3blade vs 4 blade

[motion30]

Thinking about putting some larger propellers on my boat as I run slow all the time. I can get a deal on the appropriate size propellers but they're only three blade is there a big advantage of four blade propellers and a low RPM situation? My boat is 44 ft with an advertised weight of 44,000 lb. I have twin 320 horse caterpillars

[skipperpete]

Originally Posted by motion30

Thinking about putting some larger propellers on my boat as I run slow all the time. I can get a deal on the appropriate size propellers but they're only three blade is there a big advantage of four blade propellers and a low RPM situation? My boat is 44 ft with an advertised weight of 44,000 lb. I have twin 320 horse caterpillars

There’s a bit more to it than just putting on bigger propellers and I’d prefer to avoid that discussion, however, 4 bladed ( and more) propellers are usual on motor vessels, partly because the only time your props ever trail is if you’re being towed so drag isn’t a factor… until then, but more importantly, the idea is to get as much useful power out of the engine as possible and translate it to thrust. 4 blade props do this better than 3 blade and even more noticeably with a Kort Nozzle and a Kaplan propeller where high speed isn’t required, just low speed thrust with the lowest attainable slippage. If you “run slow all the time” there’s little to be gained by messing with what you already have and overpropping the boat, which is what I assume you’re thinking of doing with the 3 blade props. I’m curious about what type of hull you have , heavy displacement, light displacement, planing but never going up on the plane? Only used on inland waterways or an actual seagoing boat that might need full power in high seas?

[HippieTye]

anytime i have a prop issue.., I always just call the local “prop” doc… he brings out multiple choices and facilitates the changing of the props while boat still in the water

[motion30]

Originally Posted by skipperpete

There’s a bit more to it than just putting on bigger propellers and I’d prefer to avoid that discussion, however, 4 bladed ( and more) propellers are usual on motor vessels, partly because the only time your props ever trail is if you’re being towed so drag isn’t a factor… until then, but more importantly, the idea is to get as much useful power out of the engine as possible and translate it to thrust. 4 blade props do this better than 3 blade and even more noticeably with a Kort Nozzle and a Kaplan propeller where high speed isn’t required, just low speed thrust with the lowest attainable slippage. If you “run slow all the time” there’s little to be gained by messing with what you already have and overpropping the boat, which is what I assume you’re thinking of doing with the 3 blade props. I’m curious about what type of hull you have , heavy displacement, light displacement, planing but never going up on the plane? Only used on inland waterways or an actual seagoing boat that might need full power in high seas?

Yes I realize there's more to it than just slapping a bigger set of props on the boat. I am reasonably knowledgeable but all my experience is on high speed race boats. My main consideration is trying to get my Motors up the proper operating temperature and get a decent load on them. Of course I'd like a little better fuel economy. I really don't expect more speed. It is a 44,000 lb boat with twin 320 horse caterpillar V8s. It is a sun deck Cruiser and I would guess you would say it's a semi-displacement

[FPNC]

As a general rule on displacement vessels. Larger prop diameter and slower turning is more efficient than small diameter turning fast.

4 blade props may give better results in mid range than a 3 blade. But if you have to reduce diameter due to increasing blade area, you may be better off with the 3 blade propellers.

Consult a propeller shop on what size 3 blade and 4 blade propeller will match your Displacement and waterline length and HP/gear reduction best. Then make those options one of your choices.