Wide-hulled trimaran?

If you can make do with less build a trimaran if you require more load carrying build a cat. One of my friends did an inventory with his wife and kids of what they had to carry for their comfort and enjoyment while cruising he ended up building a catamaran. The interesting thing is when he did a list just for himself and his wife the numbers resolved that the trimaran load carrying ability would have been more than adequate. Others may disagree but from a cruising perspective to me the arguments for the cat become very compelling once you can have a good saloon and cantilever berths while keeping reasonable fineness ratios for the hulls, prior to that point it's pretty even. Also do you want to park your boat in a marina space or on a mooring? Trimarans are generally a fair bit wider for their length which can effect your marina fees drastically.

I think something a lot of people dont take into consideration is the extra interior fitout a cat requires. Often builders get the cat hullshell up fast but spend many more years getting the interior finished. If you build a trimaran there is less interior to finish you dont need to fair the inside of the floats for example just the cabin areas and furniture thats visible in the main hull.

Unless your a tri enthusiast (like me) the cat option is pretty solid at about 35' or over up to that point it comes down to how you feel about the space. On a trimaran with a hull flare there is quite a bit of room at waist to shoulder level certainly more than most equivalent cats but not much room on the cabin sole. Some designers have taken the flare concept further for example Kurt Hughes "double shuffle" trimaran main hull sides geared towards providing a thin waterline but good internal space (it doesn't boost load carrying though, just space). John Shuttleworth also did some interesting flared cat hull designs that offered a good balance of performance and space in a small package.

I do agree with ThomD that it's a pity that the trimaran gets dismissed from a workhorse cruiser perspective. A cruising trimaran can fulfil that role pretty well if your willing to give up on any sailing performance pretension which in reality is what a lot of the current production catamarans do anyway ie if you dont have a moderate to strong breeze your hardly moving. The eye opener for me was a charter cat (Lagoon 400 I think it was) we went out to the Low Isles from Port Douglas on. In a moderate to strong breeze (it would have been 25 knots) on a beam reach that would have seen any of the trimarans or performance cats in our club doing at least double digits we were plodding along at 6 sometimes touching on 7 knots. I'm not bagging that boat out in particular but just pointing out that production cruising cats in general are not even vaguely performance focussed.

 

The picture in 39 shows a hard chine and a possible oval section to the immersed hull. There is an approach to multihull design that is rarely used but is at least interesting. Phil Bolger did a few designs and at least one was built. Start by thinking of a Wharram hull with a classic 60 degree section. Some advantages as we all know, very poor surface area to displacement numbers, and poor displacement to each added immersion inches. Nice smooth surface from only 2 panels. Bolgers approach was to go for a 120 degree hull, for the same displacement. The 120 degree hull is all you get bellow the water, and somewhat above, but beyond that you can shape it as you wish. You suffer the same surface area penalty. You get the same smooth 2 panel surface, but you get much more beam, and much more displacement for each further inch immersed. You get in a simple form a knuckle and large plan area to live in.

This tri main hull is similar, you get some drag from the oval section, but you get a lot of living space. You get good additional displacement per inch, at least for temporary loading when tied up. Just an interesting approach. But it should not be loaded to the square of it's increased beam or it will be a tug, or possibly need a tug to get moving.

There is a version of this kind of deal for all shapes. So for the two panel you have piver 90 degree hulls, lowest wetted surface areas; you have Wharram for the accute, and you have Bolger for the 120.

But the same holds true with circular forms, you have semi circular forms for the lowest wetted; you have eliptical forms for a slightly deeper hull, and you have elipses on the horizontal for more plan area. Same with any shape, whether it has been built or not.

Keep in mind for the purpose of the thought experiment, all these hulls have the same displacement, all that varies is the proportions of the immersed form.

 

Certainly one of the downsides of trimarans is their extreme width. On this trimaran the float beams appear to pivot at the hulls. I wonder if they planned to raise the floats to get in narrow berths, or perhaps when hauling. I wonder if the sea stays are actually hydraulic cylinders, but if so how would the mast be supported when the shrouds were slack?

 

Remember, as the weight and load of a tri goes up less beam is needed for stability. Review your tristars, Browns, Cross designs etc....
In Europe I would think getting the 16 foot beam for the French Canals would be the goal for folding.

