Mobo Anchor Sizing

[rwoofer]

My new to me mobo (Merry Fisher 645) comes with a 5kg Bruce copy anchor, which does seem undersized. As a reference, my weekend sailing boat is slightly smaller and lighter but comes with an 8kg Bruce copy anchor which I think is only just OK.

Tall mast vs big superstructure would tend to suggest both have similar windage, so do mobo's generally get away with smaller anchors than equivalent sized sailing boats? What do people with boats around 6-7m have?

I'm thinking of change to a 10kg Delta or given how modern anchors are so much better maybe splash out on a 7kg Manson or 6kg Rocna.

(I do have a 15kg Spade already in my garage, but that feels just a bit too big - or is it?)

 

[superheat6k]

Perhaps the majority of Moboers just drop the pick for a cup of tea in Osborne Bay or similar on a fine Summer's afternoon, rather than risk the need for a decent heavy anchor in more nasty conditions, when perhaps the safety of the boat and those aboard may rely on it.

IMHO a lot of Mobos anchors are simply too small and many don't carry sufficient scope for any sort of rough weather emergency, and that latter category I know includes my own cable scope. I only have about 40m of cable (all heavy chain) and I wouldn't want to be overly reliant on such a short scope in nasty conditions, where perhaps 6- 10 times depth might be be needed, when I definitely would not want to be in 6m or less of water dragging towards a lee shore.

If you are going to go for some more exposed boating or be at risk of getting caught out in lousy weather then up rate it and aim for scope alllowing for at least 6 times in 10m of water - or 60 - 80 m.

I will PM you some decent reading on the subject.

 

[rwoofer]

Perhaps the majority of Moboers just drop the pick for a cup of tea in Osborne Bay or similar on a fine Summer's afternoon, rather than risk the need for a decent heavy anchor in more nasty conditions, when perhaps the safety of the boat and those aboard may rely on it.

IMHO a lot of Mobos anchors are simply too small and many don't carry sufficient scope for any sort of rough weather emergency, and that latter category I know includes my own cable scope. I only have about 40m of cable (all heavy chain) and I wouldn't want to be overly reliant on such a short scope in nasty conditions, where perhaps 6- 10 times depth might be be needed, when I definitely would not want to be in 6m or less of water dragging towards a lee shore.

If you are going to go for some more exposed boating or be at risk of getting caught out in lousy weather then up rate it and aim for scope alllowing for at least 6 times in 10m of water - or 60 - 80 m.

I will PM you some decent reading on the subject.

That was my general impression.

I guess I'm used to anchoring for the night in remote places and knowing you have a secure anchor definitely aids sleep! If you have ever done Southern Brittany you might be familiar with the pyjama parties that occur off the islands (Belle Ille, Ile de Houat etc) when the winds pick up in the night.....Of course all the french boats have undersized Brittany anchors.

Thanks again for the reading.

 

[maverickofpoole]

What’s needed will depend greatly on what your intended use of the boat is. We anglers usually use far less scope than someone who is overnighting on the hook. My boat is just over 8m overall, I use a 6kg Delta with 10m 8mm chain attached to 140 metres of 14mm multiplait. We fish in depths ranging from around 9m to 35m with around 3-4 scope depending on tide strength and rarely have any trouble. If we were anchoring overnight I would probably have 5-6 scope out but wouldn’t sleep much.

I used to have a 6m Warrior 195 which is very similar in weight to your 645 and on that we had a 7.5kg bruce copy with 10m 6mm chain and 210m of 12m multi and had no problems with that set up either.

 

[Deleted User YDKXO]

This is one of my bugbears. New mobos cost zillions but the manufacturers invariably try to save a few pennies by fitting undersized anchors and chains which are far too small for regular secure overnight anchoring. On my last 3 boats, I have upgraded the chain to the maximum allowable size for the windlass and extended it to 80m; a simple change of gypsy is usually all that's required. With the anchor, I have gone up 2 or 3 sizes to the maximum size I can get on the existing bow roller. For example on my present boat, I changed the standard fit 25kg Bruce for a 40kg Delta. This is only a personal opinion because I know everyone has a different opinion on anchors but IMHO, the Delta works better in a much wider range of sea bed conditions than the Bruce.
With regard to so called 'modern' anchors like the Manson and the Rocna, IMHO you should treat any claims that they have higher holding power than traditional anchor and therefore you can buy a smaller one with a big pinch of salt. I have no doubt that once they are dug in, they do have high holding power but, as with any anchor, it's getting them dug in which is the problem and again IMHO, a heavier anchor has a better chance of digging into the sea bed than a lighter one.
In your case, I think a 10kg Delta with 50m of chain would be a worthwhile upgrade

 

[silverdee]

for my 2p worth you are on right track with 10kg delta and thats not too big to handle anyway.

