Finishing a four-string ukulele as an 8-string instrument.

[mooshalah]

Hoi!

I'm the bloke who perhaps two months ago posted a picture or two showing stuff I was making, and in this were a number of guitars and ukuleles (five ukuleles, to be exact).

I'm getting close to finishing the ukuleles, and have decided - for the sake of variety, and to stem boredom - that I should make at least one of these ukuleles a bit different from the other four. The bodies are completed and closed - so I can't open them up to add extra bracing.

What I'm thinking I'd like to do with one of them is to complete it as an eight-string instrument, but the question I have, and about which I'm now seeking advice, is whether this is wise or foolish. Another option is to complete this as a six-string instrument.

Some details are as follows;

Tenor sized body, 17" scale, 14 frets to the body;
Three symmetrical spruce fan braces are 4.5 mm wide X 8.0 mm high;
Two spruce top braces (above & below the sound-hole) are 6.0 mm wide X 10.0 mm high;
Soundboard is pretty stiff, figured Blackwood, 2.0 mm thick;
There's enough meat left on the fingerboard and neck to make them up to 41 mm wide at the nut, and 49 mm at the 14th fret.
I've not considered what (paired) strings should be tuned to what, but I'm flexible; I just want to hear what a fuller sound is like, on a ukulele (I made a few five-string instruments about a year ago, and quite liked them).

So, the question:

D'you think I'll be OK, expecting the soundboard to be able to take the pull of eight strings, even though the system was designed for four?
If not, how about six strings?
And if either, might it be necessary to add a carbon-fiber rod to the neck (which is not finally shaped, and could be left a bit thicker than normal, if necessary)?

Any input, advice, comments etc will be greatly appreciated.

Frank.

[kiwigeo]

Thoughts from my experiences with 12 strings and lutes. Allen is the resident uke professor so he'll no doubt have some sagely advice and answers to your questions.

You're dealing with nylon strings so I dont think you'll have any major issues with the increased tension unless your top is paper thin or you have very little bracing.

Tuning of the second strings......without knowing much about double course ukes a sensible choice would be either unison or octave......octave for the lower strings and unison for the trebles. If youre fussy about accurate compensation then that may be a control on your choice of string tunings.

[J.F. Custom]

There are too many variables to suggest whether you would be successful in changing to a 6 or 8 string at this stage. But I would suggest you proceed with some caution as the string tension does jump more than you might imagine. I'll quote myself from an old thread on a six string tenor I built years ago -

It is not six individual strings or notes like a Guitalele, as the pictures will show. Two courses are octave doubled like some Bouzouki's, so it is tuned - G, High C, Low C, E, Low A, High A.

David Hurd warns builders that going from a standard four string tenor ukulele at 39lbs tension, increases the cumulative string tension to the six string version at not quite double that - 65lbs. A big jump and one that requires careful consideration of bracing and a proportionate increase in bridge size/gluing area.

You can see the instrument here -

viewtopic.php?f=25&t=4919

Cheers.

[Allen]

Double the strings.....double the tension.

Most people completely overbuild for 4 strings, so you might have some success. There is simply no way for anyone to say if your bracing will be sufficient without having the part in hand. Sound boards, braces, and more importantly the pattern you put those braces down have a huge impact. As well as how much they may or may not have been scalloped. You'll have to make an informed choice, or flip a coin.

Nut width on an eight string is traditionally 38 or 40mm. I've only ever seen one out at 42mm though I'm building one right now for a client that requested that. Your spacing at the saddle is a bit more variable.

Tuning is traditionally gG, cC, ee, aa.

