Prop size

[Athy]

On ‎26‎/‎06‎/‎2017 at 17:30, Mike the Boilerman said:

 

 

I accept it proves the rule exists,

Exactly right. The exception proves the rule.

[David Mack]

On 6/26/2017 at 16:09, Mike the Boilerman said:

I don't follow the logic. How does an exception prove a rule?

Because in that phrase the word "prove" means "test".

[MtB]

35 minutes ago, David Mack said:

Because in that phrase the word "prove" means "test".

 

Yes it tests the rule, and proves the rule fails. So an exception disproves a rule, rather than proving it. 

[Athy]

39 minutes ago, David Mack said:

Because in that phrase the word "prove" means "test".

That is the other interpretation to which I alluded earlier.

4 minutes ago, Mike the Boilerman said:

 

Yes it tests the rule, and proves the rule fails. So an exception disproves a rule, rather than proving it. 

Not so.

[MtB]

4 minutes ago, Athy said:

That is the other interpretation to which I alluded earlier.

Not so.

Yes. But with either meaning an exception does not prove a rule.

[Athy]

Just now, Mike the Boilerman said:

Yes. But with either meaning an exception does not prove a rule.

I respect your opinion as usual, but cannot concur with it.

[dmr]

This is an interesting thread, I think rather than limiting it to "the exception that proves the rule" we should broaden it out to include all of the old sayings that our parents taught us...

"A bird in the hand is worth two in the bush"   almost certainly not universally true, and even where it is the factor is not likely to be exactly 2

"No smoke without fire"  Every electrical/eletronic engineer knows this to be total rubbish.

The big question here is why did our parents talk so much rubbish?

................Dave

[Athy]

19 minutes ago, dmr said:

This is an interesting thread, I think rather than limiting it to "the exception that proves the rule" we should broaden it out to include all of the old sayings that our parents taught us...

"A bird in the hand is worth two in the bush"   almost certainly not universally true, and even where it is the factor is not likely to be exactly 2

"No smoke without fire"  Every electrical/eletronic engineer knows this to be total rubbish.

The big question here is why did our parents talk so much rubbish?

................Dave

Most of the time, of course, parents didn't talk rubbish, but the exception proves the rule.

[Maudesmaster]

Thank you all very much for your input Crowther marine were unable to recommend a prop due to limitation on swing

Other company's were vague also As Daves is tried and tested although all criteria were not as my build I think I will go for the 21x19 

Now has any one got one kicking around it their shed 

thinking  T Norris as supplier as fairly local 

Thank you all again 

 

[Maudesmaster]

Ok after guesstimating a 21 x18 pitch a new propeller was purchased and boat launched September last year so 6 months trial (couple of short stays in Cowroast to have sprayer pump  and sprayers reconditioned) 

The boat very slow to uptake and tickover slight above walking pace of a Three toed sloth and 3/4 revs between 2 mile posts taking 25 minutes ( rough calculation 2.4mph)  Prop was removed last week and taken to T Norris to have a further 3” pitch  

Watch this space for next thrilling instalment 

thank you all for your input though much appreciated 

 

 

[monkeyhanger]

On 16/05/2022 at 13:23, Maudesmaster said:

Ok after guesstimating a 21 x18 pitch a new propeller was purchased and boat launched September last year so 6 months trial (couple of short stays in Cowroast to have sprayer pump  and sprayers reconditioned) 

The boat very slow to uptake and tickover slight above walking pace of a Three toed sloth and 3/4 revs between 2 mile posts taking 25 minutes ( rough calculation 2.4mph)  Prop was removed last week and taken to T Norris to have a further 3” pitch  

Watch this space for next thrilling instalment 

thank you all for your input though much appreciated 

 

 

I have aJP2 in my boat. When I had the boat  built I specified a 25 x 19 prop, as recommended by a well known Lister guru. In use the boat always felt very slow and  underpropped, so at the first blacking,I took the prop to Norris's to ask their advice. I asked them what would be the maximum they could increase the pitch to. The answer was 25x23, but they would recommend 25x21. I had it repitched to 25x23, which makes for relaxed cruising but stopping is not great, and the boat can slew across the cut under hard braking if I'm not careful.

