K&B .65 Sportster (yup, more from me)
 

This engine is on an airboat seen in this thread: http://www.rcuniverse.com/forum/m_10...m.htm#10050121
I did update that thread, but I'll run it down quick here. I ran 3 props with a GPS. 20mph - 11x7, 22.1mph 12x6, 24.9mph 12x8 All props are MAS. I would like to swing a bigger prop - 13".. I know the engine will do it but what is the difference in thrust from a low-pitch 13" to the high pitch 12"? What pitch numbers should I be looking at for this engine? I have some clearance issues - anything bigger than a 13" will require shimming the engine higher - currently its about 6 1/4" high from the very bottom of the engine stand. I dont want it to get too top heavy so shimming too much higher might get iffy.. I could possibly invert the engine to keep the balance point in check, but then I still have to shim the engine up higher to clear the current prop arsenal I have.. Any thoughts from the K&B gurus?

Thanks guys! I'll have a couple of vids up soon.

RE: K&B .65 Sportster (yup, more from me)
 

A 13 X 6 will probably give you the most static thrust but it looks like you were gaining with the 12 X 8.

RE: K&B .65 Sportster (yup, more from me)
 

I had been running the 12x8 for the last 3 or 4 tanks (7.3oz/tank) and it seemed slower than it should have been, thus the change to the 12x6 and the seemingly faster run. I tried the 11x7 just for the heck of it - the engine did give me better RPMS but the smaller diameter kinda cancels that out and given the lower thrust output, the slower speed.

I'm not going for a all-out full speed run.. I'm trying to find the best all around prop. I would like to know how fast I can get it to go, but thats mainly for a 5 second personal satisfaction moment and the rest is to just have fun..

Also - I had been running SIG fuel 10% nitro/20% oil (50/50 cas/syn) and I recently grabbed a gallon of Byrons 15% nitro 18% oil (80/20 syn/cas). I mixed up a Qt. of this with an extra ounce of castor oil to give me a 32%cas/68%syn ratio. The engine doesnt seem to load up as bad on this fuel - far more reliable (I've figured out how to get it to not stall when its loaded up) and smooth.. However, I checked the bottom of the glow plug and since I switched fuel (used 1/2 qt of new mix) the plug went from gold to a blackish gold. I attribute this to more synthetic oil burning off. The mixture was ran 3 clicks rich from peak. What is the maximum oil percentage that can be reliably ran on this engine? I'd like to add another 1/2oz to 1oz of castor to a Quart of fuel. On the current mix, I used 31oz of fuel and added 1oz of castor oil to give me 32oz and an overall oil content of 20.8% or so. I would feel better about running a 50/50 ratio of cas/syn oil but given the fuel I have, that would probably put my oil content to around 25% or so.. I can't afford to buy gallons of fuel with full castor oil because the shipping charges are outrageous and it would take me a long time to use 4gal of fuel. What should I do about fuel? I figure, using the SIG I've been using, its cheaper to get it by the qt up to 1 gallon. After that, its cheaper to get gallons from Sig directly. The gallons I'd get from sig would be full castor/no synthetic (which only comes in gallons) whereas the 50/50 ratio of the LHS bought stuff 1Qt at a time... I know I'm making this into a bigger deal than it should be but I want to run whats best for the engine..

RE: K&B .65 Sportster (yup, more from me)
 

From my many year experience, I never used any fuel that was more than 25% total oil.

With your bushing bottom end, I would try to have a good bit of castor in the fuel.

The 50/50 syn/castor ratio sounds good but by the quart gets expensive

Achive this with gallons of fuel with a Synthetic/Castor blend with enough extra castor added to get you up around 22% total oil.

The Castor oil is pretty easy to come across.

I think the 13 X 6 would make a good sport prop for that boat if you can swing it?

RE: K&B .65 Sportster (yup, more from me)
 

The K&B Sportster engines are torquers, with the intake and port timing. They like to swing a big stick, load them down. I always start with a big prop and drop dia or pitch to get best performance. I have used the Sportster series a lot and my last remaining favorite is the .28 which swings props used on a .46. Don't look for high RPM figures, look for thrust.

