Hello all, I'm about to tackle my first dual rudder class 5 ship. I've seen from other ships, having a little inwards (towards the centerline) angle helps to get wash over a single rudder. Is there any advantage to putting slight centerline angle on the shafts in a twin rudder (side by side) setup. I intend to run my rudders 2 inches apart & hopefully have the centerline of the props immediately ahead of them. Also, In your opinions, is there any good arguments to having more OR less down angle on your shafts & props? I would think the farther down your props the less chance of cavitation & greater chance of getting a good "bite" on the water. Conversely I would think Less angle would keep your propwash more on the rudders & up against the hull where the dynamic forces of the water will allow more flow against the surface of the rudders causing better turning/maneuverability. Am I over thinking this? Thanks all, Horrible Harry
Too much down angle on your shafts will cause your ship to bury it's bow at full throttle, and can make for cavitation by sucking air from the surface. Going in reverse with steeply angled props will pull you under by the stern. Aside from that, I have no real advice, in the AusBG, both rudders and props are required to be thier correct scale positions.
Having the props close to the hull allows them to take advantage of 'hull effect', where the narrowing of the hull works like the widening part of a venturi (i.e. raises pressure at the props). This increases the delta-rpm that you can accomplish without cavitation, so, better holeshots on launch. Keep in mind that hull effect works against you when in reverse. Personally, I put the driveshafts as close to the hull (and each other) as they'll go without hitting. Darren's points on downangle are good, I don't like much more than 5 -10 degrees on my ships (fast gun), for the reasons given, although you'd have to be REALLY close to the surface to suck in air and cause cavitation.
Props should be close to the hull, close to each other (if twin drive props), and close to the rudder. Try to keep them as level as possible but I wouldn't sweat a slight down angle. A slight angle on the prop shafts is good but the above points are more important. Don't forget that for optimum turning you need to keep your weight toward center as well.
Pretty much every thing said here is right on. One finall point also to think about is the size of your prop a rule of thumb we use is dont go above the size of your motors can, this seems to drag down your motor and cause it to draw more power. In the Utah we have out here we went with 3 blade flat pitch 1 1/4 props and she will easily do 26 sec in a 100 ft course so we had to slow her down useing drag dics of the same size to get her to 28 sec. This also useing a gear drive of one 550 motor and a 3 to 1 gear ratio. Buddy
