Hello,
I'm in the process of selecting a new prop, and have run up against a strange problem. The original prop was a 21" pitch and had no hole shot. The boat is a 2003 Silverline 1705, with a 3.0l Mercruiser Alpha One. Since the RPM's at WOT only read 3800, it seemed like a no-brainer to drop to 19". I bought a really cheap used prop at 19" to test out, but it cavitated badly and the engine never revved any higher. I ignored this reading and bought a new 19" 4 blade Turning Point (TP says their 4 blades should be chosen with the same pitch as a 3 blade). The hole shot with this prop is better, but still not good at all. According to the speedo, I've dropped 5 mph from 42 to 37 mph (frankly both those numbers seem overstated). The thing is though, the engine is still only revving at 3800 RPM's at WOT. It's a brand new tach too, same tach as originally spec'd for the boat. I guess I could buy a TinyTach or comparable to verify that the tach is working correctly. Could the engine have a governor on it? If so, what would it look like? Any other ideas why it isn't revving higher with the shorter prop? By the numbers, I shouldn't drop much further than 19", but it's pretty damn expensive to just try a 17" out to see if it runs higher (plus I really don't want to go much slower). Any other suggestions? Thanks
Shane
I'm in the process of selecting a new prop, and have run up against a strange problem. The original prop was a 21" pitch and had no hole shot. The boat is a 2003 Silverline 1705, with a 3.0l Mercruiser Alpha One. Since the RPM's at WOT only read 3800, it seemed like a no-brainer to drop to 19". I bought a really cheap used prop at 19" to test out, but it cavitated badly and the engine never revved any higher. I ignored this reading and bought a new 19" 4 blade Turning Point (TP says their 4 blades should be chosen with the same pitch as a 3 blade). The hole shot with this prop is better, but still not good at all. According to the speedo, I've dropped 5 mph from 42 to 37 mph (frankly both those numbers seem overstated). The thing is though, the engine is still only revving at 3800 RPM's at WOT. It's a brand new tach too, same tach as originally spec'd for the boat. I guess I could buy a TinyTach or comparable to verify that the tach is working correctly. Could the engine have a governor on it? If so, what would it look like? Any other ideas why it isn't revving higher with the shorter prop? By the numbers, I shouldn't drop much further than 19", but it's pretty damn expensive to just try a 17" out to see if it runs higher (plus I really don't want to go much slower). Any other suggestions? Thanks
Shane
Where do I check the drive ratio? (I bought the boat second hand, so anything is possible) I'm 99% certain that the new tach is set the same way as the original tach, but since I still have both, I can check that this weekend. I guess I'll invest in a tiny tach to double check that accuracy (maybe I can borrow someone's GPS to test the speed accuracy too). The thing that bothers me the most though is that 2 19" pitch props both show the same 3800rpm reading that the 21" shows. Assuming that the tach is set incorrectly and the engine is revving higher than 3800, the 2 pitches should still show up differently, shouldn't they?