What size Floats?



94 S7 Short Tail on Aerocet 1100 amphibs


While your choice of floats for 1300 lb aircraft is somewhat limited, you do need to consider some important issues before you buy. While my experience is currently with Rans S7 and S7S aircraft, these considerations apply to all light aircraft.


Two of the issues are:

1.   What does adding floats do to my useful load both practically and legally?

2.   How does the size of the float affect performance?


Except for some smaller ultralight floats which can be categorized by length, usually floats are assigned a number which is the maximum gross weight that the floats will support. This number is either the actual displacement of the float or a calculated gross weight number based on the displacement. Thus a pair of 1260 floats will nominally support 1260lbs maximum all up weight of loaded aircraft on floats.




The first issue is what happens to useable load when you go to floats.

With most S7 and especially S7S aircraft, if you put full fuel and two 180lb people in the plane with the floats installed you will be well over the manufacturer’s 1232 gross weight number.  Using typical weights, straight, non amphib floats can add about 150lbs to the empty weight of the airframe AFTER removing the wheeled landing gear. You can easily calculate what this means to your useful load. It would be rare to find a set of amphibs of the right size that could match this 150LB  added weight; amphibs are typically 75 to 100lbs heavier than straight floats.


With many certified aircraft, float manufacturers successfully argued for an increase in gross weight when on floats. Sometimes you hear the (unsubstantiated) comment that “the floats carry their own weight”. Unless the airframe manufacturer spells this out, legally, it does not happen.


So, the net of this is that you will not be able to legally load your plane on floats the way you did on wheels.




What is legal and what flies just fine are two different pieces of fruit. Both the S7 and the S7S will perform better than a lot of float aircraft even when they are 100lbs over legal gross weight. What does affect performance is the size of the float and here, bigger may be a lot better. In this context, “performance” means how well it gets off the water. Yes, bigger floats are likely to reduce top speed but there will not be major differences in speed between different sizes of floats. With any floats your top speed drops dramatically anyway by maybe 10 to 20%.


Float displacement vs aircraft weight is a very crucial comparison. As a general rule, if you have a choice and there is not a huge weight penalty go with a displacement a lot larger than your gross weight; not the same and never smaller. For example, years ago when I loved the Stinson 108 which used 2425 floats, an experienced guy in the business said that 2870 floats (normally on a 180) work a lot better because the plane sits high out of the water.


Another factor is that there may be some variation from manufacturer to manufacturer on the significance of their displacement number so, in other words, not all 1150's across different manufacturers are the same. I suspect that few manufacturers have better than only an approximate displacement number for their floats.


I have owned and flown Aerocet 1100 amphibs on a 94 S7 short tail (weight was likely 650 on wheels); 1260 Full Lotus on short tails and on a 750lb S7S; 1300 Quebec floats on a short tail, and 1500 Murphy on several shorts and the S7S. I also have a little time with Lotus 1450’s on a 100hp S7 long tail. (see more details on this float in the Float Rigging page  )


The 1100 Aerocets were a very good looking float and they would support the S7 with two on board but you had to be very careful because it was quite easy to actually submerge the bow of the floats under certain conditions (like turning in a wind) and, if you have never experienced that, the first time it happens is quite unnerving. The conclusion is that 1100 floats on a 650lb empty plane are too small. (I don’t have the complete weight and balance for that installation but can provide it for all the others mentioned here).


On the other hand, when I sold that plane, the buyer wanted to fly off both land and water so he met me at a land strip on a hot calm day. We took off there, flew to a nearby lake, did a few landings and takeoffs and flew back to the land strip. He was quite satisfied and bought the plane.


1260 Lotus work quite well on a short tail but they are certainly not too large. You will be able to sink the heels of the floats when you are helping the rear seat passenger in. At gross they are quite acceptable on the short tails but you will find situations on a 750lb S7S (like the one above) where you would prefer a larger float.


The 1260’s above were on the plane when the buyer test flew it. He agreed they were useable but knew he would want larger. He has now purchased a set of Lotus 1450 floats. The 1450 is a curious design.  See the bottom of the page on Float Geometry for some thoughts on Full lotus floats and the 1450 in particular.

All things considered, the 1650 may be a better choice for a heavier S7S.


The 1300 floats above were perfect on this 670 lb S7 but these were light floats at 167lbs so after removing 42lbs of landing gear they only added 125lbs.


The floats that give the best all around performance and handling are the Murphy 1500 shown above. At about 180 lbs complete, these floats weigh about the same as Lotus 1260’s but have 240 lbs more buoyancy. They were perfect for the fully loaded S7S but also worked just as well on the lighter short tails. You would never submerge the nose (or the tails) of these floats.  You can see from the videos on my home page that off the water time is excellent (typically around 12 seconds) depending on the plane and at lighter loading.


Here are the same floats on a 625 lb 80 hp 94 S7. It gets off the water in about the same time as the 100hp S did.


Another float that may be a good size is the 1400 Aerocomp. I have seen these in both straight and amphib but have no experience with them. A local dealer likes his straight set on an S6. One thing I don’t like about them (and some early Czech floats were similar) is the narrow, 4”  backbone for walking.




In general, you will be happier with floats that have at least 100 lbs more flotation than the total weight of the loaded, float equipped aircraft. Anything less is a compromise that you might regret. The best plan would be to fly before you buy and do it in wind and load conditions that would show up any under floating.


1450 Full Lotus


Discussion of float geometry

Discusson of float rigging


Back to my Rans  home page.