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  #1  
Old 01-21-2009, 02:10 AM
Seagem Seagem is offline
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What's the best shape for a fast & efficient catamaran hull?...

Doing research on fast & efficient hull shapes for a 25' cruising catamaran capable of 22/25 knots cruise and 30 knots top speed, while still providing reasonable performance at lower speeds, I found the following:

1) Efficient planning shape, foil assisted:Corsair 22 Foiler



The boat does 50 mph top with twin 90hp motors and cruises happily at 30 mph with better than 3 miles/gallon fuel consumption. Its asymmetrical hull design allows it to bank in turns, unlike most other catamarans..

Description: http://floridasportfishing.com/magaz...iler-2200.html

Potential drawbacks: not efficient at displacement speeds

2) Efficient displacement hull: Prowler 9 meter




Description: http://www.schionningmarine.com.au/w...cfm?pageID=195

Twin 50hp outboards will cruise this catamaran at 17 knots using 1 liter/mile or 3.8 miles /gallon with top speed of 21 knots, thanks to a fineness ratio of 15 to 1...

Drawback: Top speed cannot exceed 23 knots, as this is a displacement hull...

3) Not so efficient semi-displacement hull: VT650/950



The larger version of this design with the unusual asymmetric chines, the VT950 cruises at 17 knots, using 2.2 liters/mile (1.7 mile/gallon) and tops out at 26 knots with twin 140hp outboards.

Many advantages: sharp entry at slow or high speeds, load carrying ability aft where the weight is usually concentrated, easy access to hulls from bridgedeck and clever location of the outboard engines, which have a clear water flow while keeping the stern platforms free. This also moves the engine weight and prop further forward, reducing potential aeration in rough seas.

Description: http://www.schionningdesigns.com.au/...owler950VT.pdf

One major drawback: the hull shape has too much wetted area which affects its efficiency. I suggested tapering the "V" section to nothing about one third before reaching the stern, but Schionning says it increases the drag...

4) Efficient semi-displacement hull: Chilkat 30



This boat cruises at 30 knots with 3 miles/gallon fuel efficiency and tops out at42 knots with twin 225hp outboards. However, it still cruises at 25 knots and tops out a 43 mph with twin 150's...

Description: http://www.blackfeatherboats.com/pow...maran_boat.cfm

Drawback: the bridge deck may be too low and pound in a seaway...

There is also the Sea Knife hull design which may be suitable, but will require some expensive R&D to make work satisfactorily:

http://www.yachtforums.com/forums/te...erunner-4.html

Any suggestions of hulls that might fit the bill would be welcome...
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  #2  
Old 01-21-2009, 05:47 AM
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sabahcat sabahcat is offline
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Are you still trying to get queen sized beds, dinettes, shower toilet and a few hundred litres of water onto these hulls?

Does it still need to be a trailerable width?

If not, option 1 although I would suggest a boat with foils may not have got the hull shapes right to start with.

If you want the beds etc then option 2, but like was said in your other thread, it will be high sided due to bridgedeck clearance.

Thems the rules.
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  #3  
Old 01-21-2009, 09:27 AM
Seagem Seagem is offline
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Good question....

Quote:
Originally Posted by sabahcat View Post
Are you still trying to get queen sized beds, dinettes, shower toilet and a few hundred litres of water onto these hulls?

Does it still need to be a trailerable width?

If not, option 1 although I would suggest a boat with foils may not have got the hull shapes right to start with.

If you want the beds etc then option 2, but like was said in your other thread, it will be high sided due to bridgedeck clearance.

Thems the rules.
The beam is still 10' for 25' hull length, plus swimming platforms...

The ideal lay-out is still an athwartships queen size berth accessible through one hull and a head/shower compartment aft of the stairs in the other, with hanging locker/storage forward. The high top sides are not a big problem when combined with a bridge deck at least 2' high at its lowest...

Option 2 has a speed limit of 25 knots because of it displacement design...

So far, the ChilKat option with a much higher bridge deck seems to make the most sense and the hull is wide enough at the bottom to accommodate the head/shower compartment if some flaring is given to the top sides just above the waterline...



Btw, the performance figures quoted in the first post were run with 6 passengers and 250 gallons of fuel for a total load of 2,800 Lbs...

The hulls are a cleverer design than appears at first sight: notice the main chine coming down the bow and turning down below the slightly convex bottom to prevent the lifting water flow under the hull from escaping sideways. Also the other chine just above the waterline that has a rubbing strake: the fineness ration is a narrow 18 to 1...

