Mast support pole in cabin of Glen-L 17

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EWeitzel
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Mast support pole in cabin of Glen-L 17

Post by EWeitzel » Fri May 05, 2017 9:46 am

I am considering buying a Glen-L 17 Overnighter. One thing that bothers me is the mast support pole that is right in the middle of the bunk. I have seen other boats that do not have this. Usually the mast is over a bulkhead, very near the front cabin wall, or has some special carbon fiber reinforcement. I am thinking a retrofit to eliminate the pole is possible. I have no experience with boat building, but have some carpentry experience in house building. In a house an unsupported load would be carried by girder, beam or header. I am wondering if something similar could be devised to eliminate the pole. Perhaps some plywood strips on edge cut to fit the roof. Perhaps several strips laminated together, maybe laminated with carbon fiber, attached to and supporting the roof under the mast. This mini beam hopefully would not be that deep, as the headroom in the cabin is already pretty tight. Plywood is pretty strong when set on edge. I have seen floor beams made of 1/2" plywood set on edge that had virtually no deflection, whereas a 2x10 could be deflected with a crowbar or other lever. I am hoping some combination of plywood strips laminated together, and perhaps also laminated to carbon fiber cloth. filleted to the roof under the mast could substitute for the mast support pole and only be less than a couple of inches. Has anyone successfully eliminated the mast support pole in a Glen-L 17? Does anyone have any thoughts on this? Can anyone save me from myself and this idea?

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DrBryanJ
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Re: Mast support pole in cabin of Glen-L 17

Post by DrBryanJ » Fri May 05, 2017 11:12 am

I know nothing of sailing, but I think you have to consider lateral forces on a mast. Beefing up the cabin top to support the weight may not provide enough strength when the sails are full.
Bryan

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Bill Edmundson
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Re: Mast support pole in cabin of Glen-L 17

Post by Bill Edmundson » Fri May 05, 2017 11:36 am

That pole is called a compression post. I see 6 mast stays. When they are properly tights there is a lot of downward load and even more under sail. I'm a structural engineer and I wouldn't do it.

Bill
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EWeitzel
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Re: Mast support pole in cabin of Glen-L 17

Post by EWeitzel » Sat May 06, 2017 8:35 am

Thanks for your replies. I agree caution, in removing a support is quite prudent. Yet, other boats have done this successfully. The Sage 17 has no compression pole and the mast is on the roof no closer to a supporting wall. Bill, I thought your comment was right on point. The pole is for compression support, and offers little resistance to lateral movements of the mast base. Lateral movement resistance would be provided by the existing roof. Of course, when a structural engineer says don't do it, I'm listening. Any idea how the other boats did it?

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Bill Edmundson
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Re: Mast support pole in cabin of Glen-L 17

Post by Bill Edmundson » Sat May 06, 2017 9:02 am

I could probably come up with a load from the dead load and the stay tension. But, I don't know the other loads involved. Something of beam system like you proposed could work. I've never done timber design. I can't just say, "Yes, that will work."

Bill
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Stuart
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Re: Mast support pole in cabin of Glen-L 17

Post by Stuart » Mon May 08, 2017 6:07 am

When I changed the cabin top on the Glen-L 25 I was concerned about the stanchion post. What's important is that the weight of the sails and mast be supported by the boat. Now the boat is a combination of elements and all are supported by the water and its buoyancy over the surface area of the hull. The weight of the mast is not supported by a single point but over an entire area of the hull starting mostly at some part of the keel and may be reinforced to spread that load over a larger area. In addition there is the standing rigging pulling the mast assembly downwards and this also is supported by the stanchion but in a different way. The carling and coaming which extend along the sides of the boat while holding the mast laterally also pull the mast and sail down onto the stanchion and to some point onto the keel. What is happening is the carling/coaming are trying to pull the top off the boat by forcing the stanchion through the bottom. It can be calculated but then there is elasticity of the elements, inertia load and so on. It's not a static load but what you do know is that the wire you use has a tensile limit and you could consider it to be your weakest link. So if the wires in your standing rigging will break at 3000lbs, then your stanchion should be in that area too. However, flexing of the system is not desirable because an inertia load is a bad thing. If the load bearing elements flex a lot then the wires become loose and when they spring back the load is by the square of the numbers. You mentioned about the plywood being more rigid. It is generally agreed that the more plys in the plywood of a given thickness then the more the plywood will flex. However plywood does not like to bend on its edge and is more ridged than a similar board. I don't off hand remember if the plywood will ultimately fail at the same force but without bending. What you should consider is how to move the weight of the mast and force of the rigging to the bottom of the hull by a combination of members. You have to spread the load over a large area somewhere around the backbone of the hull and you can not allow the combination to flex like wagging dog's tail. It can be done but I would use metal in some places where the forces are concentrated until you can spread them out over the hull.

Stuart

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NAMEngJS
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Re: Mast support pole in cabin of Glen-L 17

Post by NAMEngJS » Mon May 08, 2017 12:43 pm

"Skene's Elements of Yacht Design" has a simple formula for mast design (granted it is a little dated... written in 1904) should be a good starting point for the calculation of the required load.

1.85*[(1.5*RM30)/(b/2)]

where RM30 is the righting moment at 30 degrees and b/2 is the lower shroud spreading. The 1.5 coefficient is to take into account added loading from higher righting moments and the 1.85 coefficient is to take into account shroud and stay trimming.

Hard part now is to determine the finished VCG and the Metacenter of the vessel to determine the Righting moment (which would be the displacement * the lever arm distance from the CG to the center of buoyancy transversely.) If you could possibly estimate those with any certainty (or do an incline test on another Glen L-17) your would have a decent starting point as to the required load.

Another option would be to determine a theoretical max load (compression) of the interior column and then add in a smaller steel pipe (xs or xxs wall thickness) or round bar stock that can withstand the same load and this may net you more internal area for ease of use

The biggest question is "is it worth the work (calculations and retro-fit versus the perceived inconvenience that the pole provides.
-Juan Suarez

All men dream, but not equally. Those who dream by night in the recesses of their minds, wake to find that it was vanity: but the dreamers by day are dangerous men, for they may act on their dreams with open eyes, to make them possible.

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