problem with Variprop propellor performance on Vancouver 38 Pilot

[ianw530]

Having a few issues with this and hoping to benefit from some of your knowledge out there!

The 1997 V38P I bought late 2011 has a Variprop 19in 3 blade prop, been on for several years . This seems to work ok in smooth water and under F4/5 winds. Above these, it appears to lose drive and extra revs dont help much. Short steep waves also seem to result in significant loss of thrust. Factory pitch was 13.5". ( 50hp Yanmar)
Anyone have similar issues/ideas?

currently thinking of changing forard pitch to 14.5" via adjusters, to see effect.

Possibly a separate or related aspect, above 1500rpm we get a 'grumbling' noise from the prop /shaft area (audible at the helm, not present in the gearbox area). No sign of wear in the cutlass bearing,,

ian

 

[sailorman]

do you get hull spd at WOT
the grumble might be the water compressed between the prop tip & hull

 

[ianw530]

Only in smooth water, not in stronger winds or chop.

grumble - could be, though sounds more 'mechanical'.

 

[sailorman]

does she bury her transom with full throttle
does she make black smoke at that time

 

[charles_reed]

I suggest you download Propcalc from www/drivecalc.de and run that on your existing and proposed set-up.

As I don't have the Yanmar model (to derive torque and max power revs and the gearing) I am unable to do the exercise but it would appear that the propeller is a bit small for the load it is trying to move and is cavitating when you increase the rpm.

It may be that there is no room to increase the prop diameter (you need about 20% of prop radius from tip to hull). In your position I would also increase the pitch, but you may find the result is no greater urge and more noise.

 

[ianw530]

I suggest you download Propcalc from www/drivecalc.de and run that on your existing and proposed set-up.

As I don't have the Yanmar model (to derive torque and max power revs and the gearing) I am unable to do the exercise but it would appear that the propeller is a bit small for the load it is trying to move and is cavitating when you increase the rpm.

It may be that there is no room to increase the prop diameter (you need about 20% of prop radius from tip to hull). In your position I would also increase the pitch, but you may find the result is no greater urge and more noise.

Hi, its a 4jh2be 50HP. I think the standard prop was 18 inch maybe. Will try the propcalc thanks and see what it says.

 

[TimBennet]

It's difficult to make direct comparisons between a standard fixed propellor and a feathering one.

A feathering prop is designed to have minimum drag when feathered and everything else about its design is a compromise. The blades have no twist so it is far less efficient at 'pushing water' and this marked lack of efficiency is most obvious when it needs to work hard. The real advantage of the Brunton Autoprop is that the design keeps the conventional twisted blades, which compromise its drag when feathered under sail, but retain their efficiency when motoring hard.

As in so much about boats, it's a matter of compromise.

 

[ianw530]

hi, re the drivecalc .de site - this looks as if its about aircraft propellors? am I looking at the right one?

 

[Tranona]

hi, re the drivecalc .de site - this looks as if its about aircraft propellors? am I looking at the right one?

Try Propcalc on www.castlemarine.co.uk

 

[charles_reed]

It's difficult to make direct comparisons between a standard fixed propellor and a feathering one.

A feathering prop is designed to have minimum drag when feathered and everything else about its design is a compromise. The blades have no twist so it is far less efficient at 'pushing water' and this marked lack of efficiency is most obvious when it needs to work hard. The real advantage of the Brunton Autoprop is that the design keeps the conventional twisted blades, which compromise its drag when feathered under sail, but retain their efficiency when motoring hard.

As in so much about boats, it's a matter of compromise.

You're quite right in your observations, but any of the computer models are so approximate, insofar pitch is concerned, that the inefficiency of the blades is immaterial - more importantly they are quite good in giving the optimum diameter.

Tranona,
do you know Les? I'm afraid that their pitch calculator only works with Win XP or Vista and is fairly vague on diameters.
The German site gives, from memory, both media, screws for air and for water.

 

[charles_reed]

Try this US site - not as "correct" I'm told, but may get you started.

http://www.rbbi.com/folders/prop/propcalc.htm

 

[sailorman]

The "Optimum" dia is as big as you can swing allowing tip clearance to the hull

You're quite right in your observations, but any of the computer models are so approximate, insofar pitch is concerned, that the inefficiency of the blades is immaterial - more importantly they are quite good in giving the optimum diameter.

