Double Hulled Boats

[journey on]

I just read a article in Latitude 38 about an "unsinkable" catamaran that sank. The assumption was that since the hull had 2 layers of fiberglass with balsa in between, this would provide enough flotation to keep a good part of the boat above water in case the hull was breached. That turned out to be false, as expected. Which leads to the topic of this post.

The other often overlooked feature of the Marinaut is that she really is a double bottom boat -- similar to what you see in large, modern cruises ships such that if the outer lower hull is breached, the sealed inner floor/hull should maintain water tight integrity. This to me is a great safety feature of the Marinaut 215.
Rich

It's that statement about a double bottom hull on the Marinaut that I didn't understand. The topic grew so long and varied, that I'm starting this topic to find out how the Marinaut is built and what C-Nile ment.

I know that the hull of a C-dory 25 is made of 2 ea layers of fiberglass separated by balsa. The picture below shows the external layer, the end-grain balsa and then a final layer of fiberglass is added to provide the inside the hull. I assume the C-dory 22 is made the same.

That is done for structural reasons, not flotation. Just as an I-beam has flanges separated by a web, the separation of the 2 fiberglass layers add strength and stiffness, for the least weight. This structure, on a C-Dory, doesn't qualify as a double hull, but is a normal way of building a light, strong, cored structure.

I assume that a Marinaut hull is built along the same lines, for the same reasons. It may use plastic foam instead of balsa, but the design is the same. The Marinaut builders are knowledgeable people. Remember they designed my boat.

Now my question is: does the Marinaut indeed have a second hull, inside the external hull, with enough separation to provide positive buoyancy if the hull is flooded? If the double hull is just the 2 layers of fiberglass with foam separating it, I don't believe Marinaut would claim positive buoyancy for that.

The photo shows the C-Dory 25 outside hull with the balsa laid inside the hull. In fact, this is a shot of Journey On.

Boris

 

[Sea Wolf]

Boris-

I raced dinghy sailboats for years, some of which were double-bottomed.

The double-bottom was 5-6 inches in thickness, running the entire length of the hull, and enclosing air for buoyancy.

The boats therefore had plenty of flotation, but could be compromised by being holed or penetrated, so that water could replace the air.

Sometimes the boats had styrofoam, air bags, or other solid foam in the space instead of air, but the priority was to keep the boat as light as possible, so these additions were minimal.

IMHO (without calculations), the 1-1/2" to 2" thick balsa layer in a C-Dory or other similar boat is much to small in volume to be able to support the fiberglass hull, let alone the weight of a 500 lb. outboard!

Joe. :teeth :thup

 

[C-Nile]

hello Boris,

You bring up excellent points. First, let me explain what I meant by double bottom. The Marinaut is not truly a double hull, because the inner hull does not extend all the way up the sides. Modern cruise ships don't have double hulls either, but they typically have double bottoms with watertight compartments and bulkheads that extend up to the next floor.

It is undeniable that the bottom of the CD 25 is very well constructed, but it does not have a double bottom -- just a very strong bottom that has a certain amount of flotation afforded through its use of an end grain balsa core. If you were ever to strike a sharp, submerged object below the waterline with sufficient force, it is possible, I suppose, that you could penetrate the hull and that there would be no second bottom hull to prevent the swamping of your boat.

The Marinaut has a thick layer of closed cell foam sandwiched in-between fiberglass layers. This closed cell foam is so firm, that I can't dent it with my fingernail. It is high tech construction to be sure (**see below for photo of the core.) If the Marinaut were to strike a submerged object with sufficient force to penetrate the lower hull, the floor pan/second bottom, provided its integrity remains intact, will maintain a watertight seal to prevent water from further entering the boat. She'll stay afloat.

Like balsa, the Marinaut's closed cell foam also provides flotation. I don't have the technical expertise to state the degree of flotation it provides, but it does help significantly. And seeing that the entire boat with few exceptions is cored with closed-cell foam, I am hopeful that she will float if swamped provided the integrity of the sealed air chamber between the hull and the floor pan remain intact.

If you want to see how the Marinaut is constructed, go to www.marinautboats.com and click on the site's Facebook page. Les Lampman, on Album A0001 - Hull Shell Fabrication photo 3 of 35 explains the Marinaut's double bottom as follows: "The photo shows the one-piece floor pan in place and bonded (with thickened resin) to the hull...With the floor pan bonded in place, there's a water and air tight chamber under the floor pan." Please notice in the photo that the floor pan extends for the entire length of the boat.

