The Boating Forum - Self-Righting boat?
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02-28-2006, 12:07 AM
I know there are a few "unsinkable" hulls out there. The ones I have heard of are Whaler, McKee craft, and Carolina Skiff's. I'd love to know about any others as well.
But as the title suggests, are there any unsinkable boats that are self-righting? seems like that would be a great selling feature.
Yes M'am, if the boat ever keels over in a squal, not only will it never sink but if will roll completely around. You just get backin and bail like mad.
ok kidding aside isthere a boat like that being sold or is there a reason boats aren't made like that?
02-28-2006, 12:45 AM
ok tell me if this is a stupid question or not someone. I'm new to boating but I was thinking that if a builder like McKee craft but just enuf cement in a small channel at the bottom of the v-hull part, then added all the foam, it could be weighted just enuf to right itself, and still be unsinkable. would the engine still work? If it would, seems to me that if a rogue wave hit and rolled her, it would be more desirable to climb back in her, give her throttle and let the scuppers and bilge get her back on top again. to me that would be more preferable than floating around in a raft. Now if this is just plain unfeasable then fill me in. I am new to boating but I love to dream what if...
02-28-2006, 01:04 AM
Not at all a stupid question. Some boats are self righting. Generally, you need two things for that though:
- a sealed (or sealable) hull and cabin
- a very low center of gravity so as to provide a strong righting moment
Beyond that, stuff like thick tempered windows in reinforced frames that won't break or pop out, and having all stuff inside the cabin secured to prevent injury, etc are important. If powered, you want a mechanism to seal the engine breathing from water when it rolls.
A classic example would be an offshore monohull sailboat with a deep, heavy ballasted keel, low superstructure and a sealable cabin.
Another example would be the 47 foot Coast Guard Motor Lifeboat, which is built down with engines, fuel and ballast mounted very low, sealed air tanks up high to keep the upper portion light, and a sealed cabin.
Short of actual self righting (recovery from any angle), many displacement boats if properly built can roll well in excess of 100 degrees and still recover.
Things like outboard engines, big high superstructure including flybridge, and hard chines all reduce critical roll angle.
02-28-2006, 01:05 AM
I can't imagine a power boat design that could possibly be self righting. To be self righting you need a very low center of gravity and a very narrow beam. Ordinarily it takes a heavy ballasted keel with little or no superstructure to get the COG that low. The form stability that power boat designers seek gives you great initial stability but kills your final stability. There are formulae for this called "righting moment". A Coast Guard Cutter with its narrow beam and low COG is the closest boat I can imagine....... In fact those training boats in Oregon that they take out in those humungous waves in the Willamette area probably have positive righting moments. (They are self righting).
Great question. But then the issue of bulkheads and bracing also come into play. Powerboats just don't have the bracing to stand the loads that a self righting boat would have to sustain. I would bet those self righting Coast Guard training boats have amazing bulkheads and bracing. Large open rooms in a powerboat are incompatible with this kind of strength.
Look at the North Sea Trawlers. They are steel. They have almost no superstructure. You see very little above the waterline. They have very deep drafts with engines set low in the bilge. They are quite narrow. Waves can freely flow over the top of them. Wind has nothing to grab onto. They float like an ice cube.
Today's powerboats are the exact opposite. Look at the typical powerboat today. They could only achieve true stability by turning turtle. Once upside down they'd have pretty decent stability. The low engines and fuel tanks are the only saving grace for most large powerboats' COG.
02-28-2006, 01:17 AM
Here is a plot that indicates qualitative behavior. It's a plot of righting moment (essentially the force tending to return the boat to an upright, zero roll angle) as a function of roll angle. On any given boat, the width and height of this curve differ. The better it is, the further out to the right it actually crosses zero and goes negative (indicating a flipping force, instead of a righting force), and to a lesser extent, taller is better.
