Regulations require that float buoyancy = 1.8 X Aircraft Weight. Most folks treat this requirement as the bare minimum.
But, what are the advantages AND disadvantages of fitting larger than required floats onto an aircraft?
And how much is enough? Should buoyancy be exactly 1.8X . . . . 1.9X . . . . 2.0X . . . . more?
There must be a point of diminishing returns, otherwise someone would try to put Aerocet 3500L's on a Super Cub, Right?

Thanx, Dave.

PS - Just thinking outside the box, now . . . Does anyone believe that the regulations are actually unnecessarily conservative?
Are there any (theoretical) aerodynamic or hydrodynamic benefits to "under-floating" by a factory of 1.7X . . . 1.6X . . . less?

There's lots of potential comments regarding reserve float buoyancy. What has mattered to me is when taxiing or turning with a stiff crosswind or exposed to quartering or side wave/swell action. In that situation the plane can roll and put more weight on one float than the other. It's nice to remain afloat.

Gary

Dave,

There are lots of good reasons that underfloating an airplane is a bad idea. And, there are examples of underfloated airplanes out there....that are perfectly legal.

A great example is the Cessna 170, mounted on EDO 2000 floats. The wheel gross weight of the 170 is 2200 lbs. To meet the minimum reserve buoyancy requirements, Cessna limited the gross weight on EDO 2000s to 2130 lbs. Now, 70 pounds isn't a lot, so many 170 operators on 2000 floats kinda ignore the 2130 GW on floats. And, of course, sometimes they don't actually have their airplane weighed, and as often as not, as many here know, when you actually weigh an airplane that's had lots of work done on it over the years, with just "calculated" weight and balance, when you do put one of those planes on scales, they in fact weigh a hundred pounds or greater more than the paperwork suggests.

So, our fearless aviator takes his or her fully loaded (just a skosh over 2130) stock 170B out into the lake, warms up and taxis to the far end to takeoff into the wind.....which is blowing about 12 to 15 right down the lake. Taxiing downwind is easy enough, but at the end, the pilot starts the turn around. Now, ANY seaplane leans out of the turn just a bit, pushing that outboard float further into the water. But, because we're asking a lot from this poor little EDO 2000 float, it sinks more than usual. Now, that causes the opposite wing to come up, and of course, that's the wing that's facing the wind......which gets under that wing, and further aggravates the rolling moment. Pretty soon, that downwind float is nearly completely submerged, and now we're one good gust away from the airplane rolling over. Not good.

Even when kept at 2130, the 170 on EDO 2000s barely meets the buoyancy standards, and even there, it's marginal in a wind.

Also, even if you get turned around in a wind safely, now you have floats that are significantly submerged. And, hydrodynamic drag is HUGE compared to aerodynamic drag. The task of the seaplane pilot is to reduce hydrodynamic drag to the extent possible so that lift can overcome the effects of the water on the floats. And, a stock 170 doesn't have a huge powerplant. So, the pilot pushes the power up and it can take forever for that humble O-300 to drag the poor beast up onto the step. All the while, there is a huge wetted area, creating a tremendous amount of drag.

Now, put a pair of PeeKay B-2300 floats on a 170, and you have an entirely different beast. Lots of floatation, even if run a touch heavy. These floats also require the engine upgrade to 180 hp or more, which helps with the takeoff performance, and the airplane is now a great performer on these floats.

Going to the other end of the "normal" float spectrum, I flew a Cessna 206 on Wipaire 4000 straight floats.......HUGE floats, and the airplane was significantly over floated, even at the upgross weight of 3800 pounds that airplane was approved at. That airplane bobbed around like a cork on those huge floats. Getting airborne on those floats took a good bit of experimentation to figure out how to convince the floats to turn loose of the water. Those huge bottoms just clung to the water surface. I finally figured out that if I stomped on a rudder when the plane was on step and accelerated, the plane would pop out of the water.....an unusual and actually kinda scary way to launch till you get used to it. But, what was happening was that you actually turned the plane (and floats) sideways a bit in the water, and the "leading" float bottom pushed the plane up out of the water. Once I figured that out, the plane would launch very short, even at 3800 pounds.

We had another, nearly identical 206 on Wipaire 3450 amphibious floats. Much smaller (physically) floats, but still approved for water operations at 3800 pounds. Those floats were in my opinion, the floats a 206 always wanted. Even though they were much smaller than the 4000s and amphibious to boot, they got out of the water using conventional techniques and just as short as the bigger floats. And, the 3450s still had plenty of reserve buoyancy.

To add insult to injury, those 4000 floats really slowed the airplane down through the air, and they were HEAVY. All of which reduces useful load.....the plane has to lift the weight of those floats as well.....

I flew a Beaver for several years on Bristol 4580 amphibious floats, and that airplane would get your attention when operated at 5090 in any kind of wind. On the other hand, I've also flown a Beaver on the big 6000 Norseman floats, and that beast was a cork. But, talk about drag through the air......sheesh!!

