Trying to design my own cat.

I don't want to bore all you pro boat designers....so I'll keep this short and sweet. When I get some better boat design software I will be able to refine my idea....without asking pesky questions...but for now, just 2 basic questions, if you have the time.

1. In extremely light winds and with a small sail area....which of these 2 hull types are better for speed (for a catamaran)....a double ended 'canoe' hull with rounded bottom, or the typical wider stern hull (with rounded bottom for minimal wetted sorface)? The boat will weigh 3500 pounds (including 8 passengers and camping gear). With the canoe hulls the draft comes to 11 inches at this weight.

2. If my hulls are too close together, will this create a problem with water flow between the hulls? (Please see my attachment to see the exact proportions of my boat....I have very good reasons for making the hulls this close together).

Again, thanks to anyone who is able to answer these 2 questions (crucial to my design).

- Richard

 

Richard
I believe a cat made for light airs would be best with a canoe type hull. I have done some analysis of a cat with your constraints optimised for 8kts.

I have not considered any heal so there is an assumption that you have set crew weight to keep the hulls equally loaded and in perfect trim. The hull to your constraints could achieve 8kts with a 6HP outboard. A fractional rig with about a 30ft mast would achieve this in light winds, 15 to 18kph, on a reach.

I also looked at what would be possible if you allowed the beam of the hulls to be optimised. In this case the required power to do 8kts could be delivered with a 5HP outboard. So about 20% less. The hulls come down to 20" beam.

I then looked at a no compromise displacement cat designed for 8kts and 3500lb displacement. It ends up 13.4m long and an overall beam of 3.5m (only 11.5ft). It can do 8kts with a 3.5HP outboard. Just over half the power required to drive your 28ft hulls.

I have attached some basic lineplans for the three hulls and a chart comparing the drag.

Once you start to power up then you are no longer operating in displacement mode. In this case you start to get an advantage with a wide, flat aft section that gives lift rather than squatting. However 8kts is an impressive speed for a 28ft boat designed for light air performance.

The spacing of the hulls does have some bearing on performance but it is slight and the optimum changes depending on speed. Other factors like space and sail handling ability are more important considerations than reducing drag by spacing the hulls optimally.

Rick W.

 

What a great reply! Thanks for your time Rick. That was very generous of you. You've given me a few options to think about now.

Thanks again
- Richard.

 

I think you may be a bit optimistic with the speed predictions.

But first, in very light winds, and, presumably, flat water, many people think a canoe stern is better - no transom drag.

Most performance predictions do not make enough allowance for air drag. It's obviously significant as anyone who has tried to motor an underpowered catamaran to windward knows.

Also most don't adequately allow for the wave interactions between the hulls. Many years, if not decades, ago, as a yacht design student at Southampton I ran some model tests in the big test tank there. I found that the total hull drag from a catamaran with a conventional hull spacing was about 20% more than that of two hulls at an infiite spacing.

Your design seems to be very narrow for a modern sailing catamaran.

I doubt if you will get more than 6 knots from an 8hp outboard, more likely 5.5, I'd also expect you to sail at about 1/2 wind speed unles you have a very big rig.

Hope this helps

Richard Woods

www.sailingcatamarans.com

 

Richard
I agree with your comments that hull interactions on cats are important. I do not agree that the numbers are optimistic for calm conditions. In fact I made considerable allowance for the poor efficiency of an outboard propeller.

The designs offered came straight out of Michlet/Godzilla. It takes hull interaction into account and is very reliable for displacement mode. I have compared performance with numerous boats so have a lot of confidence in the results.

I am accutely aware of windage as I make human powered boats for my puny 120W engine and have a lot of performance comparisons under windy conditions. The windage in calm conditions would be small and is taken up in efficiency allowance.

This clip shows a younger test pilot doing 12kph at a relaxed cadence on a Godzilla generated monohull:
http://www.youtube.com/watch?v=jEadJFg2F9s
With my 120W I can hold 11kph all day on this boat. It was a product of Godzilla with minor variations for development using aluminium sheet. I am now doing a version true to Godzilla from carbon firbe - see attached.

All HP numbers on the cats were based on calm conditions and smooth hulls. So no waves and still air. It is worth noting though that the long slender hulls having little reserve buoyancy in the ends are not much affected by waves.

I also did not discuss the practicality of a long cat with a 11.5ft overall beam. It might not be as stable as you would like. But then it does not need much sail and was intended for good performance in light air. The idea was to show what compromises are made by choosing various options.

I have attached the the Michlet output file for the no compromise cat. You will see the power at the hull to do 4.35m/s (8kts) is only 1.8kW. So should be possible to deliver this using a 3.5HP motor.

Rick W.

 

A very late thank you to you Richard Woods! I appreciate all opinions from people with experience. I don't expect my boat to be quick. I just don't want to end up making a really stupid design. I have a lot of very strict requirements.

 

I have been doing a lot of internet research about catamaran design and have finally come up with the crude design of my boat.
It meets all my requirements and should be reasonably efficient.
The canoe hulls are now narrower than my original sketches, with rounded bottoms and shallow shoal keels which begin quite aft and join with deep kick-up rudders.
I am doing without the dagger boards.
The center of sail area is slightly more aft than what is typical.
The hulls are devoted only to storage, as all sleeping is done on deck with the use of a very low profile tent system.
I have come up with a radical bridgdeck construction and deck layout which I think is so unique for solving wave pounding issues and allowing extreme passenger comfort, that I have decided not to show any images yet because I am considering the possibility of commercialising my ideas. (won't be any time soon!)

