If you are going to switch, I would start with a vented 19, but make sure you stick with a vented model. Aluminum inheritantly flexes under load and most load is applied during the hole shot. When you load up the prop, it will flex and essentially drop a few sizes thus producing more slip, more slip enables the motor to come up to its power band quicker and then as the boat planes out and reduces load on the prop, it will “bend” back closer to a 19 pitch giving you your top end back. I use “closer to a 19″ because aluminum will still be flexing and never really give you a true 19” pitch. Since stainless doesn’t bend as much there will be a greater load on the motor itself which may cause it to struggle until it gets into it’s power band, this is where the vents come in. The vents allow air into the path of the prop causing it to cavitate ever so slighty, this slight cavitation produces the slip required to get the motor up to it’s power band. Once on plane/higher speeds, venturi(sp?) action will actually pull the “air” out the rear of the prop hub thus taking away the slip thus giving the full effect of a stainless on top end. So, to make a short story longer, if you are going to stainless, I would start with the same pitch but make sure you have the vents. Like everyone has said, find a dealer that will let you try one and focus on your speed/rpm of your motor and go from there. Also, an often over looked reason for lower rpms is the actual rotational inertia which is greater on a stainless(more mass). This is usually negligible, but on smaller engines it can be the difference.
