jamie N
Well-known member
Just a thought, the max hull speed of 'A' boat is 6kts. This is a light weight, 24', 1/4 tonner. Is there a calculation to establish the wind speed at which the boat will reach Vmax at, given flat water?
Hull speed?
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Chiara’s slave
Well-known member
That’s rig and trim dependent, so not really.
dunedin
Well-known member
…. and presumably depends on wind angle (ie a polar diagram)
… and a light weight 24 footer doesn’t really have a V Max at 6knots, as presumably enough sail downwind will push quite a bit faster (just before something breaks or broaches!)
B27
Well-known member
Hull speed is not 'Vmax'.
It's just a point on a speed/power curve.
Above 'hull speed', diminishing returns for speed from power.
The wind speed to power a boat to 'hull speed' will depend on sail area, point of sailing, displacement etc etc.
It's just a point on a speed/power curve.
Above 'hull speed', diminishing returns for speed from power.
The wind speed to power a boat to 'hull speed' will depend on sail area, point of sailing, displacement etc etc.
Laminar Flow
Well-known member
Resistance is a direct function of displacement. Displacement/length ratio is what sets limits to ultimate speed.
Lighter boats have, consequently, a much shallower resistance curve and can reach higher (beyond traditional hull speed) speeds.
Sail area/displacement ratio is an indicator whether and when a boat will reach critical speed.
And yes, it is possible to calculate/predict and with reasonable accuracy, how fast a boat will go in a certain wind speed.
References can be found in
Kinney's, Elements of Yacht Design,
Larsson/Eliasson, Principles of Yacht Design
Dave Gerr, The Nature of Boats
I used this as a calculation basis to assess the benefits of increasing SA on our boat. The predictions were fairly accurate over much of the speed range. except at the higher end, where we exceeded expectations.
Dave Gerr offers a simple formula to calculate probable hull speed, not so much in relationship to DWL, but to the boat's displacement/length ratio (Max S/L ratio = 8.26 ÷ D/L ratio↑.311). In practice we have exceeded this proposed max speed by a considerable margin.
Our max. sustained speed/length ratio has been 1.64 with a D/L ratio of 360 (trough the water speed, of course). By comparison, trad. hull speed or speed/length ratio is 1.34 .
Lighter boats have, consequently, a much shallower resistance curve and can reach higher (beyond traditional hull speed) speeds.
Sail area/displacement ratio is an indicator whether and when a boat will reach critical speed.
And yes, it is possible to calculate/predict and with reasonable accuracy, how fast a boat will go in a certain wind speed.
References can be found in
Kinney's, Elements of Yacht Design,
Larsson/Eliasson, Principles of Yacht Design
Dave Gerr, The Nature of Boats
I used this as a calculation basis to assess the benefits of increasing SA on our boat. The predictions were fairly accurate over much of the speed range. except at the higher end, where we exceeded expectations.
Dave Gerr offers a simple formula to calculate probable hull speed, not so much in relationship to DWL, but to the boat's displacement/length ratio (Max S/L ratio = 8.26 ÷ D/L ratio↑.311). In practice we have exceeded this proposed max speed by a considerable margin.
Our max. sustained speed/length ratio has been 1.64 with a D/L ratio of 360 (trough the water speed, of course). By comparison, trad. hull speed or speed/length ratio is 1.34 .
Daydream believer
Well-known member
Ok for the less intelligent among us. ( well me at least) May I ask
What is :-
DWL, S/L, D/L & the upward pointing arrow after the word "ratio"
& when you refer to Speed/length ratio- is that waterline, or overall?
It might make the formulae a bit more intelligible --
It would be interesting to know where the figures 8.26 & .311 come from , because they could be just plucked out of thin air.Or are they products of your particular boat. Plus , how does the formulae stack up against the Op's example of a boat that might surf , or the latest 34 ft Elan that can touch 15 kts ( according to YM review)
What is :-
DWL, S/L, D/L & the upward pointing arrow after the word "ratio"
& when you refer to Speed/length ratio- is that waterline, or overall?
