fore and aft rudders on a cat...... no keel or dagger

Do you have any facts and figures, build hours or other evidence for this?
In my experience, it is considerably slower. Both in hours and in lost time waiting for resin to cure.
Costs vary. In Aus, they are about the same, once the waste (high with stitch and glue, low with Intelligent Infusion) was factored in. Factor in the labour as well, and Intelligent Infusion is way cheaper.
It is definitely not easier. Each of the following are more difficult, more effort and require more skill than any part of an Intelligent Infusion hull build
Scarfing and accurately cutting 9m/30' x 3m/10' sheets of ply,
setting up for the keel pour,
pouring and glassing the keel,
shaping, aligning, filletting and tabbing the floors and interior furniture,
coating, glassing and fairing inside and out,
building the deck using stringers and frames

Intelligent Infusion is also far safer. To build the ply boat you will/should be in full overalls, gloves and mask for almost the entire build. Intelligent Infusion has one resin mix per infusion. Gravity applies this to the job, with optimal resin/fibre ratios, no voids, gaps or filling and no contact with the sticky stuff. The completed components are then glued in place. There is no cutting, grinding or glassing of infused components, no alignment of the fitout and no fairing.

I agree that S&G is lighter than frame and stringer ply for long low hulls, but for a 30' cruising cat, stitch and glue is probably not feasible as 9mm ply does not torture very well over short distances and 6mm is probably too flimsy due to the large panel sizes. S&G also gives low volume hulls, which the OP is not interested in. For more on comparative weights and materials have a look at the thread on the same boat at Cost to build - Cruisers & Sailing Forums http://www.cruisersforum.com/forums/f48/cost-to-build-200141.html

 

Good points Rob
1. My comment on S&G being faster was not based on hard facts as i have not witnessed the exact same project built both ways by the same people and shop space. It was based on testimony by local builders they go through a S&G project fast. This really depends on the skills of the builder i think the average builder will get S&G done sooner, but i could see a really skilled infusion team beating the best S&G team. Once infusion is all set up it's a breeze.
2. What foam are you using? here PVC 2lb closed cell is expensive. And the bagging is expensive, you do save on resin. i guess the cost difference depends on the project, but you would have to save enough resin to justify the extra equipment
3. I'm not sure where the limit of S&G is, you may well be right that S&G is not feasible for a 30' Cat. My Grampa built a huge fat deep vee 25' offshore planning monohull out of S&G back before it was popularized, sold to friends but it is still going strong, no signs of being over scaled.
4. I've never heard of a infusion job coming out so perfect as to need no filling or fairing, but much less i will agree with.

I actually like foam infusion more than S&G but because of the bad old days of full framing, people think plywood is slow, i think we can all agree that S&G is not a slow process. Lets leave it at that.

Agreed that infusion is neater and safer

S&G is only faster for a one off

 

Only my second post, so please be patient. I to have pondered the concept of forward rudder(s) for a few reasons. And also mostly as an optional performance enhancing extra, given it's vulnerable nature.
1a) In following seas, the water at the top of the wave can move forward at a speed not that different from the rudders, rendering them momentarily inefficient.
1b) Especially fast and or long narrow designs (eg cats) might have a flat rocker line (almost flat underwater line fore-aft), adding to the propensity to dig in the bows down between waves, while the wave top tends to turn the boat sideways (more or less), even if not actually broaching. The good news would be that the water is moving in the opposite direction in between waves, and that would contribute to making even quite small rudders very efficient.
1c) Sadly I have very little experience sailing anything larger than dingies and surfboards, but I have gotten the impression that multihulls tend to have quicker motions. And despite having less pitching and leaning, in following seas they might actually scew more (turn back and fourth), contributing to possible motion sickness. So just like boats with foil assist tent to pitch less, (and passenger ferries with active large hydraulic foil stabilization don't roll much) forward rudders might have quite an impact on, not only safety, but also comfort.
2) The possibility of not only turning the foils opposite to the rear, but also in the same direction, should give some ability in moving slightly sideways (as an option to expensive, heavy and maintenance prone bow thruster).
3) Some boats, especially larger monohull sailboats and cats, tend to have under dimensioned latteral resistance i order to manage draught. Eg a Bavaria 45/48 has a sizeable amout of sail area, but the keel isn't very deep, not to limit harbour access or sailing grounds. And since the boat isn't really very fast either, the keel struggles to generate lift, and tacking up wind is a tedious task. An efficient daggerboard style, bow mounted rudder, might improve upwind performance quite radically.

Of course i do realize that it doesn't seam like a viable alternative for production boats or mass implementation. But for a project with certain ambitions and specific goals, by an accomplished builder/inventor, I find the concept very realistic.

And i realize that even between similar sized cats, boat movements might differ radically due to design, shape, loading, displacement, rigging and so on. But it would be very interesting to hear comments about possible improvement in reducing boat motion, as well as if stearing loss in following seas is actually an issue.

Sorry for the long post, and for the seemingly unnecessary explanations. Only i suspect a lot of readers (like myself) are reading and lurking in these forums a long time before starting to post, and sometimes aren't nearly as knowledgeable as many of you main contributors. Cheers!