Cat boat design

I have been looking at the possibility of a conceptual design for a cat boat capable of an Atlantic crossing. The idea is sort of motor sailor with a 66hp Westerbeke engine, big fuel tanks and a center board. Reasonable sail power might be 1000sqft on an unstayed mast. As I am not a sailor is their any reason a cat boat would be unsuitable from a sailing point of view.
Sailing equipment like winches and spreaders and all the other things which make sailing and maintanence costly are to be excluded.

 

Excuse me, please, how do you know the engine power or the square feet of sail area without knowing the dimensions of the boat?. I think you are starting the house from the roof.

 

how large of a boat are you talking about?

If the sail is just to assist than keep it simple and smallish so it is easy to handle. Cat rig should be fine, they do not point as high into the wind as a modern sloop rig, but they are efficient and effective.

you also might consider a modified junk rig, they are easy to handle and are self tending for the most part, easy to reef and drop without fouling, and they do not tend to flogging so are easy to control in rough conditions.

 

Yes it does look like that. However I do have ball park figures in mind.

1/ Design has to cross the Atlantic safely the boat sails must be easy to maintain so no auto furling sails I chose the Cat rig with Gaff sail. The faster this can be accomplished the better due to less time in the changeable Atlantic weather.

2/ A very small motor sailor won't have enough fuel on board to get very far across the Atlantic. Lots of fuel is heavy, needs more boat to carry it, means more power, more power costs more for a bigger engine.

3/Chose 66hp Westerbeke. According to Wyman's formula a boat 36ft LWL and displacing 30,000lbs will make 7.2kts on 52hp. 800usg will get you most of the way at 7.2 kts. The boat will need to be about 12 feet on the beam.
I need to calculate if I can get it all in on about 42ft LOA, and decide where to make design compromises, but If the cat boat is the wrong rig to cross the Atlantic then it is back to the drawingboard.

 

Petros My previous post was for Tansl.
There is something quite beautiful about a Junk sail. However one thing I don't understand about its use. If the yards and sail are on the starboard side of the mast and the wind is coming of the port bow. When you go to the opposite tack do you allow the yard to stay on the starboard side of the mast or do you perhaps drop the sail put the hard on the port side and hoist the sail again.

 

Mik, I can see that you already have fairly definite boat. The first post not allowed to know that.
As has been repeated many times in this forum, you should start with a SOR, and generating an overall layout to check first that everything fits on board.
Good luck with your project.

 

Nope.

The mast simply cuts across the sail, when it's on the lee side. It's generally far enough forward that it doesn't seem to hurt the shape of the sail much. The sail is often cut flat anyway.

It may not be the most efficient, but it is certainly convenient.

It is probably the best sail to use for an ocean going boat with just one sail, because it can be reefed while under sail.

 

A cat rig or any other type is not good or bad in itself. A gaff can be really dangerous in rough weather. What do you have against furling sails? They have been proven reliable for many decades.

 

1000 sq feet would be better broken up into three or four sails so they are of manageable size, unless you are standing four people per watch. 500 sq feet is BIG for a cruiser's headsail with furling gear. Mains up to 300 sqft are okay with some form of advanced slab reefing prerigged. That single sail would weigh 170 pounds. Then add half an inch of ice. As an exact opposite of what you are asking, look at a wishbone ketch. 1000 sqfeet split 5 ways makes a very tidy wishbone ketch. My sloop has about 900 sqft total and is intended for a crew of 7 for offshore work. Cat boats are acceptable coastal cruisers but are not generally though of as ocean boats. There are some pretty big structural issues to deal with to make one so. Go beg a ride on a big Nonsuch and see what you think.

 

A free-standing mast for a 100m^2 sail is an interesting engineering challenge. In order to reduce the weight of the mast one needs to increase its diameter.

My proposal: diameter=15% of the chord http://www.boatdesign.net/forums/bo...-wing-sails-cruising-49425-15.html#post703112 post#225

 

The cat rig isn't well suited for off shore work, in spite of some very dedicated sailors, flowing against standard thinking in this regard. Simply put, you want options when off shore and single stick, single sail boats just don't have many. In fact, every off shore sailor I know, has redundancy as a primary requirement. You need to have choices, because things will foul, break or simple become too difficult to use, on some occasions. This isn't the design's fault, just reality in blue water.

