Rule of thumb to scale a prop? (No bashing please, I know it's malpractice)

[Floating Mechanic]

Hello and good day fellows!

I remember coming across this info somewhere on the net, but it seems this disappeared along a lot of good info out there lately. So I thought I would bug you here for some advice.

Use-case:
Only for motoring my boat to the next yard with better infrastructure!

What's this about?
I just noticed that my whole propulsion line id done for it, even the prop was pitted like hell (Bad connection to the anodes). Got new prop-shaft, prop, bearings etc.
But I'm a bit stingy bout the prices of new propellers here in the Netherlands (easy 20~30% up compared to other places), so I just got a "good as new" one from an outlet that is not the right one.
The original prop was 17x10RH and behaved really nice, no lugging down the engine, fair speed.
The "good as new" one I got is a 16x12R.

The question:
Is there a rule of thumb for scaling props? I know, for slow heavy vessels (mine is 12 metric tonnes @ 35ft) bigger is better. But as mentioned, I once came across such a rule stating something like "1 inch less diameter should be compensated with 2 inch more pitch".

Can anybody point me to that rule please?

I just want to see if I'm in the ballpark or just way out there.

And yes, I'm not disregarding the golden rule of "Do it right or do it twice!"
I am going to get a properly sized propeller before I go on longer trips than to the next yard.
Promise!

Best regards, FM.

 

[Stemar]

The prop you have may not be ideal, but marine diesels aren't too fussy, provided you aren't after ultimate performance. I reckon that it'll get where you're going just fine, and the trip will allow you to compare performance with your old prop.

As for a new, "right" prop, the calculator I used a few years ago has disappeared, but this one might help.

 

[PetiteFleur]

AS stemar suggests, just see how it goes motoring to the yard, and then make a decision. My experience when I fitted a new 25hp Beta to my boat - all the sites recommended 13x10, in practice this was maximum speed at 3400 rpm but engine rated to 3500 rpm. Like this for several years but a couple of yrs ago I had it repitched to 13x9.5 but made no difference. I'm leaving it.

 

[Tranona]

Welcome to the Forum

The rule of thumb is 1" to 1", not 2. Not exactly but near enough. So your new prop is 1" overpitched compared with your 17*10 and you would expect to lower revs for a given speed by approx 250rpm.

Even a 17* 10 is nowhere big enough to move a 12 tonne boat. You don't say what engine you have nor the reduction ratio (to calculate shaft speed). To give you an idea my 31' boat is approx 5.5 tonnes and has a 30hp engine with a 2.5:1 reduction and a 17*10. It comfortably exceeds hull speed at just over 7 knots.

You are right, for a heavy boat a large diameter flat pitch slow turning propeller is better. As an example I ran some numbers on a deep heavy 35' 12 tonne boat and you need around 60hp to get hull speed with a 20*13" 3 blade propeller. Very different from what you currently have.

 

[neil_s]

According to the Propking spreadsheet calculator, my boat requires a 16 x 10 - which it has. If there is not enough space to fit this size, then a 15 x 13 is suitable. I guess your 17 x 10 prop might not be the best for your boat, as Tranona says above, but the 16 x 12 should get you to the boat yard!