Finally, the subject of float model designation..... It used to be that the float model designation actually represented the buoyancy of one float when fully submerged in fresh water. So, the EDO 2000 would (theoretically at least) displace 2000 pounds of water when one float was completely submerged. Somewhere along the line, for reasons I can't even imagine, that practice (the model designation) completely changed. So, in many cases, float model numbers don't actually represent their true displacement.

So, yes, there are very good reasons to avoid underfloating a seaplane, and also good reasons to not overfloat one. Underfloating is more likely to get a pilot in trouble than overfloating, though. And, the certification standards are actually pretty minimal. A seaplane float that just barely meets the certification standard can be a real handful in certain conditions.

Sorry for the windy response.

MTV

I’ll echo most of what MTV said, and I prefer an over-floated plane to one under.

All EDO float model numbers do reflect their buoyancy in pounds displaced, wipline floats do not. Aerocets model numbers also match their buoyancy pretty closely.

Here’s a fun photo-

On the left is a turbo-beaver on wipline model 6100 straight floats, on the right is an identical turbo-beaver on aerocet 5850 straight floats, the difference in buoyancy is pretty apparent. The additional prop clearance is a big bonus for a floatplane.

Not sure if I answered the OP’s question or not.

Wiplines suck.

Thanks Mike for that description. Helped me with this very question I’ve been pondering.
Pro/Cons.
Love this site!
Chaz

The longer, the lighter, the more buoyant a seaplane float is the better. As long as the ship flys OK in the air too. And yes, legal.

90% of seaplanes are under-floated. Under-floating becomes more dangerous as seas and wind increase.

This video of Bartlett Cove in SouthEast Alaska shows what smaller floats look like (minute 2:10). My floats were originally certified for a PA18 GW which is lighter than my Scout. I wish they were a bit bigger. But, then they would be heavier which makes my DA ceiling lower for mountain lakes. It’s a trade off for me.

So, the half of my question about intentionally going below 1.8X buoyancy, seems to be a unanimous - "no".

But, the other half about how much over 1.8X is optimum, is still somewhat vague . . . . which isn't surprising.

There are a lot of variables in play, not only between different sized floats from the same manufacturer. But, also from different float makers, on many different aircraft, or on the same aircraft with engines of different HP ratings. Another complication, is the lack of standardization concerning float model numbers and actual buoyancy ratings.

Take MTV's example of a Cessna 170 on Edo 2000 floats. If Edo 2000's actually displaced 2000# of water, then using the FAA 1.8X standard, indicates that they would be legal for aircraft with gross weights of up to 2,222 pounds. But, if Cessna limits the 170 to only 2130#, then either Edo 2000's only displace 1917# of water, or Cessna wanted to limit the 170 for some other reason? Now, switching to PKB2300 floats, which are approved for aircraft of up to 2550#, even loading the same 170 to it's land weight limit of 2200#, would still provide a reserve buoyancy factor of almost 2.1X! In MTV's words, "These floats also require the engine upgrade to 180 hp or more, which helps with the takeoff performance, and the airplane is now a great performer on these floats." But, since PKB2300's weigh 305#, that same 170 may also need an up-gross STC in order to enjoy a similar useful load as with the smaller/lighter Edo 2000's? So, do all seaplanes need 2.1X reserve buoyancy, in order to obtain decent performance on & off the water?

Maybe 8GCBC can provide some clarification: If "90% of seaplanes are under-floated". Is that because the FAA 1.8X requirement is not high enough? Or, is it because 90% of pilots overload their aircraft, either intentionally or unintentionally?

Thanx again, Dave.

P.S. - While I understand that turning cross-wind on the water during gusty/choppy conditions can cause problems. Can't most of those risks be mitigated by slowing down to displacement taxi speed, or using a plow-turn technique? Seems like a lot of waterborne mishaps occur because pilots are in too much of a hurry and keep step-taxiing when they shouldn't.

It may well be that Cessna limited the 170 on EDO 2000 for reasons other than reserve bouyancy. That said, the early Huskys (A-1) were approved on EDO 2000 floats. These planes were limited to 2000 pounds GW on floats. When Aviat came out with the A-1B, which is legal at 2200 on floats, it was never approved on EDO 2000s. That plane was approved on Baumanns and wiplines, but not EDO 2000s. It was later approved on EDO 2130 floats, a float that Kenmore Developed for the Cessna 172. Early Scouts were approved on EDO 2000s, but restricted the gross weight.

I disagree with 8GCBC's assertion that "Most seaplanes are underfloated". That has not been my experience. There are a LOT of seaplanes out there that are certainly not underfloated.

As to the additional weight of the bigger floats, yes, that's a factor, but lighter floats that don't perform well off the water because of their buoyancy are a bad idea. And, there are ways, as Bigrenna on this forum has described, to lighten the airplane up, thus improving the useful load.

And, yes, slowing down certainly helps in turning around, but once the wind catches that tail, the pivots very fast, and the pilot has little control over that pivoting. As the seaplane pivots, it drives the front of the outboard float down, and raises the wing that's pointed into the wind. Not a good combination.

