4 blade props a better choice?

T

tobnpr

New Member
Nov 12, 2006
3,246
New Port Richey, Florida
Boat Info
1988 300 DA
Engines
tw 350's w/ Alphas
I notice that most diesels have 4 blade, mine are 3 blade.
No problem with performance; boat runs 22 kts. at cruise which is on par. "Problem" (if it is one) is she needs full tabs all the time to keep her bow down. I'd heard that 4 blade props can create more stern lift- is this true, and might it help reduce the need for tabs?
The tabs obviously create drag...
 
Not sure about you big boys with the diesels but I switched to a 4 blade on my 250DA and it shot that boat out of the hole like a rocket,,, I was also able to stay on plane at MUCH lower RPMs which tells me that there is more lift as well
 
While I have heard the same is true for I/O's I don't know if the same is true for inboards
 
Not sure about the bow lift but I switched a 40 Mainship from three blade to four blade props and the difference in smoothness was phenominal! It made me a real believer in four blade props.
Given the choice, I would be four blade every time.
Hope that helps.
 
geriksen said:
Not sure about the bow lift but I switched a 40 Mainship from .....
Hope that helps.
Click to expand...

What, the 340/400/460/500 weren't enough? (TIC, and slightly jealous, but very satisfied with my 340.)
 
The change to 4 bladed props does one main thing, but gives you a couple of real pluses. The 4 blade props allow for slightly less pitch per blade since you have 4 instead of 3 blades. That means you have significantly less slippage per revolution. Less slippage translates to less power loss particularly at low speeds and that means you should have better low speed handling and a quicker planing. The other advantage is a noticable reduction in vibration.

The slippage advantage dissapates as prop speed increases, so I wouldn't expect much if any performance difference at cruise speeds or at the top end.
 
Hampton said:
What, the 340/400/460/500 weren't enough? (TIC, and slightly jealous, but very satisfied with my 340.)
Click to expand...

I have the worst case of "ITIS" you have ever seen. :)


And to agree with Frank's excellent explanation, yes switching to four blades helps the low end, the midrange efficiency and lowers vibration. However there will not be top end speed gains. In fact you may lose a little. To me, it is well worth the trade off.

I would like to see someone try four blades on a older 340DA. I think that boat could really benefit from using them. Those boats always seems to struggle a bit to plane and all seemed to have a little extra vibration.
 
on our inboard diesel, we switched from bronze 3 blade to austral nibral 4 bladed props - the boat was much quicker at idle speed and responded quicker to the controls when docking. cruise speeds and efficiency improved at all speeds. the boat holds plane at a lower rpm. our express also rides bow high with full fuel (340gals under the cockpit) and runs faster with close to full tab - this didnt change from 3 to 4 blade IMO. our top speed increased as well but it is hard to say to what degree all or some of the benefits came from 3 to 4 blades rather than old vs new and nibral vs. bronze. i suspect that new 3 nibral 3 blades would be closer to the 4 bladed props - although i am guessing that the 4 blades still give a 10-20% advantage in all categories. make sure the new wheels are the right size and allow the diesels to hit max rpm - overloading diesels is the quickest way to kill them!
 
Thanks for the replys.
I'm going to call Admiral Propeller, give them the boat specs and see what they say. They've always been "on the money"
 
I have been reading this with great interest. Gericksen, I don't know how old a 340 you were talking about but I have an '89 DA. The first thing I had done when the bottom was being painted was sent the props (3 blade) out for a PropScan computer balance. This alone made a huge difference. I was keeping same speed on the GPS as before at 3-500 RPM's less with very little vibration. I have 7.4's.

I have a very good prop shop here and I am going to talk to him and see if he thinks the 4 blade would help me. With fuel dock prices here over $4.10/gallon i am looking for any way to improve fuel economy. Stay tuned.

Shawn
 
I changed my 3-blade props on my previous 340DA to 4-blade and it was like buying a new boat. I noticed that SR started shipping later models with a 4 blade so I just used those specs and it ran great. I would look for newer models of your boat and se if SR changed the prop specs.

briman
 
We have run our boat with 4 bladed props since we purchased her 2+ years ago, came that way from the previous owner. 3 bladed spares were on the boat.

Due to a log attacking our boat back in December (well actually we hit the log) we had to remove the 4 bladed props and install the 3 bladed spare temporarily. The first thing I noticed when she went back in the water with the 3 bladed props is less 'bite' when docking and a slower idle speed. It also took about double the time to get her up on plane and she did vibrate a bit more but not too much more. When docking I hit to punch her a bit more to move quick. I can't wait to get our 4 bladed ones back on.
 
NorCal Boater said:
I have been reading this with great interest. Gericksen, I don't know how old a 340 you were talking about but I have an '89 DA. The first thing I had done when the bottom was being painted was sent the props (3 blade) out for a PropScan computer balance. This alone made a huge difference. I was keeping same speed on the GPS as before at 3-500 RPM's less with very little vibration. I have 7.4's.

I have a very good prop shop here and I am going to talk to him and see if he thinks the 4 blade would help me. With fuel dock prices here over $4.10/gallon i am looking for any way to improve fuel economy. Stay tuned.

Shawn
Click to expand...

You have exactly the type of boat I was talking about. Let me know how it works out!
The ideal situation would be if you could borrow a set from another boat to get baseline rpm and speed readings to help select the ones you buy.
 
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