RE: K&B .65 Sportster (yup, more from me)
 

Buying the SIG 50/50 by the quart is a better deal up to 4qts at a time - beyond that its a better deal to buy 2 gallons of full castor from SIG. (2gal w/ shipping is $71 whereas 2gal from LHS is about $77)

Most other fuel is an 80/20 blend and I dont think the 1oz I put in is enough. I dont like black glowplugs or pistons. (The plug element looks brand new and has about 4 1/2qts on it)

I can swing any 13" prop - I was looking for a good starting point or range on them so I can grab a couple different pitches and experiment. (I originally was thinking 13x4 or so)

I know about the "poor mans 4-stroke" deal.. I hadnt GPS'ed the boat on any of these props to know which one runs the best.. this was just learned today and given the 12x8 gave me the best speed, I needed to know where to go next. 12x8 is the highest pitch 12" prop I have seen so far and there are plenty of 13" props available.. I'll probably go get 2 or 3 props to compare with and find the right one.

I read a post on another airplane forum where someone ran a 14x5 on a .65 sportster and had good luck with it.. I imagine it was down in the 6000-7000rpm range with something like that. The 12x8 on my sportster gives me about 9400rpm which is only 3000rpm under what I've tached its max RPM at. (12,400 on the 11x7, 10,200 on the 12x6) My next question is how many RPMS would be the lowest I should run it at to get good performance from it without it being too loaded down?

RE: K&B .65 Sportster (yup, more from me)
 

If your engine runs at a certain rpm at open throttle, it is generating a given amount of hp. To reach the maximum speed possible with that hp you want a prop that efficiently turns that power into thrust at that airspeed (forward speed). I'd say you need more disc area (diameter) and less pitch. Which is where you are headed.

A handy tool is this: http://www.rcpro.org/rccalc/PitchSpeed.aspx Don't assume that you will reach pitch speed - rather, look for a prop combo that does not have a pitch speed 2 or 3 times what the boat can achieve. To do so means the prop blades are always running at a higher angle of attack than would be most efficient and thus generating more drag than they need to. Aim for a combo where your actual speed will be a major percentage of the pitch speed, as in 2/3 or more (very rough numbers!) which indicates you are matching the prop better to the airframe, er, hull. Which in short, means try less pitch - 4" or 5" - and more diameter. I would stick with reasonable rpm and trade dia for pitch. Your thoughts of a 13-4, 13-5, 14-4 are all in the right direction IMHO.

RE: K&B .65 Sportster (yup, more from me)
 

Keep in mind that this is a 65 Sportster and they are more of a torque engine. Don't be runninging it over 10000 rpm

RE: K&B .65 Sportster (yup, more from me)
 

Yeah, given that it will spin a 13-6 a bit over 10k, and that airboats can be draggier than biplanes, I'll bet the best prop might even be 14" diameter, mebbe a 14-5? I doubt a 14-4W would hold it under 10k. A lot of folks fly them in the 10-11k range, is this really out of the powerband for this engine?

RE: K&B .65 Sportster (yup, more from me)
 

10,200rpm on a 12x6, not 13x6. I bought a 13x5 last night and balanced it up. I'll try running this one and see what happens. I have to shim the engine an extra 1/4 inch to clear the 13" props, but thats ok. I dont want to run 14" props - my Center of Gravity/Balance point is already high enough... If I have still gained from the 12x8 tests to the 13x5, I might go up and get a 13x6 too..

I've been told these engines run best when loaded in the 8000-8500rpm range. Anything more is a waste. I tried the smaller 11x7 and 12x6 just for comparison purposes. I pretty much ran 3 straight line passed - just enough to get a good reading on my GPS and came back to change props. The engine cleans out faster on smaller props - but if I can get the fuel mixed the way I want, it shouldn't load up much. Military sportsters have the miscut idle disks or something, I think.. on Sig fuel it, If the engine is ran under say 4000rpms for more than a few seconds, the transition is usually pretty rough. Misfires and lots of smoke.. once it clears out, it runs like it should. I think I need to make a modification to the idle orifice in the idle disk - needs to be closed off a little bit. When shutting the engine down for the day, I close my fuel line clamp, and pull the fuel line from the carb. Most often, it takes 5-8 seconds to shut down. No speed up at the end, just a constant idle-like sound until it starts to run out of fuel and then it slows down more and more and quits. This doesnt seem normal to me, but I'm still learning the noises and quirks of these engines.... is the idle still a little rich? How long should it take to stall when the fuel supply is cut off?

Overall, I'm happy with the engines performance so far.. Hell, I can't complain.. it was FREE.

RE: K&B .65 Sportster (yup, more from me)
 

Maybe you never used it, but Fox Superfuel was 28% castor lubed.


Ed Cregger

RE: K&B .65 Sportster (yup, more from me)
 

If you are getting 10,200 with a 12 x 6...maybe you could try a 12 x 10 APC "Pattern" prop? That should load it down to the approx 8,500 or so range, as you mentioned.

