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Old 09-06-2012, 12:21 AM   #1
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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.

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Old 09-06-2012, 12:48 AM   #2
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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.
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Old 09-06-2012, 01:19 AM   #3
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Quote:
Originally Posted by zeta View Post
Not obtaining anticipated SOG for given RPM.
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.



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Old 09-06-2012, 06:38 AM   #4
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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.
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Old 09-06-2012, 11:03 AM   #5
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Try boatdiesel.com calculator. You can put in the max. diameter your can use and go from there.
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Old 09-06-2012, 11:52 AM   #6
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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
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Old 09-06-2012, 11:53 AM   #7
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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?
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Old 09-06-2012, 12:03 PM   #8
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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.
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Old 09-06-2012, 12:14 PM   #9
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Quote:
Originally Posted by zeta View Post
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


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.
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Old 09-06-2012, 01:57 PM   #10
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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.
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Old 09-06-2012, 03:22 PM   #11
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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
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Old 09-06-2012, 06:29 PM   #12
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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.
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Old 09-06-2012, 06:41 PM   #13
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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.

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Old 09-06-2012, 07:17 PM   #14
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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.

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Old 09-07-2012, 09:01 AM   #15
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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.
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Old 09-07-2012, 10:14 AM   #16
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Quote:
Originally Posted by zeta View Post
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


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?
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Old 09-07-2012, 10:50 AM   #17
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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.ahoypropellers.com/
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Old 09-07-2012, 03:58 PM   #18
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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.
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Old 09-07-2012, 06:11 PM   #19
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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.
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Old 09-07-2012, 09:31 PM   #20
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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


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