You're ignoring the basic conundrum that the prospective seaplane owner faces: There are very limited float models available for a specific aircraft model. And, price comparisons between new and used floats are interesting to say the least.

So, for example, for the Cessna 170, these are the floats that are approved: EDO 2000, PK 2300 and B2300, Wipline 2100 straight and amphibious floats. A good set of used EDO 2000s can be had for ~ $10,000, and there are a lot of them out there. PK B2300 floats are scarce as hens teeth. If you can find a set rigged for the 170 the price is likely to be similar.

But, price out a set of Wipline 2100s. Even their straight floats are north of $33,000 new, and amphibs are north of $53,000 new. And, just try to find a used set. If you do, straight 2100s will cost well over $20 K unless they require a lot of work.

The availability of new floats doesn't necessarily imply that it's going to make much sense to buy floats that exceed half the hull value of the plane. If you can buy floats that still work much cheaper, but that have some limitations.

And, let's say you really like a particular model of floats.....but those floats aren't approved on your aircraft. Getting an approval, particularly on an old airplane model is expensive, and the question the manufacturer has to ask is: How many of these are we going to sell? Wipaire approved the 2100s on the 170 because a fellow who's a very good friend of Wip had a very nice 170 that they were able to use for flight test, and that gent did the flight test. The floats were already in production, and in that case, it didn't cost them much to certify on the 170. I seriously doubt they've sold five sets for 170s.

MTV

That was an esoteric statement “90%”. Let me go further....

From a strict nautical evaluation, float seaplanes are mostly confined to very calm water. Hence my generalized claim of 90% having smaller than ideal floats for what “I would like”. I live in Hawaii, 20-30 kts & 5-10 seas are normal. Sorry my context was not adequately explained regarding my “personal definition” of underfloating. There are many variables and less of a margin when flying floats, be very careful on each “step” of the way.

When I was at Wip’s they showed me a set of floats for DH-6 Twin Otter (13000) that an Australia private owner wanted to take to Bali and set down mid ocean to go surfing. That would be nice if I had $14.5M for Hawaii too.

Wipaire is an outstanding company. At least my 2100As never collapsed in the hangar like the other brand.

A few thoughts:

Floats carry a bit more in saltwater. How much - not much but some. The math is available. Displacement is referenced to fresh water.

The width of the floats - overall spread - can determine lateral stability in winds. Wider vs aircraft weight outboard of the fuselage is less prone to tipping.

Wing tip design can influence tipping in side winds in my experience with a couple planes. Droop helped reduce the wind getting under the tip...upflare/stock made it potentially worse.

If it's wavy or long period swells are present low taxi speed may not help keep the wing down and a downwind/down-swell float from going low in the water. As Mike notes once the tail gets a whiff of sidewind we're potentially along for a ride especially with less than effective rudder(s).

Small short floats like to sink their rear getting up on step or even taxiing with aft CG heavy loads. Pull back too soon and the rears go under to the spreader bar and the bows rise up decreasing flotation. Now there's less float in the water (some is under) and the spreader becomes a sea anchor of sorts. Technique can minimize that behavior at the expense of some prop erosion especially if the struts are short like some Aqua's.

If the pilot's flat rating the flight then speed versus buoyancy helps income. If carrying a good load at the expense of speed then per flight hour rates help income.

Gary

Obviously, there are sometimes very limited float choices for some older model aircraft. But for other aircraft, there are enough approved models, for an informed choice to be made. In any selection process, it is prudent to begin with what is theoretically best for a given application, and then to choose from among the floats that are actually available, one that comes closest to the theoretical ideal. And cost will always influence that decision, too.

Gary has also introduced another variable - geometry. Are floats that are designed longer, or wider, or taller/deeper, better than others that are shorter, or narrower, or shallower, even when displacement is the same? Or is it sometimes vice-versa, depending on the intended area of usage: lakes -vs- rivers -vs- open seas, etc?

For example: I have read debates concerning Edo 2870 & 2960 floats, where many pilots preferred the smaller/shorter model for use on the same Cessna 180 aircraft. How does that make sense within the theoretical opinion that bigger is almost always better?

Thanx again, Dave.

Waterline (longitudinal, the length) of the hull is paramount and all variables will be dictated accordingly. Generally floats are too short for my taste of water. But, I’m a dreamer. The market is really (really) small and I’m very thankful to Wip for helping me achieve the incredible performance on my Scout! Flying floats is absolutely the greatest sport on water. But many, many.... trade offs. And it is dangerous too at times.

But, price out a set of Wipline 2100s. Even their straight floats are north of $33,000 new, and amphibs are north of $53,000 new. And, just try to find a used set. If you do, straight 2100s will cost well over $20 K unless they require a lot of work.

If you were lucky enough to find a set of Wipline 2100a floats used, what price range would you expect them to fall into? I am severely limited on the water levels in my lake. 8 months out of 12 the water is too low to get my plane out. I would now like to look at options to trade up the J3 to amphibs floats on PA18-150 vs. buying another LA4-200.