Propeller sizing and clearances

[Dalslandia]

I don't know a squat about squat, more then it lower the boat at speed over shallow water. it have larger effect on the lowest end of the hull if i get it right.

 

The speed formula vs power is the same for airplanes, cars and boats, after the transmission losses is taken out of the equation.

speed increase with the cube Rooth of power increase (or (p1/p2)^0.333)

double the power and speed go up with 26%, half the power and speed is 79%

how much power it will take at speed2? (V1/V2)^3

The same with a known propeller, (rpm1/rpm2)^3

problem is to know how fast it goes, GPS and 2 way helps.

power we don't know unless we measure torque on the shaft, we seldom do.

or measure fuel flow.

or measure top speed at full power, and go from there. but then the trick is to know the power, how it was measured and the losses.

Edited October 29, 2016 by Dalslandia

[crossley]

Mine really squats down as you wind the speed on.I'm not entirely sure but I think its the prop sucking the water away from the stern area combined with the bow rising pushing the stern down too. Anyway far too fast for the canal here. I'm lucky to get 1.5 mph without dragging the bottom or doing a Hawaii 5-0

impression. Makes a change from batteries this propeller thread. Mine are never ready, or flying bomb.

Remember them? They were crap batteries.

[Jason Day - Sheffield]

We're just about to dry dock for blacking and I was thinking of checking the prop. The Vicprop calculator suggests a 3 bladed 16.5" X 19.4" prop for our 54' , 17 tonne black Prince with a Kubota d1703 assuming 20 HP. It feels like it's under we have 'high' revs when cruising, but as we haven't got a rev counter it's difficult to say. I'll definitely be looking to see what the prop size is when we haul her out (I could measure through the weed hatch, but then I would, be doing that by feel, with some loss of accuracy).

 

Anyone else have any thoughts about this combination, or experience of D1703' in Black Princes? (1991, one of the dropped gunwale/big window models)

[Jrtm]

Most boats will suck them selfs down (stop it its far too easy) the faster you go the harder it will pull (hold yourselfs) but most will be around 6 inches or so. (Now i know your pissing yourselfs)

[Jason Day - Sheffield]

Any suggestions as to a good aftermarket rev counter setup for a Kubota D1703?

[Dalslandia]

Any suggestions as to a good aftermarket rev counter setup for a Kubota D1703?

VDO, but how are you getting impulse?

[Jason Day - Sheffield]

well, that would be a part of the setup - as its a diesel engine I would need to use the Alternator field output and adjust for the right ratios I imagine. I'm opento other suggestions, like the tinytach, but I dont have any experience using those.

Edited October 29, 2016 by Jason Day - Sheffield

[Jrtm]

VDO, but how are you getting impulse?

You could use a mountainbike speed o counter somehow?

[Eeyore]

We're just about to dry dock for blacking and I was thinking of checking the prop. The Vicprop calculator suggests a 3 bladed 16.5" X 19.4" prop for our 54' , 17 tonne black Prince with a Kubota d1703 assuming 20 HP. It feels like it's under we have 'high' revs when cruising, but as we haven't got a rev counter it's difficult to say. I'll definitely be looking to see what the prop size is when we haul her out (I could measure through the weed hatch, but then I would, be doing that by feel, with some loss of accuracy).

Anyone else have any thoughts about this combination, or experience of D1703' in Black Princes? (1991, one of the dropped gunwale/big window models)

The D1703 (when marinised by Beta) was rated at 39hp, so yours would be about that when new. The 1991 model is probably a 3000rpm unit as it predates most of the emission regs.

The Vicprop calculator gives 16.3 x 10 for a 2:1 gearbox; so a 16 x 11 (very slightly overproped) would do. Probably not worth changing unless your existing prop is smaller; if the existing is larger I'd leave as is.

The Vicprop calculator gave me good results when I recently re propped my 44 ft Narrowboat; cruised the full length of the Staffs & Worcs last weekend with a comfortable 1400 to 1500 rpm giving max speed without a breaking wash (depending on depth). I also found online that an extra inch on the diameter reduces engine rpm by about 450; which I used to estimate the new cruising rpm (easy in this case only the diameter was changed by one inch). The estimate was about 50 rpm high, which given all the variables is petty good.

Edited October 30, 2016 by Eeyore

[Jason Day - Sheffield]

That all fits, as I said originally, I'm only checking, not panicking. I need to get a Rev counter on to make sure the engine is within range, and the rest I'm going on feel from other boats I've had. Thanks for the confirmation though.

[Eeyore]

That all fits, as I said originally, I'm only checking, not panicking. I need to get a Rev counter on to make sure the engine is within range, and the rest I'm going on feel from other boats I've had. Thanks for the confirmation though.

