propellers

[Bee]

I have a Beta 43 hp and when I built the boat I fitted a 16 inch prop. by unknown pitch as it was available and cheap, this winter I will be fitting a more suitable prop. Beta recommend an 18 x 12 {I think} Crowthers reckon a 16 x 14 or thereabouts. Has anybody fitted anything which works really well outside these sizes? boat is 10m x 3,4m weighs a bit over 10 tons , ideally I could do with a bit more speed without revving the nuts off of it so 'overpropping' is acceptable.

[MtB]

What is the depth of your swim?

 

(Or the boat's swim, rather )

 

 

MtB

P.S. Worth pointing out for those who didn't notice, this is a widebeam. 34ft x 11ft approx.

[Burgiesburnin]

If you don't know what the pitch is of your current prop then maybe ascertain that first and then you can adjust from there.

Crowthers recommendation of 16 x 14 could well be what you are currently running.

Edited August 30, 2014 by Burgiesburnin

[Bee]

Room to swing up to about 20 inches with a few more engineering bricks in the bottom.

[MtB]

Room to swing up to about 20 inches with a few more engineering bricks in the bottom.

 

That didn't answer my question!

 

Is that the diameter arrived at after deducting the rule of thumb 15% blade diameter clearance top and bottom, or is that the maximum diameter that could physically be fitted and rotate?

 

MtB

[bottle]

Beta recommendation for a 18x12 is for narrow boat 50 to 57 feet weighing 18 tons

 

There is no recommendation on their site for a boat of your dimensions and weight.

 

Crowthers are the ones I would trust to get it right and as burgies burnin says you may well have that.

 

You could always ask Crowthers if they would take your existing and 'tweak' it if necessary.

[Dalslandia]

Room to swing up to about 20 inches with a few more engineering bricks in the bottom.

to calculate the prop size, the draft is needed, minus the skeg = hull max draft at the stern, or in front of propeller.

gearbox ratio is also important, PRM 2.09 or Techno. at 2.00 would be a good guess, but still a guess.

top speed and achievable rpm with current prop?

Edited September 1, 2014 by Dalslandia

[MtB]

to calculate the prop size, the draft is needed, minus the skeg = hull max draft at the stern, or in front of propeller.

gearbox ratio is also important, PRM 2.09 or Techno. at 2.00 would be a good guess, but still a guess.

top speed and achievable rpm with current prop?

 

This assumes the counter is just level with the waterline, which it probably is but not necessarily. This is why I initially asked for the height of the swim. So we need both the draft of the boat, AND the height of the swim. And ideally confirmation whether the skeg is of the 'offset' or 'dropped' format or not. If it is, a load more questions crop up!

 

MtB

[Dalslandia]

 

This assumes the counter is just level with the waterline, which it probably is but not necessarily. This is why I initially asked for the height of the swim. So we need both the draft of the boat, AND the height of the swim. And ideally confirmation whether the skeg is of the 'offset' or 'dropped' format or not. If it is, a load more questions crop up!

 

MtB

Right, the swim height minus 10-15% above and below blade tip to counter resp. skeg gives the maximum possible diameter, the hull draft, along with length and beam in waterline! plus displacement give the optimum diameter and pitch, with power and rpm info.

 

Installed power is possible 85% of the SAE rated power after exhaust and pumps and alternators needs is satisfied.

 

Then it comes to owners wish and demands.

[MtB]

ideally I could do with a bit more speed without revving the nuts off of it so 'overpropping' is acceptable.

 

I agree with Bottle actually. If this is the extent of your concern, getting Crowthers or the peeps in Isleworth to increase the pitch by 2" will go a long way to addressing your concern cheaply.

 

If you want to get it closer to right, you need to answer the questions above properly, and also say why you think Crowthers recommendation is wrong.

 

MtB

[Bee]

I don't think Crowthers are wrong, I just wondered if anybody had the same engine and thought their boat went well with a particular prop. I can physically swing up to about 20 ", any more and the blade tips get close to the surface, the boat has not got a flat counter, its more like a dutch barge. The boat is in Holland and although it will keep up with the rest of the herd I have to cruise at about 1800 rpm which is a bit 'busy'. I might well do what mike and bottle suggest and find out what a bit more pitch would do, I am drawn to the word cheaply!

[nb Innisfree]

Diameter is the first thing to be calculated, the larger the better after allowing for blade clearance. Then it's blade area, if there's not enough area with a standard prop of 55%? BAR to transfer max power into thrust then a wider blade is needed. Finally pitch to give boat speed and accelleration at tickover speed. If underpropped and tickover speed is ok it's no good trying to absorb more power with more pitch as it will give no increase in thrust at max power, instead blade area needs increasing.

 

It's complicated and Crowther are the ones to do this otherwise you are taking stabs in the dark.

 

ETA: We swing a max dia of 17" and 12" pitch with 40 bhp @3000rpm and 2:1 box, standard blade area. This leaves us slightly overpropped leaving us unable to reach full revs (approx 2700 to 2900rpm depending on alternator load)

Edited September 1, 2014 by nb Innisfree

[Bee]

Thanks, I'll read that again in the morning with a cup of strong coffee and a chocolate biscuit.

