Swath Time

SWATH time again "ugh".

Hi All,

I enjoined posting on the health care debate so I'm jumping in with a new thread even though I know you mono hull people are already moaning for me to stop it.

I want a large platform so I can fit out a vessel with "normal" home depot stuff, a toilet not a head, a bed not a cot, a kitchen not a galley, ect.

My SWATH is 60' L O A x 30' beam with 2 levels. The hulls are 3'x3' and as stated 60' long. By my #'s that is 66,000 lb displacement if the hulls are running submerged.

My question for the smart fellows is what thrust or HP or whatever is required to drive this pig at 10k with a reasonable safety margin. Now the multi hull people as screaming that's so slow, I know but I rowed crew and 10k was screaming for us so my base line is very low.

I am hoping with very thin hulls I can get up to target speed, after a long run up, with a very low burn rate. Acceleration is not an issue but it would be nice to be able to stop as well, I get that, so I'm thinking big wheels turning quite slow. I know I'm nuts but am I crazy?

I am going to take some pain pills and prepare to be torn up so bring it on.

Thanks for any comments, Brad

 

Twin screw, 36 inch props, electric, 10 knots, I'm guessing 20 kw total.

Go ahead Rick, show how wrong I am...

Tom

 

Thanks Tom,
20 kw would be super.
I hope Rick agrees, I was on the same electric track.
Brad

 

planning on taking a full utility 2 level swath out to the rough ?

 

Dragging two story house to weather you can pretty much forget the drag of the hulls and get 200 hp engine to have a bit of safety margin ..

 

No just coastal but that's why god made EPERB's, can I sign you up for s spin?

 

Per hull Teddy............!?!

 

If he wants to make something else than leeway in a frisky gale
There's got to be a quite high "oilrig legs" btw the submerged hulls and the superstructure to keep breakers below bedrom windows..

 

Whats so bad with making leeway? Say from Ushuaia to Perth you could save a lot of fuel!?!

 

Your target speed of 10kts works well for a conventional cat with total displacement of 30t and length of 60ft. The wave drag is not very high and there would be little point in submerging the hulls unless you desire better ride in rough weather. There will be a lot of volume locked up in the hulls.

The lowest drag cat hulls will be 1m deep and 1.2m wide. Your displacement calculation has little allowance for form.

There would be little or no reduction in drag submerging the hulls. Any saving in wave drag would be offset by the strut drag.

Power required to drive the cat hulls with big props is about 28kW total. But as others pointed out windage will become significant. The superstructure would need to consider wind drag.

Rick W

 

Upston

If you really want a SWATH and i mean a proper SWATH, then you are going to ahve to do a bit more research and design work than "...what thrust or HP or whatever is required to drive this pig at 10k with a reasonable safety margin..".

SWATHs are very complex hulls and require significantly more prelim design than any other type of craft. The variables which do not affect mono's or cat's too much can significantly affect those of a SWATH.

There is no one line reply for your one line question. Anyone who attempts to provide an answer, has never designed a SWATH before and does not understand the complexities that are involved, and such should be ignored as all they are doing is misleading you.

 

Hi Rick,

Thanks for the thoughtful post. I wanted to go SWATH not Cat to have a uniform hull to facilitate construction methods. Water tight bulkheads every 5 ft all the same in section like a torpedo. I took a look at some of your test hulls using pipe with the electric motors and it's the same simple concept.

I was thinking at low speed all those compound curves are not a big factor as long as the hull length is generous in relation to the house, but that's why I opened this thread.

I see your point on losing volume but as I get blown all about to Perth I still have lots of cubic ft above deck to make the spouse unit happy.

I know the sail boat boys are content being miserable for years in a pointy ended narrow tube just in case a 45 ft rogue grenney gets them but it's not for me. I was a Surf Life Saver for a few years on the Gold Coast and almost drowned a time or 2 trying to save dumb tourists , I can think of a lot worse ways to go.

A 4 ft wide cat hulls always make me a little claustrophobic but space is space. This is a home built as I don't have a $ mil or so like some of our members so KISS is in full effect. If I cant do it it's not going to happen.

I see you are an Aussie, my Dad played V F L footy for South Melbourne for a few years after the war so it's nice to talk to a Melbourne boy.Thanks again for you time.

Brad

 

Brad
The attached gives you an idea of the lowest drag hard chine cat hull for 15t displacement in each hull. This has very similar power requirement to any rounded chine hull. In this case you also work on 28kW total to drive the cat hulls through the water.

Working with small waterplane vessels creates stability issues that are very difficult to design for when the CofG is high. There is a strong tendency for one hull to find the surface and the other to fully submerge. Once in this attitude it will roll until the accommodation is in the water. To get the best stability from a normal catamaran you really want either hull to be able to carry the full displacement or at least close to it.

If you have a vessel like superstructure that works well as part of the hull then this effectively achieves the same thing as reserve buoyancy in the hull. However I do not imagine what you have in mind for the accommodation is hull like.

I cannot see the attached hull shape being any more difficult than the box you have in mind. A gently curved plate will have less tendency to buckle when welding. The 6ft height makes it easier to work inside. If you have fully sealed bulkheads you need to think about all the enclosed spaces you create. Ventillation becomes a major problem.

Hulls like this could have vents and be used for all the power and services. They are useable compared with a 3ft square box. You may not want to live in them but the have good space to work in.

Whatever you have in mind for the superstructure needs to have windage accounted for. You will have large wind loads if it presents bluff faces.

Rick W

 

Rick
I get it, that makes a lot of sense. You are right this would be easy to build even for me. Is this and existing power cat or one you are working on?

Any other details like framing or other details available? Happy to pay.

Thanks again, Brad

 

Brad
That is just a five minute exercise in hull shape.

Boating is just a hobby. I design, build and use small boats based on optimising performance for effort. I have found and validated methods for determining performance that are very accurate.

You may see some similarity with the shape I have provided you and the much smaller one in the linked photograph:
http://www.boatdesign.net/gallery/showphoto.php/photo/15138/size/big/cat//ppuser/18624
I can sustain 6kts in this boat all day with 130W.

The shape I have provided you is the lowest drag cat hull shape you will get for 15t in each hull, 18m long doing 10kts. It is a good starting point in the design.

The main point of the exercise was to have you consider something more practical than SWATH type hulls.

Take this as a first step in an optimising loop to narrow down what you want. Next step is to set out what you want in the superstructure. Its windage is an important consideration as there is no point in having low drag hulls if it takes a huge amount of power to push a huge bluff superstructure through the air in a good breeze.


Rick W