I have a K&B Sportster .65 drone engine on one of my airboats. I like how the engine sounds and it certainly moves the boat well. I've got the itch to collect another engine to try on the boat for a bit more speed. I havent gotten all I can from the K&B, but I'd like to have a plan B in case I cant get more from it. The K&B turns a MA 12x8 at 9400 on 10% nitro. It does a little better with APC props - it turns a 13x6 APC at 10,400 on 10% and a 13x5 around 10,600 or so. I dont want to run a prop larger than 13", but am not opposed to a 12" prop swung faster. What engines would give me say 1,500-2000rpm more than my K&B on the same kind of props? Is 12,500rpm too much to ask from a .60 sized engine on a 13x6 or similar size prop?

I have tested 11x7MA, 12x6MA, 12x8MA, 13x5APC, 13x6APC, and have a 13x7APC to try yet. Speeds were 20mph, 22.1mph, 24.9mph, 28.5mph, and 31.9mph respectively. 13x7 is untested so no numbers for that yet. (I dont use the MA props anymore - they were cheap and good to use for basline testing and learning how to drive it)

I'd like to have the power to get it to 40mph, but given the prop loads of a 13x6 to a 13x7, I think the 13x7 will be a loss of speed and thus my hunt for another engine..

Thanks for any that have any input.

With the kind of drag and weight air boats have you may need to go up a size or two to get your 40+ mph.

The size and weight of these .90 class engines is not much different then the .60-75 class. In fact the G90 uses the small crankcase. The O.S. weighs less then other brands .75's.


For the PRICE and POWER you will have a hard time doing better then a [link=http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXFV79&P=0]G.90[/link] from Super Tigre.

It will turn a 13x9 at 12000ish on the ground once fully broken in, running 10% nitro.


If you are wanting more RPM it will cost you more. An O.S. 95 AX will rev higher turning even bigger props. I have seen the AX turn a 14x8 at 13,200 on the ground. That is impressive.

http://www3.towerhobbies.com/cgi-bin...?&I=LXXMU4&P=0

Either one of these engines with the right prop are going to get you real close if not over your speed goal.

Another option would be Thunder Tiger PRO in the 90 range. I am not real sure but I am guessing it is somewhere in the middle between these two.

I didnt mention - the boat in question is about 6lb10oz with the K&B on it and wet rudder.

I'm not real keen to OS engines, namely because of the price. I read up a little bit on earlwb's Fox Eagle IV .60 but he only had RPM figures for a 11x7 prop of which is too small for my airboat application but was getting over 14,000rpm from it.


I'm not dead set on any given RPM target as I'm more concerned with staying with a 13" prop or less. I make my own fuel with castor oil so my engine choice would need to tolerate the castor lube.

Go with the ST or TTPRO engine, either will turn a 13 very very nicely for you, reaching your speed goal. Both of them are great with castor. Even the G.75 will turn a 13x8-9 with very impressive numbers. I am not an O.S. guy either.

I would rather have new stuff to build with the $$ I save on tiger type engines. But that is just me.

You will hit your goal with either of the more affordable engines with your boat under 7 pounds. I would however go with the .90 range engines turning the higher pitch 13's. I don't think a .60-70 will produce the thrust you need with any prop.

I looked into the G90 and Pro 91... Is there that big of a difference in quality or is there something that much more special about the TT Pro to warrant being $100 more than the G90? Moreover, I had seen several S90K's for sale for less than $75. Some have said the S90's were better than the G90's.. Any thoughts?

the G and S use the same insides, but different cranks, the G has a lighter Case,

how about looking into a 91 Heli Engine, they should turn more pitch than a 91 Plane Engine,,

or you could buy a Towers or Jet Muffler for your K&B to increase power,

or you could buy an OS 70 Heli Engine,,

or you could buy a 91 Heli Engine and a Tuned Pipe,,

or you could buy a K&B 100

or an OS 75AX,

or two OS Hyper 50 Heli Engines,

or raise you Nitro percentage

An other thought the ST 90 with a Davis diesel head will turn a 16x8 or 15x9 that much blade areas is a huge increase in thrust and might achieve the speed you want
plus a lot less fuel consumption martin

The S is a heavier engine, it has a big case and needs a bigger mount. But they are very good engines. I do not know if they are better but they are good.