 

[rwoofer]

Thanks everyone, you have confirmed my thoughts. 10kg Delta coming my way, all for a very reasonable £114. As much as I like spade anchors (having used my 15kg on my 30foot sailboat) the 6kg one is double the price of the 10kg delta.

 

[RogerRat]

Perhaps the majority of Moboers just drop the pick for a cup of tea in Osborne Bay or similar on a fine Summer's afternoon, rather than risk the need for a decent heavy anchor in more nasty conditions, when perhaps the safety of the boat and those aboard may rely on it.

IMHO a lot of Mobos anchors are simply too small and many don't carry sufficient scope for any sort of rough weather emergency, and that latter category I know includes my own cable scope. I only have about 40m of cable (all heavy chain) and I wouldn't want to be overly reliant on such a short scope in nasty conditions, where perhaps 6- 10 times depth might be be needed, when I definitely would not want to be in 6m or less of water dragging towards a lee shore.

If you are going to go for some more exposed boating or be at risk of getting caught out in lousy weather then up rate it and aim for scope allowing for at least 6 times in 10m of water - or 60 - 80 m.

I will PM you some decent reading on the subject.

Good reply, I have to agree with you on all principals.

For a light weight Merry Fisher tho. The real question is where you are trying to anchor and in what conditions. The Brittany coast with major tidal range is a far cry from Osbourne bay.

Basic point to remember is that the chain / scope holds the boat and the anchor holds the chain. 5Kg Bruce for your boat is fine in normal conditions and I would expect you to have a 6mm chain in place.

Over the years... I have found that everyone has their own 'favourite' anchor. I remember the blurb when the 'Bruce' was introduced and the claim was, that you could use half the size and weight of the ever popular 'CQR'. I doubted it of course, but they do work!

From my personal experience, the Bruce is hard to get bedded in kelp and also sometimes,very difficult to remove from a rocky area. If you're gonna lose one, it'll be a Bruce. (copy is the same thing, used both) So my pitch is choose the anchor for the most likely sea bed, that also fits your boat stowage. If rocks are about attach a trip line too.

Max size of chain and scope is more important. The old guide was 3X depth for chain only and 5X for rope. These days the theory boys go for 5X depth on chain only. A stop over lunch in nice weather, light airs and minimal current 3X is plenty.

However, if you're in a tight anchorage close to sailors, mobos' behave differently and usually swing first, less scope could save some embarrassment when this happens. Generally, my chain is measured in 4X predicted max depth for the duration of stay and I would let out a little more if conditions were less than good.

It's your boat and you are responsible for it, work from the basics and do your own trials. Your personal experience will give you the confidence to anchor in various locations and conditions. With my current Targa, I'm happy to anchor overnight on a 'Delta' which happens to be my first choice on this boat. On a Merry Fisher, I would probably choose the the Bruce but go for more scope before more anchor. The analogy being, 'anchoring is like a knot'. You want to tie it quickly, but stay done up and then easy to untie. Removing a stuck anchor in a small light boat when the wind and waves pick up can be quite frightening.

Don't forget to leave some rope on the fixed boat end so you can cut it free if needed. If you haven't checked for one, do so. One of the first jobs I do on a new to me boat, is check rope end and chain splice and then measure, mark up and fit chain hook.


View attachment 39463


There you go 2 more pence worth. :encouragement:

RR

 

[Nick_H]

Interesting test here, which is quite old, but found poor results from Bruce copy and CQR anchors, but good results from Delta, Rocna, Spade and some other modern designs.

http://www.rocna.com/sites/default/files/press/press_0612_wm_ym_testing.pdf

This seems to be repeated in other tests i've seen, the Delta is always up there near the top, and the Bruce and CQR are usually near the bottom. Of the high performers, Delta is cheaper and easier to store than most others, and has held me in high winds without ever letting go, so i've seen no reason to change. Delta every time for me.

