Re: Changing Tohatsu 4 to 6 hp-Do I need to change prop?
No, you don't "need" to change the prop, but depending on the application, you may want to.
Prop selection is based on a number of factors, including hp, vessel weight, vessel design, and operator expectations. What boat do you want the motor to push, and under what conditions?
With larger motors, there is usually a wider selection range, as there are major differences between hole-shot (low-end thrust) and top speed props. However... these small, one-lung beaters usually don't have as wide a prop selection range. Some older small OB's only have one prop that fits, if you can imagine that.
In all cases, the best prop selection uses on-water testing. If the motor achieves correct WOT RPM (no more, no less), then that is often the best prop. If you are over-propped (too much pitch), a planing hull MAY achieve higher top speed (eventually), however, the motor may never achieve correct WOT RPM; Likewise, underpropping (too little pitch) will give you good slow-speed acceleration, but might allow the motor to over-rev at top end -- very bad.
The Factory prop selector chart
http://www.tohatsu.com/accessories/prop.html shows pitch ranges from 6 to 9, and the Factory also offers a high-thrust 6-pitch, intended primarily for sailboats in the 1500-3000+ pound range.
According to the spec pages at
http://www.tohatsu.com/tech_info/brochure_pdfs/2009_brochure/Toh2009_Specs.pdf you want a WOT RPM of about 5500.