As a thought experiment, can you think of what the curve must look like for a self righting boat? :)
02-28-2006, 01:21 AM
ok then, I'm puzzled. If a say, 19ft whaler is unsinkable, what happen when the plugs are opened and water is allowed to fill the hull? Lets say a light center console type similar to a carolina skiff 1980v since that is all I've ridden in. Does it still sit a couple of inches above water when filled. I wasn't really picturing a larger boat with a substantial cockpit in my mind with any sort of tower myself when I was letting my mind wander, but something lighter and less heavy to begin with. Does this still hold water (pun intended)
02-28-2006, 01:38 AM
fishhawk123 - 2/28/2006 1:21 AM
ok then, I'm puzzled. If a say, 19ft whaler is unsinkable, what happen when the plugs are opened and water is allowed to fill the hull?
The water never fills the hull. There is enough captive bouyancy in the foam sandwich between the inner and outer whaler hull to float the boat, the crew, and the engine. Take a foam cooler, open the drain hole, set it in the water, and wait for it to fill up - it doesn't. It's unsinkable, because even if you fill the boat with water, the hull is less dense than water, and so it floats.
Another way of looking at that is that if you push the cooler (or Whaler) under the water, the resulting displaced water (when you push the cooler under, you basically lift an equal volume of water) weighs more than the cooler/Whaler itself, so that is unstable, and the minute you let go, it'll pop back to the surface.
Now a Whaler can still capsize in heavy seas though, in which case the fact that it's floating upside down will make it a lot less useful as a means of keeping yourself out of the drink.
Even a concrete boat can float, as long as you don't knock a hole in it, and it doesn't take a wave over the side. That's because the effective density includes all the air inside the hull. As long as that's in there, you're golden. But when it get's replaced with water, it's Davey Jones' locker for it.
The preceding paragraph also hints at how foam flotation works - it traps air inside. Therefore, this air can't be replaced by water, and the effective density has a much lower ceiling to it (if that makes sense).
02-28-2006, 01:42 AM
A Whaler or Carolina Skiff will be just as hard to right as they were to flip in the first place. They have tons of initial form stability because they carry a wide beam. They are quite low to the water with low freeboards so they will never just lay over on their side and stay there. They will either right themselves or turn turtle (upside down). But once turned turtle that same form stability works against the righting.
View the righting moment like a teeter totter where the waterline is the fulcrum. You need distinctly more weight below the waterline to self right (you want the fat guy on the teeter totter below the waterline). In the case of a turtle you'd better have a narrow boat so the low inverted form stability will allow the boat to right.
In essence you need either a sailboat with a heavy keel or a north sea trawler with most of its mass below the waterline to be self righting. (Or a coast guard boat)
It is a great question but it is entirely unrealistic for a powerboat. The best you can hope for is positive upright flotation. If a boat with positive upright flotation has a tower... watch out. You may end up turned turtle if you get swamped. Less superstructure is better for stability.
Maximize weight as far below the waterline as possible
These are the components that create a positive righting moment. For more on this read Beebe's book (I forget the title).
02-28-2006, 01:45 AM
I don't know whether I just had a brain-fart or a brain storm but what about an emergency air bag deployment system for boats. Not to cushion from a blow but to give emergency floatation to a boat in an emergency situation. puncture resistant bladder installed strategically around the gunnels is it called? tied to some sort of sensor system with a manual overide. Poof! instant unsinkability and positive bouyancy. Better than seeing your yacht under 100' of water? Can I patent that? or has it already been done? 02-27-0511:44pm
I must be getting really sleepy now...sigh
02-28-2006, 01:50 AM
Thanks Barge, that explanation was much easier on my tired brain than that equation you put up...great answer. So the ral crux is the turtle situation.
02-28-2006, 03:03 AM
A friend graduating from the Naval Academy this semester told me that in some of the Navy's ships the critical angle isn't necessarily just a function of hull/structure design which is something I hadn't really thought about. For example a given ship might pop back from 70 deg normally, but at 69 deg the ships engine mounts are now taking weight in a manner they were never designed for, will fail, the engines will fall to that side of the boat and around she goes...not so much an issue on the 19ft whaler example, but just something to consider. A smaller example is we had a john boat with a 15hp that had the safety feature disabled that kicked her out of gear if the motor went full lock one way or the other. We hit a wave at WOT, knocked the tiller handle out of my hand and put me and a friend into a WOT circle with the boat taking on water over the side that was down. Before I knew what direction was up, the 12gallon tank in the floor overcame friction and slid down. That shifting of 12 gallons of gas was enough to send water pouring over the edge and we shortly thereafter sank to the oar locks with only the top of the motor showing. It was in 5 feet of water in the summer, but I still didn't want to drag a boat and motor home out of the mud. So its not only keeping COG low but also making sure that everything on the boat can withstand more force than it will ever see, at ANY given angle. If you build one fishhawk, build it in a 25ft version with 24 deg of deadrise, outriggers and SCUBA tank racks and I'll be your first customer. :)
02-28-2006, 07:55 AM
If you want a self-righting boat, buy one of these:
02-28-2006, 08:42 AM
Or one of these ...
