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H34 Compression Post One More Time

Jan 13, 2015
60
Hunter 34 Deep Bay, BC
For anyone interested in this topic, I did the compression post repair on my H34 over the winter. I did nearly exactly the same thing dcalif did (2x2x0.25 aluminum square tube with a honking big header block at the top), with one major difference compared to what anyone else has done as far (as far as I can tell) - I left the mast on throughout the process. I estimated the mast and rigging couldn't weigh much more than 500lb, so I loosened off all the standing rigging and just jacked things up. At night I would slip the old post back in just beside the opening I was making so that when the bottle jacks leaked down (and they always did) it would hold things up. I also made a relatively small opening through the transverse beam, only about 4 inches by 6 inches (which made it tricky hollowing things out enough to get my 3 in. by 8 in. by 1-1/2 in thick header in). The net result, though, was that I only needed trim pieces 1-1/2 inches wide to cover the hole when it was done.

I'm really pleased with the result, even though it took over four months to do it. Total cost was about CAD $800.

A comment about the original design: douglas fir has a compressive strength of only about 340 psi perpendicular to the grain. This means that the wood inside the original transverse beam directly above the compression post was not even capable of 5,000 pounds, which is nowhere near strong enough to support the mast on a boat of this size. Even though the beam on my boat crushed down between 1/4 and 1/2 an inch, the wood inside the FRP looked (and smelled) brand new. The principal failure had nothing to do with rot (although the top of the compression post itself was deteriorating significantly and would have caused a failure not too far into the future). The wood inside the beam was simply not strong enough. The 2x2x0.25 aluminum post I put in is good for roughly 30,000 lb, and is never going to rot.

Since so many people have written this up in great detail already, I decided I wouldn't bother unless someone was specifically interested. If anyone is, drop me a note and I'll send pictures and drawings.
 

splax

.
Nov 12, 2012
648
Hunter 34 Portsmouth
I replaced my compression post with a 3x3 aluminum tube with the mast in place. I used fitted end caps secured to the boat with 5200 to lock the post in place when the weight was released to it. A 4x4 post held the weight while waiting on the aluminum, since the jack would have been unreliable.
 
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Jun 3, 2018
44
34 Hunter & 26 Classic Hunter 34 / Classic 26 East Hampton NY
For anyone interested in this topic, I did the compression post repair on my H34 over the winter. I did nearly exactly the same thing dcalif did (2x2x0.25 aluminum square tube with a honking big header block at the top), with one major difference compared to what anyone else has done as far (as far as I can tell) - I left the mast on throughout the process. I estimated the mast and rigging couldn't weigh much more than 500lb, so I loosened off all the standing rigging and just jacked things up. At night I would slip the old post back in just beside the opening I was making so that when the bottle jacks leaked down (and they always did) it would hold things up. I also made a relatively small opening through the transverse beam, only about 4 inches by 6 inches (which made it tricky hollowing things out enough to get my 3 in. by 8 in. by 1-1/2 in thick header in). The net result, though, was that I only needed trim pieces 1-1/2 inches wide to cover the hole when it was done.

I'm really pleased with the result, even though it took over four months to do it. Total cost was about CAD $800.

A comment about the original design: douglas fir has a compressive strength of only about 340 psi perpendicular to the grain. This means that the wood inside the original transverse beam directly above the compression post was not even capable of 5,000 pounds, which is nowhere near strong enough to support the mast on a boat of this size. Even though the beam on my boat crushed down between 1/4 and 1/2 an inch, the wood inside the FRP looked (and smelled) brand new. The principal failure had nothing to do with rot (although the top of the compression post itself was deteriorating significantly and would have caused a failure not too far into the future). The wood inside the beam was simply not strong enough. The 2x2x0.25 aluminum post I put in is good for roughly 30,000 lb, and is never going to rot.

Since so many people have written this up in great detail already, I decided I wouldn't bother unless someone was specifically interested. If anyone is, drop me a note and I'll send pictures and drawings.
AloHA ! Im in need of some advice , you can see my posts. Im a little worried. Is there a way I can see Pictures of this process ? Would be of a great help
 
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Oct 19, 2017
6,501
O'Day 19 Littleton, NH
SabreToothedEngineer,
Great write-up. Creative solution to working with minimal prep.
I suggest, however, that you revisit your numbers on Douglas Fir. Parallel compressive strength of dried wood should be in the 6000-7000 lb range and perpendicular strength between 500 and 600 lbs. I may not be remembering this quite right and it doesn't really make a lot of difference to your quandary if your rig is in the 500 lb range. I just think fir is a little better than 340 psi. Maybe if it is green wood, that would be right, but that still seems low.

