Gearbox ratios and propellers.

Gixer

Gixer

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I can't get my head around this and hope someone can help.
I'm I'm planning to replace my 1GM10 2.62:1 ratio gearbox with a Beta 14 2:1 ratio gearbox.

I've been told my current 14x9 two blade prop which works well on the Yanmar will actually be the correct prop for the Beta as well but would like a second opinion if possible.
If anyone knows of a good website explaining how to add this up in layman terms please let me know as I'm willing to learn.

I'm happy with the way my 1GM10 performs on my boat and would be happy to replace with another one but can get a Beta 14 for significantly cheaper. Seems daft not to 'upgrade' with a twin cylinder and possible hot water.

Hope you can help.
 
If you turn the propeller at the same shaft RPM it will produce the same thrust
assuming that the more powerful engine has the torque to do so then it will probably work but it’s a little bit like putting in a taller mast and using the old sails
but do the sums or better get a propeller supplier to do them for you spending money on a different engine but not selecting the best prop is rarely the most sensible thing
 
On the Beta website there is guidance on prop selection and they will also do a more specific recommendation if you fill in a template with more detail about your boat etc.
I went down this route when I re engined my Hillyard and the prop was spot on.
 
This is what I was hoping to hear, thank you so much guys.
That vicprop spreadsheet is awesome.
It’s says a 13x9 prop will give me hull speed at around 3400. Sounds like the beta won’t be at its maximum bhp according to the curve but around 12.
last question (for now) any downside to using a 14x9 as supposed to a 13x9?

BTW - both engines output shaft turns clockwise ?
 
Gixer said:
BTW - both engines output shaft turns clockwise ?
Click to expand...

But do both gearbox output shafts turn the same way?:unsure:

I had to be careful with mine and prop supplier needed to know the rotation - clockwise/anti clockwise.

"A Borg Warner [gearbox] 1.91:1 transmission output is OPPOSITE of engine rotation. A Borg Warner 2.01:1 transmission output is the same as the engine it ... "
 
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They both need right hand propellers, I’m guessing that means the shaft will turn the same direction. I’ll double check though, thanks.
 
Gixer said:
...
It’s says a 13x9 prop will give me hull speed at around 3400. Sounds like the beta won’t be at its maximum bhp according to the curve but around 12.
last question (for now) any downside to using a 14x9 as supposed to a 13x9?
Click to expand...

You may just find that the engine won't have enough grunt to turn the 14" prop at full engine revs (3600). Maybe it will only get up to 3200rpm for example. But it's fairly rare to need the engine running at full chat so no real problem. I'd use it for a season and see how it goes. You can always lop 1/2" off each blade for next season if you can't get enough revs. That's what I did a few years ago.
 
The yanmar is 9HP at 3600. The Beta 13.5 at 3600.
The gear ratio is different by a factor of 1.3, which is a smaller factor than the difference in power.
So you'd expect the Beta to push the boat at least as well as the Yanmar, with the same prop.
But what is the boat?
These are both quite small engines, so it's important to get the prop right.
It's all a big compromise, it's nice to cruise at good speed and sensible RPM, but sometime you want plenty of thrust into wind and waves or even while dragging the keel through a bit of mud.
A bigger prop will be more drag while sailing, unless it's a folder...

Also bear in mind, if your Yanmar is ready to be replaced, it might be down on its original power.
So look at all the calculators you can find, and pick the brains of people with the same hull if you can!
 
The Beta, with more power at low revs should be well able to handle the same prop at the higher gear ratio which gives 1800rpm at the propeller at 3600 engine revs, against 1374rpm at full power revs for the Yanmar. What rpm did you cruise at with the Yanmar? Try the same rpm with the Beta and I expect you will be going faster.
 
Thanks for the responses,

The boat is a British Hunter 272 and the Yanmar pushes it along at 5-5.5 knots pretty happily. I don't know what the RPM is as I don't have a tacho. To cruse at 6 knots would be great, I'm not expecting much more speed out of the Beta as the water length is only about 24ft.

I think i'll try the 14x9 as suggested and see how I get on.

Cheers again.
 
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