CP 19 compression post attachment

[tsaiapex]

Some of you have done more digging around in CP19 than I had so may have answer for this.  Do all of you find the compression post in CP19 fasten only by two 7/16"bolts to the bulkhead and does not go all the way down to the cabin floor?  Or if it varies among different models?

I removed the 7/16" bolts recently and found them bent at the center.  I replaced them with 3/8" bolts and backed up with large fender washers. 

Any idea why Hutchins not sitting the post all the way down to the cabin floor?  I am pretty sure it will sit on top of the keel.

[pelican]

This is a complex subject and a nautical engineer could answer much better then I could. I asked the same question and this is the answer I received. I am no expert and this may not be the answer in it's entirety.

I was told that fiberglass boats are basically one or two unit sections as opposed to several pieces making up a wood boat. Fiberglass boats have some rigid support but most support has some flexability. Our little boats are carefully designed to provide just the right amount of support.

If the boat is to rigid, the surrounding fiberglass will break up and fail. If the support is to soft , the fiberglass will weaken over time because of constant over flexing.

Boat stringers frequently have some type of filler material between the wood and fiberglass so the wood provides support,  but is not rigid against the glass and causing damage over time.

I guess the short answer to your question is, The compression post shouldn't be sitting directly on the keel because it wood make the design "to Rigid" and could cause damage ( stress cracking ) to the areas around the mast step,chain plates, and any areas the mast could stress. The boat as designed,  provides just the right amount of support while remaining flexible. This should be remembered when making alterations to the boat that impact the supporting  structure.  Our boats while simple in design had someone at a drafting table for many hours calculating the hull thickness, supporting structure etc: so we can enjoy the boats for many years. As you said, the bolts were bent. Well, the bolts absorbed some of those stresses. Better the bolts to bend a little then have hull damage. I hope that explains it and helps.

Warm windy winters

Terry

[tsaiapex]

Terry,
Thanks for the reply.  You are the second person mentioned differential flex between hull and deck.  It does make sense.

I sent an email to Hutchins and Gerry replied that it would be OK using a wedge under the compression post.  They left the post short to faciliate production.

[pelican]

I will stick to my explanation because it cost me three hours of hanging out at the Boat yard listening to old sailing stories and drinking beer to get.  That was the first question I asked when I bought my first Compac.

With your explanation , I lose faith in everything. I can't accept them short sticking us to save time and a couple of bucks.. ... ... But then, I have seen them shopping at Walmart.... LOL

Good sailing and enjoy your boat!
Terry

[mgoller]

I imagine Hutchins found it easier to cut the Posts short since each one might be a little different but why wouldn't they pound a shim under it?  Maybe we are not understanding how much downward force is generated.  Maybe the cabin top arch and the bolts would exceed any force by a large factor. 
I'm wondering how much downward force is there on the cabin top?  Say the mast and sails are 120 lbs.  That isn't much for the cabin top.
But a lot of the force generated by the wind is lateral and transferred to the stays and shrouds and cabin top.  The main sail is pulled forward and the jib is pulled forward and slightly up.  These two forces transfer some force downward on the mast.  I can imagine this could be quite a force.  
If you take a triangle and force it sideways and the stay is fixed to the stern the force is going to transfer down on the mast.  Seems like it could be several hundred pounds pushing down on that 1/2 sqft mast step.
I think it would be a good idea to have that compression post bolted through good wood and shimmed with something solid.
The arch of the cabin top is probably very strong, but with that much force centered over a small area, seems like it needs a little insurance.
In a heavy wind my ComPac 19 shrieks and groans.  The shrieks are from wind whistling through the shrouds and halyards.  The groans are probably from flexion of the structure and hull join.
I'm just rambling.....

[edbuchanan]

My CP23 has a center bulkhead that is constructed of plywood, and two additional hardwood molding verticals, almost directly under the mast.  The molding  functions somewhat like a pair of compression posts.
If the mast is off the boat, the molding removes and installs with no grief.  If the mast is on, they seem to be about 1/16" too long, and snap in with quite a bit of force.  Once screwed in, all is well.
My point is this: the boat's keel/deck distance changes with the downward force from the mast, even just from stay loading.  Maybe you noticed the gap with the mast off the boat?, IE. it might disappear with the mast installed.

Ernie Molly/23/II

[pelican]

An old gentleman boat designer/builder told me the problem came in with having rigid support behind fiberglass. He said there had to be some minimal flex or the glass would break up and fail.

