# 16' Semi displacement electric cat

Discussion in 'Boat Design' started by Brentmctigue, Jun 21, 2020.

1. Joined: Apr 2015
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### BrentmctigueJunior Member

I’ve been studying the Molland paper. My understanding is that the Coefficients published can be applied to the full scale ship directly. Ie for Fn0.2 in the table below the Ct for a ship with S/L 0.3 is 7.8. That’s 7.8kN of total resistance.
If someone could confirm this for me that would be great.

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### Ad HocNaval Architect

Well done.
Now the hard part... reading and understanding the paper.
As has been noted here, the LD ratio of you deisgn is way way too low:

Thus, reading the paper it shows the LD ratio of the hulls it tested as:

Lowest is 6.3 and the highest is 9.5.

It is impossible to extrapolate your design from its LD ratio of 1.97 (2.0) to 6.3 to obtain any meaningful results.

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### BrentmctigueJunior Member

Apologies I should have said that I revised the design. See below.

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### Ad HocNaval Architect

Playing with numbers is the easy part. What do the numbers mean, is the tricky part.

So, a displacement of 548kg - great!

Your 3D rendering suggests 4 people seating... so that's 75 x 4 = 300 kg.
That leaves you 248kg for the hull structure, the outfitting, the heavy batteries, the big motor etc etc..... see where this is going?

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### BrentmctigueJunior Member

It’s an academic exercise at this point. The design won’t work on many levels. I need to learn how to find resistance before moving on. So ignore the original posted material and just consider the last figures.

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### Ad HocNaval Architect

Sorry, that's the cart before the horse.
Or to use your building analogy, you're designing the length and width of stair case and locations of the windows etc, when you have no idea what size building it is nor how many rooms or floors or the size of the rooms.

You need to design your boat first... design is a very methodological process, ignore it at your peril.

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### BrentmctigueJunior Member

Hmmmm. Ok. Point taken. I’ll come back to resistance when I have a boat design that works. Min l/d of 6.3. Max. Overall beam 2.5 to trailer in Australia. Carry 4 people and gear. So 380kg + batteries and engine. Hang on I need to know how big a battery bank I need so I need to get a feel for resistance. You see my problem.

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### Ad HocNaval Architect

Welcome to DESIGN, where nothing is exact or fixed. Until it is exact and fixed!

Look at the size of motor you want to use... what power output is it, what battery size/power is needed to use it.. and that'll be your starter for 10

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### BrentmctigueJunior Member

Ok. Sounds a lot like architecture. Make it up as you go along until you have to prove it. Thanks Ad Hoc.

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### Ilan VoyagerSenior Member

Welcome to the joys and torments of designing anything from the foldable table of the living room to the Saturn V rocket.

As you are thinking of an electric propulsion you should start from the start about electric like any electric moped or scooter which have the same basic problems.
The first problem is autonomy thus the batteries which have to be found easily at a decent price, unless you are able to make your own batteries from 18650 LiOn cells but that is pretty tricky and expensive as you buy the cells, the BMS and tutti quanti.
Better to go to big chinese bats already made in solid plastic boxes, with BMS included, connectors, voltmeters etc. There are plenty at Ali Express, a LiOn 12V 200Ah of capacity 100 amps of discharge max is around 450 USD plus customs and weights a good 10 kg. As always with the chinese there the painful problem of quality, that can go from the worst to the acceptable and a few times to the best. Unhappily the same battery in an occidental country with warranty is 2 to 3 times more expensive, and often warrranties are purely nominal, ie a few words without value on a paper.
A set of 4 batts will give 48V 200Ah for a weight of 40 kg. The total wattage is 9600 watts and you can use safely 80% of the capacity 7680 watts if you want lasting batts, so a 5 HP (around 3680 watts, 77 amps at 48V) brushless motor will drain it in around 2 hours full throttle. After you'll need 5 to 10 hours to recharge it with an intelligent charger 48V 40A whose price will surprise you badly. I won't talk about all the wiring and security items needed as a short circuited LiOn batt is plainly dangerous.
In a catamaran the batteries are in two groups in the hulls and need a very big first rate wiring able to pass 48V 100 Amps quietly with minimal losses to the controller if there is one lone motor at the middle.
For a 10 KW 72V 138 amps full charge motor if you want 2 hours full throttle you'll need grossly around 72V 350 amps with the 20% security, that means 12 batteries 12V at least 180Ah, the 200 Ah batts are limited to 150 amps of discharge so it's good.
A 2 hours full throttle will give you a good 3 to 4 hours afternoon if you're careful with the throttle. I stay at 10KW 72V as it is similar to small electric motorcycles so that means that motors, controllers and the remaining items can be found in the net.
I'll leave you the calculations of the cost of a 72 V 200Ah set of batts, It's largely above 5500 USD plus customs at the chinese price, the european price is above 12000 bucks. The weight of the batts is more than 120 kg plus a beefy circuitry. 72 Volts can kill, so the are many precautions to take specially in a boat, wet by nature.
The total price of the electric propulsion is simply painful for a summer afternoon boat unless you are a Rockfeller heir.

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### DejaySenior Newbie

I'm not really qualified to give advice since I have zero experience but I also want to build an electric multihull (but a bigger cruiser and purely solar powered).

