18x14 vs 21x10 prop - sailing speed?

[sailingunity]

I'm repowering and working through General Propeller in FL to get a new prop shaft and prop, the new engine turns the opposite direction as my old one. Based on the information I gave them, they recommended two props:

Eng RPM: 3200
HP: 80
Reduction: 2.15:1
Water line length: 43 (they calculated 8 knots target hull speed)
Weight: 32000 lbs
Aperture: 25.75"

Option 1: keep the current prop diameter of 18". I'm not sure if my current prop has a 14" pitch or not (couldn't find the stamping).
Option 2: 21x10 prop.

Supposedly the larger diameter is more efficient motoring - however I'm concerned the additional diameter and flatter pitch will affect sailing speed. The new engine is supposed to be in neutral while sailing, so it'll freewheel anyway, so is this concern invalidated by that?

[sailingunity]

Might have put this in the wrong forum, mods move if you please.

[FPNC]

You are running the wrong gear reduction. Use 2.5:1 and a 21 x 14, 3 blade.

80 hp is too much power for an 18" diameter prop.

2.15:1 ratio is too high to turn the 21" diameter propeller with proper pitch.

[sailingunity]

Originally Posted by FPNC

You are running the wrong gear reduction. Use 2.5:1 and a 21 x 14, 3 blade.

80 hp is too much power for an 18" diameter prop.

2.15:1 ratio is too high to turn the 21" diameter propeller with proper pitch.

I'll ask the mechanic about that tomorrow, he hasn't ordered the engine yet. I'm guessing the 2.5 reduction will turn the prop slower this allowing more pitch.

[sailingunity]

Originally Posted by FPNC

You are running the wrong gear reduction. Use 2.5:1 and a 21 x 14, 3 blade.

80 hp is too much power for an 18" diameter prop.

2.15:1 ratio is too high to turn the 21" diameter propeller with proper pitch.

Looking at the specs of this engine online, it only comes available in 2.15 or 2.70 reduction ratios.

[Rucksta]

Originally Posted by sailingunity

Looking at the specs of this engine online, it only comes available in 2.15 or 2.70 reduction ratios.

Find an alternate gear box like a Twin Disc rated for 100% duty cycle forward and reverse.

Solves the left/right pitched prop problem

Lots of options around 2.4:1

Some with 8 degree drop angle

Start with your existing prop and work from there.

21" prop in 25" aperture seems marginal on clearance.
25-30% clearance is sometimes quoted as ideal.

[sailingunity]

Originally Posted by Rucksta

Find an alternate gear box like a Twin Disc rated for 100% duty cycle forward and reverse.

Solves the left/right pitched prop problem

Lots of options around 2.4:1

Some with 8 degree drop angle

Start with your existing prop and work from there.

21" prop in 25" aperture seems marginal on clearance.
25-30% clearance is sometimes quoted as ideal.

For reference, the engine is a Yanmar 4JH80. The only straight shaft option is Hurth ZF30M (mechancal) with the aformementioned 2.15 or 2.70 options.

I can get closer with Kanzaki KMH4A which has 2.45 ratio, but only comes in a 8 degree downangle, would require significant modification to the engine beds.

That's just what is shown on the data sheet, I'll check tomorrow if more options are available.

[Lee Jerry]

Originally Posted by sailingunity

I'm repowering and working through General Propeller in FL to get a new prop shaft and prop, the new engine turns the opposite direction as my old one. Based on the information I gave them, they recommended two props:

Eng RPM: 3200
HP: 80
Reduction: 2.15:1
Water line length: 43 (they calculated 8 knots target hull speed)
Weight: 32000 lbs
Aperture: 25.75"

Option 1: keep the current prop diameter of 18". I'm not sure if my current prop has a 14" pitch or not (couldn't find the stamping).
Option 2: 21x10 prop.

Supposedly the larger diameter is more efficient motoring - however I'm concerned the additional diameter and flatter pitch will affect sailing speed. The new engine is supposed to be in neutral while sailing, so it'll freewheel anyway, so is this concern invalidated by that?

The "proper" propeller will depend on what your goals are. Larger diameter (turning slower) is generally more efficient. Larger diameter will generally have more drag under sail. Have you considered a folding or feathering prop?

How many blades on each of the two prop options offered?

Why the large variation in options? Why not something in the middle, like 19x13 or 20x11?

Both options seem over-propped to me. At first glance, 19x11 (or 12) or 18x12 (or 13) look better for that reduction (assuming 3-bladed).

A higher reduction could be beneficial (allowing for a larger wheel), but that goes back to my original question about your goals. If you want fixed blades and low-ish resistance under sail, then the 2.15 reduction (with 19" or 18" dia) is probably the way to go. If you want to "optimize" motoring, a larger reduction and larger (21") prop is better. If you want "the best" (i.e. both), then the larger reduction and larger (21") folding or feathering prop is the answer.

A freewheeling prop under sail still has considerable resistance. In fact, it could be higher than with the shaft fixed. For multi-shaft ships (particularly naval, often with >2 shafts), that calculation was done to predict/determine which was better.

