Boating in shallow water

[3788sam]

I remember a discussion a while back about the effect of shallow water and hull efficiency.
On our trip up this Friday I went up through the Swinomish channel. Low tide entrance to the channel was about 9'. Notice we lost about 200 rpm and 5Kts when we entered.
When I looked down and saw the rpm loss I thought there might be a problem :shock: , pulled down the throttles and just keep an eye on the gauges and there wasn't any smoke comming from the exhaust. Everything was fine and when we got past the swing bridge opened up 2800 rpm as usual. Water was about 20' after the bridge.

Just an observation :?

[boating777]

I head out that way all the time, you are speaking of the Anacortes side of the slew? can you tell me where it was so shallow? right before the railroad Bridge?

777

I don't think hull efficiency in shallow water plays as much effect (if any) as the narrowing of the channel and the increased current through there. :D

[3788sam]

777 entering from Whidby side, depth sounder 7'-9' Friday at about 4pm.
There was a 30' Blow boater sitting high and dry right before the turn and no wake zone. Had a beach towel over his cockpit waiting for the tide to come in.

SS- maybe some times on the current but returning through Desception on yesterday 4 Kt against us- no drop in rpm.

[boating777]

Ken, it gets LOW where you where at , :shock: at low tide it always makes me jump up out of the chair :P and Grab the throttle's, I will have to tell you it is a show every year to see the Sailboats high and dry, let me guess he tried to cut the buoy?

777

[3788sam]

Yep - Those channel markers are there for a reson. A few season back I see red flares and a guy waving his arms. His faimly and boat are about 10 yrds out side the channel in about 2" of water. I stopped and tossed him a line. He was standing next to the nun bouy and the water was just above his knees.
No radio, pulled him off asked if check his out drive?- Nope started up said it sounds fine, took off like a bat out of hell :evil:

Been there done that... :)

I cut the last bouy on the way out on a -2.5 the first year we had the boat. Left me sitting there as well :(

Worst thing was Diane had already started cleaning up the boat in the no-wake zone and dumped all our water. Had to sit there for 3 hours waiting for the moon to go away and put the water back. :roll:

Ken: You'll have to explain that whole shallow water makes the boat work harder thing to me sometime. :D

[3788sam]

Mike, You had told me about your 3 hour tour beaching, but I was not going to mention your name to protact the innocent :lol:

I think it was on BOC about the shallows and the best I can figure out is a pressure wake must be bouncing off the bottom and interfering with the running gear? But I'm no fluid dynamic expert :lol:

[heymagic]

I read that a Mainship 34 had problems getting on plane in shallow water. It apparently is a true phenomenon but the rational escapes me also.

I suppose there's some point when the water pressure would have to go somewhere... but I'd think you'd have to be in some mighty shallow water. In a no-wake like the Swinomish, I'd think it'd be hard to feel though.

Interesting idea though.

[3788sam]

The no wake starts after you make the turn towards LaConner from the whidby side. I've been passed in the channel by MYs throwing gaint wakes going down there.

Yeah. they get pretty stupid in there. It gets pretty choppy in the entrance when everyone comes blasting through at low tide.

[Seapuppy]

I saw a film a number of yrs ago on shallow water wake compression done by the navy labs.....a planing hull looses effiency in shallow water....they were also testing the way water flows past a hull and what the effects are of wave compression...wish I could get a hold of that ...it would answer alot of these questions.....hull design is really interesting ......flat bottom ski jet boats are really effecient in fairly shallow water....5 ft or so....but deep vee's loose effeciency in shallow water due to some formula of compression and hull design and a bunch of other stuff.......man..wish I could remember what that film name was........ :argh arrr

Well... seeing as we're talking hydraulics here... there is no compression. There could be a pressure wave that could be reacting against the bottom and effectively trying to lift the boat (and its displacement) rather than push the water aside. That's the primary limiting factor for speed in aircraft carriers. Most think it's about horsepower, but its more about diminishing returns. You get to a point where it's nearly impossible to separate a column of water that's 38 feet tall (hull depth) and 134 feet wide (beam) in half the ships length. The force of the ship going forward is displacing roughly (2,546,000 cubic feet or 20,877,200 TONS) of water. AND... all of this is happening at 40+ MPH. You could triple the horsepower (USS Enterprise) and still not get much faster. You jsut start slipping the prop and standing up the wake.

