converting from compression post to strongback mast support

[Commander-147]

I'm removing the compression post in my Pearson Commander and replacing it with a strong back to support the mast. The unsupported distance between the bulkheads on both sides will end up at 35-3/4". The strong back beam itself I am making out of white oak vertically laminated to a width of 3-1/2" and a height of 3-1/2" which is glued up with resorcinol. I will tab the beam to the under side of the deck to make it all one structure and support the two ends with a double bulkhead (2ea. 1/2" thick bulkheads one supporting the forward face of the beam and one supporting the aft face of the beam with lumber spacers between them). And since I have never figured out how to post pictures here, I can offer a link to the gallery page for my commander on the Ariel/Commander forum which has pictures of what I'm trying to do.

http://www.pearsonariel.org/discussion/ ... #post22086

So here is my question. Do the VERY experienced people on this site believe this setup is capable of supporting the estimated 2500 lbs needed for the rig? And are there any suppestions on how I can test the strength of the beam to verify it's integrity?

[hriehl1]

Short answer, I do not know.

But you also now have to consider the ability of the bulkheads on either side carrying 1/2 (each) of that 2500#. More specifically, the base of those bulkheads. Your compression post was no doubt anchored in the keel with a heavy-duty load-spreading design. The footings for these bulkheads will need similar.

[Skipper Dan]

I used a carbon fiber beam over oak. With oak posts.

Just upload the file to post a picture. Use a photo resizer to make the photo smaller. I use one from microsoft that is free.

Dan

[Commander-147]

Thanks guys for your replies.

hriehl1

You are absolutely right about that and I hope my plan is sufficent to properly transfer the loads. The plan is to have the bulk head directly below the ends of the beam which are 8" wide at the top be solid plywood down to a similar setup that spans the full width of the bulkhead at the bottom which is approx. 30". The total thickness of the bulkhead assembly will end up at 3".

Skipper Dan

Your post made me consider another option for my setup. The center portion of my strongback will be 2-1/2" thick white oak and on both sides I will epoxy and screw a 1/4" thick aluminum plate that will rest on top of the bulkheads. The bulkheads will be screwed to the center oak portion. Then I will tab the entire thing to the deck.

I am open to any suggestions about the above plan of action from anyone.

[Figment]

Suggested modification to plan: After you've done all of these support beam and bulkhead modifications, build a removable (screw-adustable like a boatstand or house jack) compression post that you can easily install for heavy weather sailing.

[LazyGuy]

Ditto on Fig's suggestion.

[Skipper Dan]

You are not going to get any strength from a piece of Aluminum laying down. Put the piece across two sawhorses and push on it. Strength comes from depth. Like on an I beam The top and bottom do very little. The deeper the beam though the more strength you get regardless if you change the thickness of the top and bottom. If you really want to show off the wood, laminate up a center section and wrap it in carbon fiber. Fare it off. Now make two identical thinner pieces for each side and laminate that to the center piece. You could actually wrap the whole thing in fiberglass and use the epoxy that is clear. Beauty and strength. I am not sure they still have it up but there was a clip on U tube from on of the TV shows I cannot remember the name now something about science and does it really work kind of show. They did the is carbon fiber really that strong. They just wrapped a 2x4 and I could not remember if they could break it or not.

Dan

[Rachel]

First off, I want to qualify that I'm posting these thoughts as a fellow boater, and not as a qualified naval architect or engineer.

These are photos of a Seafarer Meridian (Rhodes/Jannace design). This is a 25' footer that is pretty similar to the Commander in overall dimensions and style. They were built in Holland in the early sixties, and the builders used fiberglass for many of the interior furnishings (although it was more "stick built" with fiberglass vs. utilizing a liner, if that makes sense).

Edited to add this drawing of the Meridian. Basic stats are (about) as follows:
Length: 25'
Beam: 7'
Draft: 3' 3"
Displacement: 5500#
SA: 250

The mast is deck-stepped, and as you can see, does not utilize a compression post but instead is stepped over a bulkhead. The bulkhead in this case is fiberglass with a wooden core (much like a deck). It's about 3/4" thick. There is no "beam" per se, but there is a thickened "shape" on the forward side that runs across the top. You can just barely see it in the second photo (I have some better photos of it that I can post later - they show the forward side). Basically, the "beam" is wider at the bottom (maybe 2") and then tapers back in before it reaches the overhead (and of course the whole bulkhead is tabbed to the hull). There is no direct connection to the keel (which is external cast iron).