I have reached the stage now where I really need to test the center of sail area / weather helm / lateral resistance. I also want to get some idea as to how it will handle a choppy sea.

So what's my point?

I just want to know what is a really good program to use for simulating the boat's performance. I imagine a 'perfect' wave simulator might be a bit optimistic. I assume Godzilla is good but does it deal with sails or just hulls? Any recommendations??

Cheers
- Richard

 

Simulating a sailboat's performance is tricky.

The Michlet software mentioned by Rick W. is free, courtesy of Leo Lazauskas. Godzilla is a set of algorithms that uses Michlet to evaluate the fitness of a hull for a given set of criteria, then "breeds" new "offspring" from the best ones in the same way as animals that are well adapted to their environment will breed most successfully. Michlet calculates hull resistance in considerable detail but is not a VPP.

To predict a boat's performance by computer requires a Velocity Prediction Program (VPP). These algorithms account for the balance between the forces on hull, appendages and sails to estimate the boat's performance in given wind conditions. They are, in general, complex and require considerable experience to use, not to mention expensive.

 

Okay thanks Matt. It sounds like someone should make a 'dumbed down' program for people like me. Like....just enter all the data for my boat design kind of like filling out a large questionare form.....and then watch the computer spit out a beautiful rendering with all the perfomance predictions.
From there I could stretch and bend the rendered image using the simple drop and drag method and let the computer tidy up my alterations by calculating the best compromise between what I have drawn and how I want it to perform. You see....it should be that simple!! hahaha
Any computer programmers out there up to the challenge?

Anyway, I've had some fun with simple hull design programs and I think it will be well worth my while now to give my idea to a pro boat designer to refine it.

that'll be the next stage.

 

I have a good VPP for propeller driven displacement boat in calm conditions. How is that for qualifiers?

I generate the Michlet data for the specific hull, load this into a spreadsheet and it provides a 3rd or 4th order regression of the michlet data to produce a simple drag formula. I generate the thrust data from a semi-analytical prop thrust integration and use a goal-seek in excel to ballance thrust and drag. It is very simple to use and not as complex as it sounds. Gives results within about 5% for powered displacement hulls.

It is much more complex for a sailing boat where things like heal, trim and sail performance has to be taken into account.

Rick W.

 

Richard, you mention 8 persons and camping gear... have you taken this weight in consideration wrt the slim hulls... knowing this bunch of hooligans I fish with and the stuff they take with it mounts to quite a bit of junk I mean weight...

 

Hi Rick
I intend to build this cat (I mean get someone else to build it) in a couple of years from now, when I move to LA. I don't have the money right now to get it designed properly cos I have other commitments. I'm just letting the idea develop like a fine cheese....

My main concern is that the boat sails well in light air so that the motor is only the very last resort. I don't want to use the motor unless the boat is virtually stopped. As for windy conditions, I don't care at all about top speed.....so this is why I'm focusing on light air sailing.

My bridgedeck structural supports, mast, and bedding on deck are so integrated with each other that I can't actually move the rig forward or aft, unless I move EVERYTHING (to get the boat balanced). So I'm thinking the hulls would have to be altered while I am altering the rig and deck to find the best compromise....all of which I will do in the not too distant future.

For the record, if I knew anyone who wanted an efficient boat which is primarily motor driven, you would be the first person I'd recommend because I was impressed with the way you write and kept everything concise and relevant to my requirements. Maybe I'm just easily impressed
Hang on.....I'm asuming you are not fimiliar with designing sail rigs?? Have I got it all wrong?

Well whatever the case, I'll just put the boat on hold for now. That is, unless you want to design the whole thing for me for free....(kidding)

regards
- Richard

 

Fanie....yeah you're right, i've only allowed about 10kg of gear for each slim person...lol. But that's fine by me. If I overload it a little from time to time (in calm conditions) it should be alright cos the canoe shaped hulls can sink a bit without transom drag.
it's probably more of a 6 person boat.

 

Richard
I guess over the years I have found sailing ocean worthy yachts to be too demanding on my time and money supply. It is hard to get value for money unless you live close to the boat or on it. My best period of ownership was when I had easement access to Lake Macquarie in NSW and my yacht on a sheltered swinging mooring a short wade/swim from shore.

My current objective is to build a seaworthy electric powered boat fueled from sun and wind. The aim will be to do extended coastal cruising hopping from point to point within the range of fully charged batteries topped up by sun and wind during the run. Cruising speed is just around 7.5kts with top speed of 10.5kts. However it has spartan accommodation. On the other hand I am aiming to make it trailerable. (I am an electrical engineer, boating is my hobby)

From a practical sense it is hard to order the perfect strength wind. If you want to get somewhere in a given time then you inevitably end up motoring. It can be very disappointing to lose a day of sailing drifting backwards with the current on a balmy night. It takes a lot of concentration to get a boat to ghost along at night particularly if there is a bit of slop around.

If you have not got FreeShip yet then it is a good tool to start to put ideas into concrete form. It will make it easier to talk to a designer when you get to that point. You can also elicit a bit of debate about the design from this forum.

Rick W.

 

Rick, your electric / wind powered boat... you just gotta see 'Waterworld'. Since the poles are going to melt (must be real soon) you could probably adapt some of the techniques yourself. I especially liked the way you recycle water into drinking water again (wonder if the same bit of water doesn't become tired ) A trimaran though, but a motorised sailer just the same... even a wind power generator too. Are we behind or what...