The TT PRO is a proven brand with a lot of history, you do pay a bit for it tho.

I have all 3 brands mentioned. I am a fan of ST but not a fanboy, If you run them side by side, the ST are CLOSE but not the same, and it is enough you notice right away. Either the OS or TT will normally have more power.

The ST are different to tune and need a longer break in.
They have a fat low end and that almost always needs tweaked several times during break in. All of that being said, The G90 is more then good enough to accomplish your goal. You will be happy with it.

Keep the KB65 and add a ST gs40 to the bow as a inline twin [8D]

Okay, maybe you missed a few things.. first off, I'm not looking to spend $250+ for an engine. Heli engines need high nitro, plane engines do not.
The K&B is not worth putting a tuned pipe on because most tuned pipes don't start making power until 14,000rpm which is 2,000rpm higher than I've been able to get this engine to turn up to. I've gotten 12,200rpm with the 11x7 which is way too small. The Sportster does not like 15% nitro or any higher. Its quite happy on 5-10% nitro.
Anything over a .90/.91 is going to require a prop bigger than 13" to run happily of which I just cannot do. Pretty much limited to the .60-.90 range.
As I said at the top of the thread.... 13" props or smaller. I do not have the space to turn a prop any bigger. As it is, the engine is shimmed up 5/8" to accommodate the 13" APC props I've been using therefore, Diesel is out. Also, I do not want to have to make my own diesel or order it. I want to stick with my homemade 5% nitro glow fuel.

What I had seen about the G90 and the S90K engines is RPM - the S90's will turn better RPM's than the G90 - namely the G90 seems similar to the K&B sportsters in respect to being lower RPM engines requiring bigger props yet. I found a used S90K for $50 and another new one for $85. These were the Italian-made Tigres, not the chinese imports. Also, I havent ran a ringed engine before. Anything different breaking one in vs. an AAC engine like my sportster? Slobbery rich/almost 4-stroking for an hour before attempting to throttle the engine...?

I actually have 2 K&B .65 sportsters, both I got new but have only ran the 1 so far. I had a thought about running both .65's on this boat, but I think the engine mount would get a bit large and heavy to hold two .65's turning 12" or 13" props. Speed wouldn't be a problem; keeping the boat upright would be the problem. Not to mention I'd have to engineer some sort of bellcrank system to pull 2 carbs simultaneously.

Great idea, but I think a bigger single motor is the ticket. Don't get me wrong - this old mifit of an engine gets the boat around good... If it had a carb for a civilian sportster engine it'd probably go better. The drone engine carbs have something amiss inside thats manufacturing related. (They were supposedly spec'd to pretty much run at WOT on a wood 12x6 prop on a foam mig 29 airframe)

OK,, if you bought one of those S-90K's you could use a 3 Blade Prop on it, the S-90K's Engines run best at 11,000 to 12,000

as for Braking-In a Ringed Engine, just running them a bit rich for a gallon will be good..

BTW, most stock Glow Plane Engines in these sizes will happily run on 15%, and Heli Engine will run on Fuel as low as 10%

Others have said it and I tried it for my own satisfaction. At least with my K&B and my TT .46 pro on a Jett muffler, I saw no appreciable performance improvements running 5% nitro vs. 15% nitro. The K&B is timed so low, it could probably swing a hockey stick and still be happy on 5%. It runs really hot and overheats on 15% no matter the prop.


I'm not set on any brand over the other, but I am basically looking for something that'll spin the 13x6 or 13x7 props I have 1,500rpm faster than what the K&B will do, or better. Those are big ol' props that are easily capable of good speed with the right amount of power. I am not opposed to an engine running a smaller prop spun faster but I would think I'd have to go overkill and get a Jett and I see that being a waste on the flatbottom hull. I'm saving the Jett for my other airboat.

