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.

On the subject of bilge runners, when building my little 6 m catamaran the design called for a dagger board, the case of which intruded quite considerably in the accommodation. I asked the designer for another non-intrusive option. He came back to me with a design for an inward angled bilge keel on the inside of each hull. These were very small, .9 m in length and about 95 mm depth at the forward end, and 60 mm at the aft end. I was a little bit skeptical about their effectiveness, but thought they were worth a try. They were very quick and easy to construct, and I have to say I am very pleasantly surprised at how well they work. The boat goes to windward just fine. The keels don't start working until the boat speed is at about 2 knots, but then any leeway disappears and my little catamaran goes to windward as well as any cruising boat I have owned. There is apparently a lot of science involved in the design of leeway prevention devices, and different devices are suitable for different hull shapes. Chine runners being suitable for flat bottom hulls, and the bilge keels I have fitted being suitable to single chine, v shaped hulls. I wonder whether such an unballested bilge keel which does not protrude below the bottom of the hull would be an option for the SibLim hull shape. From my perspective I have effective leeway preventers which are as simple as is possible, add almost no weight, and do not add to the draft of the hulls. After all my worrying about the construction of the dagger boards and cases, and their effect in the interior of the boat, I am now very pleased to have come up with such a simple and effective alternative. Image below

Graeme wrote:

I am interested in your chine runners. They look tiny to me. I put somewhat more substantial ones on my scow, horizontal fore-and-aft, at 45 degrees, so they do not follow the line of the chine, rather they run out at each end as dictated by the bottom planking - they came out looking quite different to yours. I have absolutely no idea if they will be a help or a hindrance, but keeping my fingers crossed they will do no harm at any rate. Mine are integrated with the bottom planking, so they are probably going to be permanent whatever the result! All I know is, they do work on the Paradox, I've seen it with my own eyes. I am not sure if the concept is transferable, however, so lets both keep our fingers crossed - yours at least will do little harm.

On the subject of bilge runners, when building my little 6 m catamaran the design called for a dagger board, the case of which intruded quite considerably in the accommodation. I asked the designer for another non-intrusive option. He came back to me with a design for an inward angled bilge keel on the inside of each hull. These were very small, .9 m in length and about 95 mm depth at the forward end, and 60 mm at the aft end. I was a little bit skeptical about their effectiveness, but thought they were worth a try. They were very quick and easy to construct, and I have to say I am very pleasantly surprised at how well they work. The boat goes to windward just fine. The keels don't start working until the boat speed is at about 2 knots, but then any leeway disappears and my little catamaran goes to windward as well as any cruising boat I have owned. There is apparently a lot of science involved in the design of leeway prevention devices, and different devices are suitable for different hull shapes. Chine runners being suitable for flat bottom hulls, and the bilge keels I have fitted being suitable to single chine, v shaped hulls. I wonder whether such an unballested bilge keel which does not protrude below the bottom of the hull would be an option for the SibLim hull shape. From my perspective I have effective leeway preventers which are as simple as is possible, add almost no weight, and do not add to the draft of the hulls. After all my worrying about the construction of the dagger boards and cases, and their effect in the interior of the boat, I am now very pleased to have come up with such a simple and effective alternative. Image below

Well done Frederick. Beavering away there quietly and making good progress. I should learn from you.

I am interested in your chine runners. They look tiny to me. I put somewhat more substantial ones on my scow, horizontal fore-and-aft, at 45 degrees, so they do not follow the line of the chine, rather they run out at each end as dictated by the bottom planking - they came out looking quite different to yours. I have absolutely no idea if they will be a help or a hindrance, but keeping my fingers crossed they will do no harm at any rate. Mine are integrated with the bottom planking, so they are probably going to be permanent whatever the result! All I know is, they do work on the Paradox, I've seen it with my own eyes. I am not sure if the concept is transferable, however, so lets both keep our fingers crossed - yours at least will do little harm.

Best wishes on your build, keep up the good progress and keep us all informed.

Ignore the water line - she's currently floating in 7" (just immersing the chine runner), you can see where the coppercoat has turned muddy-blue. The draft will increase a bit later, of course, when topsides, internals, rig etc are in place. 5 or 6" more I expect - nothing like the line of the coppercoat indicates. I hope.

The hole is for the internal off-centreboard pin, and leads only into the case. It will be bogged up later.

The reason for the white paint is, the tops of the chine-runners were sheathed after the boat was turned over (I could not do that part upside down.) They are ferrocement sheathed, like the rest of the hull, and the white paint is water-borne epoxy resin as a water-curing membrane, and sealer. I hope I've got enough coppercoat left to do them. Its expensive stuff and I have some misgivings about using it over ferrocement, relying on a good primary layer of epoxy sealer to keep it isolated. Time will tell.