Minimal offshore power proa

Here's some pics of a NZ tri,
45hp,
0.5L/m at 10knots
14 knot top speed
2000nm range
Can't stand the cross beam
layout though even if it is very similar to what I've drawn

 

What's the problem with the cross beam ?

 

They have made them sliding to enable marina berthing. Need to climb over them to enter the cockpit, then duck under the fwd one to enter the cabin

 

OK, but don't you have similar issues with a proa ?

 

Not in my case, boat would live on a mooring.

 

I mean the beam protruding into the cabin.

 

You need a genset to run a water maker, radar, AIS, the autopilot, nav suite, communications, to charge the dead start battery, or in case the alternator dies. I'm a sailor, so not accustomed to having the main engine running. I test it once a week or so when cruising - yep - it's still there. A genset is much more efficient than running a 40hp prime mover to charge batteries (unless you have a ton of them which you don't want, or you like a lot of hot water.) You'll want a start battery, two house batteries for the boat, and one for each person. 1000 W gen for two people, 2000 watt for more than two. This is the cheap/light/versatile way to do it. Solar panels should be matched to the current acceptance of the batteries at about 80% charge plus any base load. Use a morning gen set run to get the batts to 80%, then let solar do the rest. This is a practical long term arrangement. My TV draws 22 W, dvd is 6 W, They aren't worth mentioning. Over four months, this will save weight, either in solar kit or in fuel. If you are looking to use the main diesel to provide battery charging on the hook, your range under max fuel economy needs to be cut in half vs just using it for propulsion.

Charging batts via the main engine - Numbers based on a Beta 43, but others would be very similar:

Lets say you have a dedicated series 31 start battery, and two 'banks' of two 6-Volt deep cycle batteries. You have a three bank charger 30A-30A-10A. You upgrade to the Balmar XT 170 amp alternator because it makes 90 amps at 1500 rpm, not 50 amps at 2000 rpm. You have a second one in box. The engine runs at 750 rpm to get 1500 rpm at the alternator. Alternatively a Balmar 6-series 100 Amp unit will make 70 amps at 1000 engine rpm, 83 amps at 1250 rpm. Your house batts have 400 Ah capacity, and are sitting at 40%. You need 160 Ah times 1.1 to get them to 80% - 176 Ah. That's three hours on the charger at full chat. You can make 15 gallons of water in three hours for another 20 amps. If you want to freeze a two-gallon holding plate, that's about 6 amps. You'll burn just under three gallons of diesel doing this. In four months, that's 360 engine hours and about 300 gallons of diesel. The good news is you have about two hours of hot water. The bad news is you have 500 pounds of iron at 180 degrees sitting in your boat. You need to run an electric coolant circulating pump(s) to dump the heat.

A 2000 W genset running that same three hours will burn only half as much fuel. Two Balmar XT-170s cost about USD 2000. You can buy a portable genset for that to run on the deck. Or you're half way to a small diesel marine genset.

Solar panels would need to be about 600 W total.

 

I really can't see how a genset is going to cut the fuel bill for charging batteries, and other uses, by half, compared to taking it off a main engine that is already running to push the boat around. How so ? My query is more along the lines of what is to be done with all the fish that are presumably to be caught, whether there is the capacity to freeze and store, and what are the legalities of even doing that, in a non-commercial situation. There are strict "in possession" limits in most places, for amateur fishing. Certainly chilling down a substantial weight of fish, quickly, in a warm climate, and at the end of a day with the boat at anchor, would require a generator, both to freeze the fish, and to make ice for the next day's fishing. It is de rigeur in warm weather, to put the catch on ice immediately, not have it sitting in a fish box for hours at ambient temperature.

 

Thanks Phil
For that detail that does make sense. When boatbuilding I left the electrical detail for others, I have only interest in metal work and engines

But the title of this is minimalist.
I boat the same way as I camp out.
Call in for supplies once a week or so
Not doing a 40 day Atlantic crossing

Little bit of 12v led lighting,
Nav lights + equipment
Music
Anchor winch
Big esky filled every 5 days with ice
Live off fresh seafood
Gas barby on the transom
Composting toilet
That's it

All these extras while desirable, double the build cost, and quadruple the build time. Doesn't take long to knock out a bare hull, insulation takes a bit, simple drive systems are easy. Fit out is where the hours add up, let alone building something with a mast and sails....

I want to be out there doing it, not worrying about the electrolysis nibbling away at my 50k paint job

 

Gotta say, looking at the orange boat you posted pics of, Brendan, and depending on where the waterline is, one has to wonder about the claimed performance, I guess at ~10 knots the water has largely broken away from what appears a substantial immersed transom, but at slower speeds, like trolling for fish, that would be a source of drag. If you want the option of low drag at trolling speed, I wouldn't want that. Have the canoe stern by all means, but with a submerged flat transom, you really need to reach the speed point where the "suction" is broken, for lower resistance.

 

"lopsided engine issue"... ???
Offset engine and prop is to counteract the drag of the single ama so the boat wants to track straight,
AND to provide "offset" ballast for a diabolically volatile stability issue concerning roll.

Your calculations are off on two ama stability vs one.
I believe you may have overlooked the rising ama losing buoyancy, creating down force while the submerging ama's reserve buoyancy creates lift.
Rework your numbers, you may be surprised.

And what do you plan to put in your ama?

Yes, there is a difference between a stabilized monohull and a trimaran,
but you already know that.

 

Alik says a power proa is a problem for uncomfortable motions, I assume in beam seas, I would be taking that on trust, I really think it is catamaran or nothing, if you want something bearable, that has speed and range, and the available space increases dramatically, I'd imagine a team of "friends" on a skinny proa would be fighting like Kilkenny cats, after a time.

 

I would agree - I was just thinking that a cat version of the drawing you posted in your first post would be just the business for what you are proposing to do.
That will give you a lot of extra room for you and your mates on a long passage from NZ up to lower latitudes.
You could even add a flying bridge?

I can see that a trimaran would make more sense than a proa - but by the time you have invested time and money in building the amas / outriggers and beams on the trimaran, then you would be perhaps halfway (or further) towards building a second cat hull?

Edit - as an aside re 'lopsided' powering - there was a time when the cat in my avatar was only running on one engine for a while until another engine could be sourced. She had 2 x 115 hp O/B motors then (she now has 2 x 150 hp) and she is 50' x 16' in size. We were initially thinking that she would want to go around in circles, but she tracked perfectly on just one engine, with no helm correction required.

 

Yes, an offset engine would be unnecessary. You obviously do apply a little helm to keep a cat running straight on one engine, and depending on the hull shapes and how much immersion of the ends, but it is rarely a problem.

 

Sounds like she is nicely directionally stable then