 

The F39 can carry 3500lbs still short of your 4000lbs minimum weight factor but reasonably close and just 4ft longer than your 35' target. It can fold for short term marina berthing and canal lock transit and demount for transport, It would still offer decent performance as well. It's a tough SOR to design a decent performing trimaran for at short lengths thats for sure.

http://www.f-boat.com/pages/trimarans/F-39.html

Ed Horstmans site only lists displacements but you would assume by looking at the hullshapes that the boats could carry quite a load. Maybe Rberry will chime in with some payload figures on his Tristar 31 project.

 

Normally a tri that folds would have baby stays on the mast attached to the main hull to support it in position while folded. Hydraulic cylinders for seastays seems like a high level of complication and potential problems the dragonflys use a winch and line to pull the floats in and out and I'd imagine the posted design would be similar. You still pay more though when they pivot because you pay for length in Marina's as well.

 

Cavalier, in general you're correct. Can you explain why that is so?

.................length x width – ratio
Tri-Star 27 – 27-9 x 17-9 – 1.56
Tri-Star 35 – 35-0 x 20-6 – 1.71
Tri-Star 42 – 42-0 x 24-4 – 1.73
Tri-Star 50 – 50-0 x 26-8 – 1.88
Tri-Star 80 – 80.0 x 47-6 – 1.68

SeaRunner 25 25-0 – 16-7 – 1.51
SeaRunner 34 34-4 – 20-11 – 1.64
SeaRunner 40 40-10 – 23-11 – 1.7

 

Righting moment = the weight of the boat by the length of the righting lever heavier boats require less beam to achieve the same righting moment of a lighter boat.

 

I cant say right off hand what the usefull load of the 31 is Corley, but I think it,s about 40% of disp. which disp. for the 31 is 5200 lbs. I will be building lite and trying for a 250 lb weight savings,so if I can bring it in at a 2300 lb usefull payload I will be happy. From what I,ve read on the tristars you can load them way over the designed disp. A picture of the 27'9" on his site has maybe 10 people on deck, using the 150 lb per person rule thats 1500 lbs on the deck of a 3500 lb disp. boat. The 31 is designed to do short trips with as many as six or cruise with 2 or 3. I will have enough designed payload on the 31 for my needs and it should sail as the designer intended if I keep the boat at disp. or below. I would have looked at cats if I were building 35' or up, and I did look hard at Richard woods cats in the low 30' range. At some point you have to look at cost per pound of disp. and how much water line you get for that cost. What is the size range of a cruising cat that comes in at a 5200 lb disp.? This is the size range that the older cruising tris beat out the smaller cats with the same intended use and disp. Rick

 

Thanks for 40% payload estimate. Do you think it is the same ratio for cats?

Tri-Star 36 Cat = 5300# displacement
Tri-Star 36 Tri = 7300# displacement

 

The tristar 27'9" fits more into Ed,s XR or XRC designs so L/B ratio wont compare . The 31 may have replaced the 31XR and 32XR (4800 disp. ? ) in his stock plans,there are parts of the plans that seem to come from earlyer designs. The 31 comes in at 1.64 L/B . and less S/A per pound than the 32XR and 27'9". From the 35' up you get into the higher L/B ratio. Rick

 

The 36 cat is 7400 disp. and 3200 usefull load ,so about 43.5%. I would think the usefull load on his cats and cruising tri,s are not too far a part per pound of disp. Rick

 

The Tristar 40LW and 44LW both have usefull loads listed , the 40LW is almost 46% of disp. I think the listed disp. for the 44LW is wrong as Ed has redone his site resently. If the numbers are right for the 41LW then can a cruising cat really carry more payload than a cruising tri of the same disp. and can it out sail it ? Rick

 

Corley got the righting moment spot on.
Tristars are sort of like comparing 3 hulled cats with 2 hulled ones. The amas are immersed enough to help with the load and make a nice ride at anchor. Nicols are like this too.

 

You can't just go from one boat. Unless you have a cat that is comparable in approach.

It is unlikely if a similar boat is carrying a lot heavier load, that it will outsail the lighter, boat to boot. This is the classic tri/cat situation. The tri is hampered in carrying capacity by having one hull, and having that hull in addition carry the load of the the two other hulls. But while that cuts into the load, it means less drag, less water to move, and a wider platform to carry more sail. So traditionally trimarans outsailed cats. But there are many ways to improve performance of either type. It also depends on what you choose to compare, OAL, displacement, cost, sail carry etc... What is a fair basis for comparisons.