Prop size - Please Help Sailor.

[nortada]

OK I’m a novice so I come to the home of the experts.

But please keep it simple.

How do I find the correct prop size for my ancient 26ft Snapdragon sloop with a 10HP engine?

 

[BruceK]

Probably a question best asked in PBO.

However there are calculators around, not that I vouch for them, but what they will do is tell you what factors are required in determining a starting point.

https://sailboatdata.com/sailboat/snapdragon-26
https://www.vicprop.com/displacement_size.php

 

[Fantasie 19]

Probably worth asking here as well ==> https://www.snapdragonmirage.org.uk/

 

[Andy Cox]

Telephone Graham at C&O Engineering in Devon.

He'll ask for: waterline length, displacement, hull type, speed, engine hp, max engine RPM, gearbox ratio, fixed or folding and two or three blade. He'll then tell you the correct size of prop based on an efficiency as near to 100% as possible. Sometimes it might be just over 100%.

Cheers
Andy

 

[nortada]

Thanks for all of these pointers.

 

[CLB]

Telephone Graham at C&O Engineering in Devon.

He'll ask for: waterline length, displacement, hull type, speed, engine hp, max engine RPM, gearbox ratio, fixed or folding and two or three blade. He'll then tell you the correct size of prop based on an efficiency as near to 100% as possible. Sometimes it might be just over 100%.

Cheers
Andy

More than 100% efficient?

 

[Andy Cox]

More than 100% efficient?

My understanding from discussions with C&O is that having put all the data into the calculator it will then spit out a recommendation with the aim of getting as close to 100% as possible, say 97% efficiency. If one then then starts altering the recommended size by increasing the pitch value say, then the calculated efficiency might go over 100% to say, 102%.

Conversely, reducing the recommended pitch value will reduce the efficiency to say 93%.

The net effect of being over 100% is a slightly over size prop, which in many circumstances is preferable, as many boat engines rarely see WOT for any time, in fact if ever. Therefore you'll be at your optimum/preferred cruise speed at slightly lower RPM, using less fuel and less noise etc. etc..

I know this is true of smaller displacement boats, particularly sail boats which is why I mentioned it. Clearly this is not always the best way forward, especially with high performance high power craft.

I agree it's counter intuitive, the thing is either efficient or not, but this is how the calculators work.

Having said all that, I'm ready, as ever, to be told different by someone who knows.

Cheers
Andy

 

[CLB]

My understanding from discussions with C&O is that having put all the data into the calculator it will then spit out a recommendation with the aim of getting as close to 100% as possible, say 97% efficiency. If one then then starts altering the recommended size by increasing the pitch value say, then the calculated efficiency might go over 100% to say, 102%.

Conversely, reducing the recommended pitch value will reduce the efficiency to say 93%.

The net effect of being over 100% is a slightly over size prop, which in many circumstances is preferable, as many boat engines rarely see WOT for any time, in fact if ever. Therefore you'll be at your optimum/preferred cruise speed at slightly lower RPM, using less fuel and less noise etc. etc..

I know this is true of smaller displacement boats, particularly sail boats which is why I mentioned it. Clearly this is not always the best way forward, especially with high performance high power craft.

I agree it's counter intuitive, the thing is either efficient or not, but this is how the calculators work.

Having said all that, I'm ready, as ever, to be told different by someone who knows.

Cheers
Andy

I guess that must relate to a theoretical efficiency. I don't see how a prop can be more than 100% efficient in real terms.

 

[Andy Cox]

I guess that must relate to a theoretical efficiency. I don't see how a prop can be more than 100% efficient in real terms.

Yes I agree and perhaps I should have said 'theoretical' in my earlier posts. Of course you're right, in that in real terms you couldn't exceed the 100% mark. But the whole thing with props is fairly vague, as although a prop might be a 25 x 22, it's diameter will be 25 inches but in the real world it's not actually going to travel 22 inches in one complete revolution as there are all sorts of factors to consider like slip etc. as well as external factors acting on the boat.

As said earlier, I'm certainly no expert, but I'm aware the programs or calculators will work with efficiency figures in excess of 100% and in certain applications a figure slightly over 100% is preferable.

Cheers
Andy

 

[QBhoy]

Complete black art, but some starters.

You need to know;
Your engines rated rpm
Your current pitch and rpm
Gear or reduction ratio
Gps speed currently

All the above in normal use trim and form.
If you have no starting reference data and from scratch, it will almost certainly be a case of trial and error. Most wont get it right first time, but perhaps within an inch or two pitch.

It also depends what you want from the boat. Some go either end of the Rev range to suit their use.
So the bottom end of your rpm range would see a higher pitch prop and likely higher speed (marginally in your case), but less thrust at slow manoeuvre speed or delayed thrust at least.
Pitched higher in the rev range you will see more power and thrust at slow speed or from stand still, but less pace top end at open throttle. Likely use more fuel too.

The important thing is to be in that specified rpm range. Over it would be stressing the engine and below it would be not seeing the engines maximum power and also stressing the engine causing it to lug or perhaps even damage it.

Won’t go on too much, but on our P385, the props are pitched quite fiercely for open water speed and cruising economy. Great for that....put her in a waterway like the canals in Scotland....what a akward bugger. Lurches forwards at pace as soon as she is in gear at tick over. She will be at 6/7 knots at idle rpm in gear. Not ideal.