I just received by e-mail a '.doc' file showing a very clever 25 years old wooden cat design, the Aonix, that banks inwards in turn and is very seaworthy and still in service in South Africa as a fishery chase boat: if I can figure a way to convert to '.jpg' I will post some of the more significant pics...

It seems that BobKat in South Africa copied the hull, added a foil and patented it...

http://www.boatloft.co.za/bobkat.htm

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  #4  
Old 01-21-2009, 11:03 AM
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There is a Chilkat thing here and the owner loves it but the outboards are ripping the the back of the boat off... Most in this neck of the woods have given up on little cats that plane, by the way because of the hobby-horsing and general thrashing.
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  #5  
Old 01-21-2009, 11:18 AM
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  #6  
Old 01-21-2009, 11:32 AM
robherc robherc is offline
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Hmmm... I know you don't like the speed limit of a displacement hull, but it's going to be by FAR your most efficient through-the-water (at least under varied speeds). If I were to build a boat for myself, with the same qualifications you listed, I'd be tempted to go with fine, mostly-displacement hulls (if not fully displacement), but I'd still foil-assist it. With the foil-assist, you can dramatically reduce the displacement of the hull, and thus the wetted surface, w/o having to plane, so that seems like the way to go for me...but I still prefer to sail also, so the efficiency thing is pretty paramount to me, while I'd be quite happy with 25knots...and 0litres/nautical mi.
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  #7  
Old 01-21-2009, 06:46 PM
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sabahcat sabahcat is offline
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Quote:
Originally Posted by Seagem View Post

The hulls are a cleverer design than appears at first sight: notice the main chine coming down the bow and turning down below the slightly convex bottom to prevent the lifting water flow under the hull from escaping sideways. Also the other chine just above the waterline that has a rubbing strake: the fineness ration is a narrow 18 to 1...
But that will change for the worse as you want 25 feet, not 30
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  #8  
Old 01-22-2009, 01:59 AM
eponodyne eponodyne is offline
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I'd go with option 2. 20 knots is really hauling bass, let's not forget that 20 knots hour after hour adds up to 550 statute miles in a day.

How far do you really need to go, and how fast do you really need to get there? Who are you really trying to impress?
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  #9  
Old 01-23-2009, 02:25 AM
Seagem Seagem is offline
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Quote:
Originally Posted by mark775 View Post
There is a Chilkat thing here and the owner loves it but the outboards are ripping the the back of the boat off... Most in this neck of the woods have given up on little cats that plane, by the way because of the hobby-horsing and general thrashing.
Can you point me to where the ChilKat is being discussed, as a posts search brought no result?...

Quote:
Originally Posted by robherc View Post
Hmmm... I know you don't like the speed limit of a displacement hull, but it's going to be by FAR your most efficient through-the-water (at least under varied speeds). If I were to build a boat for myself, with the same qualifications you listed, I'd be tempted to go with fine, mostly-displacement hulls (if not fully displacement), but I'd still foil-assist it. With the foil-assist, you can dramatically reduce the displacement of the hull, and thus the wetted surface, w/o having to plane, so that seems like the way to go for me...but I still prefer to sail also, so the efficiency thing is pretty paramount to me, while I'd be quite happy with 25knots...and 0litres/nautical mi.
A foil addition remains an option and it has been done successfully on a 46' displacement cat built by Alwoplast as a retrofit: speed increased by an amazing 35%...

The fastest I was ever able to sail my Antigua 37 catamaran under spinnaker alone on an Atlantic crossing to the Caribbean was 15 knots: above that, the bows started to bury and waves would reach the foot of the mast by the time I took down the spi. I had Autoprops fitted on the saildrives locked in reverse, as per manufacturer's recommendation and they kicked started 3 times at 15 knots on 3 different occasions... until I finally locked the gearboxes into forward...

Both engines at full revs in reverse only slowed the boat down by 1/2 knot, but the first time it happened it took a while to figure out the cause of all this noise and vibration, as the tachos showed no rpm's, as the key wasn't turned on...

It would take a pretty sizable cat to be able to do 25 knots in comfort...

Quote:
Originally Posted by sabahcat View Post
But that will change for the worse as you want 25 feet, not 30
As mentioned earlier, the boat is 25' plus the swim platform, which will give a total of close to 27' and it may be necessary to still go longer to help carry the fairly substantial tankage necessary...

