Motor Cabin Cruiser
This concept is for a lightweight and very fuel efficient cabin cruiser motor boat for the Puget Sound/Inside Passage waters.
You can read more about why I want this in my community post, and some of my background from Port Townsend.
Goals are:
1. Trailerable for short distances. Demountable outer hull (not folding).
2. Cruising speeds of 10-12 knots, but capable of 15 or more. Displacement hull.
3. Very fuel efficient. I am hoping to get the speeds above with a 25hp Honda.
4. Very light build.
Currently it is drawn as 32 feet, 15.5 foot beam.
There are many many things I don’t understand yet about this idea and would like some help with. I will go through each one in turn in later posts.
Actual building of it would be a least a couple of years away. So there is plenty of time to hash out the idea.
Any questions or comments are very welcome.
Galen
The first question is about the distance between the hulls. I am sure it does not need anywhere near the beam of a pacific sailing proa. As it’s drawn the full beam of the boat is a bit over 15’, around 12’ from center lines.
Figure the outrigger is maybe 80lbs, and maybe another 80lbs per aka. Throw a dingy on the tramp and there is another 80 lbs. There will be a lightweight cockpit and folding seats, another 30 lbs. So that’s around 350lbs hanging out there. Really I would like the outer hull and akas to weight much less, so they could be lifted as a single unit by two people for trailering. That might be unrealistic. But that’s another topic.
Then there is the weight balance of the main hull. If I end up with a side mounted motor that will shift some weight to the starboard also.
I really can’t imagine any circumstance where I would need water ballast in the outer hull without sails (nor a stability pod on the main hull besides the hull flair that exists).
So the question is how much beam is necessary?
It seems the benefits of a narrow beam are mostly for launching from the trailer on narrow ramps, and for docking at times (although this is not a great consideration as I nearly always anchor out.) But even for coming in to fuel docks may be easier with less beam.
And less beam means shorter akas, so a little less weight, and they may not need to be built as strong because of less leverage.
Are there more benefits to a narrow beam? Less drag resistance from the outrigger to compensate for with the motor? Easier to turn?
Besides keeping the boat from rolling over, what are the benefits of a wide beam? Maybe a kinder sea action, so that both hulls are not going over the same wave at once?
One question I wonder about is bow wave drag interaction between the hulls. If they get too close isn’t this a consideration? Maybe someone can educate me about that.
For fun I rolled the boat over in the picture below. It looks like the hull flair will give it a fair amount of ultimate stability. But the question is at what point will it want to stay in that position instead of flopping back down? It seems there is very little chance as it is drawn, unless I have a 2000lbs kitchen stove on the port side.
Any thoughts on all of this are very welcome.
Wind resistance goes up with a wider beam.
More tendancy to turn towards the outrigger with a wider beam.
As you noted, fewer slips available for that width, and higher cost.
What kind of construction methods are you considering?
I like the design, it looks like a fast and very fuel efficient motor cruiser. I suspect that 10hp or less would be enough power for this hull. The accommodations are just right for a lightweight boat and a crew used to backpacking (not taking a lot of stuff).
One of the challenges you will face is that most of the weight of the boat is up high, in the upper cabin area. Keep in mind that glass, lexan and polycarbonate are relatively heavy. You may want to limit the size of the ports and use an extremely light material for the cabin top and sides. Foam/epoxy/glass cloth?
Thanks for your comments skyl4rk.
I would love if this could be driven with something like the Yamaha 9.9. But based on other boats in this class that I have researched I think it will take 20 or more. Hopefully I am incorrect.
As for the build materials, that is a subject I will tackle down the road. I like the idea of the Farrier build, in foam/glass, building the hull deck and cabin in one piece down the middle. But I bet that the weight savings for foam will not be worth it for the price on a boat of this size compared to Plywood.
As for the beam. Having talked my way through it above, I think the answer is actually very simple. I know that I want to have a little cockpit area near the hull, for being outside at anchor and maybe when underway when the weather is nice (autopilot?). This will probably be around 3 feet wide. And I know that I want to have a dingy on the tramp. Most likely a nesting model like Russell’s PT11, or a pram. And when I stick those dimensions into my drawing it turns out that how I have it drawn is almost exactly right.
