How big must a model be to be useful assessing a multi-hull design?

[mattwell]

I'm exploring a 16' trimaran design and would like to test out some ideas using models. I only expect to get some relative info. For example I might try longer vs. shorter amas, or add a tall curved bow to the amas (as seen on some asian multi-hulls). In each case I hope to get a sense for the tradeoffs involved by sailing the RC model on local lakes. Below what length do models start to be useless? I'd like to do a 16" long model for a quick and easy construction. My hunch is that I will learn a lot about the tradeoffs and I can make multiple models for comparison. Any bigger and the models become a project in of themselves. Opinions/thoughts?

NOTE: I did some searching on this but didn't find anything that quite addressed my question. Pointers to threads or articles on this topic would be great.

 

[BlueBell]

1/4 scale generally speaking.
But it shouldn't be necessary with a 16' displacement hull with ama's.
I've built several of these around 17'.
What is it you'd like to know?

 

[Will Gilmore]

In terms of performance vs efficiency, wave riding, turning control, sail area to heeling, and other area dependant functions, I would think you could get a general idea, but not an accurate 1:1 comparison. For instance, tiny boats tend to need deeper, heavier ballast to counter the heeling forces from a smaller proportional sail area.

If you want to know something like rigging configuration, or seating for that to-scale Stewart Little sailor, it shouldn't matter.

I would guess, but really have no idea, that CE to CLR would be pretty consistent across scale (esp. for determining helm weather), but then, you have to be very careful that you are actually measuring the forces and not the geometry.

There are certainly valuable pieces of information to be gained, but there is also a lot that can be misinterpreted.

-Will

 

[mattwell]

1/4 scale generally speaking.
But it shouldn't be necessary with a 16' displacement hull with ama's.
I've built several of these around 17'.
What is it you'd like to know?

One question I'm curious to explore is what happens if you move some weight to the amas? My design goal is a comfortable day sailing camping cruiser for flat and sometimes thin water. Moving the batteries to the ama would free up some space in the already tiny cabin. It was discussed here: What to do that Amas (Outriggers) aren't "dead space" for a Tri ? https://www.boatdesign.net/threads/what-to-do-that-amas-outriggers-arent-dead-space-for-a-tri.54139/. With a model you can move the weight to and from the amas and sail in various conditions and maybe get a feel for the impact. I'd like to move weight to the amas until the ama itself is neutral buoyancy and see what happens. I'd also like to experiment with taking up some of the weight in the ama by adding a stay to the aka. If the total weight remains the same but the amas are heavier the forces might be different but I'm not clear on why they would be dramatically larger. Anyhow, I was just hoping to get more of an intuitive sense for the impact of tradeoffs like this using RC models. I'll go for 2' model length and if I learn anything I'll report back.

 

[mattwell]

In terms of performance vs efficiency, wave riding, turning control, sail area to heeling, and other area dependant functions, I would think you could get a general idea, but not an accurate 1:1 comparison. For instance, tiny boats tend to need deeper, heavier ballast to counter the heeling forces from a smaller proportional sail area.

If you want to know something like rigging configuration, or seating for that to-scale Stewart Little sailor, it shouldn't matter.

I would guess, but really have no idea, that CE to CLR would be pretty consistent across scale (esp. for determining helm weather), but then, you have to be very careful that you are actually measuring the forces and not the geometry.

There are certainly valuable pieces of information to be gained, but there is also a lot that can be misinterpreted.

-Will

Thanks. A general idea is all I'm hoping for. I'm interested in all-else-equal relative experiments such as the ama weight I described in my response to bluebell. The total weight would be constant but the location would move. I hope that would give meaningful experiential feedback.

 

[seasquirt]

Hi mattwell, weighing down the amas means using heavier beams to support them, and they may become ploughs in flat water, and or submarines in any waves.

 

[mattwell]

Hi Seasquirt, I'm not sure I understand why the beams would need to beefier. I'll use numbers from one of my favorite designs, The W17 by Mike Waters, https://smalltridesign.com/pdfs/W17study-profile.pdf, for a rough static analysis. Let me know if I'm missing something, I'm not in any way a naval architect!

If you are able to get the middle hull out of the water and bury the ama the bending force on the aka would be ama displacement-ama weight times beam/2:

(300kg-50kg)*2m=500kg.m.

The force on the aka when the ama is out of the water would be ama weight*beam/2:

50kg*2m or 100kg.m

Moving 50kg of batteries from the main hull to the ama would reduce the force when the ama is submerged by 100kg.m:

(300kg-100kg)*2m=400kg.m (100kg.m less than 500kg.m)

The force on the aka when the ama is flying:

(50kg+50kg)*2m=200kg.m

I think it is likely that any beam designed to handle 500kg.m upward force would be easily able to handle the extra 100kg.m downward force from the batteries. In fact by moving weight out into the amas you could argue that the design strength required of the akas is reduced.

