In this part of the world there are high tides and restricted bodies of water. The two often combine to produce rapids – areas of turbulent and fast water flow, typically best left alone by yachts and small work boats. History is replete with attempts at running these rapids that have come to a bad end (because of bad timing or hubris).
In the central British Columbia cruising area, within a mile are three such constrictions.
Dent, Yaculta, and Arran Rapids all have fearsome reputations. As we are just a couple of days off a full moon the tidal range is quite high and the currents strong. Even with a boat as stable and maneuverable as Wind Horse we’d prefer not to tackle these at max flow. However, as we find ourselves in this area two hours after maximum flow, we’re going to give this a try (and maybe bring back some of the adrenaline rush we miss from sailing Beowulf).
The chart above shows the passage to the west of Little Dent Island, and the "Devil’s Hole", so named because whirlpools which form here are said to suck down the unwary.
The radar provides one means of measuring turbulence. Little Dent Island is shown to the port and Sonora Island to starboard. The land masses are solid radar images, while the turbulent eddies of Devil’s Hole are ragged and clearly show up just off our bow to port. Range is set at 0.25 nautical mile, and range rings equal roughly 315 feet (90m).
The view from the bridge on Wind Horse looks benign. There are some swirls, but nothing we can’t deal with.
A few seconds later, here is the radar view of the turbulence.
And this is the best current speed – a hair over seven knots. Interesting, but nothing in the category of an E-Ticket ride.
And our best Devil’s Hole whirlpool: this one is maybe 10 feet (3m) across and perhaps three feet (0.9m) deep.
A final radar image looking back. During this ride through Devil’s Hole our WH Autopilot did the steering, and the NAIAD stabilizers were on. As the current tried to yank us one way and then another, we would see rudder deflections as high as 20 degrees. Maximum course deviation was plus/minus 10 degrees.
Keep in mind that Wind Horse is extremely stable, with huge spade rudders, easily steered, with very efficient stabilizers. We do not recommend what we did for most boats. It is better to wait a few hours for the current to back off.
Rapids are often described as laminar or turbulent in their flow. Dent Island and Yaculta Rapids are said to be turbulent. The shape of the bottom and shoreside configuration gives rise to whirlpools, overfalls, eddies, generally exciting conditions. Arran Rapids is said to be laminar, in other words, it runs really fast but smooth, without whirlpools – at least that is the theory. That circular set of arrows off Arran Point on the Canadian chart above are an indicator that there might be more turbulent flow, at least right here. Max current is shown as nine knots, and we’re outside of this, so we should see maybe eight knots.
Here is the view from the bridge. A clear shear line and turbulence is visible to starboard, an indicator that the chart might be right about Arran Point.
Now this is interesting. The photo is carefully cropped to be parallel with the horizon. Notice the slope of the water as it rushes by Arran Point. There is a surge of water mass piling up at this restriction, trying to wedge its way through. We make this to be a four foot (1.2m) height differential between the west and east sides of the Arran Point.
A few seconds later here is what the Furuno 2117 radar shows us. You can see that it is much clearer to our north (port) but more fun where we are on the shear line. If you were forced by an emergency to run a rapids like this, knew there was deep water to the north (which there is), and saw this on the radar, you would have the best chance of success by staying out of the turbulence.
In case you missed the speed of Wind Horse through the water and her speed over ground (SOG) here is a blow up. That’s ten knots of current – and we’re well past max flow, and three days removed from the full moon. If your math skills are rusty, thus is ten knots plus of current! We need to come back here on a spring tide and check this out!
We always say take what you read and hear with a grain of salt. Above is an example of why. In the supposedly "laminar" Arran Dent rapids, that’s a whirlpool of at least 20 foot (6m) diameter and probably 5 feet (1.5m) deep. Like they say on the TV ads, "Professional drivers – don’t try this on your own".
Although our blood pressure is elevated and there’s lots of adrenaline flowing at this point, Wind Horse is hardly bothered.
So what to pass on from this little experiment?
- Wait for a better time to minimize the risks associated with these turbulent conditions.
- If there are whirlpools, stay away from the side that tends to suck you in, and stay towards the side which spits you out. In the photo above, the circulation is clockwise relative to our position – we’re on the “good” side. If you were coming through Arran Pass the other direction and were close to this same (north) edge, the whirlpool would tend to pull you in.
- Maintain good boat speed through the water so you have flow over the rudder for steering control.
- Be prepared to increase engine RPM for better rudder control.
- Look for smooth, deepwater paths through the turbulence. Radar can help here.
- The current accelerates as you get closer to choke points. If you are having second thoughts about going through, you have to reverse course with plenty of time to work out of the current drawing you in. In most low powered boats this has to be done well outside of the current acceleration zone. It is better to attack these rapids (if you must) opposite to the current flow.
- When opposing the current, if one or both shores are steep to (deep right up to shore), you can often find reduced current in this area.
- These photos show us in light winds. Wind waves and swells create much more hazardous conditions. Standing waves, with the current drawing you into them, are not uncommon.
- Do not try this in small boats that are subject to capsize. The combination of the whirling motion and tilted water surface of these whirlpools can be deadly.
- The harder your boat is to steer, the beamier it is relative to its length, the deeper the forefoot, and the lower its stability, the more you should avoid these conditions.
- Once committed, heading with or against the current, it can be dangerous to try to reverese direction. As you are turning, control is apt to be lost, and it may become difficult to get the boat back under control of the rudder. You may also be at a minimum stability attitude relative to the turbulence.
We’ll leave you with this image from the depth sounder. This is on the eastern edge of Arran Rapids. You can see a ridge coming up quickly and then dropping off. The red mass at the top is turbulence in the water from current. Tubulence can reduce rudder effectiveness, or render the rudder useless.
Wait for slack water, or at least closer to slack…