Rocna 55 (121lb) vs Mantus M1 (125lb)

[Muaddib1116]

Originally Posted by noelex 77

More boats drag because of poor performance caused by a marginal substrate than anything else so the boost in performance produced by large anchors has very significant real world benefits.

Another great point. I think one thing that Steve's testing has shown is that in good conditions most anchors put up pretty high ultimate holding numbers, so the "exponential" advantage of 100+lb anchors might be in marginal conditions where they go from not working to working, a sudden near-binary improvement.

[Panope]

Originally Posted by Dockhead

Well, there's the science, which I certainly can't argue with.

Yet everyone I know or even heard of with experience of 100+ lb anchors notices this. Maybe it's not a matter of ultimate holding power, but rather, setting behavior?

I think so.

Another possible reason folks notice a "jump" in performance when they start using very large anchors is because they have simultaneously switched to a very large boat. Larger vessels seem to not need as much anchor performance as smaller vessels because the same size waves and gusts have less effect. Larger vessels may have other aerodynamic advantages (less drag) due to Reynolds Number.

https://en.wikipedia.org/wiki/Reynolds_number

[Panope]

Originally Posted by noelex 77

More boats drag because of poor performance caused by a marginal substrate than anything else so the boost in performance produced by large anchors has very significant real world benefits.

I think so,

More and more, I am focusing the testing on the crappy bottoms.

There is a good argument to choose the anchor that "does best - in the worst" and ignore results in the "perfect" seabeds.

[Dockhead]

Originally Posted by Panope

I think so.

Another possible reason folks notice a "jump" in performance when they start using very large anchors is because they have simultaneously switched to a very large boat. Larger vessels seem to not need as much anchor performance as smaller vessels because the same size waves and gusts have less effect. Larger vessels may have other aerodynamic advantages (less drag) due to Reynolds Number.

https://en.wikipedia.org/wiki/Reynolds_number

Makes sense. Thanks.


I have an unscientific feeling, a hunch, that anchors need a certain mass to punch through some types of seabed. It would be interesting to talk to an actual soil mechanics expert on this.



Do anchors of 100 lbs and more produce more force per unit area on the cutting parts of the flukes? I guess that's something which could be tested.



Another situation where 100 lb anchors seem to do much better is in soft mud. They seem to sink deep enough to catch, more easily than smaller anchors do.


Isn't it true that weight per unit of fluke area goes up with larger anchors? That could explain that; and might also explain cutting power.


At the same time LESS fluke area per unit of weight would produce LESS ultimate holding power per pound, in larger anchors, so this may be counteracting some beneficial effect we don't understand, and concealing it.



Spade sell their anchors not by weight, but by fluke area. Maybe we should be choosing anchors by fluke area, then choosing a heavier one for the same fluke area.

[Panope]

Originally Posted by Dockhead

.........Do anchors of 100 lbs and more produce more force per unit area on the cutting parts of the flukes?.........

Larger anchors absolutely have greater force per unit AREA because when an object is scaled upward, the mass increases by the square of the area (and cube of the linear dimension).

For example, take a cube shaped object that is 1inch X 1inch X 1inch and weighs one pound. This will have a gravitational force acting on the earth of 1 psi.

If we scale this object upward so that is weighs two pounds, the dimensions will become 1.26 X 1.26 X 1.26 and will have a gravitational force of about 1.26 psi. Thus, it will tend to punch through a barrier more easily.

Another thought experiment would be a Bruce anchor that weighs, say, 1 billion tons. No matter how gently placed, the anchor will push it's fluke right through a typical concrete sidewalk.

Heck, the whole anchor might collapse under its own weight.

This is why things like bridges cannot simply be scaled upward infinitely. At some point it will collapse under its own weight unless stronger materials are used.

In the case of anchors penetrating, the material that we are TRYING to break, is the seabed, which remains the same for all anchor sizes.

[rslifkin]

Thinking about it, the holding vs weight staying similar as anchors get bigger is actually an indication of heavier anchors performing better. Anchors gain weight faster than surface area when scaled up, so holding vs surface area improves in the larger sizes if holding vs weight is constant.

[noelex 77]

Originally Posted by Panope

Larger anchors absolutely have greater force per unit AREA because when an object is scaled upward, the mass increases by the square of the area (and cube of the linear dimension).

For example, take a cube shaped object that is 1inch X 1inch X 1inch and weighs one pound. This will have a gravitational force acting on the earth of 1 psi.

If we scale this object upward so that is weighs two pounds, the dimensions will become 1.26 X 1.26 X 1.26 and will have a gravitational force of about 1.26 psi. Thus, it will tend to punch through a barrier more easily.

Nicely explained Steve and I want one of those concrete busting anchors .

[Dockhead]

Originally Posted by rslifkin

Thinking about it, the holding vs weight staying similar as anchors get bigger is actually an indication of heavier anchors performing better. Anchors gain weight faster than surface area when scaled up, so holding vs surface area improves in the larger sizes if holding vs weight is constant.

Bingo!

[Dockhead]

Originally Posted by noelex 77

Nicely explained Steve and I want one of those concrete busting anchors .

Indeed; I love it. I think we've gotten to the bottom of that 100 lb magic.


