Motor Cabin Cruiser
I would prefer a double outrigger….
Yes, there are lots of power trimarans, and they seem to make more sense.
But here is why I am leaning toward the single outrigger for what I want:
1. With a side exit hatch and small cockpit I don’t have to split my accommodations down the middle. Not having a traditional stern cockpit allows me to have that nice big double birth in the stern. I don’t much care for v-berths, particularly when there are waves at anchor.
2. From what I have read the motion at sea is not supposed to be very kindly on small narrow beam power trimarans. In particular in swells and waves I have read that they have a very uncomfortable motion, snapping quickly out of a roll over the crest. The single outrigger configuration is more like a catamaran and is suppose to be more comfortable. But I have not been on one of those power trimarans in those situations, so who knows? But it does seem like there would be more rolling in waves, like a regular powerboat. Because I believe that to keep a power trimaran efficient you have to keep the little outriggers far astern and close to the main hull to keep them out of the bow wave of the main hull??? Although I have noticed that the big ocean going power trimarans have a very wide beam and very high cross beams.
3. I like the great place to store the nested dingy in my design.
4. It just seems simpler. One set of cross beams and attachment points to build instead of four. One outer hull instead of two. One thing to take apart and stow for trailering instead of two.
5. Here is another important consideration. This boat will be used for fun day trips and longer cruising trips. With a small trimaran, which displacement do you design for? When not loaded the outer hulls will be above the water, and the boat will flop back and forth between them at anchor and underway. And if not designed exactly right when the boat is loaded the outer hulls will be planted in the water, instead of lightly kissing the top, which would not be great either. But with the single outrigger the boat self adjusts and the outer hull is always sitting lightly on the water. And I think initially I will make the attachment to the outer hull adjustable, so I can get the trim like I want it once the boat is launched. Because I am not a real designer, and doubt my ability to know exactly where the boat is going to sit once it is launched.
The question of having to steer to adjust for the outrigger drag was bothering me. But I don’t think it is going to be a big deal. The main hull is going to have almost no rocker. And the outer hull is going to be very light. So I think the boat is going to want to drive straight for the most part. Besides, unless it is dead calm and flat, when aren’t you adjusting your steering in the sea?
I have decided to just transom mount the motor because I don’t really see this being a big enough of a problem to side mount it. But I would appreciate hearing from someone with experience motoring a proa to confirm this. Although sailing proas tend to have a lot of rocker and so it would be different.
P.S. Sorry for my long winded posts.
Looking around for other motor outriggers I came across the top picture below that Chris White took in Belize. I started looking around the web to try and learn more and find more pictures. Then I realized that I had seen that boat myself when in Belize seven years ago. It is what the Glovers Atoll Reef Resort folks use to bring their guests out to the island. I had chartered a catamaran and anchored off the beach from it. So it turns out I had my own picture of it (the second image). Funny. I must be getting older.
The boat is 70’. It is an outrigger, but the outer hull is nearly the length of the main hull. But limited accommodations, dagger board, and I believe outboard power are in the main hull.
I would prefer a double outrigger….
Yes, there are lots of power trimarans, and they seem to make more sense.
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5. Here is another important consideration. This boat will be used for fun day trips and longer cruising trips. With a small trimaran, which displacement do you design for? When not loaded the outer hulls will be above the water, and the boat will flop back and forth between them at anchor and underway. And if not designed exactly right when the boat is loaded the outer hulls will be planted in the water, instead of lightly kissing the top, which would not be great either. But with the single outrigger the boat self adjusts and the outer hull is always sitting lightly on the water. And I think initially I will make the attachment to the outer hull adjustable, so I can get the trim like I want it once the boat is launched. Because I am not a real designer, and doubt my ability to know exactly where the boat is going to sit once it is launched.
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I agree with your point 5. To me, trimarans make no sense as power boats. Amas are very draggy for the amount they displace, so why have two when you only need one? In a seaway, a proa will roll nicely with the swell with both hulls firmly planted in the water, whereas a trimarans amas will either be suspended above the surface or being dragged through it, and they will slam as they alternate between one or the other.
Below, for your amusement, are a couple if images for a proa motor yacht concept I did quite a few years ago.
Below, for your amusement, are a couple if images for a proa motor yacht concept I did quite a few years ago.
Holy crap, a lap pool and hot tub on the outrigger. Why didn’t I think of that? Put your water ballast to good use! I assume the pool would have a zero infinity edge?
So here is a question. Notice on the boat from Belize that I posted above, it has a daggerboard?
It seems like a good idea for my design to have a daggerboard. Wouldn’t it help reduce side slip on windy days? Seems like it would save a lot of fuel. But I have never seen a motor cat with a daggerboard, and I wonder why? Maybe they use keels instead?
And if I used one, would putting it in the outer hull be the best spot like with Paul Bieker’s design?
I agree with Galen’s point no. 5. Varying load is a big problem with power trimarans, causing the amas to be either too immersed or too high - a problem that the proa handles automatically.
So here is a question. Notice on the boat from Belize that I posted above, it has a daggerboard?