[Lee Jerry]

Originally Posted by FPNC

You are running the wrong gear reduction. Use 2.5:1 and a 21 x 14, 3 blade.

80 hp is too much power for an 18" diameter prop.

2.15:1 ratio is too high to turn the 21" diameter propeller with proper pitch.

Everything about this post is "wrong."


The 2.15 reduction could be fine; depends on the goals of the installation, as explained in previous post.

Then how does this engine put 300 hp through a 15-3/4" diameter prop?

The "proper pitch" for any given reduction and diameter is that which allows the engine to reach rated speed (rpm). The boat performance is the dependent variable.



I put the "wrong" in quotes because the concepts as presented are incomplete. They are not true as generalizations, but can be correct in the proper context, which was missing in each of them.

[Lee Jerry]

Originally Posted by Rucksta

Find an alternate gear box like a Twin Disc rated for 100% duty cycle forward and reverse.

Solves the left/right pitched prop problem

What is the left/right problem? What difference does it make if the new installation has opposite rotation from the previous?

21" prop in 25" aperture seems marginal on clearance.
25-30% clearance is sometimes quoted as ideal.

21" diameter in a 25.75" aperture is 22.6% clearance. Anything greater than 20% is generally considered good. The minimum is often taken as 15%.

[sailingunity]

I'm not opposed to a feathering prop (don't have room for folding), but I'm not sure it's in the budget at this time at over twice the price of a fixed prop.

It does seem like a rather large difference in options - I'll inquire tomorrow if there are more options. They quoted a 16 week lead time on the 21x10 (probably custom) while the 18x14 was in stock. I started the thread because I was concerned about resistance of the bigger prop under sail.

[Lee Jerry]

Originally Posted by sailingunity

I'm not opposed to a feathering prop (don't have room for folding), but I'm not sure it's in the budget at this time at over twice the price of a fixed prop.

It does seem like a rather large difference in options - I'll inquire tomorrow if there are more options. They quoted a 16 week lead time on the 21x10 (probably custom) while the 18x14 was in stock. I started the thread because I was concerned about resistance of the bigger prop under sail.

I believe this is an accurate generalization of the differences in speed, but of course would depend on boat specifics. If we use the 18" dia as the baseline, at higher speeds, say above ~5 kt:
A feathering prop (of any dia) would see the better part of knot increase (say 7 or 8 tenths) in boat speed.
A 21" dia prop would see a 1 or maybe 2 tenth reduction in boat speed.

[Rucksta]

Originally Posted by Lee Jerry

What is the left/right problem? What difference does it make if the new installation has opposite rotation from the previous?


21" diameter in a 25.75" aperture is 22.6% clearance. Anything greater than 20% is generally considered good. The minimum is often taken as 15%.

Left right pitch is not a problem.
If your motor spins the other way to your prop just run it in reverse to go forward then swap the shift cable to produce an intuitive lever position

The problem can be the gear box.
Some are not rated to run continuously in reverse.
Some have different reductions in the nominal reverse to increase torque astern.


As you state aperture clearance is adequate

[FPNC]

Originally Posted by sailingunity

For reference, the engine is a Yanmar 4JH80. The only straight shaft option is Hurth ZF30M (mechancal) with the aformementioned 2.15 or 2.70 options.

I can get closer with Kanzaki KMH4A which has 2.45 ratio, but only comes in a 8 degree downangle, would require significant modification to the engine beds.

That's just what is shown on the data sheet, I'll check tomorrow if more options are available.

Twin Disc / Technodrive TM345 is available in 2.47:1 ratio. It is a solid built hydraulic gear. Extremely reliable.

[MJH]

Originally Posted by sailingunity

I'm repowering and working through General Propeller in FL to get a new prop shaft and prop, the new engine turns the opposite direction as my old one. Based on the information I gave them, they recommended two props:

Eng RPM: 3200
HP: 80
Reduction: 2.15:1
Water line length: 43 (they calculated 8 knots target hull speed)
Weight: 32000 lbs
Aperture: 25.75"

Option 1: keep the current prop diameter of 18". I'm not sure if my current prop has a 14" pitch or not (couldn't find the stamping).
Option 2: 21x10 prop.

Supposedly the larger diameter is more efficient motoring - however I'm concerned the additional diameter and flatter pitch will affect sailing speed. The new engine is supposed to be in neutral while sailing, so it'll freewheel anyway, so is this concern invalidated by that?

What is the boat make/model. I would query other sister boat owners/forums for what they are using with satisfaction. I would also double check my data and insure it is correct before committing to anyone. Another prop manufacturer opinion might be worthwhile like Michigan Prop which I found reliable.

Lastly, finding the proper prop & pitch matched to your engine and transmission can be a journey in itself. Take your time deciding which to choose and afterwards test on flat water and wind for the ability to operate at wide-open throttle without cavitation; that should be your goal...it took me two attempts.

Good Luck.