Approximate sailboat speed prediction

Some pretty odd statements in this topic.

Long/thin is not good for a true planing regime. Don't believe me? Look at powerboats. For a boat supported primarily by dynamic lift you want reduced lift/drag, which means a better aspect ratio, which means short and fat. The ultimate expression of this is of course hydrofoils. You never see a boat with a long thin hydrofoil.

However few sailing boats are what the powerboat people would call true planing boats. I've often danced from wavetop to wavetop on a true planing sailboat, but I don't believe many of you will have. The vast vast majority of our boats top out in a transitional region - what Bethwaite calls forced mode - where wavemaking drag is at least as important as lift drag, and in this region above all length pays. Look behind you - are you towing a big wave system behind your stern? If you are then length is vital, and thin is vital, because wave making resistance is still key. You can learn a lot about this by going out in a powerboat that will truly plane and watching the waves behind you if you accellerate slowly through the transitional region into true planing.

I sail an International Canoe these days. Its long and thin and fast, but its not a true planing boat. With the older designs the speed seems to top out in the 15 knot region, which is pretty much where Uffa Fox topped out 85 years ago. With far better rigs we get there a lot more easily than he did, but don't go much faster (excluding the very latest boats which march to a different tune). And 15 knots is a Froude number of about 3, which I believe powerboat designers would class as being the top of semi planing or semi displacement. I've sailed skiff types that will achieve Froude numbers well above three with much shorter and wider hulls, and when they go they really go, but compared to long thin boats they really suffer in the Froude number 1 - 3 region.

But all this is utterly irrelevant for Laukejas, because he is not designing the type of sailboat that will get into these region where true planing is key. Its going to top out in the low end of forced mode, and this means that length will be nearly everything in terms of performance as soon as there is enough breeze to bring it into a wavemaking regime. In that range just length will get close to 10% to predicting performance crosswind and downwind.

In the wavemaking regime where this boat will spend much of its time this is pretty much bang on the money. The only exception may be going upwind where efficiency limits how far into the wavemaking region you can push the boat.

The other interesting thing is that once you drop out of the wavemaking regime in very light winds it changes again. If the wave system is not a consideration then long/thin may stop paying, and wetted area/displacement may become key, and short wide boats may be faster than long thin ones. This esoteric region is only really of interest to racing sailors however, since any sensible cruising craft will have got out a pair of oars (in which case length wins again) or stopped for a picnic and a glass of beer...

 

Those unknown variables that are stopping me from getting an explicit purely theoretical answer are just too complex and too numerous. And other than being able to predict the speed of a boat you have not built in conditions you are dreaming up and would not be able to measure there is no use in the equation. Your boat can't optimize the crew for low windage -you still need crew and they must sit in the right place for balance. You can't optimize the waves.

What the engineer is always looking for is easy accurate answers to "how much performance can I get for each expenditure of resource?". In this case it would be something like "what is the best speed I can get out of a two sheet sail dingy carrying two crew in 5mph avg wind". The importance is having accurate equations that relate each parameter you can control to the performance feature you want. You can even use simplified estimates -for example you might use a linear estimate rather than the exponential true equation for wind power or wave drag -over a small range of values and still be accurate. When you have these coefficients the optimum will be a vertex of one of the boundaries. My recollection is that this is the 'simplex method' and it works as long as the variables are not far from linear over the range of interest.

So what I am saying is that those unknown variables are significant, but not important to our consideration so I will just chuck them all into one variable called "unknown" and estimate it's value based on the performance of prior boats. Then I will focus on the parameters that I CAN control in the design and the equation that relates them thrust=drag. Then I solve the equation for boat speed and take the partial derivative of each with respect to boat speed. That sounds intimidating and it is calculus, but as I said over small ranges it can be estimated as linear (just solve the equation at two points a little above and below the value of interest -the estimated derivative is the slope of the line between those two points). Once I have the coefficients of each variable with respect the performance I can use the simplex method to find the optimum values. By the way, the simplex method is not limited to a single performance parameter. You can derive a linear weighted performance factor from any number of desired outputs -it works in "N space". Specific to your desires it could be a weighted factor of speed and stability.

