Bungee And Blocks

Look at the lever ratio of the rudder. From the pivot pin center to the outer rim of the rudder top where the bungee is lying is one dimension. From the pivot pin to the center of area of the immersed part of the blade is the other dimension. The ratio is likely to be 5.....6...or more to one. The rudder will develope some drag that exerts a multiple of the force that is to be resisted by the bungee. 5mm bungee is not big enough if you sail at any reasonable speed. I take it that the rudder tends to raise itself while underway and not because of contact with some solid object.(Seems to me the keel or board would have grounded before the rudder unless you are sailing backwards.) Use 10MM bungee which has 4 times the area of the 5mm stuff. If that does not hold the rudder down then it is time to address the drag of the rudder itself. Or maybe rethink the way the rudder is attached to the boat. The pivot pin appears to be quite a distance above the waterline and that constitutes an unneccesary leverage problem.

 

The wake peels off the hull at an angle somewhat parallel to the slope of the bottom of the boat. If you place a straight edge along the bottom of the boat, it passes just a couple of centimeters below the cheeks of the rudder head. In fact, it is hard to keep the rudder head out of the water. The bottom of the cheeks are cut parallel to this tangent.

I also try to trim the boat such that the transom stays dry. But with crew in the cockpit, the transom can be 50-100mm below the waterline (while the boat is moving). I've considered installing a convex water-skiing mirror on the rudder so that the transom could be watched from anywhere in the cockpit. I've moved the battery and gas tanks forward (and down) out of the lazarette and into the bilge under the cockpit. There is a story of a skipper sailing one of these little boats with a 50 lb stone in the anchor locker.

As per Wet Feet and Messabout, I will try a rigid line with only a short length of 10mm bungee before completely abandoning this concept. I'm thinking that the turning block is redundant. If we design for no movement, then it is probably better to just let the line drag around the radiused cut at the back of the rudder head.

I love this forum. Thanks for the help, guys.

 

When the rudder kicks up, the top of the rudder acts like a quadrant and draws on the hold down line. The arc around this quadrant for a 90-degree kick up requires about 8-1/2" of line. If the bungee is to be pre-loaded to 125% and maxes at 150%, then 8-1/2" represents 25% of the neutral length. I need a 10mm bungee that is 34" long knot-to-knot, pre-loaded to 42-1/2" and maxed at 51". But the back knot needs 8-1/2" of travel before it binds against the rudder head. So in total, this is longer than the length of the tiller.

The concept of a long line and a short shock cord suggests that the shock cord DOES NOT allow a kick-up. It may just provide enough slack to allow the hold down to be hooked to a fixture under the tiller.

I'm considering a full length bungee, again. Or, the CL257. The 10mm diameter shock cord seemed to provide sufficient tension on the hold down.

 

If I understand correctly you appear to be seeking a system that will allow the rudder to kick up to a horizontal position with the shock cord attached.I don't see this working,the systems I have described with a short length of 10mm shock cord move at speed and the helm load increases dramatically.While you may not be moving at planing speeds,any movement will increase the helm load and with it the stress on all the rudder fittings.It would be best to regard either the shock cord or the rope as a means of holding the rudder blade in its intended position while permitting some movement in the event of hitting something and minimising damage.

 

My thoughts: -

1. A longer shock cord will provide a more constant force.
2. You can have more than one section of shock cord to achieve #1.
3. You want the maximum force when the blade is down and less force when it is raised, which is the opposite of what you probably have.
4. You can change the the circular groove in the rudder to a profile where the radius reduces as the blade rotates up to help with #3.
5. Moving the pivot upwards on the blade will do the same thing as #4.
6. If the shock cord is passing over a relatively small diameter block or pulley to change direction it may run smoother if you use line for this portion and attach it to shock cord on a straight section of the overall path.

 

Why bungee?

Maybe an easier way? No beach cat that I am aware of and most new production dingys have long since stopped using any bungee cord for their rudder hold downs- it never worked very well. This particular one is off a Holder 12/14-(I think) It is a very easy system to use on most smaller boats and is very positive. It also allows the skipper to raise the blade before it hits something- very useful when landing on a beach. It is a link between the bottom of the tiller and the top of the blade, cheap, easy to fabricate and very positive. I have adapted it to over a dozen different small boats. Bruce

 

Bruce, that is incredible. Thanks for coming in from left field.

The family cottage dinghy had that sort of arrangement before a major rudder repair 2 dozen years ago. I forgot all about the concept. You're absolutely right. Probably the best of all scenarios.

This set up allows the helm to control the pitch of the rudder. Is there ever a situation that you would want to sail with the rudder at an attitude other than full down? I have seen Wayfarers sail in extremely light winds with the rudder kicked up (by accident or by purpose).

I'm not sure how to apply it to Sapphire. But you've got me thinking. Linkages, brake cables, levers . . .

 

Only if you're proposing to use it as a paddle.

 

Kick up rudder

I have installed the links on the outside of the housing also. And yes, it does make a good scull with the blade kicked up. Bruce

 

Thoughts On Mother's Day

If we assume that an appendage is balanced around its 1/4-chord line, then the kick-up pivot point for this rudder is well behind the balance point of the blade. When the rudder is set at some angle of attack, it is producing lift (and drag). If the lift is concentrated near the leading edge and is significantly forward of the kick-up pivot, will it generate enough of a moment to help keep the rudder blade in the down position?

Maybe this rudder doesn't need much of a downhaul.

 

balance point

The rudders I showed in the pics were stock on Holder 12s & 14s, both planning boats. The solid up/down link seems to be enough to keep the blade down in any conditions that I ever sailed in. I have one now I fitted to a Beneteau "14' Wizz" that is quite fast, also with no issues. Bruce