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Old 26-08-2009, 22:10   #1
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Hull Design...

Hello, I have a healthy understanding of fluid dynamics and properties. So if you are so inclined feel free to use jargon. Though I know NOTHING about boat hull design. Before doing my eventual research and any sort of calculations I decided to do a little cheating...so here goes.

I'd like to find out basically what makes a good hull or what you think makes a good hull...or what I should think makes a good hull.

Hypothetical Example:

Quote:
Dear Event_Horizon sir,
A pretty accepted hull aspect ratio in performance cruisers is (something)x(something). This is because thinner hulls of this length, while faster, see diminishing returns in terms of speed v. boyancy. This hull is also comfortable to cruise in 10-15 kn and will still accomodate a healthy load.

PS, I think you are charming
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Old 27-08-2009, 16:52   #2
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Dear Event_Horizon sir,

I read an article in the Multihulls magazine and it claimed 10:1 and above, if what one wants to get that extra bit of speed that makes cats so sweet.

For a cruising design this calls for a hull of at least 15 meters, to make it marginaly livable inside.

Impossible to try on mono, still other factors (like narrow entry) worth investigating the speed potential.

Cruisingwise, a flat entry pounds. Thus I would say - a narrow entry mono, or a 10:1 cat with a good bridgedeck cabin.

PS, I hope you are charming
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Old 27-08-2009, 17:43   #3
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The L:B ratio is measured at the waterline. By even gently flaring the sheer it's possible to have very liveable hulls with even better ratios than 10:1 at well under 15 metres long.

ie. A 12 metre boat with 12:1 L:B is 1 metre wide at about shin height. It can be much wider at shoulder height.
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Old 27-08-2009, 20:11   #4
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Not talking about the deck ratio, why should?

The flaring or not, the overhangs - of course all influence the behaviour in the seaway, all conditions ban dead calm.

The hull shape / ratios subject completely covered in spec literature. I think now the edge is the Open 60 design, the AC design, the maxi-multis (note the recent revival of tris!) and then it trickles down to what can be done on the cruising front. Not that the Opens are so extreme but that they are not afraid to break down and then back-engineer - exactly like the ancient mariners found what works in real life and what only in their heads.

I have not seen any fast and liveable hulls. I have seen some fast and some liveable. And since a skiff is another way to go I believe the cruising design will go towards either a narow entry beamy aft mono or long sleek hulls cat.

Then there is the question of how we decide to define 'good'. To me it is max speed / max comfort which disqualifies the Open because of marginal (at best) comfort. But to get it the right way one has to go the Dashew way. And who can afford this?

So, there must be the middle way - the way of the trade-off: we trade a little speed for huge gains in comfort, or we trade some comfort for huge gains in speed.

I am on the cruising wing of god's portfolio so for me the issue is how to get a possibly fast boat with potentially most comfortable ride. My answers are: downwind - minimise roll, upwind - minimise slamming. And I found narrow entry to work for upwind work (one does have to ride a Farr 40 to get my drift) and broad flat stern (one has to try out a Pogo to get my drift).

Somewhere in between these two I find my definition of good hull.

b.
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Old 28-08-2009, 01:20   #5
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Quote:
Originally Posted by Event_Horizon View Post
Hello, I have a healthy understanding of fluid dynamics and properties. I'd like to find out basically what makes a good hull or what you think makes a good hull...or what I should think makes a good hull.
Hi

I'd be interested to find out what someone with a good knowledge of fluid dynamics makes of Beau' Boat. In this thread Beau introduces some ideas that seem contrary to the conventional wisdoms of boat design, but seem to me to have some value. I think that some of the intriguing phenomena that Beau observed may find their answer in fluid dynamics.

