Technical questions boating forces

[Bubba1235]

Gravity/weight, lift, thrust and drag define flight physics. Can the same be said of a boat/hull? I am assuming somewhere someone has done a lot of math with these when it comes to the speed of a boat/hull. Does anyone have any idea where a fella might lay hands on that sort of information? Anyone here ever used these sort of calulations to design a hull? Would love to pick yer brains for a bit.


I am trying to figure out when drag is no longer the major factor in speed limitation. IE: If a hull used a hypothetical coating with zero drag in the water, what forces would limit the boats speed?

 

[QC]

Re: Technical questions boating forces

If a hull used a hypothetical coating with zero drag in the water

Assuming zero drag, then uh, aerodynamics come into play. Especially if hydrodynamic drag is eliminated. And then there's the hangy down parts, right? Propeller, skeg, rudder, etc.

 

[UncleWillie]

Re: Technical questions boating forces

A short Goggle search for "Marine Hydrodynamics" makes John Nicholas Newman appear to be the Supreme Oracle on the subject.

 

[rallyart]

Re: Technical questions boating forces

There is a nice forum on boat design that can be a source of info.

 

[5150abf]

Re: Technical questions boating forces

I read 3 posts , now my head hurts, I am glad someone understands that stuff but way to much math for me.

 

[Frank Acampora]

Re: Technical questions boating forces

Drag is ALWAYS a limiting factor. It is present even when (for example) a hydroplane goes airborne.

Your hypothetical question seems to assume a planing type hull since on displacement hulls there is a "hull speed" which limits boat speed no matter how much horsepower you put into it. However, hull speed does increase with a larger hull so bigger boats can go faster than smaller ones.

With planing hulls, hull design is very important: Those designed to entrain air under the bottom have less drag and typically are faster than other types.

Do not forget the physics of (again for example) the drive train. Even an engine running at no load will eventually, assuming it will not destroy itself, reach a point where it can turn no faster. And, just as with airplanes, there is parasitic drag on the prop plus limitations as to how fast it can turn.

Your question is so complex that it can only be answered fully by a person who has devoted his life to this subject and has a post doctorate degree.

 

[southkogs]

Re: Technical questions boating forces

Drag is ALWAYS a limiting factor. It is present even when (for example) a hydroplane goes airborne.

Your hypothetical question seems to assume a planing type hull since on displacement hulls there is a "hull speed" which limits boat speed no matter how much horsepower you put into it. However, hull speed does increase with a larger hull so bigger boats can go faster than smaller ones.

With planing hulls, hull design is very important: Those designed to entrain air under the bottom have less drag and typically are faster than other types.

Do not forget the physics of (again for example) the drive train. Even an engine running at no load will eventually, assuming it will not destroy itself, reach a point where it can turn no faster. And, just as with airplanes, there is parasitic drag on the prop plus limitations as to how fast it can turn.

Your question is so complex that it can only be answered fully by a person who has devoted his life to this subject and has a post doctorate degree.

... ummmmmm ... 42.

Good answer Frank! I actually did a unique hull design for an engineering project back in High School. (Disclaimer: I did not go on to be an engineer ... but did win a couple of awards from it) I was fascinated by the amount of factors at play on a boat moving through the water. Hence all the different answers that have popped up over the years: Whaleback/Pigboat freighters on the great lakes, hydrofoils, hydroplanes ... and the list goes on.

I would doubt there is a complete collection of information in one single source. Too broad a scope.

 

[beardeddone]

Re: Technical questions boating forces

How is this for a bigger boat example

For my little pea brain to comprehend it is quite remarkable how an aircraft carrier of the Nimitz class with over 32ft of draft claims over 30 knots and in some cases up and over 50 knots, to me that is flat moving for that large of a ship, draft and weight, some of our boats won't go that fast, but design of such a structure has to be mind boggling to say the least..

