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Old 11-01-2012
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Forces acting on mast plate

I installed a mast plate this past spring, with the intentions of running some lines back to the cockpit. On my boat the main hatch is off-set to stbd, therefore, all of the lines will be on the port side. Will all of the blocks at the base of my deck steped mast, all pulling in about the same direction... um... pull the base of the mast out? The plate is screwed down throught the mast step into the deck with about 10 1 1/4" screws.
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Old 11-01-2012
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Re: Forces acting on mast plate

Is this a collar that goes around the mast step or the step itself? Screws or thrubolts?
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Old 11-01-2012
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Re: Forces acting on mast plate

It is a plate that goes under the step. It is one I got from catalinadirect. I had to drill 10 holes around the big hole in the middle.
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Old 11-01-2012
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Re: Forces acting on mast plate

I doubt if it's a problem. It should be thrubolted for it's own sake, but as long as the mast step is thrubolted it shouldn't disturb the mast step. I'd be concerned about it pulling out, though, if it's just screwed in.
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Old 11-02-2012
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Re: Forces acting on mast plate

I would through bolt the plate, with at least washers inside. Stainless steel plates look nicer though.

Above deck :



and inside :

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Old 11-02-2012
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Re: Forces acting on mast plate

On a 27' you can use fender washers for backing or make backing plates of aluminium as well but, as Mark says, polished S/S looks best - it's also the hardest to make.

Don't worry about the lateral forces on the step - it's a 27 footer - the forces aren't that great.
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Old 11-02-2012
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Re: Forces acting on mast plate

MarkSF - That looks good. Do you have a fiberglass headliner? Was wondering if is is strong enough to put backing plates directly on liner.

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Don't worry about the lateral forces on the step - it's a 27 footer - the forces aren't that great.
Probably proportional to bigger boats. But, so is the hardware. The thing that worries me is that they will all be in the same direction. Although, even if on both port and stbd, they would all be pulling back.
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Re: Forces acting on mast plate

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Originally Posted by Barquito View Post
MarkSF - That looks good. Do you have a fiberglass headliner? Was wondering if is is strong enough to put backing plates directly on liner.
.
On my boat the deck is solid, just gelcoated on the inside, there is no liner per se.

Bristol originally used the same backing plate design for the main halyard that came back to the cockpit, so I copied it for the new hardware I've added, for continuity of appearance.

It is possible to work with s/s plate at home with the proper technique. I cut it with a jigsaw with HSS blades (they do go blunt quickly), drilled it with HSS drills, filed it, then sanded it from 60 down to 1000 grit progressively.

When cutting or drilling stainless, the trick is low, low speed, plenty of coolant, plenty of force.
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Last edited by MarkSF; 11-02-2012 at 05:45 PM.
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Old 11-02-2012
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Re: Forces acting on mast plate

Quote:
Originally Posted by Barquito View Post
I installed a mast plate this past spring, with the intentions of running some lines back to the cockpit. On my boat the main hatch is off-set to stbd, therefore, all of the lines will be on the port side. Will all of the blocks at the base of my deck steped mast, all pulling in about the same direction... um... pull the base of the mast out? The plate is screwed down throught the mast step into the deck with about 10 1 1/4" screws.
Ah... Isn't it a bit late to think about that?

As a practical matter, screws are good in shear--i.e. loads across the screw--but not so great in tension--loads along the length of the screw. Assuming you mast passes through the base plate rather than bearing upon in, the question then becomes what the screws are screwed into and whether that material will endure the prospective tension loads applied by you lines. The turning blocks through which the lines pass will apply a load to the base plate at an angle that bisects the angle made by the line as it comes down from the mast and turns around the block. If that turning angle is 90º, the load applied to the plate will be at a 45º and equal to the load in the line (tension) divided by the cosine of 45º. Assuming the plate is sufficiently rigid, the vertical loads will be carried by at least several of the screws you installed. If the plate is flexible, however, and it looks as it it might be, only the screws in the closest proximity to the turning block/line will be loaded (in withdrawal) which is undesirable. If your mast is deck-stepped with the step sitting atop the base plate, then the screws with endure far less withdrawal loading and, in shear, should be okay.

The saving grace to the foregoing (assuming a keel stepped mast) is that, on your boat, the loads on the lines may be sufficiently low that you'll get away with your lash-up. The only loads I can see you having to endure are the jib and main halyards and perhaps a cunningham. A vang lead should go the the mast itself rather than the base plate. Other likely loads, reefing lines etc. would only be applied when other line loads are somewhat reduced.

Hindsight is (almost) always 20/20. It's foresight one needs work on eh?

FWIW...
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Re: Forces acting on mast plate

Quote:
Originally Posted by Barquito View Post
Probably proportional to bigger boats. But, so is the hardware. The thing that worries me is that they will all be in the same direction. Although, even if on both port and stbd, they would all be pulling back.
Not really - dimensions and forces on boats vary exponentially with the size of boat, not proportionately or even geometrically.
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