which prop, 4 blade or 3 blade

[pgpg5860]

I currently own a Bayliner with a 5.7L mercruiser engine and a Bravo ll leg, I'm looking at changing the prop from the stock one to a stainless. I've found 2 props that appear to fit my needs, 1 is a 3 blade the other is a 4 blade, before I purchase one how can I tell which would be better? It's unable to test props out from the dealer as I'm several hours from the closest one and then a coule of hours to water I can launch in.
thanks in advance.

 

[MikDee]

Re: which prop, 4 blade or 3 blade

You will probably lose some top end speed with the 4 blade, but it will get stronger at acceleration.

 

[Texasmark]

Re: which prop, 4 blade or 3 blade

On tail heavy outboards (like bassboats) a 3 blade with ports (one hole in the hub per blade) allows for better hole shot and higher top speed (as the holes are closed there).

The 4 blade, having more blade area, would help hole shot also, but you have a loss in efficiency at top end caused by interblade interference.

I have been told that I/O's don't do well with ported props....ventilate easily in turns and in rough water. Might check the archives for comments there. Therefore your choice would be limited to 4 blade. What I don't know is what pitch changes you may have to make just because you have the extra blade.

My 2c,

Mark

 

[Silvertip]

Re: which prop, 4 blade or 3 blade

There are a couple of issues here. Four strokes do not benefit from vented props like two strokes so that's almost not worthy of consideration. The next issue is why you want to switch. Many people switch from three to four blade because it looks cool and they think something magical will happen. So the question here is what's your reasoning for switching? That's important because you may already have the optimum setup and going four blade may actually hurt performance. You are also changing two propeller characteristics (3 to 4-blade, and aluminim to stainless). So before you spend big bucks on a prop, give us some information on why you want to change -- in other words, what performance issue do you have that you feel the switch would correct. Make some wide open throttle runs with your average load and then note wide open throttle rpm and speed. Speed is not all that important in this test but RPM is. Too much pitch causes the engine to lug or fail to reach its recommended WOT rpm band. Too little pitch causes the engine to over rev. Without any of that data, you are making a prop switch not having a clue what the outcome will be. You might also indicate if you do lots of water sports (skiing, towing tubes, etc). The prop you use for that may be different than one you use for long distance cruising. If you stick with one prop it needs to be a compromise.

 

[pgpg5860]

Re: which prop, 4 blade or 3 blade

I want to have a spare prop on board in case I have something hit or damage it while I'm cruising. I'm looking at changing my prop to a stainless The cost is within $100 of each other) as I'm concerned about damaging the prop while on the water, we currently run anywhere from 10 75 miles from the nearest marina. If I hit something with an aluminum prop I can shear a blade off pretty easily, If I hit something with stainless it might bend and cause damage to the leg but I should be able to limp into a port or an anchorage where I can change it out for the spare. The second reason is what I've heard the stainless usually gets better fuel economy. I don't use the boat for watersports only for cruising and as a fishing platform. If my memory is correct it runs at around 4500 rpm at full throttle and about 50 mph. the current prop is a 17 3/4" with a 21 pitch. Hopefully this gives you enough info.
thanks

 

[Silvertip]

Re: which prop, 4 blade or 3 blade

Let's take the durability issue first. You said you are worried about damaging the prop so you need a spare. It appears you run some treacherous waters so that's obviously a concern. Fact is, if you do hit something that would destroy an aluminum prop, there is a good chance hitting that same object with a stainless prop will take out the lower unit so whether or not you have a spare is of little help since a broken lower unit leaves you stranded -- there generally is no limping home. As for economy, that four blade will not likely pay for itself in fuel savings. Your existing prop has the engine operating at about the right WOT rpm so any prop change is likely going to alter that a tad either way so you need to be careful. Generally, going from 3-blade aluminim to 3-blade stainless you would select a stainless prop with 1-inch less pitch so a 20P would be the choice. With that change RPM and speed should be the same but perhaps a tad faster because of increased efficiency. Going to a four blade you would probably want to drop back 2-inches of pitch due to the increased blade area. You will not gain speed and in fact might lose some but hole shot will be improved. Staying with the same pitch would lug the engine and you would very likely lose top end as well. Again, the economy issue is a bogus claim in my view. If you want the durability of stainless with the same or slightly better performance than you have now, go with a three blade stainless. If you really feel the four blade will give you better economy then that's a choice you have to make but I think it's the wrong one in this case. Perhaps there is someone with the same identical setup that will be along to offer suggestions so stay tuned.

 

[achris]

Re: which prop, 4 blade or 3 blade

I would like to just add one comment here. It also may contradict what Silvertip said. I don't want to cause a blue but you should be aware of this.... Whether you have a stainless steel or an aluminium prop has nothing to do with how much damage is done to the leg internals. As a dealer/mechanic I have seen plenty of impact damage. I have seen stainless props folder up like a piece of paper and expected the gears to be completely smashed, they weren't. I had them, and all the shafts, crack tested and they came back without a problem. I have also seem boats come in with ali props that had slight leading edge damage and a sheared driveshaft. It's basically a raffle as to how much damage, if any, is done on impact.

Stainless props are more efficient then ali, because they are thinner, 3 blades is more efficient than 4 blades. What most 4 blades will give you is increased bow lift... if you need that, then get the 4 blade, if you're happy with the way the boat rides now, stick with the 3 blade. As for pitch change, if you're currently running a 21" ali, then stay with 21", whether you go 3 blade stainless or 4 blade....

Good luck and keep us informed.

Chris..............

 

[pgpg5860]

Re: which prop, 4 blade or 3 blade

Thanks a lot for the input, the relies you've given has helped me with my desision. Yes we have a lot of floating lumber, I fish with a group of 4 boats and between us we've had 3 props damaged in 4 years.

 

[Silvertip]

Re: which prop, 4 blade or 3 blade

There are way too many variables to say that a stainless prop will "always" cause internal damage or that aluminum will "never" cause internal damage. What was struck and speed at the time often dictates the type of damage, where the damage occurs, and how severe it is. Boaters not familiar with navigating the Mississippi or St Croix Rivers for example don't understand that those rivers have lots of wing dams and they don't know what the bouys mean so they ignore them. Those dams have broken more lower unit housings than you can imagine. Hit one of those and it doesn't matter whether you have an aluminum or stainless prop -- the least that will happen is the loss of the skeg. Then the prop, then the lower unit housing breaks at the separation line. And there are varying qualities of stainless so that also has an effect on what happens on a prop strike.

 

[houseboater]

Re: which prop, 4 blade or 3 blade

Aluminum hull, 55' new houseboat with 2 3.0 Mercruiser engines. 135 hp ea. Currently have 3 blade props but are looking for better excelleration and maneuverablity especially in windy situations. Would we benefit from 4 blade props?