I’ve uploaded some footage of trials of a small prototype trimaran:
being somewhat smaller than even an RG65M (multihull at 65 cm) it seems to built too light to carry it through a tack. Very early in the posted video, it comes to a complete stop, then drifts sideways. Some of the problem, is during the tack, sails (jib and main) seemed to be close hauled. Once the bows flopped to the other side, letting the sail out would improve lift and power and once sailing on the new tack, sails can be re-trimmed. Secondly, because of size the lack of weight provides little inertia to allow the boat to move through the tack - and being light it simply “wind-vanes” into the wind. Since it doesn’t tack well, you had to resort to gybing to change directions. This works for day sailing, but not racing.
Since I couldn’t see the board and rudder, it was hard to tell - but as an experiment, tape a small screwdriver (handle first) to bottom of board (keel) to add a bit of weight. Weight down low will improve stability (tipping) a bit, and may help in tacking (if board and center of effort of sails are in correct relationship. You could also tape a pair of pliers on top of hull slightly aft to see if weight improves the sailing ability. Speaking of board/CoE relationship, it is possible the location of mast is causing the boat to go into “irons” so you may want to look at that possibility as well.
Good luck with the improvements, as the boat shows some speed once it is dialed in on a tack.
Thanks for the advice Dick, I’ll try you suggestions. The boat currently does not tack well, perhaps only tacking successfully ca. 20% of the times attempted. As the boat is small the electronics are a significant percentage of the weight, so I tried to build it as light as possible. Being new to boat design I didn’t consider the consequence for tacking.
You are going to have to define “irons” for me.
[irons] Heading into the wind, the sails lose power as wind is on both sides of the sails … basically becoming a wind-vane. When tacking, a slower large turn will help, as trying to tack fast (like a monohull) will result in braking since the bows of all three hulls must pass through the wind to complete the tack. Going into “irons” often takes place during a tack, or if trying to sail too “high” or close to the wind. Multihulls, unlike their single hulled brothers, prefer to sail a bit lower (but faster) and generally don’t point as high. It looks from the outside, that even the 48 inch multihulls are finding there is a tradeoff in weight when it starts to blow hard. That said, you did have waves to contend with as well. Final thought - maybe enlarge your rudder area a bit - which will increase the brake effect if turned too fast, but on the positive, keeping the water moving over a larger rudder surface while turning will help in tacking - just try tacking slower and with a bigger radius turn.
If you want to tack faster, you would have to have a method to hold the jib to windward as you tack to help push the bows over and through the wind, then releasing the jib when on the new tack. Just a few things to try. If you go real heavy with weight and find it tacks better, then gradually reduce weight until you see a negative performance - assuming your mast and keel locations are close to correct.
Best regards, Dick
I’ve started by looking at your questions regarding CoE and keel location. I roughly measured the position of CLR (in a bath) and then adjusted the mast position (as far forward as possible with the current configuration), and rake. The following image summarises the change (and gives you a view of the keel and rudder).
I’ve tested with the modifications in light winds (and tried to improve my technique). Tacking is still unreliable, but better than before. A couple of clips can be viewed from the link below:
Next I will test you suggestion regarding the larger rudder. I’m hoping that this will also improve some problems with control.
More video, this time with an oversized rudder. tacking a little better, but still unreliable.
Next, some more weight.
At around the 2:35 mark (as I recall) it looks like you released the main sheet a bit which allowed the tack to be completed. In other views earlier in video, the mainsail looks to be fairly tight to center line of boat, causing the entire thing to “wind vane”. Hard to tell, but if my guess is correct, as you tack and the bow comes across the wind (remember you have 3 bows - not 1) release the main and jib and continue to fall off a bit to fill sails and pick up speed, then slowly sheet back in when on the next tack. Also - remember to tack slowly, allowing the boat to turn — Unlike a monohull where you can simply jamb the rudder over. Regards
I’ve put trying to improve the tacking on hold and instead focused on improving longitudinal stability in stronger winds. Some footage of testing with foils:
For the size of the trimaran, it is sailing very well in “very strong” breeze and “very rough” water.
Gradually, the boat has improved as you have put foils on it.
The main problem I see is the buoyancy of the outside hulls is not enough to (almost - at least) float the whole boat. It tends to dig in the leeward hull deep under water - creating a high level of drag - which it then “trips over”. Foils with a positive angle are helping to reduce this problem.
I think more buoyancy in the outriggers would help - but for the size of the boat, you are doing very
Thanks for the info, keep it coming!
