4 blade prop on a 20?

[Bushwacker]

Welcome to CSC! 4B props generally produce more stern lift, so would be particularly advantageous if you have a heavy 4 stroke motor, as it will allow the boat to hang on plane at a lower speed. The trade off is the drag of the extra blade, which will typically give you a lower top speed.

The 19/20' hulls were designed for the largest outboards available in the mid 60's, which weighed no more than about 300 lbs! The VDH hull is very stiff but relatively light and very efficient, so does not need nearly as much power as a typical modern Deep V. Many were sold new in the 60's and 70's with 85-100 hp motors. With a 300 lb motor. they will plane at about 12 mph without trim tabs and a 3B prop. A 115 should give you WOT speeds in the low-mid-30's, maybe even high 30's with a very light load. It's hard to find a modern motor that light today. although 3 and 4 cylinder 2 strokes will come close.

If you have a 2-stroke motor, it may be light enough that you might get a better combination of WOT speed, low planing speed and good fuel economy with a 3B prop. I'm not familiar with the Yamaha props, but the Merc Mirage series and BRP Rebel 3B props have large blades and a good reputation for good low speed planing and fuel economy. If you have a 400 lb + 4 stroke, a 4B prop will probably provide a lower planing speed and good cruise fuel economy but a bit lower WOT speed.

Before you start changing props, make sure your tach is accurate. (analog tachs are notoriously inaccurate, but the digital tachs that connect to the engine computer on the newer motors are very accurate.) Also make sure the top of the Anti Ventilation plate is above green water when you're up on plane and fully trimmed out. When motor height is right, the AV plate is typically 1-2" above bottom of hull; most dealers seem to mount them too low. Prop trials should be run with the heaviest load you normally run and WOT should be near the top of the recommended rpm range for the motor.