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Old 05-08-2016, 07:27 AM   #1
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Engine power needed to drive a 22 by 20 prop?

Anyone know, how much HP and torque required to drive a 22 by 20 prop?
trans is velvet drive 2.57 to 1 gear reduction., shafts are 1 3/8








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I have some issues apparently with the boat speed. I found out my boat from OEM factory fresh was designed to cruise at 20 knots with max speed of 25. And I can only do about 8 or 9 knots max speed. So was surprised that it is supposed to go much faster.
This is a 37 foot egg harbor sedan cruiser from 1970.
A friend bought an old brochure since he also has similar egg harbor boat. And with no trim tabs listed as an option. I do have 42 by 9 tabs to put back on at next haulout which I will mod to 42 by 12.
So I was wondering if these props which have been on here since I got the boat are really too big?
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Old 05-08-2016, 07:43 AM   #2
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https://www.vicprop.com/displacement...tion=calculate

according to this calculator using HP at 260, hull weight 17,000 lbs, hull form draft 2 feet, speed 20 knots, prop sizes are coming up 23 by 20.
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Old 05-08-2016, 07:44 AM   #3
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It's more dependent on the boat. For a bigger and heavier the boat, more HP will be required.

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Old 05-08-2016, 07:46 AM   #4
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The Propeller Hand Book by Dave Gerr.

By the way, your jam nuts are in the wrong position.
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Old 05-08-2016, 08:03 AM   #5
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Quote:
Originally Posted by boatpoker View Post
The Propeller Hand Book by Dave Gerr, free on line.

By the way, your jam nuts are in the wrong position.
I know, been like that from day one for me. I have read, you torque tight with the big nut, then unscrew it and put little nut first, then the big nut is torqued on which unloads the little nuts threads.
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Old 05-08-2016, 08:05 AM   #6
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That is a somewhat bassackwards way of asking the question. Usually you would say "I have x hp engines at y rpm, how fast can I go?". But I can live with the way you asked it.


You will need roughly two 350 hp engines running at 3,000 rpm to push a 37', 20,000 lb (guess) slow planning hull (as defined by boatdiesel's prop calculator) to a top speed of 25 kts.


To hit 9 kts at wot you will need roughly two engines producing 50 hp each at 2,200 rpm.


I assume that you have much closer to a pair of 350 hp engines than 50 hp engines, so something is terribly wrong and it isn't the props. The props are roughly what you would expect for twin 350 hp engines.


So from the data you describe those big engines won't turn more than 2,200 rpm and that means you are probably getting huge amounts of black smoke out. Also you are really hurting those engines by running them that way.


My guess is that the bottom and props are really fouled, but I see that your pictures don't show that. Then if the prop and bottom are like the picture, then other possibilities are that the engine is severely restricted in its exhaust or its intake like a plugged exhaust mixer or an extremely fouled air cooler or a stuck turbo.


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Old 05-08-2016, 08:13 AM   #7
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Old 05-08-2016, 08:34 AM   #8
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Quote:
Originally Posted by djmarchand View Post
That is a somewhat bassackwards way of asking the question. Usually you would say "I have x hp engines at y rpm, how fast can I go?". But I can live with the way you asked it.


You will need roughly two 350 hp engines running at 3,000 rpm to push a 37', 20,000 lb (guess) slow planning hull (as defined by boatdiesel's prop calculator) to a top speed of 25 kts.


To hit 9 kts at wot you will need roughly two engines producing 50 hp each at 2,200 rpm.


I assume that you have much closer to a pair of 350 hp engines than 50 hp engines, so something is terribly wrong and it isn't the props. The props are roughly what you would expect for twin 350 hp engines.


So from the data you describe those big engines won't turn more than 2,200 rpm and that means you are probably getting huge amounts of black smoke out. Also you are really hurting those engines by running them that way.


My guess is that the bottom and props are really fouled, but I see that your pictures don't show that. Then if the prop and bottom are like the picture, then other possibilities are that the engine is severely restricted in its exhaust or its intake like a plugged exhaust mixer or an extremely fouled air cooler or a stuck turbo.


David
Thanks for looking up that info.
Pics are from 2014, and the barnacles are back in 2016.
Engines are OEM IH 392 with 260HP, gas not diesel.
Port engine will push to 3000 rpm, other to 2500 rpm and only 8.5 knot.

I am not too surprised at a 350 hp rating, but these engines factory were only 260 hp.
The calculator I linked suggested 500 hp for 18 knots.


Data Input

Waterline length in feet: 35 feet
Beam at the waterline in feet: 12 feet
Hull draft in feet (excluding keel): 3 feet
Vessel weight in pounds: 20000 lbs
Engine Horsepower: 260 HP
Number of engines: 2
Total Engine Horsepower: 520 HP

Engine R.P.M. (max): 4000 RPM
Gear Ratio: 2.57:1
Shaft R.P.M. (max): 1556 RPM

Number of shaft bearings (per shaft): 1
Desired speed in Knots: 25 knots
Horsepower Calculations

This will calculate the maximum horsepower and torque available at the prop(s).

