Extend Rudders on 10metre sailing catamaran

My rudders seem very short compared to my previous multi-hull. The boat seems to meander around abit.
I was wondering whether it is caused by the length of the rudders.
The current rudders go into the water around 200mm past the bottom of the boat

If so, is it worthwhile extending them and how would I do it? I'm also worried that in rough weather steerage could be badly affected if the rudder pops out at the back end of a wave??

 

That's a cool boat. Who is the designer? My G-32 is long like your boat and the rudders are way back there and they do come out of the water sometimes. It's a very helpless feeling with the kite up and the boat on one hull, but it always seems to re-attach in the nick of time.

 

Gday Tom - your boat looks like a stretched early Grainger to me and your rudders look way too short. The rudders do not look like Grainger designed rudders.

Most cat designers make their rudders too short in my opinion. One reason is so they do not have to make cassettes to rotate up. Pescotts tend to go this way but even Pescotts have deeper rudders than you do. However considering you have rudder boxes you can kick your rudders up easily so make them deep about 900mm would be nice.

You can make whole new rudders pretty easily in a shed at home. They are a fun job to do. Your boxes should go home too so that you can reinforce them. Big rudders are like grippy tyres on a 4wd they are really important when you need traction. Strangely enough that is not only in big seas that big rudders are better, big rudders help the boat tack well which can be a safety bonus. If you want some more advice I would like to have some more pics so I know what I am looking at and some measurements too.

The reason for bigger rudders is many fold. In quartering seas the windward transom will rise out of the water, this is just them time the leeward rudder is under most load to avoid broaching - so for many times out at sea you will only have one rudder in the water. Also after tacking, the roach of the mainsail fills before the jib and rest of the main, this moves the CE of the rig aft, just as the water flow reduces over the rudders, so to tack well you need enough rudder area to keep the rudders from stalling at slow speeds under high load. My little folding cat has big rudders for its size and tacks beautifully without a jib, it can even bear away from hove to with no jib, just a few pushes to windward of the helm. However, if you pull one rudder up, she reverts to normal, slightly sketchy multihull tacking ability. Large rudder area is king for good handling at slow speeds and in big waves.

 

Rudder Size https://www.boatdesign.net/threads/rudder-size.26163/

 

Rudder is 830mm long, 300mm wide and 50mm thick. Goes around 400-460 into water
See photos
Built in 1989 on designs by Ross Turner is all I know. It has been extended by a metre at the stern and I think Bow straightened

I was looking at extending rudders. I have pieces of Huon pine which I could rebate into the existing after cutting it square????

 

Did you run any numbers for the rudders?

What is the sail area, for one?

What boat?

If they extended the hull a meter, perhaps they used the old rudders, but perhaps the hull rises up a bit with the extension.

There are some people here capable of helping you, but you need to provide some details. I am not your expert; just watching. The extent of my work on rudders is glassing a wooden one that was warping badly.

But you need to have some ideas about how big to make the extension.

 

I would double check the Ross Turner accreditation - all the other Ross Turner cats I know are square ply cats - Jarcats are one example, although they do have very small rudders like yours.

Wander around the boatyard and have a look at similar sized boats. My advice for a rudder blade about 900mm deep and about 350mm in chord at top with some area in front of the pivot. I would caution you against using Huon pine and glassing it - it is like using a Picasso as a dishrag - save it for something lovely and use some western red cedar. Cedar loves epoxy and glass, whereas the Huon has lots of oil and I wonder about its adhesion to epoxy.

These rudder are nice. Have a look at the profile.



My advice is still, go slightly too big rather than too small, I never worry about the slight increase in drag as I get such good handling with big rudders.

It may pay to make totally new rudders rather than graft onto the old ones. Maybe you can use the head of the old ones but they look so small that I would extend the chord as well as the depth, which would be a pain. Rudders are pretty easy to make and I would personally use the boat with the old ones as I made some new ones at home. At 350mm chord with a NACA 0012 section you get a 42 mm thick section. Use 40mm cedar blanks and epoxy these together to form the blank. Then shape using a cut out template

NACA0012H for VAWT from Sandia report SAND80-2114 (naca0012h-sa) http://airfoiltools.com/airfoil/details?airfoil=naca0012h-sa

This is a fun job using a circular saw to set depth and a power planer to get the timber off. Then a hand plane and lots of oohing and ahhing. Good fun. Put a slight depression in the maximum beam of the foil and laminate in about 4 layers of 600 gm uni each side. Then grind off and smooth again, then cover with 440 db. All glue and resin is epoxy.

Have fun

Phil

 

Great. Thanks.
I think I will make up new ones from scratch and put current ones back on; so I can get out on the water ASAP.
The current sail configuration is very conservative. I will have to measure both that and underwater profile area.

 

where is the best place to get WRC blanks in Australia??

 

A good timber yard will always have it. I think a good size is 90 x 38 or so. Try to get the grain running down from one 90 mm face to the other. This is called quarter sawn. Bunnings has it but you can't choose it and it probably won't be quarter sawn.

 

I'm a woodworker. Found a good piece of wrc at home. 2.4mx75mmx260,quarter sawn nice grain. Heard that wrc is very light and rudders could pop up. Some ppl weight ends down with insert of heavier material??? In Australia some ppl use hoop pine which is heavier

 

NO! NO! Whoever is telling you that doesn't get it.

Your boat is a multihull with lots of light bits. Having rudders that float is good, they should. But the weight of all the glass you will add will more than make up for the density of hoop pine. Don't use hoop pine. It does not handle water like cedar does and if you drag your rudder onto a rock and go through the glass then the cedar can handle some water ingress far far better than the hoop pine. I love hoop pine but not in foils.

The cedar is a core for the fibreglass. You must put some unidirectional glass on the sides of the rudder. Then the cedar is a core, not a beam. It makes a huge difference to strength. So save the hoop pine for later and get the WRC all set out and buy yourself some 5 to 1 epoxy, about 5 metres of 400 gm uni and 6 metres of 440gm double bias. You also need some silica powder for glue and microballoons for filler.

I can have a look for some plans from other boats if you give me an email

philthompson67@gmail.com

 

Would you mind posting pictures of the rest of the boat? I rather like her lines.

 

If you do extend the rudders, don't just extend them along the same axis as the existing rudders. Your rudders are swept forward, bringing the tips of the rudders ahead of the steering axis. This reduces the force on the helm.

However, too much area ahead of the axis will make them over-balanced. This will result in the boat being directionally unstable when you let go of the tiller. It can be tiring to steer, too, since you can't relax your attention for even a few seconds. My trimaran's rudder is well balanced under sail, but is over-balanced when motoring at high speed because the prop blows on the part of the rudder that is ahead of the axis. It can be a real handful to steer under power if you're not used to it.

One thing you can try is to simply sweep the rudder back by raising it slightly. If this improves the steering, then your problem is an over-balanced rudder rather than insufficient rudder area.

If you do make the rudder longer, you should not bring the leading edge any farther forward. It would probably be a good idea to taper it toward the trailing edge, making the current tip the farthest forward point on the rudder.

 

See photo. When fully down rudders already rake forward quite abit