I guess I should have said in begining,,I've not ran this motor on my boat as of yet (Small time in barrel) I'm going thru it first;carb.rebuild,new fuel pump & lines,impeller,etc.So I don't know about the sucking exhaust thing yet.BTW do you mind explaning what that does.I've only fooled with small motors.This is the first one with thru exh.thax
Exhaust and blades means less resistance and more rpm for a given throttle setting/boat attitude. High performance props, like several I had, deliberately drill holes in their props, near the front of each blade and slightly beneath it which is referred to as "porting". When the resistance to exhaust out the rear is high, like when you are executing a "hole shot" (coming up and onto plane from pretty much a dead stop) the exhaust that emanates through the holes flows across the blades and decreases the density of the water meaning it's easier for the prop to turn. This reduces the load on the engine and allows it to gain in rpm at a faster rate. Since hp is (torque x rpm)/5252, increasing the rpm increases the hp at the prop shaft and allows the hp to develop much faster which results in faster planing with a higher pitched prop than would otherwise be possible. Result is things like stern heavy bass boats, aka Bass Boats in particular, or any stern heavy boat adaptable to porting, can have their high pitched props for their WOT high speed thrills and still have a reasonable "hole shot"....2 props in 1.
On the flare and the ring, both were designed (opinion, not prop designer) to keep the action of the blades from sucking in exhaust at the rear of the exhaust tube which could produce undesired ventilation at a time when none was anticipated nor desired. Next time you are around an older engine without "thru prop exhaust" take a good look at the shape of the prop hub. Notice no long tube protruding from the center like on a thru prop. No reason for it. The other thing that I notice is the pitch of the props seems to be higher with thru hub because you have nice tube upon which to attach the blades. Obviously along with this comes higher gear ratios (larger numbers:1) since you don't get something for nothing, but you get better thrust (another opinion)........big mils pushing BBs excepted and the little 3 cyl Merc engines when they went to 3 cyl. loopers. On the loopers, OMC was turning large diameter props slower in the same hp range. Don't know why Merc decided on 10" diameter props on 60 hp engines. Seems OMC had the best thrust and around here they were the most popular (visual observation).
So I'm going to say that OMC didn't need the ring. Had they, I and probably a lot of other folks wouldn't have seen so many boats of the day without one, obviously the result of an impact. Around here water is man made lakes and in the man making, timber was cut and the stumps and the limb portions remained......the "firewood" was harvested all of which made for dangerous running for boats and folks.......in addition to the trees that were just left to rot and break off at the water line.
My 2c and hope it helps...I think I got the I's dotted and the T's crossed,
Mark
