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  #1  
Old 05-09-2015, 08:38 PM
Boatguy30 Boatguy30 is offline
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Catamaran dinghy 10-11' long

So it seem a good performing cat dinghy is very difficult to design. It looks like Robs is meant to be close to 13' long but maybe 11' or 3.3m is the longest I can squeeze on my arch.

I don't really need it to fully plane as I'd be happy with 8-9 knots of speed with say a 500 lb or so payload. Would ideally like to power with a 5 hp. So I'm thinking maybe some type of relatively narrow but deep full displacement hulls? Maybe with as much as a 6" hull draft loaded would be ok.

It seems most attempts are for full planning boats with very shallow hulls. I'm wondering is. Displacement hull with an 8:1 or so l/b perform how I want and carry the load? Maybe 10:1 is possible?

Any thoughts appreciated. My RIB is on its last legs!
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  #2  
Old 05-09-2015, 09:14 PM
Mr Efficiency Mr Efficiency is offline
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Why a catamaran ? I'd say nothing will do 8 or 9 knots with a 5hp motor carrying 500 pounds, there is too much conspiring against you in an 11 foot boat.
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  #3  
Old 05-10-2015, 02:49 AM
TANSL TANSL is offline
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I think you'll need more than 6" draft.
What are the red lines for dimensions ?. What materials will you use?
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  #4  
Old 05-10-2015, 03:36 AM
Ad Hoc Ad Hoc is offline
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Quote:
Originally Posted by Boatguy30 View Post
So it seem a good performing cat dinghy is very difficult to design.
Says who..and why?

Quote:
Originally Posted by Boatguy30 View Post
I don't really need it to fully plane ... So I'm thinking maybe some type of relatively narrow but deep full displacement hulls? Maybe with as much as a 6" hull draft loaded would be ok.
Do you not see the inconsistency with this statement? A deep draft hull yet you have already limited the draft to rather shallow?

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Originally Posted by Boatguy30 View Post
,.... I'd be happy with 8-9 knots of speed with say a 500 lb or so payload.
Hmmm...well,

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Originally Posted by Boatguy30 View Post
Displacement hull with an 8:1 or so l/b perform how I want and carry the load? Maybe 10:1 is possible?
Based upon these spec's you're lucky to get circa 150 kg of displacement, or some 350lbs, unless you want a hull that is a brick? With the average weight for a person US of 85kg, you're lucky to have a boat made of paper being propelled by a mouse in a wheel rotating a paddle, just to maintain the 150kgs displacement.

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Originally Posted by Boatguy30 View Post
Any thoughts appreciated.
Have a serious think what it is you really want..and then ask someone with experience in design to tell you what you can actually achieve. Since your first attempt has failed rather badly I'm afraid.
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  #5  
Old 05-10-2015, 03:59 AM
Mr Efficiency Mr Efficiency is offline
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Does the 11 foot limit include the engine mounted on it ? Quite a small boat if so.
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  #6  
Old 05-10-2015, 04:15 AM
lucdekeyser lucdekeyser is offline
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I doubt by "Robs" you meant Rob Denney's cat tender. As it serves instead of a motor sled to large Harryproa's it already features a 25 hp outboard, eliminating the need for an additional 5 hp just for the tender. This dual use of the thus bigger outboard meets your requirements given your dinghy is meant to complement a cruiser.
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  #7  
Old 05-10-2015, 04:18 AM
outside the box outside the box is offline
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Hi Boat guy
The below two options may or may not be of interest.
Both would work in with your margins.
A brief is below but a photo tells the story better.
Just a couple of dinghy’s in our range that might work
Matt one of our trade qualified boat builders finished the prototype dinghy off. It is seen here with 170kgs in it two up rowing with the prototype weighing in at 40kg being over built for thrashing as our test bed. The production built version is 20kg epoxy build.
It rows and powers well next stage of development is to get it sailing with fold out hiking seats and a centre board.
It will be built first as a rigid dinghy and is available for purchase in size ranges being standard at 2.2m with 2.4m, 2.7m, and 3.0m options. Available in standard gel coated polyester and the option of Gel coated infused post cured epoxy for the lightest strongest dinghy build on the market.
The EZIFOLD folding locking dinghy based on this prototype will be coming soon.
We also have the following
Patented Unique catamaran dinghy plans available
Quite some time ago the rights to this amazing little dinghy were purchased. As usual prototyping took priority.
Plans are available on our website.
As drawn it is 8'10" x 5'5" (2.7m x 1.7m) while the prototype was built in ply it is designed for ultimate performance to be glass foam build. It then comes in at 38 to 40kg the floats center line is 3'6" giving plenty of room in the bridge deck which is 10" (25.4cm) above the water at rest empty and is self-draining. With 250lb or (114kg) aboard the floats sit on their waterlines and are designed to plane at 15 knots with an 8hp outboard. To enhance rowing or sailing for purists hull extensions are in the plan set. When towed empty behind a sail boat it generates 1.4lb or (.6kg) drag at 5knots. It is calculated that it consumes 3hp to be towed on the plane at 21knots behind a power boat. The design states loaded at 1500lb (680kg) the catamaran floats sink and the bridge deck takes up buoyancy and stability contribution as a stabilized punt. The basic idea behind the design was for fun and functional use while being efficient to tow so it doesn't need constant stowing between cruising hops between bays or islands. Further information is available through emailing inquire@ezifoldyachts.com

