Birdsmouth Mast taper - and tapered blocking

[Redeye]

Hi All,

So I'm now starting the build of the tapered top mast (mast is in two sections for car topping - not a gaff) and the bow sprit.

My question regards getting the taper. Should I taper all eight staves evenly, or should I taper just four of them? Obviously tapering the non-birdsmouth side.

We've diverged somewhat from the original plans on this one. The original plans having 2 sets of four staves, one set plain on both sides and the other 'birds-mouthed' on both sides.

The other related question is how to set out and best cut the tapered octagonal blocking required at the end of both the top of the mast, and the bowsprit. Any tricks to make this a little easier?
It seems to be quite a tricky shape to imagine, let alone make. A frustum of an eight sided pyramid.
And how to hold this in place as it will inevitably want to slide inward during glue-up and clamping.

I've got some photos somewhere of the staves, and will post them as soon as I get the digital camera card sorted out.

R

[Thorne]

Birdsmouth bowsprit? First time I've heard of one on a small boat. Lots of good articles on Duckworks, and some decent threads here - just search for "birdsmouth".

http://www.duckworksmagazine.com/06/...outh/index.htm
http://www.duckworksmagazine.com/04/...uth2/index.cfm
http://www.duckworksmagazine.com/03/...mouth/mast.htm
http://www.duckworksmagazine.com/02/...outh/index.htm
http://www.duckworksmagazine.com/04/...outh/index.cfm

[Sailor]

My brother has some up on his site about the construction of the spars for his shellback. They have no plugs in the ends as of now but I am also curious as to how they are plugged up. Let us all know how it works out. I imagine a simple sanding drum to round the inside followed by a round plug inserted. Maybe I'm way off though never having (yet) built one completely.

[Tom Robb]

It's a 2 sectioned for breakdown mast, not a bow sprit.
I haven't heard the word frustum since Trig class, IIRC. Interesting.
I'd picture it as a 8 sided solid (what would you call that?) with 2 pyramids (cones might be easier) sort of excavated not quite apex to apex in that solid thing-y in order to minumize point loads. Fitting it would be almost as much fun as that "excavation."
Once made, I'd glue it to one of the staves before assembling the rest of the staves then gluing them up and Bob's yer uncle.
What could be simpler?

[Ian McColgin]

It will be interesting. I think the regular bird's mouth, which can easily be made in any ovoid you might want, is more readily tapered on the front and sides, leaving the back straight, or tapered symetrically if that's your pleasure, than this composit approach but the result will tell and experiment is good.

G'luck

[Jay Greer]

Clamping the spar flat on the spar bench allows the track surface to remain flat. If the taper is gentle enough, the staves will self align. The upper and lower plugs are merely a matter of accurate shaping. Even though I never install blocking at spreader points, I still use tapered fingers on upper and lower blocks so as to not form a hard spot. Be sure to bore a drain hole in the bottom block prior to assembly and glue up. Leaving the spar in octagon form at the partners allows wedges to be made up flat and so are less hassle to shape.
Jay

[Timo_N62.9_E27.7]

The other related question is how to set out and best cut the tapered octagonal blocking required at the end of both the top of the mast, and the bowsprit. Any tricks to make this a little easier?
It seems to be quite a tricky shape to imagine, let alone make. A frustum of an eight sided pyramid.
And how to hold this in place as it will inevitably want to slide inward during glue-up and clamping.

Just cut it a rough fit making sure that it is not too big, a straight octagon the size of the smaller end will do. Enough epoxy will take care of the fine fit between the staves and the plug. And the taper is usually so small that any clamping won't wedge the plug out.

Here is the heel of my bowsprit during the dry fit. It was made using a band saw.

[Thorne]

I've had VERY bad experiences with extended / 2-part masts on small open boats, with two of them breaking right at the top of the extension. So you may need to modify the birdsmouth construction and blocks to prevent breakage.

I'm certainly no NA but masts often need to flex evenly along their length, and any hard spots can focus all the forces and cause it to break at that point.

[AJZimm]

Here's what I did to get sufficient blocking at the bottom of the mast. I made another small birdsmouth section to fit inside the mast staves. That brought the inside diameter down enough that I could drill a hole and use a dowel for the centre. The flats for the heel were cut in later.

[alkorn]

I don't understand the motivation for a birdsmouth bowsprit. It's close enough to the water line that the weight savings from birdsmouth construction won't add significantly to stability. Are you just trying to reduce weight forward so that the bow lifts more easily to the waves?

I like the idea of drilling out the end of the birdsmouth for a cylindrical plug.

