How about a 180 (30cc) glow motor?

[Ian Inverarity]

Tim,

I suppose that is a possibilty, although you could use alloy bolts to hold the heads on. These would stretch or snap in the event of a hydraulic....

I actually think the power output would be greater for the 2 cylinder than a single fot the reasons Craig mentioned earlier, ie the reducing specific power of larger capacity engines. There would be gearing losses of course, but with good ground profile well lubricated gears (these would be lubed with fuel every cycle) the gear train losses may not be that high.

The ratio I based it upon was 1:1.25 simply because it is an easier number than 1.26 and should be easier to get the right tooth count. I based my calculations on the power absorbed by a prop assuming 22000 rpm and 6hp, then finding what revs would be needed to absorb 12hp. A H50 doing 27500 rpm would be ballistic!

The cranks would still need to be custom made, and that is well beyond my capabilities...

Craig

Yes, the engine cranks would run backwards, but they would run in the same direction so there would be no need for left and right hand drums. The advantage of this setup is the revs of the output shaft should be high enough to still use 90 size props, and the engine is very compact width wise, much more compact than the A180V, and I think it is probably shorter as well.

Ian.

 

[TimD]

Ian,

Never thought of using alloy head screws - not sure they would be up to the task of normal CR and heat cycling though....

Might be worth investing in a few different gear ratio's also in case we run out of prop's to use.......

 

[Ian Inverarity]

Tim,

I thought of alloy head bolts a while ago because of the number of Zenoah barrels that have been damaged from hydraulics, but I never had the chance to try it. Alloy head screws would be okay, but you would have to 'life' them, ie replace them quite regularly. Aluminium has no lower fatigue limit.

I think the gears will probably be the most expensive items in the whole engine! I dont really even know who to ask about making them. Hollinger? ???

What do you think of the crankshaft drum layout?

Ian.

 

[TimD]

Ian,

I like the look of the crankshaft/ drum.... What is the drum i.d. ?

Gears shouldn't be too hard - I have had gears cut b4 - as long as you use a common pitch it should be fine. Hollinger would be overkill I think The only thing with getting gears cut is there is usually a long wait

Tim.

 

[Ian Inverarity]

Tim,

The drum ID is 16mm, which is slightly larger than the K-90 drum ID. The drum needs to be long due to the bearings, but I could probably make it shorter by changing the angle of the outlet.

I have a sneaking suspicion that normal hobbed gears would last a couple of runs maybe... I suspect the gears would need to be ground finish and made from high tensile surface hardened steel. I have had gears cut for industrial applications but nothing that small that has to handle such high speeds and loadings.

Ian.

 

[TimD]

Ian,

I don't think the loads on the gears will be THAT bad... The most load on them will be during starting provided the cylinders are both tuned correctly. Case hardened steel gears should do the job. No need for flywheels at all on this bad boy! The weight of the cranks and gears will be enough!

16mm drum i.d is more than plenty. Using 2 separate carbs helps in that regard...

 

[Ian Inverarity]

Tim,

I might have to break out the old text books on gear design and do some calcs, but I have a feeling the numbers will be a little scary...

The A-90's dont need flywheels, the crank has more flywheel inertia than the flywheel! It is more of a starting pulley really... Which is basically what I drew as well. I dont think such an engine would be difficult to start.

I think the K-90 drum ID is about 15.7, I just rounded up to 16.

Because the cranks are geared it would also allow experimentation in firing angles. They dont have to be 180 degrees apart....

Ian.

 

[EatMyShortsRacing]

Ian,

I still think that you are better off with the output shaft going back like the 21 drives. This would allow you to get the engine a lot lower in the boat because the starting pulley will be at the front of the boat. Also the engines would not be counter rotating meaning all teh induction stuff would be standard.... Engines need to be pylon style..

Craig

 

[TimD]

Ian,

Due to the gearing we have a lot of flexibilty in the firing angle - just change it a tooth at a time, but i do suspect that 180 degrees would be optimal.

I wasn't suggesting that it would be hard to start - sorry if it came across that way. I just think that would be when the most loading would be applied to the gearing. I also meant that the added weight of the heavy gears would add to the inertia / rotating mass. Therefore a flywheel is unnessesary!

I also noticed the shape of the "carb" on your diagrams incorporates some "venturi effect" shapes - interesting. What diameter have you factored in for the carb bore?

Tim.

 

[Ian Inverarity]

Craig,

So are you suggesting put the drums on the rear of the engine and have the output shafts at the front instead? I cant really get the output shaft much lower in the engine becasue of the diameter of the starting pulley, but I could probably lower it a bit and have the shaft running back through the case. The engine would need to be tilted a bit when installed in the boat to clear the flywheel. You would also have the joys of spinning the shaft over backwards and the belt flying off when the motor starts. I see no real advantages to this layout. I can raise the output shaft in my layout to reduce the installed height of the engine. The induction timing for a reverse engine is no big deal. CMB does a reverse disc for the EVO 45's which the Multi guys use. Too easy.

Tim

I thought Honda was playing around with getting the cylinders to fire closer together on the GP bikes? I have also heard that the rotax's fire both cylinders together, but I could be wrong.

The carb is mostly the standard A-90 carb, the only difference is the taper I added after the barrel. That is shown with a 12mm bore, same as the A-90.

I think most load would be on the gears at full power. I dont think my starter is good for 12 hp.... ;D

Nitrocrazed racing: The gear noise you hear is me thinking....

