# Sailing Dinghy Design

Discussion in 'Sailboats' started by Tim B, Mar 12, 2003.

1. ### Another GuestGuest

Also aboout planing areas;

The Savitsky diagrams that everyone agrees on show that there is a massive "spike" in the amount of dynamic lift in a flat plate, just where near the stagnatiion point about 25-30% aft of the point where the solid water first touches the immersed area of the plate.

As Jim Drake (windsurfer inventor and still leading edge, former rocket scientist with NACA (literally) says, the theory is that NO hydrodynamic normal (perpendicular to the planing surface) force at the transom.

So yep, we don't plane on the fat stern, we plane on the stagnation point further forward, and the stern sort of just goes along for the ride AFAIK.

Me, I'm still trying to understand why we "need" fat sterns at all if we lift further forward - obviously they effect trim angle/water flow but would a pintail not be better overall perhaps?

Than again, theoretically the opposite (a short, wide planing surface) is better anyway when planing - the fact that it is not as good around a normal course shows the differendfce between theoretical top end speed and real life racing.

re your nosedive problem Phil, a Lechner-style widebow higher prismatic Moth would fix that because of the extra dynamic lift (at low drag) perhaps. The Moth is short and the prismatic is so low the bow ain't being "used" much, perhaps????

There may obviously be a cost in some conditions, the widebow goes against many current theories - but interestingly in the Fireballs the widebows are faster. In the D2s Moth-type bows were tried and then development returend to the Lechner type.
Perhaps one has to achieve a certain minimum width (as the Lechner and 'ball do) and then it all starts working and fatter is faster?

Some twilight we should line up and sail into some powerboat wakes to see if the Lechner slows down more in waves than the fine bow Moth.

Chris

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### astevoJunior Member

what are these theoretical models based on? and is the stagnation point theory as center of lift valid in boats like moths which do not fift the bows out of the water? and hence there isno zero velocity point.
it is not really accurate to use the flat plate approximations for the modern type low planning skiffs, as streamineflow is maintained more than in olderstyle hulls.
my theory is that if you pull the stern in to sharp you loose the flow around the hull as i tends to flow up from under the chines. this basicly is due toa pressure loss form the sudden change in shape. if you can reduce the waterline width near the back(smoothly) you can drastically improve light and midrange performance at the fore aft trim of the boats become less critical you canget away with the stern dragging. in themid you could also not suffer so much from lifting the bow to ge the positive angle of attack that is needed for planning (if that is you prefered mode of sail).
from all my drawings themoth isnt all that low as it only rreally varies in its width, depths and curvature are almost constant all the way through. the prismatic is normally 0.68 or so. not all that low really

aslo chris a few years a go the speed sailboards had a shape with the max beam very far forward (i think). why have they come back from this?
my two cents

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Moth lengths

Back to what astevo asked about different hull lengths for Moths -
I have done a few runs on a 9ft, 11ft, 14ft and 16ft moth for the drag calcs. The grapch is below. Hull s 1 + 5 are both 16ft long, hull 2 is the standard Moth, Hull 3 is 14ft and Hull 4 is 9ft.

Longer hull works up to a certain point, then the lower wet surface takes over.

So, if we believe this information, at about 10 to 12 knots hull speed, a 16ft Moth is best. At the top end, the 11ft Moth is best. But the 14ft design seems to offer pretty good all round performance.

Michlet doesn't do dynamic lift at all so no good for planing.

Next message gives an indication of the wet surface versus wave making drag....

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And here is the wave making versus wet surface drag...

I think it is fairly clear that once above hull speed, wave making drag remains roughly constant and much less important than the wet surface drag.

CHeers.

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### astevoJunior Member

if michlet doesnt model planning we must be careful what conclusions are drawin from it, consider an 11ft hull the hull speed is like 5 knots or 2.5m/s. what we draw from michlet above this point for a 11ft hull is largelly irrelevant.
with no dynamic lift being modeled the accuracy of the method is really only valid at low speed,
also in high winds it is the length of the boat(longitudinal stability) that limits our speed not some barrier that we simply cant get past, it would be possible to make a moth faster on a big reach with the same hull, just by dragging the rig back a foot or two but this would be at the expense of speed elsewhere.

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As I said - "if we believe this information"

However, I am not convinced Michlet is totally irrelevant for a very narrow hull like a Moth.

Lift is proportional to speed squared and the area of the lifting surface (broadly speaking). So, if you halve the area of the lifting surface by making the boat narrower, then you have to go 40% faster to get the same lift.

