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  #76  
Old 08-28-2015, 07:36 PM
Boatguy30 Boatguy30 is offline
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The Logical Solution

Ok, today was the day. I morphed the cat model onto the long hull blocked up 5.5" to hopefully minimize the drag and flotation provided by the extra hulls. All up weight was just about 45kg for the hulls somewhat I expect the final boat to be.

With me 65kg and the 3.3 hp I got 10.5 knots.

Add 55kg of 8mm chain and it's 8.5 knots.

Add another 25kg and its 7.8 knots. And at this load, the outer hulls are certainly dragging quite a bit and close to the max 220kg design goals.

I also hoisted the 11.5' hull onto my arch/davits and it really fits quite well. So I think at this stage, its a go for the full size dinghy. Will likely start it in the next few weeks.
Attached Thumbnails
Catamaran dinghy 10-11' long-win_20150828_133637.jpg  Catamaran dinghy 10-11' long-win_20150828_162624.jpg  
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  #77  
Old 08-30-2015, 02:50 PM
Boatguy30 Boatguy30 is offline
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Ran it the next day with a 6hp 2stroke and the pile of chain. Despite my spray skirts, I was shipping a lot of water but did get 11 knots. She does noticeably pull out of the "hole" with the bigger motor. Not really planning though either. Will report back when construction commences.
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  #78  
Old 08-30-2015, 10:05 PM
buzzman buzzman is offline
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Suspect you need higher freeboard anyway, because of the spray issue.

All the cat dinghies seem to have a pretty high freeboard compared to 'normal' dinghies.

I've been emailing with Mark Gumprecht about his cat skiffs, which you can see here:
http://catskiff.blogspot.com.au/

He sent me the drawings for it also and he's drawn the 10' cat skiff with 16-1/4" of freeboard, presumably to maximise freeboard and minimise waste from ply sheets which, being 4' means a maximum half sheet (to loft from) of around 24".

I think I'd be looking for 18" for freeboard, and decking in the bow section to help reduce spray.

But it should be relatively easy to extend the entire hulls should you wish to, and although Mark recommends no more than 3HP, it again would not be too difficult to reinforce the transom beam, even raise it a bit to make it easier to attach a larger motor.

Some 220gsm cloth either side and in the joins, and perhaps a triangular buttress, vertically from the floor up the side of the seat and to the top of the transom, either side of the seat, which would help spread loads forward along the seat sides, rather than all of the load being on the small transom across the top between the two hulls.

If you do use them, please insist that Mark accept some money for them. I tried but he wouldn't take anything from me.
mwgumprecht@gmail.com
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File Type: pdf 10 foot cat skiff.pdf (161.4 KB, 133 views)
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  #79  
Old 08-31-2015, 12:53 PM
Boatguy30 Boatguy30 is offline
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I was at marks house in 2008. He was working at the time on a demountable 12' utility cat. At the time I had a 43' ketch so it sounded like it would be perfect. He says he finished it and it "worked as designed" but never sent me any pics or performance specs.

I'm clearly the type that needs data. He seems to be concentrating too much on planning as opposed to my semi-planning narrow displacement hulls. He says the 10' went ok with the him and the owner. If you search CruisersForum for catamaran dinghy, you will find that boat. Only the owner claims to have designed it himself and that it planes easily with 4 aboard with a dry ride into a 20 knot headwind! Also that it weighed only 40kg and was all fiberglass!! Same boat, built by mark in the blog pictures!

How does 16.25 divide evenly into 48?
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  #80  
Old 08-31-2015, 08:24 PM
buzzman buzzman is offline
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Catamaran hulls don't "plane".

Planing is a very specific type of ride wherein the boat rides on its tailbone, with less than half the waterline length immersed, and usually only the very rearmost quarter of the boat.

It involves physically 'pushing' the boat up and over the bow wave created by attempting to displace the water in front of it. It then climbs up and over the bow wave that has thus created.

