Getting Screwed!

[Lee Jerry]

Originally Posted by Renegde_Sailor

Lets start with the calculations using the VicProp calculator:

The current prop is a 16" Diameter and is stamped 12.8 on the hub, so I am assuming it is a 12.8 pitch.

So currently when I am running about 2400 RPM at max speed, that makes a lot of sense.

What I am considering is going to a 4 blade PIY Max Prop and setting it at about a 10 pitch as the calculator suggests.

I worry that with the 4 blade prop that the 1" shaft will be more likely to snap, although I do plan on running a line cutter and the prop is very well protected where it is.

I did measure from the tips of the blade to the top and bottom of the aperture...

As you can see it is ~1 7/8" on the top, 1 3/8" on the bottom.

Being a 16" prop now if I wanted to go to an 18 I could simply cut out the bottom of the aperture in the rudder and add another 2" of clearance there, then move the shaft down an inch which would center the prop back in the opening.

Considering right now with a clean bottom at about 2500 rpm, she will do 7.2 knots but wind her any faster than that and she cavitates like crazy.

Additionally my bigger concern is when running against waves and current, she is prone to cavitating when you throw more power at her.

Originally Posted by Renegde_Sailor

The top speed of the boat is good, mainly what concerns me is more torque when we get into heavier seas as right now it slows me way down, and wit that when I go to a 4 blade I will be going with a PIY MaxProp so the drag will not be an issue.

Here is my $0.015...

You are currently over-propped, especially if you are concerned about performance in waves.

First, the current prop diameter is too big for the aperture. The minimum tip clearance at the top is 15% of the diameter; 20% is better. (The bottom should be the same but is a little less critical.) You currently only have 11.7%. IMHO, the max diameter that fits (properly) in your aperture is 15". The clearance would then be ~2-3/8" giving 15.8% of D.

This leads to the second - what you are describing as "cavitation" is likely a combination of cavitation and vibration. There are several kinds of cavitation. In this case, as the blade approaches vertical, the pressure increases due to the presence of the hull. This causes a vibration in the hull. As the blade passes vertical, the pressure drops, and the water could then cavitate (and collapse when the pressure rises again, either at the bottom of the aperture or at the top - either at 2x blade pass frequency or 1x blade pass frequency, respectively, depending on, well...stuff). This influence / pressure variation is typical but is magnified by the small tip clearance. Hence the reason for the minimum design values. The picture of the prop does indicate damage that could be from cavitation.

Third, you do not seem to be getting anywhere near rate RPM (3800 rpm) at WOT, but it's hard to say how much of that is due to the cavitation issue versus too much diameter/pitch.

Fourth, the "prop" that gets you max speed in calm water is not the same "prop" that gets you max speed in waves. You have to pick your compromise.

My suggestion is 15"D x 10"P three-blade, or a 15"x9" four-blade. (But I probably wouldn't argue about a 15"x10" four-blade...)

Note that the smaller diameter also has reduced tip speed at any given (i.e. higher) RPM which also reduces cavitation.

[Renegde_Sailor]

Originally Posted by skipperpete

Yanmar differs a little with your propeller and shaft size but due to your aperture, there are few options. The Yanmar installation manual suggests a diameter of 440 mm- 460mm and a pitch of 250mm for a 3 blade prop to achieve 8 knots.
About the shaft size, if the material is SUS304 they suggest a 28.58 mm shaft and for SUS630, a 22.67 mm diameter shaft (30 and 25mm)so if you have the high strength 17 alloy I think you’re ok.
I’ll post a screen shot of the Yanmar prop sizing later.
On the maxprop, excellent choice but again , the aperture size can be a problem as the blades rotate, I had to cut a bit out of my boat to install a maxprop classic because of this.

so a 250mm pitch would be 9.85" which is quite a bit shallower than my current 12.8 (assuming the stamping is correct)

I do need to sit back down and re-read the installation manual for the engine, it has been a couple of years since I have.

