calculating relative flotation values for boards

I want to build a new much shorter board that approximates the flotation -paddle power - of my favorite board. I can pull dimensions for width & thickness at various points, but i wanted to see if anyone had some sort of model/ equation that would allow me to scale a board that from a pure flotation point was the same, then i can decide on other compromises that might be needed to make a viable craft. The board in question is a 9’1" McTavish and my goal is to scale a board down into the 6’6" to 6’-8" range; obviously some sort of egg that incorporates some of the rocker,concave and rail characteristics that would make the board super fast down the line sort of the old school flow of late 60’s early 70’s single fins. I surf in central Mexico mainly reefs & points and getting north of 50 so paddle power is becoming more important. Any help would be welcome my goal is to have the design complete and built for the great summer Souths that are coming up soon.


If that McTavish is a Surftech, the volume is known. If it’s not on the board it’s on their website. To scale a 6’8’’ to the same volume, the easiest way for most people is to use one of the CAD programs like Boardcad or Shape3D. I can tell you that to match, the 6’8’’ is going to be very wide and/or very thick.

i figured a combo width around 22.5 and a thickness under the chest around 4". How do you think some sort of step-deck would work? I am not expecting it to be exact, but i’d really like to go a lot shorter without sacrificing all of the paddling advantages. Thanks for the response and i would appreciate any other advice or comments.

I think 4" might be overkill. If paddle-ability is what you’re after, you’re simply not going to approximate your longboard in a 6’8", no matter how you fit the foam in. There’s just no substitute for waterline length when it comes to paddling. If you go into this project with that in mind you’ll probably save some disappointment later.

The good news is, once you discard the idea of volume as the the deciding factor in paddlebility, you open up a lot of territory in terms of design. What you really want is a board which “rides shorter”, but which still paddles well. First thing, I’d think about going a bit longer than 6’8". In order to fit the kind of volume you’ll feel comfortable with in a 6’8", you’d have to make some design decisions which probably wouldn’t make it ride the way you want. Move up to the 7’ to 7’6" range and you’ll be able to fit some decent volume into the length, but still be able to narrow out the nose and tail, as well as thin out the rails, to make it feel like a shorter board. The extra waterline length and a lower rocker will help it glide while paddling. Again, nothing will paddle as well as a longboard. But if you go thinner/ less volume it gives you the option to duckdive. My experience in mainland mexico, a couple of well-timed duckdives can save you a buttload of paddling. Might be worth the tradeoff.

Amen brutha. I agree with the idea of a little more length. I have not had toomany duckdive issues, but you are right the occasional wouldn’t hurt. My main goal is a bit more speed and high-line drive as well as being able to wrap around the occasional cutback. I feel that shorter might assist this; i have no illusions of vertical projections and such that is not my style. How do you feel a wide sort of winged-rounded pin with a fairly flat rocker and single concave bottom config. would work for the goals i’m visualizing? I have a 8’ Walden rounded pin i really like too, but it is still lacking because it is very much a shorter/longboard. I mainly like hitting the bottom hard and carving off the top at high speed. Most of the waves i surf have easy maybe long paddles to points or reefs so i’m not busting my ass through shorebreak. Thanks for your advice more thoughts would be appreciated.

"Some sort of stepdeck’’ would definitely be a way to hide a 4’’ thick section, but it’s hard to shape unless you’re experienced. And if you’re doing it in PU, it’s hard to find a blank that would yield with decent rocker. Schwuz’ advice about going a little longer would be an easier solution, but I was trying to answer your question as asked. He’s also right that there’s really no such animal as the ‘‘shortboard that paddles like a longboard’’. Catching waves is one thing, but those long paddles to distant reefs, or paddling back up a long point against a current, is another.

Thank you for the advice and i think i will move up a little in length. Do you think that a relatively flat rocker with low entry will help with the glide in paddling; as well as down-the-line speed? Most of my breaks are reefs with channels or points with long but relatively easy paddles. I just want to be able to get into waves early when i want and surf something that has that bar-of-soap squirtyness. I have a traditional FiSH that is really fun, but doesn’t quite get me where i want to be. I am an intermediate surfer that started in the 60’s & 70’s and then got landlocked through most of the 80’s and early 90’s. I have been back with a vengeance for about 12 years surfing Mexico averaging about 90 days in the water. I like to go fast and carve and that works well with the waves i surf. I shut down at about 8’ so larger waves are not a concern. I am also not opposed to have someone create a custom for me so my lack skill with shaping shouldn’t have to be an issue. I’m just looking for something that i haven’t seen commercially whether it’s through SUrftech or through shops. I really thank both of you for your help.