Scientific explanation please

[MajBach]

Ok, I consider myself quite the academic as well as inquisitive. However, I have never really done a lot of reading etc. to the science behind what is happening on a boat in the water. correct me if I am wrong, but it's true that a displacement hull has a top speed (calculated with a formula pertaining to it's legnth) that has an upper limit. This limit has something to do with when the bow wave reaches the stern wave. I never really got a firm grasp of that conecept. What happens when you apply a limitless amount of power? Does the boat now become a planing hull? Is it true that longer boats can go faster than shorter boats? <br />I decided to post this because of a documentry I saw on TV last night about submarines. I was surprised to hear that many subs can outrun torpedoes. Is this because they are longer? Does the bow wave/stern wave theory apply when a boat is full submerged?, i.e. have an upper limit? does this mean it's possible for a sub to travel faster fully submerged than when on the surface?<br />Keep the science coming.<br />Majbach

 

[KCook]

Re: Scientific explanation please

I can't give the science behind all this stuff, but here is how it works -<br /><br />

This limit has something to do with when the bow wave reaches the stern wave.

Close, but no quite there. The limit is reached when the distance from the crest of the bow wave to the crest of the stern wave equals the length of the hull. There is a "valley" in the suface between these to wave crests. Which is now filled with our boat.<br /><br />

What happens when you apply a limitless amount of power? Does the boat now become a planing hull?

Depends on several design factors, key among which is whether the hull bottom is shaped to develop lift. The hull with the lifting shape will start to go on plane. A hull with no lift (round bottom) will be more reluctant to plane. But even pontoons can be forced up on plane with enough power.<br /><br />

Is it true that longer boats can go faster than shorter boats?

In displacement mode, yes, the boat with the longer waterline is always faster. On plane, things are not so simple. Longer hull will be more stable, which may allow more power to be applied. However, shorter hull will have less drag. Lots of tradeoffs here.<br /><br />Sorry, don't know much about subs (except to stay the hell out of their way )<br /><br />Kelly Cook

 

[JasonJ]

Re: Scientific explanation please

A displacement hulls top speed is limited to 1.5 times the square root of the waterline length. Any more power applied than what is necessary to achieve the above top speed results in stern squat and wasted fuel and power. I would image that since a submarine if fully submerged, there is equal pressure and hydrodynamic flow around the sub, so its top speed would only be limited by the amount of power it has.

 

[ThomWV]

Re: Scientific explanation please

And the only analogy I could come up with on the other side of the stick, the planing hull, is that its like a guy riding a surf board, only in this case the wave the boat is surfing on is its own bow wake. So the application of greater and greater power give incrased speed, though with constantly reducing efficiency. So more is faster but a little faster takes a lot more and two little fasters equals one hell of a lot more power.<br /><br />Thom

 

[Bondo]

Re: Scientific explanation please

Any more power applied than what is necessary to achieve the above top speed results in stern squat and wasted fuel and power.

While doing seatrials with a 13 ton barge, with Too Much horsepower.... We found that it's True.... <br />It's Absolutely Impossible to Overtake your own Bow Wake....!!! <br />Our Stern didn't Squat, the Bow went Under Water... You Can drive the bow Under the Bow Wake... <br />At rest, we showed 3' of freeboard to the deck on the bow.....<br />At 7/8mph, we were making very good headway,+ still showed 2 1/2 to 3'.......<br />At 9.5mph, Water broke over the bow ramp.....<br />At 11mph, the water was up-over the deck 15/20'.... That's when we backed out of the power.....<br /><br />

I was surprised to hear that many subs can outrun torpedoes. Is this because they are longer?

Nope......<br />That's Power.... Nuclear Horsepower....<br />Once underwater, it's more like Flying...<br />I'm Sure there are Other limits, But Wake timing is no longer 1 of them....

 

[CCrew]

Re: Scientific explanation please

"I was surprised to hear that many subs can outrun torpedoes"<br /><br />Not the MK-50 torpedoes I was working on for Honeywell/Allied! <br /><br />-Roger

 

[airman]

Re: Scientific explanation please

When a boat moves thru the water, it creates a wave. The longer a wave is, the faster it travels. (Tsunamis will travel at several hundred miles/hour, I believe.) When a boat reaches 1.34 x the square root of it's length (speed in knots, length in feet) the wave will be the same length as the boat. Below this speed, the boat simply rides along in the trough. To go faster than this means that the boat has to deal with passing it's own bow wave. A planing hull climbs on top of it. Bondo's barge tries to dive under it. Long narrow hulls can pierce through it. (lots of Navy ships do this, especially the older designs). I think with subs, it's just more power=more speed. At least until we figure out how to crack the underwater sound barrier.

 

[airman]

Re: Scientific explanation please

I think I forgot a step in the explanation. The longer a wave is, the faster it travels. The converse of this is also true... The faster an object moves as it makes a wave, the longer the wavelength will be.

 

[jollymon]

Re: Scientific explanation please

JasonJ & Airman,<br /><br />How does that formula work for a pontoon boat. I have a 25 pontoon, would it be the sqrt of 25, or 50 because of both pontoons?

 

[Knightgang]

Re: Scientific explanation please

As far as subs, yes it is like flying. Also consider that on the surface, the discplaced water has to travel around the boat on one end only, the bottom. Under water, the displaced water travels around the entire boat, creating less drag. And yes, modern subs do travel faster underwater than on the surface.<br /><br />The WWII subs were the opposite. They were actually boats that were modified to dive under water, to their greatest efficiency was on the surface. Also, the WWII subs were only designed to stay underwater for reletively short periods. Now, our subs can stay underwater for as long as they have food.

 

[Bondo]

Re: Scientific explanation please

jollymon,<br />

How does that formula work for a pontoon boat. I have a 25 pontoon, would it be the sqrt of 25, or 50 because of both pontoons?

Pontoons Plane........<br />Displacement rules Don't Apply.....

 

[mercrewser]

Re: Scientific explanation please

I disagree, I would use 25 ft for the equation. Pontoons are definatley non planing, probably semi displacement hulls. yes Ive seen the one with the Merc 496 and open exaust do something like 60mph, but in that boats advertisement it calls it "non planing". I guess some have lifting strakes riveted to help keep the stern out of the water.

 

[mercrewser]

Re: Scientific explanation please

You want that thing to plane?! Go Here!! http://www.foils.org/talaria.htm

 

[airman]

Re: Scientific explanation please

Catamarans use an entirely different formula due (apparently) to the wave ineraction between the hulls. I've never taken the time to learn much about it though. A pontoon boat will climb out of the water in a way that a sailboat won't. I'm with Bondo... the behavior at cruise speed is more like a planing hull than anything else. If you want the formula for the maximum displacement speed of your hull, I believe it's in the Power Squadron manuals. Or, contact any sailing cat maker and ask them how fast their boat will go. Hobie Cats and the real racing stuff doesn't count though. They are back into planing territory. The absolute best way to figure out how fast your boat will go is push that lever beside the steering wheel as far forward as it will go.