It seems like a good idea for my design to have a daggerboard. Wouldn’t it help reduce side slip on windy days? Seems like it would save a lot of fuel. But I have never seen a motor cat with a daggerboard, and I wonder why? Maybe they use keels instead?
That boat from Belize, I swear it’s a later incarnation of a sailing proa that used to be called Valkyrie, which would explain why it has a daggerboard. Personally I’d opt for a very shallow keel rather than a daggerboard. No crash worries and no encroachment on the interior.
That boat from Belize, I swear it’s a later incarnation of a sailing proa that used to be called Valkyrie, which would explain why it has a daggerboard. Personally I’d opt for a very shallow keel rather than a daggerboard. No crash worries and no encroachment on the interior.
Mike, I think you are correct about the Belize Proa. I saw that it was named Walkyrie. What else do you know about it?
My concern with a keel is the trailering issue. What do you imagine the dimensions would be for this boat?
I went rummaging in an old box of AYRS publications, I found a photo of Walkyrie in AYRS No. 121, an Atlantic proa of 68’ owned by Pierre-Yves Corre de Dufau de Maluquer, with a 13/12/92 date stamp. It had a stayed sloop rig. The Atlantic style would explain the large ama.
My concern with a keel is the trailering issue. What do you imagine the dimensions would be for this boat?
Powerboats always crab into a strong side wind in order to maintain a heading. A daggerboard might help with that, but really, in Puget Sound it seems it is always flat, or the wind is from dead ahead or astern, so a daggerboard won’t be of much value. The keel is more for tracking, but since you will have two long and slender hulls, tracking won’t be a problem, if anything, turning will be the problem. I’d suggest leaving the keel off, and if it is a problem then you can always add one.
I assume the pool would have a zero infinity edge?
Of course!
Powerboats always crab into a strong side wind in order to maintain a heading. A daggerboard might help with that, but really, in Puget Sound it seems it is always flat, or the wind is from dead ahead or astern, so a daggerboard won’t be of much value. The keel is more for tracking, but since you will have two long and slender hulls, tracking won’t be a problem, if anything, turning will be the problem. I’d suggest leaving the keel off, and if it is a problem then you can always add one.
I am not sure yet. I read recently about sailing catamarans that when they loose the motor in one hull, if they keep a dagger board down in the hull that has a working motor they can turn easier, and in particular maneuver better at slow speeds when docking. Also beach cats with little rocker that have no dagger boards tend to have more difficulty tacking than beach cats with little rocker that do have boards. So in my design I am wondering if that would be true also, giving the boat a pivot point for turning when docking, etc. That might just be a side benefit of having a daggerboard, and certainly not a reason alone for it.
Fuel efficiency is still a question for me with a daggerboard. I understand that motor boats do crab in a wind, but is it efficient fuel wise? Motor boat designs and users tend to use the “power through it” principle. But I am looking at very low power and plan to use this boat for long distance trips up the inside passage, so fuel economy is very important, so smart hull design is important. It just seems that if the wind is on the beam or quarter, having a daggerboard would increase fuel efficiency. But that is the problem about not being a designer. I can’t run it through a computer program. Would the added drag and weight compensate for the fuel lost to crabbing. I guess it is time to start learning how to use Free!Ship.
On this topic of a daggerboard, I posted a question on the design forum about why motorcats don’t use them. One person said they are not necessary, but one person said they use them on their powerboat (not a cat) to good effect. It is a trawler designed to go around the world without refueling. The site is french:
http://long-cours.62.over-blog.com/article-derives-100689323.html
You can see one of the daggerboards in the picture. They claim that with one board down on the opposite side of the wind direction they gain a few degrees of heading angle, which they feel reduces a good bit of rudder drag and improves the flow around the hull when the drift is less. They don’t know if it really improves fuel efficiency though.
I’m surprise this video hasn’t come up in this thread. Russ did a great little power proa.
http://proafile.com/magazine/article/325
Tom
An interesting outboard motor mount on a French sailboat - scroll down.
Thanks. What a great boat. It looks like a wonderful design for their purpose.
My most recent thinking about the motor placement is trending toward something very similar. An outboard well, open to the stern, but the transom will still be closed at the top, preserving strength and looks.
One nice benefit is that the motor is easy to access, instead of hanging off the back of the transom, if you have to work on it or pull it out.
I have been talking with Ron Mueller, who I knew in Bellingham years ago. He built an Ecocat, that is powered by 20hp. He cruises in the NW, at 10-15knts, and burns less than a gallon an hour at just about any speed. His boat weights around 1300 dry, 1700 wet. I figure my boat will weight 1800lbs dry, and I should get similar fuel economy with my longer water line? So with a 60 gallon fuel tank and I could have something like 600-900 nautical miles in range? That would be amazing! OK, maybe 30 gallons or less would be more reasonable on a boat this size. I have been trying to figure out how much fuel so I can figure out space and weight for the tank.
On a side note, the designer of Ron’s cat has a new building method that I have never seen and find amazing. You must see this if you have not already. It just seems brilliant.