 

GGG,

I don't know what you find odd but it must be semantics because your statement above is exactly what I expect an optimization to show. The low wind specification in Luake's SOR rules out planing in my estimation (to say nothing of the large load). As I said, the maximum speed will be on a boundary -in this case it is likely the limit of tolerable stability on a long skinny boat.

Righting can be divided into two components -form stability, and ballast (crew) shifting. Even if you go wide the hull will perform best when sailed upright -form stability ~0. The ballast shift is not really limited, more like "how far do you dare". GGG's IC is the best example of ballast shifting independent of waterline beam and it is a great light wind sailer. Even if you limit the crew to sitting on the edge of the hull and leaning out the math will still favor a narrow long hull in light wind with big load.

So summing up this thread
to the question (of a simple VPP for small dingy heavily loaded in light air) we have about a dozen "no thank you, too complex, not worth it" replies. Zero takers for the full academic VPP. One proposal for an empirical engineering solution. And three unsolicited predictions of what the high performance solution will be including the engineer with the only math based proposal.

It's a bunch of work but there is educational value in seeing the sensitivity of parameters. The result is pretty predictable as GGG and Mess point out from extensive experience.

 

4.6... Well, that's something. I'll get back to my boat, and trade some beam for length.

That's an interesting paper. I've started digging into it, and it seems promising, but it will take some time to make any sense of it.

As for iterative VS automatic, I'm pretty sure any equation can be solved for a specific variable, so if we manage to put everything in one big equation, it should be possible to calculate speed in a single step. Optimizing the boat to achieve best performance would be still iterative, of course.

Well, I'm not sure if it's a good idea to take ALL of these unpredictable variables and stuff into one "unknown". Because, for example, crew windage totally depends on number of crew. Wave induced resistance is dependent on bow shape (entry angle), beam too, probably, and I guess a dozen of other factors.

It'd be great if we could isolate each part of the unknown variable, but it is pretty much impossible without empirical testing.

However, I think another approach can be used: we can roughly approximate each part by making a crude calculation based on basic data.
For example, crew windage: calculate silhouette area of average man. Assume that this silhouette lies parallel to the boat centerline. Calculate the drag using some primitive formula. Then multiply the answer by the cosine (or sine, not sure) of apparent wind angle for each scenario.
Then do the same for each unknown factor.

Then, add it all up, compare to the actual real boat performance and add this so called "fudge factor" like you said.

So in the essence I'm proposing the same, but I think it is worth trying to at least roughly approximate these unknown variables before adding fudge factor. It should give somewhat truer answer, because, even though some factors are outside of designer's control, they might have different weights in equation in different weather conditions.

I'm all for the full academic VPP! I just think before we go for that, we should make some simplified calculation model for "everyday use" without too much ambition for dead-on accuracy. For starters, just to have something we can use to check how accurate it is compared to real boat performance data. Then, with new ideas and data, we can gradually add complexity and remove fudge factors to achieve better accuracy, until we have full dinghy VPP.

Just my two cents. I'm more of a "make it work now, add details later" kind of guy

 

You are still not recognizing or accepting what you are being told. You asked for a simple dingy VPP. Answer: there is no simple VPP. To get anything close to simple you will have to substitute many factors with an empirical value from the most similar boat you can find. At best you can devote significant effort to accurate characterization of the design variables you can control.

A bunch of little guesses for small factors you can't measure will not add up to a more accurate answer than my (or group consensus) one big estimate based on lots of observed and measured experience -it is only an opportunity for greater error.

This is an even more inescapable conclusion than you comprehend. The Delft series drag calculation is itself an estimate that is only valid for well designed hulls of great similarity to ones already tested. Real theoretical drag would require CFD analysis. Ditto for the aerodynamic portion of the VPP. If you start now into a good PHD program you might be able to generate solutions that are not offensively wrong within a decade. And the only error factors you will miss will be the deflections, motions, turbulence (in the incoming air)....

"I'm all for the full academic VPP!" THERE IS NO FULL ACCADEMIC VPP! Take the Van Oossaanen paper as an example -this is a paper by a PHD NA who appears to be the descendant of a PHD NA that founded a major NA company in a country that prides itself on NA excellence -and he falls back on empirical testing in several key aspects rather than rigorous fundamental theory.