I think Lisa (Blue String Pudding) has the same engine in NB Mabel Stark, which is I believe also ex Black Prince.

[Neil2]

Most boats will suck them selfs down (stop it its far too easy) the faster you go the harder it will pull (hold yourselfs) but most will be around 6 inches or so. (Now i know your pissing yourselfs)

 

I think that's far too subtle for this forum...

[Jason Day - Sheffield]

So I had a fumble about down below, and got a tape measure reading of 8 inches.....shaft to tip, which makes the whole thing 16 inches in diameter - so far so good - can't really tell the pitch through the hatch, so will have a look at it when they whip it out at Nantwich.

 

Spoke like a local :-)

Edited October 30, 2016 by Jason Day - Sheffield

[Guest]

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?

 

 

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.

 

Did someone mention squat? Can I help?

 

 

 

I think that's far too subtle for this forum...

 

I think you're wrong... it's a big effect in a shallow confined channel with minimal under-keel clearance. It may not be generally accepted on this forum, but neither are bow thrusters...

 

IMHO the draft input for the prop calc should INCLUDE the squat measurement too as a component... I think this is the "black magic" component that the best suppliers will allow for from experience.

 

My designs have been based on a fine hull and I have optimised for the 4mph canal cruise as I figure that's where I'll spend the majority of my time. Now 4mph of course is wishful thinking, I know, due to moored boats etc that we slow down for but the point was that we're more likely to be on the canal than the river. It's very likely that your hull will be more buff than mine (less streamlined under water) so you'll suffer more from the squat effect for the same draft, that's just how the equations work (function of Cb).

 

IMHO You'd be wise to add at least 4" to your draft for the calculations online, following the advice of one of the world's leading experts, Dr Barrass (http://www.ship-squat.com) I added 6"...

 

(Unfortunately the project's proof now will be delayed; I fell for a Dave Harris boat with a thumping big twin.... )

Most boats will suck them selfs down (stop it its far too easy) the faster you go the harder it will pull (hold yourselfs) but most will be around 6 inches or so. (Now i know your pissing yourselfs)

 

No, I totally agree with you !!! Bravo!

 

(an example calculation for my own design)

Edited November 2, 2016 by dpaws

[Eeyore]

 

Did someone mention squat? Can I help?

 

 

 

I think you're wrong... it's a big effect in a shallow confined channel with minimal under-keel clearance. It may not be generally accepted on this forum, but neither are bow thrusters...

 

IMHO the draft input for the prop calc should INCLUDE the squat measurement too as a component... I think this is the "black magic" component that the best suppliers will allow for from experience.

 

My designs have been based on a fine hull and I have optimised for the 4mph canal cruise as I figure that's where I'll spend the majority of my time. Now 4mph of course is wishful thinking, I know, due to moored boats etc that we slow down for but the point was that we're more likely to be on the canal than the river. It's very likely that your hull will be more buff than mine (less streamlined under water) so you'll suffer more from the squat effect for the same draft, that's just how the equations work (function of Cb).

 

IMHO You'd be wise to add at least 4" to your draft for the calculations online, following the advice of one of the world's leading experts, Dr Barrass (http://www.ship-squat.com) I added 6"...

 

(Unfortunately the project's proof now will be delayed; I fell for a Dave Harris boat with a thumping big twin.... )

 

 

No, I totally agree with you !!! Bravo!

 

squat graph.jpg (an example calculation for my own design)

Interesting stuff. I added 6" to my draft on the "Vicprop" calculator and got exactly the same result, does it make a difference with other calculators? Edited November 2, 2016 by Eeyore

[Neil2]

Interesting stuff. I added 6" to my draft on the "Vicprop" calculator and got exactly the same result, does it make a difference with other calculators?

 

That's because the displacement hasn't changed. I guess with Vicprop you would have to increase the displacement figure proportionate with the draught, I don't know if that would realistically recognise the "squat" factor though?

[alan_fincher]

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... ;-)

 

Not sure anyone has picked up on this?

 

The linked to article says....

 

Maximum bare shaft length - 4 x shaft diameter

 

which would imply with a 1 1/2" shaft typical of many leisre arrow boats you could have up to 6" of unsupported shaft between the rear of the stern bearing and the propeller boss.

 

However, when we bought Chalice, it had 4" of shaft like this, and the surveyor said it was "far in excess of accepted norms", and suggested it must be reduced.

 

I thought for canal use it was advised that the exposed portion of the shaft should be no more than about 2 times shaft diameter, which in our example would have been 3" maximum. We reduced to less than that.

