Concave Deck? Pros and Cons

Can anyone come up with some cons for a concave deck? I’ve heeled the boat to 30 degrees and only the smallest part of the jib clew would be shadowed by the gunwales. My mast is deck stepped so I don’t need the peak for a ram. I’ve seen lots of boat with a concave aft deck, but all the foredecks are convex or flat. If the concave was full length the water should shed over the stern nicely, it might even lift the bow as it exits. The pro is lowering the sail plan and possibly(wild-assed quess) providing a bit of end plate for the sails.
Any thoughts?

Which class are you talking about Don? IOMs?

It was a generic question but I play with USOM. I’ll never race in a regulated race anyway so that doesn’t really matter.

My “W-A guess” would be the hull and deck would flex slightly in unison, like a small U (deck), though shallow, stuck inside a big U (hull) while a big [i]U /i capped by a small U (d) seems like it is approaching a sphere, the strongest shape.


Complexity of build v a flat deck. Weight (the foredeck of my IOM is about 50% “hole” covered in film). Lack of bouyancy during a nosedive. (I’m clutching at straws).

There was at least one IOM design that featured a full-length concave deck. The Southern Cross IV. See the link…

I’ve also been thinking about doing this for my USOM. Should have a decision in the next few days. I’ll let you know what i decide.

As Muzza commented, the biggest reason not to do this is the loss of buoyancy forward. This is important to resist nosedives. Also this config would not shed water as fast as a raised fordeck, so you will be carrying the weight of the water longer, until it runs out over the transom.

The dished fordeck will make it more difficult to configure a mastram or jackstays to control the gooseneck forces. (see the new thread on gooseneck stay)

It is also probable that this config would weigh at least as much as a raised deck and more than a flat deck.

Finally, the placement of the jibsheet would be lower (may leak into the hull) and would negatively affect the forces acting on the jib at closehauled. (increased the downward component would close the jib leach, while a high foredeck placement provides a more horizontal force on the jib sheet, leaving the jib leach under the control of the other adjustments).

Thanks guys, I don’t see any cons that can’t be fixed(eg jib sheeting post) except the buoyancy thing. This might be a problem as with a lower sail plan I could carry the “A” rig longer and this could lead to more nose dives. I will forge ahead aand see what happens.

There’s a US1M and 36/600 offered by Dennis Desprois (Walrus Model Yachts) with a concave deck.