Where to source Boomkin scantlings for Hardwood Boomkins

[rkarakai21]

Good morning, I am planning a big refit for the winter of 2024 where all my ancient standing rig, chainplates et al will be renewed.

This is for a Roughwater 33, 3/4 keel double ended sloop

The current boomkin is stainless tube, with stainless rod supports meeting the hull close to the waterline on both sides. The boomkin at some point in the boats life suffered some collision damage and one of the main tubes is significantly bent, in addition there is some hairline stress cracking in one of the stainless rods. Really it's not long for this world.

I plan on building a Hardwood boomkin this coming winter in the wishbone style that so many similar boats utilize (BCC 28, Rafiki 37 etc). For two reasons primarily. The cost of having a new, beefed up offshore capable stainless boomkin fabricated is extremely prohibitive, and will dig deeply into our savings towards heading offshore. As well as being significantly cheaper, I can easily incorporate extra room to accomodate the windvane mounting setup, and easily dimension it to be as strong as I want.

I plan to use a pair of Ipe planks, probably with one or two lapjointed cross pieces at the aft end, one for windvane mounting and the other as support for the backstay fitting, which will probably be cast silicon bronze at PT foundry.

The biggest unknown to me is how to dimension the two boomkin pieces to be atleast stronger than the original design but not to go too far into overkill territory.

Logically I would imagine that if each boomkin halves are equal to or greater than the half the cross section of my teak plank bowsprit it would be good to go, especially considering the added strength of Ipe VS teak

Are there any good books that dig into this topic?

[psk125]

It would probably be smart to talk with an engineer or naval architect about the relative strengths of Ipé and SS tubing. If it's going to be holding your self-steering gear AND the backstay you do not want it undersized.

[rkarakai21]

Absolutely, I certainly want it to be overbuilt.

Do you know of any Naval architects in BC who specialize in Yacht construction? I have been looking around and finding that most seem to be focused on commercial vessels, tugs, workboats etc..

[TrentePieds]

This is a matter that can be settled by correspondence. It's an absolutely pedestrian engineering problem. You don't need a local NA to do it, but either Gregory Marshall in Victoria or Ron Holland in Vancouver can sort this one out for you for a very modest consulting fee.

If you haven't done it already, one of the things you owe to yourself, since you drive a woodie, is to look up "WoodenBoat" on HolyMotherNet, and to subscribe to WoodenBoat Magazine. You will learn far more about yacht design and construction there than you ever will from the "glossies" or from those of us who dive "frozen snot" boats :-)!

You might like to contact these people:

Walsteds Boatyard Saugskærvej 21, Thurø DK-5700 Svendborg, Denmark. Phone +45 6220 5168. Email: info@walsteds.dk · www.walsteds.dk · www.walsteds.com.

They are the absolute crème de la crème, and the place where knowledgeable people from all over the world go both for repairs and for new construction of woodies!

And fret not - the DO speak English :-)!

TrentePieds

[rkarakai21]

Thank you very much for the info and references! Believe it or not though she is not a woodie! Shes a frozen snot boat adapted from woodie designs, haha. She's old heavily laid up glass.

[5BTM]

Pick up a copy of Skene's Elements of Yacht Design.
I hav ethe eight edition, Chapter 23 and 24 deal with methods of and have calculated examples for a an cruising sloop "Pipe Dream"

The joint used to build hollow spars is called a "birdmouth"
You migh like to practice first:

https://www.woodcentral.com/articles...icles_614f.php

https://www.duckworksmagazine.com/04...es/birdsmouth/

[Bowdrie]

A wooden boomkin for a Roughwater 33?
Look at a Westsail 32. the main "arms" are basically a 2x4 from Fir.
Note, not like a "home construction" 2x4, but rather more like a "Real" 2x4.
You're using a hardwood?
Yes, that will give you additional strength over a softwood.
Like a bowsprit, a boomkin should be in pure compression, be careful with your end fittings/attachment points.

[rkarakai21]

Found some good info on one of the Westsail 32 forums.

Yes they seem to be 2"x4" fir. Granted as well that the Westsail has another 5000 lbs displacement on my boat and a taller mast/ more sail area.

I am quite confident in heavily overbuilding it, but wouldn't mind running numbers with a Naval Architect, to find out what tolerances are. So I am not unnecessarily adding weight. I still have to install quite a bit more water tankage in the stern, so I do have to be mindful of trim.

I think a true Ipe 2x4 would be excessive in the extreme, as it is orders of magnitude stronger than Teak, which in turn is quite a bit stronger than fir.

My guess is somewhere around 1.5" by 3.5" will be excessively strong for the intended purpose in that material.

The difficult part is going to be designing the end of the boomkin to either mount to or pass the hydrovane shaft through, with the Backstay fitting inboard of that.

