Re: Prop pitch question

[Don]

The issue that gets over looked here is that the outboard does not have unlimited power to swing a larger pitched prop to achieve those higher boat speeds everyone speaks of. The easiest way to relate this is to think of how a manual shift car performs. What would happen if your Toyota corolla was in fourth/fifth gear while coasting at 25mph, then the driver stomped on the gas to pass another vehicle? Yes, the car would stumble, vibrate, ping and knock while hardly accelerating at all. The other thing to keep in mind is that cars have the ability to coast, rolling tire resistance is a whole lot less that water resistance on a boat while running at high speeds. In other words, boats slow down pretty quickly when engine RPM is backed off quickly, almost like someone had applied the brakes. Cars can coast along nicely on flat surfaces when your foot comes off the gas.
So, it is not uncommon, for an outboard to actually perform a little better (high RPM and boat speed), on a lower pitched prop, especially if it was over propped to begin with. The outboard does not have the luxury of "shifting gears" to help develop power either. There are many cases where a particular rig might need two different pitch props to perform nicely for all boating activities (cruising with light loads vs skiing with heavier loads).
The outboard trim can be adjusted as well, but it is tough to switch trim pin positions while underway.
So, your best tools here are a good shop tachometer (it must be accurate) and a GPS. You will have to adjust your trim pin to a compromising position that allows easy planning and minimal hull drag while at higher speeds. Be sure to have a normal load in the boat, along with normal fuel tanks and gear. You must know the operating RPM range of your engine also. Generally speaking, you will want to prop your rig at least to the middle of the RPM range. In other words, if the RPM operation range on your 35hp is 4000-5000RPM (not sure if this is correct), you will want to prop the engine to at least 4500RPM. Different engines have different power characteristics, some need to be propped right up to the upper limits of their particular RPM range.
This issue became a real issue when outboard companies started selling engines to boat companies so they could "pre rig" boating/engine packages to be sold to dealers/customers. Needless to say, the boat builder wanted to present his rig/package as a great performer, so they are often over propped. Dealers are left holding the bag, and usually are unwilling to water test the rig and supply the correct pitch prop for their customer.