| |  What limits your speed? #115925
08/29/07 05:30 PM 08/29/07 05:30 PM |
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Norman,OK gree2056  OP
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Norman,OK | There has been a discussion going on the wave forum about what the limiting factor for speed on these boats.
On my 5.2 sailing solo I always thought the limiting factor was being able to hold the boat down. I have to start spilling air out of the sails long before the lee hull starts to dig in to deep.
But in some posts people have said that their limiting factor is the lee bow diving.
So what is it, does it differ by boat?
Once you go cat you never go back!
Nacra 5.2 (Elsies)#1499, running an inter17 spin!
| | | Re: What limits your speed?
[Re: gree2056]
#115926
08/29/07 06:42 PM 08/29/07 06:42 PM |
Joined: Jun 2006
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Santa Fe, New Mexico, USA WillLints
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Santa Fe, New Mexico, USA | I am no nautical architect but there is always hull speed which clamps the speed at a certain point unless you can get the boat to plane, or if it heals then the hull speed changes, also flying a hull changes the hull speed. You can move your weight to the rear in order to get the nose to come up, just like surfing. Also, raking the mast back will keep the bows up longer.
Will_Lints
one-up, Blade 706, epoxy bottoms
| | | Re: What limits your speed?
[Re: ncik]
#115929
08/30/07 08:54 AM 08/30/07 08:54 AM |
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Albuquerque NM Banzilla
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Albuquerque NM | Raking the mast has little to no effect on nose-diving, upwind or downwind.
Except that for slower boats that can't sail with the apparent wind off the breeze, raking forward lets the sails "breath" and generates slightly more power, which creates a greater pitching moment. This is only relevant for slower boats like one & two sail dinghies which don't generally have to worry about nose-diving anyway because they are fat.
Generally speaking, resistance, power and control will limit your speed.
Why do you need to de-power when your hull starts flying? One of the most commonly given answers to pitchpoling a h16 is to rake the mast aft. Am I missing something here? Thanks Sam
[b] Sail Like you have a Pair
| | | Re: What limits your speed?
[Re: Banzilla]
#115930
08/30/07 09:24 AM 08/30/07 09:24 AM |
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38.912, -95.37 _flatlander_
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38.912, -95.37 | Rake was for the other p word...point
Have reached cavitation on the 18 and this or ventilation (or something?) on the 20 as well and let me tell you IT IS VERY SCARY
John H16, H14
| | | Re: What limits your speed?
[Re: srm]
#115933
08/30/07 11:46 AM 08/30/07 11:46 AM |
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SC zander
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SC | To answer our question, My poor sailing ability limits my speed. <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />
Always borrow money from a pessimist. He won't expect it back.
| | | Re: What limits your speed?
[Re: hobie17guy]
#115935
08/30/07 02:44 PM 08/30/07 02:44 PM |
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South Carolina Jake
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South Carolina | A MEN, Zander,
That and the amount of bulge in my pants. Whoaaaa! Hey, that's too much information.
Jake Kohl
| | | Re: What limits your speed?
[Re: gree2056]
#115937
08/30/07 10:44 PM 08/30/07 10:44 PM |
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Asuncion, Paraguay Luiz
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Asuncion, Paraguay | ...I always thought the limiting factor was being able to hold the boat down. I have to start spilling air out of the sails long before the lee hull starts to dig in to deep. But in some posts people have said that their limiting factor is the lee bow diving.
So what is it, does it differ by boat? I guess you want to know if the limiting factor is the max roll torque or the max pitch torque. In this case, the limiting factor depends on each boat's geometry, mainly the beam size and bow volume. - Large beams mean lots of roll torque resistance - the boat buries the bow before capsizing. - Large bow volume mean lots of pitch torque resistance - the boat capsizes before burying the bow. An ideal design would always reach both limits at the same time, but in practice this is impossible to achieve. The same boat may be capsize prone in certain conditions and pitchpole prone in other conditions.
Luiz
| | | Re: What limits your speed?
[Re: warbird]
#115940
09/02/07 01:26 PM 09/02/07 01:26 PM |
Joined: Jul 2005
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Oxford, UK pdwarren
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Oxford, UK | Hull speed. At some point cats either have to take planing more seriously or start foiling. I prefer the planing option. This has been done to death before, but "Hull Speed" is a bit of a red herring when it comes to cats. The idea of "Hull Speed" is a rule of thumb that applies to hulls that are primarily limited by wave drag. Beach cats are primarily limited by skin drag. So, whilst you can adjust the constant in the "hull speed" formula to something appropriate for the shape of a cat hull, and that will tell you something about the wave drag of the hull, it still doesn't give you a limit on the hull speed because wave drag is not the dominant factor. To answer the original question, I think it depends on the conditions as well as the boat. I haven't sailed many different types of boat, but my experience on F18 and F16 is that in flat water with a steady wind the limit will be righting moment i.e. you won't be able to hold it down. With waves or gusty weather, it'll be bouyancy in the bows that's the limiting factor. Then again, flat waters with very strong, steady winds are hard to find, and normally we're chasing VMG not straight line speed, so this may not be correct. Paul | | | Re: What limits your speed?
[Re: pdwarren]
#115941
09/02/07 03:39 PM 09/02/07 03:39 PM |
Joined: Feb 2004
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Looking for a Job, I got credi... scooby_simon
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Hull Flying, Snow Sliding....
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Looking for a Job, I got credi... | Hull speed. At some point cats either have to take planing more seriously or start foiling. I prefer the planing option. This has been done to death before, but "Hull Speed" is a bit of a red herring when it comes to cats. The idea of "Hull Speed" is a rule of thumb that applies to hulls that are primarily limited by wave drag. Beach cats are primarily limited by skin drag. So, whilst you can adjust the constant in the "hull speed" formula to something appropriate for the shape of a cat hull, and that will tell you something about the wave drag of the hull, it still doesn't give you a limit on the hull speed because wave drag is not the dominant factor. To answer the original question, I think it depends on the conditions as well as the boat. I haven't sailed many different types of boat, but my experience on F18 and F16 is that in flat water with a steady wind the limit will be righting moment i.e. you won't be able to hold it down. With waves or gusty weather, it'll be bouyancy in the bows that's the limiting factor. Then again, flat waters with very strong, steady winds are hard to find, and normally we're chasing VMG not straight line speed, so this may not be correct. Paul Agree with Paul, but there are other isses as well. "rig" drag also comes into the mix. A low drag (but high power) rig will be better assuming other issues remain constant; but in lighter wind, power (fuller sails) does create drag. Aspect ratio of plates is also a consideration, as is angle of attack and lift created.
F16 - GBR 553 - SOLD I also talk sport here | | | Re: What limits your speed?
[Re: ncik]
#115943
09/02/07 06:54 PM 09/02/07 06:54 PM |
Joined: Oct 2006
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Norman,OK gree2056 OP
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Norman,OK | I do not feel that hull speed is a factor in cats, but I am not one to talk,
Once you go cat you never go back!
Nacra 5.2 (Elsies)#1499, running an inter17 spin!
| | | Re: What limits your speed?
[Re: gree2056]
#115944
09/02/07 07:15 PM 09/02/07 07:15 PM |
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Asuncion, Paraguay Luiz
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Asuncion, Paraguay | "Hull speed" (Froude's law) is ok for displacement boats that sail between two waves, unable to pierce or surf the front one. It is irrelevant for beach cats.
Besides stability, the other limitations are: the maximum power made available by the sails and the total drag.
Friction drag (a function of wetted area) is the main responsible for the increase in drag when beach cats sail at high speeds.
Luiz
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