[Timo_N62.9_E27.7]

I don't understand the motivation for a birdsmouth bowsprit. It's close enough to the water line that the weight savings from birdsmouth construction won't add significantly to stability. Are you just trying to reduce weight forward so that the bow lifts more easily to the waves?

That was one of my reasons. Also I didn't want to use a solid timber to avoid warping and bird's mouth is an easy way to construct a tapering spar from small sticks.

[Redeye]

Sorry for my lack of response here, totally overwhelmed with less important but more pressing matters -- work...<sigh>

The boat is the Dix paper jet, 14ft skiff, so weight anywhere is a bummer. It already feels like we're at finished hull weight, and we have yet to add the final coats of Epoxy and varnish/paint. I know some of you will scoff at me worrying about extra weight. But it can make a difference. Extra weight anywhere will add loading to the rigging. I feel that any extra weight forward will definitely add to the possibility of pitchpoling at speed if we get a little more than a chop while running the kite.

The birdsmouth 'sprit is what Dudley specifies. That's why we're building it that way, but I am contemplating not tapering it. The top mast section is definitely going to be tapered. We bought wood for staves at the lengths we needed to make the mast in two parts as per the plans, despite my reservations about it. It was very expensive so we won't be altering the approach at the moment. If it all turns to custard and we break the mast, I'll make a single piece replacement box section mast from some cheaper wood. The top mast and bowsprit will be made as one piece and then cut in half - 'Sprit from one end, top mast from the other.

Yes I think, upon closer inspection of the taper angles involved, that the angle will be easy enough to achieve just by planing regularly from all sides of the octagon in a regular and careful way. We're only talking about a 4" long block here. Timo I like your square section as this would definitely stop the block from slipping inwards.
Jay, thanks for reminding me that I should probably keep the track edge flat/ straight during glue up.

I will definitely drill small holes to allow any moisture to drain from all sections.

I agree that the mast will never flex well across the blocking and joint. But them's the breaks sometimes. I'll have to live with the result. My main concern is that the mast/top-mast joint is connected with a sleeve which covers only 4" of each. Dudley made this out of Alu but also suggests one could make it from (cover your ears) glass. I have a piece of (ears still covered) carbon fibre which I might use here but I still wonder if this will be strong enough over only 8". Too big and flex is reduced I guess. As far as I can remember, plans not in front of me, the top mast is unsupported.



Dry fit photos next week --- hopefully.

[Tom Robb]

Thorne,
I'm sorry. I misread the first post, and worse, had the nerve to "correct" you. You were quite right about the bow sprit. I got it wrong.
Tom

[Redeye]

Hi All,

Sorry to those of you who are bored by this thread already.
Well, we tapered and rebated the bird's mouth into the staves on Sunday, then cut some rough octagons and did a dry fit. It's amazing how well it fitted together and how good it looks already. I'm feeling confident the bowsprit will be strong and light.

However, one of our staves has a vein of resin saturated wood running right up the centre of it. it's not something I noticed until we routered out the bird's mouth, then it became glaringly visible. and then as it sat in the warmer part of the workshop it started to ooze out...

There is no "solid resin" part or cavity as such, and the timber still seems hard and sound, but the amount of resin concerns me. Will it adversely affect the holding of the epoxy? COuld I "rinse" or wash it with acetone first? Should I chuck it and replace it with a new stave? I have a spare stave but the grain is far inferior, about 1/6th to 1/8th inch between growth rings. sorry bout the lack of photos.

Thoughts?

[Songololo]

How long is it? What about routing it out and gluing in a spline with scarphed ends?

Lance

BTW Where are the pics !!!

[Redeye]

Hi Lance

Yes there is that option, and then I could still use the nicer ends of it, with the lesser wood in between.

it's about 1.8m long. Quite annoying as I have a 1.7m length of very nice Douglas which would have worked well for that.

We're getting closer and closer to launch, btw.
Speaking of which, do you know what requirements there are for safety inspections etc for "home built" craft on Zugersee?

[Redeye]

Oh and yeah, pics are still on Mama's camera... forgot to get them before leaving yesterday

[johngsandusky]

It's an interesting thread. I have seen a two part mast with a sleeve, on a marconi cat I believe.

[Redeye]

practicing on the boom

the staves are 50mm too long each end, and the offcuts will make funky rum vessels ... albeit with a taint of epoxy...

[andrewe]

Nice stuff, Redeye. For the blocks in my mast, I cut 'V's in each face at the ends, giving it a crown effect. Probably a bit anal, but for the one at the hounds, it reduces the possibility of a 'hard spot'.
4" does sound a short socket, but DD usually knows what he is about.
A