Please don't take this the wrong way but I feel the need to tell you

To be a professional musician and an armature engineer is to resign oneself to the world presenting cruel realities. You would find the world much more agreeable the other way around as a professional engineer and armature musician.

There is no simple dingy VPP. The simplest, cheapest, and most accurate way is to build boats to fill in where the calculations get difficult.

 

Okay. Sorry for my ignorance and stupidity. I probably still don't realize how complicated this all is. Please don't get angry. I'll do better.

In the meantime, I hope this discussion might become a kick-starter to gather minds to help you with the dinghy VPP you're trying to make.

 

I am not angry. On the contrary, I appreciate your asking such good questions and feel a little better when I hear far more experienced designers don't have any better answers than mine and are resigned to similar conclusions.

Honestly, I drew conclusion on the hull and sail form I wanted before I could finish my optimization. The only thought I have given it since is to try and explain it to you (or anyone I might sell to ) that it is not opinion but mathematical certainty. The only question I think is left for this thread is

"under what conditions/parameters does planing become significant allowing a wider shorter flatter dingy to outperform a longer narrower rounder hull?"

Wind speed? (my experience ~15mph+ wind for a 300lb displacement 80sqft sail dingy -reference laser)

"and how do we model the performance of this planing hull transition"

My inexperienced proposal would be a "discount" of wave drag based on hull lift.

 

I can't find it now, but there was a discussion somewhere, either in this forum or Wooden Boat Forum about how to make boat plane at low winds, what hull shape would allow the boat to enter planning with minimum driving force. If I remember right, they pointed out sail area (obvious) and total weight (also obvious). But there were some interesting considerations about hull shape.
Maybe if you'll have better luck finding it, there might be anything of relevance.

 

Only IME when the Froude number for the short boat gets above about 3 into true planing, which excludes all but a tiny handful of exceptionally high performance sailboats and only those in stronger wind conditions. For everything else length wins.

 

An interesting answer. I was not ready for it in that form. What I was looking for was the math for drag prediction which would allow me to model and prove an answer. Just getting an answer is an invitation to a wordy debate -not of interest to me. I also see that we need more boundaries to get to a science based conclusion.

Fn=3
that is some powerful conditions!
Ramifications:
-for anything but downwind and/or downwave that would require crew weight significantly outside the hull (assuming crew weight is 1X to 2X boat weight). To the original intent of boat that is a wide hull but gets performance from it's greater stability this says "No, not unless you get outside the hull (giving up stability). Short can only beat long (on a windward leeward course) with crew weight far outside the hull, in big wind, with a tall rig (need to sweep lots of air, sail area alone will not do)".
-my experience is not relevant, at best I have gotten to 3 briefly down wind and wave.
-heavier boats would favor length and a ->higher Fn
-down waves short and wide has an advantage ->lower Fn

I wonder if there is a lower Fn planing with a concave bottom (which many rules prohibit).

 

Not all boats performed better when sailed flat. Scows need to heel over to get better speed.

 

I predict she will do about 3.5 kts, reaching and running, and make good to windward at approximately 2.5 kts.

This doesn't seem all that impressive until you take into account all the conditions this is likely to happen in.

Having a lower prismatic coefficient will help it reach its top speed with slightly less wind than would be the case with a higher coefficient.

The single, low aspect ratio sail will help too in that regard. It's less likely to lose its lift do to hobby-horsing, in light wind conditions, if there's some motorboat slop (waves) around.

The price you pay is in pointing ability. But I think the trade off is worth it.

There is, in my view, a paradox in pointing ability. The faster a boat goes, the lower it can point, while doing so, due to apparent wind effect. This is the reason we seldom see low aspect ratio sails on multihulls. Their higher aspect ratio sails somewhat regain pointing ability lost to apparent wind effect.

Slower boats, such as yours, and ones I like to design, can point higher, per aspect ratio sails, due to their slower speeds smaller apparent wind effects.

Your boat, being a pure displacement hull, should be less effected by extra weight brought on board. In fact, except in flat water conditions, its performance might actually improve with some added weight. This is because it will have more momentum to bash through the waves, as slight as they may be.