Quote:
Originally Posted by eponodyne View Post
I'd go with option 2. 20 knots is really hauling bass, let's not forget that 20 knots hour after hour adds up to 550 statute miles in a day.

How far do you really need to go, and how fast do you really need to get there? Who are you really trying to impress?
I agree that 20 knots sustained is a good clip and a good common sense speed that would satisfy most. However, if the boat is to be successful as a production catamaran, the same hull has to appeal to a broad range of potential customers...

A pure fishing version will need to go at least 30 knots for those who run to the various fishing banks 60 to 90 miles away...
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  #10  
Old 01-23-2009, 07:47 AM
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sabahcat sabahcat is offline
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Quote:
Originally Posted by Seagem View Post
As mentioned earlier, the boat is 25' plus the swim platform, which will give a total of close to 27' and it may be necessary to still go longer to help carry the fairly substantial tankage necessary...
Swim platform like your chilcat above??, hate to tell you but thats waterline length

So................................25 feet grows to 27 feet and could well grow to 30feet.
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  #11  
Old 01-23-2009, 09:29 AM
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Alik Alik is offline
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From our experience, hull shape is secondary issue (unless it is too bad or not match the desired speed). Weight - this is key point of fuel efficiency. Make Your boat light to be fuel efficient.

For planning cat, a lot of weight savings can be obtained by light bottom. Say, using AirexR63 and Kevlar/E-glass composite on bottom will give 1200 kg of hull structure for 10m cat, while for normal WR/CSM the weight is 1900 kg.
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  #12  
Old 01-23-2009, 10:55 AM
robherc robherc is offline
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Quote:
Originally Posted by Seagem View Post
A foil addition remains an option and it has been done successfully on a 46' displacement cat built by Alwoplast as a retrofit: speed increased by an amazing 35%...

The fastest I was ever able to sail my Antigua 37 catamaran under spinnaker alone on an Atlantic crossing to the Caribbean was 15 knots: above that, the bows started to bury and waves would reach the foot of the mast by the time I took down the spi.
Hmm, I don't know about the feasibility of this for your design, but one solution to this COULD be using adjustable-attitude foils. If you can adjust the angle of attack on your foils & used 4 foils (1 for & 1 aft on each hull), you could control the pitch & roll of the boat by adjusting foil attitudes. I know this would complicate the system a bit, but with it you could (at least theoretically) provide the extra lift necessary to the bows to keep them from burying above hull speed.

Of course, at that point your boat would be sitting "on" the water, instead of "in" the water & become a foil-assisted-planing hull, regardless of hull design...you could even "plane" a double-ender that way, and the D.E. would give you FAR greater efficiency below hull speed.

Just a thought & an opinion, not a statement of fact;
Rob
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  #13  
Old 01-23-2009, 02:31 PM
mark775
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The Chilkat is by Black Feather Boats. It was an option pic posted above.
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  #14  
Old 01-23-2009, 04:08 PM
Seagem Seagem is offline
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50' Wave piercer by Gold Coast yachts on St Croix...



Please, click on the link below to enlarge the photo and see the lay-out...

50' Wave piercer by Gold Coast yachts on St Croix...

Owners comments: Mosler claims his boat burns 3 gph at 10 knots for 3.33 nmpg and 11.1 gph at 20 knots for 1.80 nmpg.

His top speed is over 26kts with a 24kt cruise with the d3's Volvo 190hp at just over 14 gph.

The length to beam of each of my hulls is about 16:1, and the bridge deck is about 3ft over the surface at the rear, sloping up towards the bow, so it can quarter some very high seas at full cruise without slapping, though the wind will blow a lot of spray on the upper aft deck at high speeds.

Warren Mosler said: I took 13 people to St. Thomas from St. Croix yesterday quartering 8 ft seas at about 20 kts. picked 4 in St Thomas, had lunch in St. John, back in St. Croix less then 2 hours later. All sat comfortably on the upper aft 'sundeck' above the aft berths (and no one got sea sick).



http://pmyeditors.blogspot.com/2008/...are-power.html
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  #15  
Old 01-24-2009, 12:28 AM
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sabahcat sabahcat is offline
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And your point?

25feet has grown to 50feet?

Similar concept


Sheer Khan
http://www.charterworld.com/index.ht...-sher-khan-715

http://www.incat.com.au/
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