Jump to content

Recommended Posts

I have a 32 ft cat.

 

Having problems with weather helm and rudder stalling at over 15 knots.

 

I thought after lengthening the boat, and moving the rudders further aft, the balance on a reach would improve.

 

Any suggestions?

 

One I already have in mind is adding a prod to get the reacher further forward. Currently tacked down on ropes from the bows.

 

Fence?

Link to post
Share on other sites

I'd expect the issue is ventilation or cavitation. Ventilation is when air is sucked down the side of the rudder casing the rudder to let go. It can sometimes be fixed with ventilation plates. Cavitation is when the pressure on the LP side of the rudder is so low that the water effectively boils - also causing the rudder to let go.

 

So, depending on when this happens (Not just speed but also rudder angle) the solutions might be different. Is it only with large rudder angles? It may be that the foil section is not a good one for the use it has, or the shape is not perfect?

 

Anyway. perhaps TC or someone else with more knowledge than me might chip in?

Link to post
Share on other sites

Hi

 

It is with large rudder angles. Have to put up to 20 25 degrees to counter the helm.

Which is why i suspect it is stalling. Losing the flow attachment.

 

Section is as per the original design by Ron Given. Looks close to NASA 0012.

 

 

I'd expect the issue is ventilation or cavitation. Ventilation is when air is sucked down the side of the rudder casing the rudder to let go. It can sometimes be fixed with ventilation plates. Cavitation is when the pressure on the LP side of the rudder is so low that the water effectively boils - also causing the rudder to let go.

 

So, depending on when this happens (Not just speed but also rudder angle) the solutions might be different. Is it only with large rudder angles? It may be that the foil section is not a good one for the use it has, or the shape is not perfect?

 

Anyway. perhaps TC or someone else with more knowledge than me might chip in?

Link to post
Share on other sites

Yep. I'd expect it's doing all it can. Flow will become detached at an angle of attack too large for the speed. The issue is likely not the rudder at all - but the balance of the boat - you should not need large rudder movements at high speed.

Losing flow attachment is caused by one of the two reasons I mentioned above.

 

What point of sailing is this normally on? What is the attitude of the boat (Level, leeward bow down etc). I'm no multi specialist, but I suspect that the most benefit could be gained from rig setup and sails, to try to reduce the weather helm at speed ??

Link to post
Share on other sites

Yep. I'd expect it's doing all it can. Flow will become detached at an angle of attack too large for the speed. The issue is likely not the rudder at all - but the balance of the boat - you should not need large rudder movements at high speed.

Losing flow attachment is caused by one of the two reasons I mentioned above.

 

What point of sailing is this normally on? What is the attitude of the boat (Level, leeward bow down etc). I'm no multi specialist, but I suspect that the most benefit could be gained from rig setup and sails, to try to reduce the weather helm at speed ??

Hi

 

Boat sails level.

3 sail reaching with genoa and drifter/reacher.

Control is OK up to about 14 knots then it starts to require too much rudder. Rounding up.  Boards are all the way down.

 

Most of the fleet has prods so wondering about setting up the spinnaker pole with water-stays from the bows, as a prod, for the reacher,  to get the centre of effort further forward. With the end on the front of the bridgedeck through the anchor fairlead.

 

I am putting up some photos of the boat on the Gallery.

Link to post
Share on other sites

Is the boat properly flying a hull at that point? How much heel?  Is your mast rake too much? Or board design, position and rake? Main in too tight? If you drop the main to leeward a bit more than normal, does that help? Playing with the main will give you  an indication as to if this is a trim/balance issue. If it still does it with the main really off, then it will be hard to fix with sails...

 

Get one of the top cat sailors to come out with you and have a look... any volunteers here?

Link to post
Share on other sites

15 knots is not slow. A prod should do the trick.

 

Drive it like you stole it, screecher on a prod definitely increases things going bad as in stressing, breaking things. Also need an expensive furler.  Screechers also have a narrow window where you can use it. But if you want to get in before dark on the coastal classic you will want a screecher.

 

If you don't have a prod I assume you don't have a genaker? You definitely need a large genaker. I usually enjoy a send on a kite more than the angle of death on a screecher.

Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...
×
×
  • Create New...