Balsa vs. Basswood -- weight comparison (both between & within comparisons)

The_Lone_Beagle · Jan 9, 2015

I recently started substituting basswood for the balsa fins in my kits (it started when a kit came with balsa (or is it "tung")) that was so soft the wood flexed alarmingly!). I was about to build another kit this way, and decided to weigh the basswood, to see how much heavier/different it was from balsa.

The model this time was a Quest Big Dog...it has 4 large fins. It came with one-eighth inch balsa fins, so for this kit I also went with one-eight inch basswood (I likely could have used 3/32, but I wanted to make this weight comparison--I'll likely cut and weigh 3/32 fins soon as well). If you put 2 fins back to back (after having flipped one), it forms a parallelogram, with a height of 3.625 inches and a base of 6 inches, so 2 fins have an area of 21.75 inches, so for one fin A=10.875. Using a gram scale, the average weight of each basswood fin (more on this below) was 9.65 grams, so an average weight per sq.in. was 9.65g/10.875sq.in.=0.887 etc., let's round to 0.89 grams/sq.in.

This compared to balsa, where each fin averaged 5.5 grams (almost half the weight), so the same calculation yields 5.5/10.875=0.5057 etc., so we can round to 0.51 grams/sq.in.

These weight differences don't seem to be big considerations for mid-powered rockets. People with low power flights or wanting to achieve higher altitudes or compete in something likely would want to stick with balsa.

However, the "big" news was, regarding the basswood--I cut the 4 fins from two different sheets (2 from each sheet). Somewhat surprisingly, one set of fins averaged about 20% heavier than the other set (basswood sheet A fins avg = 8.75, basswood sheet B fins avg = 10.55).

So, the lesson here seems to be, cut fins from one sheet, and be critical of the quality of the wood, whether it is basswood or balsa. Again, for a mid-power build, a couple of grams here or there likely won't have a huge impact, but for low power and very low weight rockets, you may have to be more careful. I found this below quote from YORF, back from 2006:

"I am picky about my Balsa however, and scour all of the hobby shops I visit (even while on vacation and business) for ' C grain' balsa. I find C grain balsa to provide the best of both worlds: strength, low weight, and a certain toughness due to the swirly grain. (My wife says: "You fingered every piece of balsa in this case and you are buying just two sheets???")"

So, phew, I was starting to think I might run out of things to obsess about...

rharshberger · Jan 9, 2015

The variances in weight are normal to all (or nearly all) wood species, much of the variances are due to conditions where they grow, how rapid the growth, etc. I work with wood building furniture and other stuff and its a good idea to get the lumber/materials from a single piece or tree if a lot of material is necessary is necessary. BTW you can also see similar variances in plywoods, less so birch/cabinet grade woods and more so in builders grades. I have starting using basswood almost exclusively in my MPR builds (or making composites of basswood/plywood skins over balsa), or plywood for long narrow fins. As for HPR its a minimum 1/8th inch birch plywood. I reserve the balsa for LPR and non-stress parts on everything else.

dave carver · Jan 9, 2015

The_Lone_Beagle said:
I recently started substituting basswood for the balsa fins in my kits (it started when a kit came with balsa (or is it "tung")) that was so soft the wood flexed alarmingly!). I was about to build another kit this way, and decided to weigh the basswood, to see how much heavier/different it was from balsa.

The model this time was a Quest Big Dog...it has 4 large fins. It came with one-eighth inch balsa fins, so for this kit I also went with one-eight inch basswood (I likely could have used 3/32, but I wanted to make this weight comparison--I'll likely cut and weigh 3/32 fins soon as well). If you put 2 fins back to back (after having flipped one), it forms a parallelogram, with a height of 3.625 inches and a base of 6 inches, so 2 fins have an area of 21.75 inches, so for one fin A=10.875. Using a gram scale, the average weight of each basswood fin (more on this below) was 9.65 grams, so an average weight per sq.in. was 9.65g/10.875sq.in.=0.887 etc., let's round to 0.89 grams/sq.in.

This compared to balsa, where each fin averaged 5.5 grams (almost half the weight), so the same calculation yields 5.5/10.875=0.5057 etc., so we can round to 0.51 grams/sq.in.

These weight differences don't seem to be big considerations for mid-powered rockets. People with low power flights or wanting to achieve higher altitudes or compete in something likely would want to stick with balsa.

