8:1 ratio

Does anyone use such a low ratio for their structural scarfs? A lot of boat building books advise 8:1 as a minimum ratio, but I'd never use such a small ratio for anything that would be very stressed.

Why do people give such a small ratio when something like 12:1 would be stronger and really isn't any harder to make? Why not advise 18:1?

 

Because it is wasteful of material.

 

Have a read here for more info:

http://www.boatdesign.net/forums/boat-building/scarf-strength-question-40707.html

 

boat building books advise 8:1 as a minimum ratio,

Its all in whats been written down and why do we not read the words and get the meaning of what they are saying to us !! The MINIMUM IS !! In otherwords dont go below !! you can do what ever you want BUT DONT GO BELOW 8=1 Ratio .

Like having a instruction manual for anything . how often do we read the book from cover to cover and then carry on unpacking what ever it is and making it work as it should , usually its only used after we have pushed all the buttons and cant get our fabulous new whatever to work the when all else fails we read the instruction book untill we get to the part where it says how to plug it in and turn it !!

Sorry i spend a good part of my day with a female translater discribing our english language in all its histerical and difficulties of meanings and how to pronounce differant words . I never realised what the english language was like till i was asked to discribe each word and why i was using those words i had chosen to make a sentance !!.

 

Beyond 8:1 the edges are too thin and hard to work. There is no structural advantage.

 

SO DONT USE IT !!

 

8:1 was developed for mechanically fastened and/or self locking scarfs in solid lumber. This ratio has carried over, partly from tradition and success, but also from a practical view (as has been noted).

Depending on the loads and the materials, I've used lower scarf ratios with good results. An example would be plywood at 6:1 with no breakage to date. It must be qualified that I used well prepped epoxy joints on these. I've also done Payson butt joints, where the amount of taper in each piece was lower then 6:1, but these also received reinforcement (fabrics).

Simply put, the 8:1 ratio is a good rule of thumb, but can be longer if you think it's necessary. A feather edge doesn't really do much in a joint, but if the scarf is hooked or nibbed or other wise attenuated, the additional slope may not be as much of an issue.

 

Please share with us your sources which have convinced you that "such a small ratio" is totally inadequate. I tend to use a scarf ratio in the 8-10 to 1 range and have never had a failure. I think that I received my initial guidance from Gougeon's West System information; most of their recommendations are based on actual testing. If better sources of guidance exist, I need to be updated.

 

With epoxy I never go more than 7:1 and never had a failure.

 

isn't epoxy laminate always stronger than plain wood, so...

perfectly OK to scarf as long as you wish?

 

Not really. It is a waste of material, makes work more difficult, the very thing edges get saturated with glue and make a hard spot, and at the extreme with an infinite to one scarf it will be a veneer laminate.

 

The most highly stressed point on a sailboat is a freestanding mast just above the deck exit. If an 8:1 scarf works there (and it has been known to do so), then 8:1 is adequate anywhere else. The mode of failure is in bending and the high stress point is at the feather edge on the tension side. Prevent a rupture at that point and the scarf will never fail. When I had a mast break in a collision with a port tacker, I did use a higher ratio of about 10:1 and it never failed. Playing safe in a critical area. A higher ratio than 8:1 in plywood is just wasteful.

 

On spars I often use 10 and 12:1 ratios and the Gougeon brothers are now also making this recommendation on spars, which is a change from their 1st through 3rd boat building book recommendations (4th and 5th editions show 12:1). On these longer slope scarfs I usually nib the ends, other wise you have a 1/5 of the scarf length in feather edge, which is useless.

 

This question keeps coming up. I routinely use 6:1 provided I can get a perfectly flat cut using a chop saw on solid lumber, and 8:1 for ply where I have to use a plane and a perfectly flat cut is not guaranteed. These are the slopes I use for stressed joints; for a joint that will not go on a bend I will occasionally go down to 4:1. These ratios are based on analysis and testing, not tradition or guesswork.

I did a structural analysis followed by an experiment to confirm the analysis results. The bare minimum theoretical scarf slope is sqrt(X/Y) where X is the with grain strength of the wood and Y is the cross-grain strength. Typically the strength ratio is around 10 so sqrt(10) = 3.16 is the minimum scarf slope. I tried it and it worked fine, sometimes the the wood broke and sometimes the joint; at 4:1 the wood broke every time. However these steep slopes are difficult to clamp, they tend to slide apart and there is a tendency for the exposed partial end-grain to draw glue away from the joint. So in practice I usually use 6:1 or 8:1 as above, which also provides a safety factor.

So far I have never had a scarf joint fail at 4:1 or longer. The very long scarf slopes go back to the days before decent glues, when scarfs were routinely nailed. But use them by all means if it is part of your belief system.

Caveat: the above applies to wood/glue combinations where the joint is as strong than the wood, which is the case for softwoods and absorbent hardwoods like Okoume. For dense hardwoods or woods that do not take glue well the use of longer slopes would be indicated; the way to be sure is to make and break test samples.

 

Sorry, I've been sick and haven't been able to post.

I was talking about things like spars and un-stayed masts, where the scarfs are highly stressed.

I'm surprised so many people favor shorter scarf lengths. If we posted this question in an aircraft forum, they'd say we're crazy for suggesting scarfs as low as 4:1! These are people whose lives are depending on them.

Perhaps the caveat is that it's difficult to make longer scarfs and therefore shorter ratios are stronger in real-world conditions?