Prop sizing, pitch, number of blades

Looking for inboard prop shop in San Diego to help determine best prop for 44 foot trawler with Volvo Penta 97 hp, 2.5:1 gear.

Not obtaining anticipated SOG for given RPM.

Told part of the problem may be the "dead wood" of the keel.

No it's not the "deadwood" but that dosn't help either.
Looks like your dia is fine. Plenty of clearance above. Should be 1 to 1.5" below and you may have that too.
Blade area may be a problem. I see you have 4 blades and ideally you should have 3 w 100hp but that's just a guess. Check w the prop shop.
To get the best performance you need to have the prop load matched to your gear and engine so when you go to wide open throttle (WOT) your engine should turn at it's rated speed. Your engine develops Xhp at Y rpm. WOT you need to get "Y" rpm.
Unless the prop shop says you've got too much blade area don't worry about getting a 3 blade. As long as the blade area is in the good zone the difference between a 3 blade and a 4 blade is small. But blade area is important.

zeta said:

Not obtaining anticipated SOG for given RPM.

Click to expand...

What is your specific anticipated and actual SOG?

Mine is "perfecto" (14-ton boat with 80 horsepower), so it can be done if one has realistic expectations.

Vicprop - Prop calculator for Displacement and semi-displacement hulls

Use the calculator .

The problem with sizing is at times the "perfect" prop wont fit.

Then Skeenes Elements of Yacht Design will allow you to run a bunch of prop choices and figure what you will loose , if not "perfect".

Great fun , once you get the hang of it.

Try boatdiesel.com calculator. You can put in the max. diameter your can use and go from there.

These are the figures on the last speed run ......

Max RPM for the engine (97 hp, 2.5:1 gearing) is 1900. Volvo states to use 200 below max as max cruise RPM.

Max RPM with current prop is 1925,

Would like to obtain 7 kts at 1350 RPM. Estimated hull speed is about 8 kts. She weighs about 26 tons.

Prop - RPM - Speed

1000. 5.3
1100. 5.5
1200. 6.1
1300. 6.4
1400. 6.6
1500. 6.9
1600. 7.2
1700. 7.6
1800. 7.9
1900. 8.2

fighterpilot,
That works good only up to a certain point. Not many boats have too much room for a prop as FF pointed out or alluded to. My own Willard is one however. The problem w that is that a prop w long skinny blades dosn't exist or is very rare in the sizes applicable to our boats. A 3 blade prop like the 5 blade in the picture would be very efficient. It's a Michigan Star and has'nt been made for many years. But w the philosophy of "diameter is everything" you reach a point where theres too little pitch because too much power is being consumed by the blade area. Visualize a prop so big it is able to consume all the engines power just spinning the prop blades through the water. No power left for pitch ... and thrust. Then a reduction in diameter and increase in pitch will produce more thrust. It looks like zeta has very little pitch but so does Mark, and most other trawlers. But if it actually is too much dia and too little pitch (hence blade area) the prop charts at BoatDiesel, Skeene's book or whatever won't present it as the "ideal" prop. Re my Willard 30 it could swing a 20" dia prop but has as original equipment an 18" dia prop. I think the prop was well chosen. The prop in the pic is 19" dia. It had too much blade area and a bit too much pitch too (I think). Wouldn't come close to full WOT rpm. So one CAN have too much diameter but most all boats won't have the space for that.

Fighterpilot,
Re-read your post and see you wer'nt saying use all the space available. Sorry.

zeta,
I see your new post. Looks like you've loaded the prop perfectly if 1900rpm is where the engine develops is rated power .. and it sounds like it.
Other questions come to mind now.
1. What's the purpose of attaining 7 knots at 1350rpm?
2. How are you measuring your speed?
3. Is your tach accurate?
4. Do you know or can you contact other people w the same hull and power?

Prop now is 26 x 23.58, four blade. Michigan suggested going to a 3 blade because the stern of the keel may be blanking out two of the blades at a time.

The 52 ft wood boat I had turned a 5 blade, so have been looking into that.

zeta said:

These are the figures on the last speed run ......

Max RPM for the engine (97 hp, 2.5:1 gearing) is 1900. Volvo states to use 200 below max as max cruise RPM.

Max RPM with current prop is 1925,

Would like to obtain 7 kts at 1350 RPM. Estimated hull speed is about 8 kts. She weighs about 26 tons.

