Hull thickness

I am contemplating laying up another hull and a question popped into my head. How thin can a hull be before it starts to “oil can” from water/wave pressure? I can imagine that on the flatter sections(ie. side of the bow)with the boat heeled that this could cause some major annoyances. Has anyone mounted a camera inside a boat looking for this? How else could you tell it was actually happening? Should a person put a reinforcing rib on the inside of the flat sections?
Just wondering
Don

Don Case
 Vancouver Island

Pardon my ignorance, but what is “oil can”?

Marino

The more I practice, the luckier I get.

Back in the olden days oilcans were made with a slightly convex bottom so that when you pushed on it,it would pop in and then pop out on its own.This measured out a relatively precise amount of oil. So now when a surface will push in and then pop back out it is called “oilcan”. If you push on a car door it will pop in and then(hopefully) pop out. If this was the case with the side of a boat when a wave or some other force acted against the side of a boat it would pop in(totally destroying your design efforts) and then pop out before you take the boat out of the water. You would never know what was causeing your boat to act erratically. I think a hull would have to be awfully thin for this to happen but I just wondered if anyone else had put any thought into this.
Don

Don Case
 Vancouver Island

Many thanks! You learn something everyday…

:wink:

Marino

The more I practice, the luckier I get.

Only if you have guts enough to ask.

Don Case
 Vancouver Island

Don -

probably not much of this happening. In the case of a hull, the complex/compound curves probably eliminate a lot of this action. If a boat with slab sides and hard chines i suppose it could happen. In the bow, it might happen but maybe a good thing? Hull would move in - not out, thereby making a thinner bow section, thereby cutting waves easier/better.

[:D] I don’t know - but it sounded good, didn’t it?

If you have a circular underwater section, it is part of a circle/curve. A curved surface is significantly stronger than a flat one. A boat 36 to 39 inches long with compound curve from side to side (cross sections) and also compound curve fore/aft probably develops a lot of resistance to oil can issues. My guess is you could take a hull with cross beams at deck level and a transom, and if placed upside down, you might be able to stand on it. Not “jump” on it, but if done carefully I would bet a hull weighing less than a pound could support upwards of 150-180 lbs. (a guesss of course)

Since most of our boats tend to be way overbuilt anyway, the flex in hull sidewalls is probably very slight - if at all.

Interesting to do a comparison of how much weight can be supported by two layers of 4 oz. glass per square inch (or square foot/meter) and when applied to our little boats is an eye opener. Do we really need 5 or 6 oz. carbon fiber for a one meter boat? Try the calculations sometime. I think I have info at home regarding this topic. Will look to see if I can find it.

My 1/2m actually does that if you prod it with a finger, but I doubt the pressure water puts on such a small,light boat would effect the shape much. The sides are a single layer of 1mm thick balsa as I was building super light as I want to test the design before making a plug, mould and all the jazz of knocking out carbon hulls. I would think you would have to use somthing like 1 layer of 50g woven glass with no ribs or anything to make it flex, but if you make it that thin the whole thing would probally fold up anyway under rig loads.

Luff 'em & leave 'em.

I’m no engineer, but laminations forming the curve are even stronger (in terms of stiffness) than the same thickness formed with a single sheet of the same material (I think). Thus a 3mm hull, formed of three laminations of 1mm balsa (for example)over a mold, would be stronger than the same hull built of a sheet of 3mm balsa over the same mold.

Thus, a light fiberglass hull may be stiffened to prevent oilcanning by laminating some ring-frames into it - just thin strips of fiberglass laminated two or three deep around the inside of the hull. Lighter than having a thicker hull, or adding additional bulkheads, A couple of ring frames, locked into a couple of light deck beams forward of the mast should be enough to remove any oilcanning. This isn’t so much a statement of fact, as an expression of hope. Feel free to correct my lack of engineering knowledge.

Muzza

Funny you should mention the camera because that is exactly what I did. I have one of the $35 EBay wireless cameras and I was told that the hull would flex on a boat I built when under a good load. So, in went the camera along with a MAG light and fresh batteries to get as much light in as I could. The picture was dark (even in broad daylight) and the winds were 12+ and there was never any indication of the hull flexing. I put a piece of black and white striped tape on the inside as well to help call attention to any flex, but never saw it happen.

Muzza is also correct… that the flexing would need to occur across the arc… which is not going to happen. The Roman arches are still standing 2000 years later because the arch supports itself.

I have an AC-15 which will flex quite easily, but there is no flex in the performance. I agree with the equation that if it is weak enough to flex from water pressure, when you tightened up the back stay it would break the hull.