RE: K&B .65 Sportster (yup, more from me)
 

I'm not really sure how you use the throttle on a boat application, but the bigger load will take longer to reach full power. That might make it seem slower. I guess it depends on how long the straight paths are, and how much throttling is being done. APC makes 12 x 9, 10, 11, 12 pitches. They also make a 12.5 x 6 as well as many 13" in a range for you to try. For a plane I love the 13 x 5 or 6 on mine.

RE: K&B .65 Sportster (yup, more from me)
 

Yeah...it might take a bit longer to unload and get up to speed with something like a 12 x 10...but I have a feeling that the top speed will be faster.

It's kinda like trying to pick the right gear ratio for a 1 speed transmission in your car...or maybe no transmission , just the gear ratio in the differential? (but ya gotta reeeaaalllyyy loose torque converter, aye?;) lotsa slippage until it gets up to speed )

RE: K&B .65 Sportster (yup, more from me)
 

Kinda like the old 2 speed Chevy Powerglide automatic "transmission". Shift it into low and rev the crap out of it and then jam in into drive.:D

Regards, Richard

RE: K&B .65 Sportster (yup, more from me)
 

IMHO: I see no potential advantage to those high pitch props. This boat so far manages 25 mph - it's draggy compared to an aircraft. The engine's only has the hp to pull it so fast. This would be like putting a 12-10 on a Senior Telemaster. To get the best speed the prop must be a good match. The best top speed will come from maximizing the disc area and matching the pitch to the speed potential. Otherwise, to use transmisison analogies, it is like trying to climb a steep hill in a fully loaded 4-cyl pickup in overdrive. Gear down and match the drivetrain to the load and speed.

RE: K&B .65 Sportster (yup, more from me)
 

Well...it weighs about 7 lbs.
That's not all that much for a .65 to power...and it doesn't look all that big to me either.

JMO...but I think an APC 12 x 10 Pattern prop might be worth trying, if looking for more top speed.

I see a Master Airscrew on there now...

APC's are more efficient... a 12 x 8 APC will be an improvment over a MAS 12 x 8

RE: K&B .65 Sportster (yup, more from me)
 

Its bigger than it looks, 32" long x 18" wide. The first couple times I ran it, it seemed like the .65 was too small for it given its weight. Since I've been switching props and the speed is getting better, I'm getting to be a bit happier with it. This same boat design and construction with a 91FX ran 38mph+.

Well, what I'm doing is using the cheaper props to find the magic number. Once I do, I'll get an APC. At my 2 stores that sell bigger props, the APC's are 6-8$ and the MAS's are 3-4$. I don't want to spend a small fortune on a bunch of props to "try" to end up with a bunch of 6-8$ props that won't be used again (or be able to be returned)

I tried a 13x5 since they didnt have any 13x6's in stock and I got 30.3MPH on 15% nitro. I think I gotta drop back to 10% nitro though.. The engine gets a tad hot on 15%. I was going to go get a 13x6 and maybe a 13x7 to try and once the top speed falls off, I'll buy an APC to replace the "best" prop.


I forgot to mention: These Military Sportsters Carb's suck. I have a feeling I'm going to have to order 2 new carbs for the two .65's I have. This thing does not throttle well. It quit on me in the middle of the pond the other day for no reason. If you keep the RPMS up it runs fine (like they said it would) but back to low rpms for any length of time to WOT is really rough - the more oil in the fuel the rougher the transition. 15% nitro and about 21% oil seemed to pep it up a bit on the high RPM, but the transition is killing me.. Does anyone know where I could find 2 carbs for these two engines that are going to work better than the ones I got and NOT have to go to Mecoa if at all possible?

RE: K&B .65 Sportster (yup, more from me)
 

I think this is where the trial and error comes in - without knowing its real speed potential with the given powerplant, its hard to pin down the proper pitched prop. Currently, I can run a 13" prop which gives me about 1/8th inch clearance between the edge of the prop and the hull. I think this is a bit close but it seems to be okay - I just need to pitch higher and try it out in order to determine what is best.


side note: I am currently using a 170cc fuel tank from an LST monster truck, but I am going to change this out for an 8-10oz airplane tank. I have several 16oz airplane tanks, which weight the same as the fuel tank I'm using except I have some bit bolts/screws holding the tank on. I'm going to try to shed a little weight in the fuel tank department and lighten the back end a little bit with a smaller tank. This should lessen the drag on the back end a little bit.

RE: K&B .65 Sportster (yup, more from me)
 

I'd bet 5 bucks the 13-5 is the fastest of the 3. You are still way below pitch speed, which tells me that increasing pitch will only slow it down. But you'll find out.

Perry carbs work very well. The 4600 is listed as a replacement for the .65.

