# I need help please. Basic power calculations.

Discussion in 'Boat Design' started by Trihulled SWATH, Jun 1, 2017.

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1. Joined: Jun 2017
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### Trihulled SWATHJunior Member

Ahoy ! And hello everyone,

I have conceptualized a yacht that I would like to build. I have had the concept for three years now. And have worked out a rough layout of spaces. But the power is completely unknown to me.

The concept for the yacht is that it would offer great comfort and space relatively economically. At least until you get a full complement of crew. To give you an idea of what I contemplate. It would be 200 tons displacement. It would be 120 feet long, with an 80 foot beam. And it stands at least 30 feet high.

I have liked SWATH hulls from the first time I learned about them. I like submarines as well. And I have an unusual desire that is incorporated in my concept. And has admittedly, driven the concept. I want to take pleasure crafts along. So I have conceptualized having port and starboard boat "garages" inboard, between the uprights supporting the superstructure. Each capable of holding two boats. I dream of having a Venetiam limousine and a "go fast" to port, and at least one center console to starboard. And space for two.

I envision having ten feet between the top of the displacement hulls (unballasted) and the bottom of the boat garage deck. Which I refer to as the boat deck. Though there are many accommodations on this deck.

Of the three displacement hulls, I want to make the center one, very much longer than the port and starboard displacement hulls. All three hulls, I would prefer to make out of steel. And probably about six feet round, or in diameter. The calculations come in about 200 tons then. And the port and starboard displacement hulls can roughly be taken off the cabin spaces found above them, and outboard of the "garages". I believe these two hulls may be approximately 70 feet. I am thinking four staterooms both to port and to starboard, and a couple more staterooms found along the leading edge of this deck, again, to port and starboard. That's 12 guest rooms.

One deck up would be the main deck, with all the normal arrangements, along the center line, with a master on this main deck forward. But huge open areas created above the garages and cabins below. Imagine walking out to the rail, and its 30 feet away. Or being in one of these entertaining and living areas, and having huge outdoor areas to either side. It is also possible to have the master on the Boat Deck forward. Which would give the Main Deck an amazing sense of space for this sized vessel.

My initial take on this was that it was to be a motor sailing barge. But then, more elegant thoughts emerged. And the next thing you know, I'd be trying to make thirty knots. ... Realistically though, I think twenty should be conceivable.

I have an idea for the mechanical spaces. There would be three engines of course. And I'm kind of taken with the idea of pod drives. Of the normal pleasure craft variety. Efficiency, steering , and station holding being the attractions. But I would want them placed such that their props are only as low as the bottom of the hulls. The hulls thereby protecting them. I know, or at least I think, the water flow to the props will be disturbed, coming up along the hulls. But I am willing to live with that.

I would like to have the benefit of any knowledgeable person(s). I know I could make 5 knots. But what do you think? I have been thinking about having access to the mechanical spaces, such as they are, through three foot steel pipes. Which would also be both structural and mechanical in that they would vent the spaces. They would be faired of course.

The intended use would be coastal cruising and the Caribbean. And it's also intended to be useful as a specialty charter. ... The idea of having any kind of turn of speed would give the ability to get out of the way of a hurricane. Which is always the preferred way to handle that. I think another 10 knots would be a lot when you're trying to figure out if you are going to zig or zag.

I would love to be able to have the drive line in a straight line. I'm fairly certain that the hulls would need to be modified (out of round and scale) to accept the power, and be serviceable. There is likely to be a "bump" there. And I saw photos online once of a SWATH with such a bump. And I hate that because of the added drag and that look. But I am flying blind so far as what a top end could be. And that is what I'm wondering about.

I know this is a different take on things. But I can't help it. It seems like it can be so nice. Any help would be much appreciated. As an aside. I hired Don Walsh and L. Bruce Jones many years ago when I had an interest in submarines. They were awesome people. ... And there will be work for the right firm when the time comes . ... Right now, I'm just thinking about my martini glass not showing the slightest hint of any motion. Oh, and I'm an audiophile. And I'll hide that room on the boat deck on the center line.

The down sides are going to be the overall look of this. It will take some doing I think to get anything at all resembling elegance. And there is nothing as graceful as the accommodation like the fantail of a canoe stern Feadship. That space, on this boat, is ten feet up if not twenty. So you've lost the connection to the water up there. And I think that is a pity.

Any help is appreciated. My thanks in advance.

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### TANSLSenior Member

Welcome to the forum. I hope you can get here all the help you need, which is a lot. To make things easier, it would be very interesting to be able to analyze a general layout drawing that accurately reflects the ideas you have.