His thinking was, if a compression post was rigid on the keel, the mast step fiberglass would be pinched between the compression post and mast leading to cracking and failure of the step and associated parts.

Perhaps, this is not an issue with our smaller Compacs and he was speaking about fiberglass boats in general.

I also noticed the bolts were bent and the washers were pulled into the wood on my boat where the compression post bolts up. I just have  to wonder where the energy would transfer and what would happen if the bolts/wood didn't bend or give way under stress.

For whatever reason , Compacs design seems to work well for the intended pourpose and it may work well for reasons not intentional or long forgotten with the passing of Mr. Mills

[pelican]

last thing

There has been a few boats come into the yard designed and built without Compression post.

90% of them had sags in the cabin top under the mast step.

[Johann]

Hello;

I can only speak from my experience with the restoration of Udara Tanda, but I had to replace the compression post after the top of the cabin was damaged in the hands of the previous owner. What I discovered, in performing the engineering autopsy, was that the bolts which hold the bottom of the post against the bulkhead near the bottom had worked the nuts loose until they fell off. This in turn permitted the compression post to drop, as it was trying to close up the gap beneath the post and the keel, as mentioned above. When the previous owner decided to take her out in some heavy winds one day and give her full sail, the resulting downward pressure forced the tabernacle down through the deck (crack!), until the compresion post was resting firmly on the top of the keel.

So, the point here is, If the post is bolted firmly to the bulkhead, it would be of sufficient strength to carry the required loads. After this discovery, I decided to take it a bit further than that during the rework of Udara Tanda and tie the whole thing in solidly from top to bottom.

I am unsure of the size of the original bolts that were used by Hutchins when building the CP-19, but there were 3/8" x 16-thread nuts sitting in the deadspace by the post when I took it out, and the holes drilled through the old post were large enough for 3/8" bolts, so that's what I used (with fender washers on both ends of the bolts) when I installed the new post in Udara Tanda. I made sure to cut the post to a height so it is resting directly on the keel and then ensured that it put a slight pre-load on the underside of the deck when using a load distribution plate I made up out of stainless steel. Here is a picture of the work involved:

I did notice that when I removed the patchwork the previous owner made to the cabin roof, the whole cabin assembly dropped about an inch under its own weight... That somewhat indicates to me it's not self-supporting and requires a support to hold up the cabin roof hence, the compression post as used for a load distribution point.

[mgoller]

Well this thread is shaping up to be one of the more definitive and important that we've had in quite some time.  Its one of those things you take for granted until you start looking and thinking.
I did go out and remove an inspection plate just in front of the compression post.  I couldn't see down there but stuck my hand down.  The post is no where near the keel.  I'd say its probably 6"-10" above anything solid.  It is bolted through a very solid chunk of at least 3/4" plywood and the wood veneer too.  I can see the bolt heads are bent as if the post has had tremendous force put on it.  The heads and washers have dug in slightly at an angle into the veneer.
The post, wood bulkhead and bolts are all pristine. 
The tabernacle around the mast step has some cracks which I had always thought were caused by the guy who drilled the hole for the antenae and by me when I wrenched the mast step out screws and all.
Some of those cracks might have been caused by the downward force of the mast.
In summary, I'd say the guys that designed this boat did a great job.  The boat is 23 years old and doing fine.  I'll just keep sailing and taking good care of the old girl.  I don't think it would be a good idea to change the design without knowing why.

[Craig Weis]

Hi Skip here. OK I see a few things that I think are overkill. [BTW the 'rig' are the wires running up the mast.]

I was reading Kerr's book, The Nature of Boats and Peter Schweer's book, How to Trim Sails, Dinghies to Offshore Cruisers. I think I understand the placement of the compression post on C-P 19's.

Com-Pac's are 100% resin without a core material. That is a good thing. Nothing to absorb water or rot. So if the tabernackle on the cabin top is loose then remove it and fill the holes, redrill and reinstall. This plate business is a lot of work with minimal return. We are really not spreading the weigh out.

My tabernackle is just screwed to the flat surface on top of the cabin. No through the overhead bolts and nuts. Me thinks. The rig carries the load.

The bottom end of the compression post is attached on to the plywood bulkhead forming the vee birth and seperation of the lower births. It is here that we hold up the mast from slamming its bottom clean through the hull. This is a structural member and is necessary to shape the hull as well as to transferr stress.  The tabernackel acts as an anchor for the mast, while the 'rig' carries the effort to power the boat. If we could magically hold the sails up without a mast, but with the rig, the boat would still move foward.

