Boat flex and stay tensioning

[Ixneigh]

While out sailing today I had a chance to ponder the rigging on my M. It seems as though the head stay can't ever be tensioned correctly because the boat is too flexible for that type of stress. I realize it's not that type of boat and I'm fine with that. However since I do plan a fairly extensive mod involving glassing over the hull to deck joint as well as installing an external sheer clamp ( for side impact resistance like getting rafted between two heavier boats) I wanted to ask the performance guys on the board if it might be worth the extra effort to bond half inch foam to the upper part of the hulls sides and glass everything over with say two layers of biax and vinyl ester resin. Is the extra wieght worth the hull stiffness gained?
Thanks
Ix

[NiceAft]

How much extra weight would this create? Also, I thought the sheer is the rise in the deck at the bow and the stern? If that is correct, I am confused about an external sheer clamp strengthening a side impact? I am not an engineer, but this caught my attention.

Ray

[mastreb]

If you're going for extra hull strength, you'll need to calculate the exact thickness of foam necessary to really add shear strength, which is the effect that gives foam and balsa core boats their strength. Without using the correct thickness, forces imparted to hull will go over (or under) the angle at which the foam deforms and looses its structural integrity.

Now, if you're just looking for impact bumpers, you can do anything you want. The strength I'm talking about is for forces not coming directly at the hull perpendicular, but for tangential forces such as forward impacts, slamming, weight above deck, and rig tension forces.

This thickness will depend on exactly the core foam material being used, and you'll need to be able to get those numbers for the material, which means using a marine-grade foam designed for core applications or a common material where the empirical data has been determined.

Marine foams usually have a table of thicknesses for various applications. Closed-cell styrofoam is also well characterized I believe. Google for books on building foam-core boats, they should have all the info you need. Typically they would have a table indicating exactly what foam thickness to use for various applications.

[Ixneigh]

Weight
Best guess is two hundred pounds. Plus or probably not minus...

The sheer clamp is a wooden boat term.
It afforded a place to support the ends of the deck beams. It also adds stiffness. On most boats the hull to deck bond, plus the width of the side decks, provide "crush" resistance.
The M has no side deck. The tiny flange between the hull and deck, compromised by the bolts, have little rigidity. One FU while docking could compromise watertight integrity, of which I had to add sealant to mine anyway.
An external 4 inch by 1 inch board, Sitka spruce if I can get it, bonded to where the black stripe under the rubstrip is now should strengthen the whole structure. The original joint will be left intact, and glassed over up onto the little edge where the chainplates are. The glass will run down to half way down the upper hull side and get faired and painted. The wood part will get four layers of mid weight biax, but the hull sides will only get one and a fairing of epoxy. The new surface will give a nice place to attach an attractive rubbing strip. Something that can actuly contact a piling without gouging. I will probably design in some attachment points to lash my paddle board more securely. I can screw them any where I need into the wood and fiberglass part instead of drilling holes in the hull itself.
Granted this is all a lot of work and expense until it is realized that I plan to cruise the Bahamas and perhaps Cuba someday, on a regular basis.

Foam thickness
Hmm I did not know that. I knew there were some smart people on here somewhere
I just figured half inch because well... I'm not an engineer
Ix

[RobertB]

Hmmm, if you are trying to eliminate variation that affects forstay tension, stiffening the hull is only part of the issue. Think about the primary triangle being worked - the forestay, mast, and connecting deck. The forestay is a flexible tension element, not much to do here. The mast is a fairly rigid but still a flexible element. Subject to forces from wind and all five stays (and variations on where the side stays are connected). Back to the primary - to stiffen up this, you would really need to concentrate on the deck. Hard to do with that pesky hatch in place. The deck is supported by the hull, but it seems to me the whole hull would need to be made more rigid in a 3-D way: would need to address twisting and flexing. To do this would involve adding thickness to the entire hull and improving/adding bulkheads. Just adding stiffening at the shear [strake] makes the boat a bit more tolerant of dock bumping (and there maybe a good idea).
Now, the second triangle that is involved is the mast, deck, and side stays. The most flex here does involve the hull but in a way that affects the distance between the stays where they mount to the hull (the chainplates). To stiffen this would involve adding a rigid beam across the interior right about eye level. Kind of ruins the cozy feel of the interior.
Interesting line of thought but my advise is to concentrate on where to add the bar and entertainment center.

[seahouse]

My thoughts as well. The greatest rigidity gain per unit of weight is to be had by adding a compression beam between the two chainplates. If it's ridgid enough, it needn't be straight, but can arch to be less intrusive. Of course, that all depends on your own priorities. And you have other benefits that you have mentioned that come from adding to the exterior of the hull.

Note that anything that protrudes from the side of the boat, even the rub rail, can be easily damaged in rough conditions by the weight of the boat dropping down on something like a dock at the wrong height. So fairing nicely, as you have mentioned, is important because if makes a dropping hull into a glancing blow, as opposed to direct downward strike.

[Ixneigh]

Some very good points. I was hoping that the strake would stiffen the upper hull edge enough to avoid having any beam or other structure inside. There is a lot of flex here as evidence when sailing.
Ix

[mastreb]

The forward cabin bulkhead in a traditional boat is placed adjacent or near to the chainplates for exactly this reason: To eliminate flex between them. On a MacGregor M, this is partially accomplished by the bolted in head bulkheads--you can see the size of bolts used on the forward head are pretty serious, and the purpose of that is to reduce flexing between the chainplates.

Not sure there's much else that could be done without seriously impacting the interior cabin space. I wouldn't do anything that reduced headroom at all.

[Highlander]

Some very good points. I was hoping that the strake would stiffen the upper hull edge enough to avoid having any beam or other structure inside. There is a lot of flex here as evidence when sailing.
Ix

Remember that flex is sometimes engineered into the design of the boat as in also aircraft , no flex could mean structural failure such as cracks or breakage ,
u could lay out say 3 or 4" flat SS flat bar @ just behind the mast at the chain plates & another @ the hatchway entrance these could be say start at the same length a the chain plates come up the side of the hull contour over the top of the rub rail & contour flush to the deck & other side basically like an up-side down U the front one would be bolted onto the the outside of the hull through to the chain plates , the rear one could be contoured to follow the deck contour to the hatchway entrance then go up to just above the height of ur dodger & stiffened with gussetts the raise section would now be stiff enough to attach a traveler , now if u r gonna go this far u might as well attach 3" or 3 1/2" SS tubing following the deck contour as well & have attachments for swing out amma arms for detachable amma,s , if u go that route then the SS flat bar at the front one could now be an L shaped brkt as the ss tube would now be ur compression post , the rear one would only require tubing going out from the bottom of the traveler arch or the tubing could now be used for the traveler arch , this would require quite a bit of fabrication but is very doable & time consuming

Now r u sorry u asked their is practically nothing u cannot do to these boats if u have the time & patience to do the design work & lots of $$$$ , just saying
J

[Ixneigh]

The traveler arch is a nice idea.
I tend to like fiberglass since I'm comfortable in that medium and can make pretty much anything I want with it.

Arch for my sunshade.

Crush proof box in the shape of a horse

I wouldn't venture into SS for a large project like that since I lack experience and equipment to work it.

I do understand the flex issue also. But looking at the boat from a builders perspective, it just seems like the upper side edge should have been built a little stronger. There's really not much substance there.
Ix