Cargo Wake Issue / Newer Boater

[Marco Flamingo]

As to angle and speed to cross a ship’s wake, I’m sure a naval architect could figure out what is best for a given vessel. It would have a lot to do with your vessel’s roll period. My little trawler has a roll period of approximately 3.4 seconds (port, starboard, and back to port). It varies some with the amount of roll, but not much.

Say a wake is approaching me from port. It initially leans me to starboard. Let’s say I’m traveling at a speed and angle to the wake such that the 6’ wake tips me to starboard. My boat naturally rights itself full upright in 1.7 seconds (half the roll period). If I am on top of the wave at that moment, I basically flop over to port into the trough, exaggerating the roll. If I’m at the bottom of the trough at about 3 seconds, as my boat right’s itself again I’ll be launched back to starboard by the boat’s righting ability compounded with the next wave. It’s this “harmonics” that needs to be avoided. Instead of a naval architect’s computation, trial and error is available free of charge.

My boat has “high stability,” meaning that the width to length makes for a snappy roll despite having a fairly large keel for its size. I can run from a wake, hoping that it will calm down over distance, but taking a 6’ wake quartering on the stern is also not pleasant, especially if I’ve run a mile out of my way for this maneuver. Best for me is to time the approaching wake so that I slow way down (basically 2 knots at idle) and take it almost head on. I have a large flare to the bow which tends to keep the helm window clean. Better for me to have the boat rock fore and aft, even with a couple of shudders from smacking the big wake, than to wallow side to side in an attempt to more closely maintain my course.

 

[wkearney99]

It’s this “harmonics” that needs to be avoided.

I don't run into challenging wakes that often, but do pretty much exactly what you describe. Get the bow into it and use power to adjust speed to help minimize jarring. Sort of like timing the waves into an inlet to surf it at just the right speed.

Learned some of that with go-fast boats where you had to not only balance how you were pounding the waves, but also the RPMs of the engines when the props would come out of the water. Not directly applicable here, of course, but you learn to appreciate effective application of speed. Which I'll grant you becomes less dynamic as your boat size increases.

 

[Blissboat]

I admit that 30 knots is a little high, since the majority boast an average cruise speed of 22 kts.

What is your idea of 'not anywhere close'??

A contemporary cruise ship capable of much more than 25 knots or so would surprise me, and even if the capability existed, it would be astonishing if the operators had any practical use for it, other than maybe sprinting to get ahead of some weather (which should be predictable enough to plan around anyway). Who would plan a cruise route that required their ship to move between ports at such fuel-gulping speeds? Travelers who are pressed for time take a plane.

The cruise industry's value proposition is about capturing customers and keeping them (and their dollars) aboard ship, not rushing them between destinations. My understanding is that cruise itineraries are designed to favor daylight port calls, leaving transits to be performed overnight at economical speeds, sometimes just paddling along at 10 or 12 knots while the guests eat, drink and sleep.

Pushing a big ship through the water at 30 knots takes a lot more energy than even just 25 knots. Lower speeds use less fuel, which means reduced operating costs, quicker in-port turnarounds and more room for profit.

Ocean liners, of which there are now very few, such as Cunard's newest Queen Mary 2, are different from cruise ships. In trans-Atlantic service, there are no alternate routes. Every passage is long and nonstop out on the high seas, and keeping a schedule requires accepting unfavorable weather along the way. The QM 2 can run at 30 knots when she must, although I'm sure that Cunard would prefer she not try to dig a trench in the ocean.

Then there are design and construction issues. A ship that can run at 28 or 30 knots needs a hull that is longer and narrower in aspect than one designed to carry thousands of people and their amusements in smooth, unruffled comfort. It needs a hull that is stiff and that has a forward section capable of driving hard into head seas without pounding or deflection. That's more like a navy warship than a cruise ship.

 

[catalinajack]

I admit that 30 knots is a little high, since the majority boast an average cruise speed of 22 kts.



