3 Bade and 4 Blade props

[Margarita Man]

I have 2 questions. With a 19 pitch stainless prop what would be the performance difference between a 3 blade versus a 4 blade prop? What are the advantages or disadvantage of a 4 blade against a 3 blade?

The second question is with a 19" pitch 3 blade prop what would be the performance difference between a 13" diameter and a 13 7/8 " diameter? I am trying to choose the best prop for my boat and do not know how these changes will affect the speed and the RPM. I know the relationship of the pitch to the rpm and speed but do not understand the relationship of the diameter and the number of blades.

 

[hitace]

Re: 3 Bade and 4 Blade props

usually a 3 blade prop will give you a bit better top end and a 4 blade will give you better hole shot usually.the bigger and heavier the boat the higher diameter prop you will need

 

[WillyBWright]

Re: 3 Bade and 4 Blade props

Diameter varies with pitch. As pitch increases, diameter decreases. That's to keep total blade surface area about the same in a given propeller family.

3-blades need to run fully or almost fully submerged. 4-blades can break the surface better and still stay hooked-up. That allows higher motor mounting height on the transom which reduces the amount of gearcase in the water. With less drag from less gearcase, the boat goes faster.

 

[al-B-kerkey]

Re: 3 Bade and 4 Blade props

So does a 4blade prop cause you to lose some top end speed?

 

[WillyBWright]

Re: 3 Bade and 4 Blade props

If you leave the motor mounted low, yes. But that's not carved in stone either. A Stainless made for your particular set-up could get you more speed regardless. 3-blade aluminums are generic props for general purpose. They work equally badly on everything. Stainless props specialize by boat and motor types.

 

[Margarita Man]

Re: 3 Bade and 4 Blade props

In ads for stainless 4 blade props they say it helps on gas consumption. Have you seen any evidence of this?

 

[WillyBWright]

Re: 3 Bade and 4 Blade props

I can see where they'll slip less and slice through the water easier, so the numbers might be a little better. But I'm not sure you'd find it to be a noticable improvement in mpg.

 

[Silvertip]

Re: 3 Bade and 4 Blade props

If whatever prop you choose results in less slippage, that can translate to less fuel consumption. How much less is a guess at best. One thing is for certain -- the difference will not likely be noticeable unless you run a fuel flow monitor.

 

[Cricket Too]

Re: 3 Bade and 4 Blade props

Diameter varies with pitch. As pitch increases, diameter decreases. That's to keep total blade surface area about the same in a given propeller family.

3-blades need to run fully or almost fully submerged. 4-blades can break the surface better and still stay hooked-up. That allows higher motor mounting height on the transom which reduces the amount of gearcase in the water. With less drag from less gearcase, the boat goes faster.

Not an expert here at all, but I have noticed just the opposite, it seems to me that as the pitch decreases, so does diameter, ie. I am running a 14 X 17 prop right now and am looking to drop down to a 15" pitch, and most of what I'm finding is in the 13.75 Diameter range.

I did actualy have the same question as Margarita Man, as to what impact diamter has on performance. Originally I was thinking I would like to find a 14 X 15 prop and keep my diameter, but maybe there's a reason why the diameter drops down?