But, the dingy could also go on the cabin top, if a narrow beam is desirable for some reason I don’t understand. Although that would put another 80lbs up high, and it would be a pain compared to the ease of slipping the dinghy on and off the tramp.
Just eyeballing it, I would say the beam you have drawn will work. You could reduce it by 10% or 20%, if you felt you needed to, or if the crossbeam was flexing too much.
I am not an expert, I am learning too. https://www.youtube.com/watch?v=YTilc7xtTHI
I found this example of a big outrigger motor cruiser. They use it as a kayak mothership.
I think those huge akas would take a pounding as the ama goes through waves.
I would love if this could be driven with something like the Yamaha 9.9.
Why not two of them? Your ama will put the centre of drag out to the side, and the single engine at the stern will have to keep compensating, unless it has a fair bit of propeller walk. You could have an outboard well in the flare just aft of the forward crossbeam for a second engine. The longitudinal separation should be enough that by steering both engines separately you could turn on the spot, and the engine should be far enough forward that you could even drive the boat sideways. No bow thruster required. And either engine alone should drive your boat at modest speed in most conditions.
Regards
Robert Biegler
Why not two of them?
Robert, that is an interesting idea. Originally I liked the idea of having two motors vs one. Coming from a sailing background, it disturbs me to think about relying on one source of power.
But, while the idea of having that much turning control sounds wonderful, putting an outboard well in the place you suggest would basically put the motor inside my cabin. I could hang a sled off of the aka beam, but one of my design requirements is that the boat not be difficult to trailer. That means I have to keep the connections between the akas and the main hull to a minimum, hopefully just the bolts to the bulkheads and nothing else. Having to remove a motor/fuel tanks and connections is not going to work. Not to mention the added structural weight that would need to be lifted.
That was one of the problems with my demountable trimaran. It took so much effort to put it all together that I never wanted to do it to get it on the water, and once on I did I never wanted to take it out, which was stressful, because I would leave it anchored out in bad weather.
Another option for a dual motor setup would be on opposite sides behind the cabin as in the picture below. The boat would probably steer better than with one.
But, I have done a bit of research on having two vs one motor. There are some issues.
1. It costs more.
2. It is less fuel efficient.
3. More weight.
4. You have to build two motor structures and link steering.
Another common comment about two motors is that they often fail together, because of fuel problems (although I am sure you could have them on separate tanks and filters, which does not help if the gas is bad).
One bigger motor will most likely have an alternator with higher output, a big plus.
The benefits of two vs one don’t appear great for this design.
So I have been thinking I will stick with one side mounted motor. It will offset the ama drag, move the weight forward, which will mean the stern does not have to be as full bodied, and prevent as much cavitation in short chop. The only problem I have with this is that I don’t know if I will be able to steer the boat effectively without a rudder.
For auxiliary power my thoughts have been leaning toward a 2-4hp dingy motor. If the main motor fails, maybe have a place to mount that little motor. Or just tie the dinghy under the akas and power the boat from the dinghy transom. It think it would probably drive this lean hull at 4-5 knots.
All thoughts are welcome.
Outboards are considerably more efficient when they are placed aft of a transom, rather than hanging off the side of the hull. How much more? I don’t know, but I do know that at Glacier Bay Cats, they tried and tried to get a center OB working between the hulls, and they finally gave up and used twins. I guess it’s just the way God and Honda intended it.
Outboards are considerably more efficient when they are placed aft of a transom, rather than hanging off the side of the hull.
Interesting. Maybe it is because they are another water piercing surface? More drag?
I know that Ron Mueller on his Economy power cat switched from twins to a single motor in the center and greatly preferrs it. Although I can’t remember why he found it better
http://ikarus342000.com/ECOmotorboat.htm
The concern that I have with the motor on the transom is in steep chop how it will plunge in and out of the water. At least that is my experience on sailboats. But that vast majority of power cats with outboards do put them on the transom, so maybe it is not as much of an issue, particularly without as much rocker in the hull?
