98 Pathfinder Stringer Job

[Chisox]

I have contacted the OP. The rest is up to him...

[Cracker Larry]

There are thousands of stringers and stringer grids out there that are of different levels, have notches, reliefs, step-downs, you name it.

Yes there are, and every one of them is built incorrectly. Just because someone builds something doesn't make it right. Sometimes I think these manufacturers don't have a mechanical engineer to share between them. Anytime a stringer has a step it will hinge at that location when it flexes, and it will fail at the step. Thousands have failed in this manner. Notches and reliefs are even worse than steps. A stringer's job is to stiffen the hull and a curved stringer can't do that. It's other job is to support the deck and I don't see those stringers doing that either.

I'll quote David Pascoe here, he has more letters behind his name than I do...

Mistakes involving stringer design and installation are legion, about which a whole book could be written. And yet the principles for creating an effective stringer system are very simple and easy to achieve. Surely there are not many designers or builders who do not understand this. Or are there? Problems usually arise as a result of other design and marketing considerations. Typical examples are when a designer wants to create a small boat with 6'6" headroom or wants to install unusually large engines. The machinery spaces, which are not subject to appearance and marketing considerations, are usually sacrificed.

In order to get the 6'6" head room or make high profile engines or other equipment fit, the principles of proper stringer design are often sacrificed. In other words, the principles of sound hull design get sacrificed for marketing considerations and the surveyor needs to be constantly aware of this fact. Its the primary reason why, in this day when all is known how to build a good boat, bad boats are still being built. Give the customer what he wants, even if the product is going to fall apart.

The principles of good stringer design are simple. They must run uninterrupted from one end of the hull to the other. They must be of adequate height to width ratio, i.e., structural modulus, to resist impact loading on the hull skin, be of sufficient strength to carry the engine load, be stabilized against lateral movement if high profile, and be securely attached to the hull so that they don't break loose. The profile, or top of the stringer, should run in a straight line. If there are any changes in the profile, then special design reinforcements must be added.

Reference: http://www.yachtsurvey.com/HullFailPart1.htm

[bly]

There are thousands of stringers and stringer grids out there that are of different levels, have notches, reliefs, step-downs, you name it.

Yes there are, and every one of them is built incorrectly. Just because someone builds something doesn't make it right. Sometimes I think these manufacturers don't have a mechanical engineer to share between them. Anytime a stringer has a step it will hinge at that location when it flexes, and it will fail at the step. Thousands have failed in this manner. Notches and reliefs are even worse than steps. A stringer's job is to stiffen the hull and a curved stringer can't do that. It's other job is to support the deck and I don't see those stringers doing that either.

I'll quote David Pascoe here, he has more letters behind his name than I do...

Mistakes involving stringer design and installation are legion, about which a whole book could be written. And yet the principles for creating an effective stringer system are very simple and easy to achieve. Surely there are not many designers or builders who do not understand this. Or are there? Problems usually arise as a result of other design and marketing considerations. Typical examples are when a designer wants to create a small boat with 6'6" headroom or wants to install unusually large engines. The machinery spaces, which are not subject to appearance and marketing considerations, are usually sacrificed.

In order to get the 6'6" head room or make high profile engines or other equipment fit, the principles of proper stringer design are often sacrificed. In other words, the principles of sound hull design get sacrificed for marketing considerations and the surveyor needs to be constantly aware of this fact. Its the primary reason why, in this day when all is known how to build a good boat, bad boats are still being built. Give the customer what he wants, even if the product is going to fall apart.

The principles of good stringer design are simple. They must run uninterrupted from one end of the hull to the other. They must be of adequate height to width ratio, i.e., structural modulus, to resist impact loading on the hull skin, be of sufficient strength to carry the engine load, be stabilized against lateral movement if high profile, and be securely attached to the hull so that they don't break loose. The profile, or top of the stringer, should run in a straight line. If there are any changes in the profile, then special design reinforcements must be added.

Reference: http://www.yachtsurvey.com/HullFailPart1.htm

Thank you cracker larry . I am so impressed by your response to my beliefs in hull and stringer design that I wanted to quote you again and david pascoe. Now I believe my questions were justified and I do not understand why the OP as osprey 250 calls him did not answer his original post?Oh I get it now OP, I am a little slow. since osprey says he made contact with him and he doesnot want to enter a two way forum on Reel Boating. Maybe he was only trying to get free advertising and is affraid to answer our specific questions. Maybe there are some questions about his product that he would rather not answer?

[Cracker Larry]

I'm not trying to put down anyone, their company, or their product. When I first read this thread, I didn't realize it was trying to be an infomercial. I think that pre-fab stringer system is probably a decent product that does well in certain applications, but I do question the methodolgy of the repair shown. Stringers are not a 1 size fits all, each have to be built exactly to the application. Flexing it to conform to the hull bottom and creating a corresponding curve on top, and then it being too short to provide deck support, well, it's probably not what the engineer who designed the product intended. That application would have been better made with epoxy encapsulated, laminated BS1088 meranti plywood, IMO. It would be much stronger and cost less.

