Leaving a Dock Against an Onshore Wind—Part 2

In Part 1 I wrote about how to decide whether, with an onshore wind, to depart from a dock in reverse or forward and concluded that, in most cases, forward was safer.

So let's look at how we rig up and then handle the lines so everyone ends the manoeuvre with the same number of fingers they started it with.

I'm assuming that we are snugly alongside starboard side-to with bow, stern, and two spring lines on.

Subscribe
Notify of
46 Comments
Oldest
Newest
Inline Feedbacks
View all comments
Matt

I am always nervous about slipping lines around bollards, in large part because on many of the docks around here, they’ll run 2″x6″ deck boards right up to the edge without a facing board. So you’ve got a whole row of nice 3/8″ slots, with exposed end grain, just begging to snag a free-running line. As long as there’s a smooth path for the line to run, though, I guess it’s OK?

I tried sketching up the forces and vectors involved and quickly realized that simulating / predicting this accurately, in the general case, is just not going to happen once wind and current enter the picture. You can get a general rough idea by drawing force arrows in the direction of the propeller thrust, the springline, and the wind, treating the fender as a fixed pivot point. But the only way to know what’ll happen, for any given boat, is to try it in the real world. Dock, undock, over and over, on some quiet day when the fuel dock attendant is bored and nobody’s in queue. Try it port-side-to, starboard-side-to, with the wind in different directions. See what works and what has you just pitting propeller against springline without any motion.

Eric Klem

Hi John and Matt,

With regards to thrust required, I see 2 factors worth considering when the wind is from the beam pushing you on.

  • The longitudinal separation of the pivot point (fender) and the center of windage results in a moment forcing you back onto the dock assuming that the wind is pushing you onto the dock. The greater the separation, the greater the moment arm which means you need more thrust to overcome this. I don’t think that this is a huge factor in most cases, you might be moving the fender 3-4′ and you might be 20′ from the center of windage so the change might be 20% or something. A very small change in wind force will easily overcome this as that is proportional to velocity squared.
  • The change in geometry of the stern of the boat, as it gets narrower. The moment that actually rotates the boat out is due to the thrust of the prop having a lateral offset to the pivot point (fender). For boats with a lot of shape, as you move the fender aft, this lateral offset can rapidly decrease, maybe even as much as 50%. For boats that do not narrow at the stern, this is not a factor at all. I am ignoring the effect of prop walk here which means that thrust is not actually perfectly parallel to the centerline.

Realistically in the real world, I doubt that you can change it all that much. It takes huge fenders or a lot of shape aft to be able to move the fenders more than a small amount.

To your graphic of the dashed line, you are right that with the geometry you have drawn, angling that line will require less engine thrust to get off. However, if you move the line to the other side of the fender, any angle will make it harder to spring out. On boats with reverse transoms, it can be tricky to get the fairlead for the dockline aft of the fender. I had honestly always just said to make it as parallel as possible as I hadn’t thought about this in a while but you are right that with certain geometries, having angle has some (small) benefit but as a default, parallel is likely best.

Eric

Eric Klem

Hi John,

Sorry if I wasn’t clear. It is generally easier to spring the bow out on a beamy boat because your lever arm increases in length as it is the lateral offset between the prop and the pivot point (fender). All other things being equal, a wide boat that carries its beam aft is the easiest to spring the bow out on as it takes less prop thrust and you need less angle to get the boat the same amount off the dock assuming your stern doesn’t hit the boat behind you as it swings out. If you had an infinitely narrow boat, then you could not spring off at all if your spring line came in at the same point as your fender, the only way would be to have it come in behind the fender at an angle as drawn in your dashed line drawing and that would require quite a bit of thrust.

I had never heard of or seen spronging off before these comments (and very rarely see springing off and I am not sure I have ever seen anyone else spring the bow out first) so I had to draw out the freebody diagram on that. I think your comment to Matt on beam mattering is right. In a narrow boat, the thrust from the prop and the dockline can end up almost in line meaning that you get no movement. The fore and aft relationship of the line attachment and prop/rudder do matter too here.

Eric

Eric Klem

Hi John,

Boy, that boat sounds not fun to maneuver. Maybe slightly better than the tiki bars that are popping up around here, basically a big square dock with a bar in the middle and an ~50hp outboard on one side, they really go sideways in a breeze.

But the actual important thing is that with an outboard, you can vector your thrust and you can actually help yourself or hurt yourself when doing it and most people’s natural reaction will actually hurt you. The goal is to generate torque about the pivot (fender) and torque is R X F (radius cross force). One key is the cross which is only equal to times when R and F are perpendicular, otherwise it is less so the goal is to vector your thrust to get them perpendicular. If we take a boat where the engine and fender are equal fore and aft, you want the thrust to be fore and aft when springing out. Of course, the temptation is to turn the outboard to vector thrust as if you were not against a dock but when you introducer the pivot point, vectoring the thrust in this way actually hurts you as a lot of that thrust just goes into compressing the fender and nothing productive. On a boat where the engine is ahead of the fender longitudinally, you can actually get to the point where you generate no torque at all (this happens when the thrust points at the fender). With an inboard, you don’t have to worry about any of this as you can’t really vector thrust in reverse but prop wash will mean that there is a better and a worse side which is dependent on relative location of the prop and the fender.