It might make the formulae a bit more intelligible --
It would be interesting to know where the figures 8.26 & .311 come from , because they could be just plucked out of thin air.Or are they products of your particular boat. Plus , how does the formulae stack up against the Op's example of a boat that might surf , or the latest 34 ft Elan that can touch 15 kts ( according to YM review)
onesea
Well-known member
The nearest thing to what your looking for is called Polars. Even then they are more of a guide.
Chiara’s slave
Well-known member
Exactly. There are too many variables, quality and type/size of sails, for instance, prismatic coefficient of the hull, salt or fresh/brackish water. You can develop a formula and a set of polars for one boat. Anything else is a semi educated guess. Though I guess if you define your formula for being applied to classic full displacement hulls only, it narrows it down a bit.Ok for the less intelligent among us. ( well me at least) May I ask
What is :-
DWL, S/L, D/L & the upward pointing arrow after the word "ratio"
& when you refer to Speed/length ratio- is that waterline, or overall?
It might make the formulae a bit more intelligible --
It would be interesting to know where the figures 8.26 & .311 come from , because they could be just plucked out of thin air.Or are they products of your particular boat. Plus , how does the formulae stack up against the Op's example of a boat that might surf , or the latest 34 ft Elan that can touch 15 kts ( according to YM review)
Daydream believer
Well-known member
Please do not make it sound like a criticism. I want to understand it first. That is why I asked for a better explanation.
Chiara’s slave
Well-known member
The formula I am familiar with for boats like mine is simply a predictor of ultimate performance. There isn’t a way of predicting the drive for an unknown rig. However, for generic types of boat there is a kind of empirical wisdom. We could safely say your Hanse will reach a notional hull speed in about 12kn of wind, knowing what type of boat it is. It can exceed that in more wind of course.
jamie N
Well-known member
It's purely an idle thought along the lines of my Folkboat reaches its max, a nominal 5kts in about 15kts of wind. Anything over that just increased the heel really. My (as yet unsailed) GK24 will reach its flat water max in a lower wind speed I reckon, but as I state it's a notional thought as I've no experience of the boat.
As I wrote this, I came across this.
As I wrote this, I came across this.
Daydream believer
Well-known member
Polar says true speed through water 8.14 kts in 20kts wind
But I have substantially bettered it (through the water of course , not because of tides etc) in 25-30 kts in squeeky bum times on my own
But apart from that -which is not really representative, because I tend to average passages of 6kts( making use of tides where possible) on most of my sailing- I wanted to apply Geem's formulae to see how it compared to my 20 year old cruising boat.
With my latest new engine I can motor at 7.5kts if I do not look too hard at the fuel consumption. But once again 6-6.5 kts is comfortable. By that I mean that the revs do not have to go up 1000RPM & the stern does not start to squat
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Supertramp
Well-known member
One difference between the folk boat and similar deep keeled, relatively narrow beam yachts is that when overcanvassed they do simply lean more and carry on. Except Dragons and Loch Longs which start to take on water!
The beamier and more modern hull/keel forms can actually start to lose speed when overpressed and need to reef to keep speed up.
I'm sure you know all that. On my heavy, long keeled boat I don't go much faster than I can manage in 15knts of wind on any point of sailing but equally I can carry full sail well past 20knts (but with negligible speed gain). All the extra drive force seems to go into making wake!
Laminar Flow
Well-known member
DWL = designed water line length
S/L = speed length ratio, as in square root DWL x factor
The arrow means "to the power of" (in this case to the power of 0.331) You will need a scientific calculator.
Where he he got the factor 8.26 from I am not sure and would have to investigate.
As to prismatic coefficients, their influence is well researched and a useful diagram for this can be found in Gutelle, "The Design of sailing Yachts"
In this sense, each prismatic coefficient has favourable speed/length ratio. For example, we have quite a high prismatic coefficient of 0.6, which means that in speeds above 1.2 to 1.25 our hull has about 20% less resistance than one with a PC of say 0.5. There are penalties for this at other speed ranges.
How to actually make the calculations for a polar diagram can be found in Larsson/Eliasson, where they provide factors for battened sails vers. regular sails etc. All these calculations are highly idealized, of course, but I have found them useful enough to me in making relatively accurate predictions as to the performance of our boat.