I think when Proline uses it in their boats, they run it straight and it bridges the entire area between hull and deck. A stringer is no more than a bridge, an upside down bridge, but a bridge just the same. That company doesn't have many hull failures

I'm not here to argue with anyone, just pointing out best accepted practices for the repair. It's not just my opinion, it's also the opinion of the real engineers in the industry. Too bad that more manufacturers don't employ one, or at least read their books

[OReely]

The OP advertises here and added a thread for people to read. He's running a business and may only occasionally read this forum. A couple of days is not that long to wait, after all this is for entertainment.

The design of the stringers and bulkheads in a small boat is not that big a deal. They perform a couple of functions. 1)stiffen hull panels. 2)resist torsional loads. 3) support additional structure (like the deck or liner).

Speaking of supporting the deck, who ever said the stringers had to support the deck. The deck could be self supporting. This is a small boat. The cockpit is probably not more than a few feet wide. You could easily build a cored deck that would be of sufficient stiffness that it didn't require additional support. There could also be things running under the deck (rigging, storage, fuel tank, etc) that need the clearance.

I've read some of Pascoe's stuff and believe a lot of it is opinion mixed with fact. For instance, this notion that the stringer must go fore and aft in a singular, uninterupted plane is not absolute or realistic. It is often necessary to change the stringer vertically and/or horizontally in a given design. There are multiple methods to accomplish this in a safe and structurally sound manner. What is important to remember is that loads need to be transfered, it's not a "only one way to do it" type of project.

Remember, there's more than one way to skin a cat. Prisma beams can do it as can plywood and glass or Coosa and glass.

Edited December 3, 2009 by OReely

[slewit]

I have contacted the OP. The rest is up to him...

Hi Guys

Sorry I was attending The Defense Manufacturing Conference in Orlando and had no time to get on line..

Some really great comments and questions that I do owe you all answers on.. Sorry for the delay ! I will respond in a couple of postings as I can today..

From Bly

What Kind of trailer accident? just loading a boat or a highway accident?

Did the original stringers separate from the hull with out damaging the hull?

I emailed Chris to get more specifics but have not heard back, here is what I do know. The boat came off the trailer, I dont know the failure mode he had but with Prisma our first failure mode is normally side wall bucking. What happens is the glass on the side of stringer buckles away from the foam, the compressive strength of the foam then kicks in and the glass buckles. This does not normally crack the glass as it is only a thin layer and can flex quite a bit. The result is reduced damage to the hull and deck and a noncatastrophic failure mode. With wood stringers and many grid stringers the stiffness is so high that the first mode of failure is either hull cracking and/or the stringer ripping out of the boat. Yes some builders do design boats like cars, designing for non-catastrophic failure could mean the difference between getting home or not and the difference between major or minor damage. Pathfinder is doing a good job in this regard.

Prisma Beam under extreme flexural loading. Note the side wall buckling and large defection.

Was the floatation foam part of the problem? was the floatation foam reinstalled before deck went down?

Chris did not say but I expect it was not as the added foam in nonstructural. The stringers take the load. Chris has not re-decked yet. I hope to hear back from him on the email I sent.

What supports the deck? the stringers do not look like they go high enough to support the deck? The glass laminate was put on top of the stringer foam? was vinylester resin used for a better secondary bonding?

The Deck/Cockpit does land on the outboard stringers. The center stiffener is to strengthen the tunnel and does not contact the deck. It is to stiffen the tunnel/hull area. Prisma Preforms are supplied with the glass already attached. Some builders do add additional tabbing and cap glass and/or glass overlays when loading conditions demand this, typically larger boats. Chris used Epoxy Resin for this repair. I highly recommend Epoxy for repair as this provides the best bonding. The next best is Vinylester Resin.

What kind of stringers were there before the accident? same type?

Yes, this boat was built with PRISMA. So the replacements are exactly the same.

I KNOW I OWE YOU ALL MORE ANSWERS.. AND I PROMISE TO GET THESE TO YOU WITHIN THE NEXT DAY OR SO.. SORRY BUT WITH THE ECONOMY THE WAY IT IS I HAVE TO DO THE WORK OF MANY.. ITS 7 DAYS A WEEK NOW.. SO THANK YOU FOR YOUR UNDERSTANDING.

If any of you need immediate technical support you can contact me directly at 321 252 4566.

Thank you all for participating in the discussion.

