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Effect of weight on boat speed

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RS400atC View Drop Down
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Post Options Post Options   Quote RS400atC Quote  Post ReplyReply Direct Link To This Post Posted: 23 May 19 at 2:51pm
Bethwaite's book has some speed/drag graphs for some hulls ballasted to various all up weights.
If you extrapolate from that, it does not take a huge amount of excess weight to lose that vital half boat length in a one-design drag race. And that is on flat water.
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Post Options Post Options   Quote Do Different Quote  Post ReplyReply Direct Link To This Post Posted: 23 May 19 at 6:55pm
Quite a way off thread I admit but sometimes frequently used quotes raise a smile from me.

 and as Uffa Fox said “the only use for weight is in a steam roller”. 

Or as some call a Flying Fifteen a mobile wind shadow.
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iGRF View Drop Down
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Post Options Post Options   Quote iGRF Quote  Post ReplyReply Direct Link To This Post Posted: 23 May 19 at 9:41pm
We/I found volume which you lot call displacement has a more beneficial effect, the greater the volume the less weight has any effect. Take your Streaker I've noticed with only casual observation that some of them appear to have more volume, especially the plastic ones, notably one that appeared down our lake made by Boatyards at Beer I think, it sits noticeably higher in the water than a wooden one and is quicker. Someone did explain to me what they'd done but I wasn't that interested tbh.

So, helm weight variations in a Laser for instance will have more effect than they would on say a Solution, and I'd guess the Aero wouldn't be as weight sensitive as say the D Zero, the former being a higher volume than the latter to casual observation.

So helm weights in a wooden streaker would be worse than in a plastic one from that Boatyard at Beer outfit, the more so because a streake is underpowered with a small sail.

My other pet rule of thumb sail square mete per ten kilos of helm weight, so a streaker with only six square metres suits a sixty kilo helm, a Solutions 8.5 suits an 85 kilo helm a Laser 7 equals seventy kilo etc. Either side of that will inevitably penalise in the wrong condition. Light helms in strong wind and heavier helms in lighter conditions.

Finally all of this is more acute on fresh water than it is on the sea.
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Sam.Spoons View Drop Down
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Post Options Post Options   Quote Sam.Spoons Quote  Post ReplyReply Direct Link To This Post Posted: 23 May 19 at 10:21pm
But Laser optimum weight is 80kg? However the way Laser measured their sails suggest that a Laser 1 sail is probably nearer 9m2 than the, oft quoted, 7m2 so you may be right. I suspect it's not as simple as that though. My take is that you need to factor in leverage to make the sums add up, and an athletic 70kg would probably handle 8m2 better than a sub-par 90kg.

WRT to displacement, it's what's under the water (and just above the surface) that counts. Windsurfers typically have a volume between negative and 3 x the 'displacement' of the board/rig/sailor combination. A dinghy, when floating to it's marks, will have a volume several times that but may have a displacement of only a few kilos more. Displacement = the mass of the boat and rig plus the sailor (I know you know this GRF). The underwater shape is the key, the BY@B Streaker may have had more rocker so it appears to float higher (it may well have been lighter too which also helps by reducing displacement), better when displacement sailing but a disadvantage when planing. But being lighter (the boat and sailor combined) allows it to plane earlier too. Like boards a flat rocker is faster in a blow but more rocker is better in the light (when 'displacement sailing'), in a drifter a Mistral Superlight was almost unbeatable in Div1 and certainly would have creamed any modern RB in those conditions but as soon as planing was possible they were toast......

And, yes, I agree freshwater vs seawater makes a noticeable difference.


Edited by Sam.Spoons - 23 May 19 at 10:23pm
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Post Options Post Options   Quote tink Quote  Post ReplyReply Direct Link To This Post Posted: 24 May 19 at 5:42am
Originally posted by iGRF

We/I found volume which you lot call displacement has a more beneficial effect, the greater the volume the less weight has any effect. Take your Streaker I've noticed with only casual observation that some of them appear to have more volume, especially the plastic ones, notably one that appeared down our lake made by Boatyards at Beer I think, it sits noticeably higher in the water than a wooden one and is quicker. Someone did explain to me what they'd done but I wasn't that interested tbh.

So, helm weight variations in a Laser for instance will have more effect than they would on say a Solution, and I'd guess the Aero wouldn't be as weight sensitive as say the D Zero, the former being a higher volume than the latter to casual observation.

So helm weights in a wooden streaker would be worse than in a plastic one from that Boatyard at Beer outfit, the more so because a streake is underpowered with a small sail.

My other pet rule of thumb sail square mete per ten kilos of helm weight, so a streaker with only six square metres suits a sixty kilo helm, a Solutions 8.5 suits an 85 kilo helm a Laser 7 equals seventy kilo etc. Either side of that will inevitably penalise in the wrong condition. Light helms in strong wind and heavier helms in lighter conditions.

Finally all of this is more acute on fresh water than it is on the sea.
What I think you are saying is the B@B boats must have fuller hulls underwater and so sit higher for the same displacement of water. For low wetted area obviously a half hemisphere is the optimum and the B@B boat could therefore have less wetted area

I remember when the Aero first came out they made a lot of fuss about the chine shape basically at the chine the hull goes vertical for a few cm so different crew weights have little effect on wetted area. They explained it better

  • The chine sits just below the water line amidships – for several advantages:
  • A 35kg sailor gains approximately the same waterline beam and hence the same hull form stability as a heavier sailor
  • The waterline beam and wetted surface does not change significantly with an increase in helm weight
  • The RS Aero is so light there is less inertial mass to pull against when moving in during a lull or header – compensated for by the flatter hull and increased form stability from  the buoyant chine area
The full list of Aero features is quite a piece, viewing it as a whole it must be one of the most thought out dinghies ever produced 
 

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Post Options Post Options   Quote andymck Quote  Post ReplyReply Direct Link To This Post Posted: 24 May 19 at 6:40am
It’s a Jo Richards design.
They are all like that.
Andy Mck
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Post Options Post Options   Quote tink Quote  Post ReplyReply Direct Link To This Post Posted: 24 May 19 at 6:46am
Originally posted by RS400atC

Bethwaite's book has some speed/drag graphs for some hulls ballasted to various all up weights.
If you extrapolate from that, it does not take a huge amount of excess weight to lose that vital half boat length in a one-design drag race. And that is on flat water.

You have me beat, what page is this graph? Thanks 
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