Prop - RPM - Speed

1000. 5.3
1100. 5.5
1200. 6.1
1300. 6.4
1400. 6.6
1500. 6.9
1600. 7.2
1700. 7.6
1800. 7.9
1900. 8.2

Click to expand...

Your present RPM and speed look OK to me as the hull speed for 44 ft is about 7. They are about the same as the Eagle.

Gary, I agree with Phil except your displacement-hull speed of 8+ knots is presently reached at your max. cruise RPM.

From here, it doesn't seem you have a prop problem.

Back to the discussion of 4 blade vrs 3 or 5......

One part that i realized i have left out is the possible cavitation which results in a hull thumping noise at about 1500 RPM.

This is why Michigan suggested three a three bladed prop and brought up the "dead wood issue".

1200 - 1350 RPM is where the engine seems happiest and most economical.

The max obtained RPM (1925) was just over the max (1900).

The prop is close. What I am looking for is efficiency, no "thumping", and hopefully 7 kts

To do that I think you could overprop until it happens. Don't know how far away 1350 is from 7 knots ... just looked at your numbers. OK 150rpm or so. One inch of pitch will accomplish that. AND the boat will be a tad more fuel efficient but if you have a warranty on the engine it may be out the window and max power will also be out the window. You will not be able to cruise at 200rpm down from max as Volvo says either. Your max sustainable rpm won't be much over cruise speed.
Another way to approach this is to see if you can find out why 1500rpm is not smooth. it may be a bulkhead or other structure that has a harmonic resonance w that frequency. If you can find it stiffening up that structure may make the boat smooth at your favorite rpm. Metal boats tend to "oil can" more than others. A good place to look for oil canning is in the large flat area alongside the deadwood. Look for a flat surface.
Changing to a 3 blade prop could help too but less likely.
You'd think a 5 blade prop would be smooth but that Michigan Star I had wasn't.
Contacting the designer before doing anything would also be a very good idea.
One nice thing about stock boats is that all (most) of this tuning stuff has been done.

Regarding prop pitch:
While Eric (from post #2) is right regarding the ideal pitch allowing the engine to reach full RPM at WOT, it's not a hard and steadfast rule. If you are not going to operate the boat at WOT and max hull speed, you can repitch the prop off ideal pitch. Engines develop their max HP at the max RPM. There is a HP curve for the engine that shows max HP at each RPM. Most of the time a boat doesn't require all the available HP an engine can produce at these lower RPMs. A rough guess would be that adding 1 to 2" of pitch would give you 7 knots at around 1,350 rpm. WOT may then only yield 1,700 to 1,750 RPM. If you plan to operate at 1,350 / 7 knots, 100 to 200 rpm off at WOT won't materially effect the engine. If you plan to operate at near WOT, disregard what I just wrote.

Also a good idea to see what local prop shops have for used props. The shop I use here let me try several props when I switched motors to figure out what I needed. Ended up running a used reconditioned one for a year till I got the pitch right. Then ordered a new one and kept the used one as a spare.

Ted

Zeta:

I think that you can improve your speed a bit with prop tuning, but maybe not.

First it looks like you require about 90 hp to hit 8 kts, your hull speed. A good rule of thumb (oft quoted by yours truly as well as FF on this forum) is that it takes 15 hp/10,000 lbs (or 3 hp per ton) to move a displacement hull at hull speed. Given your weight of 52,000 lbs. (is that loaded?) then that suggests 78 hp.

So your 90 hp is a little high, but well within the range of full displacement passagemakers noted in the Bebe/Leishman book.

I would first get a copy of Dave Gerr's Propeller Handbook. Read it and then you will be able to understand (and challenge) any recommendations given by a prop shop.

But recognize that we are talking about marginal improvements. Your speed target of 7 kts is going to take about 40 hp. Your Volvo probably makes 17 hp per gph at that rate. So your burn target is 2.4 gph. You are currently burning 15% more than average. If you can get back to average that is a saving of 0.3 gph or a little more than a buck an hour. It will take a lot of cruising to pay for a new prop.

And contrary to other advice on this thread, I don't think that you can improve fuel consumption by overpropping or that you should. The 40 hp noted above probably comes at 1,400-1,500 rpm which is right in the middle of the sweet spot for specific fuel consumption. And if you ever need all of your available hp to buck a headwind, you won't have it and may damage your engine by trying.