S/L = speed length ratio, as in square root DWL x factor
The arrow means "to the power of" (in this case to the power of 0.331) You will need a scientific calculator.
Where he he got the factor 8.26 from I am not sure and would have to investigate.
As to prismatic coefficients, their influence is well researched and a useful diagram for this can be found in Gutelle, "The Design of sailing Yachts"
In this sense, each prismatic coefficient has favourable speed/length ratio. For example, we have quite a high prismatic coefficient of 0.6, which means that in speeds above 1.2 to 1.25 our hull has about 20% less resistance than one with a PC of say 0.5. There are penalties for this at other speed ranges.
How to actually make the calculations for a polar diagram can be found in Larsson/Eliasson, where they provide factors for battened sails vers. regular sails etc. All these calculations are highly idealized, of course, but I have found them useful enough to me in making relatively accurate predictions as to the performance of our boat.
doug748
Well-known member
Buck Turgidson
Well-known member
That's interesting as my Twister loves 55°True for best VMG and she trims perfectly to sail hands off at that angle.
Birdseye
Well-known member
there is no absolute maximum speed of a hull. There simply is a point where the extra power needed to go faster rises at a non linear rate - maybe exponential but I am not sure. That speed depends on the profile of the hull - a really slender hull reaches this point faster than a chubby wide hull - the basis of the speed of multihulls and also why you see the bulbs on the bows of cargo ships.
MisterBaxter
Well-known member
A minor point but I believe that bulbous bows are designed to reduce wave-making resistance at a specific cruising speed by creating a secondary bow wave that partially cancels out the main one.
Laminar Flow
Well-known member
Bulbous bows allow for an increase in speed of about 15% in a very narrow relative speed range of between 0.6 and 1 ( Sqr root DWL x speed factor). As sailing yachts operate at the variance of the wind, a bulbous bow makes no sense. Freighters tend operate pretty much precisely in this speed range.
The main factor that governs potential (hull) speed is the displacement/length ratio. The heavier the boat, the steeper and exponential the curve of the form resistance becomes.
Resistance due to wetted surface increases at a much lower rate and is fairly constant and, consequently has much less impact on ultimate speed.
What limits the potential speed of any sailing boat is stability, which obviously limits the propulsive power.
Prismatic coefficient too, has a marked influence on resistance. The CP essentially describes how full or slender the ends of a boat are and each CP has a preferred optimal speed. Fuller ended boats (high prismatic coefficient) have a higher speed potential at the cost of higher resistance at lower speeds, whereas finer ended boats (low CP) have lower resistance at lower speeds, but much higher resistance at higher speeds where they tend to suck up a pronounced quarter wave.
Most designer choose a CP somewhere in the middle to account for most conditions. Motorsailers, that have more control over their average cruising speed tend to have a higher CP, however, the fuller bow is less optimal for windward work.
The main factor that governs potential (hull) speed is the displacement/length ratio. The heavier the boat, the steeper and exponential the curve of the form resistance becomes.
Resistance due to wetted surface increases at a much lower rate and is fairly constant and, consequently has much less impact on ultimate speed.
What limits the potential speed of any sailing boat is stability, which obviously limits the propulsive power.
Prismatic coefficient too, has a marked influence on resistance. The CP essentially describes how full or slender the ends of a boat are and each CP has a preferred optimal speed. Fuller ended boats (high prismatic coefficient) have a higher speed potential at the cost of higher resistance at lower speeds, whereas finer ended boats (low CP) have lower resistance at lower speeds, but much higher resistance at higher speeds where they tend to suck up a pronounced quarter wave.
Most designer choose a CP somewhere in the middle to account for most conditions. Motorsailers, that have more control over their average cruising speed tend to have a higher CP, however, the fuller bow is less optimal for windward work.
Chiara’s slave
Well-known member
You’ve been reading the same books as me, it seems. I find the subject fascinating. I have designed and built a number of sailing models, and my fascination with multihulls started early, my grandfather built me my first in 1968. He was a naval architect. To finish the logic of course, if full bows make for poorer windward work, but you want a high prismatic hull, then you must make the mid run less full. Therefore you either have a 1920s 30sq metre, or make 2 of them and have a cat.