[blackalex1952]

If you have seen, can access, or purchase the Gore/Gilet books, there is a section on measuring the bridge rotation. You can also check out a post by Craig Bumgarner on this forum, he builds Selmer Maccaferri guitars. In one of his posts he has a good photo of his soundboard deflection rig, a mass is placed on the bridge and the deflection can be measured. If you double course the strings, you could assume that the tension will double. Therefore, the soundboard needs to reflect this. If you have made nice sounding ukes and know the top deflection that works for you, then you can get a good idea of how much to beef up the bracing. Soundboard thickness may be an issue with heavier strings as well, but if the top is already in place and braced, then you have to live with the top thickness as is is. It should be easy enough to pop the back off a uke for brace adjustments, ie, strengthening the braces if necessary. If you are careful and made the body in a side mold, (you can put it back in the mold) it is possible to deflection test without the back in place, once you know the target deflection you are aiming at. I'm not sure about adjusting the bridge rotation with flat top ladder braced instruments, as I suspect that the top thickness combined with the bracing on the soundhole side of the bridge affects this. With X braced instruments, the bracing on the soundhole side of the bridge is where most of the compression forces from the bridge are on the braces, so that's where bridge rotation adjustments can be made... All you more experience engineers and builders please correct me down if I am wrong, but this would be my approach.

[blackalex1952]

If you have seen, can access, or purchase the Gore/Gilet books, there is a section on measuring the bridge rotation. You can also check out a post by Craig Bumgarner on this forum, he builds Selmer Maccaferri guitars. In one of his posts he has a good photo of his soundboard deflection rig, a mass is placed on the bridge and the deflection can be measured. If you double course the strings, you could assume that the tension will double. Therefore, the soundboard needs to reflect this. If you have made nice sounding ukes and know the top deflection that works for you, then you can get a good idea of how much to beef up the bracing. Soundboard thickness may be an issue with heavier strings as well, but if the top is already in place and braced, then you have to live with the top thickness as is is. It should be easy enough to pop the back off a uke for brace adjustments, ie, strengthening the braces if necessary. If you are careful and made the body in a side mold, (you can put it back in the mold) it is possible to deflection test without the back in place, once you know the target deflection you are aiming at. I'm not sure about adjusting the bridge rotation with flat top ladder braced instruments, as I suspect that the top thickness combined with the bracing on the soundhole side of the bridge affects this. With X braced instruments, the bracing on the soundhole side of the bridge is where most of the compression forces from the bridge are on the braces, so that's where bridge rotation adjustments can be made... All you more experience engineers and builders please correct me down if I am wrong, but this would be my approach.-Ross

[Steve.Toscano]

Tenor sized body, 17" scale, 14 frets to the body;
Three symmetrical spruce fan braces are 4.5 mm wide X 8.0 mm high;

Sorry i cant answer your question.... BUT, these fan braces seem way to large for a 4 string uke. Depending on brace and top materials there should be some serious stiffness in that instrument.
* im no uke expert.

[mooshalah]

Hello all, and thank you for your thoughtful, considered answers and opinions regarding my question.

It's a real privilege to be able to access the thoughts of others - many known and acknowledged experts - and have them turn their attention to an issue. I'm very grateful.

Jeremy's cautions are absolutely correct; there is going to be around double the tension on the instrument, if it has eight, rather than 4 strings. On the other hand, as Allen and Felix intimate,the instrument might be overbuilt, anyway; certainly, I've seen many more lightly braced (and unbraced, with no bridge patch) tenor ukuleles that seem perfectly stable and solid.

My feel (and it often comes down to just this) is that the top is sufficiently stiff, the (yes, scalloped) braces firm enough, and the bridge patch sufficiently stiff and thick

And thus it has, as I expected it might, come down to "the only way you'll ever know is if you try it", but with a whole lot more valuable input upon which to make the decision.

I'm gunna try it as an eight-string! The good thing is that I'm making the instrument, it's my risk, nobody get hurt, I get a little excitement in my life, and I can tell myself that although I'm getting on, in the words of the old Cat Stevens song, "I'm on the road to find out", and that's a good thing. If it all pops apart, you'll not see my red face.

I reckon that if I run four courses of doubled strings tuned as Allen has suggested, and plug them into a very slightly larger bridge than normal, and use four bridge pins into which to run the four pairs of eight strings (rather than tying them to the bridge), things might bend, but they won't rip!

That way, I can be assured of at least of a few months, if not years, of great jingly, jangly sound. I'm off to get started!

Kindest regards,

Frank