Last edited: Jun 2, 2017
3. Joined: Oct 2008
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Welcome to the forum.

If you like the idea of a SWATH - for whatever reason - why spoil it by going with 3 hulls rather than two?

Forget martini glasses and venetian limousines....concentrate on the SOR..the rest is simple details and background noise. Get the basic rights first!

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### TANSLSenior Member

He wants a "trihulled swath", let's you know what that is. That is why it is imperative to have a sketch for his GA.

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### keroseneSenior Member

200 tons... what is the going price per ton of new construction?

6. Joined: Jun 2017
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### Trihulled SWATHJunior Member

Thank you for your warm welcomes. Being that this is my first post here. I am already gaining a sense of the great amount of wisdom and good will here. It seems you have a nice community here. And though I didn't have any preconceived notions. It makes sense. Boaters are a great bunch.

And I have just had a revelation. This is one heck of an example of mission creep. I initially thought of an asymmetrical vessel like a Polynesian outrigger. A vessel with a main hull, and then a smaller hull off to one side. I thought it could be elegant. Then I decided to go SWATH with it. But maintaining the asymmetry. I wanted to have what I am referring to as a boat garage. ... Then, because I really like symmetry. And I wanted more space and boats. I decided to go with three hulls. And two garages.

One of the things I want is a shallow draft. That being relative to a boat of this size. I thought a 5 or 6 foot draft was possible. And that still seems possible. The boat would need to be deballasted about 5 feet to achieve the shallow draft.

I was thinking about what I described earlier as a "motor sailing barge". It was to be characterized by simple, robust construction. Smaller main propulsion. I was thinking one engine at one time. 6-8 knots would be OK. And three displacement hulls meant that the span between hulls might be 35 feet instead of 70 feet. It just seemed sensible at the time. Hydrodynamic efficiency wasn't an important, or even remote consideration. And a great amount of space would be available.

Then, with the slow but sure mission creep. Over the years, I started wanting more sophistication. More elegance. And eventually, more speed.

I am posting this. And will post another image of a deck plan.

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### Trihulled SWATHJunior Member

Sorry my drawings are so rough. I just used them to organize the thoughts in my head. And look at space use. The area to port is the boat garage. Staterooms can be outboard of this. And on the leading edge of the deck.

The suggestion about sticking with traditional SWATH seemed sensible. After all. We don't see a lot of Small Waterline And Triple Hulls. ... I did a calculation of frontal area. The triple had about 25% more area. That could be reduced to be competitive, by lengthening the hulls. But then, the same could be done with the conventional SWATH. ... Man. "A conventional SWATH." How about that statement. And I know the triple has more whetted area , and structure to make.

So I would say that the broad strokes are still in consideration. ... Ad Hoc, what is SOR? I know I have a fairly good mind. But I have no education in marine architecture and engineering. I think I am working on it now though. Something like critical path, and form following function?

Again, I am appreciative of any help that may be offered. I'm really curious about power and performance parameters. I'll try to see what's out there in an existing ship. That will help.

And there was another consideration. It is one of semantics. I don't think that I've seen an elegant SWATH yet. The tri-hull has a little going for it.

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### Trihulled SWATHJunior Member

These two posts are backwards. This was to precede the other drawing.

My initial thought was to have an asymmetrical vessel, like a Polynesian outrigger. Traditional two hulls. One much larger than the other. And then I switched that to a SWATH boat. With a boat garage. Then I wanted more space. And I like symmetry. So I went with three displacement hulls.

The intent, at that time, was to be simple, robust construction. And I was considering one engine. I was thinking of it as a "motor sailing barge". Cruising speeds of 8 knots would be acceptable.

I also want shallow draft. 5-6 foot unballasted seems possible.

Then, all kinds of mission creep came about. More elegance, more speed.

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### keroseneSenior Member

SOR - statement of requirements

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### Mermaid CoJunior Member

Design shall be verified through iterative approach of CFD or model testing for various hull configuration,demi hull spacings,length to hull diameter ratios,and cylindrical to eliptical hull shapes. Not much empirical data are available for SWATH and the flow pattern behind a SWATH is also complex. Resistance prediction of cylindrical or elleptical hull shapes and interference resistance are still easier to determine but wake and propulsive efficiency of deeply submerged hulls at higher speeds will require proper modelling.
Location and shape of each strut, shape of lower hull, spacing between the two demihulls, and hull appendages, all will have complex influence on flow coming to the propeller.So wake factor and thrust deduction factor will be highly dependent on arrangement.

