What's the verdict on going from 3 to 4 blade prop on 30'+ inboard express?

[RM]

Boat is a 31' Tiara with 454s. Whats's the verdict on going from 3-4 blade for an iboard boat in this size range?

Assuming the current 3 blades are properly sized and achieving WOT of 4200rpm. Have spoken with 3 prop shops and gotten 3 different answers ranging from will make little difference and lose cruising speed and efficiency big of turbulence form additional blade to will make big difference and gain cruise speed and efficiency. Not concerned with losing top speed - main concerns are cruise speed, efficiency and handling.

[tobnpr]

I've considered the same thing, but for a different reason. My 39' express tends to ride bow-high (needs full tabs all the time) and from what I've researched, the 4 blade props can add a bit of stern lift which might improve cruise speed a bit.

Other than that, from what I've read any performance differences are going to be minimal. For me, with diesel wheels I have no problems with dockside handling- but I suspect in your case (gassers) that this could be significantly improved.

You do have nibral, and not bronze, correct?

[modunlavy]

I can probably help both of you with this. If you provide me with some basic data, I can punch it all in my prop sizing program and see if you have any cavitation, excessive slip, etc and then play with different sizes and see what sort of difference they will make.

Tobnpr, prop cavitation can cause the stern to squat (think of it as sucking the stern down), so if it is excessive on your boat, reducing it could help it a bit.

If slip and caviation are not an issue, then there will be minimal or no gain in changing props....

send me a pm if you are interested

[modunlavy]

all pms replied too...