teak swim platform

[Lazy Daze]

Henry - thanks for the further clarification. As I eluded to, my comments are more in regards to furthering my understanding of the process. I do appreciate it!

Scott - there is another aspect to it - at high speed. Supposedly it helps the boat turn in a little tighter - something about helping the rear to "plant" or "set-in" better. I didn't originally comment about this because I was never too sure I subscribed to that theory.

I don't know how well this relates, but I can give you an example from another sport where it does work... Ice hockey. A number of years ago, CCM (maybe Bauer, I forget) introduced a blade that had the rear 1/4 "narrowed down" just like on your boat. It did make a difference in how tight I could turn. The downside was the blade was weaker and would crack/break - sometimes under pressure from turning, sometimes from a puck.

 

[Four Suns]

I would think more surface area would = more lift. But it also would equal more drag. Maybe the engineers at SR figured out just how much "lift" the boat needed to run efficiently and could get away with shaving some inches off, without compromising the beam of the boat.

You must be a pilot...

What you are describing is the "water ski" effect.... The faster you go on a water ski, the less surface area there is on the water (and the Cd changes). The carve out the shape of a water ski towards the back for the same reasons....

 

[cpt. sundancer]

SO,with all things said,my narrow beam,single 5.7, '81 sundancer would not need platform vents? or should i break out the jig saw?:huh:

 

[Four Suns]

SO,with all things said,my narrow beam,single 5.7, '81 sundancer would not need platform vents? or should i break out the jig saw?:huh:

Chain saw... cut the damn thing off.

 

[cpt. sundancer]

didnt think it was causing that much drag,but, if you say so,your the expert :lol:

 

[fwebster]

Thank you for the entertaining read.

However, are you just discussing Sea Ray platforms with the oval SR vent grates? Many older boats have different and often smaller drains and some have none at all.

The reason SR puts the vents in swim platforms is to drain surface water off of them so you don't bust your ass when you walk on them wet.

While the platform may have a decent amount of buoyancy, that may work when the boat is not moving, or moving slowly. But the argument that the vents drain water from underneath to "release drag or pressure" isn't valid. Have you ever looked behind your boat as you accelerate? The area immediately behind the boat is in a vacuum created by the transom as it moves forward. There is no water anywhere near the underside of the platform once you begin to accelerate.

 

[Lazy Daze]

Have you ever looked behind your boat as you accelerate? The area immediately behind the boat is in a vacuum created by the transom as it moves forward. There is no water anywhere near the underside of the platform once you begin to accelerate.

That's sort of where I was going with my thoughts, but from the other side of it. Could the platform create enough suction (loosely using the upside down cup, full of water idea) to prevent the boat from getting to a point where there is a "vacuum" immediately behind it.

As this thread has been going on, I've been thinking more and more about this. I still say there is a possibility that the platform (w/o vents) could become full of water while moving slowly ahead. But, and here is where I'm starting to hesitate on my initial thoughts, would that suction (and weight) really be enough to prevent the back end from breaking free and getting to the "vacuum" as you mentioned?

 

[fwebster]

Not al long as we have gravity.............

 

[Henry Boyd]

I recently brushed up on my planing hull theory in response to another discussion. Two things came from that: 1) it is intentional on the part of the designer that the port and starboard side streamlines created by the bow do not re-converge at the stern (like they would on a rounded stern sailboat). That creates turbulence across the transom. I'm guessing this is the same mechanism NSCAR designers are using with the very abrupt squared off back ends of their cars.

and 2) the high is optimized to lift itself up out of the water.

One thing we have not considered in this discussion has been the differences between a sterndrive boat and an inboard/ v drive. In the case of the latter the props are under the hull, and in the former right under the swim platform.

Henry

 

[Lazy Daze]

Frank, so you'd say that the boat would create enough of a void - even within the first few MPH of forward motion to drain the platform (or even prevent it from filling up in the first place)? I've never looked that closely at mine (or any boat) to see when it starts to develop.

This has actually been one of my favorite threads! :smt001

 

[Henry Boyd]

Fair enough... My point was that there is a point fatigue is not an issue... Did I mention airplanes are a machine designed to almost not work?

I seem to recall something from one of my undergraduate aerodynamics classes that helicopters weren't supposed to fly at all.

Henry

 

[Henry Boyd]

Frank, so you'd say that the boat would create enough of a void - even within the first few MPH of forward motion to drain the platform (or even prevent it from filling up in the first place)? I've never looked that closely at mine (or any boat) to see when it starts to develop.

This has actually been one of my favorite threads! :smt001

Dennis,

Not noticing that is a good thing if you are the usually the designated driver!

Henry

 

[cpt. sundancer]

sorry,but i find what little drag there is to be minimal,some are talking as if there is a parachute on the back.If it's that much of a problem, get a stronger motor or a sailboat :thumbsup:

 

[fwebster]

Dennis,

Yes, that is what happens, although I can't prove it with formulae or pictures of a drowning cat........which, BTW, is the best possible use for a cat.