Total available horsepower at the engine(s): 520 HP
Total available torque ft/lbs at the engine(s): 683 ft/lbs
Horsepower loss of 3% per gearbox: - 15.6 HP
Horsepower loss of 1.5% per shaft bearing: - 7.8 HP

Total horsepower available at the propeller(s): 496.6 HP
Total torque ft/lbs available at the propeller(s): 1676 ft/lbs
Speed & Power Calculations

Basic displacement speed and horsepower required
Displacement hull speed (1.34 X sqrt of waterline length): 7.93 Knots
Minimum horsepower required at propeller(s) for Hull speed: 43.6 HP

Calculations based on desired speed and available HP
HP required at propeller(s) for desired 25 knots speed: 1244 HP
Estimated maximum speed with existing 520 horsepower:
This is the speed we will use for the propeller size. 18.36 Knots

At this point it is important to note that all of the calculations above are based on full RPM and HP. Most engines are rated to run at a percentage of thier full RPM. This is what will determine your maximum cruising speed. The propeller sizing calculations below are based on 90% of full RPM. This gives the engine some reserve power to allow for variable loading in the vessel.

Propeller Size

Number of blades Diameter (inches) Pitch (inches)
2 Blade 24.5 X 19.5
3 Blade 23.3 X 19.3
4 Blade 21.9 X 18.9

The propeller sizes shown above do not contain calculations for cavitation or blade loading.
If you find that the recommended propeller is too large to fit your vessel, you can try increasing the shaft speed. Failing this, you can reduce the diameter and increase the pitch at the expense of your propeller efficiency. The rule of thumb is 1 inch of diameter is equal to 1 1/2 to 2 inches of pitch.

Website and all contents copyright Victoria Propeller Ltd. 2015, all rights reserved.
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Old 05-08-2016, 10:00 AM   #9
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In addition to having a fouled prop and bottom, I suspect that your other problem is not enough torque from those gassers to get over the hump. You can help this problem by taking some pitch out of your prop so the engines will speed up a bit. Maybe once you get over the hump they will let you cruise in the mid teens with a clean bottom and 2" less pitch, but I doubt you will see an 18 kt cruise and 25 kt top.


And gassers won't smoke overloaded like a diesel, but overloading is still very bad for them.


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Old 05-08-2016, 10:15 AM   #10
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D Marchand has given you a few salient starting points. Hull weight and gearbox ratio need to be confirmed. A good running set of 300 + HP gassers would be required to get it over the hump and then throttling back to 3200 RPM or so for cruise speed. In that era 460 Fords or 383 -440 Crusaders were the engines of choice.

I had a 36 Trojan with 383s and it was a good running boat. At 1800 RPM it would do about 8 - 9 knots. At 22 knots my best recollection is 3400 RPM. Getting it up was a 4000+ RPM effort.
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Old 05-08-2016, 10:28 AM   #11
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Sounds like this is a new to you boat and you don't know if it ever went faster.


The above information is correct but irrelevant if you have fouled props and bottom. Clearly the RPM tells the story that something is preventing the engines and boat from speeding up. There are may possibilities but the first thing to do is get a haul out and good bottom cleaning. It wont be terribly expensive if you don't paint just clean. Put it back in the water and see what happens and report the results.
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Old 05-08-2016, 11:22 AM   #12
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Trim tabs.
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Old 05-08-2016, 11:47 AM   #13
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Quote:
Originally Posted by semi-planing View Post
Trim tabs.
yes, trim tabs I took off the boat, they needed some welding.
I have owned boat since 1998, but I never push past hull speed being a trawler type of boater.

Many years ago, right after a haulout and bottom paint, the boat did 18 knots according to the speed wheel at WOT. And I recalled would do an easy 12 knots, with trim tabs on the boat.
I remember seeing the transom break free of the water completely, can not do that anymore.
I would never need to even make the tabs go down.
So maybe trim tabs make a whole lot of difference?

And no doubt the bottom is loaded with barnacles again. It used to be the TBT paints were great at keeping the boat clean even with little attention or use. One haul out bottom after 4 years had very little growth. Now after 2 years in Chesapeake Bay Back river, barnacles everywhere. I also felt a vibration starboard shaft which is likely barnacles on the prop.
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Old 05-08-2016, 12:22 PM   #14
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Quote:
Originally Posted by sdowney717 View Post
yes, trim tabs I took off the boat, they needed some welding.
I have owned boat since 1998, but I never push past hull speed being a trawler type of boater.

Many years ago, right after a haulout and bottom paint, the boat did 18 knots according to the speed wheel at WOT. And I recalled would do an easy 12 knots, with trim tabs on the boat.
I remember seeing the transom break free of the water completely, can not do that anymore.
I would never need to even make the tabs go down.
So maybe trim tabs make a whole lot of difference?
Our OA 440, semi-displacement hull, 30,000+, twin 250 diesels, 2:1 gears 24x20 props climbs out of the hole in fairly short order with 250-275 hp total (per the prop chart) with full down (large) tabs. No tabs takes in the range of 350-400 hp and a fairly extended period of bow up struggling against the bow wave. A bit of tab (for this configuration) also net an extra knot @ full power once on semi-plane (17 knots max). The boat has a fairly heavy dink/4- stroke hanging out the back and an aft main fuel tank. But even without the dink and at low fuel, I use the tabs to get over the hump.

I normally operate the boat at hull speed plus (~8.5 knots), zero tabs, unless the wave action demands some speed to improve the ride. And if it's just a little chop, the tabs sometimes help dampen roll.
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Old 05-08-2016, 12:45 PM   #15
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Get the bottom and wheels cleaned first. Barnacles make a huge effect on drag.
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Old 05-08-2016, 01:00 PM   #16
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eg. I had a 9' aluminum hull RIB with an 8hp Suzuki. It would do 28mph. If I left it in the water for a couple of weeks it developed a thick slime on the bottom and would not even get up on plane.
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