So walk me through the in-mast furling mainsail

[Nibiruwayne]

I am biased. I like loose footed full batten mains. Battens reduce the slapping and maintain shape. Quick and easy to reef, essential for ocean sailing and heavy seas. You Must have a good manual furling system in the event of power winch failure. A heavy roach with full battens increases speed and power amd you can point higher without sail flogging. I agree on a bigger boat ball bearing vattcars are necessary.

[David B]

Originally Posted by donradcliffe

I have used all kinds of mainsail systems--in mast, in boom, dutchman, stack pack. Just finished a 2400 mile delivery from Kauai to Santa Cruz, mostly to weather on a 42 ft boat with in mast furling. We had a lot of problems unfurling in lighter winds, with folds in the sail jamming in the slot. Things got better in bigger breeze, as the shaking sail sorted itself out better than we could.

There was a huge performance hit on the wind. Compared to other systems, the main had less area, less shape, and the maximum draft was back at the leach. Fortunately the boat had a pretty good looking genoa, otherwise I'd still be out there.

If it was my boat it would have a dutchman system, no bow thruster, and some sea berths, but hey, its a "modern" boat.

That must have been one old and worn system for all those issues. We do 9.8 in 14knts true close-hauled, and I'm happy with that. The sail comes in and out of the mast with ease, but it is a Selden system, and they seem to be the benchmark for typical cruisers size boats.

[SV-Viento]

This may be the best read on this topic I have found. Thanks to all.

We are About to finish specifying our new boat and this is one of the last big details to nail down. First I love the sail shape and reliability of a slab full batten system for all the reasons previously mentioned but we are leaning towards IMF for all the reasons previously mentioned. What I will try to add and has only been mentioned in short is that not ALL IMF sails are flat and with hollow leaches - there are some great looking IMF sails with roach and reasonable depth. We have sailed a similar example and performance/shape was Arguably better than the 3dis on our last boat.

[SV-Viento]

A couple more shots of IMF sails. We sailed these and found their shape (depth and roach) really nice not just acceptable.

[David B]

Originally Posted by SV-Viento

A couple more shots of IMF sails. We sailed these and found their shape (depth and roach) really nice not just acceptable.

This might give you some more information (Selden on a Beneteau Oceanis 50). The sail view is from around 16:30 (where this link should start from).
I regret now not getting under the boom with the camera, but for someone who would not have had IMF in a fit, I love it now - we do so much more sailing because it is so easy to get the sail in and out.
https://youtu.be/L3f6REPF2RE?t=989

[Na Mara]

Originally Posted by David B

we do so much more sailing because it is so easy to get the sail in and out.
https://youtu.be/L3f6REPF2RE?t=989

One of the best arguments for roller furling in general and powered roller furling in particular.

[SV-Viento]

Originally Posted by Na Mara

One of the best arguments for roller furling in general and powered roller furling in particular.

I think this is true particularly as the sail plan gets larger. I'm not sure at what size IMF really becomes a benefit but my guess is around 40' and age of crew may play into this decision as well