Reducing the pitch of my prop

[nicknorman]

How? By that reasoning, if he had 400 HP he would do 60MPH

 

Richard

I think it's called "failing to take into account the non-linearity of the drag curve"!

[RLWP]

I think it's called "failing to take into account the non-linearity of the drag curve"!

 

 

I know this is an old thread, but still interesting.

Trilby Tim,

As they said in the commercial, "I am not just a dentist I am a mother too"

I am not just a canal boat Skipper, I am also a airplane propeller designer, www.jcpropellerdesign.com (and a father too) and have also studied ducted propellers (Kort) and shrouded propellers a little. I designed a Shilling rudder of my own design, for my canal boat. www.dalslandia.com

onboard her I have passed 50-60.000 locks.

 

When reading about your vibrations from propeller, I can think of 2 reasons in relation to the Kitchen rudder.

At forward drive with some amount of power, the propeller sucks in (induce) a lot more water then the forward speed, you say you have about 35% slip.

if we consider the slowed down water at the rear of the keel we can think it is even higher, maybe 50% When accelerating probably 100%

this mean the propeller suck in water into the duct or shroud shaped rudder with a lot of force and speed, so the water isn't coming from just straight a head of the duct but also from the side like a trumpet shaped funnel. at low speed the water even come from behind of the leading edge lip, So the water is going around this lip at a high velocity so it can't follow the inner side of the entrance if it is to sharp, (think centrifugal force here) So it need a big radius lip for the water to stay attached and not going turbulent, a good lip here will also give extra thrust.

 

The other reason I can think of is the shape of the hull just forward of the propeller, yours look somewhat blunt, it rise 2 questions can the water follow the sides? and at what angle is the water going into the duct and propeller?

 

Will a propeller with more rake be better?

 

 

Jan

 

Not nescer-celery. Jan appears to understand the subject

 

http://www.canalworld.net/forums/index.php?showtopic=19521&view=findpost&p=1033385

 

Richard

Edited February 26, 2013 by RLWP

[Albion]

Perhaps my boat is already reaching its maximum hull speed - this might be why my friend's 60' x 12' LB widebeam with an Isuzu 70 (65hp) doesn't go any faster than mine? We had a "burn up" on the Thames last summer. Also when we travelled together downstream on the tideway he struggled to turn into Limehouse lock against the current the same as me.

 

He has the PRM 260 (I don't know what reduction ratio), with a 19" x 15" prop and also can't get more than 2000 rpm in gear.

 

Here are the curves for the Isuzu 70 which show that at 2000 rpm he is achieving about 54 hp compared to my 40 hp.

 

If 54 hp can't go any faster than 40 hp does that point to the maximum hull speed being reached for this design of boat?

 

 

A calculation that I've just done using a website and making some assumptions about your boat (displacement and waterline length) indicates a max hull speed of about 7.5 knots (8.6 mph). I don't know if you have ever been able to record your (and your friend's) boat max speed in open water but it should be somewhere around that figure. More power for the given hull characteristics would only mean a much higher fuel consumption for very little speed gain.

Roger

[bowten]

Yes, it is.

 

Why would water flow to the prop be limited any more than any other fat NB?

 

Perhaps you just meant that water flow to the prop is limited on any fat NB, but I've heard people on this forum say that LB's have short swims. Generally these people turn out to have no experience of LB.

 

I can't find a decent pi cture of the swims of my boat so this will have to do. I'm not quite sure how long the swims are but this is a 57ft boat so you can judge for yourself. I don't think they are any shorter than most other canal boats (although any fat NB will always have shorter swims relative to its beam than a thin NB). Anyway, at the sort of speeds we're talking about I really can't see any reason why water would have any trouble getting to the prop.

 

 

I've also heard people say that you can't drill LB hulls because the steel's too brittle (nonsense), and last week a "boat engineer" told me that LB's were made of tin cans recycled by the Chinese... Recycling is exactly what a lot of people do. They just hear this sort on nonsense and then blindly repeat it.

I always used L/Bs for my shells until this last one.I never took any notice of the doom merchants either.What I have found on this new boat is that the bow design is very different.The L/B has a very upturned base plate,my new boat does not have this and I have noticed that this boat has more speed,can rev in gear noticeably more and produces less wake.I concluded that it was down to front hull design.The L/B wanted to rise significantly more at the bow than this new boat.However one of the benefits of an upturned base plate is the ability to ride onto the less dredged areas of a winding hole.