 

[Deleted User YDKXO]

Max size of chain and scope is more important. The old guide was 3X depth for chain only and 5X for rope. These days the theory boys go for 5X depth on chain only. A stop over lunch in nice weather, light airs and minimal current 3X is plenty.

I've found that with a heavier anchor and chain, you can reduce that. For lunch time stops in busy anchorages in Croatia, I often don't have more than 2x depth of chain set and no more than 3x depth for overnighting unless I'm expecting bad weather. As you say, the big problem is that mobos and yachts swing differently and whilst 5x depth might be more secure, you could easily find yourself swinging into the boat next door

 

[Nick_H]

I've found that with a heavier anchor and chain, you can reduce that. For lunch time stops in busy anchorages in Croatia, I often don't have more than 2x depth of chain set and no more than 3x depth for overnighting unless I'm expecting bad weather. As you say, the big problem is that mobos and yachts swing differently and whilst 5x depth might be more secure, you could easily find yourself swinging into the boat next door

I wouldn't be happy overnighting at 3:1 scope, 'cept maybe in a very deep anchorage, say 60m of chain in 20m depth, where you'd have a much bigger catenary effect.

From the test I linked to, you can see the Delta had a 75% drop in holding power at 3:1 compared to 5:1

"At the second location at 5:1 scope, it set quickly and held at 5,000 lb in slightly more loosely packed sand. It didn’t perform so well at the shortened 3:1 scope, where it averaged around 1,200 lb"

 

[Deleted User YDKXO]

I wouldn't be happy overnighting at 3:1 scope, 'cept maybe in a very deep anchorage, say 60m of chain in 20m depth, where you'd have a much bigger catenary effect.

From the test I linked to, you can see the Delta had a 75% drop in holding power at 3:1 compared to 5:1

"At the second location at 5:1 scope, it set quickly and held at 5,000 lb in slightly more loosely packed sand. It didn’t perform so well at the shortened 3:1 scope, where it averaged around 1,200 lb"

Anchoring is a complicated subject and everybody has their own views. This is just one test carried out in artificial conditions in a particular type of seabed which may or may not have suited one type of anchor over another. Also as far as I can see, it was carried out with a part chain/part rope rode which again might have favoured one type of anchor over another. The other thing that I am a little bit sceptical about is that the test was published by Rocna who have, let's say, an aggressive marketing approach and whose products have been somewhat discredited recently!
Don't forget that what I actually said was that if you have a heavier anchor and chain, you might be able to reduce the scope. When I bought my F53, it was fitted with the standard 25kg Bruce and 8mm chain. I have had bad experiences in the W Med with a Bruce anchor on previous boats so, yes, I would probably agree that I wouldn't trust this combination overnight at less than 5:1 scope. Soon after I bought the boat, I upgraded the chain to the max that the windlass could handle which was 10mm and put a 40kg Delta anchor on the end of it. Btw even this would be considered far too light for a proper 53ft cruising boat; I looked over a Nordhavn 55 once which had a 75kg Delta and chain as thick as your wrist. Anyway, with my current combination of 10mm chain and 40kg Delta, I feel comfortable with a 3:1 scope in most conditions overnight. I do like to snorkel over to check how the anchor has set if I'm anchoring overnight and my observation is that, even in quite blowy conditions, the anchor isn't actually doing much holding. It seems to be the catenary effect which resists all the shock loading from veering and the friction of the chain moving on the seabed which does most of the holding. Yes, when the wind really pipes up and the chain becomes more taut, then the anchor will have to contribute more of the holding power and resistance to shock loading but in those conditions I would have more than 3:1 chain out or far more likely, I'm safely tied up in a marina

 

[Nick_H]

It's an independent magazine test, I just happened to locate it on the Rocna site

I'm in agreement with most of what you say, but you know as well as I do, and probably better, that squals can blow up out of nowhere, particularly in the med.

I've posted this picture before, of A&K's boat in Sardinia. We went to bed in flat calm, with no strong wind forecast, and we woke up to this. As you say, once the chain is taught, you're completely dependent on the anchor, and it arguably doesn't then matter whether you have chain or rope. The test shows that in some conditions at least, the anchor is 4 x as effective at 5:1 scope as it is at 3:1.

