Proa Hulls

Yes. In the limit where you achieve maximum speed in a multihull only one hull will be in the water. So the leemost hull becomes a monuhull with the weight of the windward hull or hulls providing flying ballast.

That is why the proa is such an efficient sailing boat. The two hulls serve distinctly different purposes and can be optimised individually for that purpose.

For a cruising boat you really don't want to be on the scary edge all the time so there will always be some displacement carried by the windward hull.

The attached linesplan is for the cat under the same objectives and constraints apart from the max bwl set at 0.7m. A cat with these hulls requires 560N to achieve 8kts. So the extra beam costs 12% more force.

Rick W

 

Cp of 0.87 seems high... but looks ok on model.
How much will this affect low wind performance? Have low SA/ws of 2-1.... will have to up it to at least 2.5.
On the plus side it should help with pitching - especially with double ender?
Are bow waves a function of cp or flare or both?
If wt is balanced evenly - is it necessary for Cb to be +-55%?

 

For a given waterline length and displacement there is not large variation in wetted surface. At low speed a cat carries a huge penalty in wetted surface compared with a monohull but not much difference between cats. If you watch mixed fleets of small cats and dinghies in ghosting conditions the good cat sailors ride the lee hull with the aim of getting the windward hull almost out of the water.

Long waterline length and some flatness in the ends is effective for pitch damping. Low buoyancy in the ends also reduces pitching. Depends on the ballance between going through waves or over them. Narrow hulls with little buoyancy in the bow easily go through waves. The challenge is to prevent them diving when running down a wave.

From a wave drag perspective it is more insightful to look behind than looking ahead. I have made hulls that have no bow wave but they have high wave resistance. There are viscous drag correction factors that are applied to the form of bows usually related to the included angle but I have not seen evidence that suggests these should be applied to the analysis I have done for the narrow hulls. I have made observation that bow waves result in lower water level along a portion of the hull so they reduce the wetted surface. There are no correction factors for this.

The drag values I have given are for both hulls equally loaded in level trim. With a sailing boat the driving force is coming from well above the line of the drag force. Hence there are heeling and pitching moments that alter the heel and trim. On a small cat the crew weight can be used to set the ideal trim. Just flying a hull and in level trim will usually give the highest speed. Larger cats need to be trimmed bow up at rest to get level trim under sail. The proportion of the displacement carried by each hull changes and you need to consider a few points of sailing with the particular rig to get some idea of how it will perform. The drag values do not include leeway which will depend on sails, keels/boards and rudders.

Rick W

 

Could we have some details of the proas please?

I have found that fore and aft rudders maneuverabilty is far better than with stern mounted, plus you have the ability to crab sideways.

As usual when Richard is bagging something, he chooses some 3rd order item and makes out it is a big deal, while ignoring all the first order advantages.

Boarding a proa is simple. You do it over the beam, via a set of lowerable steps if you are infirm. This is far safer, especially in a seaway, than trying to step sideways out of a dinghy onto the stern of a pitching cat. On the bigger harrys it is even simpler. Lower the dinghy ramp, pull the dinghy up it and step out.

A price point for you, Alex. We have just received an order for a 15m proa complete apart from paint and rig. Price $Aus25,000. Cruising rig and paint another $15,000.

If you want it shorter so you can maneuver in Richard's crowded harbour, the price will drop, along with the comfort, speed and safety.

rob

 

The size, unfortunately, comes from practicality and not desire - who wouldn't want a bigger, more spacious boat?
Why cant i build a cat more like a proa tho'?

 

I am not talking bigger or more spacious, just one longer lower, narrower hull. The other can be much shorter. What size professionally built ready to sail new cat would $Aus40,000 buy you? You can spend your money on a long, simple hull and a short comfortable one, or on a complicated, difficult to build catamaran taking 5-10 times the hours to build with 2-3 times the materials cost and weight and everything strong enough to move it.