Double bottoms are not as good as double hulls, so if the Marinaut were to strike an object above the level of the floor pan, I don't expect her to float upright and level, which is why I was soliciting opinions from people on the forum. With my limited knowledge, I think it is possible she would stay afloat, but with the bow in the air and possibly inverted. Not a pretty picture for any boat. And that would probably occur in C-Dory's as well. The best defense is to ward off complacency. That's why serious boaters use high quality radar, chart plotters and know the rules of the road.

**To see a picture of the core, go to my C-Nile photo album, go to row 3, 4th picture.

 

[C-Nile]

Boris-

IMHO (without calculations), the 1-1/2" to 2" thick balsa layer in a C-Dory or other similar boat is much to small in volume to be able to support the fiberglass hull, let alone the weight of a 500 lb. outboard!

Joe. :teeth :thup

Joe,

This is more of a question then a comment. The entire Marinaut hull, the deck and the cabin are all cored with closed cell foam. I experimented with a cross section of the hull, and noticed that the closed cell foam supported the greater part of the fiberglass, resin and gelcoat that it was sandwiched inbetween. If I had sample piece of the boat at various locations, better testing could be done, so my results are inconclusive. In addition, the watertight chamber on the Marinaut, while slight near the stern, has considerable volume near the bow. There is quite a lot of flotation there, but I can't accurately measure the volume unless I fill up the inner space with water to determine the true volume. So I and others tend to feel, in our opinion, that she would not sink, but it would not be pretty! What do you think?

Rich

Rich

 

[Sea Wolf]

Boris-

IMHO (without calculations), the 1-1/2" to 2" thick balsa layer in a C-Dory or other similar boat is much to small in volume to be able to support the fiberglass hull, let alone the weight of a 500 lb. outboard!

Joe. :teeth :thup

Joe,

This is more of a question then a comment. The entire Marinaut hull, the deck and the cabin are all cored with closed cell foam. I experimented with a cross section of the hull, and noticed that the closed cell foam supported the greater part of the fiberglass, resin and gelcoat that it was sandwiched inbetween. If I had sample piece of the boat at various locations, better testing could be done, so my results are inconclusive. In addition, the watertight chamber on the Marinaut, while slight near the stern, has considerable volume near the bow. There is quite a lot of flotation there, but I can't accurately measure the volume unless I fill up the inner space with water to determine the true volume. So I and others tend to feel, in our opinion, that she would not sink, but it would not be pretty! What do you think?

Rich

Rich

Rich-

A good engineer or physical scientist can fairly accurately calculate the boyancy provided, but to do so would have to have exact dimensions of the enclosed areas to calculate the volumes, density figures for any enclosed foam or other flotation, and weights of the hull, motors, and other equipment. The water displacement method would be easier, but not really practical.

I have to go to Sacramento this morning, but will return later today.

As a beginning, one could start from the other end of the equation and take the weight of the entire boat, motors, and equipment, and calculate how much water would have to be displaced in order to support it afloat.

Of course, you'd have to add a reasonable percentage above just an equal amount of flotation to have a boat that usefully floats enough to support itself, several persons, and upright enough to be towed, preferably, if you want a rescuable boat, which is a big step beyond simple minimal flotation.

Joe. :teeth :thup

 

[thataway]

There is rarely an unsinkable boat. Look at the Titanic, the recent cruise liner which hit the rocks off Italy...and lots more. The Balsa (and probably foam) is only 3/4 or maybe one inch thick in these hulls. Yes any foam/balsa will attenuate any impact, but it will not prevent breach of a hull, or capsize. I was on a 55 foot balsa strip planked boat (3" thick balsa), and we hit the corner of a container coming off a 8 to 10 foot wave at 8 knots--it cracked the outer layer of glass (about 1/2" thick mat and roving), water got into the balsa core, and deflected/delaminated the inner layers (about 3/8" thick). We were darn lucky that the hull was not breached. Even boats with water tight bulkheads often sink.

It sounds like a nice safety feature, and is an excellent method of construction in the Marinaut, but I doubt if it is actually non sinkable in under the correct conditions. These are wonderfully built boats, but I would be reluctant to give it properties which would prevent sinking. On the other hand--how many C Dories have sunk in the last 35 years? ---I know of the one which was rolled in the surf--are there any others?

 

[C-Nile]

There is rarely an unsinkable boat. Look at the Titanic, the recent cruise liner which hit the rocks off Italy...and lots more.

