teak swim platform

[chuck1]

Too deep for me ..... Time to fix my night cap (Grin)

 

[Henry Boyd]

Gary,

Couldn't acceleration be affected by wave action? In effect a swell functioning in the same fashion as the ram of a piston with the chamber under the platform being the cylinder? The same would be true if during power up the stern squats pushing the platform down into the water.

Even at 682 and 1364 pounds these loads could be significant when we think in a fatigue context; repetitive loads over time. The load in itself does not have to be significant, just happen a lot of times over the life of the artifact.

I've got to go, too many late nights this weekend.

Henry

 

[Woody]

I read Henrys post to mean he felt the wave itself is compressing the air and that increased pressure is causing stresses on the platform and mounts. If my boat sat low enough to keep the platform edge in the water, I blocked the blow hole and then pumped in air from a compressor there would be no worry of damage due to increased air pressure because the additional volume I’m trying to add would simply escape this “container” and there would be no pressure increase.

I’ve watched the swim platform many times to see what goes on there. I often see air displaced by the water and pushed from under the bottom edges of the platform as well as escaping via the blow hole. If you watch closely the event doesn’t take place until the wave gets to the transom. At that time the rest of the wave is passing and the wave section that is blocked by the stern slams into it. Like a wave that runs into a seawall the energy is absorbed/dispersed, water shoots up, sideways, down, etc. If there is any potential for damaging force it would be coming from the wave itself. The energy in the wave section that hit’s the boat can be calculated, how much of it pounds the platform is the question..

My blowholes are approx 22sq in total(I measured). Is that an effective relief valve for Henrys scenario? I’ve never consider they may have an important function. I’ve just thought those holes were a free bidet, a handhold or maybe just so my grandkids can stand around them and squeal.

 

[comsnark]

Hmmm. This sounds interesting.

Without fully understanding all of this. . .is there an issue with "cycles"; i.e. repeatedly cycling the loads on the swim platform as the boat goes through waves?

This is an issue with air planes, right? It's not that a plane goes from high pressure environment (ground) to a low pressure environment (30,000 ft) -> It's the fact that the plane does this multiple times a day, 350 days per year, for 10 years.

 

[Four Suns]

I don't believe anything in a pleasure boat's hydrodynamics is anything but an isentropic process... The thing with something like an inverted glass (or swim platform) being pushed into the water is that the free-surface waves will form and the water level is going to stay at the base. Like this:

The ram-air thing you are talking about would only happen if the water surface could not form waves... Then the process would not be isentropic... air would be compressed and heat would be generated. That's just not happening in the processes we are talking about here... The velocities are not high enough and the fluids are not constrained like a ram air problem... This is not PV=nRT...

I just love this LiveScribe pen I got for Christmas... It records everything I write on paper... tooo cool.

 

[Four Suns]

Fatigue is not always an issue... it can be but not always. You can cycle a 1000 lb load on a 12" steel I-beam and it will cycle forever... but on a 1" beam, it won't.

I would never compare airplane construction to boat construction. They are designed very differently. You can put the 12" beam on a boat... on a plane you put the 1" beam so it will fly. You think balsa core is bad? You should should see the paper honeycomb core they put in airplanes...

Some definitions:

Airplane = A machine that almost doesn't work
Rocket = A machine that almost does work


Plus... this thread is now about showing how 5 x 24 square inch holes prevent the swim platform from failing

 

[Lazy Daze]

I agree that if the glass/platform is held square to the water, no water will enter the "chamber". But, if the glass/platform is tilted, and then drug through the water, wouldn't water be forced into the chamber and displace the air? The air would escape (be forced) out of the chamber. If the glass/platform now starts to level off, then water would remain above the waterline, inside the chamber, until the platform breaks free (goes higher) of the waterline.

If you've already answered this, well, I'm just not pickin' up what 'yer puttin' down. Meaning, I just may not be understanding it from the way you are explaining it. It is still making sense to me that water can get trapped under the platform. But, I'm also still very interested in understanding why this doesn't happen, if that is the case.


Livescribe: So... you write on special paper with a special pen... and then what? Save it as a jpeg? Does it appear right on your computer as you write? Sounds pretty darn cool!


