jimi
Well-Known Member
What's the real constraint on speed for a displacement craft (ie non planing)? Is the wetted area immaterial in reality?
Wetted area or length?
- Thread starter jimi
- Start date
ParaHandy
Well-Known Member
my 2p worth would be that the thing can't avoid getting wet as its in something thats .. well .. watery and wet
Sui Generis
Well-Known Member
Do you want a serious answer or are we about to embark on another fishy tail
jimi
Well-Known Member
Serious answer .. Para's got a MAB and he always claims his craft's inferior performance to my AWB on the basis that it has a larger wetted area
capt_courageous
Well-Known Member
jimi
Assumming you are being semi-serious. For a normal type boat with waterline length to beam ratio of about 3 to 1 the main constraint is the square root of the waterline length. This may be called form drag. The wetted area drag is pretty obviously proportional to the wetted area, for max speed it is not as significant as the form drag - but it does matter. If the two boats have the same WLL the one with the biggest wetted area will go slowerish.
Assumming you are being semi-serious. For a normal type boat with waterline length to beam ratio of about 3 to 1 the main constraint is the square root of the waterline length. This may be called form drag. The wetted area drag is pretty obviously proportional to the wetted area, for max speed it is not as significant as the form drag - but it does matter. If the two boats have the same WLL the one with the biggest wetted area will go slowerish.
jimi
Well-Known Member
I suspect in a decent wind (F5+) that the wetted area is irrelevant and the constraint is the waterline length of the boat as it is sailing (not the LWL as it sits in the water when moored)?
bilbobaggins
N/A
Well, it's all to do with drag coefficients, innit?
The more of a boat that's in the water, the more hydrodynamic drag there is to keep it slowed down. So there's benefit in getting much of it out of the water and up into the air.
However, the more of a boat that's up in the air, the more aerodynamic drag there is.....
Some might say that that doesn't matter too much, 'cos water's thicker than air, but it certainly does matter at speeds where racing dinghies and fast sailboards operate. Have a look at that International Moth up on its foils, or the sailboard speed-sailors charging along, and tell me that aerodynamic drag isn't a limiting factor.
Hang-gliders and parascenders don't go very fast, but are limited ( VNE ) in speed by air drag; racing multihull designers like Nigel Irens draw their creations to minimise air drag and some now do extensive tunnel testing to ensure a 'slippery' shape.
So maybe the answer is to have very little boat in the water, and very little out of it - but a great big rig driving it!!
Like a Sydney Harbour skiff........?
/forums/images/graemlins/wink.gif /forums/images/graemlins/wink.gif /forums/images/graemlins/wink.gif /forums/images/graemlins/wink.gif
The more of a boat that's in the water, the more hydrodynamic drag there is to keep it slowed down. So there's benefit in getting much of it out of the water and up into the air.
However, the more of a boat that's up in the air, the more aerodynamic drag there is.....
Some might say that that doesn't matter too much, 'cos water's thicker than air, but it certainly does matter at speeds where racing dinghies and fast sailboards operate. Have a look at that International Moth up on its foils, or the sailboard speed-sailors charging along, and tell me that aerodynamic drag isn't a limiting factor.
Hang-gliders and parascenders don't go very fast, but are limited ( VNE ) in speed by air drag; racing multihull designers like Nigel Irens draw their creations to minimise air drag and some now do extensive tunnel testing to ensure a 'slippery' shape.
So maybe the answer is to have very little boat in the water, and very little out of it - but a great big rig driving it!!
Like a Sydney Harbour skiff........?
/forums/images/graemlins/wink.gif /forums/images/graemlins/wink.gif /forums/images/graemlins/wink.gif /forums/images/graemlins/wink.gif
jimi
Well-Known Member
erm .. read the question .. specifically excluded planing craft ;-)
fireball
Well-Known Member
He was drawing on the knowledge of behaviour in planning craft to (not) answer your question! /forums/images/graemlins/laugh.gif
capt_courageous
Well-Known Member
Jimi
It depends what you mean by irrelevant. For normal sailing I suspect you are correct. If you were 'match' racing the wetted area bit could matter.