SCOTT

[slewit]

Stringers tops should not curve, the tops of a stringer should always form a straight line, no curves, no steps, no change of level. The bottom needs to curve of course, to conform with the hull, but the top edge should be a straight run. And the tops of all 3 of them should be on the same level plane. Looks impressive but it wouldn't pass a survey.

And they sure seem too low to support the sole when it goes back in.

Hi Larry

As others have pointed out there really are many stringer designs. Your observation is very common on small boats. The Cockpit sole lands on the stringers. This is not the only way. We have done several race boats and stunt boats that have a suspended cockpit. I am working a Navy project now where we are suspending the cockpit for a combatant craft boat. In these applications we are trying to reduce the shock load that the operator and crew sees due to wave slamming. When the hull and deck are bonded together with the stringer you tend to get very high shock loads as the load path is from the hull bottom though the stringers into the cockpit and into your body. Think of steel wheels, no shocks or springs and a hard seat on a car, not a smooth ride !

By suspending the cockpit we now allow the hull, side and deck structure to act as a shock absorber. Shock loads have to travel though the hull bottom around the side of the boat around the gunnel down the tub deck to the cockpit. The result is large drop in G loads. You do not often find this on small boats or many recreational boats as the construction is simple and effective and we want the operator to feel the shock and SLOW DOWN.. i.e. you break before the boat does. This is not an option in Special Ops, Stunt boats or race boats, different mission !

Another example is in large boats. Once we get over 30-40 ft we no longer land the cockpit on stringers as we have cabins engine rooms etc.. We are using the internal space.

Here is a picture of a Military Rib.. Note the low section frames. This also has a suspended cockpit and has no core materials.

single skin rib.bmp

Thank you for the comment

Scott

[slewit]

The outboard stringers appear to be on the same level. The keel stringer has an obvious downward curve, which suggests to me that the dip is necessary due to the underside of the deck. I couldn't venture to say why it is formed in such a fashion as I have no idea what the underside of his deck looks like in that area.

There are thousands of stringers and stringer grids out there that are of different levels, have notches, reliefs, step-downs, you name it. If, for instance, I have a slightly raised helm area, and all my stringers are at the same level, where is the bonding surface/support for the helm area?

I'm not trying to defend the OP. What I am trying to do is point out the differences in deck designs/features, and how the stringer system has to be altered to accomodate said features.

what about secondary bonding? Resin type? Glass type? How many layers? Does this stringer system have any inherent strength or does it get all its stifness and rigid form from the glass and resin chosen? It seems to have large radiuses where it meets hull? Why? I still dont understand why you would not want the stringer to go from the hull to the deck if possible? That way the hull stringer is supporting the deck and the deck stringer is supporting the hull.Those outboard stringers in the picture sure look to be running down hill also? AND NOT HIGH ENOUGH TO SUPPORT THE DECK? Why start a thread with no feed back? Especially for the betterment of Reel Boating forum

Hi Bly

what about secondary bonding? Resin type? Glass type? How many layers?

Yes, repairs are 100% secondary bonding (at least on the first layer). Thus I highly recommend the use of Epoxy or Vinyl ester resin. I do not recommend standard polyester resin for repairs as it has poor secondary bonding.

Primary (Chemical )bonding occurs Polyester, VE and epoxy resin when laminating wet on wet. Once Epoxy cures anything you add to it is secondary bonding (mechanical not chemical). With Polyester or VE resin you have primary bonding for a few days or a week at most. This is why production boat builders can skin/bulk/brace in a weeks time and not have to worry about secondary bonding. The chemical bonding sites go away with time. Most builders will rough the surface of a boat that has sat in production over the weekend to help improve secondary bonding.

The glass on this preform is 24 oz/sqyd of oriented knit fabric. The fabric also has our special T-Mat backing. This is a high tensity thermoplastic fiber that helps impart toughness and damage tolerance. It is compatable with Polyester, VE and Epoxy.

I believe the single layer of glass we provide on the preform was all Chris used. That is what is done in the new construction of this boat.

Does this stringer system have any inherent strength or does it get all its stiffness and rigid form from the glass and resin chosen?

The strength and stiffness the stringer imparts on the hull is from the shape of the stringer and the materials. The primary purpose is to support the hull. In many small boats it also doubles to support the deck. We want the glass to carry the load, not the foam. The foams job is to keep the glass in position and not buckle thus allowing the glass to do its job. We can alter the strength of a given stringer shape by changing the fabric orientations and/or changing the fiber orientations. An ideal stringer has +/- 45 oriented glass on the side wall to support shear loads and glass running along the top of the stringer to support bending loads.

In the case of the boat where the stringer is bonded to the cockpit, the +/- 45 fabric is often all we need. Here the cockpit sole acts as a giant top to the stringer, so having 0 degree oriented fabric on the top of the stringer will not do too much. This requires that the deck be fully bonded to the stringers and bulkheads.