For now I'll keep shopping around.. any other thoughts are welcome!
Thanks for your input, guys.. I appreciate it.

how much Oil are you running in your Home Made Fuel, it should be at-least 16% ??

Airplane engines get 5% nitro, 20% all castor oil, and methanol.
Car engines get 20% nitro, 12% all castor, and methanol.

Ok I guess you got enough Castor

An airboat is very draggy compared to an airplane. From your figures, it seems clear you're stalling the higher pitched smaller diameter props to the point where RPM simply doesn't matter.

You need a lot of static thrust, and that means playing with larger diameter, lower pitches

I just do not know about the S model. What I read is the same as you however. I have seen a couple at our field and they are fine running engines. I just have not had any real time with one.
So I can't say either way.

As for breaking in.
ST says you run them WOT 4 cycling for about 5 minutes,lean a tiny bit,wait, lean a bit,wait, etc. Continue this for 30 minutes or so. It is detailed in their manual.

This will break it in good enough to start using the engine.

It will not reach peak performance until a gallon or two of fuel is used. Keep it rich, more then ideal, but not slobbery, it needs to heat up for proper breaking in. To rich and the temps are not right.

Keep it that way for a gallon or so.

You will then be able to get some good peak tuning. The ring and sleeve need to wear some, it takes a while for that fit to be perfect.

Between those two options, I'm leaning towards the TT .91. My TT .46 Pro is shaping up to be a really nice running engine. The Jettstream muffler really perked it up so I would probably get another Jettstream for the bigger engine. I am open to other ideas still, though.

You will not be disappointed with the TT PRO they really are great engines. Pound for pound they are stronger then most ST engines.


They will have their own break in in the manual. Follow it, and keep it just slightly more then perfect rich for about a gallon as well.

Happy Boating !!

Now I just need to hunt one down and convince my wife I'll die without it. LoL..

Be careful what you convince your wife will happen if you don't get to buy a new engine[8D]

Sincerely, Richard

Haha.. yeah, I suppose you're right. I have a minibike to sell, which should pay for it and leave a little extra for my pocket.

According to Tower's catalog, the TT .91 Pro has a 16,000rpm max. Are they friendly to revving up that high or are they not timed that high? I would put a Jettstream muffler on it since it doesnt come with a muffler. That muffler should give it an extra 1000rpm or so.

After doing a ton of reading, I think I'm gonna get a Super Tigre to save a few bucks although I'd rather have the TT. I'm still kinda stuck at which ST .90 to go after. The G90 with its smaller intake passages and lower RPM/larger prop scenario or the S90K. Both seem to be readily available, but I'm bearing in mind of not having the ability to run props larger than 13" diameter. Would either have a problem turning a 13x8 or 13x9 at around 12,000rpm? That or turning up a 13x6 13,500-14,000?

Those rpm figures are just left-field guesses. I have no idea what they'd turn but I'm just looking for the best all-around choice. I'm not too concerned with weight or size - the engine is for an airboat. I'd like to have the ability to wind the engine up a little bit for a little extra speed and I've been seeing more recently the G90 not being friendly to say 13x6 and being recommended a 14x6 instead. Any thoughts?

How would either of these ST engines compare in power to my K&B .65 Sportster? Is there even a comparison?

20 years ago, I built a 90 size all aluminum tig welded air-boat, It had a pusher mounted heli engine, steering via twin aluminum rudders. I beat the heck out of it and it just kept coming back for more. You could run it at speed right out of the water, up the stoney beach then down the asphalt road, it sounded like someone was dragging a large metal garbage can behind a car. It had two runners on the bottom that took most of the wear and gave it directionality (slide control) when on the water. One thing I could never do was use full throttle on the water or it would blow over, and if you were near top speed, you could not chop power or it would also blow over. I think it was in the 40 mph neighborhood where this would happen.

I sold the slightly dented, greatly bottom scratched toy at a swap meet several years ago, I'm sure it's sill around somewhere.