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Old 03-11-2009, 01:56   #1
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Shape

Which shape?

I was just wondering if anyone had any comments on the difference between the chine constructions of the Schionning against the round bilge of molded hulls.

Would the flat bottom be a benefit to planing if there is any such action involved under sail, in fact I wonder if this action would contribute to drifting (as much or perhaps more than windage) under sail, even at such a small angle as 10-15%.

The flat bottom would be a benefit to planing under power if enough power was present to get the hulls out of the hole so to speak, but this is with a flat aspect in relation the hulls contact with the water.

Also the chines are touted as a contribution to stopping the hulls from burying, but when travelling at any speed the slight lean would be enough to negate this effect, being that the hard chine is made more perpendicular to the water surface that it is being effected by, especially with the bows narrow and shall I say less chined surface.

Perhaps a hybrid highly concave side at the bow, Shuttleworth style but more pronounced, back to a Schionning rear, allowing a more spacious interior and maintaining a narrow hull shape at and below the water line.

Just some thoughts that I am having trouble clearing. Maybe I just need to change the medication I’m on!

Would anyone care to dive in and swim around this subject?
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Old 03-11-2009, 13:20   #2
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Schionning is round bilge (too). At least the one we have here. Very light and potentially fast design.
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Old 03-11-2009, 14:16   #3
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Generally large sailing cats don't plane - they can greatly exceed hull speed without planing.

Schionnings have boats with both round bilges and multi chines. It's more to do with how they are built than anything else.

The multichine boats are made from flat panel kits, the round blige boats are usually strip planked.

Round bilges have slightly less wetted area for the same volume, so are potentially a little faster. Although some designers like Bob Oram use the flat keel panel as the floor, so end up with a lighter boat, which would largely negate the advantage of the round bilge.
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Old 03-11-2009, 15:26   #4
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The answer is simple. Just look at how the pods for high speed trimarans are designed and shaped for optimum low friction high speed performance. Of course, except for the foil design for the recent plus 50 record.
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Old 03-11-2009, 16:40   #5
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Rounded hulls have more buoyancy per amount of wetted surface area, meaning they produce less drag per amount of weight. A flat bottom hull of course takes less energy to get up on a plane. A boat on a plane has less wetted surface area as well as opposed to when it is in full displacement mode. Unless you are going to be reaching planing speeds, a rounded bottom is more efficient.
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Old 03-11-2009, 18:20   #6
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what David M said

chines & flat bottoms are only an advantage if you don't sail in full displacement mode, but have some surfing / planing action going on - think Volvo 70.

Chines do however make one-off boatbuilding considerably cheaper
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Old 03-11-2009, 18:25   #7
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You don't want a sailing boat that planes.
Planeing requires a minimum of 50hp per ton and generally 60hp/ton with a deep v.
Fast displacement on average uses 25hp/ton but the hulls have to be narrow or wedge shaped to achieve proper result.
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Old 04-11-2009, 05:59   #8
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See beau,

This is what you started my mind thinking about with your mates 160 Hp motors! Now I just can’t get the idea out of my head and the whole idea of my cat has been shifted sideways. Now I’m thinking of a bigger cat if I get into strife I can just motor out and hard chines would appear to be better for that. If I’m under sail I don’t care if I lose a bit of speed, but when I want to get up and go, well getting up and going would be good. Mind you, my mind changes with the medication! We’ll see what next week brings perhaps!
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Old 04-11-2009, 14:07   #9
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I have found a huge difference with boat motion at higher speeds.
At 8 knots of speed the sea is in control, after 12 knots the boat is in control.
With a planning boat the hull shape tries to ride up every wave and then crashing down as it rides over it.
With these fast displacement the hulls cut through the waves making for a very comfortable ride.
My plan is to use sails for economical downwind sailing. Use motors when you first go out in Calm weather (no one starts out in 25 knots of wind) use sails if the wind picks up. Then use motors again if the seas increase or to get out of there.
It appears so far in my investigations that to achieve a cruising speed of 15 knots with a suitable fast displacement hull you will need 25hp/ton.
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Old 08-11-2009, 08:00   #10
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The planing vs displacement topic is regularly discussed on the F-boat forum. My F-31 begins to plane at just above 8knts with the main hull and windward float carried unusually high and a slight squat aft. By 9knts everything becomes level with neutral helm and no more change in trim. Speed continues to rise at 100-110% of true wind speed until I chicken out at about 20 or so knts. The boat never pounds and goes over or through waves depending on period and captains mood. I love a sailboat that planes, at 15knts it's smooth,mostly dry and fairly relaxed. Above 15knts you have to be paying attention and wearing foulies. The chines on these boats are rounded but the bottom nearly flat from the daggerboard aft with very slight rocker. It should also be noted that at some point the entire boat is supported by the lee float and the main hull begins to lift. That is why these boats absolutely can not be overloaded because if the float can't support the total weight the entire stability geometry changes to a boat rotating along the axis of the main hull not the float and that is bad. Modern tri's are exhilarating to sail and tremendously forgiving but must not exceed the designers maximum weight. Max weight of an F-31 is 5450 lbs, dry sailaway is about 4000 lbs so you have only 1450 lbs or so for crew and provisions. That is the trade off and the Achillies Heel and the reason so few cruise on tris and so many on cats. Dave
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