Re: Prop question
To begin with, you need to determine what diameter and pitch prop you have now. Look on the hub of the prop in the area of the nut. You will see either a string of 6 numbers or so, or you will see something like 13-3/4 x 19. If you see a series of numbers, that's a propeller part number and it will tell you the manufacturer, diameter and pitch but you will need to go to the manufacturers web site or parts catalog to find that info. If you have second example, the number 13 is the diameter of the prop (13 inches in this example). 19 is the pitch of the prop (19 inches in this example). You simply buy a 13-3/4 x 19P prop for your engine. However, to see if you have the proper prop, you need to make some wide open throttle runs and then note the maximum speed and maximum rpm the engine achieves. The ideal prop allows the engine to run at the maximum recommended wide open throttle rpm for that engine with an average load. For your engine that rpm range is 4500 - 5500 rpm but you would be advised to select a prop that allows the engine to rev around 5500. If the engine revs less than that, it is being "lugged" which is a bad thing for an engine. In that case you need a prop with less pitch. Each inch of pitch decrease raises engine rpm about 200 - 250 rpm. If the engine revs faster than 5500 you would increase pitch to lower the rpm. Whatever speed the prop provides is what you get. But before looking at props, make sure the engine is in a good state of tune, running properly and is mounted so the antiventilation plate (the flat plate just above the prop) is flush or an inch or two above the bottom of the boat. This will achieve optimum performance. There is no sense buying a prop in an attempt to increase performance if you have a tired or poorly running engine. Lastly -- DO NOT post this message again on the PROP forum until you do the wide open throttle tests. There is no way we can recommend a prop unless you tell us how the boat performs now. For all we know, you already have the best prop for your boat. If you don't have a tach or speedometer, you are obviously not going to get much help. Knowing what prop you have now is also essential. You need to examine the prop carefully to locate the numbers.
