| Subject: Boat size and safety... what size is the safest |
Author:
JD
|
[
Next Thread |
Previous Thread |
Next Message |
Previous Message
]
Date Posted: 9:06pm
Ok.......... This is the trick question:
What's the rougheest water conditions you could with some sanity take a boat into and what size and type of boat would handle it best?
Well big boats certainly seem safer. They look safer, but I'm not so sure...
First off how big is big. PB guys may buy big boats by private standards but our boats even the largest ones are just dingies by shipping standards.....
.....ad to be honest I'm not to sure bigger is more seaworthy.
Ever see the perfect Storm? After the Andrea Gale sinks Marky Mark swims to the surface and bobs around. The AG with all her size in reality couldn't handle those seas, no big boat could, but Marky could float there in those big waves no problem. Go figure...
After seeing that I thought a lot about boat design. There's a lot of people out there saying small boats don't belong in the ocean. Is that really the case?
Then again how rough is rough. 8ft seas, 10 ft seas....
What would you say is the roughest most unpredictable water conditions imaginable?
I'd say the most unstable water you can be in is river white water.
In a river not only do you have the up and down action of waves but you've got massive current and rocks. Much rougher then the ocean.
You might get bigger waves in the sea but big waves still have the same shape as a small wave. Out at sea even a fifty foot wave still has the classic pyramid shape.
Rafts work in white water but they aren't really boats. There more like a floatation device.
I'd say the best true boat for whitewater is a sit inside kayak. In fact whitewater kayaks might just be the most seaworthy boat design ever.
I bet the Eskimo's would agree
I imagine you could take a white water yak and put it in the worst offshore conditions and it would do just fine. Even in giant seas it would float up and over anything it faced. When faced with a fifty ft wave it would just rise up and over the top. Open ocean waves don't break like beach breakers but even if they did a yak would float up through it.
In conditions that would swamp or roll a large boat a yak would bob around like a cork. It's like that for a lot of small designs.
Essentually a small sealed design that can't take on water it will outperform a larger vessel in rough seas wqhen it comes to just staying afloat.
Why do think life rafts exist? Because simply put they out float big heavy vessels in bad conditions.
No doubt big rigs are more comfortable, and in moderate conditions safe as can be, but get it rough enough all that mass and weight of a larger vessel turns against you.
What sinks boats in rough weather is their mass, the wieght conflicting against the energy of the waves. That's why waves break over big boats in big seas, they just can't get out of the way of the water fast enough. Go back to that fifty foot wave. It will roll right over the heavy large craft, or cause it to surf or pitch pole.
In contrast you couldn't a wave to break over a ten foot inflatable, it's just to lite to go under. It's a cork, a bobber. You could maybe take a wave over the stern but it would still float, and ironicly the more water you get inside the harder it is to flip. Since the floatation is at the sides and is not lost when the interior fills the added weight just acts like stability ballast.
Weird but true.
Contrast that to bigger a standard larger displacement hull. The mass holds it down so big waves break over it and the more water it gets inside the less stablility it has. Eventually it gets so heavy that it can't rise over even the smallest waves. At this point it swamps and sinks.
So what's really more seaworthy. A 35ft sportfisher or a ten foot inflatable? Oddly in really rough conditions the smaller Craft, the only difficulty is staying in the thing .
This is JMO but I definately wanted to here what other guys thought about it.
Like they say it's not the size of the craft but the motion of the ocean.
follow up:
I'd say that more risks are taken by the 20 to 30 crowd then anybody. The reason is a false sense of security.
Like you say it's wave scale and period.
It's all about math.
What I would suggest is there's two ways to handle extremely rough seas successfully. They are at opposite ends of the scale. Either you have to be so big you dwarf the conditions, or so small you float over them.
Say for instance you are in 15 foot seas with the waves 75 feet apart crest to crest. How would differen't craft handle it?