I have not heard of any failures of this beam, so although one might have occurred, I feel pretty comfortable in saying it was not a typical problem in these boats. I don't know for sure what is inside the thickened profile but I don't have any reason to believe it was anything "special."





The other thing that occurs to me is that the Ariel is a nearly identical sister ship with the same hull (but with a larger cabin and smaller cockpit). The Ariel has a deck stepped mast with a passageway beneath it (and no compression post). I have heard of a number of Ariels with sagging beams, but my guess would be it is not so much the size or design of the beam, as it is the execution/attachment -- somewhat similar to the Triton (where you could find a knot right in the center of the beam or etc.). So, you could probably get some good specs from the Ariel.

Rachel

[Commander-147]

Rachel

Thank you very much for your reply. Your thought patterns are very much in line with mine. In addition to the boat you just mentioned the entire Precision line designed by Jim Taylor uses this same method to support the mast. Of the current production models the P-23 is the closest to my commander. The strong back used in that boat is SIGNIFICANTLY LESS substantial than what I am planning and none have failed.

I have also done my homework on the Ariels and out of the boats that had issues most were associated with water issues (bad maintenance) and the others were as you suggested pour execution. The Ariel strong back was not even tabbed to the deck to my knowledge which makes a huge differance in it's ability to support the mast. Again what I plan to do will be substantially stronger than what the Ariels used.

It does seem that my design is going to change significatly from what I first mentioned after having had long conversations with my fiberglass supplier and people I know and trust when it comes to this type of issue. So I will update this thread when I have finalized my design and start putting things in place.

Rachel, I have not posted a lot here but I have lurked and I must say I generally like what you have to say. You appear open minded enough to learn new things but you also seem to have a good grasp on what you are discussing and because of it offer good information. Thanks again for your reply.

[Commander-147]

Well...after feeling unsure of the actual design specs needed to support my mast and knowing I did not want a problem down the road I decided to enlist the help of a professional.

I started off by sending an e-mail to Jim Taylor who designed the entire Precision line including boats up to 28 feet that used strong back designs. He has also been involved with the design of the Sabre Spirit a 36'-8" daysailer along with 5 other Sabre boats. And that just touches the tip of the iceburg for what Jim has worked on. When I contacted him he put me in touch with Steve Koopman who is the marine structural engineer that Jim partners with on his designs. Steve's business is SDK Structures and he has been involved in projects like Alinghi, and Speed Dream - the quest for the fastest monohull on the planet. So I feel very comfortable with his ability to produce sound structural design work.

The cost to have Steve provide me with an engineered design that would allow me to safely support my mast turned out to be in my opinion quite reasonable. So a deal was struck and Steve has designed my strong back for me. Unlike most Ariels and Commanders my strong back has a pretty wide unsupported area that is designed to be as wide as 39". I wanted the more open feel that this would give me in the small commander cabin.

Steve tells me the loading on this beam came in higher than either he or I had first suspected. He told me the compression loads are maxing out in the 7000 lb range. So to manage the load with my 39" wide unsupported span I will be using carbon fiber caps top and bottom on the laminated mahogany beam. Here is the design I will be using see the first picture below.

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As of this last weekend the old bulkheads are out and the old tabbing ground down a lot to allow for the installation of the new bulkheads and beam.

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[Commander-147]

I thought I would share the completed mast support beam I built from the design that Steve did for me. The two black lines in the beam are each 20 layers of 9OX unidirectional carbon fiber.

[Commander-147]

That is supposed to read 9 oz carbon fiber. Guess I should proof read a little better.

[Paulus]

Looks really nice - I like the wood/black contrast.

By the way, if you're the author, when reading your own post, you have the ability to edit (look on the bottom right hand corner of the post).

great work!

[Commander-147]

Thanks for that tip. And thanks for the complement.