Post your design ideas

I have reworked my 4.5m tri a bit. If/when i build another boat, then it should be strip planked becuase i have done that before. It is easy, fun and goes rather fast ( if you are careful when cutting the strips) And it alow way more pleasant form of the final hull.

Dimenasions: 4.5 x 3.4m
Disp: 200kg
1 person sitting inside the aka just behind the beam/wing, steering with a steering wheel and not pedals (you can steer whith your knee just like in a car when you need both hands). Mast positon some where in the middle of the beam/wing.

This picture show a early sketch i made for this boat before starting with delf. Think it dispay some thoughts about rudder, centreboard and sails.


Critics and comments are more than welcome
/Brorsan

 

I like the center hull shape; it's very organic. Have you sailed in a boat like the one you are designing, like a wind rider for example?

 

I don't think Brorsan's design is very similar to a Windrider, except for size. The Windriders have a molded-in minikeel that is not very efficient to windward. Even my heavy little beachcruising cat, with her old-fashioned sprit rig is faster to windward than a Windrider 17.

 

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Very good looking shape, Brorsan! Would be nice to see more numbers like L/B for main hull and ama, total buoyancy of ama, etc.

 

I think Rapscallion refer to a small, low boat (more like a kayak) where you sit inside the mainhul facing forward. And the answer is no, unfortunaly. I'll try to get a ride on one this summer.

Thank you Doug, here are your numbers

Mainhull:
L/B: 1:9 (4.5m x 0.5m bwl)
Wet area: 2.3 sqm (mainhull only)
LCF: 1.93m from the stern, LCB at same distance

Amas
L/B: 1:10.7 (3m x 0.28m)
volume: 200 litre (100%) 160litres submerged useble.

The L/B ratio of the mainhull is a little bit low, but it also give me some soom for misscalculations, a very narrow vessel will be more sensible to trim. And it should be posible to bring a passanger and some gear for a daytrip without changing the trim and draft toooo much. I could ofcourse extend the boat to 5m, but that is more material, more weight, more wetted surface and more space required when parked on land. Comments on this are more than welcome.

Acutaly, maybe 4,8m x 0.43m would be better, that would give a L/B of over 1:11 with only 0.1 smw added wet area to a total of 2.4 for the mainhull...

/Brorsan

 

If it is important to you to have a single free standing mast and not to block the view from the forward cabin windows with mast supports, I reckon that an asymetrical arrangment with the mast in one hull should be a good way to go. Sketching the vectors for the wind and water forces acting in the horrizontal plane on each tack, it would seem that the effect of this asymetry is much the same as running one or other of the engines singly in a catamaran that has an engine and propellor for each hull. I believe that some sailors make this normal practice to save on fuel and presumably the rudders are able to counteract any turning effect with fairly minimal additional drag. My comment in an earlier post about increased heeling moment as you start to heel was with regard to the wind sheer - higher wind higher up - but this really is a minor secondary order effect.

I have looked on the internet to find some pictures of Roxanne/Romily and from what I see it does not look to me as though these boats have full length battens in the lug sails. I just wonder how full length battens would work in a sail that does not have a mast to support the luff. In a conventional sail with full length battens, I think the battens are compressed against the mast and this is largely what induces and controls the camber in the forward part of the battens. With a lug sail that has no mast at the luff I am not sure how you could compress the battens. Perhaps if you have a 'roach' (forwards curve) on the luff, so that tension in the luff provides a compression on the forward end of the battens that would work, but I can imagine that it might need some trial and error work in the sail making to get it right.
John

 

John,

There is that possibility where the drive from the sail is as you describe. That is truly part of what I'd like to know. With the advent of more bi-plane rigs these days, it seems that someone should be able to confirm whether this effect is noticeable when using only the windward or leeward sail. I have an old design book from circa 1906 that described weather helm as basically being visualized by considering the pull or load on the mainsheet as acting on the aft quarter of the boat and essentially trying to 'pull' it around, i.e. weather helm. We now tend to describe it as being a result of the assymetry of the hull when heeled and forces on the hollow bows 'shoving' the bow to windward.

On a cat, the assymetry while heeled is minimal to non-existent and the narrow hulls wouldn't experience the 'shoving' effect. That just leaves the pull of the mainsheet (I'll call it effect as I'm not sure it's an apt description). A higher aspect main with a short boom (such as I've drawn)might then have less effect than perhaps a lower aspect main sheeting further aft.

Currently, this is the biggest questionmark for the suitability of this setup. (btw, I'm aware that it isn't likely to be quite that simple, but it describes the effect even if the cause is elsewhere)

With regard to the battens. The sail might only have 2 or 3 near the appropriate reef points, not fully battened all the way up. They would likely be a bit more flexible than a customary full battened main but would not be tensioned up the same way. I would consider them more to reduce flogging than as either enabling mainsail roach or inducing a specified camber-- that would come from the cut of the sail, not the batten.

