Have concept for crusing cat - what next?

[rob denney]

Originally Posted by masalai

http://www.boatdesign.net/forums/sho...tch#post195286 and post #73 has some images of X-IT. - - It is about 50' something and has NO rocker about 33' beam and seems to tack quite well and sail efficiently. It is a design by John Hitch...

G'day,

Not sure it tacks without backing the headsail. Nothing wrong with that, it is a cruiser so doesn't have to tack very often. The increase in speed from the better hull shape is probably worth the trade.

A mate of mine owns Wired, (another Hitch 50 that he replaced the wire cross bracing with a full cabin same as X-IT. He has a lot of trouble tacking it, to the point that it is now for sale as it is such hard work.

The first of these was entered in a design competition I ran when I owned System Three. John made a cardboard model which was amazingly stiff in torsion. Quite remarkable.

Bruce,
Probably is a little simplistic. Difficult not to be in a few lines. I think the A cats have transoms to increase displacement aft in heavier airs and give them something to lift out in light air by sitting forward. They also need somewhere to attach the rudders and I suspect the high pressure water coming off the hull helps prevent the rudders ventilating. Could be wrong on all counts, I have never designed an A cat hull and have not sailed one for years.

I'd be interested in a detailed appraisal from you as to why the harry in the video is doing windsped effortlessly and so few crusing cats manage it.

regards,

Rob

[Bruce Woods]

Rob writes....I'd be interested in a detailed appraisal from you as to why the harry in the video is doing wind-sped effortlessly and so few cruising cats manage it.

We're getting a little off topic here. I've tried finding published data on this particular feat but can't locate any. Rob, is there any published data or is it just the verbal report from an excited owner. Was it racing another boat at the time for comparison or was it just another crew swearing on a stack of bibles that we were flying along at 20 knots but when one actually works out the average speed from time and distance travelled its more like half that reported. Gee how often do we read reports like that? Rob Try sailing /racing your boat against something/anything so we can analyze some real data and not just your mantra chant. Then I can give you a detailed appraisal.

[rob denney]

Originally Posted by Bruce Woods

Rob writes....I'd be interested in a detailed appraisal from you as to why the harry in the video is doing wind-sped effortlessly and so few cruising cats manage it.

We're getting a little off topic here. I've tried finding published data on this particular feat but can't locate any. Rob, is there any published data or is it just the verbal report from an excited owner. Was it racing another boat at the time for comparison or was it just another crew swearing on a stack of bibles that we were flying along at 20 knots but when one actually works out the average speed from time and distance travelled its more like half that reported. Gee how often do we read reports like that? Rob Try sailing /racing your boat against something/anything so we can analyze some real data and not just your mantra chant. Then I can give you a detailed appraisal.

G'day,
No owner on board, just the builder, a journo (Mark Giles, read his glowing report in last months Multihull World or drop me a line and I will email you a copy) and his very impressed wife who wants one, which is a pretty typical comment from wives who have seen the boat. Speed was from gps, wind speed estimated from the sea state (they were 3-5 miles offshore) and crew experience.

They weren't racing, it is an over weight cruiser on it's maiden voyage, the crew weren't trying and the sails needed work.

Nothing published, except the video. What more do you want? Make up your own mind about how fast it is going and the wind speed.


regards,

Rob

regards,

Rob

[gardnerpomper]

Rob,

I have been reading your posting a few times to make sure I understand. Let me have a go at rewording it and you can tell me if I have it right.

It seems that the main point, is that if I wanted to use rockerless hulls, which save one construction time (and cost), that I would not be able to tack well because the bow is vertical and the extra underwater surface area in the bow has too much resistance and will slow or stop the boat in irons. You can do this with a proa, basically because it doesn't tack. A shunt does not require the bows to go through the wind, so you never encounter that problem.

I am not as clear on the assumption that a catamaran is inherently heavier that a proa. If I use 2 long hulls with a total weight and displacement equal to the lw and ww proa hulls, and a deck with teh same weight, I would have a cat that weighs the same and whose length is somewhere between the long and short hulls of the proa. Maybe this would be clearer if I had some numbers to see why it doesn't add up. Would you be willing to post the weight and displacement of each hull in one of your designs (like Rare Bird, or the charter proa)?

I can certainly see that argument if it is a traditional cat with a central mast, which requires all the structural support for the rigging and where the accomodations are in the hull, so they get fat and heavy. But if we use unstayed masts, like the charter proa, we should have the same stresses that the proa has, half in each hull, instead of all in one hull. Then if we use your idea of only putting machinery in the hulls, and accomodations on the deck, it would seem that we should be able to do a hull to hull comparison between proas and catamarans.

Hmm... maybe I just thought of your point. Are you saying because there is a mast in the windward hull, that puts more torque on the beams, and they need to be heavier? But that applies to the pacific proa designs too.

What is the issue with the transom? Is that because it shortens the waterline length? It seems like the transom could be quite small, so it would only make a couple feet of difference. I wouldn't see how that would disqualify it. Is there more too it than that?

Thanks very much for providing info. This is very educational for me.

- Gardner

[rob denney]

G'day,

Correct on the tacking.

The weight thing is a bunch of little things rather than one big one, although if you took all the weight saving attributes of a harry and put them in a cat, you would be down to a similar weight. I did this with a 12m cat I designed and built in NZ a few years ago. Weighed 650 kgs, but was neither as fast nor as easy to sail as a harry.

The windward hull on the chater boat is 683 kgs, the ww one is 833. The areas are pretty similar. The ww hull will be lighter again as not all of it is heavily loaded.

Two rigs are almost twice as heavy as one (both have to be strong enough to capsize the boat. A boat which tacks has different loads to one that doesn't, but a proa has to be engineered for loads fromboth directions, just in case. The longer hulls put more stress on the beams.

No real issue with transoms (A class cats use them they must be fast), except that they limit overloading possibilities. There is an argumant that they 'trick' the water into thinking the boat is longer, but as long skinny hulls are not really length dependant, I suspect this may not be an issue.

If you overload a harry, and put the weight in the lee hull, the structure will not notice the difference. If you overload a cat, it will. Consequently, the cat needs higher safety factors in it's design.

Hope this helps, sorry it is a bit disjointed. Bottom line is you could probably get pretty close with a cat designed on harry princicples, but it would not have the proa advantages (weight in the right place, long waterline, shunting, comfort, lower cost), so why bother?

Keep the questions coming.
regards,

Rob