Structural role of gussets

Outboard designs up to 14'

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Shanff
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Structural role of gussets

Postby Shanff » Sat Aug 22, 2015 8:23 am

I'm starting to work the top side of my Malahini. I see I the archived photos one person trimmed the gussets back flush with the frame members which allows the floor and interior side to run the length without cutouts. Does anyone know if this will significantly affect structural stability? It seams that with the planking, decking, strong back, etc that there is plenty of static strength redundancy to allow running a flush trim router bit down the frames to remove this portion of the gussets at least for frames 2 and 3.
Stephen Hanff
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galamb
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Re: Structural role of gussets

Postby galamb » Sat Aug 22, 2015 1:26 pm

Gussets are added to strengthen a (typically) weak joint.

Depending what material was used to make your gussets with, will somewhat determine how much they could be cut back and still provide the required support.

If they were made with plywood they will manage the stress at the (underlying) joint better than if they were made with solid lumber.

Likewise, if you used plywood and doubled them up (epoxy the snot out of them when adding the "second" gusset) that would also offset the reduction in overall size of the gusset in terms of how much "abuse" they could withstand.

And while your decking/planking will give you lateral strength, that won't generally give you any benefit at the frame joint (think of it like an elbow or knee joint) - the gusset is the "tensor bandage" which is trying to stop that joint from "flexing".

So personally, if I was cutting back some of the "overall area" of the gusset, I would double it up and possibly add a few bolts through the entire joint - particularly up front in the first couple of frames since those are the ones that receive the lion's share of the "pounding" by the water while underway on anything other than a glass like surface.

(at least that's my thinking - not being a marine engineer)
Graham

Yes, Plywood is "real" wood :)

A "professional" is someone who gets paid for their work - it doesn't necessarily mean they are good at it :)

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hoodman
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Re: Structural role of gussets

Postby hoodman » Sat Aug 22, 2015 5:03 pm

Personally I don't think it's worth the trouble or "risk". I like the way the gussets look IMO.

Hercdrvr
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Re: Structural role of gussets

Postby Hercdrvr » Sun Aug 23, 2015 8:05 am

Working on
frames for my malihini. Used Maranti 1/2 ply for gussets. In pic notice pencil lines marking where frames are. I cant imagine removing that much material from gusset
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Bill Edmundson
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Re: Structural role of gussets

Postby Bill Edmundson » Sun Aug 23, 2015 10:08 am

The sharp corner weakens it also.

Bill
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Moeregaard
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Re: Structural role of gussets

Postby Moeregaard » Sun Aug 23, 2015 10:35 am

Without gussets, you have a face joint that is relatively weak. Gussets distribute stresses in this area into shear loads that are distributed over a larger area. Cutting them back so that you have a sharp corner will create a stress riser in this area. Will it fail? Who knows, but if it were my boat I would maintain some kind of corner radius. It makes installing a cockpit sole a bit more fussy, but this beats having to sister a cracked frame sometime in the future. I believe we maintained a one-inch radius on all frames, which have 1/4" aircraft ply on both sides. Seven years of use and we've had no troubles.
A boat is just a wooden box with no right angles.

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Iggy
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Re: Structural role of gussets

Postby Iggy » Mon Aug 24, 2015 3:14 pm

On my Malahini I just capped the open tops between the plywood gussets with a solid piece of Sapelle Mahogany and cut my plywood floors around them.
Image

You don't want any flaws in the wood to become a crack in your frame... the gussets brace the frames pretty significantly, and there will be times you will bump the sides on something ;)
Ian (aka Iggy)
My Malahini Build

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Andy Garrett
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Re: Structural role of gussets

Postby Andy Garrett » Tue Aug 25, 2015 4:14 pm

A Malahini is rated for up to a 60HP if memory serves. My Zip runs a 50 and takes quite a beating.

Think of the corner gusset like a wishbone. When you pull outward, trying to 'open' the joint, it is much easier if there is no material in a straight line between oposing directions of force. If the wood extends directly between oposing forces, it is exponentially harder to separate. You can test this in you own hands with the gusset plywood.

The gusset is an important element to a planing hull which will bounce and bang as it smacks wakes. I would double gusset every corner of every frame and move on to the next subject.

This is a corner I would not cut. Pardon the pun. :roll:
Andy Garrett

Perhaps the slowest Zip build in Glen-L history...

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Iggy
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Re: Structural role of gussets

Postby Iggy » Wed Aug 26, 2015 10:49 am

Andy Garrett wrote:A Malahini is rated for up to a 60HP if memory serves. My Zip runs a 50 and takes quite a beating.
...


I am using a Mariner 90 HP myself as I am towing the kids (and myself now and then) behind her. I am not sure if I did the math right, but the legal HP limit (coast guard) for a 16' boat could be 115HP. The 75/90/115 series for my Mariner were all about the same engine weight.
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galamb
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Re: Structural role of gussets

Postby galamb » Wed Aug 26, 2015 1:27 pm

To determine your max horsepower for a hard chine/planing hull (according to the Coast Guard) you use a formula.

measure the length of the boat (length overall works) in feet

measure the width of the transom (at widest point) in feet

multiply the length times the width - round off to the nearest whole number (this is called the "factor")

If you have remote steering and (at least) a 20" transom then:

multiply your factor by two (2)
subtract 90
and then round your result to the nearest multiple of 5

(sounds convoluted?)

Using the Malahini numbers from the website:

LOA is 15' 11" (15.92 feet)
Width is 6' 7" (6.58 feet)

So:

15.92 x 6.58 = 104.75 (so rounds up to 105)

105 x 2 = 210

210 - 90 = 120

nearest multiple of 5 is where the answer already is.

So 120 horses would be the max horsepower if built to spec with remote steering and a 20" transom.

When (we) rig factory boats, the boat mfg's generally recommend that you power it with at least 80% of the max horse rating in order for the boat to perform "as designed" (this is a pretty loose rule of thumb, but works in about 95% of various planing hulls).

So you would want to power with 120 x .8 = 96 horses - a 90 horse would probably be fine...

With a 15" transom (whether or not it has steering)

You multiply your factor by .8, subtract 25 and then go to the nearest multiple of 5 -

so same numbers again = 105 (factor) x .8 = 84

84 subtract 25 = 59

Nearest multiple of 5 = 60 horses (which was posted in one of the responses) - so 60 would be the max for a short shaft transom with 60 x .8 = 48 horses being the 80% (rule of thumb), making a 50 horse a "good choice" for a short transom Malahini.
Graham

Yes, Plywood is "real" wood :)

A "professional" is someone who gets paid for their work - it doesn't necessarily mean they are good at it :)

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Andy Garrett
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Re: Structural role of gussets

Postby Andy Garrett » Fri Aug 28, 2015 8:20 am

Graham,

It looks like you did your math using the beam measurement where you were supposed to use the maximum width of the transom. The beam is considerably wider than the transom as the boat tapers aft.

This error is leading you to believe that the Malahini can take a much bigger motor than it actually should.

Take a tape measure to you transom and re-calculate.
Andy Garrett

Perhaps the slowest Zip build in Glen-L history...

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Andy Garrett
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Re: Structural role of gussets

Postby Andy Garrett » Fri Aug 28, 2015 9:14 am

The transom looks to be 64" wide at its widest (judging by the pics). If true, that would mean that the hull is rated for 80HP according to the provided formula. Since it is rated for 85 according to the plans, my width estimate is probably an inch or two too narrow. 66" of transom width would be right on the money for 85HP.
Andy Garrett

Perhaps the slowest Zip build in Glen-L history...


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