Cause I Don't Know Diddley

I just checked and if I converted the numbers right, a 3 cylinder 30hp Yanmar with gear drive weighs 293 lbs. and will fit in a 24" x 24" x 30" box. (with a real problem doing maintenance I suspect) So the engine pod could be a bit streamlined. Also I have seen a few used units available on e-bay from time to time. Ya'll help me here, I'm trying to talk myself into it.

 

It seems you are trying to talk yourself into designing a low-speed, powered catamaran. One thing you may need to be aware of is the spacing apart of the two hulls, as there is an issue of wave interference that can slow your progress. Your central pod is going to complicate that as well, it is going to need to be submerged to some extent so your propellor stays in the water. You begin to see this is a juggling act where there are maybe more balls to keep in the air than you thought.

 

I've been juggling it for six months now and am no closer to finding what I want now than when I started. I need 24' to 28', and about 14' wide and I sure don't want gasoline powered outboards. Maybe one of the open type lower units with a belt drive air cooled diesel bolted to the top like the Chinese use. I saw some on Alibaba.com maybe just bolt on a pair of them, yank on the rope pull, and away ya go. OR, maybe I'll just get out the motorcycle and go for a ride.
I do thank you. you have probably kept me from wasting a lot of money.

 

One advantage of displacement power cats is the relatively high cruising speed capability compared to other non-planing boats. But the small, low hp single diesel is not going to make the most of that potential. You may run at ten knots where fifteen would be easily available with a couple of relatively small outboards. Google "Skoota" and you will see a couple of powered cats designed by a member here, Richard Woods. No diesels, they really don't mix that well with lightweight cats, and a single installation is not easy, the air cooled diesel would be a deafener.

 

Yep! I'm just making noise about the Chinese diesel. It would probably work long enough to get you to the deep water.
I have looked at and read everything about the Skoota 28 as well as the other power cats he has. It would just be too heavy and large for me to move around by myself.
One of his sail catamarans just got a new post on Utube. https://www.youtube.com/watch?v=_0sHe4q613s

 

Thank you Mr Efficiency for mentioning my name (sorry our posts overlapped)

I write this on my Skoota 28 powercat, which my wife and I are currently living on in the PNW. We go north to Canada next week

You don't say what you want to use your boat for, which would help the discussion.

Our Skoota is 28ft long and 14ft wide. Powered by two 20hp outboards it does 16.5knots flat out and cruises at 12 and 6mpg, depending on loading. I have put more on my website here, so won't repeat it

http://sailingcatamarans.com/index.php/designs/6-powercats/264-skoota-28

If you have diesel engines you'll need to have rudders and possibly beaching skegs (otherwise the props will be the deepest part of the boat) which will add weight and complication

If you only use a central engine you may well need a bowthruster to help maneuver, you will be more at risk from flotsam, and your top speed will be limited because of aeration, unless you fit a deep nacelle.

The vast majority of boats are powered by a single outboard. A twin outboard arrangement is twice as safe and allows you to run a single engine at slow speeds. My two engines weigh under 240lbs for 40hp and I do 6mpg at cruising speed, EFI outboards would be better. A diesel engine is always heavier than its base weight because of shafts, cooling, controls, fuel etc.

You started this thread by asking about rocker. I think others have already answered your question. You have to remember that a hull is a three dimensional object, so you cannot look at one view in isolation. Again, I write more on my website in the Useful Articles section.

The video you show is of a highly modified boat, even more so now. I'm not sure that it can be called one of my designs any more - after all it has gone from 28ft - 35ft. Garret told me he should have built a Romany, now he has something like a Vardo

What do you mean by "too heavy to move around by myself" Do you mean when afloat, or are you looking at a trailable boat as opposed to a transportable one? Big thread drift coming up.....

Richard Woods of Woods Designs

www.sailingcatamarans.cm

 

I do like the displacement power cat idea, but resolving the problems of a single engine installation is something that does not appear to have happened just yet. I don't particularly like the idea of needing to run two engines either, because it is never as fuel efficient as a single. And so far as overall length is concerned, below 28 feet it gets a little harder, as to keep the length/beam beam ratio up for best efficiency (12:1 or more) the slim hulls are sensitive to fore-and-aft weight shifts, and may pitch excessively, especially if the "ends" are not relatively full. And that rules out a lot of rocker. Don't look at sailing cats as a guide to power hull design, they don't travel upwind, and the line of propulsive force is not under water, either. Altogether different.

