Bonzers and Venturi effects

I have been getting interested in the Bonzer boards due to many comments made in this BB. The surf was blown out this morning so I went to Jacks and looked for a Bonzer but they didn’t have any. Does anyone know a dealer in OC? I want to examine the bottom configuration with the intent to build one. Anyone know of a place where I can rent one in order to feel what a ‘real’ Bonzer feels like in the water? Thanks, Magoo

I have been getting interested in the Bonzer boards due to many comments > made in this BB. The surf was blown out this morning so I went to Jacks > and looked for a Bonzer but they didn’t have any. Does anyone know a > dealer in OC? I want to examine the bottom configuration with the intent > to build one. Anyone know of a place where I can rent one in order to feel > what a ‘real’ Bonzer feels like in the water? Thanks, Magoo http://www.bonzer5.com/

There was a used one (in great shape) in Bruce Jones surfshop,been a while though,Herb.

Magoo - The Campbell Bros have a nice website but this link to Mike Eaton’s Bonzer shows the concave contours he uses. Eaton and Campbell Bros worked together on these at some point after the design was developed by Campbells in Oxnard. I notice the newer Campbell Bros boards are going more subtle on the concaves (perhaps the 5 fin provides the directional flow?) while Eaton is still doing the concaves pretty deep. http://www.eatonsurf.com/Bonzer.htm

Magoo - The Campbell Bros have a nice website but this link to Mike > Eaton’s Bonzer shows the concave contours he uses. Eaton and Campbell Bros > worked together on these at some point after the design was developed by > Campbells in Oxnard. I notice the newer Campbell Bros boards are going > more subtle on the concaves (perhaps the 5 fin provides the directional > flow?) while Eaton is still doing the concaves pretty deep. this is just my personal rant about venturi based bottoms which is based on engineering rather than surfing experience so please feel free to discount it as you wish. i looked at the explanation of the theory on the link and i think it is baloney. the only reason there is a low pressure region towards the rear of the board is because of the increased fluid velocity. the only reason there is increased fluid velocity is because you are forcing it through a constriction. this takes a certain amount of energy which has to come from somewhere and that will manifest itself as drag. furthermore behind the low pressure region there is another high pressure region (incidentally not shown) where the channel opens up again so the water has to be pushed from a low pressure region into a high pressure region - more drag. if the displayed explanation was all there is to the effect then after you got the flow going through the bottom contours then the board ought to be able to just keep squirting itself along (since it shows just a pressure gradient induced flow) whether there was a wave in sight or not. perpetual motion machines do not exist on a macroscopic level so this cannot be what is going on. if anything it might be the fact that there is higher velocity water moving past the surfaces of the fins which would cause their effects to be ampified. anyway i am certainly not trying to discount the fact that venturi shaped contours on the bottom of the surfboard affect the way it rides - just the explanations that i have read of what is going on. if there are any other engineers out there i would love to hear their thoughts. by the way i’m not trying to piss anyone off - just stimulate some hopefully enlightening dialogue. trev