Soooo... I don't know how much the bottom would affect a boat, other than not allowing the water to escape fast enough and causing more of the boat to be lifted out of the water. If that were the case, I think you'd feel it in the ride as well.

[Seapuppy]

mike...I think your talking apples and oranges with the carrier and the small boats running in 5 ft of water......your also trying to compare displacing millions of tons of water as compared to maybe a few thousand lbs of water being compressed under it........you could claim the same thing about submarines but with the albacore hulls..they allow the water to flow around it rather than under it......hense the incredible speeds that they can get as compared to a surface ship.......(which by the way is classified as in excess of 25 knots).....the lastest carrier was fitted with a bulbous bow to hydrodynamically inhance the speed of the ship and from the testing that was done....the bulbous bow allowed her to do her full speed run a few knots faster than her sister ships..........
.man..I wish I could remember that film's name...I watched it back in 81.....but it was really a cool demonstration of the different hulls and how the react to water...depth..speed....and conditions........ :argh arrr

Well I can tell ya which carrier is the fastest Steve... USS Enterprise. not because of anything fancy... she's just stronger and narrower (and much older)

Also... you might wanna check on your "water compression" idea. Last I understood.. it's virtually impossible to compress a liquid. You're certainly not gonna pull it off with a 15,000 pound boat and a comuple of 155 horse motors. Besides... a boat hull won't compress it anyhow, it'll would just displace it.

[Go Aweigh2452]

Well actually isn't speed limited to hull speed and not the engines? So if Enterprise has a faster hull speed then you are correct...

Yeah... she's a foot narrower, 10,000 tons lighter and the same length at the waterline (133 vice 134). Without getting into the numbers... she was stronger as well. With 8 nuclear reactors instead of Nimitz's Class of 2, she had (has) almost unlimited steam.

It's like a sailboat... long and thin, they're faster than a similarly powered boat of cruiser dimensions.

I don't know a lot about the science of pressure waves slowing down small power boats, but as a Surface Warfare qualified member of the world's largest yacht club.. I can tell you what keeps the speed of aircraft carriers limited to somewhere north of 40 mph.

The shearing action of trying to lift and separate MILLIONs of tons of water is exponentially harder as speed increases. Imagine the horsepower required to push 110,000 tons of boat (at displacement only) at those speeds. You're plowing LOTS of water. It's in excess of 280,000 shaft horsepower (times 4). The way you get that horsepower doesn't matter. Nuclear steam or conventional steam... it's all steam.

The bulbous bow Steve is talking about is for stability and bouyancy heavy seas. Traditional Nimitz Class boats porpoise pretty badly with a heavy bow. This makes sustaining flight ops in rollers tricky. Just to keep it sporting though... they also removed one of the arresting wires (three wires vice 4 now).

That's only fair... 8)

If you're ever curious about trival Naval ships. The NVR (Naval Vessel Registry) is a great resource for pokin around: Click Here (http://www.nvr.navy.mil/)

[Seapuppy]

that and 8 S2G reactor cores pumping out about 35megwatts of pwr each doesn't hurt either :shock: ......
I'm not trying to say what is better or what works.....I just remember seeing this movie on different effects they have on water and how they displace or react to different conditions............it was really a cool movie on hull design...screw design and surface hull and sub hull design....this is also where they found out that when a sub goes thru the water..there is a bulge in the surface that can be measured..it also showed that certain screw designs can cause a hi pitched sound at certain freq.'s....really cool stuff in this film....
...it had alot of info....but it was a long long time ago...... :argh arrr

[Randygh]

Do the props on carriers extend below the hull? Just wondering because if the props extend below the hull, how could the ship be dry docked?