It sounds like a nice safety feature, and is an excellent method of construction in the Marinaut, but I doubt if it is actually non sinkable in under the correct conditions. These are wonderfully built boats, but I would be reluctant to give it properties which would prevent sinking. On the other hand--how many C Dories have sunk in the last 35 years? ---I know of the one which was rolled in the surf--are there any others?

Yes -- the Costa Concordia had a double bottom, but the impact point was above the inner hull/bottom and it compromised the integrity of four watertight compartments. She was very well built, but even the safest boats can't overcome pilot complacency.

While the Marinaut's construction methods provides its owners with a certain measure of safety, I hope people don't think I am trying to give the false impression that she is an unsinkable boat. And I agree with you totally about C-Dorys. I would trust my life in any C-Dory due to its superior construction as compared to that of the typical production boat. My wife and I ventured into seas with our CD 16 where other boats of similar size would fear to follow. And we survived some pretty amateurish mistakes in the beginning that we may not have recovered from in lesser boats -- like flying into the air off of a 2 foot boat wake at over 20 knots and diving nose first into the oncoming wave, or falling off the back of a 5 foot standing wave in Plum Gut in a situation that could have led to a broach. That's why I would trust my life and the lives of my passengers with the entire C-Dory family to include their cousin, the Marinaut.

Rich

 

[journey on]

Well, C-Nile, et al, my question has been answered. The Marinaut hull is a cored single structural element just as the C-Dory has.

Both the Marinaut and C-Dory are built the same way, with a layer of lightweight material, keeping 2 layers of fiberglass apart to give structural rigidity. Good, strong light weight construction. Every time I cut another hole in the interior structure of Journey On, I find the same cored layup. Good construction, lightweight boats, both of them. Used in airplanes, spacecraft, anywhere where weight counts.

I wouldn't count on any vessel flotation qualities, though. The floatation offered is the difference in specific gravities between the composite structure and water. Between the fiberglass and the foam/wood, that should be nil. I believe that's the conclusion reached in the Latitude 38 article, if you want an actual test.

BTW, balsa coring and foam coring both offer unique advantages. There are good reasons to select one over the other, and neither is "better." So lets not go there.

Boris

 

[BrentB]

From Wikipedia

A double bottom is a ship hull design and construction method where the bottom of the ship has two complete layers of watertight hull surface: one outer layer forming the normal hull of the ship, and a second inner hull which is somewhat higher in the ship, perhaps a few feet, which forms a redundant barrier to seawater in case the outer hull is damaged and leaks.

The space in between the two bottoms is often used as storage tanks for fuel or ballast water, though fuel storage in the double bottom is not allowed for newbuilt ships since 2007, due to MARPOL_73/78.

Double bottoms are significantly safer than single bottoms. In case of grounding or other underwater damage, most of the time the damage is limited to flooding the bottom compartment, and the main occupied areas of the ship remain intact. For this reason, double bottoms have been required in all passenger ships for decades as part of the Safety Of Life At Sea or SOLAS Convention.

An even more extensive protection is available as a double hull, where the second hull layer extends up the sides of the ship as well as in the bottom.

A double bottom also conveniently forms a stiff and strong girder or beam structure with the two hull plating layers as upper and lower plates for a composite beam. This greatly strengthens the hull in secondary hull bending and strength, and to some degree in primary hull bending and strength.

see http://en.wikipedia.org/wiki/Double_bottom for the full article


m2cw is C-Dory and Marinaut designs does not met the above definition.

I could incorrect which can occur and glad we are not talking the investment double bottom term

 

[Sunbeam]

The best defense is to ward off complacency. That's why serious boaters use high quality radar, chart plotters and know the rules of the road.

I can tell you are a prudent mariner, and I agree that knowing the rules of the road is very important (I wish more did!); but I don't see how any of the three listed items ward off complacency. Also, while I loved my radar (don't have it on the 22, but have had it on other boats), and I think I will enjoy my (very first) chartplotter, I don't think they are necessary to be a "serious boater. Actually, if anything, I see more and more people out there with their noses glued to the chartplotter like it was a video game, and practically no "Mark I eyeball" in use at all! (Not saying this would be C-Brats or that a chartpotter can't be a very, very useful tool.) (And did I mention I looove radar?)

Well, C-Nile, et al, my question has been answered. The Marinaut hull is a cored single structural element just as the C-Dory has.

Both the Marinaut and C-Dory are built the same way, with a layer of lightweight material, keeping 2 layers of fiberglass apart to give structural rigidity.