RE: K&B .65 Sportster (yup, more from me)
 

Being that I get so many different peoples opinion, I'm a little confused. The guy at my LHS had said that if I were to go up in diameter 1" the pitch should go down 1-2". Based on this, a 13x5 would be a rough equivalent to 12x6 or 12x7. Which in my case, the 12x8 was 5.4mph slower than the 13x5. Since I'm not running at pitch speed (which on a 13x5 should be about 40mph @ 8500rpm in a perfect medium) What should I run on it to get the highest top speed from the given RPM limit of say 9000rpm? I understand a small diameter large pitch moves a small amount of air real fast and large dia. small pitch spins lots of air slower. With this known, a smaller Dia. high pitch sounds more promising but12x10 really move the boat faster than a 13x5?? I tached 9200rpm on a 12x8 MAS on 10% nitro on the stand. I got 29.4mph on this - I have not tached the 13x5 prop yet. The engine sounds faster on the 13x5 than it does on the 12x8, although sound is subjective. The GPS readings don't lie too badly.

I don't want to run 14" props if at all possible - The engine will be too high I think. (and they're expensive)

I'm also waiting to hear back from Mecoa on a price on a new K&B carb. At about 50$ for the Perry, I'm sure the K&B carb will be around the same price and I am not really that willing to spend that kind of money on a free engine. Mecoa said these engines were "spec'd to run WOT only and were not intended to get into a consumers hands" but thats all he really said. He did not go into the details of WHY they are that way. I'm betting it was a machining screw-up. I wonder if a Spraybar and low-speed disk change would cure the problem. I don't see how a remote needle carb is going to make that much of a difference other than the low-speed disk is different and it actually has a low speed needle.

RE: K&B .65 Sportster (yup, more from me)
 

The guy at the hobby shop is trying to show you the difference in prop load factor 2xdia+pitch, essentially if you have a 10X4 prop on an engine and you drop an inch in dia you switch to a 9X6 which has the same load factor. This will theoreticaly allow the same RPM figure on the two different props. We used this formula in controlline for years.

check this link http://www.murocmodelmasters.org/props.shtml

RE: K&B .65 Sportster (yup, more from me)
 

So given the numbers are arbitrary, there is a decent difference in prop loads between the props I've ran. 11x7 = 77, 12x6 = 72, 12x8 = 96, 13x5 = 65. Other recommendations I've is 14x4 = 56, and 12x10 = 120. Now I don't understand how prop load works in relation to RPMS and power. An 11x6 prop has about the same prop load as a 13x5 prop, but the RPM difference is significant. I never put an 11x6 on this engine, but I would guess it would spin faster with less thrust. than the 11x7 which spun about 12,400. I guess I need to know what prop load I need to get the RPMS where the engine makes its best efficient power. Obviously spinning it fast doesnt do any good. Since the boat doesnt change altitude, I don't think the prop "unloads" since it has the same amount of drag all the time.. (well, almost. less drag when on plane but not a lot. about 1/2 - 2/3 of the boats keel (bottom) is in contact with the water when going WOT.) There are a few ounces of weight savings that can be had by swapping fuel tanks but I dont know that the weight savings will change how the boat performs too much.. Even if I lost 2oz, its still 6 1/2 pounds.

RE: K&B .65 Sportster (yup, more from me)
 

Okay.. I have a new problem.... I ran the boat tonight to try out another prop.. I grabbed a 13x6 APC just for the heck of it. It ran fine for the first couple of minutes and it started cutting out at wide open throttle. I tweaked the needle richer (the RPMS were sagging off a bit.) I ended up going rich a 1/2 turn or just under and it would take off from shore fine, but once it got a little ways out, it started sagging again. I changed fuel lines, I changed the fuel tank to an airplane tank 8oz (full) I pulled the main needle assembly out, looked clean. Checked fuel flow from the tank and it was fine. Put it all back together (engine cooled off) and it ran fine for the first 1/4 tank and started sagging again.. just before I brought it in to pack up, it stalled on me.. When it quit, it quit fast - much faster than when you pinch fuel off to the carb. When I got it back to shore the fuel line was full. I'm kinda baffled because it ran fine yesterday, and on the same fuel. In the middle of fiddling with it, I put the MAS 13x5 back on it to try it out and it seemed to like that one better but it acted the same way. Seemed like the engine was getting hot, so I swapped out the byrons 15% nitro fuel that was in it for Sig 10% nitro. It was better on the sig but not perfect like yesterday. Any ideas as to what would cause this?

RE: K&B .65 Sportster (yup, more from me)
 

How does the plug look...might need a new one?