Sailing Dinghy Design

[SailDesign]

Brett,
I don't mind you "borrowing" the sails at all - you put them back where they came from (pic following was rendered in Sept. 2000)
;-))

[Tim B]

It's lovely Steve, but were designing an I14 and a Moth!! (not that I have anything against cats.)

Anyway, a week ahead of when I thought I'd start, it's here. It's as big as the last, it uses a fairer hull, it still meets the rules, it's got the trademark spinnaker chute (and a whacky spinnaker), and even better, it solves the issue of the cockpit floor.

[Tim B]

And here's the other version. Something for everyone I suppose...

[SailDesign]

Tim,
You have been busy, haven't you? Looks good.
I'm staring at the "bump" in the waterline, though, where the wings start so-to-speak, and wondering what that will do to the flow.
On the cat front, well...... I was just pointing out to Brett that the sails he "borrowed" were "borrowed" to being with. ;-)

Steve

[Tim B]

Yeah, I don't like it much either, I'll play with it a bit and see if I can sort it out. It's that section rule again.

Cheers,

Tim B.

[BrettM]

Thanks Steve, Did the on above get built?

[SailDesign]

It's curently underway, but it's going to be a looong road, as the owner/builder has been ill (very ill).
Steve

[BrettM]

Is this a symptom of building boats? I started the above design and we got to the stage where materials where being sourced/bought and then the client became ill. Struck down by a terrible chronic illness. It is waiting for him to get better (2years plus) or someone else to come along.
brett

[Jim Walsh]

False floors are the way to go and you can still have a transom to keep torsional regidity and add flaps to stop water backwashing in.

The extra weight is neglible as you should be able to reduce the weight of the inside laminate in the hull. Keeping water out is critical so a campered foredeck and false floor provides the best solution. They are also much much faster and safer after a swim

In regard to flex the most critical area is from the chainplates to the bow as this is where the rig tension is kept, if you are worried use plenty of that black stuff and it should be fine.

We can build false floor 12's to 45kgs (this includes fittings) and the pre merger Australain 14's were easliy built with false floors to 60kgs (I think this was the pre merger minimum weight) If you can't build a strong 14ft skiff to 60kgs you are doing something very wrong.

My 2 cents worth

[Tim B]

I guess I'm coming round to the false floor idea. The problem is, that sailing a LARK, which is dry as it is, and not having sailed it in really heavy weather, I tend to naturally assume irrelevance. However, the idea is growing on me, as you can see from the two rederings above.

That word 'flex' came up again, and I'm really talking about the aft-of-chainplates part of the hull. I trust that a deck and folded (Laser style) gunwhale will give sufficient rigidity up front. My basis for this, and I apologise for the bias, but I have worked on and sailed both boats, is the difference in stiffness between 420 and LARK. The 420 flexes visibly, whereas the LARK doesn't, this may be largely due to the stiffner design amidships. Just a thought. but it's quite disconcerting if a boat flexes during a race (especially on a regular basis), and further to that, whilst not significantly damaging the laminate, I can't beleive that it prolongs laminate life either. I'll get onto that waterline bump at some point.

Cheers,

Tim B.

[Jim Walsh]

"The 420 flexes visibly"

But I don't think they are allowed Carbon which is gods gift to stiff boats. Older designs are not be a good benchmark as they were probably built to a price at the time they were developed. The 470 was originally designed as a hire boat for French resorts.

You would be better looking at how development classes are built as they take advantage of the latest in materials and techniques

[Tim B]

Naturally,

We were going to discuss layup and construction a bit later, when we've finished the hull design. My point being that a good hull does not flex, whatever composite it is made of. Not only is flexing disconcerting at the best of times, but as I mentioned earlier, it does nothing for the layup strength over a period of a few years.

Cheers,

Tim B.

[SailDesign]

Hull flex is bad for more than just "disconcertion" and strength reasons - it adds drag.

[Tim B]

I'm glad you said that, Steve. I wasn't sure!! It does prove the general point though, whatever we design, we need to make it stiff and strong and light. Ah the old materials design problem again.

Cheers,

Tim B.

[tspeer]

There's another reason for a false floor - capsize recovery. The American fashion is to use sidewall flotation tanks that keep the cockpit out of the water when capsized and provide a deep cockpit and comfortable hiking. False floors fit the British dinghy sailing philosophy of allowing the boat to ride lower in the water when capsized, reducing the windage of the boat and making it easier to get to the board. Both methods come up dry. The sidewall tank never lets the water in, while the false floor drains quickly out the transom.

For a skiff, I think the false floor makes for a stiffer bottom because you can add multiple thin shear webs between floor and bottom. The sidewall tank basically acts as one longitudinal stiffener. The tank acts like a giant I beam or D-tube for longitudinal stiffness, but doesn't do much for panel stiffness.