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Old 29-03-2008, 21:07   #166
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Windage

Hi, Mike

No argument there, cats have a lot of freeboard, and putting in a big radius at the deck edge, or putting in a lot of tumblehome int the topsides can have a big effect. I read somewhere, I also can't remember just where, that a 20% radius can give you a huge reduction in windage. If I understood correctly, that would mean that a typical catamaran would benefit hugely from a 1.3' radius at the hull-deck connection.

Here is a Shuttleworth article that discusses windage on a catamaran, and the "lightbulb" shape:
Dogstar 50

His conclusion is that, "Side windage drag should be reduced by 20% in the new hull. This reduces the overall drag and will result in the new design pointing 1.2 degrees higher on each tack at the same speed to windward. Resulting in a 2.2 % increase in Vmg."

For my purposes, an extra foot of deck is worth more than +2.2% Vmg, especially given the greater number of man hours and greater weight likely to result, but yacht design is nothing if not an endless series of trade-offs weighed in terms of each design's project goals.
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Old 30-03-2008, 00:40   #167
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All very nice, but in the real world I would preffer a bit more deck space (like in red) and the ability to get out to or near the gunnel to fend or moor if required.

I have sailed on boats with the huge radius and while they may look pretty it gets very freaky and dangerous if you need to get out there for any reason.

A competant sailor and attention to sailtrim, weight, clean bum etc etc will add much more to the speed than a reduction of windage by a larger than safe radius IMHO

Dave
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Old 30-03-2008, 03:18   #168
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I tend to agree about the problems of the large radius. You don't sail very fast if you slip overboard. About a 10cm diameter is my compromise,
Robert
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Old 30-03-2008, 03:44   #169
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I tend to agree about the problems of the large radius. You don't sail very fast if you slip overboard. About a 10cm diameter is my compromise,
Robert

The CE requires a toe rail with a minimum height of 25 mm, I have designed mine at around 60 mm, these will be made of foam and glassed to the hulls, wide enough to mount the composite stanchions to. The stanchions will be drilled down through the deck as well, so they will have a supported height of around 85 mms.

The tor rail enhances safety, but doesn't do much to lower windage. Another trade-off in design.

Regarding asymmetry in the hulls, Sorry I didn't make it clear, this is only in the area above the Dwl, so this will only come into play when a hull is pressed.

Reagrding added complexity in build, I'm not so sure, as I will be using infusion, without the use of a mould. The foam panels will be CNC cut, and fixed to MDF pieces also CNC cut on a strongback. The foam panels are attached using plastic nails, and glued together. The whole outside is infused in one shot. The hulls turned, and the insides infused in one shot. Then the bulkheads are mounted. This is the "one-off process" as described on the DIAB site. This will save alot of time and money, compared to the usual build on/in a plug. Just the saving in epoxy will be over 300 kgs compared to a very good vacuum bagging.
I think this system will be much faster than even flat panel builds, as they often entail some amount of strip planking and alot of fairing.

Regards

Alan
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Old 30-03-2008, 06:27   #170
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Reagrding added complexity in build, I'm not so sure, as I will be using infusion, without the use of a mould. The foam panels will be CNC cut, and fixed to MDF pieces also CNC cut on a strongback. The foam panels are attached using plastic nails, and glued together. The whole outside is infused in one shot. The hulls turned, and the insides infused in one shot. Then the bulkheads are mounted. This is the "one-off process" as described on the DIAB site. This will save alot of time and money, compared to the usual build on/in a plug. Just the saving in epoxy will be over 300 kgs compared to a very good vacuum bagging.
I think this system will be much faster than even flat panel builds, as they often entail some amount of strip planking and alot of fairing.

Regards

Alan
G'day,

I think there may be a bit more involved than that. The nail holes may leak air, as will any tiny gaps in the glue, plus it is impossible to seal the bag against the foam. Do some sample bits before you do the big one. Also, check out the cost of Diab cutting the foam. It is an expensive process here, may be cheaper if you get the guys cutting the mdf to do it. One more thing to check is how big a lump of foam gets torn out by the nails when you lift it off the moulds.

