floppy phantom cure

-hey guys:--Ive come into possession of an old Phantom dinghy-(the sunfish clone, not the european phantom).There is quite a bit of flex in the bottom-otherwise its in pretty good shape.(like new sail, etc)-does any one know of a cheap , practical method of stiffening up the bottom-I want to maintain the integrity of the hull/deck joint if possible, so would rather not go sawing the deck off to install frames.I'm wondering if it would be possible to somehow laminate some thin wood veneer directly to the fiberglass and then cover the wood with resin--I'm willing to put up with a little extra weight as the boat is pretty light in its present form.Anyone ever heard of this being done?

 

Its glass fibre is it? Top hat stringers moulded over a foam core would be my choice. Proper closed cell PVC foam would be best, failing that a cedar core would be bearable. That's assuming you have access to the inside surface of the skin. If you don't, hmm. Just addingextra layers be it wood or glass is unlikely to do very much, and it means you then have to paint the outside, not rely on the original gel coat.

If its a sealed boat with no way to access the inner skin I'd be tempted to live with it because you could very easily get yourself into far more work than there'll be any benefit from.

 

if you have access to the inside of the hull skin, a quick and dirty way to to make it more stiff would be to take Douglass fir stringers of say 3/4x 1" and glue them to the hull with construction adhesive you buy at a big box store (about $7 a tube, you want the exterior grade adhesive). You can use weights to hold them in place until the the glue cures.

4 total stringers as long as you can fit inside would make a big difference.

 

Top hat stringers on the outside of the bottom, parallel to the keel.
Like gggGuest said, foam would be best. in this case I would even use styrofoam, since the glass would provide all the strength you need.
If you can make a half circle foam stringer that you taper on both ends it would have minimal disruption to water flow. I would try one on each side of the keel and see if that was stiff enough.
Nacra catamarans made these (on the inside of the hull) using a paper tube (looked like a toilet paper roll but longer). Of course when I cut into a hull the paper was soggy and useless, but the glass stringer was perfectly fine.

 

re:Floppy Phantom--thanks to all who responded-I may combine the sugguestions of Petros and Upchurchmr and use the douglas fir stringers tapered on both ends on the Outside of the bottom since I cant gain access to the inside of the hull with,out sawing the deck off.So whatever I do will have to be done from the outside. I'm thinking of running one stringer on either side parallel to the keel about 1/2 way between the keel and the chine.Petros-do you think it would make much difference whether I bonded the 3/4" side to the hull or the 1" side? How about using one thin,narrow plank per side,say 3/8" thick x 1 3/4,bent around the natural fore and aft rocker of the hull? (flatways,of course)

 

Tje depth of the stringer, not the width, is the overwhelming determinant of stiffness. for example: a stringer of 3/4 inch in height will be 8 times as stiff as a stringer three eights inches high that is the same width as the three quarter one. That means that the 3/8 strip would need to be eight times as wide as the 3/4 strip in order to have equivalent stiffness.

 

Yeah, I guess what will effectively be rubbing strips are a good way to stiffen the boat up if the effect on performance is not going to be an issue. Make them strong enough so they can put up with a bunch of abuse on the beach - or make them highly sacrificial...

 

re:floppy Phantom:Hey messabout-thats an interesting tid bit --I'm-curious-was the 8 times stiffness determined by testing?-and does grain direction have anything to do with it?Since 3/8" is one half of 3/4 why wouldnt it only have to be twice the width,instead of 8 times?--(please excuse me if this this is a stupid question)

 

Not a stupid question at all Sawmaster.

Imagine an experimet that you would do with a common 2 x 4. Let it be about 8 feet long and supported at the ends by sawhorses. Now lay it flat and put a heavy weight in the middle. The board will bend down a ways. Now place the board on edge and place the same heavy weight in the middle. The board will bend down a much smaller amount. In fact it will bend eleven times farther the flat way than the on edge way. But you already knew that intuitively.

You have actually asked two questions about strength of a beam. (the stringers on the boat are actually acting as beams) Every material has a certain strength characteristic. Steel is stronger than wood and wood is stronger than taffy. The measure of the relative strengths are discovered by use of tension measurements. That is trying to pull the board apart by pulling on the ends until it reaches yield point. Engineer types call this modulous of elasticity. It is represented in formulae by the letter e.

But as we know, some shapes can withstand loads better than others. We have to assign some sort of value for whatever shape we are examining. For this oversimplified explanation let's just consider rectangular shapes like the 2 x 4. When we do a little arithmetic we arrive at a section characteristic called moment of Inertia. It is represented by the letter I in calculations. O.K. I for a rectangular section is calculated by taking the breadth (B) and the depth (D) and fiddling them in this formula. (B x D^3)/12 = I That is: multiply the width times the depth raised to the third power and divide the product by 12. Now you can see that the process of raising the depth to the third power is the major determinant of the rectangles ability to resist a load. Get out your hand held calculator and play with that simple equation and you will see how come 3/4 is 8 times stiffer than 3/8 when the width is the same.

