STRONGBACK DISCUSSION etc.

[bill@ariel231]

Kurt

just a voice of caution: I recommend you take a look at Brian Toss' book on rigging or a couple of web sites...

http://www.sponbergyachtdesign.com/A...ngineering.htm

or

http://www.classicmarine.co.uk/Artic...ging_loads.htm

(there are others)

from the sponberg site a rule of thumb is:

(mast_compression) = ((weight of boat) * (righting moment from CM to CB)) / (distance mast to chain plates)

rough numbers (eye-balling the boat at 30 degrees of heel)....

mast_compression = ((5200 lbs) * (1.0 to 1.5 feet)) / (4 feet) = (1300 to 1950) lbs

This number is higher than some other more detailed models. doubtlessly, Moderator Bill has some better numbers for weights & moments, but the point is there will be a large point load under the mast and you are looking at a large span.

before you commit to a radical change, you may want to build a full size model and load it with a jack or hydraulic press.

Maybe I have misread the article.. I'm just a software guy.

cheers,
bill

[CapnK]

I took a good look at an Alberg-designed Cape Dory 26 yesterday, checking out how the strongback was engineered in a very similar boat to ours, 20 some years after our boats were in production.

The CD26 has a center-pole support for the mast, with port-offset access to the v-berth. The mast support is apparently a solid wood, 2.5-3" square pole. Interestingly, the pole is offset from centerline *to starboard*, about 1.5". There is some sort of arched structure up against the overhead which the half-bulkhead attaches to (mechanically, no tabbing that I could see), but it is small, and hidden by the cabin liner. It is an upside-down triangle in shape, approx 3" wide against the overhead, and 3" to the apex of that triangle. The corners of the triangle are radiused about 1/4".

The wide piece of wood on the bulkhead against the overhead you can see in this pic is simply trim, it didn't appear to serve any structural function. Also note the seeming lack of cabin trunk side support on the port side at the bulkhead station.

From tapping against the overhead, it was evident that the deck construction in that area - forward of the bulkhead at least as far as the forward edge of the mast base - was more solid than in other areas of the overhead.

Last, and somewhat unrelated, the size of the standing rigging was obviously smaller than that which we have on our boats.

Interesting.

[Bill]

Last, and somewhat unrelated, the size of the standing rigging was obviously smaller than that which we have on our boats. Interesting.

The Ariel used the same rigging and mast (cross section) as the Triton. Sort of explains why these are such a tough little yacht.

[ebb]

Very shallow beam, (don't really see it!) - no side supports at the cabin sides.

Lots of similarity to the Ariel of 20 years earlier - looks like heavier buttocks, to keep her from squatting. But the beamless interior is very interesting. Did you knock on the sides? They'd have to be solid, wouldn't you say? Otherwise typical Alberg. I have to wonder how it is done? maybe a single post really can act like a keel stepped mast. Do the shroud-plates come thru the deck?

Numbers say only 78 of these were built.

Hard to imagine any boat near the size of an Ariel with less than 3/16" wire.
Wonder how the general construction quality compares between the two?

[CapnK]

Bill - Right on, that was one of the attractions of the Ariel, for me. The guy with the CD26 said he is going to be staying at our marina, so I'll have to take a turnbuckle or something over, take a pic of the two for a comparison to post here on the site, it's so evident.

As an aside, I look at every boat in this size range which comes through the marina, none have the strong rigs we have... In fact, most late vintage boats (Ben-ter-lina's, primarily) have even smaller scantlings on boats which are 3-4' longer than ours. Not that I'd expect different there...

Ebb - I looked all over for an interior shot of the CD25D which illustrates how the mast/bulkhead/strongB is set up on that model, couldn't find one... wah. Looking at the line drawings I could find, it is obvious that the mast sits well *aft* of the forward bulkhead location, so it would be interesting to see if there is any kind of evident internal reinforcement on those boats...

I didn't knock on the trunk sides of the 26, but shall. It would seem that the sides must primarily be under compression from the mast, if they need no reinforcement from other structure... Still, I will build mine up a bit. Tapping on them just now, they seem to have more of a solid sound than the deck. I don't think there is coring in there, is there? Have you seen any during your (de)construction?

The sidedecks on the 26 are obviously strengthened against compressive forces from the mast/rig by some sort of side-strongback, you can see it in the pics. That fits in with my plan to have my eventualbeam wrap down around and under the sidedecks...

The chainplates on the 26 are through-deck, connected to knees on the hull, just like the Ariel.

CD's have a reputation as generally well-found boats. Judging from what I could see, it seemed put together with at least as much care as an Ariel, or perhaps a bit more.

[ebb]

Looking at the CD25 Sadler, the beam build-down is also slight. What is obviously the compression AREA is bracketed by wooden structures creating alcoves where lockers and heads are usually put. Haven't researched this at all, But the s/v Allia (with a good inside shot) has a #738 on her sail. That is a surprising difference in numbers of boats between the two designers if that is indicative.

If there has never been a problem with the CD25D mast, it is very worthy of study. And inspiration. We know that Alberg was a tyrant when it came to messing with his designs - the 'engineering' inside under the mast must be well thought out.

It's fantastic you have two live models to draw from.

I would like to see a cross-section and a diagram of this CD25D mastbeam puzzle. Maybe not, because I would be hard put not to change what 338 has now!

A bunch of luck with the project!