Photos of Anchors Setting

[Ocean Girl]

Now brian has backed down at 2600RPMs for about a minute. I have a vid of anchor dogging in, I'll try to get it uploaded. I dive the anchor everyday, makes for a good nights sleep when you can see what your ground tackle is doing! We are in the Bahamas.

[kas_1611]

Originally Posted by Ocean Girl

Now brian has backed down at 2600RPMs for about a minute. I have a vid of anchor dogging in, I'll try to get it uploaded. I dive the anchor everyday, makes for a good nights sleep when you can see what your ground tackle is doing! We are in the Bahamas.

Yep that looks pretty well dug in for a good night's sleep

[Alan Mighty]

Originally Posted by Ocean Girl

I've been diving the anchor as soon as brian drops it and pays out to the snubber. What we observed it that the engine in reverse has very little effect on the anchor until it reached 2500-2600RPMs. At 2000RPMs the chain is lifted about 50 feet shy of the anchor.at 2300, still not chain lift. Then at 2500 I start to see chain lift, 2600, the anchor digs in.

Excellent observation, Ocean Girl! You're fantastic.


A most valuable data point.


Am I right assuming that your Crealock 34 has a Yanmar 3JH5E engine, rated at a nominal 40 hp? And that your tachometer has a red line at or very close to 3000 rpm?


The Yanmar 3JH5E propeller curve suggests the engine might develop 16 kW (about 21.5 hp) at 2500 rpm, with a max of 28 kW (37.5 hp) at 3000 rpm.


Your observation that 2600 rpm is necessary to embed the anchor in what looks like sand is quite valuable.


I'd guess from the 3JH5E propeller curve that 2600 rpm would be close to 18 kW (24 hp).


Of course, that involves lots of assumptions about prop slip, real engine output vs the bench test standard and so on.


Nevertheless, your observation is a major step forward in discovering how much engine power is really needed to embed and set a known anchor in a fairly well known substrate.


Many thanks


Al

[noelex 77]

Originally Posted by Ocean Girl

I've been diving the anchor as soon as brian drops it and pays out to the snubber. What we observed it that the engine in reverse has very little effect on the anchor until it reached 2500-2600RPMs. At 2000RPMs the chain is lifted about 50 feet shy of the anchor.at 2300, still not chain lift. Then at 2500 I start to see chain lift, 2600, the anchor digs in.

Thanks for pictures and the notes about the reverse force.

For anyone in clear enough water, it is very valuable to watch what goes on when the anchor is setting.

I would agree with all your observations, although for my boat the revs where things start happening are lower. No doubt this is due to prop differences etc.

One slight caution I would express is that (assuming light wind) the first stages of applying reverse power straighten out the chain both on the seabed and to stretch out the catenary. Even if you lay it carefully, there are always some kinks and changes in direction. In the very early stages the anchor also has to swing around to face the right way (at this stage it is just sitting on the surface) and also to adopt the correct setting position. Most modern anchors are designed to start to set with the anchor resting on its side. If it has landed upright the anchor needs to "fall over" first.

If you apply a lot of reverse power quickly as the chain straightens out, the boat will acquire some rear momentum. When the chain does go tight this can create a lot of force on the unset anchor. Modern anchors are very forgiving of this, but to set most realiably they benefit from a smooth, gradual increase in force. If you apply a sudden high force to an unset anchor it will sometimes struggle to start to dig in.

So even though, as Ocean Girl has indicated, in nil or light wind the anchor will not start to dig in until you have reached moderate revs and will not set properly until higher revs have been applied for some time, it still helpful to gradually increase force on the anchor rode, especially in firmer substrates.

This is a video I took a year ago of the Mantus setting:

Mantus Anchor setting - YouTube

[Alan Mighty]

Originally Posted by noelex 77

I would agree with all your observations, although for my boat the revs where things start happening are lower. No doubt this is due to prop differences etc.

Many thanks again, noelex, for your superb and pioneering work. You and your mermaid are fantastic. Your joint efforts are very worthy of a koala stamp of approval!


Your valuable video suggests, in that case at a scope of 3:1 with your Mantus on clean sand, that your engine was in reverse at significant rpm for 2 minutes 19 seconds (from 00:26 to 01:53 by my count).


I know you've mentioned in earlier posts that you set at close to your engine's max rpm. I'd be grateful if you could mention that rpm figure and especially if you could relate that the propeller power curve/shaft power curve to give a ballpark figure for shaft power.


Al

[cwyckham]

Alan, do propeller curves exist for a prop in reverse? They're a lot less efficient that way, no? Except for feathering props, I suppose, but then we'd need to know the pitch etc.?

[wsmurdoch]

Originally Posted by cwyckham

Alan, do propeller curves exist for a prop in reverse? They're a lot less efficient that way, no? Except for feathering props, I suppose, but then we'd need to know the pitch etc.?

Like Brian, I have a Pacific Seacraft 34. It has a 3HM35F Yanmar engine turning a 17x10 2 blade fixed propeller. The transmission ratio is 2.14 in forward and 2.50 in reverse. The lower propeller speed in reverse really hurts the reverse thrust. With the engine turning at the same speed, the reversing prop pumps less water per unit of time and pumps it at a lower velocity. [F=d(mv)/dt and all that...] In addition the prop has a slight airfoil cross section, so in reverse it is like flying upside down without raising the airplane nose.

I own a 0-2000lb Dillon dynamometer and have measured my reverse bollard pull. I get 110lb at 2000rpm, 170lb at 2500rpm, 240lb at 3000rpm and 300lb at 3500rpm.