The good points of harryproas (not all of these are used on other proas) that could be used on a cat are:
1) Unstayed, balanced rig. Richard doesn't like these for a bunch of 3rd order reasons, but he does admit that they perform much better than stayed rigs for cruisers. They are also easier to use, safer, have very little to maintain or break and can be easily home built, saving you a heap of money. Plus they save a lot of weight and windage in the ends of the boat as you don't need high bows and a forebeam/striker, or a mainsheet traveller, nor all the reinforcing these need.
2) Put the rig in a hull. This is much easier, cheaper and lighter to support and opens up the bridge deck for living space. They are also a lot easier to see as you are not sitting under them trying to look past the mast at the headsail. Cruisers are slowly learning the benefits of biplane cats (we are building masts for 4 of them) and a rig in one hull is almost half the price, with better light air and reaching performance. Shuttleworth has been using a single daggerboard in one hull for a while, but has yet to make the leap to rigs.
3) 360 degree rudder rotation. This not only makes steering in reverse far easier, it makes getting out of irons a doddle. The 2nd most common cause of rudder problems is the boat going backwards and the unbalanced rudder getting slightly off centre and turning hard over. It happens under power in reverse, or worse, when caught in irons in big seas. It either breaks the shaft keyway, the stops or the helmsman's arm. If you can turn them through 180 degrees, it is not a problem.
4) Fore and aft rudders instead of stern mounted. These steer like a bike rather than a car. Large fore and aft rudders (see 6) and you might be able to tack rockerless hulls which are faster, lighter, carry more weight for their length and are much easier to build.
5) The most common rudder problem is impact. Harry's have rudders that kick up on impact (in either direction) and can be lifted clear of the water in shallow water or for inspection and repairs. Also handy for removing weed and plastic bags. They are not mounted through the hull so 2 more potential leaks are removed.
6) Replace the daqggerboards with oversize rudders. Daggerboards are a necessity for storm survival, shallow water sailing and decent progress into the wind. They are like a knife blade waiting to hit something and cut a deep slice in your boat below the water. Even before you hit somnething, the cases are two more heavy potential leaks which need maintaining.
7) Double ended hulls reverse faster and in a storm it is far easier and safer to launch a drogue or parachute off the stern than off the bow. True, you can't step onto them from the marina, but choosing between struggling on the bow in 60 knots once or having to board over the side each time you are in port, is a pretty easy decision for a serious sailor. 3rd order problem: In interesting parts of the world, stern steps are used by crocodiles to climb on board!

All these could be done on a cat and would be big improvements over the status quo.

If you can see the advantages of these, then the advantages of shunting, sheltered cockpits and hulls for different purposes should be an easy logical jump.

rob

 

You could go with a crab claw and get even more simple and inexpensive. I still think a harryproa would look phenomenal with a really well designed CC sail Rob.

 

I'll second that - Rob - have you considered them?
I like double enders but cant wrap the head around shunting.....

 

Few years ago I sailed on the most epensiove crab claw ever built (probably). Laminate sail, carbon/kevlar hull, rcb track on the gunwh'l, carbon spars, everything was as good as it could be and had been sailed for a while so the systems worked. It reached reasonably well, didn't point worth a damn, hadhuge weather helm off the wind and when it got caught aback, fell on the skippers head and nearly knocked him cold. Some of this can be corrected, but not the up wind performance.

Shunting in a sloppy seaway is also scary, with the spars swinging round.

Harryproas sail perfectly well with the coe ahead of or near the clr. The crab claw, with it's coe way up the front, is almost certainly going to be unbalanced.

A well sorted proa is fast enough to sail with the apparent wind ahead of the beam more often than not. To do this, the rig and hulls need to be efficient. That means a decent rig and decent water foils.

I could be wrong, of course, and would love to see a fully developed CC beat a bermudan rig to windward. But I have more than enough on my plate at the moment without this.

As for shunting, try it and you will wonder why anyone puts themselves through the work required to tack and gybe.

rob

 

CC for low tech DIY does have it's appeal tho'. For cruising...... Low aspect combined with efficient hulls?

 

In general, a lousy rig and an efficient hull , or vice versa will perform much closer to the lousy component than the efficient one.

rob

 

Well Rob, your summary above says more about good design - in one simple truism - than most if not all of the supposed wisdom so freely dispensed on the boat design forums I've ever read.

It certainly describes, in a simple way, how there's not really a trade off available when one is willing to give away one key aspect of performance - it's just really being given away.

May I have your permission to adopt it for use when needed - can't afford any royalties though.

Cheers to you Rob,

 

Rob - What would your rig cost for a 30'er?

 

Need a few more details like which rig, height, sail area, boat beam, cruising weight and distance between top and bottom bearings. I can tell you that the materials will be less than a couple of thousand dollars if you build it yourself, plus a vacuum pump and a full length work bench.

Tom, you are welcome to use it. Please do so as often as you can and maybe we can save some people some money/time/effort.

 

+-11m mast with 30m sq of sail - or less. Unstayed - balanced (Ballestron?).
Hull width- 120cm Boat width 5m40... Weight light - 2000kg+