I am setting up an airboat that I'll have about $60-80 and time into, I happened across a briggs 27 or 29cc 4 stroke that I want to use for this (having a standard wet crankcase wouldn't work well for the type of planes I have), any idea on prop size? I have a 18x6 on hand, would a 16" be better suited? prop size determines how high the engine sits on this thing so I don't want to have it too high with the heavy 4 stroke. I thought the hull was a bigger project than it is, it's solid foam with a plywood deck and fiberglassed outside, little duct tape and minor touchup and it's ready other than mounting radio and engine. going camping at a decent size lake in a couple weeks, figured I'd try to get this thing going.

should be fun when I get it going

doing a bit more looking it's possibly a second or third generation Ryobi 4 stroke, it's definitely not an early ryobi since I have one of those... it's n OHV with the black stamped valve cover, angled plastic oil pan (makes mounting fun), fan schroud that unbolts with the mag mount on the schroud. From what I remember it had a sheet metal con rod (two pieces spot welded into an I beam)

another question I have is the spark plug, I need to find out if it's a resistor plug, I'll look at the part number tomorrow.

I was doing some searching on here and all of a sudden the search stopped working...

If you want the engine to site lower why not use a three or four blade prop so you can cut down on the diameter? Don't know what pitch to suggest.

If you have a target rpm, and speed, you can choose a prop. If you think the airboat would run say, 30-35 mph on the power available, then choose a prop with a diameter and pitch that will turn the rpms the engine is capable of while providing the target speed.

Example: A 6" pitch prop turning at 6000 rpms gives a theroetical speed of 35 mph. Now just choose a prop diameter that will allow the engine to turn 6000 rpms.

Note that choosing a prop with too much pitch will not necessarily make the boat go any faster. If there is not enough power available for engine to move the boat through the water faster, added pitch will just reduce overall performance. One has to be realistic about the target speed, whether it is an airplane or a boat.

In this case, I have seen videos of a three point hydro airboat running around 45 mph with a G-26, hence my guess at 30 to 35 mph for the airboat in question here.

AV8TOR

that's the problem, I'm not used to these 4 strokes and their power or boats. This isn't an optimal hull and a puny little 4 stroke. The hull is about 36" long, 18" wide and will probably be in the 20# range. I don't know how tall is too tall or anything, it was a used home built airboat I got for $20, looked like it started with a .60 nitro then something like a 1.20 nitro

If possible, test run the engine with the 18 x 6. If it will turn it 6000 or more, go with that. If not, keep the 6" pitch but go down in diameter.

AV8TOR

ok, will do, thanks!

You're welcome. This video has me thinking about building one: http://www.youtube.com/watch?v=-S7YuJ3DeiQ Looks like fun!

Maybe it will give you some ideas.

AV8TOR

that's a good idea... elevator on an air boat? interesting...

Elevator? You lost me. What elevator?

AV8TOR

A airboat is the simplest most practical application of a model engine.

Everyone should have a couple

Yes to that. My first one was a cigar box with a scow nose and a Cox .049. Single channel escapment bang bang "rudder' and Lorenz 61 gas tube RX.

Current one is Dumas Big Swamp Buggy, 2 channel 75 MKHz Futaba wheel Tx, K&B 40 wirh Semco "muffler". I think I will try a .61 on it next.

Great fun at Float Fun Flys, and on the snow and wet grass also.

Sincerely,

Richard

I bet you spent all your time fiddling with that Lorenz reciever.

If the sun went behind a cloud you would have to repeak the pots to get it to work at all.

Look for the blue glow in the tube?

Yes to that also I still have the escapement and receiver, but I don't know where to get the RK61 tube.

Regards,

Richard

Look at the end of the video you linked to, there was an elevator flap thing on each side behind the engine, they were moving it around. probably not necessary. Now I have to scope garage sales for boogie boards

maidened it today with an 18x8 prop (only one I have on hand drilled to 3/8"), a bit too much prop as it seemed to turn around 5000rpm and COG too high (flipped it), thinking of trying a 14x10 or something if I can find one (got to check my stock too). Lowering the engine is pretty easy the way it's mounted. The engine ran pretty good until I flipped it, I think I might have the fuel lines backward or am pulling air at the splices in the lines (didn't zip tie or anything, easy enough fix).

I'll have to get a pic of my new pull starter... some people might find it a good way to start magneto engines... I have about an hour into it and just stuff sitting around, kinda nifty.

here's a couple pics of the boat, will get one of the starter tonight hopefully

Ran the engine with the 18x8 prop again with my tach, 4850rpm, I was close . Looking through my stash of props I have 3 15x8 props so I'll stick one on and see if it gets me in the range I want to be, I played with a calculator and it looks like it should turn around 6200rpm

now to the pull starter:


Took a Featherlite trimmer, cut the shaft about 1/4" short of the end of the handle, took out the trigger and cable, cut the heck out of the fan housing. Stuck the string spool part in the lathe and trimmed off the teeth until it was round. Found a chunk of 2" PVC then a piece of scrap aluminum that was about 2" diameter and 5/16-3/8" thick (didn't measure), drilled 3 holes for 10-32 screws to attach the PVC and 2 holes for 10-32 set screws (didn't have room for the third set screw but 2 seem to work just fine). I drew a shark fin pattern in the PVC drilling a hole at the corner to reduce the stress point then fitted it to the 18x8 prop on the boat until it fit to my liking. Works like a charm on that 4 stroke engine, starts on the first pull

Stuck the 15x8 on the engine, much better fit, the engine ran much stronger being able to rev up, will get a tach reading tomorrow if I can. I lowered the engine just over 1.5", the crankcase is sitting on those blue rails otherwise I would have gone 1/2" further... I have a feeling this is how the boat will stay.

That is a slick little starter rangerfred. Not sure if I could hold it steady enough to start a plane though.
Thus far my old 12V Sullivan running on 18V has worked but I haven't started anything over 25cc and I'll need a solution for a 33cc Homie.
I've been using a pull cord on the test stand but not sure if I could do it on a free moving (tail staked) plane.

using the starter on the boat is easy as the crank is long and self centers, I think it'll somewhat self center on other props too, I slightly hooked the flats on the teeth so it'll stay on, I'll take it with me the next time I go flying and see how it goes.

Fun looking project. Post some video if you can. The starter is pretty ingenious too.

What airplane sacrificed it's tail for the airboat??

AV8TOR

It was glued on when I got the hull, looks like an extra or yak or something, it works well though. Next time I'm at my friends house I'll grab the camera and get some video, it should be more impressive with the new prop, I actually drove it like a foot on grass last night as before it would only move about 1/4" with the big prop(it has a fin on the bottom, it's not just flat, if it was flat it would have just gone)

I got the starter idea from someone on youtube a couple years ago but they had a much more complex setup and used a standard starter cone or a socket on the prop bolt or something, I liked my idea better... now all I need is to find a cheap hand truck to haul this thing around... this dang 4 stroke engine starts on the first pull, prime, choke, pull then turn the choke off and you're going. I figured if they don't run right without an air filter since it doesn't have adjustment needles, just leave the filter on... I tweaked the regulator arm up a little to help too. Apparently the spark plug that came in it is a resistor plug as I have no radio noise, I could get used to this computer radio thing in cars/boats too, makes setup simple.

think 6400rpm is safe on this thing? it seems to turn that 15x8 at that speed... not too bad.

What made you decide to go with a puller vs a pusher?