If the above are not problems, it sounds like a great system, although it does not have the moulded finish that 80-90% of table infused panels have. You will save yourself some fairing if you get rebates cnc cut in the foam panels on the outside where there are any cloth overlaps or build up of laminate.

I disagree with the infusion vs vacuum weight. If you lay up both skins and the core in one hit (no big deal with long out time resins and a well organised crew), then bag the whole thing using bleeder cloth it will be as light as a perfect infusion (no gaps in the foam). If there are any gaps in the foam, the infused part will be heavier as the gaps will fill with resin. This is especially true if segmented core is used on tight curves. The vacuumed core can have the segments filled with bog before laying it on. The infused core will have solid resin in the segments.

regards,

Rob



regards,

Rob
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Old 31-03-2008, 05:41   #171
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I was comparing infused with vacced for polypropylene honeycomb. When you consider the joins between the sheets and any scarfs made to ease bending, the vacced comes out slightly lighter and the scrim on the honeycomb takes up extra resin in infusion. Hot pressed factory made panels using prepreg epoxy over honeycomb sheets looks pretty good for lightness and ease for the flat panels with little bending. Need to check the prices,
Robert
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Old 31-03-2008, 17:55   #172
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With that design you're going to have a massive fairing job no matter how you build it. I'm talking in terms of thousands of manhours. Even if you do a female mould, you still have to make a plug......

Then there's the internal fairing............
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Old 01-04-2008, 12:34   #173
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With that design you're going to have a massive fairing job no matter how you build it. I'm talking in terms of thousands of manhours. Even if you do a female mould, you still have to make a plug......

Then there's the internal fairing............

Which design do you mean?

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Old 01-04-2008, 12:41   #174
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Originally Posted by cat man do View Post
All very nice, but in the real world I would preffer a bit more deck space (like in red) and the ability to get out to or near the gunnel to fend or moor if required.

I have sailed on boats with the huge radius and while they may look pretty it gets very freaky and dangerous if you need to get out there for any reason.

A competant sailor and attention to sailtrim, weight, clean bum etc etc will add much more to the speed than a reduction of windage by a larger than safe radius IMHO

Dave

I agree with most of your comments. Added deckspace might be an issue on smaller boats, but I don't see any advantage in deck area per se. Lounging will be in one of the cockpits or the tramp I reckon.

Another point, is that the black coloured hull shapes in your drawing are not good for fendering, as there is reall no flat surface on the sides, so a fender will either catch on the bottom part, or "ride up".

I have chosen to make my sides practically vertical above the flare, until the relatively large radius inwards.

Regards

Alan
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Old 01-04-2008, 14:07   #175
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Which design do you mean?

Regards

Alan
The design you have drawn, in post 147. With all those compound curves fairing is going to be a big job. That's not a criticism, it's just something to be prepared for if/when you build.

I think the point Dave is making is not so much about deck area, but more about being able to safely get out to the edge of the boat when docking, or for other reasons, especially if it's wet/slippery. The big radius curves will make this more difficult.

Again, not neccessarily a criticism, but something to be aware of. I've spoken to people with a Schionning Cosmos, and it is one thing they said they would change if possible.
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Old 01-04-2008, 15:50   #176
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The design you have drawn, in post 147. With all those compound curves fairing is going to be a big job. That's not a criticism, it's just something to be prepared for if/when you build.

I think the point Dave is making is not so much about deck area, but more about being able to safely get out to the edge of the boat when docking, or for other reasons, especially if it's wet/slippery. The big radius curves will make this more difficult.

Again, not neccessarily a criticism, but something to be aware of. I've spoken to people with a Schionning Cosmos, and it is one thing they said they would change if possible.

Thanks for the feedback.

Fairing will be a big job, that's why I will get it built in a country where labour is relatively cheap.

