I guess that the rpm on the enine is aprx. ok you can still hear that the rpm raises a bit when the boat gets airborne but not a raise in rpm compared with the 1,6 prop. it sounds nice and hits just above 74 km/t / 46 mph. Then I have a 75 x 1,4 and that is the prop for my boat with std. when i hit a wave and the prop gets some air the engine gains a lot of rpm's and slows down when it hits the water again. Then I have a 75 x 1.6 2 bladed that works fine but you can hear that the engine don rev up to maks. I have a nice 3 bladed 67 x 1,5 prop for my boat and it cavitates and is not usefull for my boat but other boaters find this prop super for their specific boat. on the engine or simply by trying 3 or 4 different props and have a gps ready. It is estimated and the only way to find out these things is to use a device that mesures the rpm. now the rpm on the engine will drop a bit and the slip on the pro will encrese a bit.Įstimated: 70 x 1,6 x 12.000 x 60 - 15% : 1000000 = 68,5 KM/T or 43 mph. 15% (less on a small pitch and more on a high pitch) divided by 1000000 (estimated or sured) x minutes in an hour (60) - prop slip aprx.
If you calculate the speed in km/t it is done this way: diameter on prop x pitch x engines rpm. I think that the x470 or x 472 will work better than the x 670
