Thanks Frederik for the further information about your chine runners – and to David Th for further interesting comments – especially for the photo of that unusual appendage on your new catamaran. The proof of the pudding is in the eating. Your prior experience with catamarans will give you a pretty good idea of whether or not the appendage is earning its keep. You must be feeling very pleased with the result.
I don’t think Mark is entirely correct.
Here is the GPS track of a paradox sailing to windward.
(For full details, click here.)
This is not what you would expect for a flat bottom boat with no keel, skeg, leeboard etc and I think we can say that the chine runners are contributing something here (along with the hull shape and chine itself, and the large rudder). A certain amount of dinghy experience (we have all tried sailing with the centreboard up!) leads us fairly easily to this conclusion, without the need for controlled trials.
This is not to say that the chine runner is the sole factor, or that this device can be transferred successfully to a different flat bottom hull. (Leydon himself makes no such claim, and actually suggests the contrary). In fact we have a counter-example: Dave Zieger put a substantial chine runner on his Trilobyte 16 and found that, to cut a long story short, it didn’t work.
In my build, I am not expecting a similar gps track to windward as we see above. My hope is only that with the deeper immersion of a heavy flat bottom hull, with considerable rocker (both being characteristics of the Paradox) that I may be able to get some benefit from the chine runners if forced into very shallow water. This seems to be very much in line with the expectations and hopes of the designer of Erik’s boat. In both cases I think it is something of an experiment, and in both cases off-centreboards are still present as a primary provider of lateral resistance.
If I can get a gps track such as the one above - with boards up - I will be very pleasantly surprised. You will be able to read about it here first!
(The other factor in the decision is that my boat has off-centreboards built into the two sides of the hull, and the chine runner performs a secondary, structural role of bolstering the outside of the off-centreboard case).
Anyway, back to Erik’s build, and chine runners. I did not mean to suggest they are too small – simply remarking on them with great interest. I will be most interested in your impressions, after the boat is launched, and I think you will have a fair idea if the chine runners are contributing some benefit.
How much benefit (in the form of lateral resistance) is due to the chine runners themselves – that is a question that would require a bit more experimentation, and in that respect I think Mark is right.
Addition: Bilge-runner Application to the 5-plank
Following David Th's post here, regarding his bilge-runners: I have just been having another look at Arne's and David's 5-plank designs. There is certainly plenty of scope for a decent-sized bilge-runner on these midship sections, without any increase in draft. Maybe the extra drag would spoil the rowing a little?
I don't know, it might be worth a try, and David Th might be on to something. I'd rather try it out on a dinghy than a full-size SIBLIM and it wouldn't surprise me if it worked.
Would something like this be worth a try?
I've popped the same question onto the dinghy-design-competition thread.
