Rudders size/ Dimensions

[Tony Meola]

With all the discussions about rudders and the request of another member for the size of the over sized rudders, I thought it would be helpful to set up a seperat post on Rudders.

It is also helpful if you refer to this link, it will help you understand what you are looking Rudder wise at on the 31 and the 28.

http://www.bertram31.com/parts/rudders/

You will find one sketch in this link that shows the 3 old style rudders next to the new over sized rudder.

I know if I am off in any way, others will jump in until we have this correct.

You have to realize in following what I am posting is that the rudder as designed is not symmetrical. I am not sure of the reason for that but maybe someone else can help with that question.

The edge of the rudder closest to the prop should be the leading edge and is 15 3/8 inches in length. The trailing edge the one furthest from the prop is 11 3/4 inches in length.

Now here is where it gets a little tricky in explaining it:

The bottom of the rudder is 8 1/2 inches wide and if you measure up the length of the rudder closest to the prop, the leading edge, by 11 3/4 inches then come across from that point running parallel to the bottom of the rudder, 10 inches that will be the top or the trailing edge, the edge furthest from the rudder. This edge runs on an angle so it is not straight up and down.

This gives you a rudder that is 10 inches at its widest point and 8 1/2 inches at the narrowest point at the bottom.

Next you have to go back to the very top of the leading edge which is 15 3/8 inches high. Next draw a line vertically over 1 1/2 inches. This will be the beginning of the rudder post. If we assume the rudder post at this point is 1 1/2 inches in diameter, so know you are actually 3 inches over from the top of the leading edge, and you come down to the top of the trailing edge, you will have the shape of your rudder as shown in the link above and as you will see the top edge is on a fairly steep angle.

One other point, I am spinning a 20 inch 4 blade prop ( boat has 270 cummins). The bottom of the rudder is at the midpoint of the prop blade if one blade is straight down. This leaves the full rudder in the prop wash. If you are running a different prop, the bottom of the rudder will be either further down towards the bottom of the prop blade if you have larger prop or higher up on it if you are running a smaller prop. But in no case does the blade extends below the tip of the prop, even if you are using a 17 inch prop. which some of the early gas models did with the lower HP gas engines.

So the question is do they work. Yes they do. With one engine in gear the boat will turn and if I need to make it turn a little tighter, picking up a little RPM's on the engine in gear will force the boat to turn tighter.

I hope this helps everyone and please tell me if you think anything needs to be changed and I will edit this so that it is available for future reference should someone use the search function looking for rudder information.