Catamaran type hulls don't do this. The small wave they create never gets big enough to hold the hull back due to the ratio of beam to waterline length being so low. More beam, greater resistance, larger bow wave, more power required to get up and over it.

it's why you only need low power engines on cats and tris to get to a reasonable 'displacement-hull-type-speed'.

They are technically displacement type hulls. They displace the water to the sides. They don't tend to ride up on the bow wave.

If you add HP they simply go faster, which is why cat sail boats with tall powerful rigs are so much faster than monos of the same nominal waterline length. The monos are 'resisted' by their own bow waves, the cats aren't so much.

I dont understand the physics of it, maybe someone who does can explain it better.

As to the division of 48" by 16-1/4", I'm not sure why you'd want to do that....

What I'm suggesting is that *because* the sheets are 48" wide that you could probably get a lot more freeboard into the sides of the cat skiff, as half a sheet ripped lengthwise willl be 24" wide, it just depends how much rocker you choose to put in the hull sides.

Hence why I suggested 18" should be easy, but you might get 20".

And Mark insists that he doesn't want any money for these drawings, so if anyone wants to use them I reckon it's open slather. Knock yourselves out.
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  #81  
Old 08-31-2015, 08:38 PM
Ad Hoc Ad Hoc is offline
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Quote:
Originally Posted by buzzman View Post
Catamaran hulls don't "plane"....Catamaran type hulls don't do this.
Sorry, that's complete nonsense.

Any hull can plane if the hull lines are designed correctly to facilitate the flow at high speed -owing to the pressure wave created - and enough power.

Here are just two of many thousands of examples:-

Catamaran dinghy 10-11' long-cat-speed.jpg Catamaran dinghy 10-11' long-cat-speed-2.jpg

Think you need to go back to school on this one!
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  #82  
Old 08-31-2015, 09:09 PM
Boatguy30 Boatguy30 is offline
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And mark repeatedly refers to his skiff as planning and those 12' jobbies with 30hp yammie certainly are planning.

With the 10:1 hulls I am getting roughly 2x displacement speed with small hp (3.3) so gonna be happy with that.

I am going with 18" side as the inner side is 12" so divides up the ply nicely. I think I can get the entire boat from 3 sheets of 4mm and 1/2 sheet of 6mm.
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  #83  
Old 08-31-2015, 09:15 PM
buzzman buzzman is offline
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Talking about small cats and sailboats, not offshore racing powerboats.

I take your point, but perhaps I didn't express myself correctly. As typical catamaran hulls are of a 'displacement type' design, they don't plane, as such.

Cat hulls which are designed *normally* have a low beam to length ratio, and it is this that enables them to cut *through* the water with less power requirement than for a pure-displacement mono hull of the same length. As they accelarate the entire hull rises out of the water, reducing waterline length and drag.

The hull shape of offshore powerboats is like a deep-v powerboat hull (planing type) cut down the middle - although that's an oversimplification in itself. Deep-V forefoot and low deadrise aft. Totally different underwater shape to a 'normal' catamaran.

But sure, put a big enough engine, or wide enough beam and tall enough mast, to get a normal cat up on the plane and it will go there.

But it still won't have a 'planing type hull' design. Unless, like the offshore powerboats, you've specifically designed it to have one.

Just calling an offshore racing powerboat a 'catamaran' is like calling two logs strapped together with bamboo poles a catamaran. The only things they have in common are two hulls, rather than one, or three.

Cats can reach higher speeds with smaller motors than 'planing type hull designs' [deep-V forrard, low deadrise aft] because they don't have to climb over their own bow wave.

But basically, until you put a big enough motor on the back of a cat dinghy to force it past its displacement speed, up and over what little bow wave it has, and onto the rear quarter of the hulls, it isn't *technically* 'planing'.

And I *did* say I didn't understand the physics, so your admonition to "go back to school" is nothing short of insulting. Mere rudeness for the sake of rudeness.

As I have understood it from all the texts I've read over the years, there are basically three types of hull - displacement, semi-displacement, and planing - but you could throw in scow or flat-bottomed and even reverse-V if you like.

