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  #106  
Old 09-01-2015, 12:07 PM
Boatguy30 Boatguy30 is offline
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The new hull shape is basically flat forward as I seemed to have a lot of drag from the bow wave sliding after to a wider section of the bow on the smaller version.

I achieved speeds well in excess of your predictions with the "tri" model but not sure how equivalent that is to the cat. I'm hoping if will be about even.

Again my desired "normal" full displacement is 660lbs.
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  #107  
Old 09-01-2015, 12:10 PM
Boatguy30 Boatguy30 is offline
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I seem to be confusing myself between pounds and kilos. 300kg is the goal.
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  #108  
Old 09-05-2015, 08:31 AM
Manfred.pech Manfred.pech is offline
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In the "Introduction - Features" of his trimarans Ian Farrier explains (http://www.f-boat.com/pages/introduction/features.html ) that the main hull of his well known boats has a swept up after section to give the hull a negative lift for suction to prevent the bows from burrying.

This is tested by sailors of his trimarans and it seems to work well. On the other hand - what happens if the section is not swept up/convex but concave/hollow? From the law of Bernoulli there should be lift. And if the hull has a slight rocker from zero to the deepest point one third from the length giving hydrodynamic lift the whole hull must have reduced wetted area resulting in more speed if the whole sytem is well balanced. Test results in a towing tank have confirmed this.
Will, do you think Michelet is able to show these gains of reduced wetted area and of more speed???

Very slight hollow section:



Not two times like this:



Atkins Sandpiper:

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  #109  
Old 09-05-2015, 10:53 AM
JSL JSL is offline
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There seems to be some interference drag between the two hulls starting at just under 4kts and peaking at 5kts (20% of total drag) and then tapers off as speed increases. At top speed the program shows that there might even be some wave drag cancellation. This interference effect (both constructive and destructive) appears to decrease as hull spacing increases.
Yes, having minimal clearance between the 2 hulls (demi-hulls) does create additional drag. There are several publications by SNAME (and others) and as a rough guide the minimum clearance between hulls should not be less than the demi-hull waterline beam.
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  #110  
Old 09-08-2015, 06:36 AM
Will Fraser Will Fraser is offline
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Quote:
Originally Posted by Manfred.pech View Post
In the "Introduction - Features" of his trimarans Ian Farrier explains (http://www.f-boat.com/pages/introduction/features.html ) that the main hull of his well known boats has a swept up after section to give the hull a negative lift for suction to prevent the bows from burrying.

This is tested by sailors of his trimarans and it seems to work well. On the other hand - what happens if the section is not swept up/convex but concave/hollow? From the law of Bernoulli there should be lift. And if the hull has a slight rocker from zero to the deepest point one third from the length giving hydrodynamic lift the whole hull must have reduced wetted area resulting in more speed if the whole sytem is well balanced. Test results in a towing tank have confirmed this.
Will, do you think Michelet is able to show these gains of reduced wetted area and of more speed???
Michlet does make provision for trim and hydrodynamic lift, but both of these are manually specified at selected speeds. AFAIK, the program does not have the ability to calculate the amount of lift generated or change in trim.
So to answer your question: yes, it can calculate the drag on a hull with a reflexed aft run provided the user can supply the correct value of trim and vertical displacement due to hydrodynamic lift.
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  #111  
Old 09-28-2015, 07:27 PM
Cotemar Cotemar is offline
 
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10" CATAMARAN DINGHY, Fast, Roomy, and Dry at any speed into wind and waves