 

[Jack F]

I just realized what was missing from my diagram, Thanks Frank!

A Bravo III would help with this cat problem of your :lol:

 

[tobnpr]

I've considered doing the same project. Since you say you have access to cheap material (and teak is as far from cheap as you can get) I say go for it...

If I go forward with it myself, I'm going to use a technique developed by West System to epoxy veneer teak boards directly to the fiberglass. It's a straightforward and relatively simple system and uses thickened epoxy- tinted black- for bonding and to create the traditional look of caulked black seams. You get the traditional teak deck look, but without the maintenance of caulking the seams. It would be simple to do on an area as small as a swim platform.

You might want to consider it yourself:

http://209.20.76.247/ss/installing-a-teak-deck-on-zatara/

 

[Lazy Daze]

Dennis,

Not noticing that is a good thing if you are the usually the designated driver!

Henry

Good point!

 

[Four Suns]

I'm going to change my mind about those vents... If you look back on my 480 DB calculation, there would be about 1300 pounds of buoyancy force if the thing was underwater filled with air. What is NOT accounted for is the weight of the platform... which I put a new one on the boat back in 2003 (thank you hurricane Isabel) and I know that platform weighed over 1000 pounds... I would bet it was around 1500 pounds... The yard had to use a fork lift to get it off the truck as 6 guys could not lift it. So... Because of that, the buoyancy force doesn't do poop IMO because it is canceled out by the platform weight... and the vents don't do anything for the water flow either...

I also don't think, at least on my 480, that fatigue would ever be an issue. I don't recall the mounts for my platform on the 380 but the 480 mounts consist of some big honkin' bolts through about 4" of cored transom with back plates. If I recall, there are 6 bolts on each "arm" (3 on top, 3 on bottom).

There is usually a jet ski or dinghy (about 750 pounds) on that thing as well and it does get bounced around all the time underway so there are plenty of cycles on it (billions and billions). The cycles from waves hitting it are far far less... I also just had a moisture meter and thermal imaging done on the whole hull and there were no issues... so it's working so far.

So I'm sticking with my initial claim... the vents are decorative and probably prevent water puddles and people slipping on their ass. I can't find any other reason they are there.

You can believe what you want... but that's what I think. Q.E.D.

 

[Henry Boyd]

Gary,


The weight of the swim platform is not relevant in the calculation of the effect of an air bubble. The platform is attached to the boat. The boat is floating and in equilibrium, therefore any downward force from the 1000 pounds of the platform is countered by the other forces acting on the hull. A simple illustration would be that there is another 1000 pound downward force located forward of the center of buoyancy at a distance equal to the amount the platform is aft of the CB. In effect a counter weight.

Now for some reason the attitude of the of the boat changes (not as a result of taking on water). A trapped air bubble under the platform creates a buoyancy force of 1300 pounds. In determining the net effect on the hull we would indeed subtract the weight of the platform from the additional buoyancy. But because prior to the platform creating the bubble, the boat was in equilibrium, the result on the boat as a whole would be the increase of the full buoyancy because no matching counter force is created to offset it.

So your original calculations as to buoyancy remain correct. And like you I am beginning to wonder if it really means anything. I suppose that anything that potentially prevents reversing load cycles from being placed on the structure is a good thing, although its questionable if necessary.

Henry

 

[Woody]

Gary,


The weight of the swim platform is not relevant in the calculation of the effect of an air bubble. The platform is attached to the boat. The boat is floating and in equilibrium, therefore any downward force from the 1000 pounds of the platform is countered by the other forces acting on the hull. A simple illustration would be that there is another 1000 pound downward force located forward of the center of buoyancy at a distance equal to the amount the platform is aft of the CB. In effect a counter weight.

Now for some reason the attitude of the of the boat changes (not as a result of taking on water). A trapped air bubble under the platform creates a buoyancy force of 1300 pounds. In determining the net effect on the hull we would indeed subtract the weight of the platform from the additional buoyancy. But because prior to the platform creating the bubble, the boat was in equilibrium, the result on the boat as a whole would be the increase of the full buoyancy because no matching counter force is created to offset it.

So your original calculations as to buoyancy remain correct. And like you I am beginning to wonder if it really means anything. I suppose that anything that potentially prevents reversing load cycles from being placed on the structure is a good thing, although its questionable if necessary.

Henry

Four Suns commented on the size of his platform mount bolts so my comments are about the forces at the connection of the cantilevered swim platform to the stern. This discussion places a fulcrum at the stern. While you are correct the reaction force at that point would include the weight of the platform it doesn’t remove the downward force itself or the stresses that force puts on the bolts. The bolts would be subject to the stresses generated by the net of the upward and downward forces.