Tides and Forward Progress

Rick B, thanks for the response. Seymour Narrows is a focal point for all sizes vessels planning trips up that way. Ref post 142.

Other than a canoe stern full displacement boat, such as you average sailboat, on a typical flat transom "trawler" or (gasp) motor yacht, you will lose less speed going into a current than you gain with a following one.

The ICW inlet phenomenon cam be a matter of some importance in S. Carolina and Georgia in particular. If you are in the "journey is the destination" mode a little trip planning can give you what we call the slingshot effect: approach the inlet intersection at most favorable tide in your direction, hit the inlet close to slack, then have the tide with you on the other side. Over the course of several hundred miles, it gives you what we also call the Everett Dirkson effect: " a couple knots here, a couple knots there and it starts adding up to real money". Similar to taking a trip up the Hudson or Sacramento rivers and back, with the tide up and down.

I think the premise of the OP is one of the silliest things I've read on a boating forum. Yes, if you are just a day or weekend cruiser taking a random walk around your home waters with the only regard being your personal schedule and free time, I suppose you could cancel the current out, dismissing my first sentence for the time being. But for those of us transiting cruises at "trawler speed" measured in thousands of miles, only an extremely rich man and/or a fool and/or someone on a "schedule" (the most dangerous thing to have on a boat) would not use the currents to gain a very significant advantage.

caltexflanc said:

Other than a canoe stern full displacement boat, such as you average sailboat, on a typical flat transom "trawler" or (gasp) motor yacht, you will lose less speed going into a current than you gain with a following one.

The ICW inlet phenomenon cam be a matter of some importance in S. Carolina and Georgia in particular. If you are in the "journey is the destination" mode a little trip planning can give you what we call the slingshot effect: approach the inlet intersection at most favorable tide in your direction, hit the inlet close to slack, then have the tide with you on the other side. Over the course of several hundred miles, it gives you what we also call the Everett Dirkson effect: " a couple knots here, a couple knots there and it starts adding up to real money". Similar to taking a trip up the Hudson or Sacramento rivers and back, with the tide up and down.

I think the premise of the OP is one of the silliest things I've read on a boating forum. Yes, if you are just a day or weekend cruiser taking a random walk around your home waters with the only regard being your personal schedule and free time, I suppose you could cancel the current out, dismissing my first sentence for the time being. But for those of us transiting cruises at "trawler speed" measured in thousands of miles, only an extremely rich man and/or a fool and/or someone on a "schedule" (the most dangerous thing to have on a boat) would not use the currents to gain a very significant advantage.

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Current affects every vessel equally according to everything I've ever read or taught. ...leeway is different.

Have a reference that explains this in detail?

caltexflanc said:

Other than a canoe stern full displacement boat, such as you average sailboat, on a typical flat transom "trawler" or (gasp) motor yacht, you will lose less speed going into a current than you gain with a following one.

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I see this particular bit of silliness has already been flagged but its so abysmally stupid that it deserves being highlighted again.

The water is moving. You are afloat in that moving water. The shape of your hull is IRRELEVANT.

I agree w psneeld and Bob. Bucking current w a FD, planing or SD hull is the same if their transiting speed is the same. We're not talk'in about how much fuel is burned.

And we're not talk'in about cheating as in going at the best possible time. Wer'e talking about the question broken down into it's most simplistic terms.

It takes more time to go a mile bucking a 2 knot tide than you gain running w a 2 knot tidal current. So bucking and running into tidal current takes more time than transiting calm water. Calm water wins. Running in tidal currents does NOT average out ..... as I had thought. I think that is all there is to it ... according to FlyWright and now everyone else here. Is it that simple? I think it is.

I agree with you, Psneeld. Once your boat is drifting in a body of water, it moves with that body of water just as an airplane or a feather moves with the air mass. If the water moves north, you move north. Your thrust might overcome that influence and move you in another direction or faster toward the north, but your base reference is the same as what that body of water is doing.