Thanks for the observations. I’ve slowly been coming to the same conclusion, although I’ve been thinking about it a little differently. I think that the boat is too heavy for its length. One consequence is that “the buoyancy of the outside hulls is not enough”. Another consequence is that the main hull sits low in the water, thus creating a relatively high level of drag for the length of the boat. I think that this may be part of the reason that I am having trouble with tacking.
The foils do appear to improve the performance, although the rudder T-foil alone appeared to exacerbate the problem of the low buoyancy of the outriggers. With foils on the outriggers the stability downwind improved significantly. I kept the foils short so that the foil on the windward outrigger is out of the water and thus not creating unnecessary drag. However, I might try longer foils to see whether they improve the performance further.
I am quite pleased with the way the current boat is performing, although at some stage I intend to take what I have learnt from this boat and try to design another boat that performs better. I think that it is beyond my skill to make a lighter boat of the same size, so I will either need to make a slightly bigger boat or modify the design to handle the heigh weight/length ratio. Currently I’m not sure which option to pursue, I like the idea of a very small tri, but I want the boat to tack properly and fly on one hull. For the moment I will continue to work on the current boat to get as much info from it as I can.
I have a question for those with an understanding of the factors that determine the performance of trimarans – For the trimaran that I am working on I initially positioned the stern of the amas above the main hull in an effort to minimize pitch poling (see picture below - upper image). My theory was that when sailing on two hulls this would tend to lift the bow of the main hull. The boat does seem to be fairly resistant to pitch poling, but I can’t be sure that this is the reason. Can anyone either confirm or refute this theory? What I’m now wondering is what other effects this may have on the behavior of the boat. It certainly makes it very difficult for the boat to sail on one hull, but I’m also wondering if there are other effects? A word of caution, I’ve now lowered the stern of the amas, as shown in the image below, so any theories proposed will (wind dependent) be tested on the weekend.
I think what’s happening with the outrigger is as the boat is loading up, and the trim is being controlled by the mainhull it’s pushing the nose of the floats under. The resulting drag from this is whats stopping you flying the main hull. I set the floats so that both are clear with all the weight on the mainhull. Much less and you start clipping waves, much more and the boat heels loads before the mainhull wants to fly and it doesn’t give you much time to work with once up on one hull.
How big is this boat exactly and how heavy? I really liked the idea of small tri’s when I started playing with them but as you scale down you start fighting a losing battle against radio gear and they start looking a bit odd. Looks like a tidy job though, so good effort.
Thanks for the suggestions. In answer to your question, the size of the trimaran is:
Length: 400 mm
Weight: 520 g
I have had the same battle with radio gear. In this boat the radio gear and batteries are ca. 40% of the weight of the boat.
Compared to your setup, in my current configuration I may have brought the amas a bit too low as they just float. How do you position the amas relative to the main hull in the horizontal plane?
The design waterlines of the floats should be parallel to the main hull really. Yours look very bow down from the side on pictures!
Yes, I wanted them angled down but overdid it. I have now lowered the stern of the amas so they are now close to parallel with the main hull (lower image in Post#11). It will be interesting to see what effect this has on the performance.
Here is a short video of trying to fly the main hull. Is this something that can be fixed with foils, or, as Jon suggests, do I need larger amas?
Full clip below:
The pre-foil rule of thumb was that you needed 200% of the displacement of the boat in volume in the float to carry enough weight to really push it.
Foils will help, but bearing in mind they only really start working at 4+knots that will be a struggle with a boat this size…
You’re gonna need a bigger float!
I would tend to agree - since the main hull, electronics, batteries, 1/2 of cross beams weight and mast/sail all have a weight to them. If you want the main hull out of water, then the floats would have to support themselves AND the main hull weight. Once you get the main hull out of the water, unless the rudder is long enough you will have a loss in steering. A longer rudder for steering will increase drag, meaning your boat will go slower. The “fun” of self-design - fix one thing and something else pops up ! :rolleyes:
Aardvarkissues and Dick
Thanks for the advice. It looks like it is time to start thinking about a new design. I intend to continue working at a similar size, although perhaps 50 mm longer (I would like a little more room for the electronics, which I will probably keep the same). I’m working at this scale partly for the challenge and partly because it fits in my car fully rigged.
I have a few more things to test on the current boat, in particular I would like to understand why it doesn’t tack properly. I intend to post some details on the build and design of the current boat in the hope that you (and others) can give me some advice for the next design.
Tacking is almost always down to poor pointing ability and that’s entirely drag related. If your outriggers are half under water you won’t be pointing that high. You also then have a draggy float on the outside of the turn.
With little boats loads of rocker helps too…