Maximum surfboard not SUP width relative to planing speed?

21 - 23" wide is a good practical max. width for an easy paddler, I have a 24" wide board on loan from a friend, with down rails all the way, and the width itself doesn't slow the board down, its plenty fast. But a little uncomfortable for my bad hip to straddle.

Here in Germany is a wording from canoe and ship builder that says: "Länge läuft", which can be translated into "lenghts runs (fast)". I think this can be transfered to all water vehicles, even for surfboards and SUP's. But it does not have any ratio of width to length. Finally it means only, that at a given width, the longer, the faster. But mathematically you can revert this also into at given length, the narrower the faster. But this is for the vessel only, maybe not suitable for a 5kg surfboard, which is loaded with a 160lbs surfer...

If I build my boards, I like to have them wide enough, which makes riding easier, but not too wide, because it hinders may paddling. I orientate myself at similar boards from more experienced shapers like me. In detail I like to follow the guidelines of real experienced guys, like Mr. Thrailkill. I have build not more than 10 boards and I'm a quite poor surfer too...

Curve shape from tip of nose to wide point affects "plowing."  Width to length ratio affects the curve's shape in the front section, and therefore, the plowing effect.

I have no idea at what point this plowing effect becomes noticable/significant.  However, as others have already said, physical discomfort paddling/straddling may be the deciding factor.

Trial & error:  front-section curve shape plus width/length ratio...

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On the flip side, what's the skinniest you can go?

The older I get the wider I like. 23” for me, and next board may be wider.

I think the other guys have indicated that there’s a Practical variation that works for all surfboards, much like rocker, vee, concave et al. You can try wide variations but there’s a Range that generally works taking in variables like the wave power and the riders weight.

 Smaller waves would benefit from width for early planing, bigger waves less width to keep the board from leaping.

 To answer your original  question , I don’t think there’s a max width that slows a board but it would affect riding control once you ride meatier waves.

While I realize that Sami specified "Not SUPs" in title, I think it is still valid to reference some extremes that appear to be functioning quite well in 'normal' waves.  By that, I mean not necessarily huge waves requiring gun dimensions.  Even with that clarification, there is plenty of evidence to support the concept of wide widths on SUPs in big waves.

I am also aware of a number of bodyboards being shaped in the 6 foot range with widths just under 30".  Also consider that 'Hawaiian Paipo Designs' has been building spooned paipos 48" long and less with wide points at around 30" for a long time.  I don't think anybody would discount the speed these are capable of achieving in the hands of a skilled rider.

Sorry if I'm straying too far off track but I don't see the point in discussing 'theory' when the evidence is right in front of us regarding width and it's application in the surf zone...

That's interesting about bodyboard design, I wasn't aware of that.  We know SUPs can be wide, and fast.

I don't see width as an isolated factor determining speed.  I tried to steer the o.p. toward a "practical" max width for a surfboard.  Practical, by definition, means pragmantic, sensible, realistic, ergonomic, concerned with actual experience, ie., empirical as opposed to theoretical.

SUPs and bodyboards have different guiding design factors, as a general rule you don't lay on them and paddle with your arms into the wave, and you don't sit and straddle them in the lineup between waves.  So the practical maximum width for a surfboard seems to end up somewhere between 21 and 23 (or 24) inches, depending on a variety of circumstances, like where you surf, how much stability you desire, how wide a board you are comfortable paddling and sitting on, as well as the rest of the design criteria for the board in question.  Skimboards don't require paddling or sitting, they can be very fast in mushy waves and in juicy waves, and they seem to max out around 21 or 22 inches. 

The width itself isn't necessarily going to slow you down, but excessive width can impact things like maneuverability and comfort.

In my opinion. 

Your mileage may vary.