Propeller sizing and clearances

[IanD]

I know there's been a lot of discussion about this in the past but not much in the way of actual numbers, except for the usual advice that a bigger propeller turning more slowly is more efficient (so long as it's "undersquare" i.e. pitch is less than diameter -- optimum range for slow boats is 0.55-0.80) and that the prop shouldn't be "too close" to hull and rudder.

 

While trying to find some real numbers I came across this propeller sizing calculator which seems to give sensible results for narrowboats:

 

https://www.vicprop.com/displacement_size.php

 

For example, if I put in the numbers for a 60 foot boat drawing 2 feet with 40000lbs displacement (18 tons), powered by a Beta 43 with the standard 2.09:1 PRM150, it gives 17.9" x 11.6" as the optimum 3-blade propeller, which is *very* close to the recommendation from Beta Marine (and others) of 18" x 12".

 

With the optional 2.83:1 PRM150 (to get the diameter up and revs down, and avoid the "eggwhisk" effect by keeping prop rpm under 1000) it suggest a 4-blade 20.1" x 15.4", which if you can afford the extra 2" draught (needs 24" space from uxter plate to skeg, see below) might be a better (if slightly more expensive) solution than standard with better starting/stopping and less noise and rudder vibration.

 

Another area where a lot of hulls seem to get it wrong (going by pictures) is clearances; according to this

 

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

 

such a prop needs 3" clearance to the uxter plate, 1" to the skeg, 2" maximum to the rudder (for good steering) and 5" minimum to the hull (for clean water flow over the prop) -- in many cases the prop seems to be too close to the hull and too far from the rudder.

 

Discuss... ;-)

[Neil2]

Paging Mr Dalslandia...

[Sea Dog]

My boat is pretty close to your first example, except that it's 57ft. My prop is 17 x 11 and I'm pretty sure the Beta 43 would be perfectly comfortable with an 18 x 12.

[Neil2]

Interestingly I had a play about with that Vicprop thing a while ago, at the same time I asked Crowthers what they thought, based on the same data. The answer was the same...

 

In my case (45 footer) both recommended a 16" dia prop, which is fully 2" smaller than what I have. I think they are right it's just that I haven't had the nerve to change to what seems to be a very small prop for a narrowboat.

[Dalslandia]

Hi,

 

with more blades the clearance can be smaller, but "never" less then 2" on top and 1" at bottom. or minimum 1" up and down. there is no law about it except law of nature. others 3 blade should have min 17% on top, 27% in front of, with 4 blades that is 15% resp. 25%.

 

I use the propking spread, to that I added the SSPA lazy dog for a BAR of 50%, (Swedish Ship testing ...) that usually give a larger diameter and less pitch then propking does, however I see with propking it come close to SSPA if draft is increased as with costal boats.

Using the average of the two make sense though.

 

with your data I get 8.1 knots

17.2x 15.7 on P.K, and 18.5x 11.4 on SSPA (that don't use and hull info) average 17,9x13.5, by adding 2" to the draft i get 17,9x13.1 average.

 

something to be aware of the HP is often SAE HP, look at betas info, they have two curves red and green, often real power is about 85% of SAE.

so with 85% power we get 7.75 knots, and P.K. say 16,6x14,3 and SSAP 18x10,9, average 17,3x 12,6

 

We know that shallow and narrow waters drag down the speed, how much I don't know.

 

Now most propellers is sold in 1 inch steps.

 

some calculators i seen use 90% rpm to give a little better ability when the water isn't flat, and head wind.

Edited October 27, 2016 by Dalslandia

[Jrtm]

I did try one of these a wile back they were quite close to the sizes that the guccc used on there boats, but couldnt cope very well with steamers or slow semi engines with very high torque but extreamly low rpm.

 

Most old engines were geared to run around a max of 400-500 rpm on the prop. In reality most would run alot slower more like 300rpm on the prop.

 

Steam lumps were even slower. Its not till more modern engines that they ran at 1800rpm max abd a 2-1 gearbox reduction to bring it closer to the 1000rpm at the prop.

 

These sites when entering details also say that a 24-27 inch prop on a slow rev engine wont do more than around 4-6 mph when in reality will easly hit this and a little more.

 

This said they still give a very good idea of what size will be close to what you need.

[Jrtm]

For instance it says for mine to use a 26.9 inch prop so 27 and 22.8 so 23

 

The guccc used 27 by 21 i think it was so these sights arnt far off.

[Dalslandia]

remember that MPH is 1.151 times higher Nr. then knots, (even if the speed is the same) most prop calculators use knots, some both.

It seems that the propking spread sheet is ok on diameter for NB's but the SSPA give a good pitch.

 

those calculators is just good for estimations, people like at the Crowters have experience from different boats, and probably better software.

