HELLO!!!
This is my first boat and now that I have driven it for a while it"s gotten a bit too ""slow"" for me.The boat is made out of fiberglass and is 13 feet in lenght(3.95 meters) and about 920 lbs (415 kilograms) in weight with engine fully fueled and one driver.The whole boat was my fathers and is 30 years old but has about 200 working hours.It was in a garage for 15 years and the engine was conservated.I took it out last summer and it was running like it was new.The engine is Johnson 1975 Sea Horse 50 Long shaft 680cc or 42 cubic inches, 50 HP at (5500-6000 rpm).
First it had an aluminum 12-1/4" X 15" OMC prop and it was overreving about 6200 rpm and was hiting a top of 27 knots or 31 mph.
Then I bought an OMC SST 11-3/4" X 17" new prop and the revs vere around 6000 rpm an my top speed was 31.5 knots or 36.5 mph.
Now I am wondering could I gain more speed with a 11-1/4" X 19" SST prop without overstresing the engine or maybe a 11-1/4" X 21" SST prop?With a 21 pitch prop the bout should hit about 36 knots or 42 mph according to calculations but what would happen to the engine??
I was thinking of switching to a chopper or cleaver type prop!
Can anyone tell me what diameter prop should it be according to my engine and where can I get one in San Pedro??Of course it must be a rehubed prop,or not if its not possible ill then have to make my own modifications.
Is there a prop manufacturer that makes racing props for smaller Johnson otboards,please tell me the name or net adress,or e-mail???
If I had a 10 1/2" X 22" Cleaver prop(over the hub prop) how much speed could I gain with the same engine????When I calculate I get around 40 knots.

In the engines manual it says to use 87 octane fuel but that was 30 years ago,I say that because oils that I put in the mixture today are far much better quality then 30 years ago.So what would happen if I were to run it on 91 octane fuel,that fuel burns cleaner with less carbon stickings but at higher temperatures???And the fuel alone today is cleaner and better generally.
With 91 octane fuel how much extra horsepower would the engine have??

Generally what do you think,what would be the max speed that I could squeeze out of this engine and boat configuration with a proper prop and 91 octane fuel?????????????????????????????????????????????? ????????????
PLEASE HELP ME!!!!!!
A thing for you to have in mind,I"m a student and I live in Croatia on the Mediteranian sea and I cant do wonders over here.
Thanks!