Dinghy Development

Originally posted by rb_stretch Originally posted by Pierre So, just for the hell of it, and if I had the money and time, I would just love to make a very light and strong carbon Osprey or some such "traditional" dinghy. No restriction on weight and such like, just to see what would happen performance-wise. I happen to like boat shaped boats.   I would love to see that as well. With unrestricted carbon rig and moulded sails the CoG need not be any higher and I reckon the waterline beam would only be fractionally narrower. Let's say you could knock the Osprey down to 74kg rather than 134kg. That is only 60 litres of volume you need to lose, spread out over 17ft and I reckon you'll lose a few centimetres at most.

Originally posted by Peaky Ah, but Pierre, if you make it lighter you will make it float higher. This will reduce the waterline beam, and hence reduce its stability (as distinct from righting moment). The lighter hull will also mean a higher centre of gravity, again reducing stability. The lower mass will also mean less inertia. The upshot is that your beautiful seaworthy boat will suddenly not stay upright when launched, will be twitchy in every puff/lull and will get tossed around by every wave. Just like any other lightweight skiff. I think it was Uffa Fox who said weight was only of use in a steam roller. I would respectfully disagree. It is too easy to fall in to the trap that better = faster. Better can also be cheaper, stronger, more sensitive, more responsive, less responsive etc.

I think weight for stability in a typical dinghy is well over-rated.
The righting moment you get from heeling 5 or 10 degrees in an RS400 at rest is not much, I think you get a lot more from the action of the water on the hull when it's moving.
If you look at Bethwaite's speed/drag curves for different displacements, it seems obvious to me there is a big performance gain to be had at low speeds just by reducing weight.
For sure, optimum underwater sections would change, but that's probably a second-order effect.
Personally, I'd like to see what it's like to sail something like a 400 or Merlin built to the minimum that could be made reasonably tough.
And of course getting weight out of the ends, and removing it aloft, is going to make it ride over the waves instead of bashing into them.
Or of course, you could make a longer boat for the same weight. 505's are light for their length, and don't seem too bad to me!

Another post raised the point of ergonomics. I think that is a big area to look into. Particularly for the club sailor. How much quicker would an RS800 be in real world light weather, if the middle-aged wannabe crew could move smoothly from the foredeck to the wing? Would a 400 be faster around the cans in light air if the crew was able to get further forward easily without stressing middle-aged knees?

I dont know about this light boat being good in light wind. When we sailed our Taser in light inland winds it was fine as long as we had wind but 3 tons of old GP stored a lot of energy when there was nothing. F1 had nothing on them!

Originally posted by tick I dont know about this light boat being good in light wind. When we sailed our Taser in light inland winds it was fine as long as we had wind but 3 tons of old GP stored a lot of energy when there was nothing. F1 had nothing on them!

They also store a lot of mass to get moving in the first place.
Where a light boat picks up speed in a short puff, a slow boat will pick up less.
A bit of 'swings and roundabouts' going on I think.

But when it comes to inertia in a dinghy, take say an RS 400 weighing about 130kg ready to sail, most of us are going to be adding around the same again in body weight. So even if you could halve the boat weight, you'd only lose 25% of the inertia. But for the same length you'd make less waves and have less drag the whole time you were moving. Most classes have a minimum weight for a very good reason!

I suspect the GP's also carried about 3 tons of local knowledge? They are also a reasonable hull shape for light air if you keep the weight forward and heel a bit, they can reduce the wetted area compared to a flat skiffy hull.

Light boats are nearly always faster, that stands to reason. But faster doesn't always mean nicer. I'm not advocating that all boats should be heavy, but it is too simplistic to say they should all be light.

The Icon is about as light as can sensibly be made, so that would give a good feel for a lightweight Merlin. It's a lovely and perfectly adequately stable boat, but it was designed that way from scratch.

When I test sailed the proto 100, one of the things I noticed was that it was stable when floating prior to sailing. I understand that the production version falls over. I can only imagine the difference is because the production boat is lighter and the metacentric height therefore reduced. Possibly not a killer problem for a dedicated race boat where ease of launching is a secondary issue, but not desirable and certainly off putting if the aim is ease of use.