Whilst "hull speed" might be the time at which a wide boat like a Cherub at 900mm wide gets substantial dynamic lift appearing, a Moth at only 300mm wide would only get half of that dynamic lift if trimmed to the same angle etc etc.

I gather from Andy Patterson that Moths tend to sail at a fairly constant trim (stem about half in the water) irrespective of the wind. This would suggest that the dynamic lift would be marginal due to the very low angle of attack. (Real Moth sailors please correct me if I am wrong)

But anyway - if someone builds a long Moth we can find out what would really happen.

Another pet hobby horse of mine - "hull speed" is a convenient name for the point where transverse waves generated at the bow and stern are in phase and therefore wave making drag is maximised. If a hull is designed to slice through the water and not over it , and is sailed that way then drag calculations based on these assumptions should hold (within the level of accuracy of the theory).

By the way, Phil S - for your stressed ply Moth designs, why do you cut the 1st sheet in half and then appear to put the two halves back together again but with a "dart" cut out for only the first 800mm? Would having the ply contuinuous for the last 400mm help maintain fullness in teh bottom half of the hull - or does it just make pulling it into shape much harder?

7. ### Another GuestGuest

Astevo, your stern idea sounds right to me, also there may be "separation" (as the 12 metre guys used to call it) from laminar-ish into turbulent flow if the stern tucks in too fast. I haven't got fully formed ideas on the pintail in any way, it's just a thing I wonder about.

With the stagnation point info I wasn't really thinking about Moths, but more conventional boats. Maybe the stagnation point is not NECESSARILY trelated to the spray root and surface, but merely a point of high pressure - just as it is with flow around masts and aerofoils AFAIK. I'll have to check that one, more homework.

Have you read the stuff about high speed cat ferries? It represents a lot of work on narrow hulls, but I can't claim to know it. I will call Stu Friezer later, he's into that.

Interesting that the Moth is .68, that's quite high. BUT can you compare the prismatic of such a narrow hull with the prismatic of a "normal" hull, d'ya reckon?

The Moth's entry is so fine that perhaps (just perhaps, they are done by guys like you two who know more than I so I don't mean to be hassling) they could handle a higher prismatic without the normal problems that a wider entry brings, and with benefits. The Fireball is fairly skinny, the Lechner is very skinny, they both go well with fat bows; perhaps there's a difference between the fairly skinny/very skinny boats and the run of the mill boats, which dog it if the bow is too blunt? Snubby's boat will prove it right or wrong if it's built; what's happening with it?

Med Engineer; lift is proportional to speed squared as you say, so a boat that's skinny enough to go faster at lower winds will make up fgor a lot of the planing area problem, won't it? I'll have to work out the maths to see how much speed you need to gain enough for the lack of area.

Speed boards; a qwuick look at Bjorn's boards for the speed attempt this year seems to indicate they are similar in plan shape (generally) to those as early as '83, ie tiny pintails.

That's dedicated speed boards; I'll email / call with the Jim Drake explanation of the fat Formula shape in a few days if you want. It's all bound up with sailing style, technology, pumping and unrestriucted rigs, as well as the advantages of the high aspwect shape. Plus, the low drag/low lift of sailboard rigs ansd the extremely efficient 70 cm fins.....But mainly, I think it's the fact that they don't mind if they can't sail these things half the time and most places, and have to sit on shore whinging aboout the wind.

God this pafge is hard to write on, it's shitting me offffffff

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### astevoJunior Member

the forward sides are cut so that the spring you add can be acuratly and evenly plotted and cut. this also helps with the continuity of the hulls curvature in the long side pannels. which was considered to be more inportant to be perfect than the rocker line(which can be adjusted). we decided that the hull should be kept as fat as posible in the area under the mast. on the inside we put alot of the black stuff across between the chines and formwar, to prevent the hull V'ing. i think that if you were to make more hulls again with the stressed ply method you should extend the side seams all the way forward and make the bottom only slightly curved(elipinate the keel seam)

with the first of the stressed ply hull(the fat one) the stern tuck was too sharp and as a result the flow would come round the chine and up before it got to back of the hull. by changing the flow you get a reduction in pressure, which effectivley allows the flow to come from the flat area around the chine up toward the sides

agree that the hull speed thing is a bit of a hobby horse (ride it till the theory falls apart then get off). its just a term used to determine the speed of highest drag, related to the hulls length alone.

re moths and dynamic lift.
when my new moth was first launched it had excess rocker with too much in the back. the curve was smooth enough so that the seperation from the hull should be prevented, it was slow. a few weeks later the hull was cut open after one frustrating day at bsc. back for more the next week the boat was a massive amount faster. all i can attribe this to is the fact that the dynamic lift must be significant, as all our displacment theories tend to show that non lifting hulls go best with rocker in the back.