I believe there is an additional feature on the Marinaut, which Rich was describing as a "double." That is that there is a fairly extensive fiberglass liner (forming the sole and cabinets of the boat) that is bonded into the boat. This is what the cabinetry, etc. is attached to, and there is air between that and the cored hull. It's in addition to the two skins and the foam core that make up the basic hull.

Sunbeam

 

[NORO LIM]

When you stand in the cockpit of a traditional CD, you stand on the top of the hull bottom, i.e., you stand on the top fiberglass layer of the balsa cored "sandwich" that makes up the hull bottom. In my CC, when you stand in the cockpit, you stand on a separate fiberglass sole that is raised above the sandwich (by a couple inches near the Alaska bulkhead, and by less at the transom). This separate sole is sealed (more or less) to the sides, transom, and bulkhead. It is my understanding (and Les' pictures and description seem to bear this out) that the Marinaut has a separate, sealed, sole the entire length of the boat. Sounds like a double bottom to me. What standards it meets and how much flotation it provides may be separate questions.

I see Sunbeam beat me to the punch by a minute or two.

 

[starcrafttom]

boy I would like to hear from the builder on this one.

 

[C-Nile]

Thank you, Sunbeam and Bill. A famous line in Hamlet is "a rose by any other name should smell as sweet." Regardless of what one may call it, it is an additional safety measure that one would wish one had in the unlikely event that one were ever to encounter a hull breach. It is possible that it could make the difference between life and death, but that is also true for many safety devices on a boat such as life preservers, life rafts, flares, signaling devices, radio, GPS, chart plotters, radar, emergency transponders, and etc. With each layer of protection we apply along with a measure of respect for what we are doing, we help to increase our chances for survivability should the unexpected occur. What I have always respected about the C-Brats community was our attention to safety and our desire for well-designed and safe boats.

And Tom, it would be nice for Les to chime in on this, albeit he must be crazy busy right now building more Marinauts!

Rich

 

[C-Nile]

Also, while I loved my radar (don't have it on the 22, but have had it on other boats), and I think I will enjoy my (very first) chartplotter, I don't think they are necessary to be a "serious boater. Actually, if anything, I see more and more people out there with their noses glued to the chartplotter like it was a video game, and practically no "Mark I eyeball" in use at all! (Not saying this would be C-Brats or that a chartpotter can't be a very, very useful tool.) (And did I mention I looove radar?)

Sunbeam

Sunbeam,
This is actually a subject for another thread. While you are right that a serious boater does not need a chart plotter, to be serious about boating, one should have up-to-date NOAA charts and know how to use them. Honestly, this is where I fall short. People like me are too dependent upon technology, and we should take the time to know both methods. I'm sure many of you have taken Power Squadron courses and some may even have Captain's licenses. My hat is off to you, because you define what it means to be a serious boater. With respect to radar, I use it all the time, and it overlays my chartplotter. However, I spend most of my time looking out the windshield ahead of me and to the sides for other traffic and submerged objects. However, it is not always easy to see who is coming from behind you. The group would not believe how many ignorant boaters there are driving boats in the Northeast, who will come up from directly behind your speeding boat at high speed only to pass your starboard side by less then ten feet of space. These are often people with more money then what they know what to do with it, and who invariably own very large boats, operating them only on weekends. And in other cases, because they have radar, they think they can travel along at 30 knots in dense fog. Using radar at all times gives me the opportunity to see these people and get out of their way.

Along these lines, in Connecticut, everyone is supposed to possess a safe boating certificate. Unfortunately, people who owned boats prior to the law going in effect were grandfathered in. What good is that? I think boating should have mandatory requirements for recertification. People don't know the rules of the road. If I blow my horn to indicate if I am passing starboard to starboard, it ilicits a form of road rage, because the other boater thinks I am angry with them!

Rich

 

[NORO LIM]

boy I would like to hear from the builder on this one.

If Les doesn't have the time, here's what he has to say per the link C-Nile posted above:

Shown [above] is the "floor" (sole) of the boat. It's a one-piece molding that goes all
the way from the bow of the boat to the stern (in the photo it's inside the hull which is
covered with plastic sheeting). That way we know there aren't going to be any issues with
seams that may allow water intrusion.

In the photo the depression (at the bottom of the photo) is at the stern for the bilge pump
collection area (sump). The large flat area ahead of that is the cockpit deck, you can see
that slopes down toward the stern (to keep water shedding that direction) and that it's flat
(which makes it more comfortable to stand on and better for deck chairs, coolers, and
such). At the forward end of the cockpit deck is the low sill that the lower edge of the
rear bulkhead abuts. Just ahead of that are the flat risers for the interior cabinetry
(dinette area to port and galley/helm area to starboard). The taller risers ahead of that
are for the v-berth area.

