So how do you all determine your best prop configuration?
When we were told by Prop Shop "X" that the existing props on our GB36 were "worn out" we didn't do anything about it for another two years and then on our next haulout decided to go ahead and get new props.* I asked a number of people I knew who had had prop work done as well as friends in the local marine engine business who they felt the best prop shop in Seattle was.* They all gave me the same answer (it wasn't Prop Shop "X"), so I called them to find out what they needed to know to figure out the best props for our boat.
They wanted the following information:
Make and model of the boat.
Make and model of the engine(s).
The maximum rated horsepower of the engine(s) and the rpm at which this power was achieved.
The rpm we got at wide-open-throttle with the props we had on the boat.
The number of blades we wanted the new props to be (we had decided to switch from four-blade to three-blade)
And they wanted us to bring in the old props.
I also told them what the diameter and pitches had been of the props fitted by the manufacturer although these props were no longer on the boat.* But I thought the information might be helpful.
As it turned out they saiid that our existing props were not worn out but they had been horribly set up by whoever did it last (which would have been in the San Francisco Bay area prior to our buyng the boat).* Part of one blade wasn't even moving water, the pitches of the blades were slightly different, and so on.
So they overhauled our existing Michigan props.* Based on the numbers I had given them they repitched the props from 17" and 18" down to 16" each and after truing the blades the prop diameters were 23."* Had we decided to switch to new three-bladed props anyway, the pitches would have been different (one inch more in our case).
The point is that we used a good, reputable prop shop to determine the optimum configuration of our props based on the information we gave them.* Also, this shop has done props for dozens if not hundreds of GBs over the years so it was not a boat they were unfamiliar with.* I have no idea what sort of calculations or forumulas they used to arrive at what they arrived at.* I'm sure if one got hold of the formulas one could do the calculations themselves.* But based on our experience, my advice would be to use a good prop shop in your area to figure out what the best prop configuration for your boat would be.
Regarding three blades vs four blades, this is what we were told by the prop shop and our friends in the marine engine business:
The fewer the blades a prop has the more efficient it will be.* The most efficient prop would have one blade but it'd be a bitch to balance
A three-bladed prop is more efficient than a four-bladed prop (assuming the same size prop).* A three-bladed prop will back better than a four-bladed prop, which means there is less prop walk with a three-bladed prop than a four-bladed prop.* Less prop walk can be an advantage in a single engine boat but (in my opinion) is a disadvantage in a twin engine boat.
A four-bladed prop will be smoother than a three-bladed prop, so if a boat has a vibration problem that's not caused by something else like bad shaft alighnment, etc. switching to a four or even five bladed prop might help.
According to our boat's original owners manual, it was originally fitted with three-bladed props of 17" and 18" pitches, 24" diameter.* These props were overpitched for the boat but this was typical for GB when they were using engines like the FL120.* Since boat speeds and cruise power levels were low in those days, overpitching the props gave a bit more speed for the power developed but was not hard on the engine(s).
The prop shop told us that four bladed props are typically pitched one inch less than a three-bladed prop for the same engine(s) and boat.
Don't know if any of this is relevant to what you are trying to find out, but it was our experience with dealing with the props on our boat.
-- Edited by Marin on Sunday 10th of October 2010 10:10:29 PM