[Renegde_Sailor]

Originally Posted by Lee Jerry

Here is my $0.015...

You are currently over-propped, especially if you are concerned about performance in waves.

First, the current prop diameter is too big for the aperture. The minimum tip clearance at the top is 15% of the diameter; 20% is better. (The bottom should be the same but is a little less critical.) You currently only have 11.7%. IMHO, the max diameter that fits (properly) in your aperture is 15". The clearance would then be ~2-3/8" giving 15.8% of D.

This leads to the second - what you are describing as "cavitation" is likely a combination of cavitation and vibration. There are several kinds of cavitation. In this case, as the blade approaches vertical, the pressure increases due to the presence of the hull. This causes a vibration in the hull. As the blade passes vertical, the pressure drops, and the water could then cavitate (and collapse when the pressure rises again, either at the bottom of the aperture or at the top - either at 2x blade pass frequency or 1x blade pass frequency, respectively, depending on, well...stuff). This influence / pressure variation is typical but is magnified by the small tip clearance. Hence the reason for the minimum design values. The picture of the prop does indicate damage that could be from cavitation.

Third, you do not seem to be getting anywhere near rate RPM (3800 rpm) at WOT, but it's hard to say how much of that is due to the cavitation issue versus too much diameter/pitch.

Fourth, the "prop" that gets you max speed in calm water is not the same "prop" that gets you max speed in waves. You have to pick your compromise.

My suggestion is 15"D x 10"P three-blade, or a 15"x9" four-blade. (But I probably wouldn't argue about a 15"x10" four-blade...)

Note that the smaller diameter also has reduced tip speed at any given (i.e. higher) RPM which also reduces cavitation.

Those are some very good thoughts there, thank you, I am going to put some more consideration into this.

[Renegde_Sailor]

Originally Posted by Jim Cate

Do remember that Maxprops (and all other conventional feathering props) are somewhat less efficient than fixed props, due to their flat (no twist) blade shape. This means a bit less thrust for a given shaft horsepower. If you are actually HP limited now, it could be worse with the new prop.

Jim

The value they advertise is 4% loss, real world? Who knows.



It is certainly a thing.

[skipperpete]

Here’s what Yanmar suggests regarding a propeller size for your engine, sounds a bit small but I’m sure they spent a lot of time coming up with these numbers.

[Renegde_Sailor]

Originally Posted by skipperpete

Here’s what Yanmar suggests regarding a propeller size for your engine, sounds a bit small but I’m sure they spent a lot of time coming up with these numbers.

Ironically the size they list is larger than what I have now, because a 440mm prop is 17.32 inches, my current prop being a 16"


And that begs the question is gaining an inch and a quarter of prop worth the while for the work I would have to do in order to make it happen? Will it make that much of a difference?



As well if I were to just go for the 4 blade Max prop, the calculator calls for a 16" 4 blade, but then I get the added benefit of the extra blade pushing.

[rslifkin]

I'd stick with the smaller diameter and get the extra blade area by going to a 4 blade. It's theoretically not quite as ideal as going to a larger diameter 3 blade, but with the limitations of the aperture on your boat it's likely the better option.

[Cheechako]

Originally Posted by Renegde_Sailor

Ironically the size they list is larger than what I have now, because a 440mm prop is 17.32 inches, my current prop being a 16"

And that begs the question is gaining an inch and a quarter of prop worth the while for the work I would have to do in order to make it happen? Will it make that much of a difference?

As well if I were to just go for the 4 blade Max prop, the calculator calls for a 16" 4 blade, but then I get the added benefit of the extra blade pushing.

My experience says NO. I did go the full diameter route on one boat, eeking every inch I could out of it. I can't say there was a noticeable difference. I once replaced a wide blade fixed 3 blade with a Michigan Sailor which is fairly narrow blade intended to work well at lower rpm I think. That prop seemed better than the wide blade. I wonder why and the only thought I come up with is there is less disturbed water for it to dig into. For that reason I often wonder if the 4 blade is an improvement...? Will it cavitate more or less?