As I understand it, sailing monohulls, traditional shaped rowboats and typical catamarans have 'displacement type' hulls. Tapered at both ends, fatter in the middle. OK, so modern monohull race boats are fatter and flatter aft, so they do in fact 'plane' with enough windspeed, but we're talking 'traditional' here.

Early torpedo boats and warships have semi-displacement hulls, which are flatter aft.

Powerboats have planing hulls with deep-V forefoot and low deadrise, near flat aft sections - and once at speed they rise up and over their own bow wave and ride on the near-flat aft section of the hull, with the forefoot mostly or completely out of the water.

So if that is wrong then so is every boatbuilding text I've ever read.

But please, from the benefit of your infintely greater education and experience, enlighten we poor, unlearned students.
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  #84  
Old 08-31-2015, 09:20 PM
buzzman buzzman is offline
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Quote:
Originally Posted by Boatguy30 View Post

With the 10:1 hulls I am getting roughly 2x displacement speed with small hp (3.3) so gonna be happy with that.
Exactly my point! Two times the "traditional" *displacement hull speed*....

So at that speed, the boat will not be sitting up on its tail. Merely going faster than you would *expect* it too, given it's displacement hull speed.

It's the old 1:34 ratio thing. According to 'gospel' a *displacement hull design* should not exceed this hull speed ratio, yet cats and tris do.

But this does NOT make them a 'planing type hull design' - is the point I'm trying to make.

They need to have a deep-V forefoot and a near flat aft section in order to be, technically, as I've been lead to understand it, a *planing type hull design*.

But sure, stick a 30HP on Mark's little cat skiff and it will get up 'on the plane' and sit on it's flat bottom and skim the surface.

But it still won't be a 'planing type hull design'.
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  #85  
Old 08-31-2015, 09:22 PM
Ad Hoc Ad Hoc is offline
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Quote:
Originally Posted by buzzman View Post
I take your point, but perhaps I didn't express myself correctly. As typical catamaran hulls are of a 'displacement type' design, they don't plane, as such...
Nope..wrong again. A hull, is designed to satisfy its SOR. No magic or black art.

Quote:
Originally Posted by buzzman View Post
Cat hulls which are designed *normally* have a low beam to length ratio, and it is this that enables them to cut *through* the water with less power requirement than for a pure-displacement mono hull of the same length. As they accelarate the entire hull rises out of the water, reducing waterline length and drag..
Again, nonsense, nothing “normally” about a hull. See above!!

Quote:
Originally Posted by buzzman View Post
And I *did* say I didn't understand the physics, so your admonition to "go back to school" is nothing short of insulting.
Then you should not be giving advice to others if you do not understand the basics and concepts/theories of what you’re talking about. You only feel 'insulted' because you have been exposed as lacking in basic hydrodynamics.

If in doubt, ask!
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  #86  
Old 08-31-2015, 09:51 PM
JSL JSL is offline
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Thanks Ad Hoc, you have saved me a lot of time planning on how to explain planing hulls for catamarans.
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  #87  
Old 08-31-2015, 09:51 PM
buzzman buzzman is offline
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Ahh! Now I understand. You want to talk *specifics* while I'm talking "generalities".

For the purpose of the argument, I accept your points.

Except one. I have a read *a lot* of texts, and they all concur with the broad *general* conclusions I have merely reproduced. I claim no special skills as a designer or naval architect, or physicist, or rocket scientist.

Like the vast majority of people on here, I'm a generalist, not a specialist.

The point is that you shouldn't talk about 'planing' on any hull unless the boat is riding on its tail with less than half its keel line immersed.

This is what is *generally understood* as "planing". By us generalists. By ordninary folks. Out here in the real world. YMMV.

So, speaking *generally*, a boat hull will not 'plane' unless it is of the 'planing hull design type' OR has a whacking great motor on the back capable of pushing it over or through its own bow wave.