I’m out of my league here, just repeating what I’ve heard. The prop likes a consistent and clean flow of water, something it gets almost automatically when on the transom. The cavitation plate lines up with the bottom and it all works nicely together. The flow in the catamaran tunnel is choppy, full of foam. A way to make it work is to build a “mini-hull” sled that smooths out the flow, You can see that on some large sailing cats with the outboards mounted on transomed sleds that are slung beneath the bridge deck. IMHO this is not an elegant solution for a power boat.
Your concern about the transom outboard plunging in a chop is a good question. I’ve seen some low-power outboard boats with the outboard mounted fwd of the transom in a well, the Redwing series, for example. Might be the best of both worlds.
Kurtz Hughes has a 23’ power “proa” and says he’s thinking on future designs using this configuration. Runs with a 4hp and does 13 knots economically. It is side hung. He used an old ama. He also made this.
Then there is Arpex.
Mike, I do think that the air flow in the tunnel on a power cat does create a lot of turbulent water. I think on some the tunnel is actually designed to create lift. And the hulls on many power cats are so close together that I believe the bow waves coming together also create a lot of turbulence in the tunnel. But without the tunnel I wonder if the same issues would be a problem for this design?
Another feature that I like about a side mounted motor is less trailer length and risk of damage to the outboard backing the trailer up.
I have considered an outboard well. But I had a well on the T-Bird. The outboard tended to get starved of air and would eat it’s own gases, even with the lid off. Folks also say that the corrosion factor goes way up in a well which I found to be true. You also have to have a big slot in your hull for lifting the motor up, unless you create some type of vertical lifting system. Although I have seen the Calkins Bartenders http://www.bartenderboats.com use an open well for an outboard, and they have a special hull fairing plug that lifts up when you lift the motor. Either way, it adds lots of complexity to the build.
Another option I have considered is a split transom, moving the motor back a bit, instead of a well. Many boats do this. But it does add a lot of weight and complexity to the build, and for how much gain?
Alex - I have seen Kurt’s little motor proa.
http://www.multihulldesigns.com/designs_other/sp_powerproa23.htm
I noticed that in some of the pictures there appears to be no rudder. But in the main picture it looks like he added one afterward, and now has a wheel.
As I said above, that is my main concern with a side mounted motor, that I would need a rudder, which I don’t want. That is why the underwater shape in the top two drawings is different, so that when I turn the prop wash has somewhere to go, instead of it pushing against the hull. But this is pure speculation, because I really have no idea if that would be a problem. Unlike in Kurt’s small boat above, the motor will be much further back, so maybe it won’t be an issue?
Kurt also has a design for a 38’ power trimaran, that has dual side mounted motors.
http://www.multihulldesigns.com/designs_stock/38tri.html
It does not have a rudder. It looks like one of these has been built. Maybe I will try and contact the owners to see what there experience is with those motor positions (although I would only use one).
Mike - I did some more research into this problem of a centrally mounted motor on a motor cat. And it is like I thought. It occurs on power planing cats with very narrow beam. It creates such turbulence between the hulls that the motor is sucking a lot of air, and it is not efficient. If the hulls are spaced farther apart, like a sailing cat, they don’t have as much of a problem. Also, because they are planing hulls, the boat lifts and skips across the tops of the waves, and the motor comes in and out of those wave tops in the center.
But my design is a displacement hull, so it won’t be lifting and skipping across the tops of waves. And also with it’s wider beam it should not suffer from bow waves. Plus the speeds will be much slower than a fast planning cat.
So I am strongly leaning towards the side mount. It is a compromise for sure though.
Now the question is will the boat turn with just the motor (no rudder) in that position? I expect it would depend highly on how much rocker the hull has.
Thoughts anyone?
Maybe the split stern is worth considering. It ads weight and work, but it might be very good. It moves the motor inboard enough that it likely won’t come free of the water very much in a steep chop. Plus the motor can be set a little deeper in the water than the side mount because of the transom depth. It will be more protected from waves and have cleaner water. It would steer better. And it would keep my trailer length down. It also lets me keep a fuller shape for longer underwater.
I would prefer a double outrigger. Amas could be extremely light, and be placed above the water surface or just touching it, producing little or no resistance at all. Furthermore, the boat would require no rudder action to keep course. There are many motor trimarans, mainly big ships. Overall beam is about 1/3 length.
Regards,
Fulgencio García