 

Hello Mr. Woods, You have a real winner in the Skoota 28 it is about perfect for one or two people. The only thing I saw on it that I didn't understand the reason is, from all the pictures and videos it seems that the hulls are not usable space. I'm 68 years old and can't spend the time building one. It would be my first and I'm sure that would add even more time to it. The other problem is I would have to trailer it to the Gulf Coast and assemble it ( by myself) I want to follow the coast south to Central America, Vensuela, and then back to the Gulf Coast through the islands. I am sure I will need to haul extra fuel from time to time stored on deck, and I don't want it to be gasoline. I really don't need anything other than a big box "nailed to the deck" to sleep in and store a bit of food and water. When I get through I will sell whatever is left and head back to the desert of Texas

 

Thank you for your comments, which will help everyone

First, I'd agree, don't look at sailing catamaran hull shapes if you want a powercat. After all you won't look at a sailing monohull if you want a trawler yacht, even if they travel at the same speed.

To answer your specific Skoota questions. And remember I am writing on our Skoota right now, so I am looking around and seeing what we have on board as a liveaboard couple.

The interior space deliberately has no "boaty" bits in it. You don't lift the floor boats to find a smelly engine, nor sleep with your feet by the anchor chain. All that is outside. The aft cockpit lockers are big enough to sleep in, but actually hold the fuel tanks, BBQ, spare fuel cans, tools, cockpit cushions, sewing machine etc. They are watertight so can stow those "must keep dry" things

I always draw separate anchor locker as its dangerous to put stuff on top of your anchor. Separate lockers for fenders and warps makes things easier. The bow lockers (which would be sail lockers on a sail boat) hold the inflatable dinghy, paint cans, spare water cans, a cooler

Inside the cabin we have dry food for several months (its cheaper in the US than Canada so we buy in bulk), clothes for 5 months, 4 computers (I draw on board) and we have enough fuel to get us to Desolation Sound from Puget Sound

Our living space is basically a 12ft x 10ft rectangular box with standing headroom.

There are no skin fittings at all, no bilge pumps because we have no bilges. Each hull is divided into seven watertight compartments, so we cannot sink (even if you took both hulls away the central cabin would still float.)

I assume you will be launching your boat at a boatyard, not from a slipway? So it sounds as though you will not be trailering regularly. So why not use the boatyard facilities as every cruising boat owner does. When we assembled our Skoota we had a team available to man/woman handle it all into place, but the crane driver did it for us during his lunch break. Took him 10 minutes, it would have taken us an hour.

I have done most of the trip you plan, except the Gulf section. I would avoid Venezuela if I were you. And I think I'd go the other way round as the E wind/current along the S America coast is worse than the Gulf Stream going north past Belize. You'd also have time to get used to the boat by doing the islands first

Maybe we'll see you "out there doing it" as we plan to trail our Skoota to the east coast on a couple of years for another Keys/Bahamas trip

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

Hmm, this unsual cat sits in texas coast already, remove the silly crane mast.
http://miami.craigslist.org/mdc/boa/4350670903.html

 

I think you are stuck with twin outboard motors, at 24-28 feet, as you can't get twin diesels that would fit, or have sufficient hp, or not exceed desirable weight limits. Four strokes would be best, or maybe DFI 2-strokes. There is some scope to run on one engine at times, to conserve fuel, more likely in calmer conditions. In any case, you must have a back-up engine to be self-reliant. The thing you must do is control weight, or else you have a slug, you can't load these things up with weight and not notice a big difference.

 

If I were going to live on the boat or even keep the boat, spending $30K to $50K and working for a year would make some sense. In reality I live in a desert, when I get through with my trip I'll probably never see the ocean or even a large lake again. If plans were available for a pair of power cat hulls that were from X' to Y' long and would support X to Y pounds of weight to be used however the buyer wants, I would take a serious look at doing it. But working for a year and spending $50K for a vacation just does not make sense to me. If I find another way, I'll give it a shot.

Now, what would happen if you used to 30" diameter X 24' aluminum pontoons and raised the deck off the pontoons by 3 or 4 feet?

 

If you are not going to get that much use from it you might consider renting a boat.

Just one of many such sites: http://www.lakepowell.com/houseboating/houseboats/weekender.aspx

 

I can't sail and don't want to learn. I'm going by myself so, no crew help. That would leave any big boats out. I don't know what the per day rental rate is but I think it will take me at least 6 months to make the trip. Following the coast around the Texas gulf, mexico and to Central America is going to bang it up a lot. And, if I get tired and quit, I'll sell it or leave it and fly home. Now if I could find someone who wants to make the trip, has the time, and a boat, I'll buy in.

 

Good luck.