this is just my personal rant about venturi based bottoms which is based > on engineering rather than surfing experience so please feel free to > discount it as you wish. i looked at the explanation of the theory on the > link and i think it is baloney. the only reason there is a low pressure > region towards the rear of the board is because of the increased fluid > velocity. the only reason there is increased fluid velocity is because you > are forcing it through a constriction. this takes a certain amount of > energy which has to come from somewhere and that will manifest itself as > drag. furthermore behind the low pressure region there is another high > pressure region (incidentally not shown) where the channel opens up again > so the water has to be pushed from a low pressure region into a high > pressure region - more drag. if the displayed explanation was all there is > to the effect then after you got the flow going through the bottom > contours then the board ought to be able to just keep squirting itself > along (since it shows just a pressure gradient induced flow) whether there > was a wave in sight or not. perpetual motion machines do not exist on a > macroscopic level so this cannot be what is going on. if anything it might > be the fact that there is higher velocity water moving past the surfaces > of the fins which would cause their effects to be ampified. anyway i am > certainly not trying to discount the fact that venturi shaped contours on > the bottom of the surfboard affect the way it rides - just the > explanations that i have read of what is going on. if there are any other > engineers out there i would love to hear their thoughts. by the way i’m > not trying to piss anyone off - just stimulate some hopefully enlightening > dialogue. trev Trev, Your comment was nicely written. I am not an engineer, but I have a science background (bio)as an undergad. Your comment gave me the following thought and ideas and I would welcome your insight - as well as any other Swayholic. If you took two identicaly shaped surfboard blanks (KKL cut rocker, foil, outline, rail etc…) and carved the Bonzer type concaves into one of them, what would the outcome be when both are placed in a still body of water (controlled enviornment, no wind etc…) and pushed with an equal amount of force? I imagine some sort device could be rigged up to measure the amount of ‘glide’ (for lack of a better word) that would result from the exertion of said force. Would you anticipate that the flat bottomed board would travel farther that the ‘Venturi bottomed’ board? Would this effect be replicated at higher amounts of force or speed? I am working under the impression that the ‘Venturi’ concaves are supposed to increase speed and that is what the developers are marketing. Please correct me (anyone) if I have misunderstood the marketing strategy. Lets look at another scenario: Reading your fin comment “if anything it might > be the fact that there is higher velocity water moving past the surfaces > of the fins which would cause their effects to be ampified” made me wonder of the effect of channels cut that direct and maximize the flow of water towards the fins. So instead of channels that are parallel to the stringer they would be cut in such a way that may make them appear to be at an angle to the stringer and aimed directly at the fin configuration. I’m sure it has been tried many times, but I have not come across this design or comments on it as of yet. I look forward to anyones insight. Finally, realizing that I may be competely off base on the effects of the Bonzer board I welcome anyone to enlighten me as to what they are truly supposed to do. Thanks, Magoo P.S. Trev, Your comment has had your stated desired effect at least with myself: "by the way i’m > not trying to piss anyone off - just stimulate some hopefully enlightening > dialogue. trev However, you would be well served if you disagree with someone to post your disagreement without using a comment that may potentially cause offense: ". i looked at the explanation of the theory on the > link and i think it is baloney. I am not trying to offend you, just consider it a helpful comment from someone that you have impressed with your ability to articulate an insight. Regards, Magooo

Magoo thanks for the reply - that was exactly what i was looking for and as for your experiment - awesome. except if i might i would like to add a small variation. how about we take the two finless boards - one venturi bottomed, one flat. now rather than push them and see how far they go, let’s put a weight on them and tow them behind a boat at a speed where they are both planing since below that it is a little useless anyway. they would both be attached to the boat with a separate extendible spring of known spring constant. now measuring the extension of the spring we can determine the force of the board pulling back on the boat. with this and our boat speed we determine an effective drag constant for each board bottom. that would be a really cool experiment and i would be willing to bet my engineering degree that the venturi bottom board would exhibit more drag - my degree isn’t doing me much other good anyway. as i said before i am not saying that a venturi board is not faster i just don’t agree with the explanations which as i was rightly informed are not ‘baloney’, merely an erroneous application of scientific principles. i think that there is a certain degree of this in surfing because of the difficulty of applying the principles to an object which has contacts with more than one type of fluid medium (air and water). this makes a very compicated system of interacting variables which means the end result is part science, part experience and probably a little black art. anyway regardless of the amount of understanding of the hydrodynamics of surfboards on waves there will never be an equation which can determine if a board is going to be ‘killer’ to ride. i would love to hear anymore discussion on this subject. also i think that i have seen a channel bottom before that had the channels sort of fan out towards the tail so that the ones going past the middle fin were parallel to the stringer and the outer ones were parallel to the outside fin bases. thanks for the informed reply magoo. trev