Chris
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Old 28-08-2009, 01:55   #6
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You are quite right that length is the first consideration.
a) what can you afford to build. b) what accomodation is required.
There is also the beam. On a mono it seems to be getting wider aft with a wave penetrating slender bow for performance (to reduce keel weight presumeably).
On a Cat it seems to have gone beyond optimum to provide accomodation at the cost of rigidity (or the increased cost of good stiffness).
The 30yr old Prout has beam to o/all of 0.33 and is conservatively rigged for cruising. The FP boats seem to have a good balance between sailing and accomodation. The opportunity to go for unequal hulls (not neccessarily a proa) hasn't been taken up yet. A broad 'Master' hull and a narrow 'crew' hull has yet to appear. From your hydro-dynamic background does this present too many problems in a sea way?
I'll be following this thread out of interest. It's bound to generate much discussion.
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Old 28-08-2009, 04:30   #7
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Originally Posted by Octopus View Post
Hi

I'd be interested to find out what someone with a good knowledge of fluid dynamics makes of Beau' Boat. In this thread Beau introduces some ideas that seem contrary to the conventional wisdoms of boat design, but seem to me to have some value. I think that some of the intriguing phenomena that Beau observed may find their answer in fluid dynamics.

Chris
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I didn't have enough time to go through thread with a fine tooth comb, but I think I have a general idea of what was going on.

Basically, my meager understanding of bow wave formation is as follows. When you push a solid object forward in water it pushes the water that used to be occupying that space away. The water that was just pushed away essentially "bounced" off the hull. A bow wave is formed when the object (boat) is going fast enough that the water reflected off the bow moves too slow, is overrun, and builds up. This causes a high pressure system infront of the boat.

No real tricky fluid dynamics, water is fairly simple stuff as it is incompressible (three cheers for zero divergence velocity!!!!).

I believe the elimination of bow waves is done by reflecting the "bounced" water off highly angled thin hulls so, even though the wave is slow, it has time to clear the width of the hull before it hits. Even high performance hulls on racer will probably create a bow wake if they go fast enough (though that speed is probably in excess of 80kn or something ridiculous). Also, remember this is a 3D system and bow wakes can be formed vertically which is why having a lower draft would obviously help.

That all being explained, it looks like what he's done is build a boat where the wet surface of the boat meets the above criteria for a "no bow wake" scenario. This in itself is not revoluationary. Though another engineer suggested that he might be using wave interference to counter the bow wave, I think that is probably an incorrect assertion. Bow waves are formed by wave information not being able to transmit ahead of the boat, therefore transmitting off phase waves to counter bow wave (in my 5min of pondering the possibility) doesn't seem feasible.

Beau does seem to have completely neglected his stern. Frankly it seems like an ugly scenario back there. The water he displaced has no easy way to refill the gap. Water won't make thats 90 degree turn from the sides or bottom. That means he will have a large low pressure system pulling him back (draaaag). I also want to point out that in typical racing hulls that dont plane are long to have enough boyancy to make up for the need to be so skinny...while also allowing the water to gradually refill the displaced area as to not create that ugly low pressure system at the stern...which beau ignores entirely.

I'm never one to discourage ideas, but I will be very surprised if his boat works as advertised.

If I didn't exactly cover what you wanted. Feel free to ask me something more specific.

It also looks like his mast is not large enough to hold a sail that would make any reasonable difference is speed to what I'm sure is a heavy boat if he bothered to properly insulate his metal hull.

Disclaimer: I'm a complete fledgling noobie greenhorn when it comes to boat building/design/construction/consulting though I don't plan to be for long. I'm what you would call...a mechanical engineer with a new found sailboat fetish. Most of the opinions I've shared are based on parallel knowlege that I'm pretty much extrapolating to make sense of all this, so if I'm wrong...let me know!
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Old 28-08-2009, 09:39   #8
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I'm never one to discourage ideas, but I will be very surprised if his boat works as advertised.
I think it true to say that the performance of Beau's boat did not fulfill his hopes, but I've no idea if hull shape was part of the problem.

According to one thread, Beau had problems with extracting thrust from his engine, possibly owing to incorrect prop size or pitch. He also failed to keep the weight down to the design target, which I believe was due to too many luxuries, rather than the additional weight of insulation. The boat was primarily intended as a motor boat and the Lateen sail was seen as an auxilliary power source for lazy downwind sailing. Perhaps Beau will update us, he's still posting on this forum.

