4 blades aren't automatically better. If the boat doesn't need the extra blade area available with 4 blade designs (in other words prop diameter isn't limiting) then a 3 blade is likely to be slightly more efficient, although if it's a high blade area 3 blade design vs a 4 blade with higher aspect ratio blades that may not be the case. Comparisons like that are rare though, typically the 4 blade will have comparable blade aspect ratio and more blade area. The 4 blade will typically be smoother although this depends on the boat and isn't always significant. And cavitation goes along with blade area. Some applications are highly loaded and will benefit from more blade area while others don't and don't have cavitation problems to start with.
The 3 blade ones will be better at high speed but the four blades will give better economy in a heavy boat at lower speeds. Depending upon your vessels characteristics you may not see much difference. If you over pitch your boat you may see a decrease in top speed. I would advise you to contact the people that made your boat and ask them first rather that take what looks like a good deal only to find out like i have in the past that I made the wrong decision. ..
Are you saying you wish the engine temp to be higher than it currently is at 1400rpm? If that's the case then you may wish to use a different thermostat. All this can be calculated for any given hull design displacement and speed. Even location and sea conditions can cause dramatic changes I've often spent money with little if any improvement especially in a displacement hull. ..
I'm pretty sure that advantage is mostly related to vibration when you have a big keel or skeg in front of the prop causing dead spots in the water flow. But it'll depend on the details of each boat, there's no hard rule for what works best.
It is advisable to run the engines UNDER LOAD at full throttle for 5 minutes or so on every days run. This increases engine temp somewhat but more importantly increases cylinder pressures which is important for getting the piston rings to seat properly against the cylinder walls. Not doing so can, over time, cause the rings to not seat properly and create compression losses and increased oil consumption, which exacerbates the ring seating problem.
We are a full displacement not high speed boat. I switched from three to four blades specifically for the low speed handling. It is quite noticeable on Puffin and was also evident on Ebbtide. Made no difference on top speed but the additional volume of water over the rudder makes the stern move around quite nicely.
