I've had to change something about the gear on every boat I've had ... and I'm not an oversize anchor guy.

[Nicholson58]

Originally Posted by Mike OReilly

My 37-footer rings in at 15 tons, so you bet I consider weight as well as length when choosing an anchor. But the guides offered by anchor makers are, as you say, focused on length, with a few including a weight measure.

But these are only guides anyway. There's no way they can capture the range of circumstances I face on anchor. This is why my approach is to go for a well-designed and constructed anchor that is the largest anchor my boat and crew can reasonably manage.

Once again Mike cuts to the chase. Biggest possible = best sleep every night.

My approach is to use the sizing charts. Find these at the sites of the most reputable makers. Note the anchor weight by either length or displacement. You will probably get similar results from all sizing charts. This is not surprising since the area of dirt needed to secure the anchor doesn’t vary, only the effectiveness of the anchor to dig into various structures. I have seen sizing charts (Mantus) that include expected wind conditions.

Length has to do with windage. Mass has to do with impact as your boat swings about. Some boats are mannerly and some swing wildly.

If your boat falls near the top of a selection then size up to the next category. If you really want security then go bigger. We modified our bow pulpit to load our new anchor.

If you expect to anchor in thick grass (sod) a heavier anchor with a well aimed sharp tip will work well.

Our primary is a Rocna 54 kilo, (121#). Secondary is 54# Bruce. We have dragged over rock, duh. We moved in 48 knots, 10 foot depth, 120 feet of chain as the boat hammered the chain on the swing - soft muck. We found we could steer using the wind driven current and reduced the swing to an unimportant small arc.

We never need the Bruce. The only true Rocna failure was in the Hudson River when the anchor became fouled on a very large wad of burlap and old tarps. I didn’t inspect but it was probably Jimmy Hoffa.

Roxy is 58 feet, 40 tons.

[Izikalvo]

Originally Posted by Izikalvo

Most boats usually have a weight/length ratio that is common,

what is your way of choosing anchor size when your boat is beyond "normal" say, you have a 40Ft boat that weighs 20 tons, or the opposite, you have an 80 Ft long boat that weighs only 20 tons.
Obviously, when considering only length your are doing something wrong.

usually, anchor charts are related to boat length, and some have also weight, but where does the weight factor come from?

Any thoughts?

ignore the anchor for a moment, assuming that two boats are connected with a 300 Ft strong steel cable to a big enough buried concrete block, the wind picks up to 60 knots, and the boats are moving backward and stopped by the cable (300ft from the block), now, the same boats by size and windage one weigh X and the other weighs half X, how big would be the difference of the tension on the cable when they stop?

[thinwater]

Originally Posted by Izikalvo

ignore the anchor for a moment, assuming that two boats are connected with a 300 Ft strong steel cable to a big enough buried concrete block, the wind picks up to 60 knots, and the boats are moving backward and stopped by the cable (300ft from the block), now, the same boats by size and windage one weigh X and the other weighs half X, how big would be the difference of the tension on the cable when they stop?

But not that simple. The heavy boat will gain less speed. The wind force (not increased much by weight at a constant length) x time (gust duration) = momentum may be very nearly the same for both boats. The common observation is that heavier boats move less than flighty race boats. They are unaffected by harbor chop, damping those impacts. So weight is not nearly a 1:1 factor. Windage, pretty much, is.


So go up a size for weight or multihull (or massive enclosure).

[Nicholson58]

Please read #9

“Length has to do with windage. Mass has to do with impact as your boat swings about. Some boats are mannerly and some swing wildly. “


If your boat hangs nicely without swinging and sailing around on the chain then only windage matters.

If your boat is like mine it gets a progressively bigger swing as the wind speed increases. You could swing +/- 45 degrees. On big scope and high wind you will seriously come up hard on the chain, anchor, bow fittings. In this case, the mass & momentum is king. Not only is the anchor required size greatly impacted but you may need to get your boat under control.

Read also #9 that you may have enough wind driven current to steer the boat and reduce the swing and impact. We did this in Newport south of the high level bridge in the muck basin. The swing was reduced from random +/- 40 degrees to less than total 15 degrees. Uncontrolled impact was driving us toward the mooring field half a meter per whack. Controlled, we stopped totally. Three hour shifts most of the night until the hurricane passed. Put this in your bag of tricks.

[rslifkin]

Originally Posted by Nicholson58

Please read #9

“Length has to do with windage. Mass has to do with impact as your boat swings about. Some boats are mannerly and some swing wildly. “


If your boat hangs nicely without swinging and sailing around on the chain then only windage matters.

If your boat is like mine it gets a progressively bigger swing as the wind speed increases. You could swing +/- 45 degrees. On big scope and high wind you will seriously come up hard on the chain, anchor, bow fittings. In this case, the mass & momentum is king. Not only is the anchor required size greatly impacted but you may need to get your boat under control.

Read also #9 that you may have enough wind driven current to steer the boat and reduce the swing and impact. We did this in Newport south of the high level bridge in the muck basin. The swing was reduced from random +/- 40 degrees to less than total 15 degrees. Uncontrolled impact was driving us toward the mooring field half a meter per whack. Controlled, we stopped totally. Three hour shifts most of the night until the hurricane passed. Put this in your bag of tricks.

Thinking about it, in a situation like that where steering with current can help the boat's behavior, I wonder if you could set the autopilot? In theory if it's trying to hold a steady heading it would work.

[Nicholson58]

Originally Posted by rslifkin

Thinking about it, in a situation like that where steering with current can help the boat's behavior, I wonder if you could set the autopilot? In theory if it's trying to hold a steady heading it would work.

My autopilot was useless. It requires GPS and forward motion. Hand steering was all that could be done. If you must resort to this, it will take you a while to feed-forward program your brain to anticipate the boat’s next move. It helped that there were bridge lights to aim for.

[rslifkin]

Originally Posted by Nicholson58

My autopilot was useless. It requires GPS and forward motion. Hand steering was all that could be done. If you must resort to this, it will take you a while to feed-forward program your brain to anticipate the boat’s next move. It helped that there were bridge lights to aim for.

Interesting. I'd expect autopilot in any of the fancy modes to be useless (such as tracking based on COG from GPS to correct for wind/current induced drift). But the most basic "steer to compass heading with no other corrections" mode should be able to steer without forward motion (although it's possible the autopilot may not know what to do if it thinks the boat isn't moving).