David

DJ wrote: "I don't think that you can improve fuel consumption by overpropping or that you should."

Unfortunately I have to agree with the horde that lower fuel consumption results w overpropping but I hope I haven't given the impression that I think overpropping is good, recommended or should be done. I've argued against it for years and almost never would recommend overpropping. But as O C Diver says ... there are situations where small advantages can be had by overpropping and many do it.

As to fuel consumption OP (overpropping) will act like an overdrive on a car. The only time a propeller is properly matched to an engine in a boat is at full throttle. OP just brings the mismatch at lower speeds a bit closer to the ideal match at WOT. At lower loads w more engine speed than necessary some energy is wasted moving pistons up and down faster and more often than necessary along w a lot of other rotational and frictional losses. OP reduces that loss. I think the amount of loss is over rated and the limitations it brings about are more far reaching than most think. So my position is still prop to rated rpm.

zeta said:

Back to the discussion of 4 blade vrs 3 or 5......

One part that i realized i have left out is the possible cavitation which results in a hull thumping noise at about 1500 RPM.

This is why Michigan suggested three a three bladed prop and brought up the "dead wood issue".

1200 - 1350 RPM is where the engine seems happiest and most economical.

The max obtained RPM (1925) was just over the max (1900).

The prop is close. What I am looking for is efficiency, no "thumping", and hopefully 7 kts

Click to expand...

So is the thumping sound also at the 1200 to 1350 happy range, and does it go away at higher rpm.

I also recommend on older engine NOT to over prop, and that the rpm and speed are about where they should be. Is it worth the time and money to fine tune?

If the new prop will have sufficient blade area , a 3 blade will improve your fuel burn as the fewer blades the more efficient.

Unfortuniatly sometimes it takes a larger diameter to obtain the required blade area.

A cruising prop would be a fine idea BUT it carries a danger of overloading the engine if an unknowledgible person runs at FLANK.

A cruising prop would require an EGT gauge , and a different red line for the engine.

Go full throttle , back down the Volvo suggested 200RPM , and paint a red line.

You should first look at the Volvo site to see if it can produce the required HP at 1300 .Check with Volvo if you can for the max EGT at your target RPM.

Although engine ratings are published , adding an extra 10% for the gearbox , shaft packing and bearings , as well as most engines are operated with far higher temps than when tested in ideal conditions.

Your rating of 90 HP with about 75 at the prop is fairly normal.

" Is it worth the time and money to fine tune?"

If you want to get rid of the thumping and are willing to spend for a new prop the 3 blade would be a worthwhile experiment .

Weather to have it done as a cruising prop , is up to who uses your boat.

Our prop supplier is high quality,


Ahoy Propellers

www.[B]ahoypropellers[/B].com/
FF

The "thumping" STARTS at about 1500 rpm, but this is not the range that we normally operate the boat at.

Normally we operate at 1200-1350 RPM.

The Volvo engine was built in 2008 and has about 200 hours on it.

All Grand Banks boats were over-propped at the factory. Not a lot, but enough to gain a little more speed at the "normal" operating rpm envelope for the engines used in the 60s and 70s. As such they would not reach maximum rated rpm at WOT, as our boat didn't.

We had our props pitched down 1" and 2" respectively and 1" taken off the diameters. We now reach max rated rpm at WOT. The engines don't work as hard as they used to but we don't go quite as fast for any given rpm. So it's all a big compromise.

Over propping on boats was more common than you think. The performance data for a Willard 40 is listed in the below link. It shows only a .2 knot gain from 2,200 to 2,400 RPM. No big deal until you look above the table and realize the Perkins 6-354 is 130 HP at 2,800 RPM.

Willard 40 Performance


Ted

Okay, this older thread seems to be a good place to ask a propeller question.

I am running a 22 X 16 three blade wheel. I have an opportunity to purchase a 20 X 14 four blade wheel. To my recollection this four blade would almost be an equal in the world of performance, to the three blade. Asked the forum as finding a side by side presentation has not been located.
Regards,
Al-Ketchikan

Hi Al,
You can call Michigan Wheel.
Talk is cheap.

Personally I think those two props may load the same (possibility) but the three blade will be more efficient.