 

[EatMyShortsRacing]

Ian,

What i am suggesting is 2 engines like pylon engines.... starter pulley at the front of the boat, shaft doubles back between the engines to an output flex hex / square drive

Craig

 

[Ian Inverarity]

Craig,

Yeah, I guessed that much, but what about the induction setup? Do you mean like a 21, ie a single sided crank with an integral drum? I dont really like this setup on big motors like 90s. Not as stiff as a double sided crank.

I am not convinced of the benefits of having the pulley at the front! Convince me... ;D

Ian.

 

[EatMyShortsRacing]

Ian,

Convince you??? OK... consider that to start the engine the pulley will have to be large as the main shaft not only has 2 engines to turn over but it is overdriving the engines which means to get the mechanical advantage required to start the engine the pulley will have to have a large diameter. This I think will be too big to get the engine low in the boat if that pulley is at the prop end of the boat.

Induction should not be through the crank.

Craig

 

[Ian Inverarity]

Craig.

Oh dear. Actually because the output shaft spins faster than the crankshafts, when you turn the engine over by the flywheel you are actually geared down relative to the cranks, ie less torque is required not more. Also if the cylinders are set to fire 180 degrees apart you are only turning one 90 past TDC at a time. I have drawn in a 40mm flywheel which is already larger than that currently fitted to the A-90's, which are 36mm diameter. Thus this engine would need LESS starter torque than an A-90. My A180V was not hard to start beacuse it also was only compressing one cylinder at a time.

I have attached a pic showing a revised side view section layout, this time I have slightly increased the gear diameter and raised the output shaft in the case. This allows the engine to be mounted practically on the bottom of the boat without the starter belt clearance being a problem. It also has the side benefit of slightly reducing the load on the gears due to the larger diameter.

Nitrocrazed racing: We work as a team, and we do it my way....

 

[Ian Inverarity]

Here is the side view of the engine compared to an A-90HP. Note from the side the engine is not much larger. I have not shown a flex hex on the 2 cyl motor, but one is represented on the A-90.

One downside to the crank drum induction design is the carb is very low, which can be a problem with the rear booms of some outrigger designs.

Nitrocrazed racing: Seeking comment from the rest of the world's nitro racers too...

 

[Ian Inverarity]

Here is a top view of the engine compared to an A-90HP. It is longer and wider, but is still quite compact. Note I have not shown engine rails on the 2 cyl, which would make it wider. Actually I am not sure I would use engine rails anyway. Obviously this design is much smaller than simply mounting 2 engines next to each other.

Nitrocrazed racing: There was nothing on TV so I did some drawing.... ;D

 

[TimD]

Ian,

One thing I did pick up on from your drawings was there isn't much room for a starter belt between the pipe header's. I know it's not a big issue but thought I'd mention it.

Would it be viable to build a custom mount incorporating the centre shaft for 2 commercially available drum valve motors with reverse direction drums for initial testing?

Tim.

 

[Ian Inverarity]

Tim,

The starter belt would have to go around one pipe header so that you pull the belt tension at an angle rather than straight up. This is what I did with the A180V as well, it had the same issue. It did not cause a problem.

Of course you could build a geared mount for 2 engines that can be reversed. Or simply a geared mount that would take two CMB Vavola's which already run in reverse. But that would depend on what you wanted to test.

The reasons I am keen on an integral 2 cylinder engine over two seperate geared engines are:

It would lighter.

It would be much smaller.

It would be safer (imagine your shirt sleeve going into the open gears of two geared 90's.... )

The gears would be more relaible due to precision control of the shaft alignment in the one case and controlled continuous lubrication.

Simply testing 2 geared motors may not conclusively demonstrate the advantages of the concept.

Nitrocrazed racing: I can compromise....

 

[TimD]

The reasons I am keen on an integral 2 cylinder engine over two seperate geared engines are:

It would lighter.

It would be much smaller.

It would be safer (imagine your shirt sleeve going into the open gears of two geared 90's.... )

The gears would be more relaible due to precision control of the shaft alignment in the one case and controlled continuous lubrication.

Simply testing 2 geared motors may not conclusively demonstrate the advantages of the concept.

Ian,

I don't think testing 2 x 21's would achieve much. 2 x 90's would give us an idea of what ratio's would be optimal in "on water" testing to suit the prop's. I'm sure a certain twin owner could be convinced to use his motors in a test.

Safety issues with the gears can be ovecome via a gear cover. If the alignment is an issue we can also pin the cases to the mount.

I have concerns that the optimum ratio may require a change in centre to centre distance or to reduce the pitch which will compromise strength.

Anyone know if the VOK 15 engine is commercially available???

 

[Ian Inverarity]

Tim,

I agree that a twin 21 test would probably not prove much!

For the gear ratio I chose I used something called Fan Similarity Laws, which are used for sizing pump impellors or fans with relation to speed and horsepower.

If you feel that on water testing to optimise the gear ratio is needed, then I think the engines would need to be pinned to the mount. This mount would probably have the output shaft as the part that is shimmed. Lubrication with waste (exhaust) oil could be used on the gears. A rudimentary gear cover could be incorporated.

A small problem is which engines? Rossi 90's or OS 81's or OPS 80's can have the case reversed to have the exhaust and carbs at the back and gear at the front as Craig was suggesting. For a gear at the back setup, K-90's, A-90's and CMB 90's require mods to run backwards. Actually the easiest to change would be the A-90's.

I am not sure about the VOK 15, but the magazine article said that the SKIF 15 was available for about 190 pounds I think (I will check). The SKIF has the same basic layout as the VOK, but the picture was of only the outside so I did not scan that one. Although the VOK looked a little nicer from the outside. Anyone with contacts in Russia?

Ian.