Eric

David Lochner

John,

We use the Ares inflatable fenders as our primary fenders. They seem to be as sturdy as most of the vinyl ones from Taylor, Polyform and such, and are much easier to stow when deflated. Three or four 10 inch diameter fenders take up a lot of room on a 36′ boat.

I’ve found 3 sources, Life Raft and Survival Equipment (LRSE. com), Fisheries Supply (FisheriesSuply.com) and the manufacturer (Ares).

Dave

Dick Stevenson

Hi John and Dave,
 
I believe it
is Aere inflatable rather than Ares and we have used 4 of them for well over a
decade as secondary fenders. We have had one leak which we were unable to fix
and replaced the fender. We use soft fuzzy covers as which is nice in general,
particularly as they are often used between boats when rafting (my take is that
many boats do not have adequate fenders so I augment).
My take is different from Dave’s as I experience them as not nearly so robust as the thick-walled Taylor type which go between Alchemy and gnarly wharfs and concrete piers.
Ours are often inflated all season (mine is a 40-foot boat) as they have a place on the foredeck (where those with inflatables often store their dinghy). They are quite handy when it is necessary to have a portable fender (when another boat is maneuvering close by) as they are very light and can be moved around easily by any crew.
The lightness can be a liability in heavier winds as they can blow horizontal and get caught on the deck or above the topsides. Once every season or two, I tie my bagged lead dive weights to the bottom ring (Aere fenders have rings at both ends to facilitate use as a fender board) to keep the fender better behaved.
 
My best, Dick Stevenson, s/v Alchemy

Michael Lambert

I’m always looking for excuses to use what I call the slip-hitch(bight around something-bight from bitter end through that bight-pull standing end tight, so that holds until one yanks the bitter side and it comes off without the need for the end to pull around) I use it most often to tie the jib sheets to the bow pulpit when I want them out of the way at anchor, but I can pull them free from the cockpit.

In this scenario there is, I’d guess, enough load to make it hard to release, but if it worked (there’s the issue!), it would be slick, as it’d immediately be off the dock with most of it on board. And the crew never needs to touch a line that is ever loaded.

Another thing, I can’t recall what those boards that float over the edge of the dock are called, but in that case I’d say that standing end on top would be key. That end would run upwards to the boat in most cases, and if it were led under the bitter end could get punched by the standing. Which side is on top should be considered for all configurations I suppose.

Michael Lambert

With a slip hitch, it is only a bight that goes around whatever the object is, so the moment the crew uses the end to release it, they are holding a line that runs slack into the water up to near the previously used cleat, turns and comes back to the boat cleat. The crew can immediately start pulling in the end they are holding, shortening the bight.

Btw I also used to rig spinnaker sheets so they were tied to the stern pulpit on some race boats so they would stay tight against the stanchions even when in the water. Before the Windward Mark I’d pull them free.

I thought I got lucky and caught a mistake when you said CW around when starboard side to, since you didn’t stipulate that the bitter end must be under, but then I saw that the stern had passed the cleat so it didn’t signify. I wiped off the drool and kept reading.

Matt

Is this what you have in mind?
Load up the left side all you like. Then, to release, haul on the right side and it pops off without needing to pull the whole line around whatever you’re tied to. I see this done frequently on very small boats (<20ft), often with the release line pulled through the loop an extra time or two to make a chain. I have not tried it on anything larger, where the lines might be under serious tension.

IMG_20210625_124812178.jpg
Michael Lambert

That is correct Matt, but with line you can tie it in such a way that it’s less apt to collapse. Another situation is getting in the dink, I’ll switch to slip hitch, then if the motor starts I can just yank it free and off I go.

I wonder if there is a way to use a tapered fid tied to the pulling end with a butterfly knot or something to make it usable under more strain.

Michael Lambert

I wholeheartedly agree. But,

https://youtu.be/Po4adxJxqZk

Ernest E Vogelsinger

I just tried this @home using my 80+ kg. The “simple” version rolled itself around and slipped approx at 10% of my body weight. Doubling the slip as suggested made it hold me reliably dangling in the air, but stubbornly refused to pull free – the bight would not slip through the round turn.

I see this knot perfectly placed for securing unused sheets to the lifelines as described above, but wouldn’t trust it for springing off – either it might let go, or it wouldn’t pull free when needed.

David Steele

John, this guide specifies Starboard side tie, is that because of prop walk? Are you assuming right-hand rotation/Port prop walk or does prop walk direction not matter?

Philippe Candelier

Great details much appreciated. After reading part 1, it turned out I had to experiment such dock leaving situation 5 days ago with 20 knots wind and 1 knot current pushing under the dock. This, before part 2 was out. Had to re-read, and explain to the crew what we will be doing. They were not keen on leaving forward as this was leading to a dead end about 4/5 boat length forward. Everything went well after the 2nd tentative. We had to move the fenders much more backward than expected and not be shy on the rev.

