45' solar catamaran

Large hull has a little less wetted area and less friction drag, as minimum wetted area corresponds to half circle.

 

Thanks for your input.
So, I don't see that like an important downside.

Actually, I'm trying to find important downside for this Cat with narrow hull. Is it only those two?
-A bit more friction drag.
-Boat will sit deeper/ will be lower in the water.

 

You will have essentially zero reserve buoyancy in a narrow hull.

EVERY pound in a cat is consequential and every pound in a narrow cat is more consequential.

The displacement does not matter. What matters for someone planning to load their boat with solar and batteries and supporting systems is the immersion rating.

Take two boats, same displacement, but one has an immersion rating of 440 pounds per inch and the other 700. For every additional 440 pounds on the first, you’ll sink an inch, the other allows 700. So you need immersion data.

And most catamarans are very weight sensitive and have relatively low immersion ratings. In order to build a solar-electric cat, you need to study weights closely and allow for margins of error. If you end up 2000 pounds with pax and gear and 440 pound immersion, you may end up 5” deep which may not even be safe function.

etc…

this is not one to underthink and be dismissive about

I bought a drum roller for my cat and can’t use it. It is too heavy and now I need to resell because I misread the weight as pounds and not kg.

 

Thanks fallguy.
Based on some measures, my cat have immersion rating of 955 lbs / inch.
It's not a huge challenge to know how deep it will sink after conversion. 3'', 4'' or 5'' deeper don't matter me. What is matter me is the unknown consequence:

This is what I don't understand and I bet I have to learn about consequence to have hulls deeper in the water.

 

I've also think that a sailing cat without his mast and sail is also consequential on the boat.
Rough weight change based on 955 lbs per inch.
I remove:
-50+feets mast and sails: -1000 lbs
-Two diesel motor, transmission and tank: -1200 lbs
-Heavy full wood furniture: -400 lbs
-Two daggerboard and their structure: -200 lbs

Total remove 2800 lbs, so boat is up 2.9'' of the water

I add:
-Two electric motors, controllers and some copper: + 350 lbs
-Two 80 kWh battery: + 2200 lbs
-12 kW of solar and the structure: + 1800 lbs
-Not so heavy complete furniture: + 650 lbs

That add 5000 lbs, but remove 2800 to give 2200 lbs and the boat is 2.3'' deeper than original.
Not too bad, no?

Clearly I have to add another 1000-1500 lbs for food, water and stuff to travel, but that is still only up to 3.8'' deeper than original.

 

Personally, I would not want to be almost 4” immersed past dwl to start. To start, at most 1” deep. Sorry, but you will have nothing left to loading.

You also have not discussed where the excess loads are, but I am guessing solar is up high.

Most likely, you will also not be able to get all your loads balanced. Part of your weight study ought to be a moment study. For example, engines removed 500 pounds 15 feet from lcg, moment is 7500, adding back engines 200 pounds 15 feet, net is 4500. This will mean you cannot put solar up front. If you do and it is wrong, the framework which would be custom aluminum will require revision $$$.

so, hire a NA and get hydrostatics done

etc, etc

 

Good comments, thank.

About this, I have not much play with solar panels weight, but I have a lot of play with batteries weight.
I can move those two 1200 lbs a bit more in front or more in rear as I need.

We plan to have the boat in hands next month so we will be able to design with more details.

 

How bout I ask Bill Salisbury for you. I'm going to his hundredth birthday party early June. He built the original moulds on his peach farm in West pennant Hills with help from a few young rascals like John hitch and Shawn Arber and a bloke called lock crowther. Maybe you've heard these names before!!!

 

The best approach when there are so many unknowns is to make everything modular, adjustable, and relocatable.

But honestly we are talking about a boat that will be severely limited by hull speed, wetted surface area, and windage.

Trying to achieve any kind of fast cruising speed would be a completely inefficient use of your battery capacity, so you should be focusing more on designing a slow speed cruiser.

The good news is you don’t need to worry about things like getting on plane, pressing hulls under sail power, hobby horsing, deck slamming,.etc..

You do need to be mindful of the water conditions you intend to operate in.

In the end, if operating in calm waters, you just need it to float and to be stable.

One thing that you can count on is the more it sits in the water, the more sluggish and less responsive it will be to steering and throttle input which equates to less efficient use of your propulsion.

On the plus side it should be more comfortable at anchor.

 

That what I'm designing. 5 knot cruising and 6.5 knot max speed seem good to me.

Why? And if it's true at anchor why not also during sailing?

 

Regarding the immersion depth of the hulls, my thinking is that bridge deck clearance would be the important measurement.

 

I didn’t say it wouldn’t be comfortable during sailing.

That will depend on immersion, stability, water conditions, etc..

I was speaking to a boat with a lot of wetted surface area at anchor compared to a boat that’s light with more windage and less wetted surface area.

A lighter boat sitting atop the water tends to be more reactive to wind, chop, swell, etc.., and light multihulls tend to hunt and stretch their anchor and mooring lines more than a say a mono for example.

But the windage alone that will be created with the solar you plan to install will act like a sail when pointed into the wind and will most likely cause the boat to be more reactive to wind gusts, so again being on the heavier side should help offset that a bit.

If you want to understand first hand the affects of highly immersed hulls and how they affect stability, maneuverability, balance, etc.., maybe you should consider buying or borrowing an old beach cat and overloading it with weight to see how it behaves differently.

When overly immersed without enough reserve buoyancy, the hulls will be more susceptible to submarining as you shift weight about, throttle up, throttle back, steer, cross wakes, etc..