Now, flex in the deck, is another matter. I have seen chainplates on weak J boats flexing the deck an uncomfortable amount.

www.LudwigRCYachts.com

Larry
What was the composition of the hull you tested with the camera?
Don

Don Case
 Vancouver Island

I am pretty sure that the primary loads that are considered in a model displacement yacht are rigging, keel and rudder loads. If you built a skeleton frame that took all of these loads, you could get away with an extremely thin hull skin.

Such a boat would however be heavier than one that used hull skin to spport the loads (load bearing members are ideally at the extremeties of a structure).

So Tim - a true monocoque would be a better option than a strong frame to spread the loads, covered by a thin shell that is non-load bearing?

Muzza

I heard similar from Anthony Wright (Apollo Boat Services - UK) who builds ultralight catamarans and trimarans to the Mini40 Rule.

His oft made comment … “put the strength (carbon fiber) where you really need the strength. The remainder of the hull(s) are simply waterproof membranes and they don’t need to be built strong - just waterproof.”

I guess it makes some sense, yet somehow a hull of .5 ounce glass is just … I don’t know … “thin”? [:-weepn]

<blockquote id=“quote”><font size=“1” face=“Verdana, Arial, Helvetica” id=“quote”>quote:<hr height=“1” noshade id=“quote”>Originally posted by Dick Lemke

I heard similar from Anthony Wright (Apollo Boat Services - UK) who builds ultralight catamarans and trimarans to the Mini40 Rule.

His oft made comment … “put the strength (carbon fiber) where you really need the strength. The remainder of the hull(s) are simply waterproof membranes and they don’t need to be built strong - just waterproof.”

I guess it makes some sense, yet somehow a hull of .5 ounce glass is just … I don’t know … “thin”? [:-weepn]
<hr height=“1” noshade id=“quote”></blockquote id=“quote”></font id=“quote”>

Which brings us back to my original question.[:D]. My difficulty comes after the first lay of glass. It is quite easy to pop the boat out of the mold to look at it and then put it back in to put on another lay of glass. The problem is that the epoxy doesn’t properly set for 3 days and until then they just feel real flimsy which (so far) has made me throw another lay on.I quess I should just bite the bullet and made a superthin hull, build the whole thing in the mold so that it doesn’t collapse and see how it goes. Once you get the deck on it stiffens a bunch. I think that monocoque is the way to go. I have an idea that any tension loads in the hull could be helped by using guy lines. For example a piece of 100# test Spectra permanently installed from the bottom of the keel trunk to the jib pivot could take most of the load off the hull and add very little weight. A ridge or tube formed into the deck could take the compression loads. I had imagined that most of these things have been tried But I have this nagging doubt about the “oilcan” thing.
Don

Don Case
 Vancouver Island

The hull was 1 ply of 120 and 1 ply of 7781 and it did fine. I could not pick it up by the hatch opening (which is a benchmark of strength for me) but sail well it did.

www.LudwigRCYachts.com

<blockquote id=“quote”><font size=“1” face=“Verdana, Arial, Helvetica” id=“quote”>quote:<hr height=“1” noshade id=“quote”>Originally posted by Larry Ludwig

The hull was 1 ply of 120 and 1 ply of 7781 and it did fine. I could not pick it up by the hatch opening (which is a benchmark of strength for me) but sail well it did.

www.LudwigRCYachts.com
<hr height=“1” noshade id=“quote”></blockquote id=“quote”></font id=“quote”>

Sorry Larry, but these numbers don’t mean anything to me. Could you convert this to ounces or something? Was it CF or glass?

Don

Don Case
 Vancouver Island

Sorry, there are hundreds of types of glass, anyway… the 120 is about 3.2 and the 7781 is about 9 oz. They are both bilateral and the 120 goes around corners very nicely. Both are crowsfoot weave meaning… not a perpendicular weave.

If you are on your game, you can wet layup the same as with dry carbon. The weight is in the resin, not in the glass. Case in point I put about 12-14 oz of glass in a 2.2 lb EC-12 hull.
Less than 1/2 the weight is the glass. If you layup dry carbon, it is quite a bit more expensive and if you don’t nail the resin, you are at the same weight as the fiberglass hull.[:D]

www.LudwigRCYachts.com

Thanks Larry
Don

Don Case
 Vancouver Island

The other consideration that we(I think correctly) make is that people will at some stage handle our boats by any point on the hull. These loads are often far in excess of the sailing loads on the hull skin. Think about a modern high performance mid sized displacement yacht (say 50ft), the non-structural areas of these hulls would not support lifting the entire boat in the same way that we can generally handle our models.

I think that a detailed analysis will lead to the conclusion that SOME parts of the hull can be extremely thin for the purpose of sailing loads however this may not be practical for the purpose of a model yacht.

Maybe some lettering on the sail that says" Lift only by the mast or keel"[:D][:D]

Don

Don Case
 Vancouver Island