On edit: +1 about the cat thing!!!

 

[comsnark]

I only brought the airplane into this as an example of the fatigue thing.

 

[Four Suns]

Some more...

and

 

[Four Suns]

So what does this all mean?

It means the vents on a swim platform have a negligible effect on the force on the platform with regards to the force of water (water pressure as one person called it). Water flowing through them doesn't do didly...

HOWEVER... I was surprised at the buoyancy force compared to the dynamic force. By having no vents, when the platform goes under water, you have a buoyancy force of 1,364 pounds on a 480 DB platform. By evacuating the air through the vents, that force goes away... The dynamic force cycles between 0 and about 1400 pounds which puts it near the "800 pound limit" on the platform if there is no jetski/dinghy on it...

The total force with the vents would cycle between 0 and 1400 pounds whereas without the vents it would cycle between 1400 and 2800 pounds (rounded).... So the vents reduce the force by removing the trapped air and that does make them functional. The buoyancy force from trapped air is significant and when combined with the dynamic force, it produces over a ton of force on the platform when submerged.

Of course, in flat calm water, having that extra 1400 pounds of buoyancy would actually help me lift the dinghy:

 

[Four Suns]

Yes, but where is the drowning cat?

here:

 

[Lazy Daze]

Gary, just something I thought as I was reading... "2-4 ft/s"... does that estimate include the downward motion of the platform, or is that just the wave speed?

Also, does the weight of the platform need to be considered in finding the "net" force on the transom/mounting hardware? Or, would that be negligible since it's under water?

Another "also" question... The addition of the buoyant force would, obviously, be spread out across the entire platform and mounting hardware (assuming a perfect, uniform force). Would that mean that this extra force, in reality, is not as bad as it looks on paper?

Here's something interesting... it looks like SR may have changed the platform for your boat. Looking strictly at the pictures online, it appears there are now 7 holes in it. Hmmm...

Looks like you're smiling in that dinghy!!!!

 

[Four Suns]

The 2-4 ft/sec is my guess in how the platform is moving... it's a guess. I've been out drift fishing and have had waves come over the back (one almost washed my son out of the cockpit) and it's just a gut guess based on observation.... I said I might be wrong...

Anyone can shoot holes in this analysis.. for example, I assume the vent holes don't change the coefficient of drag and just change the area. I stand by that assumption just based on education and experience... but I might be off a few percent..

The conclusions I feel are valid... plus or minus 20% on the numbers though...

I excluded the weight of the platform and was really just looking at the "delta" in forces. Also, there is a buoyancy force with just the platform but that damn thing weighs about 2000 pounds so compared to the buoyancy force, I ignored it... This is just a "back of the envelope" calculation.

This was my first response to this thread:

Decoration... or to provide some small amount of drainage from the topside of the platform and prevent puddling/slipping.

They are not there to relieve any "water pressure" from the bottom. Well... if they are they aren't going to relieve any water pressure. The open area of those inserts is insignificant to channel a wave or something to relieve the hydraulic pressure. I've also heard people say they are there to "let the air out" which I'm not even sure why that is important...

Additionally, those inserts are held in with small stainless wood screws from the underside and if you get a wave or water hitting the bottom of the platform, you can knock them out and lose them. I've had to replace a few. They should also be inspected from time to time for loose fitting and be repaired as needed.

So I now think the "let the air out" is important... but I stand by the "water pressure" is BS comments... And I still think they are primarily there so you don't slip and fall on your ass.

 

[Lazy Daze]

I said I might be wrong...

Anyone can shoot holes in this analysis..

Don't get me wrong - that is not my intent at all. Far from it. Also, I'm not second guessing your "gut" or your estimate of the 2-4 seconds. Just curiosity, more than anything else. Really, everything I've commented on/said recently has stemmed from a desire to learn.

 

[Lazy Daze]

While I am relieved to know that the inserts on the platform are validated, mostly for creature comfort (being ours), I am still really puzzled by something.

On my boat, as well as other 340's (just so you know that I am not solely focused on my craft) there is a notch or cutout in the hull sides about 10 feet from the transom going back or a step by any other name. Marketing gimmic, or does it really serve a purpose?