My personal conclusion is that for smaller power boats, power trimarans are theoretically more energy efficient than catamarans. Instead of building two slim 5.5m hulls you can build one slim 11m hull with smaller outriggers for about the same weight because you need less structural material. Longer hull with same weight is much more efficient and faster. It's easier to get more displacement. Better motions at sea. The outriggers add little drag (or no drag at fixed speeds) so wave interaction is less of an issue. Folding mechanisms for outriggers are easier. And you sit lower inside the central hull without a deck.

The question is then of course if longer hull and a narrower deck can work for you. Check out the boat "ILAN Voyager". I don't know why there aren't more boats and building plans for power trimarans in that style. Probably berthing fees! 10-11m is about the maximum you could put on a normal car boat trailer here in Europe.

Again, just my unqualified suggestion. Of course a catamaran can work just as well but you'll need more power and more batteries. I'd stick with 48V LiFePO4 batteries for more safety.

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### BrentmctigueJunior Member

Ian,
WOW a lot to consider there. I’ve been talking to a company here in Australia “Ecoboats” on elec systems and looking at the torqeedo systems. They are expensive though. Alternative are put there though I may need to get an electrical engineer to give me some advice.

Dejay,
Thanks for your input. I hadn’t considered a trimaran. I’ll investigate. I think the maximum trailer length in Australia is 12m.

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### Ilan VoyagerSenior Member

Dejay's idea is excellent. A trimaran or better said a motor stabilized monohull with 2 small amas is a very simple boat, rather seaworthy, easy to build in ordinary materials in the small sizes. A 10/1 ratio water width over the LWL has the resistance hump at only pretty high speed so for slow speeds the needed power is fairly small. There plenty in the Philippines where they are traditional.
Stay modest for a first boat, keep it MISS (Make It Simple and Stupid) ie a basic boat for nice weather with speeds easy to obtain even if it looks at first sight slow. Better to have 4 hours at 6 knots than 1/2 hour at 15 knots...
You have a simple way without engineers for the electric propulsion: stay in the power range of mopeds and small electric motorcycles so you have the controller, breakers, motor and a lot of components easily findable at a decent price.
You'll get easily in China electric motors and controllers until 3 Kw, ie for mopeds or small scooters. After that becomes harder.
But there is solution is to buy a complete motorcycle at the cheapest possible price (preferably used to any guy who discovered that his electric dirt bike has only 30 mn of autonomy, so he needs to have 4 spare batts at 1200 USD each plus 4 chargers each for training a just saturday afternoon...). The equivalent of a 125 cc, ie around 6 to 8 Kw as you have the motor and all the electronics just to rewire at the good length. Forget the hub motors, look at the motorcycles with chain transmission.
Add the 72Volts 200 Ah battery composed of 6 batts 12V 200 Ah, plus the IP65 gold plated connectors, plus breakers all waterproof. The wallet will be drained after all but you must add a propeller, a transmission and the demultiplication between the motor and transmission. That can be made DIY if you have the tooling and a welder. The simplest is a long tail in the Thai style. On a trimaran that will be fine.
After a lot of brain juice, a bucket of sweat and lots of money you'll have a 8 Kw electric power unit.
I can say with certitude that for the same money you could get a brand new 100 HP 4S outboard purring and sipping a few liters of gas...Sometimes "ecology" is extremely expensive...
An 8 meters trimaran for 4 people would use a 25 to 50 HP outboard which will last 10 years.
The batteries will have far shorter life...Electricity is not always cheap nor so ecological.

14. Joined: Apr 2015
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### BrentmctigueJunior Member

Iian,
I’ve been working on something just like that. The difficulty I have been having is keeping a catamaran under 2.5m wide to trailer and getting the displacement. The tri seems to be working better. Longer main hull and two small outriggers.
I’m keen on the electric propulsion because of the ecology/environment. Even new 4 stroke engines are quite pollutant. I’ll keep trying to get something to work until I can’t. What I though would be a simple design exercise has turned into something much more though I’m enjoying the challenge and learning a great deal. I’ve looked into the cost of lithium batteries..... yeah well it is what it is. It may be that I end up putting a small petrol outboard on though I’ll persist with the idealist electric propulsion for now. As AdHoc said I have to get a boat design first and am concentrating on that for the moment. Rest assured I’ll be coming back to your comments a little later though.
I Had a good look at your Monica ILAN Voyager. Quite a ship. A small NZ wave pearcing tri I found is an interesting read as well. https://www.rina.org.uk/res/issue 18 nov 2002.pdf

The maximum trailer length in NSW Australia is 19m including the tow vehicle. 2.5m wide x 4.5 m high.

15. Joined: Apr 2015
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### BrentmctigueJunior Member

Latest edition to the growing family of boats. Would appreciate your feedback. Ive never had much to do with or thought much about trimarans so please do question everything.
Ive kept the outriggers within the max 2.5m trailer-able width to avoid assembly or folding etc at the ramp. Concerned they are too close for wave resistance purposes. They are well aft to balance buoyancy of the main hull which is 3.3m (-9%) from AP. The hull is still round bilge?? 7.4 deg entrance angle flattening aft. Transom clear of the waterline 40mm.

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