 

[Deleted User YDKXO]

I've posted this picture before, of A&K's boat in Sardinia. We went to bed in flat calm, with no strong wind forecast, and we woke up to this. As you say, once the chain is taught, you're completely dependent on the anchor, and it arguably doesn't then matter whether you have chain or rope. The test shows that in some conditions at least, the anchor is 4 x as effective at 5:1 scope as it is at 3:1.

Yup, been in that position a few times and tried to stay awake on the flybridge on anchor watch. In a situation like that with such an open uncrowded anchorage, I would certainly have been tempted to lay out more than 3:1 scope even in fine weather. Also the arrival of the wind and rain would have woken me up and I would have got up to check the anchor and lay out some more chain if necessary
Did you have some kind of anchor watch in your cabin to wake you if you had dragged? Also more generally, having tooled myself up with a new iPhone and iPad can somebody please recommend the best and latest anchor watch apps?

 

[Nick_H]

I normally use the anchor watch function on the plotter

I do have Drag Queen loaded on phone and i-pad though. The good thing is you can edit the anchor location to its real position, not the position of the boat (although it's a bit of a faff, as I can't work it out in my head so have to position the cursor on the plotter, then manually type the lat/long into the app. Would be much better if Navionics ipad version had anchor watch functionality, and you could just tap the location on the screen). The alarm also seems to go off a lot, when I know I haven't dragged and the radius is adequate to cover any swing. Maybe it's losing GPS position and that causes the alarm.

 

[Deleted User YDKXO]

I normally use the anchor watch function on the plotter

I do have Drag Queen loaded on phone and i-pad though. The good thing is you can edit the anchor location to its real position, not the position of the boat (although it's a bit of a faff, as I can't work it out in my head so have to position the cursor on the plotter, then manually type the lat/long into the app. Would be much better if Navionics ipad version had anchor watch functionality, and you could just tap the location on the screen). The alarm also seems to go off a lot, when I know I haven't dragged and the radius is adequate to cover any swing. Maybe it's losing GPS position and that causes the alarm.

Can you hear the plotter anchor watch in your cabin? Often thought that the plotter manufacturers should offer a remote alarm facility or better an app that works thru your phone. I often sit at dinner in the evening fretting about the boat dragging. In fact, we always have to sit at a table in sight of the boat to my SWMBO's intense irritation. Somebody must have invented a remote anchor watch system?
Thanks for the tip on Drag Queen. Have downloaded that

 

[Beachcoma]

I normally use the anchor watch function on the plotter

I do have Drag Queen loaded on phone and i-pad though. The good thing is you can edit the anchor location to its real position, not the position of the boat (although it's a bit of a faff, as I can't work it out in my head so have to position the cursor on the plotter, then manually type the lat/long into the app. Would be much better if Navionics ipad version had anchor watch functionality, and you could just tap the location on the screen). The alarm also seems to go off a lot, when I know I haven't dragged and the radius is adequate to cover any swing. Maybe it's losing GPS position and that causes the alarm.

I haven't used Navionics but is it not possible to tap the screen as you drop the anchor to mark it as a waypoint? This will give the anchors exact position.

 

[RogerRat]

I often sit at dinner in the evening fretting about the boat dragging. In fact, we always have to sit at a table in sight of the boat to my SWMBO's intense irritation.

Interesting....
Are you insured if you leave the boat while anchored?
Or are you insured for leaving the boat if it remains in sight?

Not everyone is, worth checking?:encouragement:


Nick,
I'm impressed by how much hold is lost by such little reduction in scope. I always like to see a vertical drop from the stem roller, as this ensures a good catenary curve, it's a simple check too.

RR

 

[Nick_H]

Can you hear the plotter anchor watch in your cabin?

Yep, lower helm is directly above our cabin, plotter is about 4 feet from my noggin when i'm asleep.

The lastest plotters all have very comprehensive ipad apps that allow you to see and control pretty much everything on the plotter, and I assume this includes a remote anchor watch function.

 

[TheOrs]

Don't forget to leave some rope on the fixed boat end so you can cut it free if needed. If you haven't checked for one, do so. One of the first jobs I do on a new to me boat, is check rope end and chain splice and then measure, mark up and fit chain hook.

Sorry for the possibly stupid question but could you explain this further please.

Ta