Outrigger Cabin Cruiser Design
This is a design concept for an outrigger cabin cruising motor boat for the Pacific Northwest/Inside Passage waters that I could use some help with. This is just a rough sketch to work through some ideas. Once I finalize some ideas I will start putting it into design software. Building would be a couple of years away.
1. Trailerable for short distances
2. Demountable (no folding)
3. Very lightweight. Essentially an over-sized camping boat.
4. Very efficient. My desire is 10-12 knots cruising, maybe 15 knots top, with 25hp Honda.
As drawn it is 32' with a beam at about 15'. With no sail rig, and lightly built, I am thinking dry weight around 2200-2600lbs, based on other boats in this class.
My boating experience is mostly from sailboats, both multihull and mono. I have designed and built a 23' trimaran from beach cat hulls in the past, and a number of other small boat building projects.
So I have a number of questions that I could use input on.
Let's start with the first. If you look at the drawing you see three different motor mounting options:
2. Side mounted
3. Split stern
I am strongly drawn to the side mount option for these reasons:
1. In chop and waves, from my sailboat experience, I have found that transom mounted motor props tend to plunge in and out of the waves. Maybe that would not be the case with a boat like this with little rocker? Also, the transom mount lengths the trailer length and exposes the motor to damage when backing up.
2. Side mount would move weight forward and reduce the ventilating problem described above. Also, I am thinking that the outer hull will create some drag, and the side mounted motor might offset this. How much drag would there be, and would this motor position be a good idea for that reason?
Problems with side mount:
1. Another surface piercing surface, less efficient? How much less efficient?
2. The major problem I see is less effective steering (I don't want a rudder). That is why the hull is drawn differently for that configuration, to allow the prop wash to move under the hull when turning.
The third mounting option is the split stern. This would be more difficult to build, and add weight. But it would be a nice compromise between the other two. It would move the motor forward, and allow the deeper hull shape to carry further back. Steering would be better vs. the side mount. Trailer length is preserved, and the motor is protected when trailering. The question is, is it worth it?
So I am looking forward to hearing all opinions on this motor mount question, and any other comments on this design concept. Thanks very much!
Russell Brown built a similar Proa for a guy in the PNW, called it a "pick up truck" type commuter for the guys island home, some cool video of it here at around 1:30 on the counter:
You might do better with a swing in float that folds along side makes boat much more marina freindly. Look at system used on dragonfly trimarans. The fold under set up you show basicly the Farrier design does not work for folded condition while in water. The float is on its side when folded. The swing fold system works fine at a dock.
Another motor option to consider is well mounted. A well just aft of the cabin would accomplish what you are looking for and be easier and lighter than than your "split stern."
Side mounted engines involve complexity and invention and sometimes experimentation.
I would keep it simple and transom mount the engine conventionally for a variety of reasons, starting with "conventional." No inventing, works, things like steering and controls are ready to install with no modifications, etc.
Having the engine exposed "when backing" will be .0001% of the time you are using the boat.
eyschulman - Thanks. Actually as I have it designed now it is not a folding design, and is demountable. The trailer view is just showing how it would travel. I rarely come to dock when cruising (other than fuel docks occasionally). And when not in use it will go back on the trailer. So I plan on keeping it simple and light, bolting the cross beams to bulkheads.
The outboard well is an option. I had one on a T-Bird though, and it would eat it's own gases, even with the lid off. And corrosion was a problem. And I would probably have to come up with some vertical lifting system, so the slot in the hull would not be too big. So I am not actually sure it would be that much less work than a split stern. But I will think about it more.
I side mounted a motor on my little trimaran and it was great. I actually think it would be far simpler to do than a well, or a split stern. My main concern is if the boat would steer without a rudder? Or if I am overlooking some other problem with the side mount?
Transom mounted would be the easiest, and maybe the best. But I figure that this is purely a motor boat, and I don't have all the work of a sailing rig to figure out, so if a better motor placement involves a bit more work I am willing.
The mount you describe on your trimaram and the type I am familiar with are great on cats and tri's because they retract and give a clean boat to sail. You don't need that with this vessel.
Your power Proa will have less rocker as you noted, but also when underway will be slightly bow up which will put the engine end more firmly in the water. This boat should not ventilate any more than a mono powerboat, it is a mono powerboat just a little skinnier with an added ama.
The Jedi Proa Master mounted the outboard on his "pick up truck" on the transom, suggest you consider same.
Done then! Transom mount it is.
Next question - Underwater hull shape.
From my research it seems like the most efficient underwater shape for a displacement power cat when speeds get above 7 knots is the Tennant CS hull type. I could see that working very well for this long slender hull, carrying the canoe shape all the way back, and a wider box transom that sits a bit under the waterline to help keep the boat from squatting in the stern when under power (see drawing).
