Rigs: Sloop vs Ketch
Boats are systems and when it comes to one size fits all answers, there is no right answer here. The broad generalities are for a given sail area a sloop
rig will generate a greater drive for the amount of drag generated pretty much on all points of sail. That means that a sloop will be faster or will require less sail area to go the same speed. Sloops are particularly better than Ketches on a beat or on a run. A sloop
rig would tend to be taller for a given sail area. This means it would be better in lighter air but it potentially might heel more or need to be reefed sooner as the breeze picks up.
Sloops work best on boats with reasonably modern underbodies. Both are more efficient and so can point higher and make less leeway.
Ketches work best with heavier boats with less efficient underbodies such as full keels and vee''d hull forms. These hull forms often need a lot more drive and the hull is the limiting factor in how fast or how close winded the boat will be. The ketch''s lack of windward ability is less of a liability when placed on a hull that similarly lacks windward ability. Also, the ability of a ketch to carry more sail with less heeling moment also makes it a natural for a heavier hull form which often has comparatively little stability when compared to the amount of drive required to make a heavy boat move.
Much is made of the ketch''s ability to be balanced to help with self-steering and also the ability to sail under
Jib and mizzen in a blow. This is one aspect that traditional ketch has over a traditional sloops. It is not so true of modern sloops. Modern (especially fractional) sloops can be easily depowered and that reduces the need to reef. With modern slab reefing gear, reefing is far more easily accomplished than dropping the mainsail to the deck. In a properly designed sloop balance is just not all that hard to achieve.
The performance of both
rigs, both on broad reaches and in lighter air, can be improved by the ability to carry kites of different types.
In terms of comfort at sea, ketch
rigs push the weight of the spars closer to the ends of the boat which can increase pitch angles, albeit, while perhaps slowing pitch rates. The taller
rigs of a sloop tends to increase roll angles while slowing roll rates.
Then there are structural issues. It is often difficult to properly stay a ketch
rig as the mainmast shrouds often need to be routed around the mizzen and the forward load component of the mizzen if often taken by the top of the mainmast. It is also sometimes difficult to get proper aft staying on the mizzen of a ketch as well.
Anyway, in a general sense, if you are interested in sailing performance a sloop
rig makes more sense, If you live in an area that is windier and you like traditional boats than a ketch makes more sense.
Now then, if the boat you are considering is the Roberts Spray 36 this is a rediculously overweight boat, that has enormous drag and not much ballast. This means that you are considering a boat that needs a lot of drive but does not have the stability to stand to a the taller
rig found on a sloop of equal sail area. A ketch
rig would be the better choice here. This is not a hull that will sail all that well and certainly will never be fast, weatherly, efficient down wind or very good in light air, so the ketch
rig should not be much of a liability.
Respectfully
Jeff