All that said I think the dynamic factors such as the rotational inertia might kill sailablity and I hope some testing with models would help to understand that impact.

 

[Skip Johnson]

Hopefully you will discover a fundamental truth about small lightweight craft . Putting as much weight as possible as far as possible from the center of the craft will make for far different behavior than the same craft with as much mass as possible centrally located.

 

[Dolfiman]

I think it is likely that any beam designed to handle 500kg.m upward force would be easily able to handle the extra 100kg.m downward force from the batteries.

It is not only a question of static efforts but also a dynamic mechanical one : the windward ama + its beam is a cantilever mounting which can have a low frequency of the first mode of vibration, which can be excited by the encounter frequency of the boat facing waves, which is hasardous. Pratically, with the link here below, 1) you can compute the frequency of the mounting without the extra weight, 2) you compute with adding the extra weight in the ama, and then you increase the inertia of the beam until you have the same frequency as before :
MecaTools : Premier mode propre d'une poutre avec masse http://www.mecatools.free.fr/vibratoire/mode_masse_sur_poutre.html

 

[BlueBell]

One question I'm curious to explore is what happens if you move some weight to the amas? My design goal is a comfortable day sailing camping cruiser for flat and sometimes thin water. Moving the batteries to the ama would free up some space in the already tiny cabin. It was discussed here: What to do that Amas (Outriggers) aren't "dead space" for a Tri ? https://www.boatdesign.net/threads/what-to-do-that-amas-outriggers-arent-dead-space-for-a-tri.54139/. With a model you can move the weight to and from the amas and sail in various conditions and maybe get a feel for the impact. I'd like to move weight to the amas until the ama itself is neutral buoyancy and see what happens. I'd also like to experiment with taking up some of the weight in the ama by adding a stay to the aka. If the total weight remains the same but the amas are heavier the forces might be different but I'm not clear on why they would be dramatically larger. Anyhow, I was just hoping to get more of an intuitive sense for the impact of tradeoffs like this using RC models. I'll go for 2' model length and if I learn anything I'll report back.

Thanks for your response.
Some good advice above already.
I wouldn't waste my time on an R/C model.
I'd build the 16' version with previsions to move the batteries around.
You could even split them up, some in the ama's, some in the vaka.
You'd gain much more insight into the effect on the handling characteristics
on the full size version than on an R/C version.
But don't let me slow you down.
Carry-on!

 

[Zachary Peter]

I think a 16" model can definitely give you some useful insights, especially for comparing different features like amas length or bow shape. It won’t be perfect for exact numbers or speed, but for spotting general trends and tradeoffs, it should work fine. Bigger models do give more accuracy but take way more time and effort. If you build a few small ones to test different ideas, you’ll probably learn a lot without the big hassle. Just keep in mind that water scale and weight distribution might affect results a bit, so take those with a grain of salt.

 

[bajansailor]

Do you ultimately intend to build a 16' version as well?

Have you had a look at the 16' Strike trimaran designed by Richard Woods?
Sailing Catamarans - Strike 16 trimaran using a 14ft beach cat for hulls and rig https://sailingcatamarans.com/index.php/designs-2/27-trimarans-under-25/169-strike-16

 

[baeckmo]

I support BueBells advise here; to build a model that is relevant for measurements takes as just as much work as a minimized sailing replica that allows you to get working "seat-of-the-pants" info and avoids problems with scaling plus miniaturized sensors and data collection.

 

[mattwell]

Hopefully you will discover a fundamental truth about small lightweight craft . Putting as much weight as possible as far as possible from the center of the craft will make for far different behavior than the same craft with as much mass as possible centrally located.

Thanks Skip, I agree. Like all things, especially boats, there are tradeoffs to be made. I'm simply hoping to experiment with simple models to get a little insight into the tradeoffs.

 

[mattwell]

Do you ultimately intend to build a 16' version as well?


Have you had a look at the 16' Strike trimaran designed by Richard Woods?
Sailing Catamarans - Strike 16 trimaran using a 14ft beach cat for hulls and rig https://sailingcatamarans.com/index.php/designs-2/27-trimarans-under-25/169-strike-16

Yes, I'd like to build a 16-19' boat if the ideas show some promise. Otherwise I'll just buy an old monohull and go sailing.

I like the Strike. It looks a little funky from some angles but it seems to be a pragmatic design.