 

[C-Nile]

Well, C-Nile, et al, my question has been answered. The Marinaut hull is a cored single structural element just as the C-Dory has.

Both the Marinaut and C-Dory are built the same way, with a layer of lightweight material, keeping 2 layers of fiberglass apart to give structural rigidity. Good, strong light weight construction. Every time I cut another hole in the interior structure of Journey On, I find the same cored layup. Good construction, lightweight boats, both of them. Used in airplanes, spacecraft, anywhere where weight counts.

I wouldn't count on any vessel flotation qualities, though. The floatation offered is the difference in specific gravities between the composite structure and water. Between the fiberglass and the foam/wood, that should be nil. I believe that's the conclusion reached in the Latitude 38 article, if you want an actual test.

BTW, balsa coring and foam coring both offer unique advantages. There are good reasons to select one over the other, and neither is "better." So lets not go there.

Boris

Boris,

This is not about comparing one boat to the other. I was on the Marinaut Forum and stating about a specific feature peculiar to the Marinaut, which so happens to be not present in the C-Dory, and which in this particular case is advantageous. And that feature is a sealed in sole that is separated from the hull by a gap in space above the hull floor. On your boat, you are standing on your hull, on my boat, I'm standing on the sole that is sitting above the hull.

I'm sure there are many features on your boat which are advantageous over my Marinaut. Case in point: you have a head, your beam is wider, and your greater length may give you a more pleasant ride in rough seas. In fact, I don't know if you can really compare a CD 25 to a Marinaut, because the 25 is in a different class of boat. My wife and I did not want a CD 25, because we were more interested in fuel economy and to keep our overall boating costs down. And it was also our preference to have an open berth like on a CD Tomcat, which is also a beautiful boat. I've met other C-Dory owners who love their berths closed in, and even though I'm sure they know they could change it to have a more open set-up, they would choose not to do so.

One of the problems, I think, and I can't blame you, is that for the greater part of the year, much of what you have heard about the Marinaut 215 has come from me. As more Marinaut owners come forward, it will be better for me to back away and let them speak, because I don't want to become a polarizing influence. My entire objective from the beginning has been to educate the entire C-Brats community on the Marinaut, just as I have been educated on the C-Dory line. Do you know that in the Northeast, there are no cost effective alternatives to the C-Dory line or Marinaut? I traveled across country to see the Marinaut in person. I greatly admire people out in the pacific Northwest for crafting American made products and for their hard work ethic, and that is equally true for the manufactures of both the C-Dory's and Marinauts. I feel that both manufacturers have a lot of integrity in craftsmanship, and would be very unhappy if more jobs were shipped overseas. I truly want both manufacturers to succeed. The Marinaut is a cousin of the C-Dory's, and deserves equal consideration for her unique features and capabilities, much the same as anyone would choose one C-Dory model over the other. All boats are wonderfully constructed and have unique capabilities.

Thanks for reading by long-winded response,

Rich

 

[BrentB]

How is the sole attached to the hull?

 

[C-Nile]

How is the sole attached to the hull?

Brent,

A picture is worth a thousand words. Please click on the below link

http://www.c-brats.com/modules.php?set_ ... _album.php

 

[thataway]

Brent,
There are a number of pilot house boats built in the Eastern part of the US--none are semi dorys. But there are some very good and very useful boats built in the North as well as the Souther East Coast.

 

[C-Nile]

Brent,
There are a number of pilot house boats built in the Eastern part of the US--none are semi dorys. But there are some very good and very useful boats built in the North as well as the Souther East Coast.

Dr. Bob,

I think you were directing that toward me and agree with you. However, name me one trailerable, cruising, pilothouse boat made on the East Coast that 1) costs under $80,000 fully equipped, 2) gets 4+ mpg, 3) sleeps two comfortably with most amenities one is looking for in a cruiser like stove, heater, dinette, ample storage space, running water and a spacious berth, 4) has an outstanding quality of construction, and 5) has a draft under one foot? My wife and I went to boat show after boat show -- the largest in the Northeast to include New York, Boston, Providence and Hartford, and we concluded that the only two boats that would meet that criteria were the C-Dory 22 Cruiser and the Marinaut 215 -- both made in the PNW.

Rich