However, the "big" news was, regarding the basswood--I cut the 4 fins from two different sheets (2 from each sheet). Somewhat surprisingly, one set of fins averaged about 20% heavier than the other set (basswood sheet A fins avg = 8.75, basswood sheet B fins avg = 10.55).

So, the lesson here seems to be, cut fins from one sheet, and be critical of the quality of the wood, whether it is basswood or balsa. Again, for a mid-power build, a couple of grams here or there likely won't have a huge impact, but for low power and very low weight rockets, you may have to be more careful. I found this below quote from YORF, back from 2006:

"I am picky about my Balsa however, and scour all of the hobby shops I visit (even while on vacation and business) for ' C grain' balsa. I find C grain balsa to provide the best of both worlds: strength, low weight, and a certain toughness due to the swirly grain. (My wife says: "You fingered every piece of balsa in this case and you are buying just two sheets???")"

So, phew, I was starting to think I might run out of things to obsess about...

Same holds true for balsa. I've read of people going to hobby shops with a Ohas Quad Beam Scale to weight the balsa for specific weight for specific parts for free flight gliders, I think the plans were from MIT or something like that.

I'll use 1/16th bass in place of thicker balsa all the time. I still have a bunch of basswood from an order I got from AC Supply. I just got a good sized order of balsa from SEMROC in the 3 common thicknesses so I'm good there for a while. I just use Gorilla Wood glue on the basswood. The basswood is mostly used on my rockets I intend to go really fast

astronwolf · Jan 9, 2015

The_Lone_Beagle said:
"I am picky about my Balsa however, and scour all of the hobby shops I visit (even while on vacation and business) for ' C grain' balsa. I find C grain balsa to provide the best of both worlds: strength, low weight, and a certain toughness due to the swirly grain.

I raid hobby shops and crafts stores with a digital scale. I do look for the light c-grain, but I will grab just about any balsa that has a density < 5 lbs/cu.ft. I'll buy some of the super dense balsa too. The light balsa is great for composite components. I'm not a fan of basswood, and never find that I feel like I have to use it for anything. Anything basswood can do, balsa can do better.... like the song...

-Wolf

Level 1 - 3-inch Big Bertha on a Plasmajet H
NAR 28643
TRA 15520

TopRamen · Jan 10, 2015

astronwolf said:
I raid hobby shops and crafts stores with a digital scale. I do look for the light c-grain, but I will grab just about any balsa that has a density < 5 lbs/cu.ft. I'll buy some of the super dense balsa too. The light balsa is great for composite components. I'm not a fan of basswood, and never find that I feel like I have to use it for anything. Anything basswood can do, balsa can do better.... like the song...
-Wolf

Level 1 - 3-inch Big Bertha on a Plasmajet H
NAR 28643
TRA 15520

When I first got back into Rocketry for the whatevereth time a year and a half ago, I thought Basswood was just great, but now I use Balsa with lightweight FG Cloth, and don't see any reason to ever go back to Basswood.

RodRocket · Jan 11, 2015

If you really want the top grade balsa, might look for the contest grade balsa suppliers. For the indoor gliders and rubber power fliers. Higher costs but if you want to squeeze out every inch of altitude.

I like to use whatever I have available. Kit wood is usually fine. If I kit bash into a two stager, then I will build the booster with basswood. Little bit tougher for the tumble recovery.

Winston · Jan 11, 2015

Balsa density gradations:

Balsa less than 8 lb/cu.ft density is usually considered “soft” or “contest grade”
Balsa from 8 – 12 lb/cu.ft. density is usually considered “medium”
Balsa over 12 lb/cu.ft density is usually considered “hard”

TheAviator · Jan 12, 2015

Winston said:
Balsa density gradations:

Balsa less than 8 lb/cu.ft density is usually considered “soft” or “contest grade”
Balsa from 8 – 12 lb/cu.ft. density is usually considered “medium”
Balsa over 12 lb/cu.ft density is usually considered “hard”

I got a piece of balsa from a distributor once that was 18lb/ft^3. The stuff was so tough that it was impossible to cut with a #11 blade. I had to use a miniature table saw and a box saw.

The only disadvantage that I've ever had from bass is that it tends to split relatively easily. For structural members that need more impact resistance or fatigue resistance, such as wing spars and glider booms, I use spruce.

Daddyisabar · Jan 12, 2015

For a poor, lazy cheapskate like myself, I always try to get the scraps of squishy balsa from TLP kits the other guys toss out. Then some self stick mailing labels may follow me home from work. CA up those square leading edges and Yippee! Light and strong. They ain't be breakin' till I really crash it on that motor from the garage sale someone gave me.

No, no. Old School is the way to go. At least three good coats of sanding sealer. On the V2 fin go composite with a core of thin Baltic Birch sandwiched by shaped and embossed balsa with dope. All natural fiber will keep you regular

TopRamen · Jan 12, 2015

Daddyisabar said:
For a poor, lazy cheapskate like myself, I always try to get the scraps of squishy balsa from TLP kits the other guys toss out. Then some self stick mailing labels may follow me home from work. CA up those square leading edges and Yippee! Light and strong. They ain't be breakin' till I really crash it on that motor from the garage sale someone gave me.

No, no. Old School is the way to go. At least three good coats of sanding sealer. On the V2 fin go composite with a core of thin Baltic Birch sandwiched by shaped and embossed balsa with dope. All natural fiber will keep you regular

I've been given a pile of TLP Balsa by cavecentral, and it is quite nice in my opinion. Likewise the TLP Balsa in the Kits I've baught has been nice too.

The_Lone_Beagle · Jan 12, 2015

Daddyisabar said:
Then some self stick mailing labels may follow me home from work

That made me laugh...

Another reason I was trying out basswood was that it sealed a lot quicker & easier using the carpenters wood filler, than balsa. I may still try out the sanding sealer with the balsa fins, when I can work with the sanding sealer outside (to avoid fumes).

Daddyisabar · Jan 13, 2015

TopRamen said:
I've been given a pile of TLP Balsa by cavecentral, and it is quite nice in my opinion. Likewise the TLP Balsa in the Kits I've baught has been nice too.

Nothing wrong with TLP balsa, but as long as some folks think it is crap I too will be glad to take it off their hands, along with anything else in kit they don't like.

neil_w · Oct 12, 2015

Sorry to resurrect and old thread but hey it's not *that* old.

I just bought some 3/32" contest-grade balsa for my upcoming Diamond Cutter build, and was astonished at how soft and flexible and spongy it is. I guess I had become accustomed to the feel of the more rigid tung wood in the recent Estes kits. Anyway, I'm planning to paper the fins with self-stick label paper, but still am suspicious of the balsa. Should I have any concern about the strength of papered 3/32" balsa for my fins? Alternatives would be 3/32" basswood (though I'd prefer not to add the weight), or I could even go to 1/8" balsa, which would add less weight.

The rocket will fly on D's and E's and maybe F's if I get brave, so I'm not really up to mid-power construction but it does need to be reasonably strong.

Thoughts?

rharshberger · Oct 12, 2015

I thought "contest grade" balsa was the lightest and weakest variety, meant for projects like free flight aircraft. I could be very wrong on that though.

neil_w · Oct 12, 2015

I think you're probably right. My reading on which kind to get was probably polluted by model aircraft sites.

Michael's sells Revel balsa, but I don't think they're specific about the weight. It seems stiffer but I dunno.

The_Lone_Beagle · Oct 12, 2015

Depending on how adventurous you are, you could try it. I would think about reinforcing the balsa somehow, like using a stiffer wood (basswood? harder balsa?) spar on the edges of the fin, and then using a strip of balsa as a fillet (i.e., glue an extra bit to both the body tube and side of the fin, and then sand it down to shape it like a glue fillet).

Of course this would be a bit of extra work, and you would arguably be better off getting the other balsa and not pouring a ton of effort into the fins like this.

samb · Oct 12, 2015

neil_w said:
Sorry to resurrect and old thread but hey it's not *that* old.

I just bought some 3/32" contest-grade balsa for my upcoming Diamond Cutter build, and was astonished at how soft and flexible and spongy it is. I guess I had become accustomed to the feel of the more rigid tung wood in the recent Estes kits. Anyway, I'm planning to paper the fins with self-stick label paper, but still am suspicious of the balsa. Should I have any concern about the strength of papered 3/32" balsa for my fins? Alternatives would be 3/32" basswood (though I'd prefer not to add the weight), or I could even go to 1/8" balsa, which would add less weight.

The rocket will fly on D's and E's and maybe F's if I get brave, so I'm not really up to mid-power construction but it does need to be reasonably strong.

Thoughts?

I say, no, you should not be concerned with the strength of papered balsa fins. I've found that any kind of lamination adds a surprising amount of strength. Let me add that I have never used self-stick label paper but many TRFers have. I've done it several different ways over the years, because I like to try new things and I'm easily distracted:

- regular printer paper and Elmer's white glue, placed under a book while drying to prevent warping

- model airplane tissue and dope

- white glue and used dryer sheets (very strong but required alot of primer and sanding to get smooth, only tried this once)

Something I did on a stretched Fatboy is add square hardwood stock to the leading and trailing edges. Adds durability to those surfaces and strengthens the fin span-wise. [edit] Just like The_Lone_Beagle suggested. :cheers:

How about this: build one with papered balsa and one with basswood fins ?

EXPjawa · Oct 12, 2015

These days, I only use store-bought balsa when papering. I can't seem to find anyone carrying a grade similar to what Estes uses, its either soft or medium soft. But I've had that fail when papered, so that helps but isn't a cure-all. Anymore, I just use basswood or light ply. I think in some cases I can get away with a thinner stock when using basswood, and still have sufficient strength.

As a tangent, what I really like to know is how basswood compares (generally speaking) to light aircraft plywood. It seems like that invariably, the ply has to be stronger, and by rights has to be heavier. However, the data in Rocksim suggests that basswood is heavier. I have not, however, tried to weigh comparable parts to see.

neil_w · Oct 12, 2015

samb said:
I say, no, you should not be concerned with the strength of papered balsa fins. I've found that any kind of lamination adds a surprising amount of strength. Let me add that I have never used self-stick label paper but many TRFers have.

I guess I'll just build a fin from the existing sheet, paper it, and see how strong it feels. If satisfactory, I'll continue with that balsa. Otherwise, I'll resort to a stronger wood (even if that's just a denser piece of balsa). It's easy enough to cut a fin and try it out. I need to practice my papering technique anyway.

Something I did on a stretched Fatboy is add square hardwood stock to the leading and trailing edges. Adds durability to those surfaces and strengthens the fin span-wise.

That is an interesting thought. More work but not bad. On the other hand, for the cost of a new sheet of balsa I'm not sure how much effort it's worth.

How about this: build one with papered balsa and one with basswood fins ?

Ha. For what this build is costing me I think one will be enough, thank you. :bangpan:

samb · Oct 12, 2015

neil_w said:
...

Ha. For what this build is costing me I think one will be enough, thank you. :bangpan:

OUCH ! I withdraw the suggestion.

Micromeister · Oct 12, 2015

I had a Missionary friend in Costa Rica who would harvest and send me a Balsa tree every so often. He and his Parrish would cut the wild balsa trees, saw them into large board blocks about 5feet long, sell most to the local Balsa Shipping company market saving one to box up and mail it to me. He's moved on to another assignment now

but I'm still working on the last two trees. Most of the time these trees don't get any bigger around the 7-9inches. In the Last box his letter said it came from one of the largest diameter balsa trees (10"dia.) anyone in the village had seen in decades.
This was pretty KEWL as it gave me an enormous amount of different grade balsa to work with. Some Contest C-grade 8lb beautiful boards, lots of the 8-11lb medium weight in A & B Grade, and a fair amount B-Grade really heavy thick stuff 12lb/cu-ft PLUS.
This 12lb plus stuff is the planking used in the ballast holds of most cargo ships and why it's become a bit harder in the last few years to get good grade balsa as this Heavy stuff (along with the lighter grades) has not be as available as normal due to weather and increased world wide shipping company demand.

Contest Balsa 8lb/cu-ft or less, cut C-grade is the lightest, softest and most flexible balsa out there, used for contest BG, RG, HD and rocket fins on PD & SD birds. In Contest use it is rarely papered. That said many FAI and BTC competitors have been know to compress C-Grade 1/32" contest balsa between two very thin layers of Carbon fiber cloth to make ultra light weight super thin and strong fin and rotor material.

For sport flying models really the 8+ to 11 lb is just fine. If your concerned about larger heavier models with balsa. Papering, or Card-stock laminating is an option. I've even seen Balsa Plywood lay-ups that seem to be considerably stronger then single thickness balsa fins. Or another option is c-grade 8lb or less cores with 1/64" of 3/64" ply skins. I've made some very large light weight fins with this method.

Personally I like Basswood for Sport flying models because of it's tighter grain, which makes for a very strong fin that doesn't soak up CA or Primer as much making the end product only slightly heavier then the same size and thickness Balsa fin. It is possible to use a bit thinner Basswood in place of Balsa. if the kit comes with 3/32" balsa 1/16" Basswood would be a perfect replacement without the need for the extra work of papering the fins.

Balsa-b_Balsa Trees of Costa Rica 2-pic (128dp)_02-06.jpg

mkadams001 · Oct 12, 2015

This company is in Loveland, CO. I have visited his shop and he has a lot of balsa. https://www.specializedbalsa.com

neil_w · Oct 12, 2015

I picked up a sheet of 3/32" Revell balsa at Michael's just now, and it is *so* much more rigid than the other stuff it's not even funny. I am not worried about it at all, especially if I do end up papering it. So I'll use it for this build, and save the other sheet for something smaller and less critical.

Oh, and since (as it turns out) they had a 1-item 50% off coupon in effect just for today, I picked up a pack of C6-5's at the same time for $7 after coupon. So it all worked out for the best...

neil_w · Mar 23, 2016

I just weighed a 1 in square piece of my Revell balsa and it came out to .6 grams.

By my calculations, that calculates to a 22 lb/ft^2. To which I say: holy crap, no wonder I'm killing myself trying to cut the stuff. But it sure is strong! :eyepop:

So what is an *appropriate* weight for balsa fins that will be papered? I'm getting fins laser cut and I suppose I should be specifying the balsa weight that I want. If I'm not cutting it myself then I suppose it can veer towards the hard end of the scale, but what's "normal" here?

My two balsa sheets are so far to the ends of the scale that I don't have a feel for where in the middle I *ought* to be...

[edit]Never mind, I see Micro answered this question above. I'll shoot for something in the 10 lb density or thereabouts. One of these days I'll try basswood.

tab28682 · Mar 23, 2016

True contest grade balsa is considered to be 4-6 lb stock and not 8 lb stock.

Few non competition models would ever really need contest grade balsa. Heavier stiffer wood in the 8-12 lb range is great for most LPR uses.

Basswood can be found in weights close to balsa weights, but it is not common.

TopRamen · Mar 23, 2016

Here's some pics, of two identical sets of fins, one balsa and one basswood. Both before and after getting a layer of lightweight glass cloth.
Even after the glass cloth, the balsa is lighter than the basswood without it.

3g. vs. 11g.

7g. vs. 15g., before cleaning them up with sanding.

lcorinth · May 13, 2016

The fins on my Big Dog were pretty hard and dense, I remember. I didn't weigh them - it hadn't yet occurred to me to do that. Now I weigh everything, and compare the combined weight with the weight of the finished model.

I also use basswood sometimes, and recently did a build up that I really liked with a 1/16 inch basswood core and 1/32 inch balsa fins. It was nice and tough, and because of the difference in color of the layers, I was able to free-hand sand a radial taper. I had run out of everything but the thin basswood and some 1/32 inch balsa I'd bought on accident, and thought I'd never find a use for it.

I also recently weighed a few pieces of balsa I had, and was surprised by how much different they were. All boards were 1/8 inch thick, 3 inches wide, and, I think, 36 inches long.

The piece I'd gotten from Menards or Michael's (I forget which it was) was pretty soft, and weighed somewhere between 20-23 grams. The C-grain I got from SIG Manufacturing was over 29 grams. And the Very Hard Balsa from SIG was over 70!

The really light stuff I don't like so much, because I can't even make a pencil mark on it without denting it. There's more of a dent than a line.

As for filling grain, I'm still trying to refine my technique. I used CWF for everything for a while, but got frustrated with it because I'd inevitably get pock marks in it from air bubbles, and wouldn't notice I hadn't sanded them out until I put on primer. Since I often do tapered edges, I'd get warping, even if I carefully did both sides evenly and pressed them between books and waxed paper (since there's a bit of wiggle room between a tapered edge and a the flat surface I'm using to press the fins), and I would find fine dust everywhere in the room.

I've been trying sanding sealer as well, and I like it, but I find I can sometimes still see wood grain. I'm not sure if I'm supposed to be able to sand all that out one the sanding sealer is on (I know it seals the pores rather than fills in the grain), or if I need to do a few more layers of filler primer and sand that smooth.

I'm OK at papering if I leave the fin edges square, but I usually don't, and on rounded or tapered edge fins, I always kind of mess up the fin when I paper, as I try to sand off the overhanging paper edge.

I've also tried wood glue thickened with either flour or talcum-based baby powder. The flour frustrates me, because I find it really hard to sand, and must have sanded off a chunk last time I did it, because when I lifted up the sanding block, I found a weird gouge in the surface of the fin. I must have rubbed a chunk around on it with the sanding block.

The wood glue and talcum is pretty good. Not easy to sand, but easier than with the flour, makes a stronger fin, and pretty smooth surface. I have only tried it on scrap balsa and one rocket (which I screwed up in another way), but it looks promising. Probably adds a lot of weight, and is too labor intensive for every rocket, but I'll probably do it again.

TopRamen · May 13, 2016

lcorinth said:
The fins on my Big Dog were pretty hard and dense, I remember. I didn't weigh them - it hadn't yet occurred to me to do that. Now I weigh everything, and compare the combined weight with the weight of the finished model.

I also use basswood sometimes, and recently did a build up that I really liked with a 1/16 inch basswood core and 1/32 inch balsa fins. It was nice and tough, and because of the difference in color of the layers, I was able to free-hand sand a radial taper. I had run out of everything but the thin basswood and some 1/32 inch balsa I'd bought on accident, and thought I'd never find a use for it.

View attachment 290893

View attachment 290894

View attachment 290895

View attachment 290897

View attachment 290898

View attachment 290896

I also recently weighed a few pieces of balsa I had, and was surprised by how much different they were. All boards were 1/8 inch thick, 3 inches wide, and, I think, 36 inches long.

The piece I'd gotten from Menards or Michael's (I forget which it was) was pretty soft, and weighed somewhere between 20-23 grams. The C-grain I got from SIG Manufacturing was over 29 grams. And the Very Hard Balsa from SIG was over 70!

The really light stuff I don't like so much, because I can't even make a pencil mark on it without denting it. There's more of a dent than a line.

As for filling grain, I'm still trying to refine my technique. I used CWF for everything for a while, but got frustrated with it because I'd inevitably get pock marks in it from air bubbles, and wouldn't notice I hadn't sanded them out until I put on primer. Since I often do tapered edges, I'd get warping, even if I carefully did both sides evenly and pressed them between books and waxed paper (since there's a bit of wiggle room between a tapered edge and a the flat surface I'm using to press the fins), and I would find fine dust everywhere in the room.

I've been trying sanding sealer as well, and I like it, but I find I can sometimes still see wood grain. I'm not sure if I'm supposed to be able to sand all that out one the sanding sealer is on (I know it seals the pores rather than fills in the grain), or if I need to do a few more layers of filler primer and sand that smooth.

I'm OK at papering if I leave the fin edges square, but I usually don't, and on rounded or tapered edge fins, I always kind of mess up the fin when I paper, as I try to sand off the overhanging paper edge.

I've also tried wood glue thickened with either flour or talcum-based baby powder. The flour frustrates me, because I find it really hard to sand, and must have sanded off a chunk last time I did it, because when I lifted up the sanding block, I found a weird gouge in the surface of the fin. I must have rubbed a chunk around on it with the sanding block.

The wood glue and talcum is pretty good. Not easy to sand, but easier than with the flour, makes a stronger fin, and pretty smooth surface. I have only tried it on scrap balsa and one rocket (which I screwed up in another way), but it looks promising. Probably adds a lot of weight, and is too labor intensive for every rocket, but I'll probably do it again.

Those Fins are beautiful!:clap::clap::clap:

lcorinth · May 14, 2016

TopRamen said:
Those Fins are beautiful!:clap::clap::clap:

Thank you! I'll eventually post a build thread. I don't know where I got the idea. I know some kits do three-layer fins, but I've never built any like that. It was just something I came up with out of necessity, and had an unexpected benefit.

tab28682 · May 19, 2016

I have done a few models over the years with laminated fins. 1/64 ply and 1/32 ply in between 1/16 and 3/32 balsa or basswood works great and gives that nice visual reference when air foiling and thinning the fins.

Balsa vs. Basswood -- weight comparison (both between & within comparisons)

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