re prismatic : why the hell not? as far as i know it wouldnt be used in velocity prediction(dimesionless number) but is realy only just a term to represent volume distributiuon. also its a good way to compare basic hull forms. boats like a i14 are proably around .75 or so which is going to be one of the most squareish boats around short of the aus cherub.

i think i the lechner and fireball you can get away with the big bow as they have abit of length to play with, at 11foot the loss of waterline from the spoon bow will be significant. also in waves where it will go over most waves , the big one it goes through will really pull the boat up. also the frontal area is a potential point for the boat to trip up on as it goes downhill. this is of course unless you have heavily V'd sections forward

the stagnation point is pretty fundamental to vpp progarms based around integral approach to fluids (bernoulli equations). although ive never used these programs at all and have even less idea how they work.

maybee some of the more scientifically/technically informed thinkers out there can explain how they work?

because of the oversize rigs the formula boards can carry, they never have to worry about when they dont plane (just sit on shore and drink cans, whis is perfectly good form for windsurfers), but it does concern us.
also could somebody explain the weird step in the back of the formula boards?
my 2 cents (atually this one is more like 50)
as

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### Tim BSenior Member

We must be careful to remember that the design needn't match course, but must exhibit generally low drag.
For example, assume you get to a regatta on Day1 sailing a course where the longest leg is a broad reach. So the rig is powered for offwind performance. Day 2 it's a beam reach, so the rig is optimised again. Day 3 goes upwind, so again the rig is re-tuned for optimum power around the course.
The goal in the above situations is to optimize boat speed around the course (simple bit of code could tell you the settings, interpolating known speeds and directions) by optimizing the average rig power. In a race, this may mean that you are overtaken on a windward leg, but blast past the opposition downwind.
This situation has a very small amount to do with Hull shape/drag, and rather more with rig tuning. That is not, however, the end of the story. The hull is free to move in three directions, and rotate around three axes. Assuming that the hull is kept flat (which it is isn't it?) and assuming we're sailing a dead-straight course, then the pitch and forward motion are the only things we're concerned with. The question then becomes, on any one hull, what pitch gives least drag at a certain speed. you'd be surprised to find that the pitch/drag curves are U shaped. but the optimum point moves predictably. A tank test is in order I believe.

Cheers,

Tim B.

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### Phil SJunior Member

Re stressed ply moth. In addition to what Andrew (AStevo) has posted, the ply sheet is cut and the halves turned 90 deg so the grain matches the long sheets across the splice. This matters so that it bends as one without hard points.

If this makes no sense to the rest of you see the stressed ply moth plans are at http://www.moth.asn.au/building_registration.html

I agree with Chris and Andy Paterson that we try to sail our moths at constant trim, about half way up the transom, depending on crew weight. This results in about 100 to 150mm of the bow in the water.

Because there is only about 50mm spring, both ends are in the water most of the time and that is why the prismatic coeff is higher than expected.

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Phil / Andrew - thanks for the clarification on the plywood, it makes much more sense now.

Another Guest - yes a narrow boat goes faster in lighter winds thus benefitting from speed squared - but for the same HULL speed the reduced area reduces the lift.

Cheers

12. ### Another GuestGuest

Phil, re your query about the step at the back of Formula boards - from Jim Drake

"The reason for the cutaway is to move the center of pressure (cp) forward and trim the board to a higher angle of attack for better lifting efficiency and still provide a large enough flat area around the base of the fin to prevent spinout."

The fact that fluid dynamics uses stagnation points indicates that even if an entire planing surface is submerged, we will get a stagnation point and therefore the spike in the dynamic pressure, doesn't it?

Prismatic as a comparison between skinny and fat hulls; I was just wondering about this, because while the prismatic does " represent volume distributiuon. also its a good way to compare basic hull forms.", can we use it to compare basic hull forms that differ so drastically in other parameters ie beam?

We "know" that a Cherub-beam boat with a prismatic of (say) 95 would be slow, but that's partly/largely 'cause of the very blunt entry and wide stern that such a high prismatic implies in a beamy boat.If we tow a Cherub-beam blunt box it would have huge drag.

If the boat is 6" wide, then it could have a prismatic of 95 without so many problems, couldn't we? If we towed a telegraph pole it would tow OK even if almost blunt at the end, unlike the Cherub box.

What's the situation in cat prismatics, what sort of numbers are they developing towards? I thouoght the offshore cats were getting quite high.