[Dalslandia]

Diameter is the first thing to be calculated, the larger the better after allowing for blade clearance. Then it's blade area, if there's not enough area with a standard prop of 55%? BAR to transfer max power into thrust then a wider blade is needed. Finally pitch to give boat speed and accelleration at tickover speed. If underpropped and tickover speed is ok it's no good trying to absorb more power with more pitch as it will give no increase in thrust at max power, instead blade area needs increasing.

 

It's complicated and Crowther are the ones to do this otherwise you are taking stabs in the dark.

 

ETA: We swing a max dia of 17" and 12" pitch with 40 bhp @3000rpm and 2:1 box, standard blade area. This leaves us slightly overpropped leaving us unable to reach full revs (approx 2700 to 2900rpm depending on alternator load)

 

Well written, your engine is probably as strong as the Beta 43 (SAE) 41 HP at 2800, this is probably 36 HP installed

 

Btw. going from 2700 to 2900 with the same prop. takes 24% more power, meaning some 8 hp or so from 2700 to 2900,

 

BAR is blade area ratio, blade area in relation to swept area.

Edited September 2, 2014 by Dalslandia

[nb Innisfree]

 

Well written, your engine is probably as strong as the Beta 43 (SAE) 41 HP at 2800, this is probably 36 HP installed

 

Btw. going from 2700 to 2900 with the same prop. takes 24% more power, meaning some 8 hp or so from 2700 to 2900,

 

BAR is blade area ratio, blade area in relation to swept area.

Yes our alternator absorbs around 8 bhp at full output. Imossible to have a fixed pitch prop that is exactly right in all conditions, has to be a compromise.Maybe a lower pitch would suit us, perhaps 11.5", a slightly slower tickover speed would be handy Edited September 2, 2014 by nb Innisfree

[Dalslandia]

It is many concerns, all numbers aside.

 

low speed at tick over is nice when going in the locks,

good speed at low(er) RPM at normal cruise/canal speed

maximum power at max RPM and speed.

 

the first require low pitch and or RPM, maybe a trolling valve gearbox?

second, a higher pitch at cruise reduce RPM and ad load on engine, so more efficient propeller and engine.

Third, the correct propeller is needed for max power and RPM at max speed. it might be a good prop for the first requirement too, maybe.

 

it all also depends on where and how the boat is used, in the canal/locks a LAGOM tick over speed, and a nice harmonic RPM at canal speed, might feel better then being able to reach max rated RPM on wide deep water.

[Dalslandia]

The PropKing spread sheet,

 

with Bee's Nr, and 2 feet hull draft?

get 6,9 knots (7,9 MPH)

and 2800 RPM, 36,5 HP, 2,09:1 gearbox, (assuming PRM)

gives a 17"X11.25" with a 33% DAR (developed area ratio)

 

the same with 2600 rpm max 34 HP

gives a 17,5"X12" and a speed reduction of 0,1 knots

 

A wider blade will probably give a smaller diameter, but 33% DAR is what this soft strive for

[nb Innisfree]

The PropKing spread sheet,

 

with Bee's Nr, and 2 feet hull draft?

get 6,9 knots (7,9 MPH)

and 2800 RPM, 36,5 HP, 2,09:1 gearbox, (assuming PRM)

gives a 17"X11.25" with a 33% DAR (developed area ratio)

 

the same with 2600 rpm max 34 HP

gives a 17,5"X12" and a speed reduction of 0,1 knots

 

A wider blade will probably give a smaller diameter, but 33% DAR is what this soft strive for

I believe a standard prop has 55% BAR which would give a higher DAR? If so 33% DAR would be non standard and quite a low BAR? Edited September 2, 2014 by nb Innisfree

[Dalslandia]

Yes, most props I seen have a higher BAR then 33%, 55 being more of a standard.

If I get it right BAR is a common name, and can mean PAR, DAR & EAR,

 

PAR, Projected as seen from behind.

DAR is Developed, as if pitch is set to zero

EAR, is Expanded, as in DAR but with the twist taken out, so completely flat, the real blade area.

 

http://hydrocompinc.com/wp-content/uploads/documents/HC135-BladeAreaRatio.pdf

 

Interesting about under propping, not all boats is canalboats.

http://hydrocompinc.com/wp-content/uploads/documents/tex0601.pdf

 

More time killer here; http://hydrocompinc.com/library

 

Ps, sum. in England or in EU at least, it seems to be so that if you want a correct propeller for you and your boat, contact Crowthers :-)

I never talked to them and is not connected to them in anyway!

Edited September 2, 2014 by Dalslandia

[Bee]

A big thank you to all who have given this some thought and ventured into the world of maths. Thanks for your help, I will see if I can find a pitch measurement stamped on it when I get it out. Now If I was to get rid of the propeller and fit a pair of paddle wheels ......

[Dalslandia]

Diameter and pitch is usually stamped on the side of the hub.