The compression post is bolted here to the plywood wall above the keel for a very good reason. To let it move. Cooefficient of expansion of wood and cement and resin due to sunlight heat and night time coolness is considerable. In other words, let everthing move or watch it crack. Plus every deck and dog house of a 19 is slightly trimmed differently when it comes from the mold. Hense we need adjustment to fit the post.

Strain...a mast head sloop [Com-Pac 19's] place more strain on the rig [stays] then do 3/4 or fractional rigs. Fractional rigs like on Star boats and Rhodes 19's allow the upper 1/4 of the mast to deform, bend and change sail shape as wind velocity varies. Sail shape equals speed. Keeping the rig stiff equals plotting along under the best conditions. And bending the mast puts the engergy into the sails and rig.

Our Com-pacs are 'corrested' so stiffly that they just plow along the seas in a stolic manner, but it makes them bullet proof easy to sail. Want to bet that a C-P 19 with a well tuned rig could be pulled from the water with just one cable attached to the mast's head? The boat only weighs 2000lb. And it is not the tabernackle that allows the boat to be pulled from the water but the rig.

When we build our 150 ft Sport Yachts of 160 to 174 ton the finial alignment of the motors, gear box, and shafts is left till the hull is supported by water. She moves.

[mgoller]

Thanks Skip,
A lot of these posts are convincingly definitive.  To sum up, I take it that the boats were designed with reason and designed well.  Over time the compression post should be given attention as any important structural mechanical component should.  But, the overall design should not be modified from its original design.

When those supporting bolts start tearing out and bending they should be replaced and the support bulkhead can be reinforced so that the bolts don't tear out. 
We shouldn't work to create a rigid support between keel and mast or fiberglass will be left to flex which it doesn't like to do.  Glass breaks!  Bolts bend, wood flexes!

So, what's left to answer here is - how far do we go and what method do we use to reinforce that plywood bulkhead?

[tsaiapex]

Thank you all for the lively responses relating to the compression post subject.  Here are more findings and how I eventually tackled this project.

The area directly under the compression post on this 1989 CP19 is boxed in all four sides and the floor with marine plywood, same material used for the bulkhead.  The only reason I can think of for such arrangement is able to leave this boxed-in area untouched when flotation is put in so workers can fasten compression post after the deck is put on.  Yes, my boat has plenty floatation under the v berth and under both side of quarter berths are all are in very good shape.

So when I placed a wedge under the compression post, it does not make direct contract to the concrete keel, instead it is cushioned by plywood floor.  I also secured the wedge with SS screws through the bulkhead to keep it from moving.

As an added lesson for you all who uses Damp-Rid to control moisture, put it in a container to contain any accidental spill of dissolved calcium chloride.  I spilled some which got absorbed by carpet then wicked up by teak panel covers bulkhead.  What a mess!

[pelican]

Here is another thought. Down here in the sunshine state, we  had several larger sailboats sink at the docks because of storms or neglect. Water enters the boats interior and saturates the wood. The interior wood work/bulkheads swell.

When the boat is raised and dried out, you can frequently see the locations of the bulkheads from outside the boat, if you look down the side and catch the light right. Sometimes this is very obvious, sometimes not. Sometimes this is the result of poor construction, sometimes not. Sometimes there is gelcoat cracks and other times humps.

Folks that construct wood boats know they can't place planking tightly togeather because the wood will swell in water and the boat will blow apart. There has to be some type of filler/chalk/cotten between planks to absorb that swelling and keep the boat water tight.

I can recall vividly in the 60s,  launching wood power/sailboats and having to run  mud hogs (pumps) to keep the boat from sinking until the wood swelled and the boat would seal. Water has a profound effect on wood

I put that out there to think about , and to think about how it relates to the wood/glass componits of our boats. If the mast is raised and the rigging is under tension...... and water gets to the compression post mounted directly on the keel...... could it cause damage if the mast was rigid down  to the keel ??  .... I had a small leak around the power socket going the mast on My C19 and it ran straight down to the area around the compression post.

[pelican]

Oh and Marcus, I wouldn't add any reinforcement to the boat. I would only maintain whats there.  If anything, Replace the bulkhead every 20 years or as needed.

Terry