Catamaran dinghy 10-11' long

What on earth is a "hull speed trap"...??

Again, where do you get this nonsense from??

There is your main problem. You're guessing on a subject with no knowledge of what you speak. As I noted above, give a designer/NA your thoughts/ideas and them tell you what is or is not achievable.

If you don't want to go that route, then just build a box with a pointed end and see what it does. It will work, just may be not as you intended though.

 

Well you take a 8' pram and stick a big load in it. Try to make it go faster than about 4 knots. what does a naval architect call that?

 

Depends upon beam and draft and design dwl and also depends on the ratio of the big load to its carrying capacity

Overloading and not understanding the limits of ones boat.

 

What is a "big load" in pounds?

 

I've only given this site a quick scan but I can't see a problem with a cat dinghy... Livingston has been building several models 8' and up, since 1968

 

Boatguy

I get the idea of a boat speed trap - you are talking resistance humps usually at the square root of 1.34 x waterline length. Cats usually have a far smaller resistance hump than typical monos. Getting around a resistance hump can be done by planing or by going thin - usually less than 10:1 in length to beam ratio.

You can't get down to 10:1 in a cat dinghy so you are going to have to plane if you want to go fast - flat bottom and straight rocker run. Putting a load in it is hard.

A dinghy has to do heaps - it has to be stable for snorkelling - a cat shines here. It has to be fast for scouting - 8 knots is fine for me. It has to be stable for guests. It has to carry 100kg of water and me. It has to carry 80 litres of fuel so I can anchor out and get in to a busy fuel dock or one with a cross wind. It has to carry 90kg - me, 150kg - my wife and me, 300kg us and friends. It has to be easy to lift and cheap enough to leave on the beach and be happy to walk away from for the whole day. I really want it to keep me dry.

This is a generalist boat and as such will do all things relatively well but not shine in any area. To plane at 20 knots with 250kg will require a scow/sled and 25hp. (That is what our flat inflatable does) If you ditch the fast heavy requirement the boat can be light and cheap. If you need to go fast when heavy then it becomes heavier, and much more expensive. I could have taken our inflatable and 25 on the cruise and never wished we had it. It weighs a ton.

Like the great Dick Newick said "You can have any two of speed, economy and acommodation, but not all three." Choose your two

cheers

Phil

 

Well 10:1 is what I was going to try. I've just gotten back for a 4 month cruise on my 3 in 1 Woods Vardo cat (fast, cheap, and comfortable) so I'm just getting sorted out on land and going thru my list of work and boating projects. I do hope to build a model soon will need to be 70% length to have enough displacement for testing. As an FYI I plan to make it with no overhangs so 11' waterline with the motor mounted forward of the transoms.

I'm thinking of just doing something slab sided and wondering maybe if I shouldn't even have the bottoms almost flat with no rocker? I can always carve up some foam blocks and glue to the hull bottoms to play with the models hull shape a bit during testing.

 

Oh, again I only want a top loaded speed of 8-9 knots not 20, but I realize this is a hard target to shoot for. So that's why I started this thread with "hard to design" as I've seen lots of attempts at "cat dinghies" including the livingstons, but at the end of the day they all basically require the same HP as a rib of the same size.

 

In my opinion speed is not the best gain from a small catamaran. But you can have a lot of stability from only 3,20m loa (10.498ft) and 1,20m (3.937ft) boa:

 

I have no doubt you can get 8 knots out of a 3m cat tender, just not with 250kg on board.

If the boat is to plane I would have it pretty flat bottomed and straight aft rocker. It may be nice to have some deadrise for pounding and to put a keel strip on.

For 300kg weight (250 kg payload plus light dinghy and small outboard) you need 300 litres. If you try for 10:1 with 3 or 3.3m long hulls you get a hull width of 0.33m.
Volume = length x breadth x depth
Depth = Volume / length x breadth
Depth = 0.150 / 3.3 x 0.3
Depth = 0.150 / prismatic (lets say 0.8 for a very blunt shape)
Depth = 19cm

That is for a planing shape. If you want to try displacement then the prismatic will be about .6 so the depth will be

150/ .6 = 25cm.

That is one heavy and deep dinghy. I think going 20 knots is going to be the same difficulty as going 9 for the planing type as you will be on the top a huge resistance curve and reduce drag by going faster - the planing hump any short board sailboarder knows well.

Try drawing the displacement hull on graph paper and count up the areas to get the displacement you need. The dinghy may not be possible.

If you really need the speed then you could think about going long and using an outrigger canoe. I am thinking a long 18ft single outrigger canoe with a small outboard that pivots in the middle on the water and locks in place with an over centre latch. Gary Dierking is the king of these boats and they certainly would get you to 9 knots with a heavy load. You also get some of the advantages of the very capable tender Rob Denney likes.

My bet would be a folding Wapa

http://homepages.paradise.net.nz/garyd/wa_apa.html

cheers

Phil

 

I appreciate that help. Ironic how I suggested a 6" hull depth and others told me I was limiting the design by wanting so shallow a draft! basically a displacement hull was my original plan, I'm just not sure it will work. I have never seen someone try a displacement hull in a little powercat like this. I personally think that's the only reason to fool with a cat dinghy is if you can push it fairly fast with low HP.

 

As has already been pointed out, load 250 kilos into it and it won't be a catamaran any longer, as the tunnel top rapidly submerges. It will be more like a barge ! You can't win. You can't get the speed advantages of a displacement cat in such a tiny thing, with such a load.

 

I think the displacement has grown slightly in the conversion from imperial to metric. Perhaps 10%. I'm planning to knock together a 7.7' model which should hopefully just float my 68kg and a 2 hp motor for testing. Hull beams less than 8"! I plan to make the hulls similar in shape to the outer hulls of a Bolger Bantam.

The one further input I could use is how much to taper the hulls aft of the max beam? Most production power cats seems to taper little if any, but I suppose its less drag if you taper at displacement speeds.

Since I'm looking for easily driven and max hull volume, what is the right compromise? Hard to change the hull taper of the model once built.

 

I think it is just a waste of effort trying to make a tiny displacement cat with skinny little hulls, even shifting your 68 kg to the stern will cause it to tilt alarmingly. If you scaled a 33 foot displacement cat down to your 11 feet, that would be like putting 27 people your size right down the back of it. How do you reckon that would affect the trim ?

 

My mistake, the 7/10 model will have a hull beam of 9.25". The hull shape aft is fuller and deeper than a sail cat so the comparison you draw is valid, but not quite as extreme. Also the immersion is based on area not volume so it will be the square not the cube?

I reckon I can knock together a model in 4-5 hours of work with wood glue and nails. Chuck it in the canal off the dock it will be built on.

Only one way to find out!