30 Knot = 34.5233536 Miles Per Hour
50 Knot = 57.5389227 Miles Per Hour

 

[Slide]

Re: Technical questions boating forces

As an aerospace engineering grad, I'm pretty qualified to answer your question, and the answer is that you just cannot discount hydrodynamic drag. Any drag force is a function of the fluid density, which for water is literally 1000x that of air.

This also only considers form drag. Any lifting body creates induced drag through the displacement of the fluid - vortices are produced at the trailing edge of the lifting body which act against the direction of thrust. Again, this is a function of fluid density, and it is also a function of lift - a heavier boat produces more induced drag. Even for a theoretical hull with zero drag coefficient (i.e. no form drag) there will still be significant induced drag. So, to answer your question directly - induced drag is the limiting factor.

Most boats do not move fast enough for aerodynamic forces to be a limiting factor, because the density of water is so many magnitudes greater than that of air. Form drag from air is minuscule compared to all of the forces exerted by the water.

 

[H20Rat]

Re: Technical questions boating forces

Your hypothetical question seems to assume a planing type hull since on displacement hulls there is a "hull speed" which limits boat speed no matter how much horsepower you put into it. However, hull speed does increase with a larger hull so bigger boats can go faster than smaller ones.

That is probably the biggest misconception about hull speed... It is NOT a speed limit by any means, and there are boats that are perfectly happy running at double their hull speed. Lets say hull speed on a non-planing canoe is 10 mph. Strap a 50 hp merc outboard on the back, and I can assure you that canoe is going to go faster than 10 mph! (yes, canoes are bad examples as they will plane with enough power, but its for sake of discussion. There is no artificial limit, if you add power, you will go faster, just not necessarily in a linear manner. (double hp != double speed) Torpedos are another example, the can easily run 50+ knots, obviously well above their hull speed based on length, and no planing going on!

Wiki has a pretty good article about it, but the very short version is the hull speed is where the bow and stern waves interact with each other. That can require a bump in power to get past, but once you do, the actual drag can sometimes go down slightly even. The reason it is commonly believed as a speed limit goes way back to early sail boats. You couldn't really easily add power, but people figured out that long and narrow sail boats went faster than shorter ones, even if they were heavier. In other words, there must be something limiting the speed if the shorter boat is slower, even with identical sail area.


http://en.wikipedia.org/wiki/Hull_speed

 

[V153]

Re: Technical questions boating forces

You forgot poontoon boats. Not really a displacement, or planing hull. Yet with enough HP you can push one to well over 100 mph.

As SC correctly said there is no 'speed limit' on anything. You just have to build it strong enough & supply enough thrust.

 

[UncleWillie]

Re: Technical questions boating forces

Yes, I made the assumption displacing the water limits speed but for what I have in mind I'm not concerned with it,,, for now.

Resurrecting the Ekranoplane, are we?

 

[tomhath]

Re: Technical questions boating forces

For my little pea brain to comprehend it is quite remarkable how an aircraft carrier of the Nimitz class with over 32ft of draft claims over 30 knots and in some cases up and over 50 knots, to me that is flat moving for that large of a ship

I don't know what the hull speed is on a 1000 ft long vessel, but it's pretty high. Plus a nuclear powered carrier has an incomprehensible amount of HP available. The limiting factor for them is propeller cavitation. I would bet a nuke carrier could go way over 50 knots if it had to.

Nuclear submarines are probably faster yet because they don't really have a bow wave so the hull speed is entirely different (which is why torpedoes are so fast too).

Planing hulls obviously live by yet another set of rules...

 

[beardeddone]

Re: Technical questions boating forces

I don't know what the hull speed is on a 1000 ft long vessel, but it's pretty high. Plus a nuclear powered carrier has an incomprehensible amount of HP available. The limiting factor for them is propeller cavitation. I would bet a nuke carrier could go way over 50 knots if it had to.

Nuclear submarines are probably faster yet because they don't really have a bow wave so the hull speed is entirely different (which is why torpedoes are so fast too).

