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Anchoring a small boat .... particularly a trimaran


QUESTION: Can you advise me as to what sort & size of anchor equipment I should shop for?       Richard-W, CA

ANSWER: Here are some suggestions and guidelines.

While the basics are similar to any sailboat, the extra windage of multi-hulls becomes especially noticeable when at low or zero speed, quickly drifting a boat off-course or backwards, so this is something you will need to adapt to when preparing to dock, moor or anchor.

There are really 3 levels of anchoring to consider, so as far as an anchor choice, each level will have a different requirement.  

For starters, I am going to throw this rough guide out for you.     For a main anchor, a weight (in lbs) equal to L^2 / 30 is generally about right, assuming this is an anchor of efficient design (not a folding or mushroom anchor!).    A common and inexpensive one that still works well is a Danforth … but they do take space on a very small boat, with sharp pointed flukes and rod ends poking out, and it might need to be 10% heavier than the formula gives.   The lowest level picnic anchor can certainly be a one-piece Claw anchor of about ½ that weight (L^2/60), while a storm anchor should be at least double your regular weight (L^2/15).

On a small boat, I personally prefer an anchor that is one piece and non-articulating, as the articulating types tend to catch a finger or jam up with the anchor line.  So a small Bruce or Claw anchor can do the job.    While these two look identical, I’ve noticed some subtle differences in fluke (blade) angles, so if using this type for your main anchor, you might want to make sure your getting the most efficient Bruce.  

Today there are also many claims for ‘new-generation’ designs and anchors like the Mantus, Rocna and the ultimate Ultra in 316L stainless.  Though these certainly perform very well they are typically only available for larger boats - or in the case of the Ultra, just very expensive. 

NOTE: anchor cost always needs to be balanced against risk .... to the boat and the crew.  The latter may not be very high with a small boat, but could be massive for a large boat that you cannot fend off with just manpower.  So if you are fortunate enough to own a large valuable yacht ... then IMHO, only the very best available anchor and chain should be fitted.   A drifting anchor in a storm in a crowded bay can cost you WAY more than your anchoring gear).  

But back to small boats.  The Delta are just a level lower but more available in smaller sizes.  They may permit you to lighten your anchor by as much as 20% compared to the weights noted above, but I would take claims of double/triple-the-holding-power with 'a grain of salt' as different locations will give different results, even if the new anchors are quite likely to be one level better overall than the older styles.  Depending on their past experience (& luck), you will have sailors who swear by their own choice of anchor so it’s impossible to pick one type as always superior.   Regardless of the anchor, the reality is that the greatest variable is the actual sea-bed and you can be lucky or unlucky with that.   Typically a boat with power can back-up their boat to pre-test the holding, but a lightweight sailboat seldom has that reverse ability, so only a good pull has to suffice and then a close observation of any drift.

So for a W17, (using +/- 10% as an acceptable range), my guide line above gives roughly 2kg, 4kg and 8kg for the lower end anchor weights for the 3 uses.   Particular hi-tech might allow a further reduction, but there will be times you will need the weight, though more chain can help compensate.   

You may never need the storm anchor, (I presently do not carry one), but that will depend on where you sail and how you use your boat.    Many experienced sailors that live on their boats will need one though and then, it never seems heavy enough.   This can be a real ‘clunker’ to stow on board and old timers used to swear by the classic old heavy CQR anchors that have indeed performed well.  However, I’d highly recommend one of the new style anchors that are made of 3 parts that all bolt together, as then you can dismantle it for stowage and only need to assemble it when a serious storm is forecast.  The 3 parts are the stock, the flukes and a roll bar and they look like this.       See the Mantus M1 shown here.

For your main anchor, I again personally prefer an anchor that’s not articulated, but it needs to be well designed and solidly built.   While a good galvanized hook is fine, I tend to lean towards an anchor of stainless steel that is both strong and easier to clean, and in the small size needed, are available below $100.

Here’s a pic of the stainless Delta anchor that I personally use as my main anchor even if a bit underweight.  To compensate, I added a little more chain and I do tend to ’choose my spot’ and accept to relocate with a major wind shift.

