Help with an atypical outrigger concept

Hi,
After lurking for some time I have finally mustered the courage to post a question to the forum.

We are currently designing a (low)- power outrigger boat project, and are a little unsure in figuring out the design of the arms.

Overall dimensions:
It is not the biggest boat with 20' LOA.
The main hull is ~ 2.3 m beam and 6.3 m // 20' length
Side hull is 0.73 m // 2.6' beam and 5.65 m // 18' length

Weight distribution :
Total disp. is ~3300 kg
Main hull is 2250 kg
Side hull 450 kg
Cockpit at ~500 kg
and arms 100-200 kg

The boat is designed to be a slow disp. boat with crusing speed of ~4 kn.
It should cross big oceans, but be capable of offshore sailing - somewhere around category B.
we are not in a hurry, but the boat needs to be rugged enough to survive the trip.



Between the two hulls, a cockpit shell is suspended, attached to the arms.


Right now we are working with two options - either a steel truss beam or a composite beam with a more custom shape.

We imagine the loads on the beams will be quite high, but it has recently been pointed out to us, that many bigger boats with sails etc. (ie. dragonfly tri) have rather small beams with hinged attachment- in this case we don't even have a sail, which should reduce some of the expected righting loads, but to be honest, I am a little unsure on how big forces we can expect from the boat being tossed around in bigger waves.

Another point is the rigidity - For a truss system i would be a very stiff connection, whereas with composite beam it could probably flex more?

Do any of you have suggestions for either beam type, shape, expected loads, or maybe you can give us a direction for how to approach verification of the beam design?

All help is appreciated.

 

Welcome to the forum Niklas.

Some questions :
Can you post a side profile drawing of the vessel please?
And perhaps something showing what the hull lines are like?

What construction material are you proposing to build with?
For example, it could be single skin fibreglass, or foam sandwich fibreglass, or aluminium, or plywood / timber composite with epoxy?

Re your displacement of 3,300 kg (which seems to be a lot for a 20' boat, even when you allow for the side hull / outrigger), have you calculated that this is the actual displacement at the waterline shown in the section drawings above?
Re the 450 kg weight of the side hull, does this correspond to the volume displaced at the waterline shown, or is it a calculated weight based on the materials used in the structure?
The weight of the cockpit and arms (600 - 700 kg) is going to be significant - have you calculated if the vessel will remain level (ie not heeled) with this weight offset to starboard?
The side hull is going to have to support some of this weight, and the depth of the side hull relative to the main hull is going to be fairly critical.

Please do not even think about steel truss beams for connecting the hulls - there are MUCH better and more efficient ways of connecting them, and I am sure that you will receive good advice on this thread in due course.

It sounds like it will be a powerboat then, which has to have the range to cross an ocean - this sounds very ambitious for a 20' boat!
What type of propulsion system are you planning to use?

Have you found the Harry Proas in your research?
These vessels are generally bigger than what you are proposing, but I am sure that the design principles here will be very similar.
http://harryproa.com/
@rob denney is the gentleman behind Harry Proas, and he might be able to offer you some useful advice if he sees your thread?

 

I have to ask.

What is the outrigger for?

If you took its waeight, the weight of the connecting structure, and the weight of the cockpit, and turned it into ballast, the boat would likely have sufficient ballast to self-right.

The main hull design is far too blunt to get anywhere in any kind of a head-sea. It will need at least a more pointed bow. Maybe making the boat a meter longer will help win back the internal volume lost.

 

In the spirit of full disclosure, I am not a naval architect, but I will try to answer your question somewhat, lol.

For computing the required strength of the truss, I would use the buoyancy (force) of the outrigger if submerged completely multiplied by a safety factor of 2 to 5. Since this would be an ocean crossing boat I would use a higher safety factor and sacrifice weight.

 

Thanks for all your feedback and questions, i can try to elaborate a little :


Drawings looks something like this - i am aware that the bottom towards stern needs a rework to soften the flow around the indentation.
We are working with electrical propulsion - hence the low speed, and the outrigger is to support a massive surface of solar panels.
The waterlines are calculated with a fairly accurate weight model of everything - there are minor uncertainties, therefore small stuff is still shuffled around.
@bajansailor "The side hull is going to have to support some of this weight, and the depth of the side hull relative to the main hull is going to be fairly critical." - this sounds interesting - could you elaborate on this ?

We have discussed a more pointy bow - but i am unsure with the low speeds how necessary it is ? That being said, i can imagine that in head-seas the front surface would slam a lot.

I love the proposed method of using the outrigger buoyancy to estimate loads, but this is mainly vertical load - are there not also lateral loads from hitting waves, and the outrigger crash-diving into waves?