Tranona,
do you know Les? I'm afraid that their pitch calculator only works with Win XP or Vista and is fairly vague on diameters.
The German site gives, from memory, both media, screws for air and for water.

 

[Tranona]

You're quite right in your observations, but any of the computer models are so approximate, insofar pitch is concerned, that the inefficiency of the blades is immaterial - more importantly they are quite good in giving the optimum diameter.

Tranona,
do you know Les? I'm afraid that their pitch calculator only works with Win XP or Vista and is fairly vague on diameters.
The German site gives, from memory, both media, screws for air and for water.

Can't see that operating on XP or Vista is a constraint if you have a PC. Works on both mine PCs. Don't understand the comment about vagueness on diameter. You enter the maximum diameter you can swing as a constraint, but it may come up with a diameter less than the maximum. In my experience using it for my two boats it has been spot on and agreed with the prop supplier recommendation in both cases.

I think the OP should go back to Variprop and ask for their recommendation having first checked Propcalc to get a ballpark figure. The description of behaviour suggests it is underpitched - the diameter seems about right and probably as big as will fit. The Variprop is I think adjustable pitch so should be relatively easy to experiment with a bit more pitch. The engine has enough power to get hull speed at maximum revs if the prop is right.

 

[charles_reed]

Can't see that operating on XP or Vista is a constraint if you have a PC. Works on both mine PCs. Don't understand the comment about vagueness on diameter. You enter the maximum diameter you can swing as a constraint, but it may come up with a diameter less than the maximum. In my experience using it for my two boats it has been spot on and agreed with the prop supplier recommendation in both cases.

I think the OP should go back to Variprop and ask for their recommendation having first checked Propcalc to get a ballpark figure. The description of behaviour suggests it is underpitched - the diameter seems about right and probably as big as will fit. The Variprop is I think adjustable pitch so should be relatively easy to experiment with a bit more pitch. The engine has enough power to get hull speed at maximum revs if the prop is right.

many have a more recent computer, with Windows 7.

Any calculation needs to specify a diameter because this will be constrained by the gearing - a 1:1.2 box will have to throw a smaller diameter prop than a 1:2.1 or it will never achieve its max power rpm. In fact diameter is a more likely constraint than having too coarse a pitch.

Diameter is a more critical dimension than pitch. Suggest you cross check with a marine engineer.

 

[charles_reed]

The "Optimum" dia is as big as you can swing allowing tip clearance to the hull

That's a point of view - how come all those boats with saildrive legs have such small props? Most of them could be at least twice the diameter.

 

[sailorman]

Go for greatest dia & adjust the pitch to suit.
an egg whisk wont have the punch in head sea / wind

many have a more recent computer, with Windows 7.

Any calculation needs to specify a diameter because this will be constrained by the gearing - a 1:1.2 box will have to throw a smaller diameter prop than a 1:2.1 or it will never achieve its max power rpm. In fact diameter is a more likely constraint than having too coarse a pitch.

Diameter is a more critical dimension than pitch. Suggest you cross check with a marine engineer.

 

[cmedsailor]

an egg whisk wont have the punch in head sea / wind

Though I agree with this, what about flat water or calm seas (where the engine is used more)?

 

[charles_reed]

Go for greatest dia & adjust the pitch to suit.
an egg whisk wont have the punch in head sea / wind

I obviously did not make myself clear - whilst not disagreeing at all with your dictum that one should swing the largest prop you can, there is an upper limit in diameter, set by the engine and gearbox, above which it would be rank stupidity to exceed.

 

[Tranona]

many have a more recent computer, with Windows 7.

Any calculation needs to specify a diameter because this will be constrained by the gearing - a 1:1.2 box will have to throw a smaller diameter prop than a 1:2.1 or it will never achieve its max power rpm. In fact diameter is a more likely constraint than having too coarse a pitch.

Diameter is a more critical dimension than pitch. Suggest you cross check with a marine engineer.

If you have used the Propcalc programmers you will know that it asks for the gearbox ratio along with the engineering and revs so that it can calculate max shaft speed. It then calculates optimum diameter and pitch for 2 or 3 bladed as selected.

No need to ask a marine engineer to explain the obvious.