- Will (Dragonfly)
 
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Jan 13, 2015
60
Hunter 34 Deep Bay, BC
SabreToothedEngineer,
Great write-up. Creative solution to working with minimal prep.
I suggest, however, that you revisit your numbers on Douglas Fir. Parallel compressive strength of dried wood should be in the 6000-7000 lb range and perpendicular strength between 500 and 600 lbs. I may not be remembering this quite right and it doesn't really make a lot of difference to your quandary if your rig is in the 500 lb range. I just think fir is a little better than 340 psi. Maybe if it is green wood, that would be right, but that still seems low.

- Will (Dragonfly)
Sorry, I've been travelling and didn't see this earlier. Thanks for your kind comments, and for inspiring me to double check the numbers. I originally just used an old reference I had, "Engineering Materials and Their Applications" by Flinn and Trojan. That says 3600 in the grain direction and 330 perpendicular to it (the 340 in my original post is actually a typo). It's from 1975, but I didn't think the values ought to be out of date. I didn't bother going to other sources then, but your comment prompted me to. The Engineering Toolbox says 1700 and 625. Another really detailed source (https://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr113/ch04.pdf) gave numbers between 6200 and 7430 with the grain and 330 to 390 perpendicular. That reference is great - it lists virtually every kind of wood in North America. I'm keeping a copy for my library, which is why I'm glad you prompted me to look. It seems like there are lots of numbers out there, but 625 is the highest I saw, and it doesn't change my mind about the conclusion.
I remember from a class on Naval Architecture back in the Stone Age when I went to school that a rough rule of thumb for the compressive load at the base of the mast was the displacement of the boat (for a displacement hull). Since this boat weighs 12,000 lb, that's what I used. I also did a bunch of reading in "Skene's Elements of Yacht Design", and concluded that 12,000 was a reasonable guess. It's not the weight of the mast that's the problem, it's the compressive load under sail which is a much, much higher number. The original compression post (and therefore its bearing area) is 3.5in square, less a bit for the wire pass-through. That will be something less than 12 in², so even using the biggest number of 625 lb, that's only 7,500 lb. And that number is to failure, not a design number.
The new post is strong enough that it's even money the shrouds would part before the post failed. And therefore I'm a happy guy.
 
Jan 13, 2015
60
Hunter 34 Deep Bay, BC
AloHA ! Im in need of some advice , you can see my posts. Im a little worried. Is there a way I can see Pictures of this process ? Would be of a great help
Seeing there is some interest, I will certainly post some pictures when I get a chance. I'm in between work trips (home for one day) so I don't have time to do it now, but I'll try to get to it next weekend when I get home. I can also send anyone interested the actual fabrication drawing I made of the post in order to have a shop build it.
 
Oct 19, 2017
6,501
O'Day 19 Littleton, NH
It is awesome that you looked up several sources. I have seen a wide variation too. Just to add to the confusion. Green wood has significantly less integrity than dried wood. The lower numbers you are giving seem more in line with green wood, to me. I don't know is air dried or kiln dried is different. As you point out, it isn't that much of a difference that you would change your approach to the problem.

- Will (Dragonfly)
 
Jan 13, 2015
60
Hunter 34 Deep Bay, BC
It is awesome that you looked up several sources. I have seen a wide variation too. Just to add to the confusion. Green wood has significantly less integrity than dried wood. The lower numbers you are giving seem more in line with green wood, to me. I don't know is air dried or kiln dried is different. As you point out, it isn't that much of a difference that you would change your approach to the problem.

- Will (Dragonfly)
Just to close the loop, air dried and kiln dried are essentially the same thing. In either case, it's considered to be 10% moisture (compared to 50% moisture for green wood). The only difference is time; air dried is literally that, left to its own devices to dry, versus kiln dried where you cook it to drive the moisture off. I just happen to be in the pulp and paper business, and wood characteristics are my speciality. Which is why it's a bit embarrassing I had to confirm the numbers on strength, but also why I appreciated that I was prodded to. I spend my time on fiber properties, not normally wood properties. If you want to know the fiber dimensions and characteristics, I have that covered. That is not so useful in boat building or maintenance.
 