At a certain size they may not bust concrete, but they will bust your seabed in a way that smaller anchors can't.

[Panope]

Originally Posted by Dockhead

Indeed; I love it. I think we've gotten to the bottom of that 100 lb magic.


At a certain size they may not bust concrete, but they will bust your seabed in a way that smaller anchors can't.

Related to this,

Is Nick's (Jedi) description of his "hard Sand" setting technique where he free falls the anchor so that it makes a crater!

[Panope]

Late last Summer, I was forced to let Panope lie to her single anchor (with no crew aboard) for a couple months just offshore of my friend's house. Couple miles of fetch.

I chose to use a 66lb Genuine Bruce because these anchors do very well in that particular seabed type, and because this was one of the biggest anchors that I have, that will fit.

This is a 35', 7 ton boat, so massively oversize.

Although the boat never budged, I did not have a completely satisfied feeling.

With only a small modification to my bulwarks, I believe that I can fit the 50kg Bruce (it weighs 116 lbs). If I remove my secondary anchor (65lb. Forfjord) the result will be a weight REDUCTION.

I mentioned this idea to an accomplished cruising friend and his response was:

"That will solve every problem"

[hpeer]

Steve,

Thanks for that explanation re: heavier anchors.
Our big boat came with a 44lb CQR, which was a joke.
I upsized with a 66 pound Spade, but found it to just slide through some poor bottoms.
I then went to.a 125 pound Mantus. Even here I find there are times when, because of poor bottoms, rubble for instance. I will not get a good first set. But then there are times when the set is just rock solid, the bow dips when chain slack is taken. Always brings a smile to my face. That will do.

[Panope]

Originally Posted by hpeer

Steve,

Thanks for that explanation re: heavier anchors.
Our big boat came with a 44lb CQR, which was a joke.
I upsized with a 66 pound Spade, but found it to just slide through some poor bottoms.
I then went to.a 125 pound Mantus. Even here I find there are times when, because of poor bottoms, rubble for instance. I will not get a good first set. But then there are times when the set is just rock solid, the bow dips when chain slack is taken. Always brings a smile to my face. That will do.

6 years ago, I (infamously?) stated that the 45lb M1 was "too spindly" for my taste. A (YouTube) commenter predicted that I would "come around" one day, and embrace the anchor.

Well, he was sorta right.

I have since become quite fond of the 55lb version due to it's "chunkiness". I used it for a quick San Juan cruise last season and sat through a blow with at least one solid 40 knot gust across the deck. Worked great.

The problem for me, is that my new, pivoting bowsprit that I built a year ago, does not work with the roll bar. When the spirit is pivoted upward, there is no problem. But in sailing mode, the anchor must be released several inches, which is no good.

On my project list, is to play around with a new rollbar design that pivots. It would have a link (soft shackle?) to the tripping hole to immobilize the rollbar when anchoring. Also, the rollbar might be constructed of smaller diameter tube or rod (or plate?) so that a possible boost in performance might be realized.

Ultimately, this may lead to me "designing" a whole new anchor, but from what I now know, it will probably resemble an M1 (or Viking).

And it will be big.

[hpeer]

Originally Posted by Panope

6 years ago, I (infamously?) stated that the 45lb M1 was "too spindly" for my taste. A (YouTube) commenter predicted that I would "come around" one day, and embrace the anchor.

Well, he was sorta right.

I have since become quite fond of the 55lb version due to it's "chunkiness". I used it for a quick San Juan cruise last season and sat through a blow with at least one solid 40 knot gust across the deck. Worked great.

The problem for me, is that my new, pivoting bowsprit that I built a year ago, does not work with the roll bar. When the spirit is pivoted upward, there is no problem. But in sailing mode, the anchor must be released several inches, which is no good.

On my project list, is to play around with a new rollbar design that pivots. It would have a link (soft shackle?) to the tripping hole to immobilize the rollbar when anchoring. Also, the rollbar might be constructed of smaller diameter tube or rod (or plate?) so that a possible boost in performance might be realized.

Ultimately, this may lead to me "designing" a whole new anchor, but from what I now know, it will probably resemble an M1 (or Viking).

And it will be big.

I had the unfortunate “opportunity” toreplace my bow sprit. I was very dissatisfied with how the anchors came up through the bow platform, the shanks protruding through cut outs. The damn anchors always fouled the bob stay hp and down and the anchors were not easily mounted and it limited anchor size. So on my new bow sprit I moved the rollers outside and forward. I got orofioes of the then real popular anchors and laid them on the drawing tk make sure they would fit.

I am a little dissatisfied, the roller could set a couple of inches higher to avoid the chain rubbing in one place. I fit the contact area with some HDPE which solves the issue.

That is the beauty and problem with custom boats, we are always figuring it out for the first time. OTOH, this has very much become MY boat, none kther like it.

[teneicm]

Just our take: I initially had the same concern about bolts vs. welds, but in the end we used the Mantus (105 lb) for 10 years going around the world (always at anchor somewhere) and found it nearly perfect. The bolts actually ended up being a good thing as we bent the roll bar on something in Indonesia and Mantus shipped a replacement roll bar free of charge and we easily changed it out.