Also as noted, you are pushing three bows through the wind, which, obviously, is two more than when tacking a monohull. Just another factor to consider.
Below are some details of the design of my current boat. Some aspects of the design are a little different to most rc boats, so I thought that it may be of interest to provide a description. It may also help to understand some of the challenges faced when working on a tri of this size. I’ll just provide an outline of the design for the moment describing the rationale and internal layout. I will post more details of the build and the exact shape of the hulls etc at a later date.
To begin with - The length. I wanted to build as small a tri as a I could using conventional rc equipment. I went to a model shop and purchased the cheapest 2-channel rc unit they had, which was a Futaba 2-channel am unit. I also purchased a mini servo for rudder control (LxWxH = 24x27x12; Weight=11g). I wanted to use a conventional continuous loop sheeting system, as I thought that this would provide flexibility wrt the position of the mast during testing. The smallest sail winch I could find was a GWS 125 (LxWxH = 40.5x20x42 mm; Weight=50g) I used it with the drum it arrived with (ca. 50 mm diameter) and after some experimentation I established that this required a loop of ca. 250 mm (pulley to back of drum). After estimating the length required for the rudder and batteries, I came up with a length of ca. 400 mm.
The design. I couldn’t find a design for a tri under 650mm, I did look at a few larger designs but it didn’t look like they would scale down and still provide sufficient room for the electronics. I was also keen to try to design something myself as I thought that I would learn more. So, without doing sufficient reading or calculation, or consulting you guys, I came up with a design, essentially by looking at as many tris as I could find on the net and making lots of guesses.
Many aspects of the design were dictated by the size and position of the electronics. The image below shows the layout of the electronics inside the main hull.
In order to be able to access the electronics and sheeting system I used a two-stage hatch approach. A large hatch provides access to the sail winch and the rudder servo. This hatch is designed to be removed infrequently. The mast is positioned on top of it, and the sail control cords passed through it. This arrangement is depicted in the next image. The larger hatch has a second smaller hatch in it for easy regular access to the battery compartment.
The tension on the continuous loop is controlled by a spring attached to the pulley. The spring is attached to a cord which passes through the top of the bow of the boat. This cord is then looped around the front cross beam and tensioned with a bowsie. This approach allows the continuous loop to be set up without needing access to the inside of the front of the bow.
The top of the rudder shaft sits in its own compartment. But to be honest it works but is a bit of a mess, so I won’t bother you with the details.
In order to fit the keel box, and still have the keel in a vaguely sensible position it was necessary to have the keel at an angle (I only realised this during the build).
More detail of the build and shape of hulls etc to come.
@Mij,
I am really impressed with how you have this little boat sailing, well done!
Some more detail might help.
The angled keel provides some food for thought on what happens to the CLR (centre of lateral resistance) as the boat heels. Wondering if this may contribute to your tacking difficulties… When the boat is sailing at 45 degrees to the wind with some heel the CLR is well back. As you tack, the boat slows, the fin goes deeper and CLR moves forward (and thus the tack pivot point). Combined with pushing 3 bows through the wind and the small mass, there is no doubt it would be hard to tack.
Even on a monohull (I have a 500mm Discovery) I find that is tricky to get through the wind on a tack and that is heavier than your boat I would think… More ballast helps with tacking, by providing more (low down) inertia to keep the boat moving through the maneuver. With the tri, you have comparatively very little mass so it will be somewhere between hard and impossible… maybe try temporarily adding an 80 gram fishing sinker on the bottom of the fin to see the effect?
Another observation - you may be able to move to a smaller winch servo using an arm winch (with double haul if necessary to get enough travel) - should finish up lighter with quicker sail changes… The drum type servo would be far more power than needed I would think…
All guesswork I am afraid, but may help…
mrpenguin,
You beat me to it. The next thing I am planning to test is the keel. The shape, position and/or size may be adding to the problems with tacking. Something I haven’t noted here is that I seem to need an oversized rudder to control the boat properly. I imagine that the hull shape, lack of buoyancy in the floats and the keel could all be causing this.
I’ll have to think about the CLR issues you raise. The keel may well be too far forward (a consequence of the size constraints). Unfortunately with the current boat I can’t do too much about it. Something to think about for the next design.
Jim.
A brief description of the build. Note: this is a lightweight build for testing, I’m not sure how it will stand test of time.
I used the carbon/epoxy over EPS method. I went straight for carbon as I expected that weight would be an issue. The build was a bit fiddly. The two-hatch approach described above required several steps to implement.
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Created the hull shape in foam;
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Covered the bottom of the hull up to slightly above the edge of the deck) in carbon/epoxy – 1 layer of carbon. I use a technique I was taught for fixing kayaks – stretching plastic sheet (I use plastic bags) over the carbon epoxy layer and holding in place and applying tension with tape. Bubbles and excess resin can be worked to the edge by gently pushing on the plastic. Not quite as effective as vacuum bagging, but produces a nice finish, much like a mould, that doesn’t need painting.
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With the hull set, remove plastic and tape and clean up the deck (remove excess carbon and set resin and make smooth again). Then stretch cling wrap over the deck and hold in place with tape. Lay the carbon/epoxy (1 layer of carbon) on the deck and ca. 10 mm over the carbon layer on the hull. Cover again with plastic and tension and hold in place with tape and remove bubbles and excess resin.
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Once set, add another layer of carbon/epoxy (2 layers of carbon) over the area where the main hatch will be. Cover again with plastic and tension and hold in place with tape and remove bubbles and excess resin.
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Once set, prise the two layers apart, and off the boat. Trim the deck and hatch cover to the required size. Cut hole in deck for hatch.