 

Your design looks similar to a scow bow sailboat. I don't know the reason a designer chooses that bow shape, but it must out weigh the extra resistance that you would get in waves and wind. It may not be the ideal choice for electric.

 

It is a rather bizarre shape. If you need to move 3.3 tons with low power a longer and narrower boat will perform much better. Also, a monohull would be more appropriate, cheaper and less complicated. Write a Statement Of Requirements (SOR) and see where that takes you. The design will develop itself.

 

As per what Gonzo says above.

If you REALLY want to have a proa, look at the Harry proas (and others) for inspiration.
They are all invariably long and skinny.
I have never seen a boat with a hull shape like yours - like Willy says, it is similar (sort of) to a scow, but scows are effectively skimming dishes - and your boat is definitely not a skimming dish!

Re the quote attributed to me above, think about it - if the side hull is suspended too high on the cross beams, then the boat will heel over until there is enough immersed volume of the side hull to resist the heeling moment.
If the side hull is 'too low' (relative to the main hull), then it will be immersed much deeper than it should be, with consequent increase in resistance.
You will probably have to do iterative calculations to determine where the position should be.
And your main hull looks like it will already have a 'lot' (relatively) of resistance, when compared to other more suitable and sea kindly hull forms.
Especially when you have to motor into a head sea!

 

I acknowledge all of you feedback - they are very good points. It might be a bizarre hull shape - i have thought of it as something resembling a life boat.
The hull is iteratively calculated to balance the heel - the above pictures shows cog and cob vectors. We might also take the advice to lengthen the boat and move more weight torwards the main hull, to help a bit with balance and wave piercing.

The reason for the outrigger is to have a big enough surface to mount solarpanel - at the same time we want to keep the boat as small as possible.
We have considered both monohull and a catamaran, but the mono does not give enough solar area, and we wanted the outrigger over the cat to keep the construction more simple.

I would love to hear some more opinions om beam design.
Is it really mainly vertical forces that act on the outrigger, or what horizontal forces can we expect?
Also - any suggestions for topology/material?

 

Okay, so if you want a massive surface for solar, how does the weight of the solar not bear on the tiny outrigger?

Why did you not design a drua expecting this issue?

Nothing about the main hull looks efficient.

You can't put any solar panels on the cross structure the way I see it. The small ama will sink.

You are putting the cart before the horse. Only when you know the loads can you build the hulls...and you don't declare the solar system loads, or battery loads, or even the crossbeam/cockpit loads? So, start over with the loads.

 

I am becoming more interested in the capabilities of a solor powered power boat. So I am curious to know how many 100 watt solar panels you think you will need? I bought my first 100 watt solar panel over the winter and I was surprised how heavy they are. I guess I never really thought about it. Does the water line on your side hull reflect the weight of the solar panels?

 

There are forces on all six axes.

 

Ok, where to start with this?

First the crossbeam problem. One one hand you have a simple cantilever problem, one hull in the water, the other in the air, so the beams and their attachment points have to be strong enough to support the entire weight of the boat. OTOH, the two hulls can be in different waves and they try to twist the beams. This can be solved in different ways, but usually by using some sort of box beam. You could of course beef up your trusses to whitstand that, but it would be more elegant to use a space frame truss, a box beam, or, since the cockpit is to be covered by solar panels, to use their frame and the cockpit floor to create a single big box beam (that's how bridgedeck cats work).

Second your overall design. Multihulls have a high slenderness ratio, and you are really far away from it. A L/B of 2.7 on the main hull is low even for a monohull, while the outrigger at 7.7 is just below 8, wich is usually considered the low end for a multihull.
Even if you decide to go for a monohull, given that you have a low power situation you should not go under a L/B of 4.

Lifeboats are not designed to go anywhere, it's the wrong model for a passagemaker. Your inspiration should be transatlantic rowboats and other solar electric boats.

 

Hi Niklas, welcome ! Cool project, and i think you're getting some great feedback above.

Only thing i thought may not be so evident is *how much* the inefficiency of such a wide hull shape matters, *especially* for low-speed use, just like in any other long-range electric craft or anything powered on on-board renewables.

Here is a basic catamaran resistance calculator: Catamaran resistance calculation http://msi.as/Calculations/CatResistance.aspx You'll notice the impact of the spacing between hulls isn't going to matter much at the speeds you're targeting. It may not even be able to calculate a shape like you drew, but even playing with more average multihull shapes, you'll notice slimness can reduce resistance by a factor 2 or 3. That is 3 times less solar panels to buy, install, put away in case of storms so you don't get blown over etc. It's also much less load on the transmission so you could probably use highly efficient gear like large RC plane propellers (that could be 80%+ of efficiency, not just 60% like a good normal boat prop). And it's 3 times less batteries, which may well make up half the weight of your boat !