There are a couple of magazine articles where the reverse bollard pull of sailboats have been measured with different propellers.

http://www.flexofold.com/upload_dir/...onthly_low.pdf

http://www.propelspecialisten.dk/dow..._test_2008.pdf

https://books.google.com/books?id=yJ...20pull&f=false

They are great evening reads. It is hard to draw solid conclusions from the data as the all the props are not perfectly (or equally) matched to the engines, but there is still lots to learn.

[Alan Mighty]

Originally Posted by cwyckham

Alan, do propeller curves exist for a prop in reverse? They're a lot less efficient that way, no? Except for feathering props, I suppose, but then we'd need to know the pitch etc.?

Apologies for not replying earlier - my previous post was just at the end of a long continuous ops job (kept awake by measured quantities of the dreaded brown powder of course).


1. Mea culpa.
At the end of conops and susops, my IQ is usually low. That's why I erred calculating how long Noelex had his engine in reverse at significant rpms: I should have calculated that Noelex's engine worked hard for 1 minute 27 secs doing the final phase of his power set (which is sort of from fluke bury to shank bury).


2. Concerning propeller power curves.
I think the gold standard for knowing how much effort is needed to embed fully a drag embedment anchor is Bollard Pull in Reverse. I started a thread dealing with that issue a while back. And I applaud WSMurdoch for supplying his Bollard Pull figures - knowing those figures is a huge step towards a more scientific approach to embedding a drag embedment anchor.


Now the sad news: Propeller power curves, as published by Yanmar and other engine manufacturers, are not about real propellers on real boats. Prop power curves are useful, don't get me wrong. But prop power curves are just fictional, idealised indications of the power output of an engine if the engine was (1) performing to blueprint (i.e. a brand new blueprinted engine, not an elderly wheezer); (2) losing power by driving through its standard gearbox (i.e. flywheel power minus losses in the gearbox); and (3) driving a perfect prop in some standard water (i.e. prop shaft power minus losses from slip found in a 'perfect prop' and with that perfect prop rotating in perfect water).


And yes, that would be in forward gear, not reverse gear. And yes, that would be in forward gear driving a boat through water, which is not the situation when a boat is in a bollard pull situation (i.e. tethered to an immovable bollard or a hopefully immovable anchor). In that 'perfect water', the blades of the prop fly (in an aero-hydrodynamic sense) through the water. In a bollard pull, the blades of a prop are in a horrible turbulent mess.


Let me summarise and repeat: the prop power curves from engine manufacturers are a work of fiction, an idealised guideline to give an aspiring engine purchaser (a boat builder etc.) a rough idea of what power might be available for transmission from the prop to the water.


I don't know of many authorities for the view I've just expressed. You can wait for the inevitable comments in this thread (hopefully the responses will not detract from the very valuable efforts of Noelex and all other posters of anchor photos) or browse such a website as: Propeller Loading


Bottom line: (1) knowing the rpm, the engine, and its power prop curve, you can make a stab at guessing how much power (in kW or hp) are available to be transferred into the water. (2) If you have bollard pull figures, you have a good handle on the linear pull being exerted on the anchor chain. (3) If you know the anchor (brand, model etc), and the bottom conditions (sand, mud, silt, multiple layers etc) you might have an idea of how much power needs to be exerted for how long to power set an anchor.


Al

[Ocean Girl]

Howdy, internet here is spotty.
We have a 30HP yanmar
2 blade fixed prop.( should be same as Murdochs)
Will post my vid tomorrow and Heading out soon for Green Cay and will make a better timed vid of our anchor setting as the RPMs increase.
Cheers
E

[noelex 77]

This anchor was dropped by a private boat in 6m @7:1.

I think it is a genuine Bruce, but it may have been a claw (copy). It was a bit to covered in weed to tell.

It was quite a small anchor (20 kg?). I have a soft spot for the Bruce anchor but it needs to be large to work well. It does not seem to scale down to smaller sizes and work the same way.

I suspect in the smaller models the proportion of shank weight is too high. At least that is way the way they will behave. Often they are on their side with close to 90 degree list. In this orientation they will dig in one fluke, so the anchor has some grip, but it is not reliable in stronger wind.

This anchor has developed more than a 90 degree list. It is almost sitting on its back.

[hoppy]

Originally Posted by noelex 77

I have a soft spot for the Bruce anchor

I bet you do 😉

http://m.youtube.com/watch?v=_f_p0CgPeyA

Sent from my iPhone using Cruisers Sailing Forum

[noelex 77]

Thanks Bruce.
A reference to Monty Python, preferably complete with a video, should be compulsory in every anchor thread .

[hoppy]

No problem Bruce.

This thread is so long lasting that it will probably out live a parrot


Sent from my iPhone using Cruisers Sailing Forum

[Ocean Girl]

Here is the vid of the anchor digging in. It's kinda lame, sorry, will make a better one showing the different RPMs effects on the rode and anchor. This is at 2600RPMs, anything less didn't even budge the chain much, let alone move the anchor.

https://m.youtube.com/watch?v=fE1qOt6Ig2M

[transmitterdan]

Originally Posted by Ocean Girl

This is at 2600RPMs, anything less didn't even budge the chain much, let alone move the anchor.

This is a very important point and often overlooked by a lot of people talking about anchor setting. It takes a lot of force to lift or drag 100 feet of chain. If you don't believe it lay out 100 feet of your anchor chain on a grassy knoll. Get a case of beer and offer passers by a free beer if they will help you try to drag it.

For even more fun fix one end of the chain to something and get 2 cases of beer and see how many people it takes pulling from the other end to lift the entire chain up off the grass. You might need yet another case of beer.

The point is that a lot of times we try to set the anchor with too much scope. A small engine cannot even drag/lift a lot of chain let alone bury the anchor.

An anchor that can set with really short scope is important. Once set then it's ok to let out rode until the desired scope is reached.