Trev, You are correct in estimating that venturi bottoms not force generators. However, in entering this discussion you need to understand the difference between a displacement hull and a planing hull. Surfboards in general are planing hulls. In other words they reduce drag by riding on top of a small layer of turbulence. Edges on surfaces always produce drag as any fluid medium flows across them. Bernoulli’s principle is a detailed explanation of how this works. However, there is another drag vector that must be considered. Laminar flow is the flow of viscous fluid in which neighboring layers are not mixed. As a fluid flows across a surface the water closest to the surface will slow due to skin friction. At slower velocities the flow will conform to shallow angles of variation across the surface. This boundry layer will produce considerable amounts of drag. If you can create a surface with edges that minimze the amount of drag that they create and focus the turbulence along the remainder of the surface the the fluid is flowing across that surface will be able to plane at lower speeds and with greater efficiency as the velocity is increased. So, if you took your two identically shaped blanks and did your tow test you would find that at very low velocities the board without a channeled bottom contour would create less resistance. As you increased velocity you would reach a critical velocity where all of a sudden the channel bottom would have much less resistance than the “flat bottom”. Venturi bottoms take this concept one step further in that they focus that vectored flow at your fins. Now we need to modify the parameters of your test. Performance surfboards are rarely used on a flat plane. Typically they are always is some degree of a turn. As you turn a surfboard you are burying one rail and allthe components that affect that third of the board. If you consider that the flow of water is resisting the downward pressure of the surfer initiating the turn the force of the flow will spread and the “flat bottom” board will loose some force out the sides. The contours on a venturi bottom are design to direct more of the flow back under the board and across the working foil rather than out the sides. Above the critical velocity this becomes more and more efficient. I hope this helps answer your question. And by the way Magoo, I just picked up my first Bonzer this weekend. If you want to check it out, I am just down the street from Basham’s. Tom

Over drinks one night, I posed the question of Venturi bottoms and C5 fin set ups to a group of engineer friends of mine. After much analysis and some debate a consensus was reached that much of what people claim are the benefits of such contrivances to be at the least misrepresented and more likely false altogether. Like Trev wrote previously, the benefit realized is most certainly not because of the reasons purported by their advocates. However, this is not to say that such ideas are wholly without merit or benefit. For example regarding the C5 or like set ups; it was commonly agreed that like the Venturi bottom, more drag is created and not less in virtually all circumstances. The possible exception being when the board is moving at some critical speed that is likely never to be reached on most waves. However, you will hear those that say they can surf and even paddle faster through the water because of their bottom shape or fin set up. The fallacy in this can easily be seen when comparing the potential producers of drag (when paddling). Foremost, the torso, clothing, legs, feet and accessories create most of the drag experienced, while the fins create the least. If someone were really interested in reducing drag they would simply lift their feet out of the water while paddling. Or, while surf riding they would go without a leash. After all, a leash produces far more drag than the fins do while riding. So, why dont you hear people claim that they can rip far more without one? The truth is they can’t. A good surfer will rip no matter what he or she rides. As a non-Engineer, I personally cannot comment on the scientific validity of these designs. However, as a BBA/MBA (Marketing) I think I know what is going on. The answer is not on the drafting table, but rather in the business plan. Anyone wishing to compete in the competitive market place must differentiate their product from the competition and market it to a homogenous segment of a population. This reminds me of a childrens shoe line of the late 70’s called “Zips”. In their commercials, the kids with the Zip shoes could run much faster than those without. In fact, red streaks streamed from the base of the shoe when they did. After a lot of begging, my parents bought me a pair of said shoes. And, for at least a week I was convinced that not only could I run faster but would swear that that I could see the red streaks trailing behind me when I did.

maybe the best way of reducing drag is stay out of the water…those same engineers will tell you there’s no way a bumblebee can fly either. i don’t think any of us can really say whats going on under a surfboard on a wave.

