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: 28 Jun 25 at 12:15pm
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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
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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?
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Replies:
Posted By: andymck
Date Posted: 22 May 19 at 8:35am
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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. ------------- Andy Mck |
Posted By: davidyacht
Date Posted: 22 May 19 at 8:51am
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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. ------------- Happily living in the past |
Posted By: ifoxwell
Date Posted: 22 May 19 at 8:53am
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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 ------------- RS300 |
Posted By: Sam.Spoons
Date Posted: 22 May 19 at 9:11am
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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. ------------- Spice 346 "Flat Broke" Blaze 671 "supersonic soap dish" |
Posted By: H2
Date Posted: 22 May 19 at 10:24am
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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? ------------- H2 #115 (sold) H2 145 OK 2082 |
Posted By: H2
Date Posted: 22 May 19 at 10:25am
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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! ------------- H2 #115 (sold) H2 145 OK 2082 |
Posted By: Sam.Spoons
Date Posted: 22 May 19 at 1:50pm
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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. ------------- Spice 346 "Flat Broke" Blaze 671 "supersonic soap dish" |
Posted By: tink
Date Posted: 22 May 19 at 6:20pm
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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. ------------- Tink https://tinkboats.com http://proasail.blogspot.com |
Posted By: tink
Date Posted: 22 May 19 at 8:08pm
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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 ------------- Tink https://tinkboats.com http://proasail.blogspot.com |
Posted By: RS400atC
Date Posted: 23 May 19 at 2:51pm
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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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Posted By: Do Different
Date Posted: 23 May 19 at 6:55pm
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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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Posted By: iGRF
Date Posted: 23 May 19 at 9:41pm
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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. ------------- 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
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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.
------------- Spice 346 "Flat Broke" Blaze 671 "supersonic soap dish" |
Posted By: tink
Date 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 |
Posted By: andymck
Date Posted: 24 May 19 at 6:40am
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It’s a Jo Richards design.
They are all like that. ------------- Andy Mck |
Posted By: tink
Date 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 |
Posted By: RobertHan
Date Posted: 24 Oct 20 at 2:33pm
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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 |
Posted By: Sam.Spoons
Date 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.
------------- Spice 346 "Flat Broke" Blaze 671 "supersonic soap dish" |
Posted By: Guests
Date Posted: 24 Oct 20 at 3:59pm
| To be very pedantic, that’s not *why* a boat floats, that’s *where* a boat floats. |
Posted By: Sam.Spoons
Date Posted: 24 Oct 20 at 4:35pm
 Yup.... 'precisely' ------------- Spice 346 "Flat Broke" Blaze 671 "supersonic soap dish" |
Posted By: NickA
Date Posted: 28 Oct 20 at 10:06pm
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There are two kinds of 3000 class dinghy hulls, one weighs about 79kg and the other about 55kg. The light version is faster, but mostly because is requires less wind to get planing. When it's windy enough for both to plane the weight doesn't matter so much and they're about the same speed. In very light wind the difference is also small .. I guess, because it's hull length rather than displacement that's speed limiting. In marginal planing conditions a lighter boat will be surprisingly faster... Otherwise not to much, though acceleration in gusts is better. ------------- Javelin 558 Contender 2574 |
Posted By: Sam.Spoons
Date Posted: 28 Oct 20 at 11:01pm
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Yes, that's my feeling too, sub-planing and waterline length is king, when everybody's planing hull form is the decider (and irrelevant in a strict one design as they are all the same shape) it's only in marginal planing where hull weight makes a significant difference*. Of course that is slightly frustrating for me sailing a boat who's whole raison d'ętre is to plane early and fast, with an old and moderately overweight hull. Equally eating a few less pies (and drinking less beer, but that would be a step too far) could easily compensate for the 5kg difference...
* it was much the same when I was racing Raceboards except the benefits of early planing were magnified tenfold compared to dinghies, and the benefits of abstinence were similarly magnified on those marginal days...
------------- Spice 346 "Flat Broke" Blaze 671 "supersonic soap dish" |
Posted By: whitsundays
Date Posted: 05 Nov 20 at 9:40am
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A boat sails since it displaces more water than it weighs. More weight consistently implies more inertia, so the heavy boat needs more capacity to get to a similar speed. On the off chance that you analyse two in any case comparable boats, the heavier one will sit lower in the water, making more frictional and wave obstruction also. ------------- Whitsundays Yacht Charters |
Posted By: Sam.Spoons
Date Posted: 05 Nov 20 at 1:42pm
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Somebody said this elsewhere and it is incorrect, at least insomuch as a boat floats because it displaces exactly the same weight of water as it weighs (a boat's weight is often termed it's 'displacement').
------------- Spice 346 "Flat Broke" Blaze 671 "supersonic soap dish" |
Posted By: Guests
Date Posted: 05 Nov 20 at 2:21pm
| Yes indeed. And, as mentioned before, that is not an explanation of *why* a boat floats. It floats because there is hydrostatic pressure acting on the wetted surface area of the hull, and the resulting force is equal and opposite to the weight of the boat. |
Posted By: Sam.Spoons
Date Posted: 05 Nov 20 at 2:39pm
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Perhaps I should have said "when a boat floats it displaces exactly the same weight of water as it weighs"? ------------- Spice 346 "Flat Broke" Blaze 671 "supersonic soap dish" |
Posted By: 423zero
Date Posted: 05 Nov 20 at 3:00pm
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Plus it's less dense due to air space in Hull. ------------- Robert |
Posted By: Sam.Spoons
Date Posted: 05 Nov 20 at 5:58pm
Windsurfers float and most don't have air space inside  ------------- Spice 346 "Flat Broke" Blaze 671 "supersonic soap dish" |
Posted By: 423zero
Date Posted: 05 Nov 20 at 6:05pm
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They are full of foam. ------------- Robert |
Posted By: 423zero
Date Posted: 05 Nov 20 at 6:08pm
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https://www.britannica.com/video/181395/Discussion-forces-bodies-water#:~:text=When%20an%20object%20enters%20water,This%20is%20called%20displacement. ------------- Robert |
Posted By: epicfail
Date Posted: 05 Nov 20 at 6:21pm
| Maybe slightly off topic but.. We get some double handers doing handicap racing being sailed singlehanded - normally in light to moderate conditions. Assuming that the PY is based on these classes being sailed by two people - removing the weight of an average adult must have a significant impact on boat speed. |
Posted By: Sam.Spoons
Date Posted: 05 Nov 20 at 6:23pm
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The second paragraph is wrong for starters "The object pushes out a volume of water that is equal to its own volume." that is only true for an object that sinks. For an object to float it must displace less than it's volume of water. They go on to qualify but it's all a bit 'lies to children'...