It depends what you mean by irrelevant. For normal sailing I suspect you are correct. If you were 'match' racing the wetted area bit could matter.
machurley22
Well-Known Member
A larger wetted area will always be slower than a smaller one, all other things being equal, which they never are. However hull shape has a much larger effect IMO. How could Silkie be so fast if it were otherwise?
Notwithstanding the above, I really can't imagine Para claiming that his craft had inferior performance to yours.
Notwithstanding the above, I really can't imagine Para claiming that his craft had inferior performance to yours.
Sui Generis
Well-Known Member
At slow speed wetted surface area is the most significant drag - due to friction.
Nearing maximum hull speed the boat is creating waves at bow and stern (and in the middle but less significant)and the power required to build those waves is far greater than that required to overcome friction from the wetted surface area, and is proportional to waterline length.
So it's also down to power available - you need more power to go faster, so an undercanvassed boat will be slower at slower speeds, but for the same waterline length the MAB will have as much power as an AWB from about F5 and above.
That's enough seriousness for me tonight. . . /forums/images/graemlins/tongue.gif
Nearing maximum hull speed the boat is creating waves at bow and stern (and in the middle but less significant)and the power required to build those waves is far greater than that required to overcome friction from the wetted surface area, and is proportional to waterline length.
So it's also down to power available - you need more power to go faster, so an undercanvassed boat will be slower at slower speeds, but for the same waterline length the MAB will have as much power as an AWB from about F5 and above.
That's enough seriousness for me tonight. . . /forums/images/graemlins/tongue.gif
Ships_Cat
Well-Known Member
All else being pretty much equal the limiting factor is likely to be the weight of the boat. That because the boat displaces its own weight of water and all that water has to be moved aside as the boat progresses and that takes energy. The heavier the boat the more water that has to be moved aside (or under).
When I say all else being equal, obviously and as you already know, waterline length allows the possibility of a faster boat as do efficiently drawn lines (and that infers fin keeled and skegless rudder), as does a shiny bottom compared to a dirty one, more sail, etc.
John
When I say all else being equal, obviously and as you already know, waterline length allows the possibility of a faster boat as do efficiently drawn lines (and that infers fin keeled and skegless rudder), as does a shiny bottom compared to a dirty one, more sail, etc.
John
KCook
Well-Known Member
Start hanging keels under the bottom and the wetted area goes way up. And speed will drop. The hull form counts for a lot, even at displacement speeds.
Kelly Cook
Kelly Cook
Salty John
Well-Known Member
I think the ultimate hull speed is determined by the waterline length but the acceleration, how quickly the boat gets to hull speed, is determined by wetted surface and hull shape. As well, of course, as how much sail you have up, how well it is set and whether or not you are secretly running your engine.
Twister_Ken
Well-Known Member
I suspect the reason that Passing Wind is slower than Glen Poseur is that Para is a far more considerate driver, who doesn't believe in screaming around, carving up passing racing sailors, fridges, ladyhelmspersons (aka harridans), Fatipa's glasses and entrance channel marker posts.
tcm
...
correction re Passing trade
It might have inferior performance - but that dosen't actually mean it is slower, ahem.
It might have inferior performance - but that dosen't actually mean it is slower, ahem.
jimi
Well-Known Member
Re: correction re Passing trade
Well, on the single occasion you refer to .. you cheated by lightenining the boat by chucking everything moveable overboard... fenders, warps they all went .. whilst I considerately shortened sail and made my guest comfortable!
Well, on the single occasion you refer to .. you cheated by lightenining the boat by chucking everything moveable overboard... fenders, warps they all went .. whilst I considerately shortened sail and made my guest comfortable!
AlienP2
Well-Known Member
We used to race a sillouette* (aplogies for the oxymoron). When running down wind, there was a distinct advantage in getting the crew to move right up into the bows. This lifted the fat stern out of the water and reduced wetted area.
* 17ft day sailor - distinctly non-planing.
* 17ft day sailor - distinctly non-planing.
jimi
Well-Known Member
When racing the Laser in light winds when running I kited and knelt well forward on the opposite side to the boom ..reducing the wetted area makes an enormous difference in LIGHT winds ..