My own interest in this subject has been from sailing my Lagoon 420, which in terms of accommodation and liveability is a class leader, but I think its hull design affects performance. My ideal would be a design that offers the liveability of a Lagoon with better performance. Watching the bow wave and the turbulence at the stern makes it clear that a lot of energy is being consumed.

I know it doesn't have to be this way, because last year I saw the Antigua to Barbuda catamaran ferry (designed and built by Bernard "Sonny" Eymann in Key West, FL of Sunny Days Catamaran Cruises), which generated almost no bow wave or wake - it was eery.

Chris
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Old 28-08-2009, 13:25   #9
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I assume you're talking about this boat:



As you can see the hulls are very thin and 85 ft long (almost 26 meters). I'd say this hull design is more akin to a racer cat with a high L/B ratio, obviously more than enough to lose the bow wake and gradual enough to greatly reduce the stern wake. The reason why a Lagoon 420 doesn't share these properties is because its too heavey. If you had hulls thin enough to create this effect on the same length (42ft-ish/12.8m) they would be impracticle--too thin for you to put things in, have a double berth, walk down etc. Even if you did find a way to pack the ample lagoon 420 luxury onto two very thin hulls I'd bet that they would stuggle to keep that small draft.

I think the best way to have Lagoon 420 luxury and have remarkable performance would be to take all the accomidations on the 420, then fit them neatly into 55ft (16.7m) hulls.

There is a phrase you hear often in engineering for times like these: "There is no free lunch"

From all the reading I've done so far, it seems designing the shape of the boat is similar to a chef preparing a nice stew. There is a blend or balance to be achieved where too much of one thing spoils the dish. Making the boat faster decreases its livability and interior room (for the same length boat), and conversly, making it slower adds room and boyancy for luxury.

Note on Beau: I agree that he had problems beyond hull design. I also believe his hull is also massively inefficient and that his "Triflection" design probably isn't as good as modern mass produced boats of a similar size.
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Old 28-08-2009, 19:40   #10
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Ok, Event Horizon and Octopus, where do I start?

Probably you should have a look at www.earthrace.net for starters.
I am using the same concept as they are but I have designed a boat that is suitable for living aboard, is fast and can be shipped in a 40 ft shipping container.

Some years ago an Australian company "Incat" and "Austral" started producing fast displacement ferries which "work" on the basis of "the 16-1 rule", which is, if you design a hull which is 16 ft long and 1 ft wide you will not produce a bow wave so you can go a lot faster in displacement mode. (general boating theory is 5 hp /ton for displacement speed whereas a planning hull requires a minimum power of 50 hp /ton) these designs are now in regular use worldwide including the US Navy.
They are all Catamarans.

However, it is difficult to build "the fast ferry concept" in anything under 50 ft, because of buoyancy considerations.

I came across what I call ,the "triflection" concept which was aimed at the large ship market .
I set about modifing the theory around what i wanted in a live aboard cruiser.

The original "triflection theory" came from an American mathematician who worked on the America's cup race.
I built 5 prototypes testing the theory, all were approx 15 ft long.

I found that the angle of entry is critical, flat sides necessary (reduction of friction) and the flat bottom ideal for shallow draft.
In initial testing with a 10 hp outboard (15 ft prototype) there was no bow wave produced at all, even at speed.
However initially there was a massive stern wave as suggested by Event Horizon, BUT as told by the original theory at about 10 knots the boat lifts up and the stern wave flattens right out and the boat takes off. The boat is NOT planning the bow is in the water at all times.
Exactly why the boat lifts up I am not completely sure, but is does happen.