Humps and hollows of wave making resistance are crucuial to determine and in this scenario where you are experimenting with hull shape and arrangement,CFD or model test will be more accurate to ascertain that these humps doesnt occur at lower froude numbers. So carefull design of all aspects are important.

However Michell integral method can be used for these kind of hulls.This theory of Michell in deep water has been extended and further developed for a multi-hull system. Interference resistance of any pair of hulls can be calculated after calculating "Kochin" function of individual hull.

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### TANSLSenior Member

I would just like to comment on a couple of things, although it may seem that they are not very relevant and although many more could be commented on:
- the last two letters of the word SWATH mean "Twin Hull". That is, your boat should be called "SWAThreeH", or something similar.
- ships with SWA do not allow large changes in payload.
- If you decide to have a garage in each side, you must load and unload both garages at the same time so as not to cause very unpleasant heel angle on your boat.
- In addition to space for motors in submerged hulls, you must provide sufficient space on the "legs" for a man to access the engine room(s).
- For this type of boat to be effective, the legs must be deep, so that the hulls do not reach the surface even if there are large waves. This results in drafts larger than normal.
I do not know if I have helped you but that was my intention. Surely other members can give you very useful advice.
Good luck and go ahead with your dreams but improving them as you progress in your knowledge .

12. Joined: Jun 2017
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### Trihulled SWATHJunior Member

My apologies. I had written a post, that then did not post somehow. And was lost. I meant to first and foremost, thank you for your warm welcome. When coming to a new forum, you don't know what to expect. I judged from the activity that I could see, that there was an active forum here.

And boating people are always good people it seems. Full of wisdom and willing to be helpful. So again, thank you for your help.

And yes, that may be the mother of all mission creeps. Originally having a slow boat in mind. Then seeing if I could get a wee bit more speed out of it. Say 30 knots. Man, I have to laugh at myself.

The tri-hulled concept probably has a better chance of looking anything at all like pretty. And thus having market acceptance in the charter market. As it can be more readily be made to mimic the look of a traditional boat. Think pointy bow.
... I was actually thinking about rounding the corners. Otherwise SWATHs are real boxes. And in particular, sweeping some lines forward to the bow. Some have no purpose other than being aesthetic. So that there really isn't any relationship above the water, to what really must be happening beneath the water. ....... These lines are still a work in progress. And they are three dimensional of course. And move in all directions at once. Think about modern art that you may have seen. It's hard for me to do as an artist. Because my mind has always worked better on a two dimensional field. And I draw in two dimensions of course. I'll try to work something up though. .... I agree that this thread needs pictures. .... The picture provided of the deck plan needs to be redressed. It needs to be longer, and the corners need rounding. For starters.

I was also thinking about using the top of the center displacement hull, as a dock, when the ship is deballasted. I expect to use a five foot range of motion. The SWATH design require vastly less fluid transfer to obtain this movement. I envision this vessel spending a lot of time anchored. And there really needs to be an effort made to connect with the water. SWATHs seem to be work boats. And they haven't made inroads that I am aware of, in to the pleasure market.

Kerosene, I would like to respond to your inquiry. But I am sorry, I am not the best able to answer that question. But information about new builds is readily available. Find a yacht of 200 tons or so. And see what it cost.

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### Trihulled SWATHJunior Member

Thank you. And yes, I know that there will be testing. I'm trying to start with observations of things that I assume have been tested. But that's only the starting point.

14. Joined: Jun 2017
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### Trihulled SWATHJunior Member

Thank you for your input. Yes, the launching of a 10,000 lb. boat off center could provide a challenge. I have theorized that movement of water, both on to a hull, (port or starboard) and off of a hull (port or starboard), simultaneously could maintain this trim. But as it is intended that such operations are conducted with the top of the displacement hulls just above water, this is alleviated to a great extent. I had also contemplates a simultaneous launch of boats, one port and one starboard, to help trim. But again, this will likely not be necessary, as long as some portion of the port and starboard hull are above water. That area will increase dramatically more than the 10,000 lbs. of pleasure boat.
The changes to payload would require an ability to trim to the desired draft, regardless of the payload. But my concern here is not the boats. It is the people. There needs to be enough ballasting area available to maintain proper trim. None of the work SWATHs that I have seen have this concern.

This boat should not be out in anything more than a sea state of six feet. A ten foot wave may wet the bottom of the boat deck. Or expose the top of the displacement hulls. A greater wave could possibly lift the boats on, or in the boat deck. That must be avoided. And is just another real good reason to avoid hurricanes. I consider this to be a protected water pleasure craft. Unlike other SWATHs.

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### Trihulled SWATHJunior Member

Thank you kerosene.

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