Regards
Craig and team Ezifold

Quote:
Originally Posted by Boatguy30 View Post
So it seem a good performing cat dinghy is very difficult to design. It looks like Robs is meant to be close to 13' long but maybe 11' or 3.3m is the longest I can squeeze on my arch.

I don't really need it to fully plane as I'd be happy with 8-9 knots of speed with say a 500 lb or so payload. Would ideally like to power with a 5 hp. So I'm thinking maybe some type of relatively narrow but deep full displacement hulls? Maybe with as much as a 6" hull draft loaded would be ok.

It seems most attempts are for full planning boats with very shallow hulls. I'm wondering is. Displacement hull with an 8:1 or so l/b perform how I want and carry the load? Maybe 10:1 is possible?

Any thoughts appreciated. My RIB is on its last legs!
Attached Thumbnails
Catamaran dinghy 10-11' long-10557254_741399769266667_4261057831046378497_n.jpg  Catamaran dinghy 10-11' long-10690294_743501102389867_2797268142300831003_n.jpg  Catamaran dinghy 10-11' long-10704040_743501209056523_9195846308829739857_n.jpg  

Catamaran dinghy 10-11' long-10350426_741924012547576_7756374663032168759_n.jpg  Catamaran dinghy 10-11' long-10450538_731085843631393_7884256733928831476_n.jpg  Catamaran dinghy 10-11' long-10672358_731097120296932_7260254780195331702_n.jpg  

Catamaran dinghy 10-11' long-10710959_731085733631404_8660986895203679990_n.jpg  
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  #8  
Old 05-10-2015, 05:59 AM
catsketcher catsketcher is offline
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Read this thread

Catamaran tender to replace rib?

I would not want to use a dinghy sled. When we rescued a turtle last year my wife was motoring Kankama whilst we were motoring in the dinghy. If you go for a dive then the mother ship has no way of moving under power if the anchor drags - very dumb idea. I also don't think you could easily connect the dinghy in big waves so again the mother ship may be left without a dinghy just when you need it. I like the thought that if I am ashore a friend or concerned observer could jump on my boat and motor it if she was dragging - I always leave the keys in the motor.

Also trying to use the same engine for a 4000kg cat and also a 200kg tender and crew plays havoc with the prop choice. You will have to go the fine pitch prop for the heavy cat and so the tender will have a 25hp outboard on it and go no faster than if it had a 6 instead. Then what do you do with motor controls etc. Seems like a good idea until cruising reality strikes.

Even worse is the problem of dinghy theft. If you lose your dinghy it is bad but you buy a cheapy and work out what to do later. If you have a sled and it drifts away or is stolen you can't motor your mothership. You can't buy a cheapy but have to stop and make a custom new one. You hope the thief is nice enough to have stolen it whilst you are close to a boatyard where you can build a new one.

After cruising with the 10ft cat dinghy for 7 months I still like it. There is nothing special in the design and the bridgedeck is low enough that with three people it turns into a punt and has a high immersion rate. It is really nice when carrying 80 litres of water and fuel. Much better than a mono dinghy. Also guests can't go wrong as they do in our much nicer rowing dinghy. With the cat one I don't have to harrange them about how to sit or stand. It just copes.

Go the 12ft cat dinghy made from ply or foam and keep it rough.

cheers

Phil
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  #9  
Old 05-10-2015, 06:42 AM
Boatguy30 Boatguy30 is offline
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Who said sled? Not me. I thought Deneys dinghy was just a dinghy not a Bolger yawl bot thing.

Anyway, I figured 6" or so was a deep draft for an 11' dinghy. This is the running loaded draft as opposed to a planning V hull which might draw 3-4" when planning?