When you want to anticipate the amount of bend in the 2 x 4 experiment you combine I with E and a few more little things to determine the result. Engineers and architects must be able to do all this in order to select appropriate sizes for floor beams and all that sort of thing. Not to burden the point, just for fun in the case of the homely experiment, we could have anticipated the extent of bending (deflection) with this equation......Let the weight concentrated in the center be called P the Letter L is the length of the unsupported part of the beam.....(PL^3)/ 48EI= deflection at the center of the beam. Notice the evil little exponent 3. That is the one that makes the big difference here. Our 96 inch board is multiplied times itself like this....96 x 96 x 96 = which comes out to be a big number...884736 actually. E is typically a very large number (except for taffy) so in the quotient the deflection number is not so bad.

There I have over done it. Forgive me, I will go to my room now.

 

Sawmaster,

Messabout's explanation is all completely true.

But the simple answer to your question is that the increase in stiffenes is true as shown by theory and proved by experiment (about 2 million times - everybody checks this to start with).
This is a very early lesson you get with a mechanical engineering degree.

 

re:-Floppy Phantom--Thanks,Messabout, for the explaination.As you,(and obviously upchurchmr) can tell by the nature of my question,I do not possess a mechanical engineering degree,however,I think I grasped the general idea.Lets see if I've got this right.The modulous of elasticity,represented by the letter e,describes the resistance a particular matierial has to being pulled apart,while the moment of inertia is a section characteristic that can be used to determine the relative strengths of different shapes.So,to determine the strength of a rectangular shape,we multiply the breadth times the depth,raise to the 3rd power and divide by 12.Since the quotient thus derived is 8 times the number you get when you reduce one of those dimensions by half,(the thickness) that means you would have to increase the width 8 times to get the same strength. So basically,this is a mathmatical proof of an empirical experiment?

 

Yep, you got it Sawmaster.

The whole concept was oversimplified in my unintentionally pedantic reply. But that is the general idea and you can rely on the concept. Just dont start designing bridges yet. Reinforcing the boat, yes, do.

Matter of fact if you put some stringers on the bottom and finish them nicely with hat section glass, the boat just might go to windward a little better. Terminate the stringers before they get to the bow or transom or they might cause the boat to be cranky about tacking. Of course you will run them parallel to the centerline of the boat.

We look forward to hearing about the result of the Phantom resurrection.

 

You may not be an engineer, but you have a start.
Correct.

Of course if you change the material at the top of the stiffener you affect the stiffness also.

Think about the same wood (the taller than wider one). Laminate glass on the top of the wood. The glass is stiffer than the wood. Its effect at the top of the wood is to increase the stiffener stiffness even though it is a thin layer. Graphite at the top will be even stiffer.
The stiffness of the outer layer is so important, that you could probably replace a piece of cedar with balsa and loose little stiffening of the hull. Of course, then you have to think about other issues, like hitting a rock, crushing the balsa. In this case the Cedar would have been better, and oak even better - for the smashing the rock case.

Nothing in the practical world gets designed by only one concern. Sometimes we think so, but we usually pay later for our lack of foresight.

 

I was away from the forum for the last week, sounds like your questions are answered. Glueing two streamed lined stringers on the outside of the hull is a simple soloution.

The easiest way to show how stiffness is affect by depth I think is in beam theory equations, validated by many experimental tests:

deflection equation for simple span, load at center: Delta = W x L^2/48xExI

I is the cross sectional property of the beam, for a rectangular beam I=BxH^3/12

for more complex shapes, like an I-beam or hat section, the I equations are also more complex.

If you want compare a 2x4 wood beam, for deflection with the same load at the same span, one on edge, and one laid flat; all of the values are the same except the I, so it comes down to a ratio of I(edge)/I(flat). you can substitute and get this:

Delta ratio = H^3(edge)/H^3(flat)

this would be 64/8= 8

You would get 8 times less deflection with a 2x4 on edge than flat. This is assuming a full 2" x 4" beam, not the modern net 1.5" x 3.5", but results would be similar.

Also notice that the deflection is a function of a square of the span, so deflection, and stress as well, goes up very quickly as the span increases on a typical clear span.

The equations for a uniformly loaded beam, or a cantilever beam is completely different, but give similar resluts.

None of which is what you have here, but it will give you an idea of the effects of depth.

I would try bonding the 1" face to the hull, with a 3/4" depth, not as stiff, but in practical terms it would give you more bonding surface. If you round the outside corners off it will not have as much effect on steering, though these will have some effect, so it is best to balance the amount of stringer forward and aft of the keel or dagger board location. I would just put one on each side of the center line about one third of the way out to the turn of the bilge to start, if not good enough you an add another outboard of that.

If you really want stiffness, bond a number of rows and than sheet over it with thin plywood, fair it to the hull and fiberglass it. it would add a lot of weight, but it would make the floor very stiff!

 

Floppy Phantom Thanks again--I'll keep the forum updated on the project