|Posted on Monday, March 06, 2006 - 09:28 pm: ||
we have a 1921 trotter engine with a schebler carb.its in a 19ft steel hull and thats what we bought!we added a 16" prop because it was free.test run in water....the motor seemed underpowered.started backup motor(16hp outboard)and rev'd it up,which caused trotter engine to stall?!?headed back under trotter power and the carb froze up?!? how does one figure out prop size? Where do i start from here? thanks for any imput. Ed
Post Number: 212
|Posted on Tuesday, March 07, 2006 - 05:20 pm: ||
You start from a couple of places.
FIrst, a 19 ft displacement hull has a terminal speed of about 7 knots or about 10 mph.
From this you can figure out that you go about 880 ft per minute
You know the engine speed, say it is 800 rpm.
That measn that you travel about 1 ft per engine rev, which is a pitch of about 12 inches.
With most propellers of this type the slip is about 20% - 25%, so the propeller pitch is 14 - 15 in.
Again, from practical experience, most of these props were square i.e. the pitch and the diameter were the same. So your 16 dia prop might be a bit big, depending on its pitch.
The pitch is usually stamped on the prop somewhere. My guess is that you ae overloading the engine with a 16 in prop.
Remember, speed is where the horsepower is , i.e. torque x speed = horsepower. So, if you have lowered the speed by overloading the engine, you are also losing HP.
As for carb freezing, it means that you have very moist air that is quite cool comming into the carb and buildng up - the carb did not freeze all of a sudden, the ice built up over the time the engine was running.
Providing some heat to the incomming air would probably fix the problem.