Over drinks one night, I posed the question of Venturi bottoms and C5 fin > set ups to a group of engineer friends of mine. After much analysis and > some debate a consensus was reached that much of what people claim are the > benefits of such contrivances to be at the least misrepresented and more > likely false altogether. Like Trev wrote previously, the benefit realized > is most certainly not because of the reasons purported by their advocates. > However, this is not to say that such ideas are wholly without merit or > benefit.>>> For example regarding the C5 or like set ups; it was commonly agreed that > like the Venturi bottom, more drag is created and not less in virtually > all circumstances. The possible exception being when the board is moving > at some critical speed that is likely never to be reached on most waves.>>> However, you will hear those that say they can surf and even paddle faster > through the water because of their bottom shape or fin set up. The fallacy > in this can easily be seen when comparing the potential producers of drag > (when paddling). Foremost, the torso, clothing, legs, feet and accessories > create most of the drag experienced, while the fins create the least. If > someone were really interested in reducing drag they would simply lift > their feet out of the water while paddling. Or, while surf riding they > would go without a leash. After all, a leash produces far more drag than > the fins do while riding. So, why dont you hear people claim > that they can rip far more without one? The truth is they can’t. A good > surfer will rip no matter what he or she rides.>>> As a non-Engineer, I personally cannot comment on the scientific validity > of these designs. However, as a BBA/MBA (Marketing) I think I know what is > going on. The answer is not on the drafting table, but rather in the > business plan. Anyone wishing to compete in the competitive market place > must differentiate their product from the competition and market it to a > homogenous segment of a population.>>> This reminds me of a childrens shoe line of the late 70’s > called “Zips”. In their commercials, the kids with the Zip shoes > could run much faster than those without. In fact, red streaks streamed > from the base of the shoe when they did. After a lot of begging, my > parents bought me a pair of said shoes. And, for at least a week I was > convinced that not only could I run faster but would swear that that I > could see the red streaks trailing behind me when I did. you bring up an extremely valid point. a double blind test on boards with and without their modifications would be an interesting experiment. people can easily fall under the spell of stoke and that will definitely influence their experience. of course, we’d have to take out an important variable…the wave. since “every wave is different”, the testing grounds would be best at say, typhoon lagoon or some other artificial wave that is produced via the same process every time…even so, given identical shapes, i too would have to draw a line when considering a paddling performance change. this i would attribute to “standard stoke deviation”…

It’s proven it’s self to me,and lots of others with typcal results,no big money hype,no payoffs,just some good R&D work over the last 5 years… And yes,I would like to try/build a Campbell type Bonzer5 for myself… I like all surfcrafts… Also,you left out the most important thing… THE HUMAN FACTOR…Herb.

Tom thanks for the reply. you really have me thinking now. first of all i would like to reiterate that i’m not trying to discount the fact that the bottom contours work - just the explanations that are given. as you said the surfboard is almost never going straight throught the water and is usually in some degree of turn which renders up to 2/3 of the board to be free of the water. the channels are redirecting the waterflow under the board rather than out the side so that the foil of the board can generate more force from the passing water. I think that is a the best explanation i have heard yet but you have to admit that it has very little to do with a venturi and it’s associated change in fluid velocities and pressures. basically the inside vertical surface of the channel is acting like a very long and low aspect ratio fin that doesn’t really have an outboard surface which i believe is where most of the turbulent drag is generated. basically you have a channel bottom with curving contours rather than straight channels which happen to look like a venturi if the flow direction was directed parallel to the stringer. i guess this gives the effect of having more fin area with a minimal increase in drag? you always see channels on surfboards intended to make alot of progress along a fast peeling wave. this would cause the flow of water to be at a larger angle to the stringer where the channels could keep the water under the board and moving along it from end to end rather than off the sides. however you never really see too much of that stuff on really big wave guns (the fastest of the fast). the bottoms always seem to be very straight, smooth and unobstructed. i imagine that this is because most of the time flow patterns under these boards is very straight and parallel to the stringer since any turns are larger radius and in this case a flatter planing surface has less drag. anyway thanks for all the insight and i love to read anything you ever write about fins. trev

maybe the best way of reducing drag is stay out of the water…those > same engineers will tell you there’s no way a bumblebee can fly either. i > don’t think any of us can really say whats going on under a surfboard on a > wave. This engineer will not stay out of the water. it is Swaylock’s Surfboard DESIGN forum. i think that there are some who have a pretty good idea of what is going on under a surfboard and they also happen to be the ones consistently making good, versatile surfboards. the sharing of that insight i believe is some of the aim of this forum and is why i and i imagine alot of others frequent this website along with the obvious pleasure of sharing stories and opinions with other people stoked on surfing. no hard feelings but i bet there are scientists and engineers out there who can explain a bumblebee’s flight - it just isn’t me. trev