The fitting itself will probably be cast at Port Townsend foundry, and will incorporate lugs underneath to install the two lower Stays to the hull, which will be parcelled and served Dux.

[Bowdrie]

Originally Posted by rkarakai21

I think a true Ipe 2x4 would be excessive in the extreme, as it is orders of magnitude stronger than Teak, which in turn is quite a bit stronger than fir.

Not so sure about "orders of magnitude".
Perusing "The Wood Data Base" for compressive strength;
1, Fir, ~6,900 psi.
2, Teak, ~5,900 psi.
2, Ipe, ~13,600 psi.
Of course, if using Fir one must get the best/clearest stock, with high ring count.
Thankfully that's not hard to find in the PNW.
For any wood you choose be mindful of grain run-out and cut.
Vertical grain and straight as can be.
Caveat, Teak can have higher, (sometimes much more,) compressive strength than Fir, and frequently does, it depends on where it came from, (Southeast Asia vs Myanmar as just an example,)
With Fir it depends much on ring-count.

[rkarakai21]

With teak and Ipe would it still be necesary to have vertical grain? Should the bolts holding the flat of the boomkin arms down to the aft deck not be perpendicular to the grain?

[Bowdrie]

Originally Posted by rkarakai21

With teak and Ipe would it still be necesary to have vertical grain? Should the bolts holding the flat of the boomkin arms down to the aft deck not be perpendicular to the grain?

You make a point there.
I didn't necessarily mean fastener orientation, but rather not having "radial" or "slash" grain in the wood.
Like looking at the end-grain and seeing straight lines that are parallel with two opposing sides, not tight arcs or grain that runs at an angle to any edge/side.
Shallow/gentle arcs ok, if so orient the wood so that the ends of the arc/curve are downward.

[rkarakai21]

Ah ok, I get what you’re describing now.


https://photos.app.goo.gl/svgK43cfqFNZhRmR6

https://photos.app.goo.gl/MrTY8bbFzRcJk1yU6

Just for a point of reference, I have this teak plank, I’ll give you a couple views of the end grain. Would you say there is too much curve in the end grain to be appropriate for this application?

[Bowdrie]

Originally Posted by rkarakai21

Ah ok, I get what you’re describing now.
https://photos.app.goo.gl/svgK43cfqFNZhRmR6https://photos.app.goo.gl/MrTY8bbFzRcJk1yU6
Just for a point of reference, I have this teak plank, I’ll give you a couple views of the end grain. Would you say there is too much curve in the end grain to be appropriate for this application?

For your application, I would reject the plank in first photo without a second thought.
The second plank? I might use it only if it was the best I could do.
Of course, you'll not use "planks", but rather something that has more thickness relative to its width.

[catsketcher]

A boomkin is a compression member. It is pretty easy to work out the maximum compression load of any single member. If you keep the geometry the same, all the new member has to do do is have equal load characteristics.

You can plug the dimensions of the stainless tube into an Euler compression load calculator but they are pretty tricky to use if you don't know about end conditions. If you are not going to change anything then all you need to do is keep the second moment of area multiplied by the Youngs modulus the same in both situations.

Second moment of area calculators are pretty easy to use

https://skyciv.com/free-moment-of-inertia-calculator/

PLug in the dimensions of the tube and get a value.

Then compare the values of Youngs modulus for stainless and the wood you will use. Say 304 stainless has a Youngs modulus (E) of 190 GPa and Douglas Fir about 9 GPa.

The Euler formula for compression failure is P cr = π 2 ⋅ E ⋅ I L 2 where E is the modulus of elasticity in (force/length2), I is the moment of inertia (length4), L is the length of the column. E is the Youngs modulus and I is the second moment of area, so if you want to subsitute a douglas fir boomkin then you have to make its second moment of area 190/9 = 21.1 times times larger.

So you now go back to the second moment of area calculator and increase the wall thickness (or go solid) with different shapes till you get an I value that is 21 times greater than the stainless one you have now. Then increase it a little more to get an easy to find or produce size and the wooden one will be stronger.

It is not rocket science at all. Anyone who understands basic Physics can understand it. You will need to pay attention to the attachment points though and a good boatbuilder will be helpful here. If you can find a wood with higher E value than douglas fir then great, I just used it as an example here, but do get nice clear quarter sawn grain and epoxy any bolt holes holes properly.

[rkarakai21]

Originally Posted by Bowdrie

For your application, I would reject the plank in first photo without a second thought.
The second plank? I might use it only if it was the best I could do.
Of course, you'll not use "planks", but rather something that has more thickness relative to its width.

Ok, thats good to know. Those are in fact two ends of the same teak Board. I will be very picky in my selection of the hardwood for these pieces. Thanks for the insight.