At the speeds my boat travels, I can't imagine it does anything other than reduce the wetted surface area of my boat towards the stern. I suppose that would reduce drag, but wouldn't it also reduce lift and cause the boat to squat?

These are the things that keep me awake at night.....

(and yes Dennis, this is a major, major attempt at derailing the OP's thread, how am I doing? Although I am interested in the answer to this query.)

Yeah, I'm as much to blame. Oh well, hopefully the OP is at least enjoying the interesting reading material.

The cut-outs were designed to help eliminate the slow-speed wandering that so many boats are prone to as it disrupts the water flow. I've been on very similar SR hulls with and w/o them. I can honestly say it makes a difference. But, at the same time it is, by no means, a "night and day" difference.

Gary? :smt001

I can't comment on how much it does/doesn't affect planing time. But, I would bet that that was taken into account by the engineers and, at best, is minimal.

 

[Four Suns]

A. That's a really ugly paint attempt. Go to livescribe.com and buy and pen.

B. I don't know. Other than it ain't got sh!t to do with the dynamics when the hull is on plane because that part of the boat is out of the water.... If it was something like a stepped hull (i.e. Formula boats), it would introduce air/air bubbles under the hull and reduce skin friction drag... which is why my boat will go faster in a 1-2 foot chop than flat calm. If it is reducing the wetted area of the hull on plane, they may have a purpose... I can't tell from your craptacular drawing.

 

[firecadet613]

A. That's a really ugly paint attempt. Go to livescribe.com and buy and pen.

I want one.

 

[Henry Boyd]

Gary, just something I thought as I was reading... "2-4 ft/s"... does that estimate include the downward motion of the platform, or is that just the wave speed?

Also, does the weight of the platform need to be considered in finding the "net" force on the transom/mounting hardware? Or, would that be negligible since it's under water?

Another "also" question... The addition of the buoyant force would, obviously, be spread out across the entire platform and mounting hardware (assuming a perfect, uniform force). Would that mean that this extra force, in reality, is not as bad as it looks on paper?

Here's something interesting... it looks like SR may have changed the platform for your boat. Looking strictly at the pictures online, it appears there are now 7 holes in it. Hmmm...

Looks like you're smiling in that dinghy!!!!

Dennis,

The buoyancy force is very real, or better said, the distributed air pressure is not trivial. For example, we use air bladders regularly for testing wall strength on cargo containers. 5-6 psi distributed over the door end, or front end of a standard 20' or 40' ISO container duplicates the effects of a 50,000 pound cargo shift. 7 PSI by the way will blow the doors off the hinges! This is in part what got me thinking that the vents were to relieve air pressure and not water after Gary originally demonstrated they had no hydrodynamic effect.

I did use the wrong analogy with my piston and cylinder illustration. What I wanted to express was that while the inverted glass idea duplicated the swim platform it didn't quite show the whole picture. After all it is attached to this floating thing called a boat. My thought was that under initial acceleration, a squatting stern would effectively apply an vertical external force adding a downward component to Gary's diagram.

While the buoyancy loading does not by itself create a failure load, I believe it would over time and repetition it would result in a failure, most likely cracking in the area where the platform mounts to the transom. This is by the way the definition of fatigue. Fatigue is often thought of failure through vibration (and it is), but there is more to it than that.

After spending a number of years managing a sizable fleet of 12" steel I beam frames on wheels (container chassis), I can report with authority that it takes much less than 1,000 pounds applied repetitively over time to break them. But that's a digression for another day.

Henry

 

[Four Suns]

After spending a number of years managing a sizable fleet of 12" steel I beam frames on wheels (container chassis), I can report with authority that it takes much less than 1,000 pounds applied repetitively over time to break them. But that's a digression for another day.

Henry

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?

 

[highslice]

Did I mention airplanes are a machine designed to almost not work?

Actually, the closer you can get an airplane to not working, the more maneuverable it is. Modern fighters almost don't fly.

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.

I'll look into the livescribe, it looks pretty cool. I thought though, that maybe I would get extra credit, because my diagrams are in color...

Those of us with I/Os get to vary the amount of running surface in the water. There is a point where it becomes counter-productive.