Why won't my Yanmar reach redline?

Zonker

Super Anarchist
10,904
7,468
Canada
I'd agree that a 2 blade x 17" on 55 HP is suffering from lack of blade area. Doesn't explain the problem with not reaching RPM.

On my last boat with a Yanmar 27 / 2.62 ratio I had a 3 blade x17" autostream. It was 1" too big so the pitch was pretty low - but used 3 blade feathering props are pretty rare so I bought it.
 

DDW

Super Anarchist
6,946
1,397
The blades on the Autostream look like a child drew them with a crayon. That does not seem to hurt their efficiency - it is at least as efficient as the 4 blade Variprop. A lot of thought went into the engineering though. All stainless, all rebuildable including the blade bushings. Can change the pitch independently and continuously forward and reverse, can even reverse the direction of the prop RH/LH which on a saildrive can come in handy. On a saildrive, hard to fit a line cutter, although I managed to do it.
 

mckenzie.keith

Aspiring Anarchist
1,411
535
Santa Cruz
Rumour has it that they're going back into production. At least that's the story I got off a friend of mine who has one and wanted some spares.

I have a saildrive 18" Autostream on the bench ATM. I stripped it down to give it a full service before I decide what I'm going to do with it. I'd like to get a 20" one for a shaft drive so I'm hoping production restarts. Otherwise maybe I'll machine a shaft with the splines for the saildrive prop.

FKT
getaprop.com appears to be offering autostream props.

 

DDW

Super Anarchist
6,946
1,397
Last time I looked at the Seahawk website it said closing production as of July 2023. That was in November. Now the website says the new owners will start production in 2023. I've never had a Flexofold, but I have had a Martec folder, a CDI feathering, a Maxprop feathering, and a Variprop feathering as well as the Autostream. If I was buying a new one I'd buy another Autostream.
 

mckenzie.keith

Aspiring Anarchist
1,411
535
Santa Cruz
Last time I looked at the Seahawk website it said closing production as of July 2023. That was in November. Now the website says the new owners will start production in 2023. I've never had a Flexofold, but I have had a Martec folder, a CDI feathering, a Maxprop feathering, and a Variprop feathering as well as the Autostream. If I was buying a new one I'd buy another Autostream.
Money will enter into this. Two new blades for my existing prop run about 800 USD, more or less. A whole new three blade prop is more like 2200 more or less. But I won't pull that trigger until after I rule out throttle lever travel and tachometer error. If I am facing 2200, then I will certainly consider other brands. But if I decide I could be happy with a two-blade, well, that is a lot cheaper. They estimate I will get 300 more RPM if I stay at 17 inch and reduce pitch, or maybe 400 more RPM if I go down to 16 inch. But they definitely don't like that option. They are just letting me know because I inquired about it.

It may be that an 18 inch two blade with reduced pitch could work OK. I have to check if there is room. Do you have any idea how much clearance is reasonable from prop tip to hull?
 

Zonker

Super Anarchist
10,904
7,468
Canada
0.15 = 15% is sort of a reasonable minimum (clearance/prop diameter)
The blades on the Autostream look like a child drew them with a crayon.
Do you mean the blade shape or blade cross section? Lots of feathering props have flat blades with no twist.

Honestly I see nothing wrong with them. Flat blades w/o twist are not as efficient as blades with twist but are lower drag when feathered.

1679523370456.png

Maxprop

1679523404364.png

J Prop
1679523550772.png
 

DDW

Super Anarchist
6,946
1,397
It's the lollipop shape that is pretty unique. The blades are quite thin, investment cast from duplex stainless. The planform is different from everyone else, so either they are wrong, everyone else is wrong, or it doesn't matter. That said it works at least as well as the (more traditional) one it replaced, with less vibration, so I'm voting for the latter. Because of the longish skirt hub, it integrates a little better onto the saildrive than most, and spaces the blades further from the leg, which may have some effect on vibration.
 

DDW

Super Anarchist
6,946
1,397
On tip clearance, 15% is rule of thumb. But it is a nebulous number. Here is a drawing of my setup, showing the Variprop and Autostream. 19" and 20" and I think the tip clearance was about 12%. You can see much more blade area further out and closer to the hull on the Autostream, which caused some worry about more vibration, but also the blade is much further from the drive leg. Both Variprop and Autostream thought I was pushing blade loading to the limit with a 4 or 3 blade respectively. The result of the experiment was the Autostream vibrates significantly less, and the boat speed is the same or slightly better at every rpm. I bought the Variprop for $4200 and sold it on eBay with <100 hours for $700.

Prop.jpg
 

mckenzie.keith

Aspiring Anarchist
1,411
535
Santa Cruz
So 15 percent of 18 inches is 2.7 inches.

So AFTER I install an 18 inch prop I would want there to be 2.7 inches of gap. That means half the prop diameter plus 2.7 from shaft center to hull. So that is 11.7 inches from shaft center to hull. If I have that I can run an 18 inch prop.

