MC for Stringers?

Ondarvr's giving you some very good advice James. You have a choice. Do it right or do it cheap.

 

Thank you! When you say scarf them 8:1. Do you mean the way you scarf a keel or the way you scarf planking. I don't know how to term for it. But one is on the flat or the full width of the beam (9"s). One is on the edge the thickness of the beam (1.5"S) using a locking type of scarf (less reliant on the adhesive). I am a woodworker so difficulty of execution is not an issue.
James

 

Thank you very much for that detailed reply! So far from what I have read it is the quality of the workmanship that is most important. Rather than the gains achieved with advanced materials. I like the idea of using the plies of fiber glass/resin for the strength reliability regardless of the core. However, how do I know how may layers of glass and resin will achieve what is required? At this point I am leaning toward using the Ash I have or DF 2x10's with polyester resin. With the the notion that they will eventually rot out. Therefore the lamination will need to be substantial enough to do the job. I do have 2" polyunsaturate foam that has a paper? skin on it. It is used for flat roofs with tar over it. Could that be used as a core? But again. How many layers of glass and resin would be needed?

In the end. I am looking for the least costly but still effective solution. Since most boats are built with polyester. I am thinking that is the least costly yet effective solution. I am formerly a carpenter and aviation structural mechanic and now a woodworker. So I feel I am capable of executing this type of work. I have also replaced the deck in my current boat. So I do have some experience working with glass and resin.

Besides that I have all of my Youtube badges on boat building/restoration! Boat works today, Acorn to Arabella, Sampson boat Co, and Lou at Tips from a shipwright and Bristol boat works! Ha Ha! Did I miss any good ones?
All kidding aside. I appreciate you and the others sharing your knowledge.

James

 

What are you suggesting is the right way?

 

Doesn't really matter, a properly glued long scarf is stronger then the wood. Whatever you like is fine.

 

There are numerous ways to approach the stringer replacement. What I would suggest is that you do some reading first. This book will give you some insights into design and construction. I relied on Mr. Gerr's book and advice that I received from some people on this site when I did an extensive restoration. I did just what you are considering on a larger boat and followed Mr. Gerr's scantling rules. Absolutely no issues.

ELEMENTS OF BOAT STRENGTH https://gerrmarine.com/ELEMENTS_OF_BOAT_STRENGTH.html

Just off the top of my head, I'd say that 3 layers of 17/08 biaxle stitchmat over your cores would be sufficient for your boat. I'd verify that though with Gerr's calculations. You're just putting a sole or floor over the stringers so it's not carrying any large loads as an inboard powered boat might. If you plan on keeping this boat (think about this for awhile) epoxy is the way to go.

You'll use a fair amount of epoxy and it's expensive but you're only doing this once.

The stringers that you see supporting the engine there were constructed of Owens Corning Formular 250, right out of Home Depot. The wood inserts are required as the engine beds needed something strong in compression to bolt to. There are five layers of 1708 biaxle stitchmat on those stringers and I added an additional layer at the engine mounts. You don't need that much as there is no engine. Those stringers were built over 10 years ago and have been 100% trouble free. Many people can't seem to get their heads around the idea that when combined with a sufficient fiberglass covering, the core of a stringer carries none of the loads placed on the assembly.

Good luck with your project.

MIA

 

MIA,
Thank you for your input!
Are you advocating for epoxy for it's flexibility?
For argument sake. Lets imagine your guesstimate of three layers of 1708 is correct. How many gallons of epoxy and thickening agent for the fillets would you think would be required? My four stringers will average 9"tall by 11' long.

Also how does the deck attache to the stringers? Would I need to let in some wood for where chairs get attached?

Another thing that I have been wondering about is access panels. The gas tank sits between the two center stringers. Where can I find info on how to design those removable panels? Especially something like a storage locker or perhaps a fish box.

BTW that workmanship looks great! What is making that stringer with the let in blocks white?

Thanks,
James

 

Resin in a layup by hand is 1:1 by weight.

Calculate the weight of the glass you need.

Calculate the length of all fillets.

Use 1 oz resin for 17" of fillets.

say your glass weighs 18 pounds

18 pounds@100% resin is 18 pounds resin, typically resin is 9 pounds per gallon is two gallons

then say you have 680" of fillets, that is 40 ounces resin for fillets

then you need 2 gallons and 40 ounces resins and a waste factor of 10-15% or 40 ounces. Resins are sold in gallons and halfs, so for my example, you'd buy 3.

If the work is to be done indoors or attached to a dwelling, polyester is a big no-no. It is really smelly stuff.

ps 1708 is 25oz a sqyd

 

This is one of the most confusing parts of a rebuild, and really there is no right or wrong answer.

I suggested using polyester, or you could use VE. MIA says epoxy is the way to go.

For someone doing it the first time they can become paralyzed by this, and second guess themselves constantly about which resin to use.

Here's what you need to consider.

Hundreds of thousands of fiberglass boats were built with polyester resin over many decades. What failed on these boats after 30 years was the wood rotted away, the polyester is still there doing its job.

And after 30 years the wood was rotten due to poor workmanship and neglect. Frequently 10 of those 30 years were spent in the side yard under a tree with no cover, and half full of water.

