From 2 blades to 3 - but keeing same size

[Philip_Oceanis 361]

Just curious,
if you switch from a 2-blade fixed propeller to a 3-blade fixed but keep the same diameter and pitch what changes shall I expect?

 

[John T1594]

When I switched from a 2-blade to 3-blade Campbell sailer prop on my 2000 H340, they reduced the diameter by one inch in addition to a pitch change.
The original prop was 2 Blade, 16 X 13, replaced with RH Campbell Sailer 15 X 9 X 1. I asked them about that and they said that their blades were cupped and therefore the smaller diameter would give better performance with less drag. It works great!

 

[Philip_Oceanis 361]

When I switched from a 2-blade to 3-blade Campbell sailer prop on my 2000 H340, they reduced the diameter by one inch in addition to a pitch change.
The original prop was 2 Blade, 16 X 13, replaced with RH Campbell Sailer 15 X 9 X 1. I asked them about that and they said that their blades were cupped and therefore the smaller diameter would give better performance with less drag. It works great!

If you have changed from a 2-blade "normal" (eg michigan or radice) prop to a 3-blade CS yes I understand this due to their special shape. Otherwise the CS would have really overproped the boat.
But in theory what will happen if you change from a "normal" 2 blade to a "normal" 3-blade and keep the same size. My guess is a drop of max rpm (overprop?) but how much (if any)?

 

[Ross]

You are asking a question that Dave Geer wrote a book concerning. First of all your blade area ratio changes and the three blade will push more water and therefore require more HP. It won't be 50% more but it will be substantial. If the blades on the three blade prop are the same size and shape ason the two blade it is easy to understand.

 

[Richard Bryer]

There are a number of variables. I just sent my CS back to Norm for some adjustments as I could not reach 2500rpm without being almost right up against the stop. WOT became about 2700. My original 2 blade was a 15X12 ( interesting that the yanmar was supposed to use a 15X15- must have been using up what they had in inventory- this 83 was an early Yanmar model H34) Norm recommended a 14X9. It is going to be cut back an inch, the pitch reduced by an inch and the cupping halved. According to Norm that will let me get back up to the proper WOT range ( and hopefully get me back to cruising at 2500- 6knts. The condition of the engine plays a part also.

I would certainly recommend Campbell Sailors and Norm- the service has been great and other than the over propping I love the prop- less prop walk, much faster stopping and acceleration. Smoother also

 

[Steve Dion]

Typically adding a blade to the same size prop will require 1 degree less pitch for the same prop (ie: Michigan Sailor).

I would not purchase any prop without a qualified Prop Shop sizing it for me.

If you go to a different type of prop all of this is not valid.

 

[Philip_Oceanis 361]

Typically adding a blade to the same size prop will require 1 degree less pitch for the same prop (ie: Michigan Sailor).

This sounds like what I was told. I am planning to replace my 2-blade 16X11 radice (similar to michigan) with a 3-blade radice and was told (by the prop shop) to get a 16X10. My question was more academic..."what if I was getting a 16X11 instead". Anyway, I certainly don't have any reason not to believe the experts.