One thing that I should have also added in my previous post is that Team Phillips was designed to do long ocean voyages and not to race around bouys. If it was a pre-requisite that it was required in that form of racing to first sail triangles etc. I doubt you would see many cat’s designed.
Peter
I must admitt, I have never thought of inducing the rotation through the sheeting line attachment point.
The Ackerman will help a lot for sure.
Sounds like solutions for all the problems. [:D]
The boat was never intended for sailing around the triangle, but rather for straight line speed. The whole layout was ment to carry as much sail as possible without loosing control.
Peter, I was thinking of adding moveable ballast to the boat for heavier conditions. If the ballast is moved over the inner hull when tacking, I wonder if that might move the pivot closer to a Trimaran-like condition where the CG is also above the “Inside” hull (asuming the windward hull of the Tri is out of the water until the boat got its bow right through the wind).
ABC, The gybes work well, because the sails never loose power. In fact the sails drive the boat so hard, that you could probably tow a bucket without noticing.[:D][:-bigmouth] Just kidding. But I really couldn’t tell the difference in speed.
By the way. This is the first boat I use an unstayed rig on. In my first gybe in windier condition, i hauled the main in to reduce the load on the masts when flipping. Nevertheless my heart stopped for a second, when the sails smacked over for the first time and you could here the bang across the lake (its a very small lake thou [:D]). But maybe it is better to hear that bang, than the cracking noises of the loadbox. [:D][:D]
Thank you all, you really helped me a lot.
Marcus
Andy,
Since you are building a Team Philips concept too, I thought I share some experience with you that I got from building the boat:
Right now I have the masts mounted in 2 ball bearings each, inside the hulls. A Carbon rod that is laminated inside the masts is sticking right through them. Pretty much like it is done on Team Philips. I actually figured out, that this causes a lot of trouble:
First of all you have a hole in your hull that you have to deal with and second of all you have a lot of load on the bearings since the lever arm between them is just maybe ten centimeters against a 1.8 meter mast. Its way easier to laminate the rod inside the hull sticking out on the top and then mount the bearings inside the mast. That way you can seal the hull and have alot more lever length between the bearings.
Maybe this helps you. Just a thought.
Marcus
Don,
I just ran a few calculations in Excel and it looks like you can set the Ackerman at the servo horn without a problem.
As you say, with normal Ackerman, the key is that the straight line pull causes more rotation due to the angle of the pitman (or tiller in our case). When you set the Ackerman angle at the servo what happens is that at high servo rotation angles you get more straight line pull than if you had the servo set up square. So you get the same effect. If you look in the link that I gave, they show an alternate way of achieving Ackerman (at the bottom of the page) with the tie rod attachments offset at the steering box. They show a steering box output crank with two holes - one for each tie rod. If you hold your hand over one of the two holes and just consider the other hole and the center of rotation, what you have is basically a servo horn set at an angle.
Do you have any legos at home? If you do, you can build a test rig to try it out. If you do, you will find that I am right…
Will Gorgen
Will
You are right. I was thinking of two rudders tied together by a tie rod and then a link going to the servo. As long as you have individual links to the rudders it appears that the Ackerman can be set at both ends. Now a question. How accurate is Ackerman? By this I mean is, if the Ackerman is adjusted right will the rudders be pointed correctly at all angles or does one slow down and then catch up? I’m thinking that maybe one rudder(same area) would be better than two. A little brain exercise to get my mind off sail block’s radii for a minute.
Don
Hey Don,
In cars, the Ackerman produces the correct turning radii for both wheels for all turns (tight versus wide). This is somewhat the resul of the fact that the rear wheel always want to drive straight. That is why you set the focal distance to the rear axle.
In boats, the equations are not so clear. The rudders develop their lift by slipping through the water at a slight angle. If you turn the rudders slightly to compensate for helm (to keep the boat going straight) the two rudders are going to be at slightly different angles. But they will both be generating lift. One will generate slightly more lift than the other and the sum of that load will be the reaction to the helm. since the drag is not linear with lift, it is likely that this will result in slightly higher drag for the rudders than if they stayed parallel (for straight line sailing).
The situation gets more complex when you consider the forward rudders on this boat. Ideally, you would want all 4 rudders turning the same direction when sailing straight, but turn in opposite directions when turning. But this is impossible to do with a single RC channel for the rudder. You could “mix in” some collective on a second channel so that the forward rudders would turn in parallel with the rear rudders. This is what the CBTF boys do on the full size, fore/aft rudder boats and it is quite effective. In this case, you might “need” to do that since all 4 rudders would turn with the rudder stick in ways that could make the boat go through the water with relatively high drag.