There will be a 60 mm high toe rail, do you think that, and the fact that the decks will be painted with an agressive non-slip paint will minimise the problem you mention?

When going alongside, I either go nose or rear end first, so someone can jump ashore etc. There the deck curve is not an issue. The midships cleats are outside the toerail.

What do you think?

Regards

alan
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Old 01-04-2008, 16:03   #177
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Another point, is that the black coloured hull shapes in your drawing are not good for fendering, as there is reall no flat surface on the sides, so a fender will either catch on the bottom part, or "ride up".
Everyone thinks that, but you just have to put a bit of extra thought into how you do it.

The advantages to the hull shape are good, extra strength and panel stiffness without extra supporting framing, small waterline footprint yet massive increase in reserve bouyancy when pressed and huge internal volume from the waist up.

The internal beam on my (15m ) hulls is 2400mm at max width, yet 1160 at dwl

On my last cat (similar hullshapes) I found that the slight annoyances with fenders was FAR outweighed by the advantages.

Dave
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Old 02-04-2008, 10:25   #178
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Everyone thinks that, but you just have to put a bit of extra thought into how you do it.

The advantages to the hull shape are good, extra strength and panel stiffness without extra supporting framing, small waterline footprint yet massive increase in reserve bouyancy when pressed and huge internal volume from the waist up.

The internal beam on my (15m ) hulls is 2400mm at max width, yet 1160 at dwl

On my last cat (similar hullshapes) I found that the slight annoyances with fenders was FAR outweighed by the advantages.

Dave
I just measured the same points on my drawing. Hull beam from centreline and outboard is 1220, and more inboard but where to measure??
Hull beam at Dwl is 1060 at the widest point at present, but we might modify this, as I want the prism. coeff. brought up to around 0,62 as several people kindly suggested on this forum.
So we have chosen practically the same dimensions.

I have added a fender list on the outside of the hulls, to even further increase panel stiffness, deflect spray?? and of course for mooring between poles that is the standard way in some places. "leaning" against a pole on the fender list is nice to be able to do in a strong crosswind.

Nice pictures of the new boat build in your gallery. Have you got some more of the inside?

Regards

Alan
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Old 02-04-2008, 15:43   #179
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Thanks for the feedback.

Fairing will be a big job, that's why I will get it built in a country where labour is relatively cheap.

There will be a 60 mm high toe rail, do you think that, and the fact that the decks will be painted with an agressive non-slip paint will minimise the problem you mention?

When going alongside, I either go nose or rear end first, so someone can jump ashore etc. There the deck curve is not an issue. The midships cleats are outside the toerail.

What do you think?

Regards

alan

There's an aerodynamicist's adage: "An inch of frontal area is worth a mile of aerodynamics." By adding a 60mm toerail you are effectively raising your sheer by 60mm as far as windage goes. I'd think that would more than account for any windage saved by having the large radius at the hull/deck join.

Another point - when you are concentrating on doing something with your hands, your peripheral vision helps you keep track of where you are - but you will be seeing decks that look much wider than they effectively are.

Ayway, as I said, the owners of a Schionning Cosmos love the curvy look, but would quickly swap it for a smaller radius if they could.
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Old 02-04-2008, 16:03   #180
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There's an aerodynamicist's adage: "An inch of frontal area is worth a mile of aerodynamics." By adding a 60mm toerail you are effectively raising your sheer by 60mm as far as windage goes. I'd think that would more than account for any windage saved by having the large radius at the hull/deck join.

Another point - when you are concentrating on doing something with your hands, your peripheral vision helps you keep track of where you are - but you will be seeing decks that look much wider than they effectively are.

Ayway, as I said, the owners of a Schionning Cosmos love the curvy look, but would quickly swap it for a smaller radius if they could.

Some very good points! I will look into changing the radius, or maybe incorporating the toerail,into the sides, so the decks will look "sunken". But its a hell of a job to change.......

Thanks

Alan
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