Many years ago, I attended the christening of the Iwo Jima. One of my cousins was a Lt. and he was stationed aboard ship when she was brand new. He was a SCUBA/underwater demolition diver. He said that when the ship would put in at a foreign port, the divers were continually checking the hull for explosive placement by unfriendlies. Does the Navy still have divers checking on ships' hull all the time while in foreign ports?

[Seapuppy]

carriers can be drydocked easily enough.....the dock blocks are monster in of themselves....the area that is cantelevered off the last block is minimal....the props do hang below the keel..... :argh arrr

I've walked the keel of both the USS Constellation and the USS Abraham Lincoln.

When on blocks in the yards, they make you walk the entire length of the keel and identify all the eductors, shafts, pit sword ports and pickups.

Very cool to see.

[Seapuppy]

I haven't been under a carrier..but I have been under destroyers and subs...lotsa subs.....we had to go up into some of the ballast tanks and check cable penetrations at the top...talk about serious dark and smelly.....once my flash light went out while I was at the top.......reeeeeallly freakie to say the least.....
it's an amazing sight to see something that big on dock blocks.....
:argh arrr

[Go Aweigh2452]

When Chuckie V (Carl Vinson, CVN 70) was in dry dock, I took my dad under and while we were walking just about center, we had a small earthquake... I was not worried bout Chuckie falling, I was worried about the sea wall failing... Amazing how big those shafts are and how thin some of the hull is... They had cut a few holes in her bottom and I thought the hull looked awful thin...

Yeppers. But if you think that's thin. Wait until CVN-21. They're talking about changing alloys and thinning the hull even further to reduce weight.

1-5/8" steel is the thickest steel plating you'll find in most places, but we're talking high-carbon alloys here. It's stronger than old battleship steel of twice the thickness. It's a pretty impressive chunk of iron to see on blocks for sure.

[Seapuppy]

Seawolf...the SSN21 class is made of 2.5-3" of HY80 to HY100 steel...HY=Hi yield steel..hardest stuff made today that won't become brittle ......awesome stuff....when welding this stuff...it has to be heated from ambient temps to over 800 degrees in 12 hrs..then held that way for 8...worked...then held to 800 deg. for another 4 then brought back to ambient in another 8hrs......then xray'd for cracks...the care and procedures are amazing.....I was once next to one of the heaters on the hull looking up in the overhead of a sub(name withheld on purpose only cuz I can't remember the boat I was on)...I smelled something burning...looked down and my pant leg was on fire!!!.....stompted it out and went on looking for my cableway.....it's amazing to see the yards tear apart a sub or ship and think to yourself.....no way is this thing ever going back to sea....then ....18 months later there she goes....sailing away......simply amazing.... :argh arrrr

Those heaters are pretty neat. Whenever they cut or weld on any of the structural parts of the ship, they install them both before and after the welding. They have these huge DC converters that pump ungodly amounts of current through the steel itself. They weld these tabs on... and then turn on the juice. Over the course of days, the metal gradually heats to several hundred degrees and burns away the paint and everything. Then the guy will come in and weld some hatch in place. :)

It'll crack if they welded directly to it for any length of time without breaking the temper of the metal first.

[Seapuppy]

we called the process "anealling the metal"........pretty awesome stuff for sure...... :argh arrr

They were doing what's called "normalization" if they were welding it. The expansion properties of hardened steel (high carbon %) will not allow you to weld directly onto the material. You only want to break the temper enough to get the work done, you don't want to actually anneal it and render it soft. You'd have to re-temper the metal if you did that, which would mean a furnace and quenching. Not too practical on the inside of a ship. :)

[Seapuppy]

yeah..that's right.....either way it's hot work.....I didn't envy those welders one bit.. :argh

That's what I did before joining the Navy. I worked in the oilfields in the Gulf of Mexico and as a yard welder. Not a great way to make a living. Hard, Hot and Hazardous work.