I figure is a single Yamaha T8 can push my 5 tonne sail cat at 6 knots, I must be able to get 8 knots with something 1/16 the weight on 5 hp?

Maybe built a 1/2 scale model and see what it will do with 2 hp.
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  #10  
Old 05-10-2015, 06:52 AM
Boatguy30 Boatguy30 is offline
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I guess my main idea is to get the speed not planning but from having narrow enough hulls to avoid the standard hull speed trap. Maybe this is where I'm all wrong.mim under the impression most large, fast, sail cats never plane. They are simply able to exceed a typical 1.34 hull speed due to the fineness of the hulls. For example I was sailing at 15 knots this past winter on my 34' cat and a 42' Newick tri. On my cat we were semi- surfing maybe semi planning, but on the newick we were in totally flat water and certainly didn't seem to be planning.

The only small power cats I have seen rely on planning to get to speed much like catsketchers seems to when he pushes it solo with the 3.3 horse he mentions. I did do some calculations and come up with a few vague numbers a couple months back, but can't find them.


Cheers,
Jeff
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  #11  
Old 05-10-2015, 08:10 AM
rob denney rob denney is online now
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Quote:
Originally Posted by Boatguy30 View Post
So it seem a good performing cat dinghy is very difficult to design. It looks like Robs is meant to be close to 13' long but maybe 11' or 3.3m is the longest I can squeeze on my arch.

I don't really need it to fully plane as I'd be happy with 8-9 knots of speed with say a 500 lb or so payload. Would ideally like to power with a 5 hp. So I'm thinking maybe some type of relatively narrow but deep full displacement hulls? Maybe with as much as a 6" hull draft loaded would be ok.

It seems most attempts are for full planning boats with very shallow hulls. I'm wondering is. Displacement hull with an 8:1 or so l/b perform how I want and carry the load? Maybe 10:1 is possible?

Any thoughts appreciated. My RIB is on its last legs!
I presume I am the Rob referred to. The harryproa tender http://harryproa.com/?portfolio=tender is indeed 13' long. A shorter version would be easy enough to build. "Just" knock 2-3' off the sterns.

Hull shape on a catamaran dinghy is not going to make a big difference to handling under motor. It will be harder to row than a mono dinghy, but speed will depend mostly on the motor and the weight carried.

Wider hulls are going to be a better bet than deeper. A scow (cat with no tunnel) is the best load carrier/speed machine available if you don't mind slamming at high speed and probably worse rowing performance than the cat. It is certainly easiest and lightest to build.

Tell you what. Build an Elementarry (see other thread) and i will throw in tender plans for a version that best suits your needs for no charge.

Quote:
Originally Posted by catsketcher View Post
I would not want to use a dinghy sled. When we rescued a turtle last year my wife was motoring Kankama whilst we were motoring in the dinghy. If you go for a dive then the mother ship has no way of moving under power if the anchor drags - very dumb idea. I also don't think you could easily connect the dinghy in big waves so again the mother ship may be left without a dinghy just when you need it. I like the thought that if I am ashore a friend or concerned observer could jump on my boat and motor it if she was dragging - I always leave the keys in the motor.
Not sure of the relevance of this to the Op, but the boats the tender as a sled are meant for are boats where light weight, low cost and big tenders are required. And that sail well, especially off a lee shore. And even more especially, short handed.

"Dumb" is leaving your boat poorly anchored in the hope that some stranger will see it dragging, jump on board and figure out how to motor it to safety and re anchor it.
"Smart" is a tender that is capable of getting back to the anchorage at 20+ knots when you see the weather change coming. Then stay in the tender, tie off the anchor rode to the substantial stern cleat, lift the anchor and tow the mothership out to safety. Reset the hook, put the crew on board, lower the big storm anchor and all it's chain into the tender and take that out and drop it as well. Then 20+ back to your dive spot, motoring past your mates "normal" tender which is trying to avoid swamping while travelling at 5 knots to get back to see what damage has been done to his boat by some well meaning but dumb "concerned observer" who thought the anchor was dragging.

Connecting requires picking up a floating line and using it to pull the tender under the beam and hoisting it. Same as davits, but not as high and only the bow needs lifting. There will be a seastate when you would get out of the tender rather than staying in it and going under the beam, but getting off amidships is far easier than trying to get on the stern steps of a pitching cat.

