Ill try on your questions...:
The "cav plate" is actually an "Anti-Cavitation Plate" On real outboards its purpose is to help keep the surface air from entering the prop thrust causing cavitation/slippage. I have played with cav plates a little, particularly on my SAW boat. I dont have one on there. The prop does "blow out" easier, and it definately cavitates when turning the boat....but its for SAW so it doesnt matter so much. I have seen cav plates on lawless lower units come loose (multiple times) during a heat and the boat handling went to hell..... bouncing/porpoising and the prop would blow out easily. However, I think if you set up the boat without one, you could make it work. The turns would be the biggest challenge. We run considerably high engine heights compared to regular real boats with outboards, and alot (most) of todays props tend to run with a blade out of the water anyway....so it could be argued that a cav plate is a bandaid to help fix another problem since we run our motors so high. some guys have cav plate extensions that basically further amplify the effect they have on handling. Im sure youve seen a real outboard, the cav plate is no where near the size of what we run when compared the rest of the engine size... Youll probably get a thousand different answers on this one, but my main point is....Try it with one and try it without one, then if you're feelin froggy like I was, try different sizes...
As for is it needed for FE? it doesnt matter if its squirrel powered, it will need (or not need, depending how you took the above) one regardless of power type.
there really is no set ratio or formula to use when setting sponson end to prop distance. Although I will say, based on what you typed, a 46inch boat with the prop only 2-3 inches from the rear of the sponsons seems REALLY short. What brand is the hull? Most manufacturers will give you a recommended distance to get the prop hub from the back of the sponsons, some race bred, high performance boats that are finicky must be really accurate on this demension, but this holds true with most high performance/pushing the limits set up specifications.... If your manufacturer doesnt have a recommendation, you could always take an average of a couple different manufaturers on say... a .21 size boat and do the comparative/ratio math to find out where yours should be. just a suggestion...
As for your drive depth/angle.... well thats a HUGE can of worms. There is no optimum height or angle. Some boats, like my sport .21 run the bottom of the prop shaft 3/16 above the bottom of the sponsons... some boats, especially the bigger/heavier boats can/will run a considerable amount deeper. Prop type/size is a big factor when considering height, obviously you dont want to bury a surface peircing type prop...and vice versa. The angle can make or break a tunnel boat. You must do very fine adjustments until you get where you need to be. this will change as conditions change (wind, prop, water surface roughness, mineral content, ect, ect) There again, you must spend hours testing a given prop at a multitude of different settings made in very fine adjustments, then once youve run out of adjustment options do the same thing on the other 15-20 props...you will find the set-up you desire.
People who have never run tunnel boats seem to think they are easy to run/set-up for race conditions....this is not the case. It takes just a much, if not more, time and patience as any other hull type.
If you are thoroughly confused about my aswers, its because they arent really answers...(think: politician) they are more of a guide/opinion.
I would suggest the following real world settings to get you started.
1) Cav plate: on a heat racing boat, leave it alone, its there for a reason.
2) A good rule of thumb for the prop hub face distance would be to set the lower unit pivot pin, even with the back edge of the sponsons. whatever that distance is, should put you really close.... You will/may have to add spacers or custom make a mount to get this disatnce.
3) I would set the height of your motor as such. adjust it where the where the bottom of the prop shaft is the same height as the bottom of the sponson. basically, if you had 0 degrees of angle, and the skeg was choped off, the sponsons and prop shaft should sit flat on a table.
4) I would set the angle at 0 or -1 degree at this point, once you start blowing the boat off, you can adjust the negative tilt SLOWLY, I use a digital angle finder to move the motor 1/10 OF A DEGREE at a time.
Hope this helps....
~James