 

so when ordering a prop it is better to let the experts do the calculations, they know most of the time the real power, and speeds,

but for planning these calculators is fine.

Edited October 27, 2016 by Dalslandia

[IanD]

remember that MPH is 1.151 times higher Nr. then knots, (even if the speed is the same) most prop calculators use knots, some both.

It seems that the propking spread sheet is ok on diameter for NB's but the SSPA give a good pitch.

 

those calculators is just good for estimations, people like at the Crowters have experience from different boats, and probably better software.

 

so when ordering a prop it is better to let the experts do the calculations, they know most of the time the real power, and speeds,

but for planning these calculators is fine.

Indeed, I'd always suggest asking the experts for a final opinion -- this was more intended so people can do what looks like a reasonably accurate "what-if", either thinking what the options might be for a new boat (e.g. whether to use a bigger ratio gearbox and bigger prop) or whether the prop they've got is well off the optimum, which is a question that seems to come up regularly.

[mykaskin]

Not completely related to the question, but note that while my 28x16 was perfect on Victoria unloaded, it was too overpropped for loaded work and towing. It's now 27x16 and spot on, though still too big in diameter as it chews through the crud on the bottom of the canal too often.

 

Remember that an inch on the diameter is worth around 200 rpm, and two inches on the pitch.

 

Also, a lot of prop calculators are designed for deep water, and not canal use, or leisure use, where over propping to reduce revs has it's advantages.

 

Mike

[Dalslandia]

Not completely related to the question, but note that while my 28x16 was perfect on Victoria unloaded, it was too overpropped for loaded work and towing. It's now 27x16 and spot on, though still too big in diameter as it chews through the crud on the bottom of the canal too often.

 

Remember that an inch on the diameter is worth around 200 rpm, and two inches on the pitch.

 

Also, a lot of prop calculators are designed for deep water, and not canal use, or leisure use, where over propping to reduce revs has it's advantages.

 

Mike

 

Diameter change the required power with the factor^5 so a diameter change have big impact on power demand.

27/28 means a 20% reduction in power demand, all other input keept the same.

Edited October 28, 2016 by Dalslandia

[casper ghost]

Remember that an inch on the diameter is worth around 200 rpm, and two inches on the pitch.

 

Mike

Several years ago, a prop manufacturer told me the opposite, that an inch of pitch was worth 2 inch of diameter, which is why I swapped my Sabb's prop from a 16x14 to an 18x14, but then, this month, Crowthers said that an inch of pitch was worth an inch of diameter..

This is the problem with props, it's hard to find experts who agree.

Casp'

[Dalslandia]

Several years ago, a prop manufacturer told me the opposite, that an inch of pitch was worth 2 inch of diameter, which is why I swapped my Sabb's prop from a 16x14 to an 18x14, but then, this month, Crowthers said that an inch of pitch was worth an inch of diameter..

This is the problem with props, it's hard to find experts who agree.

Casp'

I have a feeling that it depends on the P/D ratio, different from a slow boat and a speed boat with a small diameter - large pitch

[crossley]

You can play around with the numbers,just out of curiosity,to fine tune the dimensions,but here the science let's us down, each individual application needs to be looked into with an experienced eye if every ounce of performance is required.unlike the commercial ships, We are not looking for every last knot of top speed,or pound of bollard pull,or the absolute minimum fuel consumption in any given propeller.

Beta marine, Crowther's, et al have a vast store of knowledge in specifying props that will give a good all round performance, and if you don't deviate wildy from their recommendations you can't go far wrong.

What do we actually want from a narrowboat propeller?

Good slow speed performance at idle.

Relaxed cruising speed rpm.

Plenty of thrust in reserve for rivers etc.

Good stopping ability in reverse.

All the above point to keeping the diameter as large as possible, for plenty of "grip" in the water. As an example,

I'm running a 65' heavy trad of 2'6" average draft, say 22tons displacement.

60 hp @ 2000 rpm, 2:1 reduction, 3 blade 24"x 19" prop. Cruises at 400 to 600 rpm on shallow, and upto 900 rpm on deeper canals.

Crash stops are quick too, that's when you need a prop with plenty of grip. Crowther's re pitched my prop from 22" and I couldn't be more pleased with the result.

[Dalslandia]

Great info Crossley.

 

with those data the PropKing say 8,92 kts (10.25 MPH)

propeller PropKing 22,5"X22,4" 33%BAR, Slip 40%

 

SSPA 23,8"X16,4" 50% BAR

 

Average 23,1"x19,4"

 

Pretty close.

 

At the end it is also a question of what the owner want from the boat/engine/propeller.

