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Dalslandia

By Dalslandia
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Albion Influencer

Albion

Posted
Posted

 

 

 

 

No way. Although I take care in the way I phrase my written comments you seem far too close an supportive of the company. That offer was to Athy, NOT you.

 

 

 

Sensible decision Tony. If I remember correctly, during the last extensive discussion of those props with the spade-like blades, an enthusiastic supporter was eventually found out to be the company owner posting under a fake id.

Roger

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Dalslandia Rising Star

Dalslandia

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What is an propeller?

 

Is it a screw? it sure can screw up a thread. :-)

is it a paddle? or a wing? a fan?

 

well it is a wing, a rotating wing, with tips that screw up the flow, a wing that work often in a fluid that got the clean air or water stream disturbed, the fluid the propeller work in is almost always going in different directions and with different speeds all over the propeller disk area.

 

axial speed entering the prop can be anything from 100% to 5% of the forward speed, but how much reduction in speed is the tricky question, calculate the propeller is a Childs game compeered to calculate the reduction in fluid speed behind a body. that's why they use CFD today, the cost to do so is paid back in better propeller efficiency

 

The wing is most efficient if the load along the span is elliptical, that to reduce the induced drag, it should also have a constant load per area unit, in theory.

but the load will be different when the propeller goes into and out of the variation of inflow speed. so it must be a average. on a ship/boat the slowest inflow is above the prop shaft, behind the swim. the lover part of the prop disk see the fluid come up in an angle from the bottom of the boat.

 

To make an good place for the propeller to work in, the hull should not restrict the inflow to much, the swim angle is said to be maximum 15 degree each side, or max 30 on a slow boat, the finer the better. if the fluid can't follow the hull side into the propeller it loose efficiency. and ad drag to the hull.

 

The wash the propeller make is the result of the thrust it make, to get thrust we must accelerate a mass of fluid, no wash no thrust.

The increase (Delta speed) of the fluid speed over the forward speed and inflow speed, is half at the propeller disk, the other half happens far behind the propeller.

best efficiency is had if we accelerate a large area or mass as little as possible. it means a big propeller is better then a small, but draft is always the limiting factor if rotational speed isn't, the pitch should not be so big that the blades stall at low speed either. making it into a paddel wheel.

 

The load along the blade should be constant, except for where it will cavitate or make large losses, as at the tip and the other tip of the blade that is attached into the hub, it should not make negative thrust either so the angle should not be to small, and not to big so it stall, that is a problem if the blade station goes into and out of a large variation of fluid speed along the circle.

 

if we now have in theory a constant load, or adjusted load along the blade, the blade area is set by the dynamic pressure at each station, dynamic pressure depends on the forward speed and fluid pressure (static pressure) dynamic pressure is speed ^2 (squared) .... meaning it is 4 times higher at the tip then at mid blade, so the blade area don't need to be so wide at the tip, or near the tip, (there is no area at the very tip) so in theory the blade width should be 1/4 of what it is mid blade at the tip, (near the tip) but if it problem with cavitating the load will be lower and the chord longer to make the same lift. at the tip there is no load or lift, so no chord is needed.

 

same at the center of the hub, there is no rotational speed, no lift so no chord is needed, except for holding the blade into the hub, unless it is rim driven, then there is no chord, area or hub in center.

 

The airfoil should have the camber needed to make the lift efficient at each station, and at no place along the chord have so low pressure that it cavitate.

 

The pitch or angle of the blade station will be so that it create the chosen load at the most efficient angle, with the average of the axial inflow speed along the circle it make. simple as that!

 

I think without knowing, that most mass produced boat propellers have constant pitch along the blade, but some might have an reduction in the inner stations.

What I have seen from propellers on ships, is that the inner pitch is about 60-80% of the nominal pitch,

 

The propeller slip, well a propeller don't slip, a wing isn't. the slip is the increase in fluid speed into the propeller along with the angle of attack the blade have.

A wing, airplane wing or hydrofoil, lift up the fluid in front of the wing and suck/press it down over and behind the wing.

 

so what is called slip is the difference of the theoretical pitch times the rotational speed, and forward speed. and depends on the speed of the fluid going into the propeller disk and the delta speed the propeller create.

Edited by Dalslandia
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wrigglefingers Experienced

wrigglefingersDonate to Canal World

Posted
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I'm gonna chuck my conventional propeller and weld on a giant self tapping screw in its place. If it comes off I only have to pop into Wilco's for a new one.

Yep, but what happens if the head falls off and it continues to self-tap? Does it enter a different time/space continuum? Like a wormhole?

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j04n Collaborator

j04n

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I am getting conflicting information on this prop design. Some are saying that the clearance between the tip of the prop and the uxter plate is very important and should be between 1.5 to 2 inches whereas others are saying spin as large a prop as you can and dont worry about minimal clearances. What is your opinion on this? Thanks

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MtB Veteran II

MtBDonate to Canal World

Posted
Posted

I am getting conflicting information on this prop design. Some are saying that the clearance between the tip of the prop and the uxter plate is very important and should be between 1.5 to 2 inches whereas others are saying spin as large a prop as you can and dont worry about minimal clearances. What is your opinion on this? Thanks

The bigger the gap, the bigger the size of tree branch it is that won't get wedged in it and stall the engine.

 

So it's up to you. What is the maximum size of tree branch you plan to unwittingly steer over?

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MtB Veteran II

MtBDonate to Canal World

Posted
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The rules of thumb say

3 blade 17% of diameter

4 blade 15%

5 blade 13%

 

but always 2" min

 

or 10% minimum if big is better, but still 50mm / 2"

 

but that min must depend on the size of the prop too, what if it is model boat.

 

 

Yes but WHY do the rules of thumb say that?

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Dalslandia Rising Star

Dalslandia

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in Swedish an inch is a thumb :-)

 

​http://www.fao.org/docrep/x0487e/x0487e04.htm

 

I guess it is years of testing, with smaller gap it sound like you have rats onboard, it come from the big naval institute. even if it is % here and % there, it also say minimum 50 mm

Edited by Dalslandia
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j04n Collaborator

j04n

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The bigger the gap, the bigger the size of tree branch it is that won't get wedged in it and stall the engine.

 

So it's up to you. What is the maximum size of tree branch you plan to unwittingly steer over?

We get some serious twigs coming up the Ouse on a spring tide so that doesnt leave me much scope! I see its going to be yet another compromise! ........ I'm learning boating seems to be one big compromise! frusty.gif lol

post-27205-0-06181600-1474900095_thumb.png

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wrigglefingers Experienced

wrigglefingersDonate to Canal World

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in Swedish an inch is a thumb :-)

 

And in Sweden, they measure across the thumb too. Big thumbs, know what I'm sayin'?

 

We get some serious twigs coming up the Ouse on a spring tide so that doesnt leave me much scope! I see its going to be yet another compromise! ........ I'm learning boating seems to be one big compromise! :banghead: lol

Welcome to boating ... Edited by wrigglefingers
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j04n Collaborator

j04n

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And in Sweden, they measure across the thumb too. Big thumbs, know what I'm sayin'?

 

Welcome to boating ...

Thank you! ........ although I was looking forward to cruising and not spending every weekend with my head under the bonnet! icecream.gif

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cuthound Veteran II

cuthoundDonate to Canal World

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Not really,just seems like that when you are trudging Home through the Snow on the way back from the Pub.

 

CT

Looked it up. A Swedish mile is 10km (6.2 imperial miles)

 

https://en.m.wikipedia.org/wiki/Scandinavian_mile

 

Edited to include whole url.

Edited by cuthound
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