Re: prop suggestions
Jim, your MOTOR is rated for 5,000 to 5,500 RPM from the factory, your outdrive has a gear ratio of 1.61:1, and that means you need a prop that will let your motor turn between 5,000 and 5,500 RPM with the gear ratio of 1.61:1. Most 13" Pitch 3 blade aluminum props will let that happen. When you change to a four blade propeller you will lose some RPM, it just depends on blade geometry as to how many RPM you lose. Blade Geometry controls RPM and prop slip much more than pitch or small diameter changes. When I discuss blade geometry I am talking about how much Rake and whether it is flat rake for moderate bow lift, zero rake for stern lift or progressive rake for a lot of bow lift, Pitch and whether it is regressive pitch for better hole shot, progressive pitch for a better hole shot and top speed, or fixed pitch for a narrow speed range at a particular RPM, Cupping and whether it is on the trailing edge for stern lift and less ventilation when running props at high motor heights, or blade tip cup for maximum bow lift and water control, and the actual amount of blade surface area( low blade surface area for light boats and high speeds, high blade surface areas for heavy boats and less prop slip) that are designed into a prop for specific applications.
Most of the recommendations made in this forum are for Pitch only, as the person asking the questions is just trying to get the RPM for his motor in the right manufacturer recommended WOT power range, and they use aluminum props which have very little blade geometry and thus can be exchanged between one brand and another with very little unexpected results, except the Turning Point and Michigan Match series. When changing to a Stainless Steel prop is when all of the above items I mentioned come into play and that is when the Black Art of propeller recommendations matters. When changing out propellers in SS I have seen a 700 RPM difference between two props that have a diameter within a 1/4" of each other and the 16" Pitch prop is the one with the 5,050 RPM versus the 17" Pitch running at 5,750 RPM.
Hopefully this increases your knowledge about props and the differences involved between blade geometry and what it can change in how a boat will react to certain blade configuration changes in geometry.
H