The resin selection has a minor impact on strength and stiffness. However, you will get better fatigue and durability with Epoxy and VE resin. Also dont forget we are bonding to old laminate and need the adhesive quality of the resin so we dont debond.

It seems to have large radiuses where it meets hull?

Actually this is a really good feature. Think of the glass fibers as the highways of stress. They are the stiff material in the composite and will attract the loads. These stresses are transmitted nicely when the fibers are straight. So when we have a stringer to hull or bulkhead to hull connection we want a radius to reduce the stress concentration and give us a nice "off ramp" for the stresses. Sharp corners are undesirable in composites as the fiber is not effective at transferring the load. So instead of the fiber taking the load the resin takes the load, the resin is not strong thus sharp corners tend to lead to sites for cracking.

I still dont understand why you would not want the stringer to go from the hull to the deck if possible? That way the hull stringer is supporting the deck and the deck stringer is supporting the hull.Those outboard stringers in the picture sure look to be running down hill also? AND NOT HIGH ENOUGH TO SUPPORT THE DECK?

I think my previous posting on this thread addresses this. The cockpit is supported by the outboard stringers and is suspended over the tunnel stiffener. If it is not clear let me know and I will explain better.

Why start a thread with no feed back? Especially for the betterment of Reel Boating forum

Yes, this is my fault.. It has taken me 2 days to get back on. I wish my workload was such that I could do this all day but keeping a business floating in the boating industry in these tough times is more then a full time job. Just yesterday we lost two great companies and customers. Century boats and Caravel boats, both closed for good, some really good boat builders out of work.

I THANK YOU FOR YOUR PATIENCE AND WILL TRY AND RESPOND WITHIN A DAY OR TWO OF POSTS. I DO HAVE HEAVY TRAVEL SCHEDULES AT TIMES AND WILL DO WHAT I CAN WHEN ON THE ROAD.

I HOPE THAT MY LIMITED EXPERIENCE AND KNOWLEDGE OF COMPOSITES CAN HELP THOSE OF YOU THAT WANT TO LEARN AND UNDERSTAND HOW COMPOSITES WORK. IT IS A REALLY AMAZING FIELD TO WORK IN AN GOES WAY BEYOND BOATS. I LEARN AND GAIN EXPERIENCE EVERY DAY I WORK IN THIS INDUSTRY AND AM ALWAYS AMAZED AT WHAT A GOOD EXPERIENCED BOAT BUILDER CAN TECH ME.

THANK YOU BLY

SCOTT

[Cracker Larry]

Thank you for your comments Scott. As I clearly said, I think the product has some good applications. I also think the Pathfinder project was maybe not one of them.

I highly recommend Epoxy for repair as this provides the best bonding. The next best is Vinylester Resin.

Me too. I only use epoxy for boats. Polyester is best left to liesure suits. It's not even waterproof.

We have done several race boats and stunt boats that have a suspended cockpit

Yes but that Pathfinder is not one of them. That cockpit deck is not designed to be suspended. Those race and stunt boats were designed from the start with that in mind.

By suspending the cockpit we now allow the hull, side and deck structure to act as a shock absorber. Shock loads have to travel though the hull bottom around the side of the boat around the gunnel down the tub deck to the cockpit. The result is large drop in G loads.

No, the G loading remains the same, W weight X V velocity does not change, it is just distributed differently so the operator doesn't feel the shock. The boat must be designed from the ground up for this to work and the forces have to be distributed and absorbed elsewhere, often resulting in failure of the hull to deck joint, or the chine to hull section. I've seen a few hundred of those cracked. I've also done some work with Navy boats, special ops and minesweepers. They even suspend the engines in some of them

Here is a picture of a Military Rib.. Note the low section frames. This also has a suspended cockpit and has no core materials.

The RIB is also not built or designed like the PF in question. Again they were designed from the ground up for a suspended cockpit. It might be just an optical illusion with the photo, but it does not appear that the outer stringers reach deck level on the Pathfinder. If they do, sorry, my mistake. The deck will still need additional support and the tunnel stiffener still should not be curved.

Another example is in large boats. Once we get over 30-40 ft we no longer land the cockpit on stringers

That's not a large boat, but I agree. The engines usually mount to the stringers on a boat that size and the cockpit is raised. But the stringers still make a straight run across the top. Not curved. The PF is not built that way so your point is irrelevant

I'm only commenting on the PF in the photo, not military boats, not stunt boats, not special ops, not RIBS, not inboards. Just that PF application.

The design of the stringers and bulkheads in a small boat is not that big a deal

OK, right. If you believe that, there are many manufacturers who can satisfy you. I'm done, build it however you want. And hope you don't get me when your insurance company sends out a surveyor.

[bly]

Thank you scott for coming back and answering the many questions I had.I am going to re read and learn from your answers.Thanks again.