Take a ULCC tanker roughly 1500 feet long and 550,000 tons. Because of it's size to scale of waves it's uneffected. We're talking 100 times long as the waves are high. The wave period is such that at any one time it's got 15 or 16 swells under it's hull. shouldn't effect it. It doesn't have to respond it just stays on coarse.
In contrast your average large sportfisher: a thirty footer that weighed something like 10 tons would be twice as long as the waves are high, and half as long as the space between them.
In order to progress through these seas the boat has to react to and rise over each incoming wave, it can't plow over them like the tanker, but the question is can it really react quick enough.
What kills you here is the period. To get a boat to change direction takes energy. Think about how much. How quick does the boat really respond when you hit the throttles.
Mass, needs energy to change direction. In the case of boats in waves that energy comes from floatation. The displacement supplies the lift to get the boat over the incoming waves. The problem here is the wave period is too tight for the large mass of the 30 ftr to react with diplacement alone.
As you come off one wave you get downward inertia which has to then be stopped by the next wave. The displacement has not only to float the boat over the wave but change it's direction at the bottom of the trough. That's why waves break over boats. They can't change direction fast enough.
If your headed into the wave the shape of the bow and foreward inertia helps lift the boat up. That's why boats head into bad seas.
If your running downswell you have to keep enough speed to outrun the swell and prevent the swamp. Either way the engine supplies extra energy to get the boat over the wave. Like I said it takes energy to change a boats direction.
A thirty footer might seem really seaworthy but when you do the math, it's not really in that great of a position to handle the seas. It's scale is wrong. It's size is and weight play against it in the inertia game.
What about a ten foot inflatable?
It's relative length to wave height is not great .666 to be exact, so the waves are effectively bigger to the boat.
It's length to wave period is much better though, the space between the waves is roughly seven times as long as the boat. This means it effectively has more time to react to the oncoming waves.
What really makes a difference though is it's mass. At under 200 pounds it takes very little energy to get it to react to the waves.
It's more "tender", bounces around more but it floats over the wave rather then pounds through it. It's also by design inherintly more stable then a top heavy sportfisher.
So if you can't afford a super tanker get an inflatable...LOL
So once again are bigger boats more seaworthy?
What I'm saying here is that anglers can't afford to buy boats big enough to handle any seas thrown at us, and what we consider large is not that big and actually may be less seaworty then most think.
Maybe bigger is not better? Smaller light weight boats may pound around rock and roll, but they are inherently seaworthy.
I'm just trying to get some people to think here, but maybe this is that time your teacher told you about in highschool where algebra would save your life
follow up 2
Dorie: though tender the have a great deal of reserve stability, get the weight low like your talking about and they are almost impossible to flip over. Guys have crossed the Atlantic in dories.
Here's some thing to consider. What about wave scale to boat scale.
It's in the math..... wouldn't there be an advantage to a smaller craft in tighter wave pattern because it's relative scale and mass.
Say the wave period puts the peeks 75 ft apart: for a 30 ft boat that's two boat lengths, for a twenty five foot boat that's three boat lengths, for a sixteen footer roughly five, and a ten foot inflatable seven in a half.
In scale the smaller has almost three times the effective wave period of the larger one. So in simple math terms it has three times as long to react to a wave of the larger boat.
Once again the on the other hand is mass. The larger the boat the more mass weight it has, and in reality the short sea problem is a mass problem. When the waves come fast there's less time for the boat to react to get out of the way. The larger boat with it's heavy mass can't react to the incoiming waves quick enough so either smashes through them or gets swamped by them. In contrast the smaller craft with it's lighterweight has less mass and more effective time, so reacts better and floats over the same stuff that would break on the bigger boat.
So which is safer small or big??
I find it interesting that guys say big boats are safer, but when push comes to shove abondon ther big boats for small tender craft that float like a cork.
Tight lines, Jim
[
Next Thread |
Previous Thread |
Next Message |
Previous Message
]
| |
Feedback: 