All this talk about the difficulty reefing and fore deck work, just seems counter intuitive to me. Most boats can be setup to handle everything from the cockpit or even in a pilothouse if desired. Of course, there's compromises to this approach, but it certainly can and previously has been done, many times. The idea these things doesn't exist for small craft is simple silly. Also the idea you need to be blow decks to be safe is equally silly. Crossing oceans is going to present some bad days and with the speeds of small craft, expect several and storms too. The boat can "dogged down" and left to find her own way, self steer, let drift under warps and poles, etc., lots of tactics to survive, when you're spent. Besides, you're not going to sleep in these conditions, but at least you can get some rest and food.

I have a large yacht with a single 75' mast and true cutter rig. I have no issue reefing in any condition, any of the sails, both electrically or manually. Granted I would want to haul down a 1,300 sq. ft. gennie in SCA conditions, but I wouldn't be flying this sail in 23 knot wind strengths, I'd likely already be down to the 600 sq. ft. baby, with the jib rolled on the headstay and probably a reef in the 800 sq. ft. main. Modern sail handling gear makes this possible. Slab reefing an 800 sq. ft. sail is very difficult by hand, but with tackle could be done, though motorized reel winches pay for themselves, on these occasions. Anyone thinking they can handle these sails without the appropriate setup, is nuts or inexperienced. I do have an emergency tackle (more than one) for just such occasions. They live in a locker and remain there for 99.99% of their lives. I did use one to pick up a swamped dinghy once.

Simply put, if I can handle sails of this size, with a well though out and properly setup for offshore work rig, why can a small boat sailor use similar tactics and arrangements, without resorting to doubling the line length for everything, because they are afraid to go out into the rain. You're just not going to avoid getting good and soaked, with really cold green water, if crossing oceans, so forget about making it unlikely and focus on what can and previously has been done. Simplicity and several options for every task, is the safe and successful path, not a bullet proof, yet slug of a boat with a non-redundant rig and cowering below decks thinking your safe.

 

Thank you for your replies. I think my problem is trying to be a designer I know what I want BUT I have obviously missed some practical requirements and other things real sailors know about. I don't have anything against furling sails.
The concept was to try and design something safe for an ocean going trip without all the expensive stuff which can add up to the cost of a diesel engine. There are motorsailor designs aplenty I am just trying to be original.

 

42' . I want to have 2 masts. Cat ketch/cat schooner. Small engine is fine. Minimal is alright. After all, on my last trip to France, I almost bought an original (1967) Moitessier's Joshua.

 

Motor sailors are sail assisted powerboats or engine equipped sailboats. The hulls can be canted toward one side or the other of the equation, but basically you get a mediocre sailboat and powerboat, doing neither very well. Again, you can design it to be a better sailor, but the powered end of the spectrum suffers and vice a versa.

Mik, these types of boats are usually custom designs. If you want, a stock motorsailer can be selected, but then you're faced with making it suitable for deep water passages. This might include some rigging changes, a level of redundancy to the various systems aboard and other considerations, to make you feel better and the boat more hearty and safer.

A cat ketch/schooner is a good choice with some hoist options and easily handling too. Paying for those spars (assumed free standing carbon) will be painful, but in the long term they'll pay for themselves. Eric Sponburg might have something in his stock plan drawer to interest you.

 

Re: safety and furling

Disclaimer. I'm not a naval architect, my opinion could be a mistake.


NACA 4421 profile 21% thickness http://www.worldofkrauss.com/foils/1693




Small circle - a cylinder (D=0,025) that has the same drag.

I mean the following. The wingsail* in the feathered position has less drag than the mast alone, without the sail. If you have a freely rotating wingsail, you don't need to retract it in case of a hurricane.

* Definition. Wingsail - a sail that can keep its profile at all angles of attack including ~zero angles.