Edited February 26, 2013 by bowten

[nicknorman]

Not nescer-celery. Jan appears to understand the subject

 

http://www.canalworld.net/forums/index.php?showtopic=19521&view=findpost&p=1033385

 

Richard

He has just taken the cube root of the ratio as the multiplying factor. This only gives the increase in speed assuming the drag curve is reasonably linear in the range, but as the fat boat will be approaching its theoretical max speed, I would imagine the drag curve does not resemble a straight line in this region.

[bizzard]

I used to get this same question asked me with car owners. Oooh my so and so car does 70mph on half throttle but it only does 90mph on full throttle, shouldn't it do 140mph? is there something wrong with it? I doubt it, I'd say its only a Nissan Micra

for a start plus wind resistance ect.

[Timleech]

Yes, it is.

 

Why would water flow to the prop be limited any more than any other fat NB?

 

Perhaps you just meant that water flow to the prop is limited on any fat NB, but I've heard people on this forum say that LB's have short swims. Generally these people turn out to have no experience of LB.

 

 

 

Yes, it was the short-ish swim, unsophisticated stern shape, small prop etc which are common to most fat NB's not a dig at Liverpool Boats.

 

Tim

[Dalslandia]

Yes, it was the short-ish swim, unsophisticated stern shape, small prop etc which are common to most fat NB's not a dig at Liverpool Boats.

 

Tim

[Dalslandia]

Having problem editing and responding

 

Up to the hulls max speed the speed increase with cube roth (sp) of power increase.

 

Jan

www.jcpropellerdesign.com

www.dalslandia.com

Edited February 26, 2013 by Dalslandia

[Dalslandia]

How? By that reasoning, if he had 400 HP he would do 60MPH

 

Richard

If he have 400 hp he will do 13 mph (if not planing)

[nicknorman]

Having problem editing and responding

 

Up to the hulls max speed the speed increase with cube roth (sp) of power increase.

 

Jan

www.jcpropellerdesign.com

www.dalslandia.com

However, you are assuming a progressive increase in speed according to the cube root of the power increase, and then "hitting a brick wall" at the max speed - in reality the as the max speed is approached the speed is much less than according to the cube root of the power increase - that only works for speeds well below the max speed.

[Dalslandia]

The drag, air and water drag increase with square-root of speed.

(so the car increse speed with 1,414 times with double power ?)

but thrust from the propeller is reduced with speed T=P*efficiency/V

so airplane and boats increase speed with cube root of power increase.

so it takes 8 times more power to go 2 times faster.

it means we can save a lot of fuel by just a small reduction in speed.

Propeller efficiency also come into play, a big slow rotating high pitched propeller is often more efficient, stopping and accelerating will be better with a finer pitch.

 

Jan

 

However, you are assuming a progressive increase in speed according to the cube root of the power increase, and then "hitting a brick wall" at the max speed - in reality the as the max speed is approached the speed is much less than according to the cube root of the power increase - that only works for speeds well below the max speed.

 

Yes, correct, I think i wrote that, but i lost the post. you can also see it as the increase in extra speed near the max speed will urge for much more power. you mean progressive increase in drag?

 

Jan

Edited February 26, 2013 by Dalslandia

[bizzard]

On full throttle the stern would go deeper and deeper until water flows in and it would then probably become a submarine or a'twitch on the riverbed, a nervous wreck.

[bowten]

On full throttle the stern would go deeper and deeper until water flows in and it would then probably become a submarine or a'twitch on the riverbed, a nervous wreck.

Yer a scientist you are.

[bizzard]

Yer a scientist you are.

I apologize if I get far too technical at times for you Ian and anyone else, I'm a stickler for precise highly scientific technical data

Edited February 26, 2013 by bizzard

[Dalslandia]

I apologize if I get far too technical at times for you Ian and anyone else, I'm a stickler for precise highly scientific technical data

 

In theory there is no difference between theory and practise, but in practise there is.

[bizzard]

In theory there is no difference between theory and practise, but in practise there is.

Precisely.

[nicknorman]

I'd leave it as it is if I were you.

...