Update on Beau's boat
We are living on board a 39ft Trimaran which has far more liveable room at much less cost to build than a similiar catamaran.
The outriggers fold in, so we can use a monohull berth.
It will fit inside a 40 ft shipping container for long distance travelling.
The boat was built by a professional boat building company who build larger vessels and came in much heavier than estimated by the Naval Architect, and the two 30 hp outboards only give me a top speed of 10 knots with a cruise speed of 7 knots. When I get some money I will replace the 30hp motors with two 90 hp for a top speed of 20 knots.
Beau Lyons
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Old 28-08-2009, 20:01   #11
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Event Horizon,
I would be very interested to know what other more efficient design are available of similiar size.
A friend of mine has a 40 ft Grainger Catamaran Sailing boat.
It cost four times more to build that my Trimaran. He has two 175 hp Steyr diesels and has a top speed of 20 knots.
It is a great boat but it cost four times more.
It does have 4 double beds, mine only has one double, but who needs four double beds.?
The trimaran is cheaper because it is basically one hull with outriggers. And we all know that the main cost in building a boat is the fitting out,
I have one hull to fit out not two and a bridge deck.
I have a 39 ft boat for good seakeeping ability and you just cannot build a practical catamaran under 40 ft. without having two narrow tubes(hulls) and a bridgedeck without headroom or sufficient sea clearance, and I pay monohull marina rates.
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Old 28-08-2009, 20:06   #12
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I think saying your boat uses the same technology as the earth race boat is a an interesting comment.

Please explain to me (or give me the gist) of what triflection is or means. I

Lastly, I suspect your boat is trying to plane and thats why it lifts up. The flat surfaces of your boat are providing the lift. Torque and weight of your engines is imparting a rotational moment on your boat, this probably gives it a mild angle of attack thus generating lift on the flat surfaces parallel with the water surfaces. It wouldnt baffle me if your boat started to plane if you got her up to speed.
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Old 28-08-2009, 20:14   #13
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Event Horizon,
I would be very interested to know what other more efficient design are available of similiar size.
A friend of mine has a 40 ft Grainger Catamaran Sailing boat.
It cost four times more to build that my Trimaran. He has two 175 hp Steyr diesels and has a top speed of 20 knots.
It is a great boat but it cost four times more.
It does have 4 double beds, mine only has one double, but who needs four double beds.?
The trimaran is cheaper because it is basically one hull with outriggers. And we all know that the main cost in building a boat is the fitting out,
I have one hull to fit out not two and a bridge deck.
I have a 39 ft boat for good seakeeping ability and you just cannot build a practical catamaran under 40 ft. without having two narrow tubes(hulls) and a bridgedeck without headroom or sufficient sea clearance, and I pay monohull marina rates.
Beau Lyons
Please understand when I say "more efficient" it has NOTHING to do with cost/ease of construction/how much the marina charges. I was commenting purely on the physical properties of your design as it moves through the water.

As far as being practical under 40ft, thats subject to opinon. Personally I'd say claiming they are impractical is a bit hasty because more than a handfull have been taken around the world which is sailed all over...which is the practical use for a cruising vessel.

I'm also surprised that 350hp doesnt take your friend a bit faster.
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Old 28-08-2009, 20:16   #14
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...
I'd like to find out basically what makes a good hull or what you think makes a good hull...or what I should think makes a good hull.

Hypothetical Example:
Check out catamaran hull design formulas here:

Cruising Catamaran Boat Design
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Old 28-08-2009, 20:52   #15
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Event Horizon,
In regard to the similarities to Earthrace, my boat uses the SAME angle of entry, and basically a flat bottom. And is very efficient.

Excepted theory is that it requires a minimium of 50 hp per ton to plane and generally more.
My boat currenty has 60hp for 6 ton and will have 180 hp. I could use two 75 hp (150 hp for 20 knots)
A 6 ton planing monohull cruiser generally has 2 X 300 hp motors.

My trimaran cuts through the waves, it has a very soft motion, and it does not hobbyhorse because of the wide stern. It does NOT plane I can assure you.

Austral shipbuilding in Alabama recently were awarded a contact to build ships for the US navy using a similiar trimaran design.
This is state of the art.

Have you ever seen inside a 30ft Catamaran.
They either have two thin hulls which are like coffins to sleep in or wide fat hulls that are dogs, or have a bridgedeck which hits every second wave or have open bridgedecks and they are still expensive to build.

Beau Lyons
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