Need Help Understanding Tides

[Steadfast]

Hi all. We are so happy we chose a vessel with thrusters. Docking in the strong tidal current at the mouth of the Piscataqua in Portsmouth NH, however, continues to be a challenge. Can someone help me to understand the concept of slack tide? In particular how to time our docking to make it as easy as possible, at least until we gain more experience. Thanks in advance!

 

[pleeson]

Your Garmin navigation system should have a screen that will give you current tides for the nearest reporting station, or you can get an App for your phone. The tides are the result of the gravitational pull of the moon and the sun and the net result is that every 6 hours the tide swings from low to high and then back to low six hours later. Within a few minutes of the absolute high or low tide the currents slow, as the basin you are in finishes filling or emptying, then the flow stops completely, before slowly reversing and then accelerating to maximum flow, on the way to the next turn of the tide. You therefore have four periods of relatively slow current (or slack tide) every 24 hours. The length of this period of relative "calm" is variable depending on the tidal range at a given phase of the moon because with larger tidal range at some times of the month or season larger volumes of water have to rush in our out of channels to accommodate the need to rise or fall a greater amount in the same 6 hours... If your Marina is located in a relatively narrow channel the currents can be pretty strong at times other than slack water, and like with strong winds, your thrusters can be easily overwhelmed. Consult and observe other boaters to see how they use brains over brawn to accomplish what needs to be done when operating too far outside the slack water periods. There are little tricks of the trade that might include backing down using the main engine to counter the flow and then using willing helpers on the dock to finalize your positioning.

 

[Red Raven]

In addition to tide reporting stations there are often current reporting stations which may be more helpful. The timing of the slack is often quite different than high or low tide depending on the geography. If no direct current table is available there may be a correction table giving the offset time from another location. If no current or correction tables are available for your location you may need to rely on local knowledge.

Generally when I have to dock in high current I keep the bow into the current and use the throttle to match the speed of the current. Be sure to stop all momentum of the boat first so that the boats momentum is not confused with thrust against the current. With the added current past the rudder the sensitivity of the wheel is enhanced so care must be used to make only slight changes in direction or the current will pull the bow off the heading. Also when the forward speed is balanced against the current the thrusters may be more be effective to move side to side. It is important to practice in current away from other boats first. Over time you will learn what conditions can be safely navigated. Always give yourself an out so you can start over or chose another docking alternative.

 

[stwendl]

One important fact I learned from sailboat which had two outboards ( catamaran ) that as long as the wind blew below 25kn I could safely spin the boat with both engines in opposite thrust. Each engine produced 285lbs of thrust. And due to its freeboard 25kn was the envelope

Similar to that, numbers pertaining to our tugs should be known too and published as envelopes or safety margins.

Numbers of importance are

Bollard thrust of engines in forward and reverse and angular force when rudder is hard over
Bollard thrust of each thruster individually and together as well as opposite
Wind load front and side

With those numbers and practical experience one can then come up with safety guidelines in terms of
For instance If it blows 30kn on the beam the re is no way they engine, the thrusters or both can push the boat of the dock

Unfortunately, there is no wind indicator on the boat which would really be a safety tool if one were to be in such a situation.

Of course the most important decision a skipper has to make is the decide when to go out or not. However there are sometime where one might be forced to be in such situation and knowing the limits can make a difference

 

[tugnnaweigh]

Launch, retrieval and slipping are the most challenging areas of boat handling. The wife (boat driver) and myself, (arm waver) have learned the top rule is to never place any body part between a boat and any immovable object. The force of moving water has caused our empty trailer to take off downstream, ending under the dock and when the trailer captured the boat the same thing happened.

With much momentum behind them, from whatever source, vessels of any size can be unstoppable with human effort, keep in mind the force of moving water multiples when the vessel arrives at whatever it’s going to hit and water begins building up on the far side.

Prior to every close quarters maneuver we try to quantify the other forces acting on the boat this time. Wind, tide, waves, current, what the heck was that, all affect the course the boat ultimately undertakes with an overarching concern of losing power or thrusters.

Try for high slack tide as this reduces the seaweedy slippery ramp area uncovered by low tide. When in that situation I wear good tractionary boat shoes and chant Don't Slip, Don’t Slip, seems to help.

For a good tutorial on what not to do, just go watch any boat ramp on any busy day.

 

[JamesTXSD]

In our litigious society, I don't see any boat manufacturer publishing a wind guideline for safe operation... can't you just see someone with no experience saying, "The manufacturer said I'd be fine in 30 knots of wind!!"

Safe operation of any boat is going to be a combination of experience and judgement. Yes, I am aware that some people get experience through bad judgement. :twisted:

No one comes into boating knowing it all. If you aren't sure of the effect of tides/current on boat handling, I would suggest taking some safe boating classes (US Power Squadron or Coast Guard Aux). Build your experience in a safe manner; hire a captain to go out with you in varying conditions, if necessary.

No one would think about getting into an airplane and taking off without instruction and building experience, yet some thing nothing of doing just that in a boat.

You can read all about tides on-line, but nothing will be getting experience in different current situations. Thrusters can be helpful, but they don't replace experience and good judgement.

Congrats on the new boat - wishing you many years of great experiences on the water.

 

[Champ]

Hi all. We are so happy we chose a vessel with thrusters. Docking in the strong tidal current at the mouth of the Piscataqua in Portsmouth NH, however, continues to be a challenge. Can someone help me to understand the concept of slack tide? In particular how to time our docking to make it as easy as possible, at least until we gain more experience. Thanks in advance!

We recently received our R-23 and are experiencing the same issues with tide and currents on the Hudson River. Add a little wind and it's not fun. Can't always time it to our schedule but slack current seems to be an hour after high or low tide. I never worry about getting out, just in. Went out Sunday in not the best of conditions but got back half an hour before slack current and conditions were favorable and we looked like pro's backing into our slip. We are new to this so take my observations for whatever it's worth.

 

[Sea Jo]

Last December ,we had a slip on the Carrabelle river . Good current in river and when winds picked up, challenging .The slips were opposite the river current,with pilings further out than boat size. Slips could handle large boats. It was nerve wracking at times,but after that week there everything seems easier now. Had to learn what forward speed was needed,and when turning, in leaving enough room for getting pushed against current. 85% good slip entrys,the other were learning exp.

 

[Levitation]

That is the third fastest flowing river in The World! It is a tidal bore river.
None of our cruising knowledge on mainly calm waters is going to be of any help to the OP.
To him I say, talk with the locals. Find a commercial tug captain or a local yacht captain and offer to take him for a steak and a glass of Jamison's if he will spend an hour with you on the tug going through docking and how to time the LOCAL tides.
And as far as boating on that river, better you than me matey :mrgreen:

 

[tugnnaweigh]

Solid point regarding local knowledge. Went up the Petaluma River to slip at the Petaluma Marina. Timed the voyage to the tide and found it shallower than expected. Spoke to locals who said the movement in local water level is up to two hours behind the tide times!

Keep in mind throttle and thrusters can affect and modify the course of travel but typically cannot overcome or defeat other forces present. Compare the difference between arriving at the trailer in good order to the difficulties present in coming around for a second try, a situation no one wants to find themselves in.

As much as I am a fan of low speed and low throttle in close quarters, when forces are affecting control it can become advisable to move smartly, increasing rudder efficiency and decreasing overall time to arrival. I tell the Captain; bring it in Honey, I don’t have the trailer as deep in the water as usual, it’ll catch her.

Once the boat is held securely by the trailer I back it in a bit further while the wife uses throttle to move up to the cradle.

Odd thought; in operating a Tug it seems like there’s an obligation to look extra competent.