Multihull power requirements

[Ad Hoc]

....do you know of some basic formulas that i can use to calculate speed and powe

See my reply in post #7.
A start is also shown here:

https://www.scribd.com/document/885...ponents-of-High-Speed-Displacement-Catamarans

This outlines what you need to do. But you still need more of the lower range of LD ratio hulls from 4 - 6.

 

[Brian Alsum]

And see the catastrophic loss of range.

Range is relative. Im not sure if I made it clear but im talking about a sailing vessel. In an all-out one-time sprint under diesel only, yes a diesel is hard to beat. However how often do you really run your tank from full to empty? Like most cruisers, you hop from location to location, all along sitting around for extended periods of time. Under these conditions, a solar/battery/edrive system has a huge advantage. it refills its self. The people I work with that have gone Edrive love it. they never have to re-fill. Some keep a diesel generator on board. And no running a diesel generator in series to charge batteries and in turn spin an electric motor is nowhere near as efficient as a regular shaft drive diesel. However, this is so often a fringe case. Also, when taken into account the idle self-recharging capability of a properly designed battery and solar/wind charging system, you end up hardly ever needing to spin the diesel up. I know crews that have gone a year without ever needing to turn on their onboard generator. Along with that a DC generator is far more efficient at direct battery charging. allowing for 100% of the generator power to be transferred to the motors. no charges needed, my goal is to have about 100MN range at 5kn on battery power only and a 600nm under diesel power again at 5kn.

 

[Brian Alsum]

It seems to me that if you have an existing boat powered by diesel(s), and you have the speed vs RPM and better still the fuel flow at the RPM points, you should be able to calculate the resistance of that actual boat, and use that to calculate the electric motor and battery etc needs

I plan on doing just that. The problem is I'm landlocked and the boat is in a charter fleet most of the time.
Tell me if I'm wrong here but the power required to push a boat through the water is the same as a monohull... the advantage comes in that the multihull just has a higher "hull speed" (I use that term knowing that it is not the speed limit everyone thinks it is) I believe this explains why a Cat will have closer to 2x the power installed per ton than a mono, no? for example
Boat 1: Leopard 48 (17 Tonne Cat) has 85kW installed
Boat 2: Breehorn 48 (16 Tonne Mono ) has 36kW installed

 

[Alik]

Brian, frankly speaking I don't think you can do powering analysis yourself. Better to hire expert for this job.

If You still want to get into it, start from reading basic books on naval architecture. You keep asking magic formulas but seems You don't know what hump of resistance is.

 

[Brian Alsum]

Brian, frankly speaking I don't think you can do powering analysis yourself. Better to hire expert for this job.

If You still want to get into it, start from reading basic books on naval architecture. You keep asking magic formulas but seems You don't know what hump of resistance is.

It's called learning. To learn one must first start with what they don't know. Admitting I don't know something is the most empowering thing I can do. Also, my analysis won't affect anyone but myself. If I'm off by 10%... the only thing it will change is the range. I can overcome that with more storage both in diesel and in batteries. I build my own batteries so they are extremely easy to modify.

I actually think I found what I was looking for. I don't necessarily need to worry about the hump. I'm looking for that gray area somewhere between super fine detail and eh... close enough. I know what power I need to match existing performance as I can just match kW for kW what's installed.

The formula I've come up with seems to fit existing installs and current performance.

13kn is about the most I've been able to achieve in a normally loaded boat

Using the S/L Ratio working backwards for HP I came up with this.

When you work backwards with drive train eff losses, hull resistance..... 74Hp lines up nicely with the 100hp of the installed engines. (114 Hp in the newer boats and 100HP in the boat i have

I know there is much more to it but as I said If I install at least 100hp of electrics the only thing that suffers is the range if im off. In reality, I don't motor long-term at anything more than about 7 kn. this means with my ideal install of 200kWh of batteries I should be able to get 6 hours (42nm) on a single charge (60nm at 6kn) and with 1x 20kW DC Diesel Genset i should be able to get about 450nm at 6kn using diesel only and 550nm with both the diesel and the batteries... add in 2kW of solar..... its just get better and better

 

[Ad Hoc]

.... but the power required to push a boat through the water is the same as a monohull...