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Carve out holes for the electronics and space for the sheeting system in the foam, leaving as much foam as possible (much of the strength of the boat comes from the foam). Seal the foam surfaces with epoxy.
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I used the method Siri has described from making the keel and the sheath for the keel. A hole was cut through the hull and into the foam and it was glued in place using epoxy.
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Cut a second hatch in the main hatch for the batteries.
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Glue the deck in place, again using epoxy.
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The main hatch cover is fixed in place with tape (good electrical tape seems to work fine). The second hatch is sealed using a piece of wide tape.
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Sheeting cords are threaded through hoes in the main hatch (I drilled a hole in the hatch and I glued in place a piece of Teflon and drilled two holes through it).
Most of the parts for the rig were relatively inexpensive micro magic spares. Nothing else particularly different or interesting, other than what may well be a novel use of Lego (courtesy of my kids):
Detailed plans of the hull and floats etc to come (may be some time).
Here is a plan of the main hull. It is a bit rough (first attempt to use DELFTship), but shows the basic hull shape (drawn at 10x the real scale).
At first glance I’d suggest the deep V is probably not helping your tacking much.
These type of hullforms sit a lot lower in the water for a given displacement and you’ve got to drag all that through the water every time you tack. U sections would be a better shout.
I only use V’s in the bows of my tris and even then not much on the main hull!
As noted above - from front beam back (as rule of thumb) use the U" shape or the needed portion of an “O” shape (below water) to allow the hull to “slip sideways” when tacking. This is a non-nautical term, but place yourself just below the surface of the water looking at the side view of the portion of your hull under water. The hull portion under water should create very little lateral resistance until the freeboard (flat) side of the hull is pressed underwater to add to lateral resistance. Back to your underwater view, even though you will see some rocker below the water, your primary lateral resistance is created by the daggerboards (or keel) and the rudders. Once you begin to turn, your rudder is following the arc of the turn that your stern creates (loss of lateral resistance), and you are (in essence) spinning on your daggerboard or keel, and thus want very little resistance to this turn/tacking change of direction by the lengths of your hulls.
I would offer the following link from the “good old days” when multihulls were just coming of age. A splendid article, and be sure to note the comments about deep V shaped hulls and inability to easily change course. Note they used sail trim to make changes of direction, once the main was “loaded” up. This on a cat sailing up near the 30 mph range at the time. Enjoy.
http://setsail.com/multihull-history-how-cruising-started-for-us/
Dick
Thanks for the suggestions, and Dick thanks for the link to the article. I enjoyed it both for the boats and the hairstyles. It must have felt like a new frontier.
Below is a modified main hull design with a U-shape. Any suggestions?
For the new design I’d like to free up space in the main hull. I’m considering a couple of options: to use two rudders, one on each float; to have boards on the floats but not the main hull. Any thoughts on what would be the likely consequences of each of these options?
I’ve had a go at making a wing sail for the boat. They turn out to be more accessible than I had imagined. Taking advantage of the size of the boat I was able to use materials and techniques typically used for simple model planes. The structure is made of 6mm and 3 mm depron foam sheet and a few carbon tubes of various sizes. The whole thing is basically stuck together using double and single sided tapes that I purchased for making conventional sails. It is a bit crude, but only took ca. 4 hours to put together. I’ve taken it out in very light winds and it sails surprisingly well. It is going to take some practice as it is quite different to sailing with conventional sails. In the current configuration I don’t have a dedicated servo for the flaps. Their position is controlled by the same sail winch as the main wing, consequently the angle between the two is fixed (and varies slightly for different positions of the main wing). I will definitely be leaving room for another servo in the next design.
Some footage below:
To finish this thread I thought that it might be useful to reflect on what I learnt building and testing this boat. To begin with the following are what I perceived to be the strengths and weaknesses of the boat:
Strengths:
Fairly resistant to pitchpoling - Before building the boat I read a lot about the tendency of small trimarans to pitchpole, but I was pleasantly surprised to find this boat to be fairly stable. That is not to say that I didn’t spend a reasonable amount of time paddling out to turn it back over, but from what I have seen of other larger rc trimarans, it didn’t seem to be any less stable.
Fast - For such a small boat it seemed to be quite fast. Having said this, the one time I sailed on the same water as an IOM it looked like it was standing still.
Portable - It fitted easily rigged in the back of my station wagon or in the front seat.
Weaknesses:
Didn’t tack well - Eventually I gave up trying to tack, it was simply not reliable enough. Clearly my technique wasn’t particularly good to start with, but I think that this problem mostly came down to a design flaw.
Wouldn’t fly the main hull - Not once.
A bit tricky working on the internals - This became particularly difficult when I was trying to fit 3 servos in.
Final assessment:
On reflection I think (and others have suggested) that I made two major design errors: insufficient volume in the floats, and V-shaped main hull. I also think that the boat was a little on the heavy side for its length. I do wonder if the weight was somewhat responsible for the stability of the boat. As one final test I removed all the electronics to test the boat with less weight (ca. 400 g). It was significantly less stable, but still didn’t appear to be light enough to fly the main hull, although it got close (see footage below):
[video=youtube_share;qk2iTrT9M30]http://youtu.be/qk2iTrT9M30[/video]
I will be making a larger trimaran next, probably a 65M, as I would like to be able to fit more servos. However, I do wonder if there is a place for a smaller class of multihulls?
Spot the similarities: http://www.thundertiger-europe.com/index.php/katalog/artikelinfo/13440-1-show-tri-maran_volans_abs_rumpf_lackiert_baukasten.html
Apparently I’m not the only one to make this mistake:
http://www.thundertiger-europe.com/img_big/905892f069342ad5968dffb024f1383d.jpg
