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Effect of weight on boat speed

Printed From: Yachts and Yachting Online
Category: Dinghy classes
Forum Name: Dinghy development
Forum Discription: The latest moves in the dinghy market
URL: http://www.yachtsandyachting.com/forum/forum_posts.asp?TID=13334
Printed Date: 09 Dec 19 at 3:58pm
Software Version: Web Wiz Forums 9.665y - http://www.webwizforums.com


Topic: Effect of weight on boat speed
Posted By: stonefish
Subject: Effect of weight on boat speed
Date Posted: 22 May 19 at 7:56am
Does anyone know how boat weight effects speed on a planing dinghy for example a Comet, Solo, Streaker etc?

For example how much faster would a boat that weighs 95kg go compared to a boat weighing 100kg if all other variables like helm weight etc are the same between the boats?



Replies:
Posted By: andymck
Date Posted: 22 May 19 at 8:35am
There are many more variables than just boat weight.
A lighter helm boat combination will plane earlier and accelerate faster.
Depending on where the weight is in the boat, there can be other affects. We all talk about trying to keep the weight out of the ends of the boat to reduce pitching moments.
The bottom line is 5kg will not make as big a difference as skill level for most of us. But an equal skilled sailor in a lighter newer boat will be quicker, all other variables being equal.



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Andy Mck


Posted By: davidyacht
Date Posted: 22 May 19 at 8:51am
Newtons Laws apply and as Uffa Fox said “the only use for weight is in a steam roller”.  That being said, any difference will be most apparent when “like for like” racing, and will manifest itself as a lack of acceleration compared with similar but lighter boats, and possibly lateness to get on the plane.  Obviously it is the all up weight that also counts ... so the combined weight of helm and boat has to be taken into account.  If I were looking to buy a boat, I would avoid one that is overweight.

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Happily living in the past


Posted By: ifoxwell
Date Posted: 22 May 19 at 8:53am
I agree. In most conditions, all other things being equal, the effect would be negligible... but on a marginal planning day the ability to push past your bow wave earlier could make a big difference in a one design fleet

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N12 3441


Posted By: Sam.Spoons
Date Posted: 22 May 19 at 9:11am
A 5% difference in hull weight won't make 5% difference in speed over the wind range, in fact I'd suggest it would be completely unnoticeable in most conditions. But if I was buying a new boat I would expect it to be bang on minimum weight. A used boat OTOH there are so many other factors that 5kg would not put me off buying an otherwise good boat.

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Spice 346 "Flat Broke"
Blaze 671 "supersonic soap dish"


Posted By: H2
Date Posted: 22 May 19 at 10:24am
Focus so far has been on acceleration but you also need to factor in how a boat responds in the lulls. Many of the older designs which are heavier also are able to maintain some momentum in lulls which means they can ghost across the calm patch and get into the new breeze and they also remain able to steer longer. The lighter boats on our pond accelerate faster but also slow down faster!! Hope that makes sense?

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H2 #115


Posted By: H2
Date Posted: 22 May 19 at 10:25am
Related to this is how heavier boats deal with waves on a light wind day. Try catching a K1 upwind on the sea in a light breeze if you need convincing! 

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H2 #115


Posted By: Sam.Spoons
Date Posted: 22 May 19 at 1:50pm
And usually heavier classes of boat (older boats) have a more displacement biased hull form which means they carry way better in the lulls anyway.

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Spice 346 "Flat Broke"
Blaze 671 "supersonic soap dish"


Posted By: tink
Date Posted: 22 May 19 at 6:20pm
I know the post is about boat with but bare with me, I bought a Streaker because with its ideal weight is 73kg (based on class recommendations and ties in with the average to ten helms weight for a number of years at the nationals) so I would keep my weight down and healthy. My boat is 20 years old, all wood, been repaired a few times and I suspect heavily overweight. For most of my time I was at the top end of the weight range but after a few years of getting to know the boat frequently I would be at the windward mark with the lighter, more experienced helms in much new boats and better rags. When we went down wind however despite being right alongside hot shots and in clean air they would get on the plane and just get away from me. I would copy exactly what they where doing but still couldn’t get it on the plane as quickly as they did and it made a big difference. 

I have recently moved clubs and can keep up most of the time with much newer all plastic boats especially when it is blowing. I am still learning wind patterns etc of the new club but feel I doing OK 

So I would say upwind probably no difference it as all about technique 
Off wind it makes a difference especially in conditions when others can get on the plane and you can’t

I am doing nationals in a few weeks and I am unable to replace my boat at the moment I have been on a weight lost plan 12kg since January and the hull and foils are being polished within an inch of there lives. Hopefully the boats weight will only have a small percentage of detrimental performance as some above are suggesting.


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Tink
https://tinkboats.wordpress.com/

http://proasail.blogspot.com


Posted By: tink
Date Posted: 22 May 19 at 8:08pm
Lots to read here

https://strathprints.strath.ac.uk/59980/1/Day_OE_2017_Performance_prediction_for_sailing_dinghies.pdf" rel="nofollow - https://strathprints.strath.ac.uk/59980/1/Day_OE_2017_Performance_prediction_for_sailing_dinghies.pdf

But graph on page 59 though crew weight shows how insignificant effect though the preamble says ‘The effect of crew weight is studied by comparing time deltas for crews of different physique relative to a baseline 80kg sailor. Results show relatively high sensitivity of the performance around a race course to the weight of the crew, with a 10kg change contributing to time deltas of more than 60 seconds relative to the baseline sailor over a race of one hour duration at the extremes of the wind speed range examined.’

This therefore by extrapolation hull weight is, however I think this must only be displacement mode




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Tink
https://tinkboats.wordpress.com/

http://proasail.blogspot.com


Posted By: RS400atC
Date 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.


Posted By: Do Different
Date 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.


Posted By: iGRF
Date 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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https://www.corekite.co.uk/snow-accessories-11-c.asp" rel="nofollow - Snow Equipment Deals      https://www.corekite.co.uk" rel="nofollow - New Core Kite website


Posted By: Sam.Spoons
Date 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.


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Spice 346 "Flat Broke"
Blaze 671 "supersonic soap dish"


Posted By: tink
Date 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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Tink
https://tinkboats.wordpress.com/

http://proasail.blogspot.com


Posted By: andymck
Date Posted: 24 May 19 at 6:40am
It’s a Jo Richards design.
They are all like that.

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Andy Mck


Posted By: tink
Date 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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Tink
https://tinkboats.wordpress.com/

http://proasail.blogspot.com



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