------------- Spice 346 "Flat Broke" Blaze 671 "supersonic soap dish" |
Posted By: Sam.Spoons
Date Posted: 05 Nov 20 at 6:28pm
In the light it certainly does but so when the wind picks up the lighter 'crew' is at a disadvantage. I suspect that at clubs where such practice is common they will have a lower PN to apply when the helm is sailing alone. OTOH would it be fair to give a couple of 50kg teens the straight PN handicap while penalising a 100kg adult for sailing alone? ------------- Spice 346 "Flat Broke" Blaze 671 "supersonic soap dish" |
Posted By: 423zero
Date Posted: 05 Nov 20 at 6:33pm
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https://en.m.wikipedia.org/wiki/Displacement_(fluid) ------------- Robert |
Posted By: 423zero
Date Posted: 05 Nov 20 at 6:42pm
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https://en.m.wikipedia.org/wiki/Buoyancy ------------- Robert |
Posted By: Sam.Spoons
Date Posted: 05 Nov 20 at 6:48pm
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Yup, that's pretty much what I was saying. ------------- Spice 346 "Flat Broke" Blaze 671 "supersonic soap dish" |
Posted By: 423zero
Date Posted: 05 Nov 20 at 6:49pm
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More specific to why ships float
https://letstalkscience.ca/educational-resources/stem-in-context/why-do-ships-float ------------- Robert |
Posted By: Sam.Spoons
Date Posted: 05 Nov 20 at 6:58pm
That's wrong too..... It says How does buoyancy relate to density?If a block of wood measuring one cubic centimeter (1 cm x 1 cm x 1 cm) is placed in a container of water, the amount of water displaced will equal the weight of the block of wood. But what about if a block of the same size is made of lead? Lead has a much higher density than wood. If a one cubic centimeter block of lead is placed in a container of water, the amount of water displaced will equal the weight of the block of lead. The amount of water displaced by the block of lead (weighing 11.34g) will be exactly 1cc and will weigh 1g, so it will equal the volume of the block of lead but not it's weight ------------- Spice 346 "Flat Broke" Blaze 671 "supersonic soap dish" |
Posted By: 423zero
Date Posted: 05 Nov 20 at 7:05pm
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I think they mean, the amount of air (buoyancy) to weight of object, to make it float, ie you will need more air (buoyancy) for lead to float than wood. ------------- Robert |
Posted By: Guests
Date Posted: 05 Nov 20 at 7:22pm
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Good grief. How can a STEM education site get it so wrong? It’s very simple, the boat will sink until the weight of the boat divided by its submerged volume equals the density of the water it is sat in.
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Posted By: 423zero
Date Posted: 05 Nov 20 at 7:31pm
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https://courses.lumenlearning.com/physics/chapter/11-7-archimedes-principle/
Archemedes 'eurica' moment ------------- Robert |
Posted By: Rupert
Date Posted: 06 Nov 20 at 8:07am
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When Brunel built the Great Britain, people said she would sink because iron doesn't float. I had hoped we had a better understanding nowadays, but reading this thread I'm not sure we do, entirely.
It's also fairly clear that movable ballast has a different effect on boat speed than fixed, or Phantom sailors and water ballast wouldn't exist. ------------- Firefly 2324, Puffin 229, Minisail 3446 Mirror 70686 |
Posted By: Sam.Spoons
Date Posted: 06 Nov 20 at 10:14am
It is indeed very simple. A floating object displaces it's weight of water a sunk object displaces it's volume of water. for an object to float it's volume must be greater than that of the water it displaces. Density is just a measure of how much an object weighs relative to it's volume. Wood floats 'cos 1 cc of it weighs 0.7g so it only has to displace 0.7cc of fresh water. Lead sinks 'cos 1cc of it weighs 11+g but can only displace 1cc of water. Ice floats 'cos 1cc of it weighs around 0.9g (or put another way 1g of it takes up slightly more volume than 1g of water, that's why your pipes burst when they freeze). ------------- Spice 346 "Flat Broke" Blaze 671 "supersonic soap dish" |
Posted By: maxibuddah
Date Posted: 06 Nov 20 at 11:06am
Phantom sailors only exist to ensure Ginsters remains in business. Moveable ballast is to improve the righting movement against the rig though isn't it? Simple levers surely? ------------- Everything I say is my opinion, honest |
Posted By: NickA
Date Posted: 16 Nov 20 at 9:00pm
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I mostly sail my super light 3000 single handed. Not just the reduced weight that gets it planing early, also the fact that the crew weight is all on a trapeze. Low weight but lots of righting moment.
Hence why my club has me sailing a boat designed at a Py of 1032 (and currently languishing at 1064) off 978. On the other hand .. F5 or more and I'm screwed. ------------- Javelin 558 Contender 2574 |