Propeller question

I turn 2400 with my 22x18 4 blade (1.91 : 1 )
What will I turn with my new 22 x 23 3 blade ??
Not the boat in pic but lobsterboat.

Hard to say. One way to know is to join boatdiesel and use their prop calculator. Another way is to ask the prop shop where you bought your new prop if you bought it from one.


A rough rule of thumb is that each inch of pitch change results in 100 rpm change. But a 5 inch up pitch change plus 4 to 3 down blade change is more than any rule of thumb can handle.


My guess is less than 2,400. Going from 4 to 3 blades won't offset 5 inches of pitch.


David

My SWAG is 2000.

djmarchand said:

Going from 4 to 3 blades won't offset 5 inches of pitch.

David

Click to expand...

No of blades doesnt affect pitch

HP? WOT? Boat spec other than 'Lobster"(weight)? Did I miss any pertinent information required? Other wise a WAG.

Al-Ketchikan

Brooksie said:

I turn 2400 with my 22x18 4 blade (1.91 : 1 )
What will I turn with my new 22 x 23 3 blade ??
Not the boat in pic but lobsterboat.

Click to expand...

You'll just have to try it and see. Apparently no-one on here is omniscient.

My guess is 2050

Whatever you get , just run on the pin (full throttle) for 30 seconds and note the number.

And look astern to see if heavy black smoke is following.

Reduce the observed WOT (wide open throttle) RPM by 200 for your new "Flank" speed and you should see no black smoke.

Reduce WOT by 300 and you can cruise there till out of fuel.

These reductions assumes no EGT gauge , and will keep you out of trouble.

Al said:

HP? WOT? Boat spec other than 'Lobster"(weight)? Did I miss any pertinent information required? Other wise a WAG.

Al-Ketchikan

Click to expand...

150SHP @2400, WOT 2400 w/ 22x18 4B, 14000 lbs, 33' LWL, 11' WLB

Probably too low at wot and slower top speed. Tell us what was used to pick the 23" pitch?

Why are you changing propellers? What is the manufacturer's rated WOT rpm for your engine?

As a starting point it seems you would want a propeller that would give you mfg. rated WOT (and maybe 30-50 rpm more). If the current propeller allows you to reach, but not exceed, that rated WOT the 5" pitch increase may result in your being over propped.

Lots of old threads re pros and cons of over/under propped. Maybe you want to be over propped?

stone beach said:

No of blades doesnt affect pitch

Click to expand...

Of course it won't!!! But it will affect load and therefore RPM. You must have missed his point.

Brooksie said:

150SHP @2400, WOT 2400 w/ 22x18 4B, 14000 lbs, 33' LWL, 11' WLB

Click to expand...

This will give you some idea. http://www.vicprop.com/calculator.

Thanks for replys.

Yes I know I am going to have to take some pitch off just wanted to get opinions on how much...

I "chose" the 23" b/c I had a chance to get a brand new 22x23 for $175. and I always wanted to try a 3 blade in place of my 4 blade.

Yes I do want to reach 2400 @ WOT, as I do now and yes 2400 is the engine mfg. WOT max spec. so was just looking for guesses as to what it would do w/ the higher pitched 3 blade and how much pitch to take off.

Booksie
This was offered to provide some idea of 'What' might be. I would agree with FF regarding a back pocket formula to assure engine protection. It very closely meets my own perspective running our boat.
With a 22x16 wheel our WOT is 2000 RPM with an engine rated at a minimum 2500 RPM and more preferable in the application of our boat of 2800 RPM.

Obviously very over propped by any standard yet using FF's approach, we achieve hull speed of 7 knots at 1400 RPM which leaves a margin of 600 RPM.
Running the engine beyond the 1400 results in a very small gain I.E. 1600 RPM, the edge of FF's suggested maximum RPM, will give us a mid to near 8 knot SOG but at a cost of fuel,noise, fussy water, and a picture of a ugly movement through the water.

With a lobster hull design, (SD/planning hull) turning the RPMs at a higher level would seem a better direction.

Al-Ketchikan 27' Marben Pocket Cruiser

I will love to hear what you find out! I have a Yanmar 4JH with a three blade prop, the other Willard's I am aware of with the same power plant have 4 blade props and don't get as good of fuel economy as I do. I will be very interested in whether you find your efficiency has improved, stayed the same, or decreased.

AKDoug said:

I will love to hear what you find out! I have a Yanmar 4JH with a three blade prop, the other Willard's I am aware of with the same power plant have 4 blade props and don't get as good of fuel economy as I do. I will be very interested in whether you find your efficiency has improved, stayed the same, or decreased.

Click to expand...

My Willard has a 5 blade original prop. Smooth as an electric motor. It's a "shrimper" style with very narrow blades. The conventional wisdom on lobster boats is that 4 and 5 blades always work better behind a keel. but you know what they say about conventional wisdom.. I had a chance to pick up this 3 blade cheap and just wanted to try it. I have all my existing numbers with both Michigan Dynaquad 22x18-4 and Columbian 22x18-4 so will inform you of my results. I won't repitch more than 2" b/c I'm not sure it can be done correctly.
Your 3 blade should be more efficient than the 4 blades all other things being equal.

OK, sitting outside today, waiting for 5:00 to come around, I decided to play with your question.


Using boatdiesel's prop calculator I plugged in your 4 blade prop parameters, the engine hp and maximum rpms, then I played with boat weight until the calculator showed 22x18 pitch. I then changed the input to 3 blade and holding everything else constant, I adjusted hp and rpm so it would calculate a 23 pitch.


Lots of different hp and rpm combination would result in 23 pitch, but I wanted it to produce a hp that your 150 hp engine could make at wot. All NA, mechanically injected engines produce less hp at less rpm. I had a NA Ford Lehman hp curve that peaked at 150 hp at 2,400 rpm, just like your engine so I used that as my stalking horse.


I finally found an rpm/hp combination that produced a pitch of 23 that fit with the Lehman's wot hp curve. That was 140 hp at 2,200 rpm. That, as best as boatdiesel's calculator can tell, is where your engine will rev with a 3 blade 22x23 prop.


Then I adjusted the pitch so it would hit 150 hp at 2,400 rpm. That took a pitch of 20".


So, if you took 2" of pitch out of your prop you still would be over propped but not by much. If you keep the rpms down to 2,000 or less, you should be fine.


Time for that cocktail!!!


David

Baker said:

Of course it won't!!! But it will affect load and therefore RPM. You must have missed his point.

Click to expand...

Nope.

Well, to try to answer this question about 3 vs 4 blade props, I plugged the OP's boat data into the boatdiesel calculator for a 4 blade prop. As in the post above I adjusted the boat weight until I got 18" of pitch. Then with all else the same I changed it to a 3 blade prop. The boatdiesel calculator came up with 20" of pitch for the 3 blade prop.


Pretty much as I would have expected. Fewer blades result in less blade area, so to compensate you have to add pitch.


David

djmarchand said:

Fewer blades result in less blade area, so to compensate you have to add pitch.


David

Click to expand...

Very obvious to you and I....but not others.

David, Thanks for a nice deductive job. I think you are right on based on some figuring using the "Propeller Handbook"
Whenever I have run my boat through a calculator, it always seems to want to come up with a 22x20 3 blade, which I always assume is the same as my 22x18 4 blade as far as load is concerned.
So if I can't get 3" pitch taken off this prop, I'll sell it...

"Fewer blades result in less blade area, so to compensate you have to add pitch."

Only so much blade area is required to put a certain HP in the water.

Any extra is just more drag. Thats why there are 2blade 3 blade 4 blade 5 blade ,,,,,,