5 bladed props, performance enhancer?

[TedZ]

Well there is hocus pokus IMHO as demonstrated by the following:

Five blades should be less efficient than three blades, but that didn't happen. Whether it was the thinner blades offsetting the efficiency loss, the water "hanging" on each blade longer, the value of the more blades entering at a larger effective pitch (the angle of the prop shaft being greater than the angle of the boat to the water), the fact that there is less disturbance to the water the next blade sees with the large thin blades or what...the designers and manufacturing are speculating, but in most instances a quality 5 bladed prop is outperforming a quality three bladed prop.

You want to talk to the US's finest prop shop (design and build) call Bob Herring. He'll talk to you and you will learn more in 5 minutes about what the industry knows and doesn't know then you can possible imagine.

A lot is known. A lot isn't.

Ted

 

[jim rosenthal]

How do you reach Bob Herring? (funny that a prop specialist is named after a fish, but what the h-ll...)

 

[Boss Lady]

There is a lot more known on the military side, but they ain't sharing any of it. If you ever get to see a submarine prop you will understand. They don't look anything like what the industry is still trying to sell us and call it high tech. LOL The carbon fiber props are getting close, but they are made in Germany. The science guys will tell you that 3 blades have less drag. So I ask them to explain why a 5 blade out performs a 3 blade then? I am still waiting on an answer. Just like WWII the planes went from 2 to 3 to 4 and then 5 blade props, but they will still tell you the book answer is the 2 blade is more efficient. LOL

 

[Boatsb]

I think less blades are more efficient as the have less resistance as they cut into the water but you cant move as much water with 2 blades as you can with 3 or 4 or more ijn a limited diameter.

 

[Boss Lady]

I think some engineers need to use a slide rule, not for math, but as a crowbar, so they can get their head out of their ass. LOL

Can you just imagine a P-51 Mustang with a 2 bladed prop? Well it does have less drag, and I can prove to you it is more efficient with my abacus. Good thing nobody listened to them.

 

[Trojan]

I don't believe you read what I said in the previous post. I'll try again. If you are OVER POWERED, have more HP/torque than you can use. Then going to a 5 bladed prop will increase your performance. Most boats are under powered. EVERY blade and edge you add requires additional HP/Torque to turn. It's not free. Your just using the HP that was left over when you reached the WOT RPM. If you gained performance then you were under propped with your previous 4 bladed wheels. There is a point when you can't increase the pitch without degrading the performance. When this happens then you go to a larger diameter wheel or add another blade. But if you don't have the extra HP adding another blade will degrade the performance. The more blades you have the more turbulence you generate around the props. Every blade is trying to dig a hole in the water and mother nature and gravity is trying to fill it up. That's a lot of turbulence. Also the prop blades are loaded the same all the way through the entire RPM range. That is another reason you adjust the pitch at WOT. For 25/55 ft. boats depending on the engines the new 3 bladed prop should still be the best props for us. Do as you like it's your boat. Sub props are of no comparison to ours except there props. They are multi bladed like 36 blades maybe more. They are designed not to leave air bubbles, make under water noise or leave a slipstream trail. I don't think I'll worry about that today. You can't compare airplane props to boats they have to worry about Dia., rpm., pitch. and prop tip speeds. More blades less rpms, more pitch, less tip speed. What can I say.


BILL

 

[luckydave215]

In the case of the P-51 (or other fast prop airplanes) the problem is geting enough blade area to absorb the horsepower without increasing the blade diameter. Not only is there a ground clearance issue, but the greater the diameter, the lower the rpm at which the prop tips go supersonic and lose all lift while inducing huge drag. Boat props don't suffer the effects of transonic speeds, but rather cavitation.
In either case, the fix is more blades and more blade area.

 

[TedZ]

Hering Propeller is at 4806 56th Place, Marysville WA 98270. Phone is (360) 659 4315. Bob or son Jim. Sorry is used two "r's" it is only one.

The notion that you need to have excess torque to get benefits out of a 5 bladed prop i don't believe is correct as there are efficiency gains which help in every circumstance, i.e. lots of power or needing a little extra oomph!

I have personally seen increases in speed with 5 bladed props and increases in acceleration (not much use to us) and no change except smoothness. Lots of factors affect the result.

Ted

 

[Trojan]

As far as paying for the new wheels. The 4 blades 24X26 would cost Me around $2,400. bucks each. The 3 blades will cost around $1,600. buck each. That's a savings alone. But that price could change a little. Because when I move to the new wheels I would like to go to a 1 inch bigger wheel. But first I need to go to the boat and measure the space I have. I think I can pay for them in two years or less In fuel savings alone. It depends if I cruise at plane or hull speed. But ether way I should gain speed at less rpm. Its ether new props or new Flow Scan. Is this the old chicken or the egg thing. Luckydave215 your right it's the same principal. The single blade prop is not the most efficient because of the RPM it must turn. The racers had to go to a single blade because of the hole they dug. There was no water left for the next blade to grab so they just caveatted. I always found that hard to believe. Another prop mystery. There predicting 6-8 inches of snow wed.

BILL

 

[Trojan]

If your properly propped at WOT you won't have any HP left. But some of the bigger boats with the big engines could have excess HP and not be able to use it because of the prop clearance. I don't say you can't benefit from a 5 bladed prop. I only say it DOES NOT COME FREE. You must have the horse power to turn it. And if you show an increase in performance, including speed without any rpm gain or at WOT. You were NOT propped correctly in the first place. You had to have the extra Hp to make it work. This does not work as well with gas engines it mostly applies to diesels. Because the way a diesel makes power you could reach the the WOT and not use all your HP. But it still can happen in a gas boat. More blades or more pitch or more diameter does not come free. You pay with H/p or torque.


BILL

 

[Boss Lady]

You could have cavitation with a 4 blade because of blade loading, that the 5 blade could eliminate. Just remember that a fixed prop is only right for a given rpm, it is wrong for the rest, meaning that if you had a controllable pitch prop you could adjust if for every condition and extract maximum efficiency. This best demonstrated by the amount of rotational force needed to spin a prop at a given rpm. It may only require say 75 hp to spin the prop at 1400 but your engine is actually capable of delivering 200 hp, you would be under propped by 60%. If you could adjust the prop on the fly you could get a lot more thrust at the same rpm and be a lot more fuel efficient and less wear and tear on the engine. Using a fixed pitch prop we have to prop for max rpm and not overload the engine, and this makes us extremely inefficient from idle until we reach max rpm.