Four blade may be a bit smoother because the prop blade tips will be further from the hull. Tip spilling/wash won’t be slaming the hull as much right above the prop. And the dia being less may keep imbalances down.

Smaller props are better at loading over a broader range of rpm. That’s why you can mount a small OB on just about any kind or even size (within reason) but you pay the price of less efficiency. You seem more interested in how it runs than efficiency so you may like the four blade.

Nomad Willy said:

No it's not the "deadwood" but that dosn't help either.
Should be 1 to 1.5" below and you may have that too.

Click to expand...

the recommended clearance on each side of the propeller (top and bottom) should not be less than 10-15% of the diameter of the prop. Not 1" to 1.5"....

SeaMaster101 said:

the recommended clearance on each side of the propeller (top and bottom) should not be less than 10-15% of the diameter of the prop. Not 1" to 1.5"....

Click to expand...

Agreed, 10 - 15% for slow boats, 15 - 20% for planing hulls. But in some cases, particularly with slow turning props, you can get away with less without any noticeable issues. And then there are tunnels, which often run tighter clearances.

Al said:

Okay, this older thread seems to be a good place to ask a propeller question.

I am running a 22 X 16 three blade wheel. I have an opportunity to purchase a 20 X 14 four blade wheel. To my recollection this four blade would almost be an equal in the world of performance, to the three blade. Asked the forum as finding a side by side presentation has not been located.
Regards,
Al-Ketchikan

Click to expand...

Al,
Why would you want a four blade? The only reason I can recall about this is that a four blade is better only if you need more blade area. 22X16 shows your blade area is about right. With a four blade you would need to have less pitch and that may .. I say may take you out of the most desirable pitch to dia ratio. You’d probably need about 14” of pitch. Probably too much blade area. Blade area is for big engines.
Thanks for the kind words Al.

Vicprop & Michigan Prop web sites will likely give you 2 different answers.
Typically if you add a 4th blade you would drop an inch in dia and an inch in pitch from your 3 blade but there are too many prop and hull configurations to make general conclusions.
I suspect dropping 2 inches in both dia and pitch would result in lower speed. If it is strictly a get home spare then it doesn't matter. If you want to leave it on, then get a price for repitching.

Nomad Willy said:

Al,
Why would you want a four blade? The only reason I can recall about this is that a four blade is better only if you need more blade area. 22X16 shows your blade area is about right. With a four blade you would need to have less pitch and that may .. I say may take you out of the most desirable pitch to dia ratio. You’d probably need about 14” of pitch. Probably too much blade area. Blade area is for big engines.
Thanks for the kind words Al.

Click to expand...

He's looking to go down in diameter, so probably not a big increase in blade area with a 4 blade. I'd think it's probably a little less efficient than a larger diameter 3 blade though, unless the 4 blade is of a better design or has fairly slender blades. So I probably wouldn't do it unless extra hull clearance is needed.

rslifkin,
Thanks for chirp’in up.
He’s been an overproper for many years. A lot of people like the sound of engines at lower engine speeds. Never really got my head around Al’s wants and needs. His boat and engine combo is ripe for overpropping having a bigger engine than most boats (repower) and most boats are plenty big enough.

Yes he could have blade clearance problems As he went from the small Perkins to the largeer 4cyl Perkins. So he could probably use the four blade more gracefully than most. And I think efficiency dosn’t Matter much as he typically goes from Ketchikan to Wrangell Ak. Al’s a special guy and I want the best for him. If he’s having troubles solving those troubles w a fix that looses a bit of fuel effiency would be a very small thing.

A four blade Michigan MP would work well for him .. but would cost $1000. He knows a prop man down near the yacht club. Old prop guy that’s been there forever. He may have an old prop from the days when double end wood fish boats were everywhere. But a 4 blade in his dia could be hard to find. One can cut down a prop blade quite easily but that would make it even less valuable if it didn’t work out. Far less.

Maybe Al will fill us in.

"The only time a propeller is properly matched to an engine in a boat is at full throttle."

This is mostly correct for boats that operate at or near near full throttle most of the time.Sport Fish , Ski Boats

Very few cruisers operate at near full throttle .

A proper selected Cruising prop does carry the danger of an uninformed operator overloading the engine,
but the results of lower RPM, better engine life and loading , less onboard noise seem worth it.

Just don't give the boat keys to your drunken brother in law to pull water skiers..