Propeller upgrade- size change and nibral vs bronze

[docofthebay]

I am strongly considering upgrading my 20x25 3 bladed props to a 4 blade. My current props seem to be correct as I can get to the recommended 2800 rpm with them. I understand the recommended change when going to 4 blades would be a one inch drop in pitch so does 20x 24 seem correct to y'all?
Pietro changed his to 21x24 4 blades but also upped the shafts to 1.75 inches. His boat also has 40hp total more than mine (355 hp vs 375 hp per engine) so I'm not sure if the extra 1 inch in diameter would be perfectly fine or would be an extra strain on my motors and shafts.

My other question is the difference in Nibral vs Bronze. There seems to be a 20-30% price increase for nibral. Is this a worthwhile extra expense or is bronze OK??
Thanks! Darrel

 

[fwebster]

Nibral is an alloy of bronze, aluminum and nickel and is significantly stronger than bronze alone. You need the stiffer blades of nibral with diesel engine torque.

We see these prop questions pretty regularly where boat owners try to select the correct prop for their boats. Learning is great and being involved in the decisions made for your boat is terrific, however, this is a place where you need to leave the details to the pros.....for a couple of reasons. First and foremost, the prop guys do this every day, all day long for all types of boats. Their business is prop selection and adjustment to achieve the owners needs as far as speed, rpm's etc. You do it once or twice in a lifetime, so, who is going to be better at the engineering aspects of prop selection? Second, if you work with a prop professional to change from 3 to 4 blade wheels and net 2800 rpm's, he is hooked up for that task and will tweak the props as needed to get the desired goal. On the other hand, if you order a 22 X 24 XYZ prop, he has no responsibility, other that getting you the prop you ordered. I think I would just call the prop guys and tell them you want the smoothness of 4 blades wheels and wold like for them to work with you on prop selection and purchasing and let them do their thing.

Also, you happen to be in one of the best spots in the country for prop professionals. In Ft. Walton Beach there is a company that does computer tuning on props. I think the business name is "CompuProp" (I don't know them and have never used them, but know people who have, with good results). In Panama City, "Marine Wheels" is owned by Buck Holmes....850-265-0095. Buck has been doing this forever and there is no one better at it. He does do computer scanning, but has the experience (art?) to know what to do to get what you need. I have always found Buck to be reasonable and most of the time cheaper than competitors in this area.

Hope that helps..... but, there is one thing to be careful of on a 390EC......your tunnels are deep and it doesn't take very much to cause ventilation of the blades. 4 bladed props will allow you to reduce pitch which will decrease slippage and improve efficiency and they should get you on plane quicker and work better than 3 blade wheels. Just don't let anyone talk you into going to a larger diameter props in your tunnels and don't expect much more than a kt or 2 improved speed.

 

[docofthebay]

Thanks for the advice and referrals!!!

 

[Jimmy Buoy]

You may benefit from calling Ron Jacobs at http://www.wolverinepropeller.com/. If I recall correctly, he may have the same model boat as yours and he is VERY knowledgeable about prop changes. Even though you will obviously not be using his shop (due to your location), it would still be worth the cost of a long distance call to gain another professional's opinion.