What do you mean by "a boat"...?

.. I don't necessarily need to worry about the hump. ...

Indeed... you do have a lot to learn!

 

[Brian Alsum]

What do you mean by "a boat"...?



Indeed... you do have a lot to learn!

A boat, a relatively small water going vessel. In my chase a 48' sailing crusing cat. I just need a number to help estimate (
es·ti·ma·tion
/ˌestəˈmāSH(ə)n/
Learn to pronounce

noun

1. a rough calculation of the value, number, quantity, or extent of something.

) to help me size motors, battery and charging system. I did not need a precise number on a boat that will spend 99% of its life under sail power...

Glad I could clarify that for you!

 

[Mr Efficiency]

Another way, get someone to tow your boat, use a strain gauge, and note the figure at each speed gradation.

 

[Ad Hoc]

So when you say:

.... the power required to push a boat through the water is the same as a monohull... ...

....In my chase a 48' sailing crusing cat. ...

You actually mean.. a cat, a catamaran, not "a boat". Helps to use the correct terminology.
As noted.. you have a lot to learn but seem unwilling to do so, all for the magic formula that does not exist.

There is a big difference between a monohull and a catamaran.
At higher Fns and LD ratios... a cat has 50% less resistance. ...so, as always..it depends!

I assume you mean "case" not "chase" too...

 

[Brian Alsum]

So when you say:




You actually mean.. a cat, a catamaran, not "a boat". Helps to use the correct terminology.
As noted.. you have a lot to learn but seem unwilling to do so, all for the magic formula that does not exist.

There is a big difference between a monohull and a catamaran.
At higher Fns and LD ratios... a cat has 50% less resistance. ...so, as always..it depends!

I assume you mean "case" not "chase" too...

I award you Cpt Keyboard Warrior. I'm not saying I'm unwilling to learn I'm saying I'm unable to collect the data needed to do a FEA or even a basic simulation based on the boat I want the data for, All I have with me is basic hull dimensions, no models, and I can't get them without a haul-out and a modeling scan. I would love to dig into the deeper math but so far your solution is to "hire someone" that will most likely cost far more than even the overall project. and way more than if I even threw a dart at aboard. again I'm looking for data that is within 10-15% of the real world... at best. so far the best data I've been able to come up with ballparks that are far worse such as 4hp/ton and does nothing to tell me about what power is needed at what speed. Like is said if I have a boat that has a 75kW of diesel install and I install a drive system that has the same. I know I will at worse have the same performance. I needed a way to find the value (even if they don't take in things like the prismatic effect and the hump effect) I love engineers in their field. if they can get into an analysis paralysis they don't like it. I'm an EE and I do the same. I have to pull myself back and realize that when 10.1 will do I don't need to bury my head to find 10.111111111111111. If you have books you recommend I would love to get them and study but for the timing being, I'm using the basic D/L ratios to find hull speed by diving the D by N where n is the number of hulls. like I said before it lines up with the installed power and speed numbers close enough that I feel at least comfortable that I won't be massively underpowered. I will continue to refine my numbers and build new formulas with time as I learn more. It's a sailboat so the only times I motor are on and off the anchor, and in and out of port. I might motor sail some if I need to make a location by a time but other than that... I'm sailing.

 

[Brian Alsum]

Another way, get someone to tow your boat, use a strain gauge, and note the figure at each speed gradation.

Not a terrible idea. The question is how to turn the pull numbers into power numbers

 

[Alik]

I would love to dig into the deeper math but so far your solution is to "hire someone" that will most likely cost far more than even the overall project.

Hmm, dreaming Someone who does this work every day will spend 30minutes to provide you the resistance/power curves. And it will be reliable, unlike your maths.

 

[Ad Hoc]

.... I'm saying I'm unable to collect the data needed to do a FEA or even a basic simulation based on the boat I want...

What has FEA got to do with resistance calculations??

 

[Brian Alsum]

Hmm, dreaming Someone who does this work every day will spend 30minutes to provide you the resistance/power curves. And it will be reliable, unlike your maths.

If its so easy why can't you share the apparently super simple and fast math?

 

[Brian Alsum]

What has FEA got to do with resistance calculations??

Now you're just picking. You know what i mean, dont be an ***