By the way, Steve Brewin was musing about a planing-hull A on Sunday.

Mad Engo; re "but for the same HULL speed the reduced area reduces the lift", my point is that the hull speed of the narrow boat WON'T be the same, all else equal it will be faster.

By the way, my feeling is that the long Moth (it was out again this weekend) has a similar attitude to the "real" Moth. It MAY be more bow-up but it's effectively much lighter (much lower DLR) so I assume dynamics are differnt.

Tim, championships are held on the same course; either P, ww/lw, trap or triangle from day to day, and it's very hard to get arouond the top mark behind and go "blasting through the fleet" because the fleet (and their bad wind) very rarely have the good manners to merely wave admiringly as you go past them downwind.

And about the code, when you that simple bit of code please pass it to Phil, astevo and I, we know many IMS and America's Cup designers and world champions who have spent years looking for it.
.
What we need is a plywood skiff class. One design rigs (perhaps), ply construction or soliud foam, dollar limit - cheap playground for ideas.

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Another Guest - I totally agree in the same wind and water conditions, where an 11ft long 3ft wide hull is doing 4.5kts, the 11ft long 1 ft wide hull would be going faster, but would it be doing the 9kts needed to generate the same lift??

The best comparison is the skiff moths versus the scows or the Europe.

Are the narrow skiff moths twice as fast as the scows or a Europe?

PY Number of the Moths in the UK is now 1000, whereas the Europe is at 1139 which would suggest about 15% faster. In marginal conditions though where the Europe is hitting the hull speed increase in drag, the difference is probably greater.

Anyway - on prismatic - this would only seem to be much use to me in comparing hulls of similar length and displacement or at the least the same DLR. As you said a telegraph pole would be easier to tow, but for the same displacement, it would need to be much longer than the Cherub box....

14. ### Another GuestGuest

You're right, the Europe would be only 15% faster or so, and the skinny boat will not develop the same lift. I knew the skinny boat would not lift as much overall, I was wondering how much difference the extra speed it would make and I expressed myself badly. However, as far as I know, every class has gone to the narrowest possible beam which means it must work, even if you lose planing area.

ALSO do your calculations allow for the fact that the Europe/standard style boat has a lot of vee in the area that generates dynamic lift? The lift is developed at right angles to the hull skin, so there is a loss of lift proportional to the angle of the Vee. The skiff Moth, and just about all really skinny dinghies (UK Cherubs, Canoes) are pretty flat or very flat (along the keel at least) in section, so all the dynamic forces are being directed upwards much more and therfore more effective.

The fact is that the moderate beam flat planing hulls, such as the scow Moth, Australian Skate and Vee Jay etc are all slower in many or most conditions than the round-bottomed or skinny boats, aren't they? Empirically, if the calcs don't match reality must be not look at them to work out where to go.

I still reckon beam may be vital for comparing prismatics but I shall check this. Is displacement related to/measured in prismatic? I thought it was only measrued in the block coefficient?

I agree, a telegraph pole woould be longer than a box of the same Displ, but that's only a problem if you're racing under class rules that restrict length or designing a boat to fit into a marina berth. Otherwise, isn't extra length one of the simplest, cheapest, easier, most atttractive, "best-handling" ways to increase speed?

Talking to many of the top designers, from Bethwaite to Patterson and back again, there seems not a lot of doubt that the big Moth (or may I call it skinny light Canoe?) is the way to go for singlehanded speed, and maybe for all fast boats.

How important speed is, is of course another matter entirely. I'd rather race my Tasar than the cat, although the 16' cat is fast enough that we beat the world Tornado champs to the first mark last time we raced them (we were lucky, I hasten to add, I'm pretty average at cats through lack of time and incentive!!!!!!)

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### Tim BSenior Member

So comfort is a factor, I'll second that, for two boats of similar spec. the more comfortable boat will be faster.

And that piece of code isn't that hard to write if you know the optimum rig tuning for specific headings. A simple analysis of the course is not difficult, and I think you'll find you can power past a fleet, although it's not always easy. It should be doable if the settings are right for max power.

I like the idea of a plywood skiff, shall we say £1500 top limit, including all gear. That should give plenty of cash to play with. I'm assuming that we don't really want to try a spinnaker with it. It would be hard though to improve the design without competitive racing against each other... This is where idesign begins to fall over.

Oh, and could I add, that adding length is the short-cut route to boatspeed. The true answer can only be analysis and improvement of a fixed length hull. If length is not fixed then the entire problem changes and the offender has not properly understood the reason for a decrease in drag.

Cheers,

Tim B.

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