Originally Posted by jwchessell
Second, is there something rational to this difference in propeller pitch...
Yes. We have a twin engine GB36, one of the first fiberglass GBs made. It has a pair of FL120s and BW Velvet Drive transmissions.
Twin engine boats usually have counter-rotating propellers. In some instances, the engines actually rotate opposite directions. But in most cases, the engines both rotate the same way and one of the props is rotated the opposite direction by its transmission.
Most twins have the starboard prop rotating clockwise (as seen from the rear of the boat) and the port prop rotating counterclockwise. The engines, at least the FL120 and probably most engines, rotate counterclockwise.
So our starboard transmission has an extra gear in it to make the startboard prop turn clockwise.
BUT.... fitting an extra gear into the transmission changes the final drive ratio a little bit because of the sizing limitations imposed by the transmission's case.
So even though both engines are going the same rpm at cruise (1650 in our case) the props are not going the same rpm. So to get the same thrust from both props, the slower turning prop is given more pitch.
On our boat as delivered in 1973, one prop had a pitch of 18" and the other one had a pitch of 17." The prop with the 18" pitch had the slower rpm when the engines were going the same rpm.
The original three-bladed props on our boat were changed to four-bladed props by a previous owner. They were supposed to have the same 18" and 17" pitches of the original props, but they didn't. Long story short, when we had the props totally reworked a number of years ago, the prop shop said that they had found having the pitches different between port and starboard really didn't make any difference with a GB36 like ours, and they recommended pitching both sides the same.
Which is what we had them do. They are now both 16" pitch because they're four-bladed instead of three-bladed. One blade still goes slower than the other one at the same engine rpm, but the shop is right--- we don't notice a lick of difference in steering or the directional characteristics of our boat with the slightly different thrust between port and starboard.
But if you have questionss about the props on the boat your interested in, I would strongly suggest you talk to an experienced and reputable prop shop. Information they asked us to provide when we decided to have our props worked over were:
What is the make and model of our boat?
What is the wide-open-throttle rpm (in gear with the boat moving) of each engine?
What kind of engines and transmissions are in our boat?
What is the horsepower rating of our engines?
And finally, they wanted to see the props currently on the boat.
This last obviously had to be complied with by having the boat hauled, but we had it out of the water for other things plus a bottom job so we simply had the props pulled and took them to the shop.
Bottom line--- there may be good reason why the props on the GB you're interested in have the prop diameters and pitches that they have. Don't automatically assume that because the pitches are different, this indicates a problem.