3 blade propeller equivalent to 4 blade 21x14???

[yodani]

Hi there,
I have a boat with the following specifications:
Engine: Volovo Penta D2-55 (55 hp) @ 3000 rpm
Reverse gearbox: 2.7 to 1
Prop: 21x14 four blade
Boat made of aluminium - 8m waterline X 2m wide
Draft of the boat 80 cm

Current cruising speed is 12 km/h and max speed about 15 km/h. The engine spins the prop without effort.

The problem:
Need to switch to 3 blade prop because these can handle weeds much ... much better. I found one on eBay but it is a 19x19... Would this prop work? If not what would be the 3 blade prop equivalent to the 21x14? A smaller prop with a higher pitch is less efficient ?

Thanks and hope to get some answers soon as the auction ends in one day

Daniel

[Joakim]

The number of blades will not change propeller sizing much in this case, especially, if the blades are as wide (=3 blade has 3/4DAR). 19x19 will be less efficient and the maximum rpm will drop about 10%. 19x16.5 would be about equal in max rpm, but less efficient.

[pistnbroke]

if you change from 3 to 4 blade the difference is generally taken a 2 iches of pitch...so a 14 pitch 4 blade would need to be replaced with a 17 pitch 3 blade

[gonzo]

It depends on the blade area. The number of blades is irrelevant. The pitch should be the same and the diameter is dependent on blade shape. The total blade area should be about the same. If the diameter is less, it should be a bit more area.

[pistnbroke]

yes gonzo " should be " I use to run The Noosa Prop Shop and sold thousands of props and did many test runs admittedly on outboards...The 4 blade props did have more area and if changing 3 to 4 you always had to drop 2 in pitch.......

[gonzo]

Blade design and material also make a difference. Stainless blades are more efficient at high speed.

[pistnbroke]

Yes gonzo got two posts confused so have modified my answer ...yes stainless tend to be thinner particularly on the leading edge

[gonzo]

Old age does you in

[yodani]

Thanks guys for the clarification. So I think I would not go wrong if I switch from 21x14 four blade to 19x17 three blade. The only problem would be efficiency. I found a nice 19x19 three blade prop and was tempted by the price... but I will wait to see if something else comes up.

Any of you have experience with line cutters? Are they effective also against weeds?

Thanks

[pistnbroke]

Get the oxy out if its stainless and bend it a bit if its too much or too little ..never had any balance problems ..all dont by eye just like a solas professional

[gonzo]

That is a lot of change in pitch. What is the BAR, blade area ratio? There isn't enough information to calculate what the change will do.