Top Speed

[SEAHORSE50]

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!

[Bear 45/70]

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!

If stepping up to a 17" pitch didn't drop the rpms at least 400 rpms. You need more prop pitch yet. Try a 19 but you may need a 21.

Now top speed is not just a function of what prop you install, but horsepower to weight ratio and hull type and you are a bit short on hp and the hull is rather dated to be going much faster than 35 knots, if that much. The extra octane will not help unless you pull the head and cut it to up compression. Otherwise all the extra octane will do is make you coolling system work a little harder to take the extra heat away.

Short cure for faster is a bigger motor and a newer hull.

[Mark75H]

Otherwise all the extra octane will do is make you coolling system work a little harder to take the extra heat away.

BTU's would mean a difference in heat value, octane number does not (different octane numbered fuels usually contain the same BTUs).

I agree, fuel change alone will not change performance.

Along with changing props also try changing the height of the motor. It sounds as though you may be pretty close to the best performance your boat will give and Bear's main advice may be the best: bigger motor, smaller boat .

[JohnsonM50]

When you calculate speed based on pitch and rpm its all accurate untill you consider resistance and slippage, the 2 variables that make fuzz out of math. You can up the pitch, raise the rpms, jack and / or trim the motor and not get much. but when you get somthing document it, which means dont do many changes at once so you know what works. You might get 10 mph without a new rig if you persist. Look at your bottom, most racers dimple the aft 20 - 30% of the bottom to break surface tension.

[Mark75H]

most racers dimple the aft 20 - 30% of the bottom to break surface tension.

I have never seen a dimpled bottom on any race boat on the east coast. Must be a west coast thing

Dimples are for golf balls, not race boats

[JohnsonM50]

oops, wrong word for it but taking the shine off is what I meant, seen it on most boats at Lambertville.

[JohnsonM50]

1 thing I forgot to mention, I have a 50hp 1972 evinrude and it turns out that with minimal altering a stainless steel prop from a 1977 115hp fit. Since this is an aluminum Macenzie Cherokee 15ft boat it does 43mph average with strong acceleratoin. Take a look at weight.