29er GBR 074 Tynemouth |
Laurent Giles 'Jolly Boat' Exeter |
J24 (Sail No. 4239) Dartmouth |
List classes of boat for sale |
Effect of weight on boat speed |
Post Reply | Page <1234 5> |
Author | |||
RS400atC
Really should get out more Joined: 04 Dec 08 Online Status: Offline Posts: 3011 |
Post Options
Quote Reply
Topic: Effect of weight on boat speed 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.
|
|||
Do Different
Really should get out more Joined: 26 Jan 12 Location: North Online Status: Offline Posts: 1312 |
Post Options Quote Reply 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.
|
|||
iGRF
Really should get out more Joined: 07 Mar 11 Location: Hythe Online Status: Offline Posts: 6496 |
Post Options Quote Reply 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. |
|||
Sam.Spoons
Really should get out more Joined: 07 Mar 12 Location: Manchester UK Online Status: Offline Posts: 3398 |
Post Options Quote Reply 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 |
|||
Spice 346 "Flat Broke"
Blaze 671 "supersonic soap dish" |
|||
tink
Really should get out more Joined: 23 Jan 16 Location: North Hants Online Status: Offline Posts: 788 |
Post Options Quote Reply Posted: 24 May 19 at 5:42am | ||
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 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 |
|||
Tink
https://tinkboats.com http://proasail.blogspot.com |
|||
andymck
Far too distracted from work Joined: 15 Dec 06 Location: Stamford Online Status: Offline Posts: 397 |
Post Options Quote Reply Posted: 24 May 19 at 6:40am | ||
It’s a Jo Richards design.
They are all like that. |
|||
Andy Mck
|
|||
tink
Really should get out more Joined: 23 Jan 16 Location: North Hants Online Status: Offline Posts: 788 |
Post Options Quote Reply Posted: 24 May 19 at 6:46am | ||
You have me beat, what page is this graph? Thanks
|
|||
Tink
https://tinkboats.com http://proasail.blogspot.com |
|||
RobertHan
Newbie Joined: 22 Oct 20 Location: USA Online Status: Offline Posts: 1 |
Post Options Quote Reply Posted: 24 Oct 20 at 2:33pm | ||
A boat floats because it displaces more water than it weighs. A heavier boat has to displace more water to remain afloat that a lighter one. The more water that a boat displaces, the further down into the water it is, and therefore the amount of water that it has to move out of its way to move is greater. Water has mass, and it takes effort to move water. That, a larger boat can have a larger engine, which can deliver more torque to the propellers, thereby possibly overcoming the increased effort needed to move it through the water, and thereby allowing it to achieve higher speeds |
|||
Sam.Spoons
Really should get out more Joined: 07 Mar 12 Location: Manchester UK Online Status: Offline Posts: 3398 |
Post Options Quote Reply Posted: 24 Oct 20 at 3:32pm | ||
You may not have noticed that this is a 17 month old thread?
No, a boat floats 'cos it displaces exactly the same mass of water as it weighs.
A simplification but, basically, yes.
Edited by Sam.Spoons - 24 Oct 20 at 3:34pm |
|||
Spice 346 "Flat Broke"
Blaze 671 "supersonic soap dish" |
|||
Guests
Guest Group |
Post Options Quote Reply Posted: 24 Oct 20 at 3:59pm | ||
To be very pedantic, that’s not *why* a boat floats, that’s *where* a boat floats.
|
|||
Post Reply | Page <1234 5> |
Forum Jump | Forum Permissions You cannot post new topics in this forum You cannot reply to topics in this forum You cannot delete your posts in this forum You cannot edit your posts in this forum You cannot create polls in this forum You cannot vote in polls in this forum |