Question about waves and depth

[labatt]

Say you're in 10ft of water and the waves are 3ft from trough to crest. Does this mean that generally you're in 8.5ft to 11.5ft of water? When there are waves I generally don't see our depth meter change much, even when the waves are 6-8ft. I know we cut through them some, but I would think that I would see some sort of depth change as we pass through them. I was very nervous the other day when we were in about 10ft of water and 42kts of wind and the fetch was picking up to about 4-5 feet, very steep. We draw 6ft. I've always been curious how exactly waves affect water depth.

Thanks!

[Mipcar]

If you read advice on crossing bars with waves/swell you will see they often caution about "bottoming out" (not the word they used) in a trough. So my thoughts are that if your depth limited then you'd need to factor in wave effect.
I agree I've not noticed changes in my depth log when sailing in deeper water wave/swell but that might be as much the dampening of my unit as much as anything else.

Mychael

[bheintz]

I suspect that with the dampening of the Depthsounder your assumption of average is correct, however the actual depth may depend on the "offset" of your Depthsounder. For example, some Depthsounders can be set to display "depth of transducer" (default), "depth from keel," or "depth from surface."

Here are two recent examples of crossing a bar in waves:

Spanish sailor's accidental adventure (read more)


Yacht departs Southport (read more)

[sailingdog]

There are probably two things going on... first, the depthsounder probably has some dampening in the readings to prevent wild fluctuations. Second, being a relatively heavy displacement monohull, you may not be seeing the changes since the boat doesn't really rise and fall with each wave... since the boat is too heavy to respond quickly to a rapid rise or fall due to inertia.

While any given wave or swell pushes up on the boat, it would have to accelerate the entire mass of the boat to get it to move the four feet it rises in the short period of time it is actually under the boat, which is not gonna happen.

[KeelHaulin]

Well if you were sailing in 10ft and had 4ft chop; you would probably be OK if your boat were long enough to bridge across the trough between crests. But if you were sailing abeam to the waves; you certainly would be down in the trough for a short time between wave crests, and in that situation you might momentarily ground the keel. If in shallower water or higher waves; it could cause serious damage slamming down on the bottom multiple times.

I'd say being in 10' water with a 6' draft in 4' seas would be close to the limit of what you would want to be sailing in; I'd be looking for deeper water to be sure I did not find an uncharted shoal or rock. Remember also; that the wave height is divided roughly in half for the trough depth relative to flat water. So a 4' wave would have a 2' high crest and a 2' deep trough for a total wave height of 4'. The depth of the water at the bottom of the trough would be 2' less than the mean water height (in 10' of flat water it would be 8' deep in the trough of a 4' wave).

[Valiente]

^ ^ ^ Abeam was the only situation I was going to mention as problematic. In such situations, I would think proximity to shore and "backwash" or reflected waves would come into play, because a shallow anchorage like that could turn into a real washing machine quickly.

So you might not bottom out, and your anchorage might hold well, but there's still compelling reasons of comfort and security to seek a deeper and/or more distant anchorage when it's getting that sloshy.

[bubb2]

Originally Posted by labatt

Say you're in 10ft of water and the waves are 3ft from trough to crest. Does this mean that generally you're in 8.5ft to 11.5ft of water? When there are waves I generally don't see our depth meter change much, even when the waves are 6-8ft. I know we cut through them some, but I would think that I would see some sort of depth change as we pass through them. I was very nervous the other day when we were in about 10ft of water and 42kts of wind and the fetch was picking up to about 4-5 feet, very steep. We draw 6ft. I've always been curious how exactly waves affect water depth.

Thanks!

You are correct. If you are in 10 feet of water and have 4 foot waves. You will have troughs that have only 6 feet of water.

The most noted example of this is the wreak of the Edmond Fitzgerald and 6 fathom shoal. It is belived my many that she bottomed on the shoals in a trough because of the 35 foot seas.

Decades later, bells still toll for crew of Edmund Fitzgerald | Articles & Archives | Professional Mariner, Journal of the Maritime Industry, Maritime Industry News

[sailaway21]

Pretty good explanations offered and I'd only mention to the OP that fetch is the amount of open water that the wind blows over before it reaches you. It has nothing to do with describing sea state other than it is a causative factor in sea state. I think the intent was to describe an increase in sea state but the term fetch does not do that.

[AdamLein]

Where is your transducer located? The boat will tend to pitch around its center of gravity. If, for some reason, you put your transducer all the way in the bow, you'd expect to see much greater fluctuations than if you located it, somehow, right under the mast.

[scottyt]

i just put in a fish finder/gps plotter and spent some time riding some waves. on my boat the fishfinder shows the bouncing in depth contours but not on the depth display. i would like to know wither the waves go plus and minus, meaning if the depth is 10 feet with 4 foot waves does it go trough 8 feet peak 12 feet. or do they all go neg or pos, i would bet it it goes both but maybe weighted more one than the other