Clearly, a scow-bottomed hull will also 'plane' with a relatively low-powered outboard on the back, even though it is not of the 'planing hull design type'. But it will be sitting up on it's aft bottom third while doing so.

*Most* catamarans don't do this.

Even larger cats like the Incat ferries don't 'sit up on their tail' but still travel at a fast speed and using lower-powered engines than they might otherwise for their SOR load capacity, fuel consumption and speed requirement. The point being that they go faster and use less fuel doing it while carrying a given load.

So while a flat-bottomed scow's SOR might be "carry load in skinny flat water with low-powered engine" it will also 'plane' with a big enough engine. And will be sitting up on its tail whilst doing so.

A 'three-point hydroplane' is also not a 'typical' *deep-V forefoot, flat run aft* planing hull shape, but it too 'gets up on the plane', with only its aftmost transom in the water and the keel of the sponsons barely kissing the water. But this too is called 'planing'. At least, by us generalists.

A fishing boat might require 'speed to fishing grounds' as its main SOR, in which case you might design a deep-V forefoot, low aft deadrise, large engined boat capable of getting up 'on the plane' in rougher seas and maintaining that SOG. And much of the time, only the aft third of the boat will be wet, thus 'planing'. As we generalists understand it.

So my point is that, whatever YOU say to the contrary, we ordinary folks call that 'planing', and cats don't *normally* pass through the water with only their aftmost third immersed.

So, on that basis, cats don't plane. Normally.

But of course they can, if you put a big enough engine in them, and if you design the hulls so they will - specifically - - - 'get up on the pane'

But *generally*, they don't.
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  #88  
Old 08-31-2015, 09:56 PM
Mr Efficiency Mr Efficiency is offline
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A planing cat typically has less waterplane area/kg of boat than a planing monohull, so lends itself to the notion it is ploughing through the water like a displacement cruiser, rather than skimming over it like a speedboat. Which is not entirely wrong, but it is still experiencing dynamic lift. I have met more than one fast power cat owner who reckoned their boat did not plane, but they were wrong. If the typical planing cat was dramatically lightened, it would become obvious it too was planing like a lightweight "tinny" runabout, but it would be a scary conveyance at speed. Similarly, if even a relatively heavy planing cat was given sufficient power and a high speed, the planing would be clear to see.
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  #89  
Old 08-31-2015, 10:15 PM
Ad Hoc Ad Hoc is offline
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Oh dear…I think you should quit while you’re behind!!

Quote:
Originally Posted by buzzman View Post
The point is that you shouldn't talk about 'planing' on any hull unless the boat is riding on its tail with less than half its keel line immersed..
Has nothing to do with that. As previously noted, you should read and by that not blogs and websites, but actual text books that have all the correct theory explained, since reading “lots of text” has not educated you sufficiently at all.

Quote:
Originally Posted by buzzman View Post
So my point is that, whatever YOU say to the contrary, we ordinary folks call that 'planing', and cats don't *normally* pass through the water with only their aftmost third immersed..
Everything is about definitions. Just because YOU wish to call it something does not alter the facts of what it is nor the theory behind such. And this is where you’re going wrong. In order to dumb down the basics of hydrodynamics for you to understand it, you have thrown the baby out with the bath water. Definitions have meaning and in engineering definitions and meaning is everything. Get them wrong and you’re lost and simply following the equally blind crowd and unsure why too…as noted by your posts above.

You can either get hold of a book and read the theory of the hydrodynamics of displacement to planning hulls – as it is not for a simple one line blog reply and requires some understanding of maths and physics as a prerequisite - or you can continue in the vein you doing now and also throwing misinformation out the others who do actually ask and seek the correct answers.

Your choice….thus any further “insults” that you feel, are all generated by yourself.
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  #90  
Old 08-31-2015, 10:27 PM
Mr Efficiency Mr Efficiency is offline
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Incidentally, imo of course, the planing cat that doesn't seem to have a decent planing action, is the desirable vessel, as it negates a lot of the flighty characteristics that bedevil lightweights, and not only feels safer, but is.
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