outrigger placement relative to main hull

I am building a small tri. Basically I am building the main hull from one of Gary Dierking plans, but I am turning the proa into a trimaran.

I am trying to determine the best placement of both outriggers relative to the main hull. Attached is a drawing that shows what I believe would be acceptable, but I would just like to get some other input. As you can see the center of gravity of both outriggers is forward from the C.G. of the main hull which I believe is the best placement, but is there a scantling for the placement of the outriggers?

Also the outriggers are not 160% volume of the main hull. More like 90%.

Any advice would be appreciated.

 

sorry here is the file

 

Looks Fine

Looks Ok, give it a try. You can always address tripping when the time comes. What are you using for a sail plan?

Delane

 

If it were a proa maybe staying in the original placement works, that is to say if there weren't anything wrong with it. Just additonal one more and it's a tri.. after all the other outrigger is supposed to fly..

 

Proas shunt to change tacks. They need to be symetrical

 

your layout looks quite good

90 percent outriggers ought be fine as long as you sail conservatively. You can use the 'bankas' of the Philippines as inspiration, in that they use bamboo as outriggers and thus have less than 90 percent volume.

I might suggest that the forward crossbeam is a little far forward. When I took my boat out in rough weather, the forward crossbeam (which is also far forward) hit a wave and slowed the boat down.

My 2 cents worth would be low move the front crossbeam 18" aft, and increase volume of buoyancy compartments. For background, here is something that is similar to what you propose, this is a dory styled tri and a bit beamier, they used 3 crossbeams.

http://www.tacking-outrigger.com/red_tri.html

Remember that when fully loaded the bottom of each outrigger should just touch the water. If they are too low then the outriggers will take all the weight and the mainhull will be supported in mid air (to a degree at least) and this will increase stresses

 

Thanks guys. Peter that is some good info, I will take that into consideration once I determine the final location for the crossbeams.

When you say 90% outrigger displacement versus the main hull displacement, at what loading is the main hull displacement determined? Is the main hull displacement determined with full gear, including people, or just bare bones, no gear just boat?

 

The value is based on loaded displacement.

So a high performance boat might have 200 percent volume outriggers, this way the entire weight of the boat rests on the outrigger with volume to spare, with the other 2 hulls in the air. Of course when this happens you obviously need a lot more than 100 percent volume, otherwise the outrigger would be almost totally submerged and with no reserve bow buoyancy it would trip.

Your boat, say hull is 120kg, crew 120kg and gear at 50kg, gives a total of about 300kg, Now nine tenths of 300 is 270kg, or 270L volume. This may be a bit high and 90 percent may be too high for you. However if you know the numbers then you can make decisions.

I have a single outrigger on my boat, and it is 220L. For my purposes I have found it to be very good. It is 14ft long, deep V in section, a lot of rocker, and is from memory about 40cm wide and 40cm deep at midsection.

So this gives you a rough guide to dimensions. Looking at your sketches your outriggers look longer, and in the center there is a straight section of several feet.

I just had a play with the software. A 16ft outrigger with a width of 30cm a depth of 30cm, with the center 7ft straight, and with a chine (not a deep V, but with a chine so that a midesction V is created) have a volume of approx 220L

What do I mean by a chine. Well the top would be 30cm above the base of the centerline and 15cm offset. The chine is an extra point which is 10.5cm from the centerline and 8cm above the base. If you plot this out you kinda get the shape.

A deep V would have less volume of course.

When you sail, if you are have lots of wind, more and more weight will go to the outrigger, If it is too small the outrigger will submerge. By limiting the volume you can limit the stresses and reduce structure weight. So it is a balance between the plusses of a big outrigger and a smaller outrigger.

If you are able to move your weight out to windward with a hiking seat, obviously you can go faster with less weight on the outrigger

I can throw up a 30cm wide and 30cm deep outrigger for you on the computer and give you the offsets if you are interested. I am off to the snow from Thursday to Sunday (winter here) but before of after is no stress, it only takes 20 mins or so to produce a hullshape.

I think Gary Deirking has pretty small volume outriggers, at a guess maybe 130L, I would have to have another look in his book (its on the shelf).

regards
N Peter Evans

 

Hull design software

Firstly, many thanks for the info about the size of outriggers, their placement and volume displacement. I must admit this is an issue I've been thinking about with my current design project ( http://outriggeryacht.blogspot.com.au/ ) and the possibilities of tripping on the ama when the outrigger is on the leeward side--a problem all trimarans seem to have when being driven hard in heavy weather.

Peter, I see you mentioned some software for hull design and would be interested to know just what that is and if there any alternatives. I was planning to use offsets proportional to the length and frame placement in the hulls and then draw them up manually. Software would be so much better and perhaps more accurate as well.
Any advice you could give me in this regard would be most appreciated.

Best,

Scot.

 

Hi Erik, normally the center of buoyancy of the float is placed forward from the C of B of the main hull.

90% does not seem to be a problem in a small boat as you can shift your weight to increase stability when it is needed.