Catamaran Dinghy Specifications

1) Is 10’ feet long x 54 inch beam
2) Weigh 95 lbs. without motor
3) Very easy to row
4) Dry when going to windward in a chop
5) Easy to clean
6) Holds four people
7) Plane's easily with a 6 hp
8) Easy to walk off the bow
9) Maintenance free. No air, no leaks
10) Hold up well in the Caribbean sun
11) Turns on a dime
12) Is fast and stable
13) Fits on a car roof rack and is easily moved by only 2 people.
14) Has two Aluminum Oars
15) Stainless-Steel Oarlock Side Socket base
16) Four SS Lifting Eye Straps 1-7/8"L x 7/16"W for hanging dinghy off davits
17) 3/4 – Round Gunnel Guard all the way around
18) Two Rectangular Access Hatch’s, Overall: 11-1/8" x 14-3/4"
19) Stainless-Steel Rub Strakes to protect bottom when dragging up on a beach
20) Made from composites, epoxy and fiberglass layup
21) Optional: Mount a folding ladder off the bow for stable diving and snorkeling

This Catamaran Dinghy is very stable and fast. You can turn her hard over at any speed and she turns flat and on a dime.

She is just an effortless pleasure to row. I easily rowed her for over a mile, you can go full speed into a 20 knot chop and be dry as a bone.
-------------------------------
Why a cat dinghy verses and inflatable.

1) Most inflatable’s give a wet ride in 15+ knot chop
2) Inflatables are noisy when tied behind the boat and water laps up under the tubes
3) Inflatables need more HP to push them
4) Inflatables row poorly
5) Inflatables turn poorly
6) Inflatables get stains and are hard to clean
7) Inflatables do not hold up well in the sun. Need expensive chaps in the Caribbean sun
8) Inflatables get leaks and most need to be repaired in just 4 years.
9) Cats Plane 600 pounds with only 10 horsepower
10) Cat seating arrangement gives you the advantage of choosing the direction you wish to face.
11) The Cat Tunnel increases the hull planning efficiently, by causing the water flow through it, to form a pressure which enhances the lift
12) Cats are Super Stable, roomy, safe and can carry a big payload and still plane with low horsepower.
13) Cats have no feeling of getting up on step, they transitioned smoothly from slow to high speeds.
14) Cats turn on a dime in both low and high speed.
15) Cats make great stable dive and snorkel boats with a folding ladder off the bow
16) My 10" inflatable would just barely plane with 2 people with a 6hp engine. This 10" Cat dinghy easily planes with 2 people in it using the same 6 hp engine.

http://www.ebay.com/itm/191703337093...CA:MOTORS:1123
Attached Thumbnails
Catamaran dinghy 10-11' long-catatude-dinghy-1.jpg  Catamaran dinghy 10-11' long-catatude-dinghy-2.jpg  Catamaran dinghy 10-11' long-p5030103.jpg  

Catamaran dinghy 10-11' long-p5030144.jpg  Catamaran dinghy 10-11' long-p5030136.jpg  Catamaran dinghy 10-11' long-p5030113.jpg  

Catamaran dinghy 10-11' long-p5240252.jpg  Catamaran dinghy 10-11' long-catatude-dinghy-13.jpg  
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  #112  
Old 09-29-2015, 04:32 PM
Boatguy30 Boatguy30 is offline
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the first few pieces

Here are the first few side panels of the final boat. I'm hoping to get the last small inner side partially from the same sheet as the bottom so need to put one hull together and trace it all out.

I like the bow storage lockers on the other boat. Will do something like that and should be somewhat larger as they will be further aft in my hulls. The forward 30" of each hull is basically just extended waterline.

Also got the rudders on the big cat upgraded and repainted and just rehung. Hope to get back sailing when the weather dries up in October.
Attached Thumbnails
Catamaran dinghy 10-11' long-win_20150929_011453.jpg  
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  #113  
Old 09-29-2015, 04:50 PM
Cotemar Cotemar is offline
 
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2 things to watch out for when making a cat dinghy with a motor on it.

1) DO NOT taper the sterns, as water will run up the sterns into the boat when you let off the outboard engine throttle. You will be ankle deep in water within the first minutes of using the dinghy.