In all cases, you want to have a few feet of chain shackled to your anchor.  This serves two purposes.  One is to keep the end of the rope mooring line away from the anchor and bottom where it can get prematurely chewed up, but the most important thing is that the relatively heavy weight of chain will help to lower the pull angle on the anchor itself, making it more efficient in its grip of the bottom.    Just 3ft is fine for a picnic anchor but I’d suggest 5ft for your main anchor and at least 8ft for your storm anchor.    For a small boat, chain size could range from 3/16” for your picnic anchor to 5/16” for your storm hook.

Your anchor line can be 50ft x ¼” for your picnic hook but 100ft x 5/16”dia (or 3/8”) nylon for more serious overnight use.  For your main anchor, I suggest to measure off 10’ and 25’  …. (include in that your anchor & chain)  .. and then put markers around your main line …. either with a whipping in black nylon or using 1” wide black Gorilla tape rather than plastic insulation tape as it stays put longer, assuming the rope was dry when you taped it.   These markers will allow you to use your anchor as a lead line to precheck the depth when choosing an anchor spot.   To prevent the anchor rope from chafing on the deck edge, install a handed pair of bow chocks up close to the stem.   These are available in strong but lightweight nylon from www.Duckworks.com

But having splashed out on an expensive anchor, you now need to make sure you don’t lose it!  Nearly every long time boater has lost an anchor at some point .., I remember having lost at least two.  They can get hooked into ‘something’ just too far down to see or reach … be that a rock crevice or some old steel junk that found its way there and you can pull on that line all day, but it will NOT come free.  Ultimately, you’ll generally have to give in, cut the line, tie the end off (to prevent it unravelling) and go shopping for a replacement.   SO, what can be done to prevent this ?

That’s where a trip line is important and unless I can see that the bottom is either soft sand or is of accessible depth, I now almost always use one.    An anchor trip line is basically a small line with a snap hook .. capable to withstand about a 50-100 lb pull.  When lowering your main (or storm) anchor, you snap this on to the back-end of the anchor (that will be the farthest from the boat when anchored), so that the anchor can be extracted out backwards when there is no load on the real anchor line. Typically there is already an eye or small loop on the anchor for this purpose, but if not, get one welded or bolted on. 

For a small boat like my W17, there’s seldom any need to anchor in water deeper than 25ft (8m) so that’s the length of my trip line.   But this thin line (1/8th braided polyester) can get all tangled up with the anchor and rode, especially if you’re anchoring in water of less depth, so we need a solution.

I call this my ‘Paddle-Reel’.    You can make one out of ½” or ¾” rigid foam (styrofoam is fine) and cut to the shape shown here.  I fiberglassed mine each side with one layer of glass to give it a decent lifespan, and it works great.   Paint it a bright fluorescent yellow or orange – you will appreciate why later.   After well attaching the end of the line permanently to the flat reel, you wind up your trip line and slide the snap hook either under the layers, or bond an eye to the reel edge so that you can snap it there.  This way you can store it by your anchor or in some adjacent compartment without it coming undone.    So now for the magic ;)

When lowering your anchor, snap the tripline to your anchor eye, drop the reel in the water beside the bow and lower your anchor.   Your Paddle-Reel will go ‘flop-flop-flop’ as it flips over ½ a turn at a time, but as soon as the anchor hits the bottom, it stops like magic, and the unused line stays happily on the reel.   Not only is this ‘very neat’, but your highly visible fluorescent reel now shows you exactly where your anchor is, regardless of how far away your boat finally swings.    It also gives a nice visual of where you will need to be to recover your anchor.    Pull in your line, lift in your anchor and grab your ‘Paddle-Reel’.  Wind the line back-on for the next deployment. You don’t even need to unclip the trip-line, so it will be ready for the next set.

Although each boat will show you its own needs, trimarans often lay quieter if you arrange a bridle from the ama bows.    S/S strap eyes on the ama bows work fine on a small boat under 1100 lbs. (500 kg).

The above guidance notes apply to boats under 8m, though the larger boats will need more chain  But if you need guidance for anything over that, I recommend checking out this YouTube by Richard Macfarlane that makes fair sense to me.

For more on small trimaran MOORING  go here for some further suggestions.


mjw .... Nov 2020


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