pvanv
Admiral
- Joined
- Apr 20, 2008
- Messages
- 6,567
Last season, I installed a new Nissan 9.8 on my 8,000-pound, 28-foot sailboat. A 15 would have been better for the load, but would not physically fit in the boat. I also installed the 5" pitch x 8.75" diameter, 4-blade HT Solas/Tohatsu prop. I was turning max RPM's in excess of 6,000, maybe up to 6,200 -- just below the rev limiter point. The correct WOT range for that motor is 5,000 to 6,000.
This season, I changed to the 7" pitch version of the Solas/Tohatsu HT prop. As expected, it lowered the WOT RPM's, with a slight increase in boat speed. What's surprising is how MUCH it slowed the motor down. Now the tach only makes it up to barely 5,000 at WOT. 5,500 would be ideal.
So that old rule of thumb for changing pitch and how it relates to RPM is not correct when using 4-blade HT props. A 2" pitch change resulted in a 1200 WOT RPM change. Apparently my ideal HT pitch will be 6", which is not a stock, off-the-shelf prop.
This season, I changed to the 7" pitch version of the Solas/Tohatsu HT prop. As expected, it lowered the WOT RPM's, with a slight increase in boat speed. What's surprising is how MUCH it slowed the motor down. Now the tach only makes it up to barely 5,000 at WOT. 5,500 would be ideal.
So that old rule of thumb for changing pitch and how it relates to RPM is not correct when using 4-blade HT props. A 2" pitch change resulted in a 1200 WOT RPM change. Apparently my ideal HT pitch will be 6", which is not a stock, off-the-shelf prop.