The other thing to consider with a boat is: does it alwys turn the same way. If you turn a very long slow turn, both hulls will track well. But at some point, as you turn the boat through tighter and tighter turns, one or both of the hulls will begin to slip. If you turn the boat with one hull well loaded and the other unloaded (such as if you are flying a hull or have all your weight on one hull) the loaded hull tends to track through the tack and the unloaded hull tends to do all the “slipping”. So it may not be the case that you have a consistent tracking point that defines the center of the turn (the way the rear wheels do on a car. So it may be the case that you want the Ackerman to change for different degrees of turning and differeing conditions. This of course is far too complex to worry about…
As to your question about 2 rudders vs. 1 rudder: Normally, twin rear rudders are used on boats that heel alot such that one rudder would not stay in contact with the water all the time. Beach cats often sail on one hull so they need two rudders so that one is always in the water. Scows are always sailed on a heel so again 2 rudders (but one one is really used at a time). So in general if you are using 2 rudders it is because you don’t have the luxury of asking “is 2 better than 1” becasue 1 would not work (or would only work on 1 tack). I remember that for some time a group of Nacra guys were conviced that 1 was better than 2 so they would raise their windward rudder each time they tacked. They had slightly larger rudders than the rest of the boats, but it was less than the total area of the 2 standard rudders. I was never really convinced that they gained anything by doing this. Most of the time they ended up making a mistake at least once during a race in either failing to get their new rudder down and locked before a tack or spending too much time getting the old one up (and thus failing to sail their boat well on the new tack) that it always seemed to cost them more than they gained. So after a fairly short trial period, the idea was dismissed. The rest of us were convinced that as long as you did a really good job aligning your rudders, you were not going to loose anything by having both rudders…
Obviously with a tri, you can ask the question “should I have 2 rudders (one on each ama) or 1 rudder (n the center hull)?” But for a Cat, I really think that 2 rudders is the only way to go…
Will Gorgen
Marcus,
Yeah, I’d planned to configure the masts that way with a fixed post sticking out of the hull and some sort of bearing/sleve arrangement inside the masts. Seems like an easier solution for our scale of boats.
At one stage I was thinking of using a central mounted rudder hanging off the control pod but think I’ll hang a pair of rudders off the back of each hull instead.
Can’t wait until November when Uni finishes and I can start building again!
ABC - Andy
Taipan 4.9 Catamaran
‘Team Philips’ Formula 40 in building phase.
<blockquote id=“quote”><font size=“1” face=“Verdana, Arial, Helvetica” id=“quote”>quote:<hr height=“1” noshade id=“quote”>Originally posted by ABC
At one stage I was thinking of using a central mounted rudder hanging off the control pod but think I’ll hang a pair of rudders off the back of each hull instead.
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Andy & Marcus - here is an example of a Mini40 using single daggerboard and rudder located under the control “pod”??
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?.. and what happens if boat heels quickly ! There is an instant loss of rudder control once the majority of rudder surface area leaves the water. Only ability to save it, is a very fast sail winch or the masthead float once rudder leaves the water! Since you aren’t on board and feeling windward hull getting light, there is a good chance it will need a rescue.
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[:-banghead]
The boat in Dick’s post looks like one that was built by Graham Howard here in Australia. I have raced against boat’s identical to this one and they tack well, but not as quick as the tri’s. The boat’s can be controlled by feathering the sails to keep the windward hull from flying to far out of the water.
When setting up a cat with this configuration you need to extend the rudder so that the wetted surface is closer to the combined wetted surface of a boat with twin rudder’s.
Setting up twin rudder’s isn’t hard. I had twin rudder’s on the cat that I used to sail and it tacked fairly well after a lot of trial and error. What I did was have a servo in each hull. The wiring ran into a two to one then to the receiver. In this way both rudder servo’s were fed power, and you avoided trying to tie the rudder’s together. The servo’s then had wires connected on both sides of the servo and then to an arm on the rudder shaft. The wires were set up so that the outboard wire was set further in on the servo the the inboard wire, but the location distance on the rudder arm was identical.
This then meant that during a tack the windward hull turned tighter than the leeward hull. The correct positioning takes some time but it does help.
The twin servo idea is very easy to set up and is better than trying to tie the two rudder’s together.
Peter
Yeah, that’s the sort of thing I was thinking about, but as you say, going back to only one rudder poses more problems than it sovles.
I think housing all the radio and control gear in a central pod has merit given that the hulls are only 45mm wide at their widest point. It would also help having two sealed hulls and only the pod to worry about waterproofing etc.
The other advantage that I thought of with having wingmasts is that the boat might not completely invert if the mast is made of foam/fibreglass. In the twin rig configuration it’ll probably end up with the two bows and the two mast tips in the water but it might not completely invert which could be an advantage. You never know, it might be able to tip itself back up again??
ABC - Andy
Taipan 4.9 Catamaran
‘Team Philips’ Formula 40 in building phase.
<blockquote id=“quote”><font size=“1” face=“Verdana, Arial, Helvetica” id=“quote”>quote:<hr height=“1” noshade id=“quote”>Originally posted by wgorgen
Ideally, you would want all 4 rudders turning the same direction when sailing straight, but turn in opposite directions when turning. But this is impossible to do with a single RC channel for the rudder. You could “mix in” some collective on a second channel so that the forward rudders would turn in parallel with the rear rudders.
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Will,
This is actually the way I have set it up. I am using four servos on two channels. I have a mixer in my Remote controll so that when I move the stick side to side, the front and rear rudders turn against each other, when I move it up and down, they move in parallel.
I actually figured that this second function is hard to control and needs to be planed well when drawing the boat, otherwise it will for example have more lateral resistance in the bow and thus sail not exactly sideways when you turn the rudders parallel, but add a little turn to it.
I’ll probably play around with that when I get my hands on a digital R/C controller since there it is possible to adjust a fixed relation of rotation for both channels. On my analog R/c you have to move the stick diagonally to add a sideway component to its motion, which is quite a challenge…
Marcus
I tried having a small bow rudder and a large stern rudder on my twin rigger cat, you could pretty much make it spin on the spot. Just the problem was in stong winds the forward beam would catch and trip the boat splash
Great looking boat by the way, get some pics of it in a blow!
Luff 'em & leave 'em.