Subject: Re: [harryproa] Re: Flat bottom hulls?
From: "=?UTF-8?B?QmrDtnJu?= bjornmail@gmail.com [harryproa]" <harryproa@yahoogroups.com.au>
Date: 3/21/2019, 2:15 PM
To: harryproa@yahoogroups.com.au
Reply-to:
harryproa@yahoogroups.com.au

 

None of Speers' lift to drags look to beat a good symmetrical foil, and the symmetrical wont have issues like weird balance, lift curves, and drag buckets. IMHO. Do you see it differently?  

I see it differently, because I think the CL/CD curve of a foil profile is a small part of real life performance. In real life, the span is in most cases pretty short (unless it's an efficient glider airplane or a wingmast without a sail). And this makes induced drag, when the foil is developing lift, the major drag in the equation. Even big differences in coefficient of drag of two profiles has little effect of the total coefficient of drag in a short span when the foil is developing lift.

I took Tom Speer's profile P30212 and compared it to NACA0012 in XFLR5 at Re 800 000:
So yes the Curves of the P30212 profile are maybe a bit "weird". (But I know too little to know how much weird curves matter.)

Then I made 3d foils of 2m by 0.4m. Which I thought would be close to the rudderfoils of a Harryproa in the 40-50 feet size.
Here they are, at CL = 0.8, and the resulting flowlines which shows vortexes in the ends, inducing drag.

Then I simulated the foils in water at 3m/s or about 6knots (About Re 800 000), which gave these results:
As you can see, the induced drag at high coefficients of lift (which I assume is necessary to counter the sailforces at low speed) is much larger than the viscous/profile drag. So the sum of them, the total drag, is very similar for the two profiles.

I also did another analysis where I assumed that the boat was sailing at the limit of righting moment. This should mean a constant force on the rudder, more or less. So I used a "Fixed lift" analysis in XFLR5. In this analysis the angle of attack is changed in steps, and the software calculates the speed required to reach the required force. I assumed the Harryproa would have 3000N or 300kg of force on each rudder in this case. This is the result:
The speedrange where the software reached a result became 6-30 knots. The plots are is similar to the above, but generally shows a lower coefficient of drag since the profile drag decreases as the Reynolds numbers increases (along with the speed). I think it's interesting to see the relationship between speed and angle of attach of the two profiles. At speeds above 12 knots, the cambered profile needs to be set to a negative angle of attack to prevent "windway" (negative leeway).

In hindsight, I think simulating a rudder aspect ratio of 5:1 was maybe too much. A rudder might have 3:1? And if I've understood Rick right, the rudders are 1:1 or 2:1 on the 18m boat he's involved in. But then the rudders have endplates in one end (the hull) on that boat, so the effective span should be double that.
Anyway, a lower aspect ratio would only further increase the induced drag, so the profile differences would be even smaller.
Another thing to mentions is that I haven't taken cavitation or suction of air from the surface (like mentioned in the emails) into consideration, simply because I don't know how to.
   
Lets me know what you think about this analysis!

/Björn


On Wed, Mar 20, 2019 at 9:37 PM '.' eruttan@yahoo.com [harryproa] <harryproa@yahoogroups.com.au> wrote:
 



| I'm not convinced a unidirectional uncambered foil has less drag than a cambered bidirectional. But I guess it requires some analysis. A cambered foil can develop a higher coefficient of lift, so can be smaller, which might balance the higher coefficient of drag.

http://www.basiliscus.com/ProaSections/Paper/ProaSections.htm
I am sure you have read the paper. Speer stopped his camber at 2%. Why do you imagine 4% would be better?

My read is Speer didn't seem too optimistic about his sections beating unidirectional. They only beat them in that they could work without flipping.
His pro3 has problems with balance at 50% cord. There are possible practical solutions as discussed in the paper, and, I thought, that is what Rob was working on.

None of Speers' lift to drags look to beat a good symmetrical foil, and the symmetrical wont have issues like weird balance, lift curves, and drag buckets. IMHO. Do you see it differently?

So it is reasonable, to me, that Rob develop a neat unidirectional system and a, probably more complicated, symmetrical rudder. One for cruisers, one to go fast(er). But it might be one system ends up way simpler in the wash.

| But it could also be that for hydrofoils this potentially higher coefficient is not that significant, because of effects like cavitation and suction of air from surface piercing foils?

Yep. I am still getting used to the ideas of foils in water. In air, a low pressure spot can cause a bubble of air to act differently than the surrounding air. In water, that can pull a air bubble all the way down the span from the surface. Which can totally change the expected performance. So they do cuffs and forward point and other tricks.

And then cavitation adds even more complication. You can only push the foil so hard. You really want to stay in the drag bucket. Or, you HAVE TO, if I am understanding things right.

But the old unidirectional foils are well understood, so I imagine the understanding and solutions are well tread.

| Regarding reefing, I think some kind of system with ropes and pulleys must be used on a Harryproa. Otherwise, how else would you reach to the end of the boom to attach the reef if the boat is stopped with the wind abeam? (And then, how to tie in the loose sail in that position?)

Seems obviously true now that you say it.

| Regarding tillers on 50' boats, it's not completely strange.
| Tiller is the proper way to steer a sailboat, imo.

All right. A few sailors have said that now. Then why are most all of Robs'/Steinars' renders have wheels?

Are wheels super sexy in some way?

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