11.7 inches = 297mm (rounded to nearest mm).

1679528761047.png

So I can make myself a little 297 mm length reference stick and bring it down there and just hold it up to see if I have enough clearance. If it is touching the hull and does not extend beyond the shaft center, then I should be good to go with an 18 inch prop. The hull is fairly flat in that area so that removes one source of error.

I am hesitant to bring a tape measure underwater.

If I can fit an 18 inch prop I will probably stick with the two blade of whatever pitch they recommend. And if not, then I may very well go to 17 inch three blade.

I don't think any feathering prop is going to be more efficient than the flexofold in forward or when sailing. The only advantage would be in reverse, I think. Also, I guess adjustability would be an advantage. I have to get the pitch right with the flexofold. Some or all of the feathering props can be adjusted to the right pitch instead of replaced.

That is my thinking right now, anyway. Also just to re-iterate, I absolutely will confirm correct operation of the tachometer and correct travel in the throttle lever on the motor before buying a new prop.
 

DDW

Super Anarchist
6,946
1,397
I love the CAD drawing.

The 15% isn't a hard rule by any means. If you had 2.6" for example. A larger prop is always better until it causes real problems. Also, blades can be cut off, but are hard to stretch. In most tests, the Flexofold with cambered blades does better in forward thrust than a feathering. The adjustability of a feathering can be nice though - perfectly feasible to dive and reduce the pitch, say, for a bash up the CA coast.

On that blade shape, if you have an EAR > 1 then it is going to have a pretty full shape. The wildest ones are the scimitar blades on the Volvo folder, look like an attack submarine's.

Props aren't new tech, you'd think this would be settled science and they would all look the same.
 

kiwin

Member
448
303
Auckland
So 15 percent of 18 inches is 2.7 inches.

So AFTER I install an 18 inch prop I would want there to be 2.7 inches of gap. That means half the prop diameter plus 2.7 from shaft center to hull. So that is 11.7 inches from shaft center to hull. If I have that I can run an 18 inch prop.

11.7 inches = 297mm (rounded to nearest mm).

View attachment 581401
So I can make myself a little 297 mm length reference stick and bring it down there and just hold it up to see if I have enough clearance. If it is touching the hull and does not extend beyond the shaft center, then I should be good to go with an 18 inch prop. The hull is fairly flat in that area so that removes one source of error.

I am hesitant to bring a tape measure underwater.

If I can fit an 18 inch prop I will probably stick with the two blade of whatever pitch they recommend. And if not, then I may very well go to 17 inch three blade.

I don't think any feathering prop is going to be more efficient than the flexofold in forward or when sailing. The only advantage would be in reverse, I think. Also, I guess adjustability would be an advantage. I have to get the pitch right with the flexofold. Some or all of the feathering props can be adjusted to the right pitch instead of replaced.

That is my thinking right now, anyway. Also just to re-iterate, I absolutely will confirm correct operation of the tachometer and correct travel in the throttle lever on the motor before buying a new prop.
The flexofold three blade is a very good prop. In use manoeuvring it feels as good as a fixed prop, while having less drag when sailing. I much prefer the flexofold to a feathering prop. The difference in reverse is not discernable.
 

Zonker

Super Anarchist
10,904
7,468
Canada
Props aren't new tech, you'd think this would be settled science and they would all look the same.
You'd think so but I don't think they are settled. New CFD tools etc mean newer prop designs are often better in one or more ways.

From a Yachting Magazine August 1959 which a colleague gave me:

1679695352951.png


From the web Michigan Wheel's site, 2023:

This is a very garden variety prop.

1679695394922.png



This is a Veem brand. The little blue stripe is an interceptor strip of hard plastic. By changing the depth of the little lip you can change the pitch cheaply and fast (within limits). This is clever.

1679695529458.png


1679695587879.png
 

longy

Overlord of Anarchy
7,439
1,557
San Diego
The Gori is an excellent option, just wish their proprietary zincs weren't so expensive. To add insult, the central zinc often must be filed a tad to allow the blades to fold. Aside from that, they give excellent performance & durability, and the US dealer nailed the prop settings exactly
 

Zonker

Super Anarchist
10,904
7,468
Canada
Exhaust elbow clogging the engine. But I don't think it affected engine RPM that much.

 

Fah Kiew Tu

Curmudgeon, First Rank
10,968
3,900
Tasmania, Australia
The Gori is an excellent option, just wish their proprietary zincs weren't so expensive. To add insult, the central zinc often must be filed a tad to allow the blades to fold. Aside from that, they give excellent performance & durability, and the US dealer nailed the prop settings exactly

The Autostream zincs are a bit exy as well. Friend with one asked me to machine hi up an adapter that allows standard donut shaped zincs to fit. Seems to be working out OK.

Personally I couldn't be bothered as I've a 10+ year supply of the zincs and could easily cast my own by making a simple mould. Not like zinc is difficult to cast.

FKT
 
Top