Therefore very hard to see how you can have conventional narrow boat stern gear, standard shaft diameter, and also achieve a large separation between prop and rear of hull, without increasing the chances of damage.

The issue of course is that if the blade sits at the end of a lot of unsupported shaft, and then is subjected to a major collision with a log that jams it against uxter or skeg, the chances of the shaft being bent are progressively increased as the prop is hanging off a longer and longer unsupported length.

[Eeyore]

 

That's because the displacement hasn't changed. I guess with Vicprop you would have to increase the displacement figure proportionate with the draught, I don't know if that would realistically recognise the "squat" factor though?

Ok, I guesstimated an extra ton of displacement, which gives a reduction in pitch of 0.2". So I have a 17x10 fitted (an exact match to the original calc), the revised calc suggests a 17x9.8 and the manufacturers recommendation is 17x9 (based on achieving max revs with throttle wide open). I can see the point, but it wouldn't have made a difference in my case.

[Eeyore]

 

Not sure anyone has picked up on this?

 

The linked to article says....

 

 

which would imply with a 1 1/2" shaft typical of many leisre arrow boats you could have up to 6" of unsupported shaft between the rear of the stern bearing and the propeller boss.

 

However, when we bought Chalice, it had 4" of shaft like this, and the surveyor said it was "far in excess of accepted norms", and suggested it must be reduced.

 

I thought for canal use it was advised that the exposed portion of the shaft should be no more than about 2 times shaft diameter, which in our example would have been 3" maximum. We reduced to less than that.

 

Therefore very hard to see how you can have conventional narrow boat stern gear, standard shaft diameter, and also achieve a large separation between prop and rear of hull, without increasing the chances of damage.

 

The issue of course is that if the blade sits at the end of a lot of unsupported shaft, and then is subjected to a major collision with a log that jams it against uxter or skeg, the chances of the shaft being bent are progressively increased as the prop is hanging off a longer and longer unsupported length.

Fit a longer stern tube? It was certainly an option suggested by a prop manufacturer for my little project boat.

Going for a more radical rebuild, but may still be incorporated to a lesser degree.

[Jason Day - Sheffield]

Update from the dry dock today - 16X11 prop, so pretty much what it should be for the engine. There's a lot of clearance between prop and rudder, which may account for the steering judder, but regulation 'just over 2" blade tip to underside of swim', and only avbout 1 1/2 " of propshaft exposed. all in all leave as is I reckon....if only we hadn't found damage around the weedhatch and wear plates to the underside of swim, with a rebuild cost of £1200, I might actually be happy.

 

Got any room in the miserable corner of your field, Eeyore?

[Jrtm]

16x11 is quite a diffrence from 16x9.

My understanding was an inch of pitch is around the same as 1-2 inches in hight. I could be wrong.

 

Its not far out just means your a bit over propped really.

 

Your vibration is prob from the amount of water being pushed past the rudder but it the rudder sit a fair way from the blade this wont help.

Edited November 2, 2016 by billybobbooth

[Eeyore]

Update from the dry dock today - 16X11 prop, so pretty much what it should be for the engine. There's a lot of clearance between prop and rudder, which may account for the steering judder, but regulation 'just over 2" blade tip to underside of swim', and only avbout 1 1/2 " of propshaft exposed. all in all leave as is I reckon....if only we hadn't found damage around the weedhatch and wear plates to the underside of swim, with a rebuild cost of £1200, I might actually be happy.

 

Got any room in the miserable corner of your field, Eeyore?

Always room for one more ?

[Jason Day - Sheffield]

Just a thought - has anyone here any opinion on extending the rudder blade, by welding a section on forward of the pivot point? I beleive (from sailing boat experience) that this could better balance the blade, and reduce turbulence against the rudder causing juddering. It also moves the rudder closer to the prop, and will reduce the force needed to hold it against the force of water.

 

But there's a good chance I'm completely wrong.

 

Any thoughts anyone?

[Jrtm]

Not sure about adding it but most old boats have a round 3-4 inches the otherside of the rudder stem.

[Dalslandia]

Rudders if starting from the end, sailboats seems to have about 17% balance, (17% of total chord, forward of stock) many motor boats have 20%, 23-24% would be max with a hand pumped hyd. system.

Seen info saying 10-15% minimum of prop diameter between rearmost part of prop blade to LE of rudder.

27% of propeller diameter from propeller foremost to swim (dead wood)

max 1.5 times the prop shaft diameter free, but seen 4 times, but that seems wrong. better to move the shaft bearing back out, there is holders...

 

The draft make a different in prop calculations from slowdown of inflow, and also water pressure.