I've had 5 three blade Max Props over the years, most of them replacing fixed 3 blade, one replacing fixed 2 blade.
From an unscientific standpoint it seems to me all 5 performed better than the fixed blade prop, forward and reverse, regardless of what the calculations say.

[rslifkin]

The flat blades on a Maxprop may be a disadvantage at higher speeds. But flat, symmetrical blades aren't much of a downside for thrust at low speeds. Plus the symmetrical blades will give better reverse thrust.

[Renegde_Sailor]

Originally Posted by Cheechako

My experience says NO. I did go the full diameter route on one boat, eeking every inch I could out of it. I can't say there was a noticeable difference. I once replaced a wide blade fixed 3 blade with a Michigan Sailor which is fairly narrow blade intended to work well at lower rpm I think. That prop seemed better than the wide blade. I wonder why and the only thought I come up with is there is less disturbed water for it to dig into. For that reason I often wonder if the 4 blade is an improvement...? Will it cavitate more or less?

I've had 5 three blade Max Props over the years, most of them replacing fixed 3 blade, one replacing fixed 2 blade.
From an unscientific standpoint it seems to me all 5 performed better than the fixed blade prop, forward and reverse, regardless of what the calculations say.

Everyone I have talked to with a MaxProp has had nothing but nice things to say about them, I ask every chance I get and it always seems to be the same, they are loved.

[Renegde_Sailor]

Originally Posted by rslifkin

The flat blades on a Maxprop may be a disadvantage at higher speeds. But flat, symmetrical blades aren't much of a downside for thrust at low speeds. Plus the symmetrical blades will give better reverse thrust.

That reverse thrust would be great because that is the one situation where my boat really struggles, you have to be careful to not have too much way on coming into a slip or she isn't stopping.

[rslifkin]

Originally Posted by Renegde_Sailor

That reverse thrust would be great because that is the one situation where my boat really struggles, you have to be careful to not have too much way on coming into a slip or she isn't stopping.

You'll definitely see some gains there both from the Maxprop having symmetrical blades and also from gaining additional blade area from the 4 blade, so you'll have more prop bite at low speed.

[Renegde_Sailor]

Originally Posted by rslifkin

You'll definitely see some gains there both from the Maxprop having symmetrical blades and also from gaining additional blade area from the 4 blade, so you'll have more prop bite at low speed.

I suspect that is probably going to be the best course of action, get the 16" 4 blade max prop, leave the shaft where it is just replace the bronze shaft alley with the G10 and call it a day, as well looking to add a spurs line cutter to the mix if there is room to fit it.

I feel like that is probably going to be my best option as it doesn't require a huge modification to the boat and rudder.

[skipperpete]

Originally Posted by Renegde_Sailor

I suspect that is probably going to be the best course of action, get the 16" 4 blade max prop, leave the shaft where it is just replace the bronze shaft alley with the G10 and call it a day, as well looking to add a spurs line cutter to the mix if there is room to fit it.

I feel like that is probably going to be my best option as it doesn't require a huge modification to the boat and rudder.

By “ Bronze shaft alley” do you mean the actual stern tube? That would normally be a substantial amount of work unless its just a matter of unscrewing the existing tube ends and threading in a new tube.

[Renegde_Sailor]

Originally Posted by skipperpete

By “ Bronze shaft alley” do you mean the actual stern tube? That would normally be a substantial amount of work unless its just a matter of unscrewing the existing tube ends and threading in a new tube.

Yes, the stern tube.



It has the style with a bronze fitting on each end and a bronze tube going through the hull, going to remove the whole works and bore it out with boring bar to accept a G10 tube that Fisheries sells that is pre-bored for a cutlass bearing.