Jun 3, 2018
44
34 Hunter & 26 Classic Hunter 34 / Classic 26 East Hampton NY
Im not able to open the File ?
Ahh i found it.. Wow. Looks really good man.
So. Heres where im at. And I havent seen anyone yet with this. People are going out and paying to have a rig fabricated.
I just received my 15Ton Basement Jack. They hold HOUSEs up. Their used all over the world. It was $75 !!!! It does not ever sag. Will post pictures once i begin. Seems like it should do exactly what you had fabricated.
Im amazed to see how small of a section you removed from the Header @SabreToothedEngineer . I feel much better after seeing that. We'r you having any water leakage from the beam ?
Im still confused on how I stop a water leak in this location with out removing the Mast ? As my wood down post was completely rotted.
Really appreciate the PDF . It brings clarity to a 1st timer.
 
Jan 5, 2018
166
Hunter H34 0828 New Orleans
On water leak: he said he left the lower portion of the 1.25” pvc in place. Then the header slipped around it. So the pvc is now below the header and water only flows down... And, he has a small hole at base which allows any water that does find its way in the aluminum post “out” to the bilge.

Ahh i found it.. Wow. Looks really good man.
So. Heres where im at. And I havent seen anyone yet with this. People are going out and paying to have a rig fabricated.
I just received my 15Ton Basement Jack. They hold HOUSEs up. Their used all over the world. It was $75 !!!! It does not ever sag. Will post pictures once i begin. Seems like it should do exactly what you had fabricated.
Im amazed to see how small of a section you removed from the Header @SabreToothedEngineer . I feel much better after seeing that. We'r you having any water leakage from the beam ?
Im still confused on how I stop a water leak in this location with out removing the Mast ? As my wood down post was completely rotted.
Really appreciate the PDF . It brings clarity to a 1st timer.
 
Last edited:
Jun 21, 2004
1,610
Beneteau 343 Slidell, LA
I wrote up what I did in the hopes that it will help some others in the same way that the write-ups by several others helped me. I'll also send the drawing I made to anyone interested.
Brian,
I had a '88 Hunter 33.5 that had the same problem, so I am quite familiar with the process. Just wanted to complement you on the design and craftsmanship. The write up & photos should provide a path for anyone who has to do this repair in the future. The results were fantastic!
 
Jan 13, 2015
60
Hunter 34 Deep Bay, BC
Ahh i found it.. Wow. Looks really good man.
So. Heres where im at. And I havent seen anyone yet with this. People are going out and paying to have a rig fabricated.
I just received my 15Ton Basement Jack. They hold HOUSEs up. Their used all over the world. It was $75 !!!! It does not ever sag. Will post pictures once i begin. Seems like it should do exactly what you had fabricated.
Im amazed to see how small of a section you removed from the Header @SabreToothedEngineer . I feel much better after seeing that. We'r you having any water leakage from the beam ?
Im still confused on how I stop a water leak in this location with out removing the Mast ? As my wood down post was completely rotted.
Really appreciate the PDF . It brings clarity to a 1st timer.
I think the reasons we haven't heard of anyone who has used a floor jack are first, that it's steel, not stainless or aluminum, and will therefore be difficult to maintain if it tends to get wet, and sooner or later will have to be replaced. Second, anything that's adjustable will slip in the presence of vibration, so you would need to find a way to positively lock it in position (welding?). You might think it'll never slip, but you can't take that chance. There's too much depending on that post.
My own philosophy for any project or purchase is that I will always choose the option that produces the highest quality result I know of, with little, if any, consideration of cost. Given a choice of "good enough" or "best possible", I will always choose "best possible" based on the information available to me. Many years ago I used to try to do things at the lowest cost, but my experience has been that quality always pays off in the long run, so I changed the way I do things. I know lots of people who take the opposite approach, and good on them; there are probably a far greater number of people who try to do things at the lowest cost than the other way around. So when I went into this project, my mindset was on building something that will never fail, will look good, and will outlast the boat.
Good luck with your project. Hopefully you can isolate the leak where it exits the mast and find a way to route it to the bilge. I'm also curious how you plan to build trim around it to hide it. Or are you planning to just leave it exposed?