This hull type is talked about here:
hull shape of cruising power v sail cat
And here is a variation of the same hull type:
Schionning also uses this hull type on smaller cats with outboards like the Prowler 9000.
Does anyone see any problems using that type of hull on this craft?
Your design looks very good, am pretty impressed.
Cant really find ways of providing improvements (maybe outrigger a fraction longer?), all I can do is make you aware of a similar 10m outrigger, (though it has sails). This one is a bit unrefined, though large and simple, I think the outrigger pivots horizontally as per drangonfly trimaran.
With that long skeg, your turning radius may be a fraction large (may/maynot be an issue). Your crossbeams look a fraction thin just near the outrigger, a big wave from the port side may tend to bury the outrigger and put a lot of load on the crossbeams
Possibly you could eliminate some of those compound curves. Plywood curved one way (or straight) would be easier to build. (just a polite suggestion). Given all that, it would be an efficient, fast, stable and nice boat to travel on.
Just in case you are not aware of it
If your serious about building you can contact the owner of this 10m tacking proa on facebook to ask any questions.
n peter evans
My other most polite suggestion, might be to move the superstructure aft say 18 inches.
Generally the bow is going to be for wave piercing and have the most pitching. The area 2/3 aft would be the most comfortable part of the boat. I woud not really go further than 18 inches... just to me it would look a fraction nicer.
Is now the next day here, and after sleeping on it I think that the lines do look very good.
If you chose to move cabin aft, then the hull would have to be changed so that it is a fraction finer foward, and a fraction fuller aft, probably resulting in lower efficiency. So maybe it looks best where it is. Yes having weight forward seems to give a better ride, I noticed this when sailing my first tacking outrigger
Big Power Outriggers
Here are a few pictures of some bigger versions of what I am thinking about. The first set is a 70 footer I saw in Belize seven years ago. The other one I believe is in Texas or Florida.
Two interesting boats,
the Belize one looks like a lot of hull, and not much structure, I guess that makes it efficeint and fast (is that a daggerboard aft?). I guess simple hulls without cabins, plumbing, electics are easy to make, thus having a big large outrigger only adds a little to the cost. It looks like a big boat, but chances are that the owners are very happy with it (low fuel bills, good cruising speed, stable platfrom, shoal draught, to clean the hulls you leave it on a sandbank at high tide, wait for the tide to go down then start scrubbing). I can see a lot of flat panels and simple curves that would make it easy to make (assume plywood)
I never knew such boats existed, would be great to hear from the owners and see how they like their boats. My guess is that they are happy visiting tropical islands and internet comes a long way second.
Note that these two boats have outriggers of almost same length. The crossbeams on that Texan boat are sure not going to break in a hurry (do they really need three huge crossbeams?)
Did you take photos of Belize boats?
I assume you are aware of this one too, Asean Lady. 7 star hotel ship/boat. Very ugly but apparently very very upmarket. You would think they could improve the styling.
it has a small outrigger like yours.
I dont think much work has been done on power proas, as to what is the ideal main hull/small hull size ratio,
here are 3 small ones
I dont really know if these boats have a proper name. Do you call them power proas, do you call them assymetric catamarans. I think power proa sounds better even if the proa enthusiasts get upset.
aside ... its 41C max temp here in Melbourne today. darn global warming
Peter - I think the Belize boat was just a ferry designed to get people efficiently and comfortably out to Glover's Reef to their camping resort. I don't think they desired any accomodations. It is about 30 miles offshore. I bet it is nice. I once was a passenger on a 30 foot planning boat out to the Blue Hole on another Atoll in Belize and it was a horrible experience, dropping off waves and slamming. I had blisters on my hands and my muscles were so tense for hours that I could not relax to pee at the end, which I desperately needed to do!
And yes I believe it does have a daggerboard. That is something I have been wondering about. What would be the value of a daggerboard on my boat? I assume in strong winds my boat could probably benefit from one. I would put it in the outer hull. But I have never seen one besides this boat on a motorboat.
I took the second picture of the Belize boat seven years ago when I was anchored out on Glover's Reef.
I agree, the second boat seems to have gone very overboard on the cross beams. I bet those really slam when the outrigger goes through waves.
I think for the size of my outrigger I am going to do something like Paul Bieker's new proa design he is working on. It just happens this boat is the same length at 32'. My boat will probably weight more, but I may not need as much displacement because there is no risk of getting caught aback without sails, and I don't have any intentions of going offshore.
P.S. I live in Tucson, AZ, so heat is nothing new to me. Fortunately it is nice and cool here right now.
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