The shape of your hull or the mass of your vessel has no effect except in the time it takes to react to the changes in water movement. If you have a slick hull or a very high mass vessel, it might not react as quickly to changes in current like a high drag hull or a light weight boat. But in a boat, I suspect we're talking mere seconds to react, not an effect on your cruise that would last over a long period of time.

Similarly, large aircraft are more susceptible to wind shear because the inertia of the aircraft slows its reaction to sudden and significant changes to velocity and direction of the air mass. Small aircraft with less inertia respond quicker to the changing wind direction and suffer less impact from wind shear. But like boats in water, small aircraft of all shapes and large aircraft of all shapes are equally impacted by the wind once the craft achieves stability within the air mass.

I probably didn't do a very good job of explaining that, so please ask away if you have questions and I will attempt clarify the concept further.

Keeping all things equal; at an established/specific “through water speed” (10 knots at 2,200 rpm in the following examples) the friction of a boat’s hull surface against the water touching it is only equal to the surface area of said hull touching the water, nothing more and nothing less. In following examples a boat is at 90 degree angle to the opposite direction 5 knot currents, water surface is considered always flat, and there is no wind.

1. Traveling Against 5 Knot Current: If at 2,200 rpm the boat’s “through water speed” is 10 knots then that boat is traveling at 5 knots “over land speed”
2. No Current: If at 2,200 rpm the boat’s “through water speed” is 10 knots then that boat is traveling at 10 knots “over land speed”
3. Traveling With 5 Knot Current: If at 2,200 rpm the boat’s “through water speed” is 10 knots then that boat is traveling at 15 knots “over land speed”

Friction component of hull against water alters not and therefore does not change the stated 10 knot “through water speed” no matter what direction, or no direction, the water mass may be accomplishing. Only the “over land speed” will alter as per the current’s direction, or no direction.

bobofthenorth said:

I see this particular bit of silliness has already been flagged but its so abysmally stupid that it deserves being highlighted again.

The water is moving. You are afloat in that moving water. The shape of your hull is IRRELEVANT.

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First, for a discussion of the effect of boat and hull design and its resistance to water, Dave Gerr's "The Nature of Boats". The shape of your hull is highly relevant, along with the weight of your boat.

And, we are not talking about merely floating, but running the boat. But you can see the phenomena for a brief time while your boat is at rest. Here's a little experiment you can do sometime, which I've done while waiting for a bridge a couple of times. Point the bow into the current. A little piece of 2x4 is handy, but any small squarish piece of floating something will do. Tie a fishing line to it. (I first noted the phenomena when we were fishing and how a bobber floated) Stop your boat, throw the block over board. It will start floating down stream faster than your boat does. You can even get a stop watch and see how long it takes to get between two points on your boat. Now turn the boat stern to the current and do this again. The block will move more slowly relative to the boat than before, and your boat will catch up to it faster. Now yes, eventually your boat (and most boats can end up beam-to the current if left to float on their own, so the effect is neutralized) will eventually more or less float on the water at close to the speed of the water, but the fact is you boat is designed to have the least resistance when the water is flowing on its pointy nose, and more when a wider squarish transom is being pushed along.

I've also logged this effect while running and the STW gauge was properly working, measured vs SOG at the same engine RPM, going into and with the same current. Indeed, it is not significant, on my big tub of a boat with a giant butt, maybe a couple tenths of a knot at hull type speeds if the current is really ripping.

You can observe this also on a straight stretch of river during a flood, though the current speeds can vary along the width as noted above. Watch how different sizes and shapes of objects float down at different speeds. Didn't any of you have little races of some sort when you were a kid with toy boats or sticks or pieces of wood sent down a creek?

caltexflanc said:

First, for a discussion of the effect of boat and hull design and its resistance to water, Dave Gerr's "The Nature of Boats". The shape of your hull is highly relevant, along with the weight of your boat.

......... followed by a bunch of silliness which I snipped to spare the rest of you from having to read it more than once.