3) The shorter pitched blade will work less well in reverse - damaging stopping performance. I suggest this because I ended up reverting to over-propping Aldebaran in order to get the braking performance back, after fitting the optimum blade for forward engine power on the K1

 

 

Propeller efficiency also come into play, a big slow rotating high pitched propeller is often more efficient, stopping and accelerating will be better with a finer pitch.

 

Jan

 

So who is right? I would say Jan. Certainly that is the case with aeroplane propellor. Is it the opposite with water? (seems unlikely).

Edited February 26, 2013 by nicknorman

[blackrose]

How? By that reasoning, if he had 400 HP he would do 60MPH

 

Richard

 

Not necessarily. Since Jan didn't show his/her workings why do you assume that the calculated relationship between hp and speed is linear?

 

Edit: sorry I'm on my phone so I didn't see the rest of the thread before posting.

 

Anyway, here is an emailed reply from Noris.

 

Michael

I have run some calculations for 55 H.P. @ 3000 RPM and the computer comes up with 19 x 12. I think you are right to keep the diameter as large as possible and that you will never gain much more speed, but restricting the revs as much as the 19 x 13 is doing is not good for the engine or fuel economy. As you say, you are not developing the top end horsepower.

 

Maybe a reduction of 1 ½” of pitch would be best. Assuming you propeller is “sound”, we could re pitch it, re balance, re track and recondition for £150.00 plus return delivery and V.A.T.

Edited February 26, 2013 by blackrose

[bizzard]

During the war an American 4 engined Liberator took off from RAF Morgan or Mawgan Cornwall and almost straight away it crashed into a cliff. The investigation discovered that the propellers variable pitch control was set at ''course pitch'' instead of fine pitch for take off that allows the engines to rev out and develop their maximum power and so the engines were struggling and loosing power causing the plane to grapple with the air and stall.

I'm not old enough to have witnessed it by a good many years but was told the story by an old publican called Des of the 'Pheasant inn' near Newquay who saw it happen.

[Dalslandia]

So who is right? I would say Jan. Certainly that is the case with aeroplane propellor. Is it the opposite with water? (seems unlikely).

 

Sure the optimum propeller is best.

 

but are we talking about 10-20% of the pitch or double the pitch? at some stage the pitch will be to high even if we reduce its rpm with gearing, in theory an blade angle of 45 degree will be best, the blade will travel the shortest way. less blade drag.

45 degree on a 20" D will have a pitch of 47", here we talk prop walk.

 

Some say on an airplane that a P 1,5 times the D will give the most efficient prop, (if the gearing can be changed to suit speed and RPM)

so a 20X30" typ. or 18x27

what prop size did the old steamers have?

 

If the boat is under propped it can lead to poor stopping, and over propping too where we don't get full power.

On a diesel that is under propped the rev stop will limit the power aviable.

[nicknorman]

The investigation discovered that the propellers variable pitch control was set at ''course pitch'' instead of fine pitch for take off

"Course pitch" - is that a propellor that knows the route and heads off in the right direction? Of coarse it is!

[bizzard]

Not necessarily. Since Jan didn't show his/her workings why do you assume that the calculated relationship between hp and speed is linear?

 

Edit: sorry I'm on my phone so I didn't see the rest of the thread before posting.

 

Anyway, here is an emailed reply from Noris.

 

Michael

I have run some calculations for 55 H.P. @ 3000 RPM and the computer comes up with 19 x 12. I think you are right to keep the diameter as large as possible and that you will never gain much more speed, but restricting the revs as much as the 19 x 13 is doing is not good for the engine or fuel economy. As you say, you are not developing the top end horsepower.

 

Maybe a reduction of 1 ½” of pitch would be best. Assuming you propeller is “sound”, we could re pitch it, re balance, re track and recondition for £150.00 plus return delivery and V.A.T.

Norris know their onions I've used their propeller service in the past.

[blackrose]

Now I'm just wondering, if I can co somewhere shallow and get the prop off with the boat in the water? I can put a pair of waders on but is there such a thing as waders for arms? It's bloody cold at the moment!

[nicknorman]

Norris know their onions I've used their propeller service in the past.

Im confused now. Are you proposing that OP removes his propellor and fits an onion instead?

 

Now I'm just wondering, if I can co somewhere shallow and get the prop off with the boat in the water? I can put a pair of waders on but is there such a thing as waders for arms? It's bloody cold at the moment!

Its called a dry suit! Do you know any friendly Scuba Divers in the local area?