I'm hoping Someone Who Knows might be able to shed a little light on these issues from a practical experience viewpoint. The only other option is to build a 1/3 to 1/2 scale working model and I have neither the time nor resources to go that far before even getting to the larger boat.

 

Perhaps also the reason that successful new ideas in sailing are often originated by amateurs then later refined by professionals.

I have great respect for your extensive sailing experience, as well as your successful design work. However, I would say, with tongue half in cheek, that space craft are not designed by astronauts, planes are not designed by pilots, trains are not designed by engine drivers, factories are not designed by factory workers. I suppose houses are designed by house dwellers and cars by car drivers, but it could hardly be otherwise. I have designed all sorts of things over the years and if I am lucky I might get the chance to discuss the design with an end user(s), but quite often the constraints of business do not allow even that. It seems that designers are expected to be good at guessing what will work for the end user!

I think your comments may be directed to Dryfeet since I am not in the PNW, but I do realise that I could do with gaining some experience of sailing on a multihull before seriously considering building one for myself. I have an invite to sail on a cruising trimaran sometime this summer, that should be a start.

John

 

Dryfeet, you might want to read what Thomas Firth Jones has to say about his experience with a biplane rig on his last little cruising catamaran. I won't reprise it here, but it had many problems, after a year or two, he converted the boat to a conventional sloop. One problem was that with only one sail up, he said the boat was "uncontrollable."

On the other hand, tacking outrigger canoes seem to do fine.

 

Ray,
I have TFJ's books and know what you're referencing. I suspect that the issues went beyond just the rig. Also, he was very unhappy with the blanketing aspects of the bi-plane rig. That design may have had more than one issue leading to his dissatisfaction.

:idea:Maybe you'd consider putting Slider's rig in one hull and testing the concept for me?????

 

I'm trying to move on. In fact, I've thought about selling Slider, but my wife won't hear of it.

 

I recall Steve Brown saying that he had a friend who had built a small tri with 2 windsurfer rigs, biplane style. Around the time that trifoiler had that kind of rig, though not to suggest it was drawn from there, just that general idea. He said it was very practical for cruising. In that case I think it was more a cheapness deal. That said, a fractional rig is cheap and very practical.

 

"I want the weight of the boat to be no more than 1500 lbs. Considering the L7 is less than 1400 lbs I think it's achievable."

Considering the Kurt Hughes 24 is 800 point in underlayment, it sure as heck should be.

" I was hoping for 200% buoyancy in the floats, but I have to reduce the volume in the mail hull first and put it in the floats."

The percents are a very rough measures, where the volume is the shape are more important.

"NO SEASTAYS!!"

Seastays are a huge way to save weight. It is basically the same issue as a rig, a lot more money and or weight to do away with them. Of course you can do away with them. You can also consider different riggings for different uses.


"width will be 8'6" when "folded" and about 18' when fully retracted."

Be careful, 8'6" is a lot of width for highway trailering. Be sure it is an easy sell all the places you are going. One of the reason I am building a Kendrick 18 is because it is under 7 feet on the trailer.

I think a hughes would be cheaper to build than what you have in mind and a lot faster, and more salable. saving 600 pounds is like 3-4K materials, nominally, and a lot of performance, and his hull shape is way better but he doesn't currently make a model with decent folding or accom. You would sure blow away F boats.

 

I have several study plans from kurt, I also have his DVD on cylinder molding and his plan book. Kurt is very good at designing light fast boats on the cheap.

But I want a trailer sailor. A boat parked in my garage, not in a slip. and a fast setup. I already have a 30' sailboat in a slip.. and I don't like it.

The L7 slider system is cheap... and the big floats give you a dry ride for the size and type of boat... or so I'm told. I have never sailed on one. But I have sailed on a trimaran with seastays on lake michigan before.... and I'm not getting a tri with sea stays. The extra weight is a small price to pay when compared to the drag caused by the spray.... that is like a firehose... coming from those sea stays at speed. I wore goggles over my eyes in the cockpit of this 27' trimaran... because I needed them.

basically the L7 is the closest design to my wish list... I want to build an L7 type boat on a seaclipper 24 budget. The plans are coming along... the center hull looks more like sardine run now... scantlings are 6mm ply, 9oz on the outside, 6oz on the inside... and it is looking like I will easily be below 11oo lbs, most likely under 1000.

 

I was going to recommend you look at the Trinado, but I see it has the waterstays. (I think I share your aversion to them for a variety of reasons)

http://www.biekerboats.com/Bieker_Boats/Trinado.html

http://www.biekerboats.com/Bieker_Boats/Trinado.html