Edited October 28, 2016 by Dalslandia

[IanD]

I have a feeling that it depends on the P/D ratio, different from a slow boat and a speed boat with a small diameter - large pitch

No rule of thumb will be right for all sizes since it depends on pitch and diameter, absorbed power is proportional to rpm^2 * pitch * diameter^2 -- so a 10% increase in diameter (20% increase in area) is equivalent to a 20% increase in pitch, and at the same rpm each will absorb 20% more power, or drop the rpm by 10% at the same power. So for a "square" prop (e.g. 18" x 18") an inch of diameter increase to 19" x 18" will have the same effect as a two inch increase in pitch to 18" x 20".

[crossley]

Who can't resist the temptation to open it up just to see what it'll do flat out? On open water with a gps phone app speedo I think we managed to touch 7 and a bit mph.

At which point the counter was going under water and it was becoming uncontrollable. It does sit stern heavy though, and squat down by the stern as speed increases.

Interestingly, tied up alongside we got 1700 engine rpm at full load. On our "speed trials" at say 7 mph we managed 1850rpm before giving up.

This is with the same prop that will pootle along a shallow canal with barely a ripple at 450 engine rpm.

So its well suited to our application. There's no such thing a the perfect propeller for all conditions,only a good compromise.

If you tightly specify a prop on a new build boat it may be ok when new, but 10 years later with a tired engine it may be struggling to drive it and be smoky. You can't change down a gear like in a car if the engine is labouring. It's one of them things where your happy with it and never give it a second thought, or it's nagging you as you cruise along.. If only I had a bit more diameter, less pitch, 4 blades, etc. Whatever you had you wouldn't be happy with.

[Jrtm]

Not completely related to the question, but note that while my 28x16 was perfect on Victoria unloaded, it was too overpropped for loaded work and towing. It's now 27x16 and spot on, though still too big in diameter as it chews through the crud on the bottom of the canal too often.

 

Remember that an inch on the diameter is worth around 200 rpm, and two inches on the pitch.

 

Also, a lot of prop calculators are designed for deep water, and not canal use, or leisure use, where over propping to reduce revs has it's advantages.

 

Mike

So not far off what it would have had when new then.

I know dad had his repitched to a bigger number and its no really good on the 2 pot where before we had 3 pot and was more than useless at stopping or reversing

[Dalslandia]

Who can't resist the temptation to open it up just to see what it'll do flat out? On open water with a gps phone app speedo I think we managed to touch 7 and a bit mph.

At which point the counter was going under water and it was becoming uncontrollable. It does sit stern heavy though, and squat down by the stern as speed increases.

Interestingly, tied up alongside we got 1700 engine rpm at full load. On our "speed trials" at say 7 mph we managed 1850rpm before giving up.

This is with the same prop that will pootle along a shallow canal with barely a ripple at 450 engine rpm.

So its well suited to our application. There's no such thing a the perfect propeller for all conditions,only a good compromise.

If you tightly specify a prop on a new build boat it may be ok when new, but 10 years later with a tired engine it may be struggling to drive it and be smoky. You can't change down a gear like in a car if the engine is labouring. It's one of them things where your happy with it and never give it a second thought, or it's nagging you as you cruise along.. If only I had a bit more diameter, less pitch, 4 blades, etc. Whatever you had you wouldn't be happy with.

 

So you are slightly over propped, I guess that open water wasn't the English channel, so you can still be effected by shallow water, I don't remember the number, but saw that even with 10-15 meter under the keel the ship was effected, didn't say size though, slowed down at least 15%,, we all know what happened with inches under the keel and narrow canals.

 

I don't know your 60 HP, if it is SAE HP, but I guess it is, so with 1850 RPM you prob. have 55. and put one or two generators on, so minus 5-10 HP, with say 45 HP you will then in theory do 90,8% of the speed with 60 hp 9,3 MPH (8.1 kts) out of 10.25 MPH, and that is what the spread say too. So what that is that drag the speed down from theory I don't know, hull shape? swim shape? shallow water? maybe all 3.

 

It is not all bad to be over propped, in diameter, unless it goes to fast on tickover/idle speed, 2-2.5 knots is ok, 3-4 and it goes to fast for driving in gear all the time. but over propped in the canal helps with propeller efficiency, and engine efficiency as well.

 

M/S Dalslandia is doing 7.9 kts with her 163 DIN HP. Scania. spread say 8.9 kts, but she have a wide deep almost flat transom, and the deadwood in front of propeller isn't nice hydrodynamic shaped.

Edited October 29, 2016 by Dalslandia

[Neil2]

Who can't resist the temptation to open it up just to see what it'll do flat out? On open water with a gps phone app speedo I think we managed to touch 7 and a bit mph.

At which point the counter was going under water and it was becoming uncontrollable. It does sit stern heavy though, and squat down by the stern as speed increases.