Trev, The descriptions of how venturi bottoms work on various surfboard manufacturers web sites are simplified, generalized and intended to sell boards rather the educate the reader on fluid dynamics. It would appear that the impressions taken from that information (if taken to the ulitmate conclusion) would mean that you ould tow a funnel through the water and it would begin propelling itself. I do not believe that that was the intent of the board manufacturers. But, I do understand the misinterpretation of that information. Basically, channels, concaves and venturis are all about vectoring the flow of water and reducing skin friction. Venturis are focused on directing that flow across the boards working fin. The greater the flow of water across the fins flow the more effective it becomes to a point. Once the velocity of the flow reaches a critical speed the flow across the foil will not follow the trailing tapered edged. A greater amount of turbulence will occur and the fins will loose efficiency (i.e. wash out). Consequently, channels on boards designed for medium size waves are effective. When you start Chasing Rihnos you are no longer trying to generate speed. You are attempting to control speed. Drag becomes much less of a concern and a good positive bite is what you really need. The wave will provide you with all the power you will ever need and then some.

It’s proven it’s self to me,and lots of others with typcal results,no big > money hype,no payoffs,just some good R&D work over the last 5 > years…>>> And yes,I would like to try/build a Campbell type Bonzer5 for myself…>>> I like all surfcrafts…>>> Also,you left out the most important thing…>>> THE HUMAN FACTOR…Herb. great thread guy’s…years ago I had a 10foot wave magnet with a cluster set up, a 6.5 cutaway center, and a set of channel’s just ahead of the sweet spot… one of the best boards i ever had…it rode like my 6’4" pat ryan quad…it also had an acrylic of merlin(mickey mouse) right above the channel’s…year’s ago you wouldn’t see channels on a long board any thing you can do to a flat bottom to increase lift or redirect flow goes a long way to enhancing performance…it also helps between the ears

Bob Simmons was doing concaves and slots (channels of sorts.) Velzy’s boards and Greg Noll’s “Da Cat Model” sometimes featured concaves and speed slots (again, channels of sorts.) Salesmanship or hydrodynamic enhancement???

Bob Simmons was doing concaves and slots (channels of sorts.) Velzy’s > boards and Greg Noll’s “Da Cat Model” sometimes featured > concaves and speed slots (again, channels of sorts.) Salesmanship or > hydrodynamic enhancement??? I don’t know about salesmanship but i’m 6’2" 250 and when i can ride a 10 footer like it’s a raped ape i know it works for me right now i’m building a 8’2 fish with the same principles as the other board lift is good …so is life…

isn’t it true (to an degree) that things which cause drag also increase the ability to turn? for example, the vee bottom: the vee causes some drag, yet allows the board to roll over on it’s rail quicker, therefore turning quicker and easier. i’ve also read that longer fins will give a board a little more drive (speed). that said, isn’t it possible to create more drive in a board by directing the water flow through a specific fin configuration (and specific sized/shaped fins)? so the venturi/bonzer bottom might create drag, but it could turn better, and potentially, with the proper bottom design, create more speed?>>> Magoo thanks for the reply - that was exactly what i was looking for and > as for your experiment - awesome. except if i might i would like to add a > small variation. how about we take the two finless boards - one venturi > bottomed, one flat. now rather than push them and see how far they go, > let’s put a weight on them and tow them behind a boat at a speed where > they are both planing since below that it is a little useless anyway. they > would both be attached to the boat with a separate extendible spring of > known spring constant. now measuring the extension of the spring we can > determine the force of the board pulling back on the boat. with this and > our boat speed we determine an effective drag constant for each board > bottom. that would be a really cool experiment and i would be willing to > bet my engineering degree that the venturi bottom board would exhibit more > drag - my degree isn’t doing me much other good anyway. as i said before i > am not saying that a venturi board is not faster i just don’t agree with > the explanations which as i was rightly informed are not ‘baloney’, merely > an erroneous application of scientific principles. i think that there is a > certain degree of this in surfing because of the difficulty of applying > the principles to an object which has contacts with more than one type of > fluid medium (air and water). this makes a very compicated system of > interacting variables which means the end result is part science, part > experience and probably a little black art. anyway regardless of the > amount of understanding of the hydrodynamics of surfboards on waves there > will never be an equation which can determine if a board is going to be > ‘killer’ to ride. i would love to hear anymore discussion on this subject. > also i think that i have seen a channel bottom before that had the > channels sort of fan out towards the tail so that the ones going past the > middle fin were parallel to the stringer and the outer ones were parallel > to the outside fin bases. thanks for the informed reply magoo. trev