Quote:
Originally Posted by catsketcher View Post
Also trying to use the same engine for a 4000kg cat and also a 200kg tender and crew plays havoc with the prop choice. You will have to go the fine pitch prop for the heavy cat and so the tender will have a 25hp outboard on it and go no faster than if it had a 6 instead. Then what do you do with motor controls etc. Seems like a good idea until cruising reality strikes.
It would take less time to change the prop (which is easily reached from the deck of the mother ship) than it would to launch a deck stored dinghy over the lifelines, get the outboard and it's fuel from the bow locker and attach it to the dinghy. Regardless, it is not about making the mothership go fast (it is likely only going to motor in and out of marinas) as it is about having a large, fast capacious tender on a boat with limited space and budget.
Motor controls are easily sorted, with a little initiative.

Quote:
Originally Posted by catsketcher View Post
Even worse is the problem of dinghy theft. If you lose your dinghy it is bad but you buy a cheapy and work out what to do later. If you have a sled and it drifts away or is stolen you can't motor your mothership. You can't buy a cheapy but have to stop and make a custom new one. You hope the thief is nice enough to have stolen it whilst you are close to a boatyard where you can build a new one.
What a funny way to think. If your tender is stolen, bolt on an outboard bracket (if you are that paranoid, bolt it on before you leave) and buy a new outboard.

Quote:
Originally Posted by catsketcher View Post
Go the 12ft cat dinghy made from ply or foam and keep it rough.
Or, go the 14'ter, with the big outboard and the option to water ski, go to distant or inaccessible places, handle big waves, carry large payloads, tow your boat and in the event of a fire, man overboard or other calamity, have a quickly launched dinghy that the crew has some hope of surviving in.

rob
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  #12  
Old 05-10-2015, 04:08 PM
catsketcher catsketcher is offline
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I think getting 8 knots with 250kg aboard is going to be very hard in 11ft. 20 knots will be just as easy as you are going to have to plane.

Do you need 8 knots with two of you in the tender? Is 3 okay instead? If you need to go faster then it will cost you as even a cat tender has to plane to get to this speed.

A friend has the same dinghy as mine, same mould. he is a fair bit heavier than me and his doesn't go faster than 5 knots with the same outboard. With his wife as well the thing is slow but he, and me, don't mind slow.
hil
cheers

P
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  #13  
Old 05-10-2015, 04:15 PM
Boatguy30 Boatguy30 is offline
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So what is the hull l/b or your dinghy. It's probably real fat like 5:1 or 6:1, right? That's my point a bit of this thread as all cat dinghys Ive seen are basically no better in most respects than the scow suggested. It seems to me the inverse V thing is actually worse as it has less displacement and more wetted surface than a scow of the same overall dimensions.

So I'm thinking of making a model. Even something 7'7" long will only have 1/3 the displacement. So enough for me to hop in with a 2hp and give it a go. (Unsure the time frame) maybe the next few weeks.
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  #14  
Old 05-10-2015, 04:23 PM
catsketcher catsketcher is offline
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About 7:1 - so very fat for a cat hull.

Doing scale tests with planing boats is tricky because of the square cube rule. I would continue looking and asking as I don't think you are going to do anything that hasn't been done before.

I would be more concerned with weight of the dinghy rather than speed. I do like light things and putting a heavy dinghy down the back of a cat is bad for stern squatting and pitching. I would prefer a dinghy to weigh nothing and go slow so that my sailing cat goes faster. Then again I am definitely on the outer here. Many 38 ft cats would weigh almost double my boat and their dinghies (RIBS with 8-15hp) over 4 times the weight of mine. The owners like them and so they are valid for them but terrible for me.

Another friend who started off with a cat dinghy (this mould has been passed around the group) now has a v nosed punt that goes a bit faster than mine with the same outboard, With two on it slows right down again.

I met a guy on the beach who came up to a bloke I was talking to and said "Which is the best dinghy on the beach?" I looked to where he had come from and there was a small RIB thing about 3.5m long with a centre console. When he pointed it out to me and my new friend he asked us to come and help him lift it up the beach as he couldn't do it himself.

I thought is was terrible but loved it. Each to our own

As to the scow - my friend with the V nose punt probably gets a rougher ride than me and has to jump the resistance curve to get planing. ON MY OWN my tender acts like a cat and is easy to push around at 5 knots and little wake. With more people in it the boat becomes a scow anyway so you get a higher immersion rate like a scow.

cheers

Phil
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  #15  
Old 05-10-2015, 04:39 PM
Boatguy30 Boatguy30 is offline
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My current rib weighs only about 32kg and the replacement would be about 40. But at $3,500 and needing atleast a 9.9, I was hoping for a better way. interesting at 7:1 it still performs like a brick either planing or not.
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