How much buoyancy is needed for catamaran hulls?

Counterintuitive for a design forum I know, but it's so true, and so often ignored.

If it's short answers you want then 2.5X for reserve buoyancy.
Or, if it makes you feel better 2.538X.
But if you find a proven design that fits your needs (SOR), you won't need to worry about how much reserve buoyancy you "need".

 

This is low efficiency. Remember that a single horse can pull a 20 ton barge 4 knots this is 754 watts. Your proposed boat is not even ne ton. Trolling motors have low (20-25% effficiency) typical. There are some brushless ones that are up to 60% but, they are designed to push smaller boats so you would need several of them to achieve even this result which is not
very good.

I used 32 inch carbon air prop and pushed my boat (4000lb) 2 knots using 120 watts.

It is not practical to run starlink for me, and with your boat an even stricter weight and power limits it will not be practical for you either. Sure you can do it, but the negative (cost, weight, power) are too much.

You probably wont get as much power from solar as you think you will.

I can tell you from experience that this is unfounded. I sail everywhere. Yes you can sail in these places, and its not that dicey. In fact it would be more comfortable to have working sail power than a comparatively less reliable electric motor. Regardless, it is energy consumption to power a motor, and sailing you already have, so it makes sense to sail in the tightest areas too. The motor may help you avoid actually crashing or tacking with unexpected wind shifts but there is not a pressing reason to use the motor to cover miles. Because the motor is only used in bursts to maneuver (when you make mistakes sailing), the battery and solar requirement is not much.

It is too heavy.

I know this, and I reduced the amount of water to increase production. Eventually it still only made half a gallon and the still weighed more than 100 pounds.

I would be very interested if you can build multiple effect still, perhaps it can just run on driftwood fire and you can make water on the beach.[/quote][/quote][/quote]

 

Yes, in my opinion counter to the whole point of this forum, to drive ingenuity, creativity, and new concepts for design, backed by people with experience who can guide you.

I was hoping for concise answers, not necessarily short, backed by reasoning. I'd prefer to know why I'm doing something rather than just taking someone's word for it. If everyone who comes here just does what they're told without knowing why they will have learned very little. You and rwatson have given me the same answer, so at least I have 2 people telling me the same thing, i think... Do you mean that each hull should have 2.5 times the buoyancy of the total loaded boat weight? Would you share your reasoning behind this number?

None of the designs I've seen thus far meet all my requirements. And part of the whole point of this endeavor is to design and build something myself, not just to sail it.

 

I have my doubts about this. Where did you get this information?

To my understanding the poor efficiency of trolling motors is due primarily to the controller type and prop design. The difference between brushed and brushless is on the order of about 10% if memory serves. The size of the boat is irrelevant, the weight and the actual resistance are what matter. The weight really only matters for acceleration, but more weight usually equals more frictional and residuary resistance. Many trolling motors(and their props) are designed to push around fishing boats which are significantly heavier with significantly more residual resistance. If I choose a motor that has sufficient power output, the right controller, and the right prop I should be fine. Am considering using two NK180 motors. 600 watts apiece, 48% claimed efficiency. If I want speed greater than about 6mph I'd need to adapt different props. These motors are designed to push around heavily laden fishing kayaks which have fairly inefficient hulls.

If it takes much less power to drive this boat, as you indicate, I'll be very pleased and you'll get a "you were right".

The power usage numbers I used were mostly from kayaks using torqeedo motors, or regular trolling motors modified with aftermarket props and controllers.

That's a huge prop. 2 blade? How many watts are you using at 5 kts? I feel like using really low speeds for reference can be a little misleading due to the nature of resistance curves.

The plan would be to disable the aiming mechanism and mount it flat. I believe the vast majority of the power use is from this mechanism. This will decrease the data rates quite a bit at times but be more than sufficient for my needs. There are a number of people doing this with land based mobile applications. I am fine with the cost, not a lot to pay for peace of mind having reliable internet for weather and other information, and watching movies here and there would be nice. To my knowledge the new generation starlink kit weighs under 20 lbs.

I'll run through some calculations here.

Solar energy per square meter per day reaching the surface in the bahamas in kw: Mar 6.3, Apr 6.8, May 6.2, Jun 5.4. This takes into account all factors like cloudiness, etc. Let's call it 6kw on average. Module efficiency(as opposed to cell efficiency) of the renogy 200w panels is 16.7%. 4 panels has an area of 50ft2, approx 5m2.

5m2 x 6000 watts = 30,000 watts

30,000 x 16.7% = ~5,000 watts

Flat mounting is about 20% less efficient than optimal stationary tilt angle, so: 5000 x .8 = 4,000 watts

Now let's take off the temperature penalty of mounting panels directly to a surface without ventilation, to my knowledge this incurs a potential loss of another 20% due to decreased cell efficiency at higher temperatures: 4000 x .8 = 3200 watts

A good mppt charge controller is about 95% efficient so: 3200 x .95 = 3,040 watts.

Not far off from my original estimate. I should consider ventilating my panels somehow. Would probably make them last much longer too.