 The moral.....listen to Norris's advice, and try not to have more pitch than you really need.

Edited September 26, 2022 by monkeyhanger
Added info.

[Maudesmaster]

I had Jonathan at Norris re pitch to 21x21 as boat was really slow, however very scary when first out on right hand bends boat would just go straight very nearly hit another narrow boat (left hand prop) giving side thrust to port

after a few months I’ve discovered if tiller more than 45 degrees boats stern would drift port more revs harder it would drift

now used to the prop im fairly happy I think to make perfect I need to change gear box from 2:1 reduction 2:1.5 to give me more revs on prop 

[Tracy D'arth]

3 hours ago, Maudesmaster said:

I had Jonathan at Norris re pitch to 21x21 as boat was really slow, however very scary when first out on right hand bends boat would just go straight very nearly hit another narrow boat (left hand prop) giving side thrust to port

after a few months I’ve discovered if tiller more than 45 degrees boats stern would drift port more revs harder it would drift

now used to the prop im fairly happy I think to make perfect I need to change gear box from 2:1 reduction 2:1.5 to give me more revs on prop 

Scary!  I am not surprised that the prop walk is so noticeable, at a 21" pitch you almost have a paddle wheel on the stern.

I always reckoned that the diameter was the thing to go for, gives better stopping and less walk.

But if the hull will not give you enough blade tip clearance  there is not much you can do.

Is it worth thinking about a bigger rudder?

[Bee]

On our boat the prop dia. would be reckoned to be too large because the tip clearance is too close to the skeg, in thousands of hours of travelling we have had no problems. The clearance under the counter is not a problem so I reckon you can probably use a bit of discretion with the theory. Basically i would hazard a guess that if you spin the prop a bit quicker you would see a big difference. We cruise at about 1600 rpm and that gives 800 at the prop. I seldom go over 2000 rpm, but that is with a 4 cyl. Beta, I guess thats a bit more than your engine. Quite how you change a gearbox ratio I would dread to think. Personally I would stick with what you have got and spend the winter months looking at some way of fitting a chain or belt increase between the gearbox output and the prop shaft if you have the space. Doesn't have to be very sophisticated to see if it works and any power transmission place could supply clamp on pulleys or sprockets. Some steel plate and a bit of shaft would do the rest, not frighteningly expensive. if it works then do it properly and keep your fingers and feet out of it.

[Maudesmaster]

Not feasible I’m in correspondence with Newage as to possibilities of changing gearing in my box or purchase of new box exchange 

this would be easiest solution as no need to drydock boat but not too worried as yet 

On 13/06/2017 at 20:11, jonesthenuke said:

We have a 58' boat, a Gardner 3lW, 1.5:1 PRM500 and a 22x17  3 bladed prop.  Hope this helps.

Shoul have gone for your gear box suggestion first but chap who supposedly refurbished engine fitted box Hey ho carry on for now hoping to change one day 

[BEngo]

Have you looked at a 4 blade prop?

That will have a bit less pitch than a 3 blader but will still absorb the power. Cheaper than a new gearbox too, I expect.

 

N

[IanD]

30 minutes ago, BEngo said:

Have you looked at a 4 blade prop?

That will have a bit less pitch than a 3 blader but will still absorb the power. Cheaper than a new gearbox too, I expect.

 

N

 

A 4 blade high-BAR prop would help even more, many suppliers can supply such a beast. The OPs basic problem is that the prop is turning too slowly for the diameter he can fit in...

 

I wouldn't recommend them because of cost, but Vetus do high-BAR (0.85) 4-blade props up to 21" x 26", see here...

 

https://webshop.vetus.com/en/products/stern-gear-systems/propellers

Edited September 27, 2022 by IanD