Eric Klem

Hi John,

With regards to fenders, this is a situation where maximum friction is good to keep them from popping out as the angle increases between the boat and dock. I have found that some fender covers intended to protect topsides make them very slippery against the topsides which is not good for this. I have not tried the stowable inflatable type that you mention so I can’t speak to how slippery or not they are.

Eric

Bruce Bayne

I bought 3 Aere fenders about 15 years ago when, I believe, they first came out. They seemed nice, easy to inflate and stored in a small space. My problem with them is they are forever loosing air and in need of topping off to keep them inflated.

About two year ago a French company, FenderTex, stated selling inflatable fenders in the US. They are a bit expensive, but are ruggedly built. I bought a pair of cylindrical fenders and a pair of spherical fenders.They are very rugged and pack into a very small space, much smaller than the Aere fenders. An added bonus is they can be washed in a washing machine! These fenders can be purchase at Fisheries Supply or PYI Inc. PYI is the importer, so they have more selections than Fisheries. There might be other resellers, but those are the two I know of.

I had trouble with the first set of cylindrical fenders leaking, so PYI sent me a new set. They also leaked and upon conversation with the manufacturer found that the valve was at fault. New bladders and valves were dispatched via the manufacturer, which were quite easy to replace. No problems since. They stay inflated with no need to top them off. PYI is now offering, via the manufacturer, replacement bladders with the new valves for all customers with that particular size of FenderTex fender. Both PYI and the manufacturer really stand behind these products. I can highly recommend them.

On another note, the manufacturer recommends putting water in the fenders as a way to weight them down if they are too light in wind. This seems to work quite well and is easier than hooking diving weights (or other heavy objects) on the fenders.

Bruce Bayne

John,

Glad to be of service. 🙂 Here’s the manufacturer’s website with lots of info:

https://fendertex.eu/gb/

Dick Stevenson

Hi Bruce,
That is interesting information about FenderTex fenders: thanks for sharing. And PYI is a good outfit: I have dealt with them for decades for Max props/ zincs etc. and always felt well taken care of, so their standing by their product is not surprising.
In 5 Aere fenders over a lot of years (we would guess ~~2006-2010, but if they did not exist till 2015, that memory is subject to review) one had a very slow leak at a seam which we could not repair. None of the others ever needed attention. But we do use them as secondary fenders and subject them to the more easy-going jobs (rafting for ex).
And I only remember needing to top off the air maybe once a season or so, if that.
Putting water in the fender is an interesting idea. At first blush it sounds more difficult, messy, and time consuming than tying on a bagged lead pellet dive weight which takes but seconds (chosen as these will not mar the top sides if things get really boisterous).
My best, Dick Stevenson, s/v Alchemy

Bruce Bayne

Dick,

I believe I purchased the Aere fenders between 2006 and 2007, about 15 years ago. Your memory is not suspect. 🙂

Ralph Rogers

What a great article! I have been doing pretty much this for years. Just never gave it that much thought as to why it works, or how to do it better and no way could I have explained it like that. Brilliant.

Terence Thatcher

I have used Stowaway inflatable fenders satisfactorily for years, Tough, durable, easy to inflate, But …they make nothing big enough for this maneuver. On the other hand, if you want rugged cylindrical fenders to hang between a dock and a boat displacing less than Morgan’s Cloud, they work well and are very easy and fast to fill and deploy.

Brian Guck

Hi John,

I have had good luck, so far, with inflatable fenders from North Atlantic Inflatable Boats in Portland, ME. The fenders store compactly when deflated and inflate quickly with a foot dinghy pump. They are rugged and I don’t use the fuzzy covers on them. A round ball fender is the best for the maneuvers you are describing if you have the storage capacity.

Brian Guck

Philip Wilkie

Thanks for this John. Last time I slipped a line like this (and I was very pleased with how seaman like I had set it all up) we watched with open-mouthed amazement as the release-end flicked itself around the bollard and tied itself into a perfect figure of eight. I recall bursting into laughter at the impossibility of it all.

Fortunately it was a very benign situation and nothing dramatic happened, but yes a plan to deal with this happening is needed.

Rajesh Mehta

John,
New to your page – much obliged for your advice and the various comments here.
Eric’s point about narrow sterns and the effect of fenders too far aft is well taken (I have an older C&C 35-2).
I get we need to get a read on clearing the stern of the boat ahead when springing off but would you employ this approach to pivot out and then run a forward turn in the other direction? I face the scenario you describe in the picture in the previous section (15) where one is pinned between boats, and where reversing presents some complication – the boat behind yours in the picture is on an angled wharf much as I face at my regular slip, save the channel is rather narrow where I dock. I know the idea of pivoting a 180 degree turn would depend on the nature of the onshore wind and the tide, but all things being equal, any recommendations? 

Rajesh Mehta

Thanks John. It’s that leeward blow I’ll have to keep in check!
Would you suppose springing off the bow end would be a better option here? Swing the stern out – reverse and better be able to use prop walk (right hand prop) to kick the bow around the other way. Again, tide and wind might factor into the choice!
Cheers.

Rajesh Mehta

Yes, did catch that! Hoping against hope I suppose. Thanks.