Here are some examples of places I'll be going. Originally I was going to build deep vee hulls to serve as leeway prevention but soon realized this would very significantly impact my windward ability, an important thing in the bahamas, and my overall efficiency. So I was convinced to use boards. But boards and rudders are kind of useless in 12" or less of water. I'd be sailing whenever I can but won't always be possible. And as previously noted the motors are also for safety if for some reason my sailing rig, boards or rudders fail.

That's odd. All the data I looked at shows production significantly greater than what you got. I'll have to run my experiment and see what I get and post my design and results.

I don't know how your still was so heavy. I'd love to hear a full rundown of the design and materials. Do you have pictures? Even if mine ended up weighing 70 or 80 lbs and producing a little less it would still be worth it.

 

Really? Where did you see this Mission Statement?

You're asking for an education then, not guidance.

Wow, gross generalization and not entirely correct.
Some learn by doing but not everyone learns at the same rate or depth.

No, I meant the total buoyancy would be 2.5X the loaded/sailing weigh of the boat.

Sure: It seemed reasonable given the loose parameters.

There could be a very good reason for that. Wanna take a guess why?

Well then get on with it.

We can help you with feedback but we can't teach you how to design a boat, it's much, much more challenging than I think you may realize.

Given your situation, I don't feel there is anything else I can help you with.
Best of luck with your project and do start a build thread.

Cheers!

.

 

Since you’ve made it clear you are not just seeking advice but would also like to understand the reasoning behind it, and since you did not acknowledge my advice or respond with questions about it, I will go ahead and explain the reasons behind it.

First of all, if efficiency is important to you and like most you would like to see the highest speeds possible as efficiently as possible, then overall efficiency of a trolling motors is not what should be important to you.

I see various general figures being quoted of the efficiencies of trolling motors but they are likely overall efficiency ratings and are not what you should focus on.

Instead, as I mentioned before, you want to identify the Peak efficiency of a motor and then build around that.

What motor qualities result in good efficiency?

Ideally you want a brushless, direct drive, PWM ESC controlled motor with the electronics in the motor housing operating at the highest/safe voltage possible.

While the NK180 motors do check a number of those boxes, IMO they are not powerful enough and too low a voltage for your proposed application.

As I stated before, you want your cruising speed (fastest speed you can achieve on sun only) to be somewhere around 70-75% throttle.

Why?

Because a PWM (Pulse Width Modulated) ESC will allow the motor to achieve the highest RPM possible while still modulating and drawing as little power as possible.

What does this mean?

Think of it sort of like riding a bike where you reach a speed and only have to pedal occasionally to maintain that speed.

By pulsing the power, the motor is able to maintain a high RPM at less power draw.

This feature can greatly increase efficiency and range.

Most PWM ESC’s allow a motor to reach its peak efficiency at about 3/4 throttle. After that, the motor becomes less efficient. This is why the popular electric outboards on the market can only run full throttle for 1hr but are able to run all day at reduced throttle.

And this is why you want to target the Peak efficiency throttle range/RPM as your ideal cruising speed.

Why am I advocating for higher power higher voltage motors?

Well, the bigger the motor the higher your cruising speed/RPM will be, plus you’ll be able to swing a more aggressive prop giving you more speed and bite, and if you go with a higher voltage motor, you’ll also gain some efficiency, not to mention with electronics it is always best not to run them at their max specs as heat is a killer. So you instead go with bigger motors that you continuously run at lower throttle.

If you choose the right motors and you operate them in this manner, and you are able to offset their consumption with solar, you will be able to achieve the highest cruising speeds possible at the highest efficiency relative to Watts in versus Watts out, while still leaving some reserve power for when it is needed.

How much more efficient might a motor be when operated this way?

As much as 20%.

As I mentioned before, usually solar is the limiting factor as you can only fit so much on a boat.

Once you settle on that, you can then choose appropriately sized motors that will result in the highest most efficient cruising speeds possible on sun only.

If you choose motors that are marginal, you will get marginal results both in propulsion and efficiency, and you will run a greater risk of less reliability and margin of safety.

But just remember that the efficiency of your propulsion system is only one part of the equation. In order to take advantage of an efficient system, you need efficient hulls.

 

I ask myself this question too... and I wouldn't be satisfied too with a number thrown out without further explanation.

Have you found an answer?

I see the most critical situation in surfing in front of a wave - the boat moves on a curved surface and is largely supported by the bow and stern sections, while the middle areas are almost free ... I don't know how to calculate this. The easiest way seems to me to build a simple model where the hulls are a little higher than planned (just carve them out of Styrofoam), let them sail in small surf, and watch with the video camera.

Notice how deep the bows dip, and how high the waves pass along the hull. Take this measurement, and maybe add 5'', or whatever seems right to you.

If the sterns sit too high on the wave and push down the bows - or if they sink too deep at high speed, you may need to play a little with the geometry of the waterlines, or the ratio of depth to width.

I'am courious about "better" ideas. Adopting values from "proven" designs that have a completely different design goal (inhabited vs. uninhabited hulls) or randomly thrown-together numbers (2.5 vs 2.538) are not the answer.