Polaris 43 Compression Post

[Brew]

I currently have an opportunity to purchase a Polaris 43. I know this is one of the designs that was stolen from Bob many years ago. I’ve very sorry about that but I still think it is a beautiful boat. I had a survey completed and he came back with a modification to the compression post that I’m not sure is proper. I’ve attached a crude sketch of the problem.

The surveyor said that the top stainless steel plates at the top of the cabin and the one screwed to the teak beam below deck was too thin. He said he saw the bottom one had been flexed about 1/8" when under a heavy load. He thinks new thicker plates should be welded on. My thinking is that the plates are only used to hold the post in place when there is no pressure. They are not structural. The post itself is the structural member. The way the blocks resting on the keel are spaced to distribute the load evenly. He also suggested placing another block directly under the post so the beam would not flex under load. I think that is a mistake. That would place pressure only on that point on the keel, not disbursing it. I believe his change could even cause damage to the keel under extreme conditions.

Any advice would be appreciated.

[perchance]

How thick are the plates and can you provide a top view?

[Brew]

I'm not sure of the gauge, but it is frairly thin. I think the main purpose of them is to hold the post in place when there is no compression.

[donradcliffe]

I had a friend who had a Polaris 43 built in Taiwan, and Bob Perry was heavily involved in the design-he redid the deck layout.

The post carries the load, but he purpose of the plate is to distribute the load over a larger area, especially when the material which has to transmit the load (like the teak beam) doesn't have the compressive strength of SS. If the bottom plate has permanent deformation, it is obviously not thick enough to do its job. If the plate deformation is due to the post pressing into the teak, its time for a thicker plate. If the deformation is due to the teak beam bending, maybe its time further stiffen the plate and/or to improve the beam's support structure.

What are the dimensions of the teak beam and how far apart are the blocks? In your sketch, only the 2 blocks next to the post will be transmitting any significant load to the keel. What is the keel material?

A picture showing how the plate is deformed would also help.

[CharlieCobra]

Ah, Bob wasn't involved. The design was pirated. Bob had a major dislike for the boat and the yard for a long time but has mellowed in his senior years unlike some of us...,

[SV Sedna]

Here is a photo of the compression post and the plate. It is my opinion the plate is non structural and is designed to maintain the total vertical column of the mast. It is 4mm SS.

Charlie you gotta stop hating on the Polaris 43. Every time a post comes up the only thing I see you give is a rant about Bob not liking his own design. The only part Bob was not involved in with this design is the interior layout and the manufature of the 12 additional layups beyond Sunflower (the original Polaris 43). He has told me he likes the design but was bummed the way it was handled by the Liggets and he didn't get his slice of the pie. Its a great boat and if you would like to get onboard and really check one out PM me.

[TassieBloke]

Disclaimer: I am not a boat builder, but I am an Engineer. Take my comments with a pinch of salt and large amount of Tequila

For the sake of the discussion, lets name your fibreglass supports looking left to right, A, B, C & D.

In the drawing that you have shown, blocks A & D will not distribute any of the vertical load, only horizontal loading. If you place the compression post in between supports B & C, you will need to spec a plate with much thickness to resist bending. It is best to place a continuous support under the post, something like a square section (sort of like a stringer) of fibreglass or ss section. This will distribute the loading a little better than what is shown.

The plate which is bolted at the top & bottom, is not 'structural' as such, it serves to first, obviously keep the post in place, and secondly to distribute any horizontal loading that may occur. Other than that, you can keep it nicely polished!

I can see what the Surveyor has tried to achieve, but with the wrong solution. Essentially you can say that any point loading that is placed on a beam, will distribute at 45deg in all directions in the horizontal plane (think of a cone spreading out from under the post dia) After that, the load is not spread too well, and becomes ineffectual.

I hope this has helped a little, or helped to understand the problem.

Cheers,

Bloke

[sailvayu]

Does the surveyor have a engineering or navel architecture back ground? If not why is he redesigning the boat? As a surveyor I never make recommendations to change a boat. If something does not look right or I think it could be a problem I recommend the buyer have a naval architect or engineer look at it. A surveyor is there to report the condition of the boat not redesign it. I would get a second opinion before making any judgments or changes.

[CharlieCobra]

Originally Posted by SV Sedna

Here is a photo of the compression post and the plate. It is my opinion the plate is non structural and is designed to maintain the total vertical column of the mast. It is 4mm SS.

Charlie you gotta stop hating on the Polaris 43. Every time a post comes up the only thing I see you give is a rant about Bob not liking his own design. The only part Bob was not involved in with this design is the interior layout and the manufature of the 12 additional layups beyond Sunflower (the original Polaris 43). He has told me he likes the design but was bummed the way it was handled by the Liggets and he didn't get his slice of the pie. Its a great boat and if you would like to get onboard and really check one out PM me.

Attachment 33698

Actually, I didn't hate the boat. I saw one at the boat show and wanted one for myself when they first came out. Years later I was looking for one, heard Bob was the designer and asked him about the boat on a BBS. The rant I got back was EPIC!.

[Brew]

Thanks for all the replies. I think I'm going to take the advise and have a naval engineer look at it. Better to be safe then sorry.

Charlie. I do understand that this is one of the designs that was stolen from Mr. Perry which I think is horrible. I have mad respect for the man and his body of work. But the boat is in existence and it's a beautiful design while the Chein Yu company that stole it is out of business. I guess that's karma.

[Cheechako]

If the deck is not caving in after all these years, and whatever is under the bottom plate is solid, leave it alone. It's quite likely that the plate was "cupped" from the welding stress from new. It will take a pretty thick plate to really distribute the load much outside the post foot. Sure, if you were building a new boat you might make the plate 1/2" thick, but if it aint wrong dont fix it. Love those boats... If the plate is really bending like the surveyor says, then you need to look at what's under it more importantly.

[casual]

O Raa for Sail Vaya