WTB Rudder Blades and Mainsail for 12m Catamaran

[snowcat 0]

Hi,

I am looking for new rudder blades or alternative rudder ideas for a 12m Malcolm Tennant catamaran. The current blades are very heavy and, in my opinion, too large. They create excessive loads, making it difficult for the person steering the boat. I’m not sure what the correct size should be, so any advice would be appreciated. The current blades are over 1800mm in length and around 400mm in width.

I also need a replacement mainsail, as the current one ripped beyond repair on our last sail. I’ve been watching sailbrokers.co.nz for a while, but nothing suitable has come up, so I thought I’d try here. The sail needs a 13m luff and a 4.2m foot.

Thank you in advance for any advice and guidance.

[djp 4]

if the rudders are too heavy for steering they might need kicking under more to create more balance on the leading edge

[LE Bb 31]

We have a mainsail here off a tennant trimaran, it's slightly higher in the hoist but very similar on the foot. I should be seeing the owner in the next couple of weeks and coud ask if he still wants it.

 

Try sailing with the rudders up 200mm then 400m and see if it gets better or worse.  the rudders can be modifyed but you will need a baseline...

[snowcat 0]

On 2/03/2025 at 3:38 PM, djp said:

if the rudders are too heavy for steering they might need kicking under more to create more balance on the leading edge

Thank you. I have experimented with both mast and rudder rake. So far no improvement. I will continue to explore this option.

[snowcat 0]

5 minutes ago, LE Bb said:

We have a mainsail here off a tennant trimaran, it's slightly higher in the hoist but very similar on the foot. I should be seeing the owner in the next couple of weeks and coud ask if he still wants it.

 

Try sailing with the rudders up 200mm then 400m and see if it gets better or worse.  the rudders can be modifyed but you will need a baseline...

Thank you

[B00B00 329]

Looking at the rudder setup I would think that the blades would need to be all the way down to have the correct balance with enough blade forward of the rudders pivot point. Half rudder would make most of the blade being behind the pivot point and a huge load on the helm.

Hard to tell from the photos but it looks like the centreboards have some angle aft? Again the centreboard depth makes a big difference as you need the centre of effort/lateral resistance of the boat to be in the right place in relation to the sail plan and not enough centreboard blade makes the rudder work as the lateral resistance which loads it up.

Mainsail shape is also a big player. A really deep draft aft high road main will overload the helm.  A taller, skinny, flatter small square head main with the draft in the right position and using your rotating wing mast to create the depth when needed might make a big difference. And again a headsail with no entry/shape forward and all the draft aft also puts the sails effort quite far aft loading the helm. 

It's all got to work in harmony. 

I'm just up the river from you in Stillwater and out and about in my dinghy all the time (usually towing kids in sailing dinghys and beach catamarans around). Happy to come down and have a look with you sometime to see if we can suss out the issue and possible solution. I'm a sailmaker and ex multihull sailor so can have a look at the sailplan and sheeting angles ect.

I have the 45ft Jeanneau just past the Stillwater wharf called Rogue 3. 

[Island Time 1,293]

What BooBoo said.

The amount of rudder blade forward of the pivot point is what provides rudder balance (ease of turning). Those rudders look like they drop down to probably fwd of the pivot point - it's essential that  they lock down properly and cannot move aft in their cases - even a few mm makes a big difference. So the balance on the rudders controls helm load, but as Booboo says, the centre of lateral resistance v the centre of effort of the rig denotes how MUCH rudder is needed to counter any rotational forces. If you need a few deg of rudder to keep the boat sailing straight, then the rig & or sails need adjustment to suit.

Personally I'd not change ANY naval architecture until all other sources were exhausted... 

[snowcat 0]

On 5/03/2025 at 10:28 PM, B00B00 said:

Looking at the rudder setup I would think that the blades would need to be all the way down to have the correct balance with enough blade forward of the rudders pivot point. Half rudder would make most of the blade being behind the pivot point and a huge load on the helm.

Hard to tell from the photos but it looks like the centreboards have some angle aft? Again the centreboard depth makes a big difference as you need the centre of effort/lateral resistance of the boat to be in the right place in relation to the sail plan and not enough centreboard blade makes the rudder work as the lateral resistance which loads it up.

Mainsail shape is also a big player. A really deep draft aft high road main will overload the helm.  A taller, skinny, flatter small square head main with the draft in the right position and using your rotating wing mast to create the depth when needed might make a big difference. And again a headsail with no entry/shape forward and all the draft aft also puts the sails effort quite far aft loading the helm. 

It's all got to work in harmony. 

I'm just up the river from you in Stillwater and out and about in my dinghy all the time (usually towing kids in sailing dinghys and beach catamarans around). Happy to come down and have a look with you sometime to see if we can suss out the issue and possible solution. I'm a sailmaker and ex multihull sailor so can have a look at the sailplan and sheeting angles ect.

I have the 45ft Jeanneau just past the Stillwater wharf called Rogue 3. 

Thank you for the information. Boat is no longer in Stillwater.  I bought the boat about 1.5 years ago (old photo). I think I will wait for a new mainsail before playing with steering. The main is definitely not in good shape very stretched. The boat was lengthened by a previous owner thus  moved the rudders about 1.5m backwards from original position.