02-28-2006, 09:22 AM
44 ft self-righting motor lifeboat
02-28-2006, 09:42 AM
"I see" said the blind man. So what about my air bag idea for an emergency. A salvage me now-or salvage me later call.
02-28-2006, 10:08 AM
WATCH THE VIDEOS
of the motor lifeboat training school
02-28-2006, 10:12 AM
02-28-2006, 10:28 AM
Research and Development-Not determinable
Marketing-Also not determinable
Market (customer) Acceptance-Not guaranteed
Great idea, really is, but don't quit your day job...
02-28-2006, 01:02 PM
I'm not very well-versed with this subject, but that won't stop me from offering my opinion anyway. :) I think the "safeboats" the Coast Guard and other agencies use are self-righting. These are outboard powered boats, and not cutters. Here is a link to what I'm talking about:
02-28-2006, 01:12 PM
Cool boats! Now make on with stealth so I can sneak up on those fish!
But I like my martini's shaken - not stirred moneypenny.
02-28-2006, 01:16 PM
I think they're probably priced a bit steep for recreational fishing, but they look cool for sure.
02-28-2006, 01:44 PM
Those SafeBoats do look cool - they are kind of like the Aluminum Chambered Boats, but with a foam collar instead of air filled aluminum. Unfortunately, they are not self-righting though. A self-righting boat will turn itself back upright, from *any* angle. One of SafeBoat's videos shows some guys climbing on the bottom after it is flipped upside down.
Looks like someone beat you to it...I hope this works cuz this is something to see!
02-28-2006, 02:03 PM
I See that you can flip them. Must be pretty hard though. One of the neatest concepts I've seen. Good old American ingenuity.
02-28-2006, 06:24 PM
There have been air bag systems for offshore boats for some time. Never really caught on because of cost, can be punctured, and take a fair amount of space. Steve Dashew (Setsail.com) designed a 83 foot power boat which has positive righting moment at all most all angles--but it looks like a patrol boat.
One of the problems with the Unsinkable boats, is that filled with water, they cannot get back on a plane to drain the water out. Also after a capsize, the engines often are not startable.
One of my offshore sailboats went over to about 90 degrees and came back up in hurricane force winds--in retrospect, that is probably what broke an engine mount--this is not something which you want to experience often.
02-28-2006, 09:23 PM
Self righting would be nice, but it's out of the reach of most. I would settle for something that has a large enough range of positive stability to survive most knockdowns. Outside of certain sailboats or specialty boats like the CG one mentioned above, that leaves you with full displacement trawlers as the best option.
02-28-2006, 09:36 PM
Well it was just a thought and a question and I thank y'all for enlightening me. all the vids are cool and at least the coast guard has boats like that to help people like us over on the "how far out on a single engine in a small boat thread" lol
02-28-2006, 09:37 PM
Like this. This is the system shown on the back of the RIB photo I showed earlier.
02-28-2006, 09:38 PM
A lot of the offshore sailboats I've seen carry a good bit of ballast. Some of them are probably self-righting. Is this a problem for sailboats in particular because they are vulnerable to being blown over by a hard gust of wind catching their sails? I would think powerboats moving at a sensible rate of speed would be less susceptible to flipping in the wind.
02-28-2006, 09:42 PM
coastie motor lifeboat was the original but being phased out too slow 6-53 DD was the only engine to stay running when it rolls also ask a coastie who has rolled all the way around if they would do it again
02-28-2006, 09:56 PM
trophy1703 - 2/28/2006 9:38 PM
I would think powerboats moving at a sensible rate of speed would be less susceptible to flipping in the wind.
I think powerboats are normally capsized (if so) by waves.