I just built a Goldberg Seahawk seaplane. I took it out for it's maiden test flight and the OS Max .035 (about 20 years old but runs perfect) and it wouldn't get the floats up on the step and therefore it would not take off. I want to keep the weight down. I am considering putting a new OS .032 SX on it. My question is whether or not the new OS .023SX will significantly increase the power without adding weight. The .032 is very close in weight to the .035 engine that is on the plane.

Douglas,


In your post you seem to enter an extra, redundant '0'.

The current OS size in 0.32 and not .032 - It is the OS.32SX engine.

I believe your engine is the .35FP as appears in the photo.


The .32SX is significantly more powerful than your current engine, horsepower-wise.
It would spin the same prop sizes only a little faster, since torque is more displacement bound.

A more effective prop would be quite helpful, getting your seaplane on the step...
You did not write what prop size/brand you are using.

Were it me, I would step up to .40 or .46 engine, swing a bigger prop and generate more thrust. You can always throttle back after getting airborne. It takes a lot of power to get unstuck from the water, the water creates more drag than one expeiences from a wheeled take off.

Good luck,

Chip

I appreciate the input. I originally set up the airplane with an OS 45FSR engine. I decided to drop down to the .030 after reviewing the instructions again. Primarily because with the 45 I was going to have to add weight to the tail to get balance. I built a seaplane years ago that failed for that same reason. I painted that plane and once I got it balanced it sat too low in the water. Another issue with swinging a bigger prop is that the 9" prop on the 35 is already chewing spray coming off the floats. Another reason that I am so interested in not adding weight.


OS.045FSR changed to OS 45FSR by moderator....

Sorry about the extra zeroes. I have decided to try the .32 SX with a 1" bigger prop. I think it will do the trick. The plane was pretty close to getting off the water as it is. A little more power with no more weight and I think that I will be fine. I will post the results. I hope with weather permitting to try it this weekend. I am flying the plane off of Lake Huron which is one of the largest lakes in the world. Not many calm days to give it a try.

Barred.

The .32 SX will put out lots more power than the old .35 but you will have to let it rev to produce the power.

Make sure you have a selection of props - if you're trying 9 and 10" diameter props, don't lug the engine down with too much pitch. Let it do 13 to 14K rpm static.

Good luck.

You should have used the 45FSR. The weight difference is negligable and it is a very powerful engine. One the all time best OS two cycles.

I have a ST-75 and am getting 8220 RPMs with a attached to a Astro Hog Bipe. 13 X6 prop. I can barely get it up. I would think that I should be getting at least 9/10,000. I have no leaks, new fuel, and new plug. I am useing omega 15% with 16% synthetic. I was told a long time ago by a flyer that works for Hobby bench in Phx that all 15% has 16% synthetic oil. should I drop to a 12 X 8 to get more RPMs??????? Thanks.

Ron

A 13-6 is a bit large. I would think you would be running a 12-6 or 12-7, but I have never had an ST-75 so this is a guess.

I think you are correct so I will try it first on the test stand to see how much diff it makes. Thanks.

Ron

Your rpm seems low. That engine should turn a 13x6 about 10500-11000rpm "right out of the box". Maybe you can look at the piston ring if it is worn out or if it might be something else restricting the power? Also one more thing, super tiger always recommended that part of the oil content in the fuel is castor oil.

13x6 isn't too small for a 75. An 11x7 or 12x6 about as small as I'd run on a 60. A 13x6 is about right for a 75. I run most of my 60's on 11x7.5's

I put the 45 FSR on the Goldberg Seahawk last night and I will try that. I actually have the 32SX also. I believe that I should have put the 32 on it now that the 45 is on there. There is a significant weight difference between the motors, the CG shifted an inch forward. I added a weight to the tail to split the difference. I am going to try it with the 45 this weekend if I get decent weather. I put a 10 x 6 prop on it (I have an assortment of pitches and lengths to try). I am very concerned that the prop is too close to the water already and it still could use more weight on the tail. I think in the end that I will wind up putting the 32SX on it and the CG will be right and the power will be sufficient. I also found another problem that I addressed that may have caused a lot of drag. I did some measuring and found that the pontoons were out of parallel by 1/2". I reformed the mounting straps to compensate and now they are right on. That has to make a huge difference in drag since it must have been plowing through the water.

While I don't doubt an ST .75 will turn a 13-6 I think your figure is a bit optimistic. I have a ST .91 and it will barely do better than 11,000 with a 13-6. Though it has been a while since I ran it and the .75 may be a better performer.

Douglas,


This engine will give you much better performance for a sea-plane, with an 11" diameter prop. An 11x5 would be a good choice - much better than a 10x6, about 25% more static thrust.

Change the angle of the floats, to bring your plane's nose up!

You don't do this by changing the C/G location. Move the airborne battery pack back in the fuselage, instead of just adding lead.

The battery is already as far back as it can go. This is a pretty small plane. The 11 x 5 prop will be spinning in the water or chewing spray. I am a firm believer in power like everyone else but this situation creates limits to weight and prop selection. If you lay a straight edge on top of the floats the end of the prop is very close to it. With the 35 they sat pretty low in the water. With the 45 they will sit even lower. I may have to change the mounts for the floats to get more clearance. Everything you do like that makes it heavier and it is already a small plane with big floats. The way the floats are mounted limits how much they can be lowered to gain clearance. I will keep playing with it until I turn it into a flyable airplane or make it a submarine. Will keep you posted as to what I find.

Douglas,


With full size planes, the float/amphibious models always get a larger diameter prop.
The Cessna Skylane and the like gets a 82-84" prop, while the try-gear gets an 78-80"...

The added thrust is needed when operating from water.
It is your own experience that emphasizes this.


Water spray, or small "ground" contacts will not bother a GFN prop.

I think you should modify the float mounting to allow the use of an 11" prop.

I know this does not answer the origonal post.

My Super Tigre 75 will spin an APC 13x6 in the neighborhood of 10500, using either 10% or 15% fuel, it does not seem to matter on this engine.
In comparison, my Fox 74 has always turned the same prop around 500 faster than the ST 75, using 10% fuel.

The difference between 10% and 15% nitro is negligable.

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My ASP (first generation) .61 ABC would swing a 13x6 at a little above 10k rpm and an 11x7 at over 12.5k rpm utilizing the stock muffler. Something doesn't sound right about your ST.75. Not broken-in sufficiently? Seriously, it should do much better than that. The ST.75 is a good engine.

You might want to drop back to an 11x8 for a while. Don't want the engine overheating. Also, a bad glow plug can put the crunch on the upper numbers too. ST engines tend to run with the ignition too far advanced when running 10% or higher nitro and a warm plug in warm weather. Backing off the prop load should help considerably. The 11x8 is not too small, by the way.

Try a 9x6 or 9x7 3 blade prop for clearence. It will be good on a 45 and keep RPM up. My son has a sea plane we added small wheels in grass takeoffs. I would have guessed the ST 45 and 3 blade MAS prop would struggle. Boy was I surprised.