What is your idea of 'not anywhere close'??

The difference between 22 and 30 knots is quite significant.

 

[Hippocampus]

Over the years my thinking has been to divide wakes. Those that will be annoying. Those that represent a risk to the stability of the boat +/or safety of the crew. In others words those that will produce a knock down or even just putting the rail down and throwing people around.
Here go back to Allard Cole’s teaching about how to handle large seas as written in “heavy weather sailing “ with some modifications for being in a SD hull. In a blue water sailboat an option is to surf the wake. Even with my limited experience in power at least for the current boat this isn’t often the best option. Have tried with large waves (not wakes) and it’s bit scary and requires focus. The lack of a sufficiently sized rudder and low speed of water passing over the rudder might mean periods of poor response to rudder input. However power has a significant advantage over sail when positioned to meet the wake head on. One can apply sufficient power to ride over the wake even if large. Given the lwl is longer than the beam risk of being overcome is minimized and safety maximize.
So our protocol has become if caught unawares so diversion isn’t possible is:
Look and judge if the SeaKeeper will be enough. If so turn moderately into the wake with slight increase in throttle until over the crest. Then back off.
If it’s not on or deemed insufficient a hard turn into the wake attempting to meet it head on. Increasing throttle as needed to make some head way. Then backing off once the crest has passed. If surfing bear off to not bury the bows. If not surfing continue on setting up for the next. If surfing at trough add power to turn into the next one.
This is pretty much what Cole suggested in his book. It’s contrary to Dashews writing but think small SD trawlers behave more like the boats Cole talks about.
Of course if you decide to run from the wake and have sufficient HP Dashews technique works quite well as long as you can go fast enough to have water flowing by the rudder. However I’ve yet to have enough experience to feel confident doing this. We try real hard to avoid being subjected to really large wakes.

 

[rslifkin]

Surfing a wake can be an option, but how well it works depends on the boat (hull shape, rudder size, etc.). When surfing at low speeds in a powerboat, you generally want to run with 1 hand on the wheel, 1 hand on the throttle(s). Even at low boat speeds, a quick squirt of throttle will move a significant amount of water past the rudder (as the prop wash generally covers most or all of the rudder, unlike a sailboat, and the larger props typically found on a powerboat can move a significant amount of water).

So if the boat starts to get out of sorts and adding more rudder isn't doing enough, add some throttle. Plus, if the boat is responsive to speed changes you can also adjust speed a bit as the waves pass to find what's most comfortable.

 

[Hippocampus]

Thanks R I’ve found the same. A great option not as available on low powered sail.

 

[rslifkin]

Thanks R I’ve found the same. A great option not as available on low powered sail.

Agreed. That trick is much less effective on sailboats both due to less prop thrust available and due to the very different prop / rudder relationship (leaving the rudder more at the mercy of water flow and less driven by prop wash).

 

[psneeld]

I hope no one is reading this thread that is relatively new and now is fearing wakes.

If you pay attention and decide early to put distance between you and the wake thrower or are very close and have to decide on the angle best to take it at the last minute.... I firmly believe the worst that will happen is you will be badly rocked.

I have encountered plenty of wakes, often while towing with little opportunity to do much maneuvering and never really been even scared. Rocked hard and had plenty tossed around...but never buried a rail or broke a windshield....though I may have had water in the cockpit.

The worst wakes I ever had were when I was tied up and not paying attention to passing traffic. Little I could do but any prep is better than none.

The best advice I can think of..... is try to avoid being near shipping lanes where they can run fast, keep things battened down, your head on a swivel and if really a big, steep wake...take it on your best angle which can differ if on the bow or stern.

 

[wkearney99]

I hope no one is reading this thread that is relatively new and now is fearing wakes.

Don't think anyone's intention was to raise any fear about them, rather to point out there are situations where wakes might be much larger than you'd first guess or expect.