2) If your going to hang your cat dinghy from davits on the back of your mother ship then DO NOT put those pointy cat ears on the bows. Connect both bows and leave a straight line from bow to bow. The reason is that pointy bows will do major damage to your mother ship as it sits between the mother ship hulls and bounces on any wave action as you lift it up on the davits.
The square bow also allow you to walk on and off the cat dinghy easily.
It also allows you to push your mother ship or save other cruisers boats.

The hatches are a real + for PFD's and anchor with line and flashlights and rain gear.
Attached Thumbnails
Catamaran dinghy 10-11' long-cat-dinghy-design-problems.jpg  
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  #114  
Old 09-29-2015, 06:27 PM
Boatguy30 Boatguy30 is offline
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I guess that explains the difference between the look of your dinghy now and as built by Mark.

I will have the pointed bow as it has to straddle an arch. I take the point about getting on and off. I'm thinking of maybe having a drop in piece to fit between the bow if it's a problem. The other option is backing into a dock which might actually work pretty well in most situations.

My sterns aren't quite angled the same way as there is a swim platform and a higher transom.

Cheers,
Jeff
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  #115  
Old 09-29-2015, 06:51 PM
Cotemar Cotemar is offline
 
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I have saved many a boat by pushing it with the dinghy bow, so a none pointy bow is very important for a cruising boat dinghy.

Dinghy docks are packed everywhere cruisers go. You have to use your motor to push your way into a slot. Walking off the front of a Cat Dinghy at a dock is a MUST HAVE.

My Wife would shoot me if we backed into a dock and she had to walk over the gas can and engine.
Attached Thumbnails
Catamaran dinghy 10-11' long-dinghyhdock.jpg  Catamaran dinghy 10-11' long-exumamarketdinghydock.jpg  
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  #116  
Old 10-05-2015, 08:26 PM
Boatguy30 Boatguy30 is offline
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here you go

As you can see, I've followed the sage advice and connected the bow for a nice fendering surface. Might cause me a bit of extra drag. What do you think. (joking of course just holding the stems on correct CL spacing)
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Catamaran dinghy 10-11' long-win_20151005_031112.jpg  
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  #117  
Old 10-06-2015, 04:34 AM
saltdragon saltdragon is offline
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A very interesting thread, as someone who has rowed, motored, tipped over and sworn at many a yacht dinghy it's good to see people addressing the problem as opposed to just going with what's available.

Cotemar that's a well thought out dinghy, did you design/build it? I like the idea of putting the outboard forward of the transoms

Last edited by saltdragon : 10-06-2015 at 04:35 AM. Reason: spelling
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  #118  
Old 10-06-2015, 04:43 AM
Mr Efficiency Mr Efficiency is offline
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Cotemar's little cat seems a well thought-through little boat, but the claim it "planes easily with a 6hp motor" seems a stretch ( with two people aboard ). That would be a wonder boat.
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  #119  
Old 10-06-2015, 06:37 AM
Cotemar Cotemar is offline
 
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Not much new here. Catamaran dinghy's have been around for a long time now.

Working on a 12 foot model over the winter with an even faster hull.

Just like an inflatable, catamaran dinghy's have to have hull extensions past the engine. With out the stern extensions the Cat dinghy would just bow up when you give it throttle.
Attached Thumbnails
Catamaran dinghy 10-11' long-inflatable-dinghy-w-engine.jpg  
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  #120  
Old 10-06-2015, 07:06 AM
Boatguy30 Boatguy30 is offline
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the designer and builder Mark (plans on an earlier post) confirms the performance. Cotemar originally claimed the boat did 15 knots with 4 aboard into a 15 knot wind! On other forums. Also it weighed only 80 pounds and was all fiberglass. He was close to taking deposits and supposedly built a mold.

Of course a 9' AB VL will plane 2 adults with a 6 hp also. Pretty much the point of this thread. A "stock" cat dinghy will only slightly outperform a similiar sized RIB.
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