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Catalina/Capri 25/250 Sailor's Forums

Catalina 250 Specific Forum

1/4-20 or 28?

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Bill Arden
1st Mate

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USA
83 Posts

Initially Posted - 06/04/2008 : 10:20:50

Any mechanical designers out there?

I'm going to bolt some fittings to my C250 boom. They require 1/4 inch bolts, and I need to decide between 1/4-20 and 1/4-28 bolts.

The boom material is not very thick, as you know. So am I better off with 1/4-20, which it would seem would leave more metal in the hole, or 1/4-28, which would engage a couple of more threads?

There's got to be a well-known answer to this, it's just that I don't know it!

Thanks and regards,

Bill Arden
Prana - C250WK #898
Apostle Islands, WI

Edited by - on

SEAN
Admiral

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USA
772 Posts

Response Posted - 06/04/2008 : 17:07:15

how bout a pop rivet ?

I added a t track to the boom and used 1/4 by 20 3/4 long .
with silicon . so far so good .

Edited by - on

Bill Arden
1st Mate

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USA
83 Posts

Response Posted - 06/04/2008 : 18:43:25

I'm a little worried about the loads. It's a T-track for a second reef (which means that if things are bad enough that I need it, I sure don't want to worry about it letting go!).

Edited by - on

Al
Captain

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USA
269 Posts

Response Posted - 06/04/2008 : 19:40:30

I think I'd go with 3/16" SS pop-rivets. Check out http://www.dwyermast.com/items.asp?cat1ID=40& cat1Name=Hardware& familyID=61& familyName=Fasteners. They also list some machine screws (coarse thread) but I don't know what they're used for.

Edited by - on

SEAN
Admiral

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USA
772 Posts

Response Posted - 06/04/2008 : 19:45:07

ya-a 28 would get a extra thread .

there is a little meat to it (the boom).

Edited by - SEAN on 06/04/2008 19:48:46

OLarryR
Master Marine Consultant

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USA
3440 Posts

Response Posted - 06/05/2008 : 05:52:12

There are other screws, etc in the boom but most are probably smaller diameter than 1/4". I am not all that familiar with exactly the type of fitting and how it is to be attached....one screw ? Is it possible to attach at different points with smaller diameter screws ? Is this thru bolted ?

I would think that for installing a fitting for reefing, the strain due to the any forces would not be all that great at that point. After all, it is a second reef point and the main will have been dropped down significantly which puts a lot less load on that area even though it would be during windy weather. The other thing is that the addl thread may not make much of a difference. Still, if this is one wood screw type (not thru bolted), I would be leary of counting on the minescule threads to hold.

I added a boom vang and that was 6 screws (smaller diameter) and boom seems fine. Not sure how the strain on that area would compare to your situation. But the fitting wraps the boom and so the fitting and screw holes does not put strain at anyone specific point and the screws are at different locations with some taking the load in shear versus tension (just pulling out).

Edited by - OLarryR on 06/05/2008 06:26:37

Prospector
Master Marine Consultant

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Canada
3159 Posts

Response Posted - 06/05/2008 : 07:09:15

from "Machinery's Handbook" page 1636

"<b>Coarse thread series:</b> This series, UNC/UNR is the one most commonly used in the bulk production of bolts, screws, and nuts and other general engineering applications. It is also used for threading into lower tensile strength materials such as cast iron, mild steel and softer materials (bronze, brass, aluminium, magnesium and plastics) to obtain the optimum resistance to stripping of the internal thread. It is applicable for rapid assembly or dissasembly, or if corrosion or slight damage is possible."

"<b>Fine-Thread Series:</b> This series, UNF/UNRF, is suitable for the production of bolts, screws, and nuts and for other applications where the Coarse series is not applicable. External threads of this series have a greater tensile stress area than comparable sizes of the Coarse series. The Fine series is suitable when the resistance to stripping of both external and mating internal threads equals or exceeds the tensile load carrying capacity of the externally threaded member. It is also used where the length of engagement is short, where a smaller lead angle is desired, where the wall thickness demands a fine pitch, or where finer adjustment is needed."

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Second Wind53
1st Mate

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Canada
45 Posts

Response Posted - 06/05/2008 : 10:00:59

Can you not remove the end cap of the Boom and put a regular nut on your bolts with needle nose or even a stick with tape on it?

Just thinking out loud.

Peter

Edited by - on

Bill Arden
1st Mate

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USA
83 Posts

Response Posted - 06/05/2008 : 11:47:35

Folks -

Thanks very much for all the suggestions and advice!

I think we can end this thread, but let me fill in the blanks of what I'm doing just in case you're curious:

I'm putting a track on the boom to carry a block with a bail. The track is a standard WM part, 8466799, and requires six or eight 1/4" machine screws. Since it's a second reef, it'll be fairly far forward on the boom, which limits my ability to bolt it on from inside. Since the boom material is fairly thin (3/16? 1/8? I haven't been able to find information about that), I was leaning toward a finer thread to provide more thread surface. Prospector's comments tend to support that.

The good news is that there's fairly little tensile (pulling) load on the screws; in addition, since the reefing line from the bail pulls down (under and around the boom, then up the other side) and the load on the block pulls up (the reefing line comes into it from above), the shear load should be somewhat balanced, too. At least I hope so!

Again, thanks to all for the suggestions and information.

Edited by - on

Nautiduck
Master Marine Consultant

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USA
3704 Posts

Response Posted - 06/05/2008 : 13:04:12

I can't wait to see this mod. Please post photos. I would like a second reef too but am not sure why you need a track to do it.

FYI, I have had the mast-end casing off the boom. It is crowded in there!!

Make sure you cut the bolts so they are just as long as you need and do not interfere with the lines running inside the boom or the sheeves in there.

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