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Rupert View Drop Down
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    Posted: 23 Apr 14 at 12:32pm
Now that is interesting... it is almost identical to what the cvrda number for the boat with the bigger sail - 108- works out to in modern money!

Otherwise, I'd say that it shows that unusual hull shapes don't typeform as well as ones which are basically similar (which is to be expected) and maybe that the current PY of 1168 is harsh in all but a drifter, which is also pretty much the case. I guess static lwl is shorter than the length when sailing in a scow style hull, slightly heeled.

Many thanks!

Could I just ask for one more, please - promise I'll stop then!

Minisail - length waterline must be about 3.8m once heeled, the sail area 7.5 m2, and boat plus sailor must be in the region of 140kg, though with the hulls varying by 20 kg and the sailors by rather more, it is a bit of a guess!
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rb_stretch View Drop Down
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Posts: 742 		Post Options Post Options   Quote rb_stretch Quote  Post ReplyReply Direct Link To This Post Posted: 23 Apr 14 at 12:33pm
Originally posted by Peaky

Righting moment is important to boat performance – without it you simply can’t harness the power available.   The reason it doesn’t appear explicitly in the formula is perhaps partly explained by the fact that most boats have their sails sized to the RM. Most of the boats I looked at would have a ‘design wind’ of about 11kts – so if boat A has more RM than boat B, it will either carry more sail or be sailed by heavier folk. So sail area and crew weight can be thought of as a crude proxy for righting moment.


I like the logic here. A very pragmatic alternative to what would have been something quite complex I imagine.
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Post Options Post Options   Quote Guests Quote  Post ReplyReply Direct Link To This Post Posted: 23 Apr 14 at 12:39pm
No problem - how does 1143 sound? Or 1167 if weight is 160kg.
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Chris 249 View Drop Down
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Posts: 2041 		Post Options Post Options   Quote Chris 249 Quote  Post ReplyReply Direct Link To This Post Posted: 23 Apr 14 at 12:56pm
Originally posted by Peaky

Righting moment is important to boat performance – without it you simply can’t harness the power available.   The reason it doesn’t appear explicitly in the formula is perhaps partly explained by the fact that most boats have their sails sized to the RM. Most of the boats I looked at would have a ‘design wind’ of about 11kts – so if boat A has more RM than boat B, it will either carry more sail or be sailed by heavier folk. So sail area and crew weight can be thought of as a crude proxy for righting moment.

I agree that RM is very important to performance and understand that most designers ensure they have enough of it to balance their sail plan, or vice versa. I would hazard a guess that British designers have come to a consensus on the right numbers, which are very dependent on environment and culture, and perhaps also represent a desire not to create a boat that is too far removed from the norm to be reasonably competitive all-round.

I know it's a UK forum but I was also thinking of a wider range of boats, including some that demonstrate very different ideas of "sizing sails to RM"; the ultimate could be the Kiwi Rs and the 12 Foot Skiffs. The Rs can have higher RM than the 12s as they have no maximum beam, but the R carries about 140 ft upwind and 520 downwind whereas the 12 is carrying about 225 and 830! Considering that 12s can and do sail as Rs by simply not using their big rigs, it's an interesting illustration of how diverse RM to SA ratios can get. The FDs and Sharpies show similar variation in ratios. The old 16 Foot Skiffs and the US Thistles were another example of boat that were not too dissimilar in dimensions (and even general hull shape at times) except for RM, which created a vast difference in performance.

These observations aren't meant as a criticism of your numbers, just a comment about how difficult it would be to create a rating system for dinghies. 






 




Edited by Chris 249 - 23 Apr 14 at 1:32pm
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Rupert View Drop Down
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Posts: 8956 		Post Options Post Options   Quote Rupert Quote  Post ReplyReply Direct Link To This Post Posted: 23 Apr 14 at 1:04pm
Sounds like the sail needs to be a better shape for the boat to live up to its potential! And better foils.

I'll be trying the Lnisail frankenboat soon, with a 7.1m2 Lightning rig, which is more powerful despite being a fraction smaller. Got the mast ready to plug in, just need to sort out the controls. Be interesting to see how it goes!

Thanks again for plugging the maths in!
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Rupert View Drop Down
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Posts: 8956 		Post Options Post Options   Quote Rupert Quote  Post ReplyReply Direct Link To This Post Posted: 23 Apr 14 at 1:24pm
At work with the printers doing everything, too much time to ponder all this...

If you decided to form a development class based on a set number at the end - say, you had to have a boat that came out measuring 1100, you would have to balance off length, sail area and weight to give you a boat that measured. As has been said, a designer would probably ride a coach and horses through the rule, but would length come to dominate? A long, heavy boat with small sails? Or a light weight boat with both shorter length and sail area? The permutations are endless, really - what are people's thoughts?

You could have a whole series of development boats, each incorporating the classes that currently fit each number. Bit like the Metre rule boats - so a 900 class of speedsters, a 1000, an 1100 and a 1200 class of smaller, slower boats.

Yup, bored...
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Post Options Post Options   Quote Guests Quote  Post ReplyReply Direct Link To This Post Posted: 23 Apr 14 at 2:00pm
I like it! All these would qualify for the 1100cc...

L       SA       Disp
4.0      7       112
4.0      8       143
4.0      9       174
4.0     10       205
3.8      8       119
4.2      8       167
4.4      8       191
3.8      8.8     143
4.2      7.2     143
4.4      6.4     143


Edited by Peaky - 23 Apr 14 at 2:03pm
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hobbiteater View Drop Down
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Posts: 107 		Post Options Post Options   Quote hobbiteater Quote  Post ReplyReply Direct Link To This Post Posted: 23 Apr 14 at 3:44pm
this gives a development class where hull shape, foil efficiency are ignored and the latest tech boats should be better.

how about adding a factor for age, should be simple to take the PY fall of development classes (Merlin, N12 etc) and correlate a speed vs age factor - of course they had weight adjustments in those classes too. 


Edited by hobbiteater - 23 Apr 14 at 4:06pm
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Rupert View Drop Down
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Posts: 8956 		Post Options Post Options   Quote Rupert Quote  Post ReplyReply Direct Link To This Post Posted: 23 Apr 14 at 4:02pm
More to the point, max overall and min w/l beam are ignored, though the former is to an extent governed by sail area. Trouble is, because it is overall weight including helm which is measured, if a lighter helm sails the boat, we will be on to weight equalization and all that goes with it. If a heavier helm, he or she will have to saw a leg off...
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2547 View Drop Down
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Posts: 1151 		Post Options Post Options   Quote 2547 Quote  Post ReplyReply Direct Link To This Post Posted: 23 Apr 14 at 4:16pm
What is good about this formula is simplicity, visibility and openness ...

Seems to me it would be a fun development class rule ... really simple.
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