What needs to be looked at, is the resin only needs to be able to handle the loads that it's going to experience when in use.

History shows us that polyester is more than up to the job of surviving in use as a complete boat for at least 70 years. And any repairs done correctly with polyester have the same life expectancy.

The other part is a design is calculated to meet a certain load based on what it will experience in use. If you upgrade the materials you still build to the load/stress it will experience. The upgraded materials frequently allow you build it lighter for the same strength, you don't need it to be stronger. But to know how to take advantage of the increased strength and lighten up the build, it needs to be engineered correctly.

You have in front of you a design that lasted for a very long time and didn't fail in use, the wood rotted. This tells you that whatever materials were used, they were good enough for its intended use/abuse, upgrading will have little benefit if you are rebuilding it back to what was there to start with.

I compare it to the lug nuts on a car. Let's say the car has 5 lugnuts on each wheel. You need do a complete brake job. In putting it back together do you add a lugnut so now there are 6?

6 will certainly be "stronger" and that means "better", right....

This is sort of what using epoxy is like. It has better physical properties in just about every category, but do they add any value in this type of application, does epoxy, with its added cost and hassle in use, add any benefit when using it in a boat that is otherwise 100% polyester.

That added lugnut is stronger, you can't argue with that, but is there any real world benefit of the added cost and hassle. The wheels didn't fall off with 5 lugnuts, so is 6 actually better.

There is the fact that you can build something lighter out of epoxy compared to polyester. To do this you need to have it carefully engineered and use the correct fibers, methods and other materials, plus post cure the entire part. You need to remember, those physical properties you read on the data sheet are after it being post cured with a specific schedule on temperature and time, not an ambient temperature cure.

To be continued.

 

I recommend poly for that old boat. But only if he is not attached to his living spaces where work to be done.

 

Fallguy,
Thank you! That is just the ticket!

Ondarvr,
That really puts things into perspective. You sound like a very rational thinker.
With that said. I totally understand why MIA and others are advocating for the premium products. There boats are worth it! I would akin it to a boat that is a mistress vs a fishing buddy.
If this boat had a value of over say 15K when completed. I think I would be going with the epoxy no question. I still may. Time to do some math!

Can anyone point me in the right direction regarding designing/making access panels and hatches?

Great forum! I am glad I found it.

James

 

I probably will not do it in my garage. However If I did. Would a window fan in one of the four windows running during install and cure period handle that concern?

 

When using epoxy the biggest mistake, well its not really a mistake, is that the polyester is replaced with epoxy and no other considerations were taken into account for taking advantage of the increased physical properties of the epoxy. The same type and amount of materials and techniques are used, you only replaced the resin type. So all you really did was increase the cost and hassle, you didn't gain a benefit.

Now if you did re-engineer the structure to take advantage of the better properties you would be saving weight.

The problem is in this type of application you may only save the weight of a 6 pack of beer, which could be very important in some situations.

In a complete build that was engineered correctly the weight savings can be substantial, but the cost skyrockets. That's why very few boats are built with epoxy and engineered to take total advantage of it. This market is small, and it's mostly custom, or semi custom builds.

Now, back to your rebuild.

You have a complete blueprint of how to rebuild it in front of you. Use the same materials in the same way and you'll get about the same life expectancy again.

Increasing the attention to detail, which is easily done when the owner is doing it themselves, and the expected lifespan is drastically extended.

All you need to do is look at the laminate currently over the stringers closely. If the only failure was rotten wood, then copy that and it will survive in use. Adding to it doesn't really add any benefit.

Now if the stringers failed in a different way, you may need to make some changes, but you need to determine whether the failure was in the design or poor workmanship.

After saying all that, the resin choice becomes almost irrelevant because if it held up with polyester, it will easily hold up with epoxy. In 30 years there will be no real world difference in how the two methods survive the abuse. It wasn't going to fail with 5 lugnuts, so why is it going to fail with 6.

Now the real world factor comes into play.

Some people can't sleep well at night thinking they didn't use the best methods and products available, it has to be the "best".

So you need to understand what will make you feel the most satisfied with with the final product.

If the satisfaction is in building it with what you believe are the best materials, then use epoxy, if you are more concerned about being out on the water more affordably, then use polyester. The outcome will be the same.

The other part that comes into play is on the first rebuild every person is thinking they're building their forever boat, so no expense and effort should be spared.

But the minute it hits the water the dreaming begins on how next time they will do it differently.

 

Would I be doing that if I where to replace the wood with foam? Or would there need to be additional layers of glass?

 

On many old boats there was little engineering done, they just built something and if it failed they added more chop. If the stringers were set in place and they used a choppergun, then the amount of glass was totally dependent on the the operator's gut feeling of how much to put down. This could vary a great deal.

If they used CSM and Roving, then you can count the layers.

Many times the stringers would rot away and so would the transom. The boat may get used another 10 years before someone notices by having their foot punch through the floor.

If the stringers have more than 3 layers of roving you're close to not needing the wood.

The other part is the floor/deck/sole going to be setting directly on the stringers?

Bonding it to the stringers increases the strength dramatically.