Interestingly, tied up alongside we got 1700 engine rpm at full load. On our "speed trials" at say 7 mph we managed 1850rpm before giving up.

This is with the same prop that will pootle along a shallow canal with barely a ripple at 450 engine rpm.

So its well suited to our application. There's no such thing a the perfect propeller for all conditions,only a good compromise.

If you tightly specify a prop on a new build boat it may be ok when new, but 10 years later with a tired engine it may be struggling to drive it and be smoky. You can't change down a gear like in a car if the engine is labouring. It's one of them things where your happy with it and never give it a second thought, or it's nagging you as you cruise along.. If only I had a bit more diameter, less pitch, 4 blades, etc. Whatever you had you wouldn't be happy with.

 

I suspect the vast majority of narrowboat owners never give this a single thought, too busy discussing solar panels inverters kitchen layouts etc.

 

Most of them are way overpropped I reckon. Our boat is definitely overpropped but even so we will go past other boats on the river. I will have to go smaller though as I still can't get maximum revs and the Bukh engine does like to reach max rpm.

[Jrtm]

I know that 15hp bollys on both wood and steel will do over 5 easly pos even 6 unloaded.

 

With a butty we could chug at 5 happly.

 

The 3 pot would do 4 while towing at just not even 1/4 opened up was well over propped.

 

National with what ever prob on deeper canal will do 4 but very shallow were restricted by the shallowness. A river she will happly move quick on.

 

Chiswick with the jp would move butthe smallest shallow spot and it would suck you into the shallow.

 

Best 3 boats were mendip, dane and lamprey all nice and smooth and you could let go of the tiller and they would stay dead strait no pulling nothing. Were lovely to steer.

[Dalslandia]

 

I suspect the vast majority of narrowboat owners never give this a single thought, too busy discussing solar panels inverters kitchen layouts etc.

 

Most of them are way overpropped I reckon. Our boat is definitely overpropped but even so we will go past other boats on the river. I will have to go smaller though as I still can't get maximum revs and the Bukh engine does like to reach max rpm.

 

Yes, boating have many aspects, from kitchen to loo, as a living, the heat and electric energy is a big part of it. racing on the river might not, being slightly over propped don't change the top speed much, say you are 10% down in rpm and power, means speed is 96.5% ~3.5% reduction or 5.8 instead of 6.0 or less difference if the prop efficiency is up a bit, say you have 21" instead of 20" diameter, the efficiency can be up with 3%, as long as something ells isn't wrong.

[crossley]

I think if we trimmed it level it would make a big difference to top speed and we might then hit the magic numbers. Yes, slightly over propped as we're a couple of hundred rpm down, but I'm not sure how accurate the tacho is. It's powerful enough for us . I'm looking to try it on the Thames at some point.have included plenty of cooling for river work having both skin tank and heat exchanger coolers. I feel I've got the prop about right, any fine tuning will only give small improvements. I would like to try a 4 blade just out of curiosity.

[Neil2]

 

Yes, boating have many aspects, from kitchen to loo, as a living, the heat and electric energy is a big part of it. racing on the river might not, being slightly over propped don't change the top speed much, say you are 10% down in rpm and power, means speed is 96.5% ~3.5% reduction or 5.8 instead of 6.0 or less difference if the prop efficiency is up a bit, say you have 21" instead of 20" diameter, the efficiency can be up with 3%, as long as something ells isn't wrong.

 

That's very interesting, using the figures above at top speed of 6mph a journey of 15 miles would only take 5 minutes less than if you had 10% less power. So how do you work out the effect of power on top speed?

 

I think if we trimmed it level it would make a big difference to top speed and we might then hit the magic numbers. Yes, slightly over propped as we're a couple of hundred rpm down, but I'm not sure how accurate the tacho is. It's powerful enough for us . I'm looking to try it on the Thames at some point.have included plenty of cooling for river work having both skin tank and heat exchanger coolers. I feel I've got the prop about right, any fine tuning will only give small improvements. I would like to try a 4 blade just out of curiosity.

 

Yes - whose the guy that does the boat modelling on here? he said something about to what extent the hull "squats" makes a big difference, and narrowboats do like to bury their arses in the water don't they. Having said that, I've tried experimenting with water levels in the bow tank on our boat, (the tank is huge, holds well over a ton I reckon, so you can make a big difference to the trim with it) and it hasn't been very conclusive. But of course you can't do the same journey twice so it's never a precise like for like comparison. And, the boat is overpropped anyway so i can't get up to maximum speed.

[Sea Dog]

... narrowboats do like to bury their arses in the water don't they.

 

 

Shallow water effect. Same thing happens to Warships in the Gulf, for example.