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Of course the shape of a hull moving through the water has an effect on its resistance. That's not what we're talking about. Either you don't understand the discussion or you are deliberately trying to divert it to take attention off your own foolishness.

The rest of what you wrote was so patently silly that its not worth a response.

I'll make it easy for you but you'll have to use your imagination. No bits of wood or pieces of string are required.

Imagine a very large bathtub full of water - well not quite full but fullish. Put your boat in the bathtub and let it float. Now load the bathtub on a semi-trailer - a really really big semi-trailer. Drive the semi down the road at 4 or 6 or 8 knots. What difference does your hull shape make?

OMG!!!!!

Sometimes this place amazes me for all the insightful knowledge of experience cruisers....then the next moment...OMG!!!!

I've noticed that when going downstream, the person overboard in the water goes down the river faster than the boat. Presume this is due to the apparent wind caused by the current and the difference of the wind resistance of the person versus the boat.

From which side of the boat have you observed this, inside or outside? Does it happen often?

FlyWright said:

From which side of the boat have you observed this, inside or outside? Does it happen often?

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On a dinghy/raft, it is always outside.

With several rafts, it seems there is at least one person going overboard every day assuming there is some good whitewater. On the first day going down the Salmon River (Idaho), the guide was bucked off the boat's stern and went overboard three times in the first half hour. We grabbed/recovered him before he was swept past the boat.

Do lardasses go downstream faster than skinny broads? Inquiring minds want to know.

Based on this thread alone...I wish the Mayans had been correct..

Based upon this thread, the Mayans were correct.

Did you hear the one about the Bi Mayan priest who went to San Francisco to find a virgin to sacrifice, oops that is another thread.

All I can say is spend more time actually on the water and observe how the world, and your boat, really work.

Or, not: I know that doing something so radical as that is a silly concept for many, so many bad things can happen that just don't pencil out!

psneeld said:

OMG!!!!!

Sometimes this place amazes me for all the insightful knowledge of experience cruisers....then the next moment...OMG!!!!

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HS and JC... I agree!!! OMG!!!!!

I'm done... no more clear info given from me!

I'm amazed at what some boaters think or believe!

I give up!!

caltexflanc said:

All I can say is spend more time actually on the water and observe how the world, and your boat, really work.

Or, not: I know that doing something so radical as that is a silly concept for many, so many bad things can happen that just don't pencil out!

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It was hard for me to understand at first, but I now realize that this forum is much about Arm Chair and jewelry boating. Don't let it confuse you...join in.

caltexflanc said:

All I can say is spend more time actually on the water and observe how the world, and your boat, really work.

Or, not: I know that doing something so radical as that is a silly concept for many, so many bad things can happen that just don't pencil out!

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Do you think any of the professional mariners on here with maybe over say 30 years working on the water have a basic understanding of what you are pointing out???

And then there is always the practical side and the "pencil" side of doing things...

I think I understand tides but are we making forward progress here?

xfedex said:

I think I understand tides but are we making forward progress here?

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Forward progress is assisted if you bow is pointen in direction of da tidal current!

It's been an interesting, if at time confusing thread. All I know is at cruising revs in still water we do 6.5kn. If I'm going faster than that over the ground as per the GPS, I know I'm getting a current lift, and rejoice - if slower than that I know the current is agin me, and I accept it will take longer, and I usually resist the temptation to put the hammer down, 'cause of my frugal nature....on occasions I have achieved 11+kn, surfing in front of large waves - now that is fun - and no, she never broaches. I put that down to both fuel and water tanks now being well back.

I think there has been enough said to assume that the Mayans were correct.

Benn

bobofthenorth said:

Do lardasses go downstream faster than skinny broads? Inquiring minds want to know.

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Not sure, but witches float 'cause they're made of wood...

And what ELSE floats???

Keith said:

And what ELSE floats???

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Checks - Just ask Bernie Madoff... oh yea, and Ben Bernanke!!

Keith said:

And what ELSE floats???

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Crap of course

CPseudonym said:

Crap of course

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Toilet humor... this close to Christmas... Please! :lol: