Sitka VS ash for ice boat a question of strength vs weight

[Woodrow]

If any would know, boat bulders would.I have plans to buld a ice boat(iceflyer type)and it is a known fact that sitka spruce is best, but costly. I have axcess to white ash ,local mill run #1, and dry.As near as i can figure the ash weighs up about 25% more than sitka.Most of the lumber for boat is Glued up 3/4 inch sitka.I am wondering if i could go whith 5/8 ash to save wt.and still be strong.

[Bruce Hooke]

I'm no iceboat expert, but I ran some quick numbers in a spreadsheet I use for calculating beam strength and here is what I found.

For the ash member to have the same strength as sitka spruce it would need to be 0.65" thick (about 21/32" or a bit over 5/8") to replace a 0.75" thick spruce member. If I've done the math right, this would make the ash about 30% heavier than the sitka spruce.

HOWEVER, for the ash to have the same stiffness as the spruce the ash would need to be 0.72 thick (about 23/32") to replace a 0.75" thick sitka spruce member. If I've done the math right, this would make the ash about 45% heavier than the sitka spruce. Ouch.

So, since stiffness is likely to be as important as strength in many places you would probably need to go with 23/32" ash, which is basically the same as not changing the dimensions at all. I'm sure the boat would sail, but it would be a good bit heavier than the design weigth and thus no race winner.

For the record, the density of white ash is about 37 pounds/cubic foot, whereas the density of sitka spruce is about 25 pounds/cubic foot.

A better solution might be to use lumberyard spruce, rather than sitka. If I've done the math right you would probably need to go with roughly 13/16" thick "local" spruce, but a lot depends on the quality of the spruce you can find at the lumber yard. This would probably only mean a weight gain of around 10%...

Hope that helps.

[Paul Pless]

Bruce Hooke,

What reference do you use to find those numbers. I am interested in adding that book to my library.

Paul

[Bruce Hooke]

Originally posted by Paul Pless:
Bruce Hooke,

What reference do you use to find those numbers. I am interested in adding that book to my library.

PaulThe raw data for the calculations came from two sources, Understanding Wood by R. Bruce Hoadley, and The Encyclopedia of Wood by the Forest Products Laboratory of the USDA. The former (Understanding Wood) is an excellent book for understanding this sort of stuff and had pretty much all the numbers I needed except for the density information, which is available from various sources including the Internet. The latter goes into a lot more depth and includes "numbers" for many more wood species, but it does not do nearly as much to explain the data. I believe both are still in print but I think The Encylopedia of Wood may have a different title now. I would recommend starting with Understanding Wood -- this is a book that I think should be in any woodworker's library.

One word of caution -- the sort of comparison I did in my previous post is reasonably straight-forward using the information and formulas in Understanding Wood. Where things get trickier is for "quantitative" calculations -- meaning how much weight can a piece of wood of a given size and species support and how much will it deflect under a given load. The numbers in these books are based on small, clear, straight-grained samples -- in other words, perfect pieces of wood. The wood we use is generally not that perfect so some adjustments need to be applied along with a safety factor. For woods of a species and grade that is commonly used for buildings these values are fairly well established via the building codes (assuming the safety factor used in the building codes is suitable for the task at hand). For the woods boatbuilders and cabinetmakers commonly use things get a lot messier. The building codes do list values for hardwoods, but they are ridiculously low in many cases, I think because hardwoods are generally not strength graded (they are just graded on appearance), so the codes must assume the worst. By way of example, if one tried to use the quality of wood the building code assumes for white oak, the ribs would all break when you tried to steam bend them in place! So, to get to the point, for calculations of actual strength a fair amount of judgement as well as a variety of additional references are necessary.

Also, while Understanding Wood gives a very nice explanation of the strength of simple beams loaded in the middle by a "point load," it leaves it there and does not go into the permutations that come up in the real world, such as cantilevered beams, uniform rather than point loads, off-center loads, round beams, etc. At this point you are really moving into the engineering realm -- my reference for this sort of thing is my grandfather's Mechanical Engineer's Handbook!

[Paul Pless]

Thank you for the reply and explanation. I will be looking for these books soon. One of my weaknesses tends to be a prediliction for book collecting.

Its funny you should mention, Mechanical Engineer's Handbook , I also use my father's copies of this wonderful reference book.

[Dan McCosh]

With the exception of the runner plank, most of the loads in an iceboat end up being compressive, much like a mast. The cross section ends up the critical dimension, and the weight gain from going to ash is considerable. That said, weight is mainly an enemy for acceleration, rather than top speed, as the runner friction itself doesn't change proportionately to the weight. Current DNs, for example weigh about a third of what the older ones did, and while you wouldn't win a race, the old ones were still fast. Actually, clear white pine or cedar also make a reasonable substitute in the hull. The runner plank is much different, as it is an unsupported beam that needs to flex and support weight. There I'd stick to sitka spruce.

[Bruce Hooke]

A very good point Dan. I, without really thinking about it, assumed beam loading. Compression loading is a very different story and, as you noted, the overall cross-section of the member is all important with compression loading.

[Bob Smalser]

I'd be looking for a local sawyer with airdried Black Spruce of whatever spruce species is cheap. I'd high-grade it on the tablesaw, cutting out all the big knots...then epoxy-scarf and laminate it all together to make my big beam.

Saving several dollars a BF is probably well worth the additional 80-dollar gallon or so of epoxy.

Sitka is too expensive even here where it grows....and clear mast-grade stock is 8 bucks a BF. I've used old-growth WR Cedar as a substitute for oars by upping the scantling size a tad, but I believe you would be much better served by spruce and it's stiffness.

[ 12-10-2003, 04:12 PM: Message edited by: Bob Smalser ]

[Woodrow]

Thank you Bruce and every one else.Bruce you are a real smart gie,can you run me some more numbers?I called most of the lumber yards here local.and thay were not much help.How about clear #1 red pine(norway pine)?White pine? Aspen? all these types of wood can be had real easy in NW Wis.Dan has a point in that once its moving who cares?.Woodrow.

[Todd Bradshaw]

Unless you're seriously class racing, a few extra pounds isn't a big deal. Many of the older home-built D.N's that we see around here are overweight compared to the newer boats and their sailors still seem to be having plenty of fun - even if they aren't finishing in first place. The same with Renegades and other classes. Considering how long a properly constructed iceboat will last, sitka is possibly worth the extra money, but people are using cheaper woods as well as plywood-on-frame construction for bodies and laminated hollow-cored runner planks. I saw a Nite a couple years ago with a laminated ash plank and it was doing just fine.

Construction of the runner plank and springboard are the critical parts. The body can pretty much be a box. It's just as important that the plank and springboard flex enough as it is that they be stiff enough, and that's where coming up with a new, untested lamination or construction plan gets tricky. If the plank is too soft, it flexes in use and will actually belly-out and drag on the ice - with obvious potential concequences. If the plank and springboard are too stiff, the ride is rougher, the runners spend less time making good contact with the ice and these make the boat slow. In general, if you sit in the boat (not moving) the plank and springboard should flex enough to flatten out any crown that's built into them, or possibly sag just a little bit. At speed they will sag even more, but shouldn't drag on the ice.

Replacing solid spruce components with solid ash ones may not be the best all-round bet, but would probably work. You could start with the full dimension (or even a bit more) on the plank and springboard then test-fit and shave them down until they flex properly.

Before building any iceboat I would buy two things as reference material (which for iceboats is rare). I would go to the IDNIYRA (D.N. Class Association) website
http://sailingsource.com/ice/index.htm
and their publications section and order the book "Think Ice" ($15). Then I would check Gougeon Brothers to see if they still sell plans for their version of the D.N., which at least used to sell for about $25. Even if you have no intention of ever owning or building a D.N. these will both be quite valuable in showing you things you need to know and which will help you build and sail any iceboat. "Think Ice" covers safety, right of way rules, ice quality, sailing techniques and other stuff. Naturally, it's from a D.N. sailor's perspective, but much of it applies to any iceboat. It's also got enough info on a stretched, side-by-side two seat version (with a 10' plank and a springboard and which would make a great recreational boat) to figure out how to build one.

The Gougeon plans will show you how they build and reinforce the hull and runner planks of top competition boats. The plank construction plan alone is worth the cost of the set. Personally, I find D.N. a bit cramped and uncomfortable to sail, but they are the standard for home-built iceboats around the world and understanding their construction will help you build almost any iceboat. Why mess with the small stuff. Here's a nifty boat from Europe called the "Ice Zeppelin"
http://www.imagestation.com/picture/sraid92/pf2bb1b54a5c3436a648b1d99be761663/fa552c38.jpg
I think this guy nust have owned a sail batten factory....

[Wayne Jeffers]

Originally posted by Bruce Hooke:
. . . I believe both are still in print but I think The Encylopedia of Wood may have a different title now. . . My 1987 edition is titled The Encyclopedia of Wood. I think I bought it from the WoodenBoat Store. It may still be available there.

The 1999 edition is available on-line under the title Wood Handbook at http://www.fpl.fs.fed.us/documnts/FPLGTR/fplgtr113/fplgtr113.htm I'm not sure if the printed version carries this title, but it appears so.

Wayne

[Woodrow]

Thank you Todd . I think I'll go whith the zepplin.(just kiding)I have a mast from a wind surfer board and the Iceflyer boat hooks right up.I hope Bruce can give me more #.Where do you get Sitka near Mad Town my brother in law lives there,Iam a cheap bastard but will spring for good materals if i need to.Are you familiar whith the Ice flyer?Any thoughts?(iceflyer.com) Woodrow

[Todd Bradshaw]

Ice Zeppelin Explodes at Mooring Mast due to Wood Substitution - Oh the Humanity!!!

Here in town we can buy sitka spruce from McCormick Lumber on Milwaukee St. (608) 244-4741. The McCormicks are very serious iceboaters. How many regular midwestern lumber yards do you find that stock sitka spruce and WEST epoxy?

I've been keeping an eye on the Ice Flyer for several years and have seen at least one sailing here in town. It certainly wasn't having any trouble moving but he had already been out for most of the morning and went home before we got our boat assembled so we didn't get to do any close inspection or speed testing. The idea of sitting upright is a plus, to be sure. When wearing a helmet, your head is pretty heavy. Sailing from nearly flat on your back, as is the case in most boats is really tiring on your neck. I actually know some Nite sailors who attach a bungie-cord to their helmet and then to the deck to hold their heads up!

Small, highly manueverable iceboats are a lot of fun to sail, even though iceboating tends to be more of a long-tack sport. A little boat that tacks and jibes quickly, and in a small space, can be a blast. We used to own a Lockley Skimmer 45 that was crude by any standard, but would turn on a dime. It was quite fun, even though it wouldn't keep up with the big boys speed-wise. The entire "hull" was made from welded steel tubing and didn't flex at all (talk about a rough ride on bumpy ice) so an Ice Flyer should ride much more comfortably.

I liked the upright seating concept enough that I borrowed it along with features from a couple of my favorite iceboats a few years ago and came up with this small-to-mid-sized design. Hopefully one of these years I'll actually get around to building a prototype to see if it works. Spruce plank and springboard/keel piece probably running all the way back to the plank with a stitch-and-glue plywood body over frequent bulkheads and D.N. runners. My problem is always trying to get motivated to work on an iceboat in the summer when it's warm enough in the garage to build something. Somehow it just seems like insanity when you're sweating.
http://www.imagestation.com/picture/sraid50/pd33ae2c611a453951f8a8e81411e8111/fcab2b9a.jpg
http://www.imagestation.com/picture/sraid50/p407842031d2841b7fd9604b1d9be9b29/fcab2e6f.jpg

[Bruce Hooke]

Originally posted by Woodrow:
Thank you Bruce and every one else.Bruce you are a real smart gie,can you run me some more numbers?I called most of the lumber yards here local.and thay were not much help.How about clear #1 red pine(norway pine)?White pine? Aspen? all these types of wood can be had real easy in NW Wis.Dan has a point in that once its moving who cares?.Woodrow.I can try to run a few more numbers when I have a few minutes, but first I need to confirm one point. Are the pieces in question loaded as beams (i.e., in bending) or are they loaded in compression (like a column or a mast) or in some other complex way, because that would change everything.

Todd raises a very good point -- some parts have very critical parameters so some experimentation may well be necessary if you want to substitute woods. It is also important to sort out which parts are critical in terms of strength and flexibility and which are just necessary to make things work (such as the cockpit). The latter can, of course, be made from almost anything that is handy.

It seems to me that there are really only three key, large structural wood members - the plank, the springboard and the mast - everything else is either small parts, or basically non-structural, such as the cockpit. I am guessing it is the plank and the springboard that you want to substitute materials on, right? Are the dimensions for these pieces fairly uniform or do that vary along the length? If they are reasonably uniform what dimensions are specified in sitka?

[Woodrow]

Bruce,the boat looks alot like the one Todd shows so this is good ref photo.The rear beem on Ice flyer is 9ft long and looks like a air foil 11.25inch X 1.937inch,it is made from 3 pcs glued up whith a 1.5inch crown in center so when a person sits in it it sqwishes down.The front spring board is 78in long X 5in X 1.5in whith a x-tra .75 X 20in board glued on top where it enters the box .So all togeather its 9ft wide x 140in long total.42in of this length is the 5X1.5 sticking out for front runner.Spring board and rear beem do not vary in cross section. Todd I will call on sitka in Mil. lumber yard.Thank you so much.woodrow

[alteran]

Woodrow, I'm in northern Wisconsin also and when I get done building a couple canoes I was also thinking of building an ice boat with a mast and sails from a windsurfer. Have you looked for wood in the Twin Cities? There are some good sources there. I'm going there tomorrow to to buy epoxy and stuff.

Let me know if you would like to compare notes.

Al.

[Woodrow]

Alteran,,, all I have done so far is copy the patterns down and made full size paterns for boat.I called Madison today and sitka is selling for $8.50 a foot.This boat may weagh up big when its done.Canyou give me a name+ number for wood in Mn.? Woodrow

[Todd Bradshaw]

OUCH! One would think that spruce is made from beef! Anyway, that's the reason I suggested getting the Gougeon D.N. plans. Their runner plank is made with a handful of skinny sprice strips and small blocks, sandwiched between layers of birch veneer (door skins?). It's a fairly complex thing to build, but makes one of the best planks available without having to buy big expensive slabs of sitka to laminate. Fairly similar constructions have been used for building both downhill and cross-country skis (which in use need the same kinds of flexible strength) for a long time using various woods for core stringers, including poplar. Here is a scan of part of the Gougeon plank layup from their plans.
http://www.imagestation.com/picture/sraid92/p9fbae77d15e84a45d474886a51ad2c04/fa533d26.jpg

[alteran]

Lumber places in Minnesota for you.

Scherer Bros. Lumber Co.
651-633-9680
Arden Hills

Siwek Lumber
612-781-3333
Minnneapolis

Root River Hardwoods
Preston
888-809-6637

Scherer should be able to steer you to others that can supply what you need if they don't have it.

Al.

[ 12-11-2003, 05:30 PM: Message edited by: alteran ]

[Bruce Hooke]

OK, I ran a few numbers and here is what I came up with. First off the assumptions:

1. As a "test piece" I used the vertical load on the rear beam. This obviously ignores the horizontal load on this beam, and adjusting the sizing for the horizontal load would of course impact the vertical load capacity and deflection, but trying to address both directions at once is more than I am up for. So, the results of my calculations should just be viewed as general guidance rather than hard numbers because you clearly need to change both the width and the depth on the real boat. I ran my numbers on a rectangular beam. Trying to find the numbers I would need to run these numbers on an airfoil is more than I am up for (what's the section modulus for the airfoil in question! :D You mean you don't have that handy!). I don’t think the results would be that much different in any case.

2. I assumed that all of the wood would be of about the same quality in terms of straightness of grain and lack of knots. These parts are too highly stressed to use anything but the best pieces of whatever wood species you use.

3. In the analysis that follows I use the terms "strength controls" and "deflection controls". What I mean by this is that I tried to match the deflection to sitka unless that would make the piece weaker than sitka, in which case I increased the size to match the strength of sitka. Of course we don't know if the original design was based on getting the strength right or the diflection right. I'm guessing the latter and that the strength is probably more than necessary but I did not want to go with a weaker beam without being sure.

3. I am not an engineer, just someone who has spent some time messing around with beam calculations and suchlike as applied to wood. So, it's quite possible that I've made mistakes. So, take this all with a large grain of salt!

With that out of the way, here is my take -- all relative to sitka:

Ash:
- Deflection controls so the member will be stronger than sitka in order to keep the deflection from being more than sitka.
- The height (thickness) can be 96% of sitka.
- The weight ends up being 144% of sitka.
- Strength ends up being about 123% of sitka.
- As discussed before ash would probably work but it would make for a heavier boat by a good bit.

Eastern White Pine:
- Strength controls but by so little that it hardly matters. So, the strength and deflection are both about the same as sitka.
- To get there however, the beam needs to be 108% as high as the sitka beam.
- The weight ends up being 95% of the sitka.
So, at the expense of making the beam a good bit bigger you end up with a slightly lighter beam.

Western White Pine:
- Strength controls so the beam will flex less than sitka.
- The height needs to be 104% of sitka to get the same strength.
- The deflection is about 93% of sitka.
- Weight ends up being about 99% of sitka.
- So, at the expense of increasing the size you end up with a stiffer beam that weighs about the same. Of course a stiffer beam may not be desirable because it would give you a harder ride.

Red Pine:
- Deflection controls so the beam ends up being a bit stronger than sitka to get the same deflection.
- The height needs to be 99% of sitka.
- The strength is about 105% of sitka.
- The weight is about 114% of sitka.
- So, red pine gives you a stronger but heavier beam with little change in size.

Quaking Aspen
Note: Quaking aspen and bigtooth aspen have fairly different numbers and both are sold under the general name “aspen” so you don’t know which you are getting. So, I used quaking aspen because it is weaker.
- Deflection controls so the strength is greater than sitka.
- The height has to be about 110% of sitka to get the same deflection.
- The strength ends up being about 105% of sitka.
- The weight is about 104% of sitka.

So, what can we make of all this? In terms of best performance (short of sitka) eastern white pine is rather promising because at the expense of increasing the size you actually get a lighter boat. BUT, the size change is bigger than any of the other options listed, which greatly magnifies the effects of also needing to adjust the width. It also means more changes to the plans for the boat and more possible problems with everything fitting together and working as designed. So, I gravitate towards red pine because, while it ends up being a bit heavier, the dimensions can stay about the same as for sitka. This is nice both because it makes the plans easier to follow and because it gets around all the messiness of trying to adjust for both the horizontal and vertical loads. Also, it means that the strength factors like column loading and torsion are more likely to be similar to sitka (although I did not check this).

Of course, all of that said, it must be remembered that there is a lot of variation between individual boards and defects in a board can have huge impacts on the strength of a member. So the numbers listed above almost certainly give a false sense of precision!

[Woodrow]

Thank you. Red pine it is.There is a lot of red pine here. I can see a 70footer frome here and the first limbs starts 2/3 up the tree.Now to shop for wood.Woodwow.

[alteran]

Thank you Mr. Hooke. I have a few red pine logs laying out here ready to saw and was wondering what I should make from them. Now I know.

Al.

[Bruce Hooke]

Woodrow and Alteran,

Your welcome. I hope you will let us know how the red pine works out in practice...

- Bruce