Planing hulls obviously live by yet another set of rules...

Saw a thing on the tube just this last week about aircraft carriers and what they showed was about 5 years old and not nuclear, but it showed the engine compartment and they had two massive jet engines in there and claimed it could go 30 knots.I can't imagine how they tied them into turning the props

Most all info I can find on aircraft carriers the top speed is classified and if the nukes brag about 50 knots and the top is classified I can't imagine what they could really do if needed to, but that is a heck of a lot of metal moving way to fast in the water for an object of that magnitude. Military specs have there own rules.

Would there even be any propeller cavitation 30-50ft under water or where ever the props are under water and if nuke powered and if turbines are used with steam, would there really be a limit, no wonder it's classified..

As USS Enterprise Retired, A Question: Why So Many Aircraft Carriers? | United Liberty | Free Market - Individual Liberty - Limited Government

 

[Frank Acampora]

Re: Technical questions boating forces

I have been "schooled" on hull speed. ---Just goes to show that the old maxim "A little knowledge is a dangerous thing" is true. LOL

 

[UncleWillie]

Re: Technical questions boating forces

Hull Speed on a 1000ft vessel would be 42 to 47 knots.

The added water pressure on the prop at 30 ft under water will help a little; but nowhere like a sub 300 ft down.

Nuclear reactors have power limits just like any other device.
50 knots would not be an unreasonable number.
The government would like you to think it could do 80 as much as they like people thinking area 51 has flying saucers.
It distracts you from thinking about the real secrets.

 

[frantically relaxing]

Re: Technical questions boating forces

There are some vids online titled "outboard overkill", in one of those vids there's one boat doing like 112 mph (guy is holding a GPS), and from the camera angle in the next boat shooting the video it appears NONE of the hull is in the water, ONLY the outboard's lower unit. If that was the case, the only factors limiting speed seem to be air drag, water drag on the lower unit, HP and prop slip... ?

 

[V153]

Re: Technical questions boating forces

the only factors limiting speed seem to be air drag, water drag on the lower unit, HP and prop slip... ?

Something like that, gravity too ... I hope.

This was was mine w/90hp: Sorry no new good pics with the 140. Stay tuned.

And pray for no wind gusts ...?

 

[ssobol]

Re: Technical questions boating forces

Get almost any hull going fast enough and it will turn into a planing hull, even an aircraft carrier. However, it may be impossible to dissipate that much energy into the water due to mechanical limitations of the propulsion system.

Almost anything can plane if it is going fast enough. When you skip rocks you are causing the rock to act as a planing hull.

Most modern fast torpedoes can achieve very high speeds by either disrupting the water ahead of them with gas generators (bubbling the water) or by generating a cavitation bubble around the torpedo (the torpedo is then not moving though water). If you look at modern torpedoes you will notice that the front of the torpedo is flat.

Some Russian torpedoes can go 200 knots while fully submerged. VA-111 Shkval - Wikipedia, the free encyclopedia

Even though the boat hull may be out of the water the drag from the lower unit or rudder can be quite high. Some years ago some guy was trying to set the speed record in a boat. He was aiming for a very high speed (can't remember the number). Anyway when he found that he could not attain the speed he was trying for, he decided to cut 1/2 an inch off the depth of his rudder. This reduced the drag of the boat by 8 tons.

 

[CHARGER2]

Re: Technical questions boating forces

Something like that, gravity too ... I hope.

Gravity doesn't directly affect speed of a boat. Drag will affect speed, and more gravity will increase drag. A zero gravity boat given forward thrust will plane instantly and negate any water drag.

Also, not every hull can plane given enough speed. Most objects create lift, either positive or negative. Any object creating negative lift (like a true car spoiler) will never plane at any speed. Often lift is dependant upon object orientation. A traditional planing deep-V hull will not plane when upside down, as the lift will be downward.