Sizing Toothed/Synchronous Belts for prop drive

[steamboatjack]

Malcolm,
you cannot decide on a belt width until you know the pulley diameters as this influences the torque transmitted.
I use a 30mm belt on 64 tooth pulleys on Grayling which is about 5 IHP.
Regards
Jack

[DetroiTug]

Quote: "Do either of you have thoughts on dog-clutch arrangements."

Do you mean a Lovejoy or spider coupling? Or some sort of clutch that allows the drive to be disengaged?

The easiest thing to do and following 99.999% of every other inboard boat set up (and the way God intended ) is mount the engine in line on the correct angle and use a rigid coupling. Everyone wants their engine to set level, the reality is, the engine mounted on an angle, unless it's some ridiculous angle is normal.

Mounting the engine off to the side and running a belt over is not a simple set up. As simple as mounting two pulleys and running a belt. New problems arise with this set up that a typical mounting with rigid coupling alleviates. Added resistance: There will be side radial loading on the propshaft and engine crankshaft. This radial loading is going to introduce additional drag not present in a conventionally inline mounted engine resulting in power loss at the prop. Some sort of thrust bearings are going to have to be incorporated in to the prop shaft to take the linear force/thrust from the propeller. In my opinion, these marine engines. especially those with babbet bearings are not really designed for radial loads and too, some even incorporate an integrated thrust bearing for conventional mounting.

You're still suggesting some sort of UJ (I take to mean "Universal joint") That means you're intending to run a stub shaft on some angle in pillow blocks off the engine or propshaft? This means at least three pillow blocks ( they can serve as a thrust bearing), fabricating mountings for these pillow blocks, timing belt, two pulleys, potentially troublesome and expensive (the good ones aren't cheap) flex coupler, power loss at the prop etc.

I'd mount it conventionally.

-Ron

[barts]

Perhaps you should explain the reasons you'd like to use an indirect drive.

- Bart

[malcolmd]

Malcolm,
you cannot decide on a belt width until you know the pulley diameters as this influences the torque transmitted.
I use a 30mm belt on 64 tooth pulleys on Grayling which is about 5 IHP.
Regards
Jack

Yes, I understand, I did my computation on the basis of 28-tooth wheels.... as you suggest larger would be better, but I need to look at the clearance between the keelson/hog and the propshaft to see if I could fit larger ones. (hence "hands and knees" comment).

[malcolmd]

Quote: "Do either of you have thoughts on dog-clutch arrangements."

Do you mean a Lovejoy or spider coupling? Or some sort of clutch that allows the drive to be disengaged?

- I mean a clutch to allow disengagement.

Mounting the engine off to the side and running a belt over is not a simple set up. As simple as mounting two pulleys and running a belt. .... In my opinion, these marine engines. especially those with babbet bearings are not really designed for radial loads and too, some even incorporate an integrated thrust bearing for conventional mounting.

- surely the thrust from the pistons generate a fair amount of radial load - although I was imagining incorporating a outrigger bearing for the crank.

You're still suggesting some sort of UJ (I take to mean "Universal joint") That means you're intending to run a stub shaft on some angle in pillow blocks off the engine or propshaft? This means at least three pillow blocks ( they can serve as a thrust bearing), fabricating mountings for these pillow blocks, timing belt, two pulleys, potentially troublesome and expensive (the good ones aren't cheap) flex coupler, power loss at the prop etc.

- I was intending to put a thrust bearing (roller in pillowblock) on the end of the propshaft, then a Universal or CV joint to get a stub shaft parallel to the sole (horizontal), mount the HTD pulley on this with another outrigger bearing/pillowblock.

So, yes, in total 3 bearings. I take your point about complexity, but this also allows some flexibility in "gearing" the prop if necessary and also allows me to set the engine off the centreline, to easy traffic thru the boat for passengers, and ease access to ashpans etc.

[malcolmd]

Remember to employ service factors suitable for a multi-cylinder gas engine; this will help compensate for the variation of turning effort w/ crankshaft position.

- Bart

Yup, I went for "engines with less than 4 cylinders" when choosing the load factor...

[malcolmd]

Thanks everyone, that was really useful - I have a month or 3 of woodwork building the cabin/decks/lockers etc., then it will be time to make my final decisions on the plant!

I think I might go with the belt drive for reasons of introducing a bit more flexibility in engine position and future proofing in case I find I need a few more RPM at the prop. But the attractions of a simple installation might mean I finish with a direct connection, either way you have exercised my grey cells and provided good input - thanks again...

As a BTW I have found Punch!'s viaCAD very valuable in combining the paper plans from Selway Fisher with my own drawings of the Engine/Boiler, fit-out and mast structures. It's not quite as intuitive as AutoCAD, but it's a lot cheaper, and the solid modeller helped a lot in designing the pump and alternator drives for the engine, my first experience of such things... I would certainly recommend it as an affordable 2D/3D CAD system.

....and this week another first for me was taking a memory stick to a local steel fabrication company and taking away 30-odd components for the tabernacle plasma-cut in 6mm steel from my own CAD drawings, very satisfying!

thanks again

Mal

[DetroiTug]

Quote: "this also allows some flexibility in "gearing" the prop"

Yes and that is one big advantage I left out. Belts are a lot cheaper than props. I accrued a stack of test specimens trying to determine the correct one for the tug. And in these sizes, it wasn't inexpensive. Two of them I paid around 500 (used) each for.

-Ron

[Mike Rometer]

And in these sizes, it wasn't inexpensive. Two of them I paid around 500 (used) each for.

-Ron

Ron that sounds like $500 in used notes?

Sounds like an underworld deal, rather than underwater.

[DetroiTug]

That was a bad sentence placement of the term "used". Yes they were quite used and in my possesion briefly as usual.

-Ron