Back in the days I was in the pdracer discussion group, I once claimed that a heavier duck (pdracer) would row faster than a lighter one. One member decided to test his notion. He had the lightest duck in the fleet, at about 60 lbs hull weight. He was not a very big man himself, so he started off at a very low weight, maybe 225-250 lbs total. He rowed the boat at that weight, tracking his speed with GPS. Next, he loaded his duck up with another 100 lbs, then did another rowing speed trial. The boat went faster. He loaded another 100 kbs, and the boat went faster still. Only after he added a third 100 lbs did the rowing speed start to drop.

OK, back to my prediction.

It is based on three assumptions:

1.) the boat will only be able to reach about 90% hull speed, due to its low prismatic coefficient,
2.) it will able to point no closer than 45 degrees from the true wind, due to its low aspect ratio sail, but should foot well after that, and
3.) Its waterline length is about 9.5 ft.

So here's the math:

((9.5^0.5)*1.34*0.9=3.72 kts top speed

3.72 kts*0.707 (sin of 45 deg.) = 2.63 kts made good to windward.

 

Well, if it'll sail 2.6 kts VMG at 45° TWA, I will die a content man. You probably have no idea how much I suffered with my rigged fishing boats earlier, beating to windward for hours, tacking forth and back, sometimes making only as much as 80 degrees to the wind. This would be heaven, compared.

Actually, in the conditions I'll sail in the pointing ability is more important than VMG. I know, it seems unlogical, but there is a good reason for it: these lakes are very narrow, and wind corridors are often even narrower. I often had to sail in 15-20m wide wind corridor, tacking every half a minute. If I dropped out of the corridor, I'd have to row back to it. Tacking every half minute, you don't have the time to accelerate and attain decent speed. It's much more important how long can you stay in that corridor before you have to tack again, because every tack means lost time. Pointing higher allows to do just that.

So, having a boat that is easily motivated at the expense of top speed is exactly what I need.

Of course, high aspect rig would also help, but it requires long spars, unless it's a gunther or high peaked gaff, and that's going to cause troubles with transportation. So for now, I chose simple rig, and since I'm making sail myself, I can always "upgrade" to a different rig later.

 

You will probably find this new boat (if it ever gets built) a revelation.

It should short tack quite easily.

Short tacking is an acquired skill.

I learned it with my Siren 17, which loved to get caught in stays, meaning it would not quite make it to the new tack.

The remedy for this was backing the jib. The jib, then sheeted to the upwind side, would fill on the new tack and push the high bow over. As soon as this happened, it had to be quickly sheeted in on the downwind side.

After a while my crew and I got so good at this that we sailed up a channel that was maybe three boat lengths wide, dead up wind. People came out of their cottages to watch.

With your boat, you will probably just have to throw the helm over, or ease it over, depending on the wind strength. Since your boat will still be quite light for its beam and length (even with two people aboard), it may have a problem coming about in strong winds. If that that is the case, you simply hold the boom to windward until the bow comes around, then release it. Once you get the hang of this, it becomes second nature.

Sailing in the light winds this boat was designed for, you will probably find yourself seated on the bottom more than on the side decks, while sailing alone. This is mainly because you will want to induce some down wind heel in order to keep the sails full, so they will be ready for the next zephyr. Keep this in mind before considering making the boat narrower. You will need some room to slide your butt between the tanks. Also keep in mind shorter boats tend to change tack faster than longer ones.

People sailing faster boats will have a hard time keeping up with you, as their boats will have more drag than yours in these conditions. Your boat, if properly sailed, could quickly get a reputation for being "slippery".

Light wind sailing is an art in itself. So don't let yourself get too discouraged during your first season.

 

Yeah, I understand that with all the never-ending designing one might get a feeling that this boat will never actually go further than drawings in computer... Sometimes I feel like that too. But when the time comes to build (after my graduation, 1 month from now), I'll definitely put an end to all the designing and start putting it all together.

That I already noticed! It was actually very interesting to sail in the light winds for the last few summers, I felt like it was a true learning environment. The only frustrating downside was the bad boats, which wouldn't sail well even if handled perfectly.