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Old 07-02-2019, 11:24 AM   #1
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Sea Ray 430 Convertible 1988

I have a Sea Ray 430 Convertible 1988 with 2 CATís 3208 AR#7W 7500 375 HP with Twin Disc 507-2-1 PERF# OT 6428 with Shaft 1,75 inches and 100 inch long with Propeller size 26 by 25

When I run one engine at see 1200 rpm I make around 9 kts and if I run both engines at same rpm and will do around 11 kts so only 2 kts more but burning twice more fuel and else like motor times, wear and tear and so on

Is there a solution to this like doing some type of modification or else?

Thx

Paul (July 2nd, 2019)
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Old 07-02-2019, 11:48 AM   #2
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How did you conclude the burn rate was 2X at 1200rpm for both engines? Floscan or manual calculation?

Burn rate table:

RPM HP GPH
2,800 375 21.1
2,700 337 18.4
2,600 301 16.0
2,500 267 13.9
2,400 236 12.1
2,300 208 10.6
2,200 182 9.3
2,100 158 8.1
2,000 137 7.0
1,900 117 6.1
1,800 100 5.3
1,700 84 4.5
1,600 70 3.8
1,500 58 3.2
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Old 07-02-2019, 02:11 PM   #3
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There's a recent thread on this, titled 'one engine running ' or something similar. Many opinions both for and against. Make sure your model of TD gears will tolerate freewheeling. Your owner's manual or the TD website will tell you. If you have dripless shaft seals, make sure they plumbed to cool the off-side seal.
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Old 07-02-2019, 03:58 PM   #4
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Question Ken

Thanks

What I am mainly looking for is to see if there is something I can change like the prop size or else so I can run both engine at lower rpm and get 10 to 14 kts with this boat I rather use both engines than one

The goal is to reduce the fuel consumption

Paul
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Old 07-02-2019, 04:34 PM   #5
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This is why I bought a single engine boat. No math involved.
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Old 07-02-2019, 06:38 PM   #6
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If you're interested in some prop permutations in pitch, diameter, gear reduction, horsepower, hull shape vs speed and fuel consumption, boatdiesel.com and other sites have prop calculators that let you jockey these variables. I used them when I repowered to get in the ballpark of what props I needed. But, further to your interest in reducing fuel consumption, you will find that there is no silver bullet for props. It takes a certain amount of power, and fuel, to drive a given hull through the water. You can fine-tune the props but your fuel improvement will be limited. Maybe a knowledgeable voice like Ski can elaborate further.
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Old 07-02-2019, 08:19 PM   #7
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Quote:
Originally Posted by Scallops View Post
I have a Sea Ray 430 Convertible 1988 with 2 CATís 3208 AR#7W 7500 375 HP with Twin Disc 507-2-1 PERF# OT 6428 with Shaft 1,75 inches and 100 inch long with Propeller size 26 by 25

When I run one engine at see 1200 rpm I make around 9 kts and if I run both engines at same rpm and will do around 11 kts so only 2 kts more but burning twice more fuel and else like motor times, wear and tear and so on

Is there a solution to this like doing some type of modification or else?

Thx

Paul (July 2nd, 2019)
You are not burning twice as much fuel with 2 engines. When you pull one engine to neutral the other engineís increases itís fuel flow internally to maintain RPM. You will use less fuel because the boat is traveling at 9 kts instead of 11. If you run your boat at 9 kts with 2 engines you will find you use less fuel than if you run the boat a 9 kts with one engine.

A boat your size takes significantly more fuel to run at 11 kts vs 9 kts.
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Old 07-03-2019, 02:48 AM   #8
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Smile

Thanks You for your answer

You is probably right What I did is to compare how much fuel I burn with
one engine at 9 kts and 2 engines at 11 kts for the same distance

What I should have done is to compare how much fuel I will burn for the same distance with one engine and two engine at the same speed and let you know

I will do this trial over the next week or two


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Old 07-03-2019, 05:28 AM   #9
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Paul, are you even on plane at 11kts? Whats your minimum planing speed?
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Old 07-03-2019, 09:10 AM   #10
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I am not sure Jason if I am at even on plane and also I am not sure of my minimum planing speed I will try to check it today
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Old 07-03-2019, 09:33 AM   #11
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I am not sure Jason if I am at even on plane and also I am not sure of my minimum planing speed I will try to check it today
Okay, well thats important. Planing hulls (like ours) are most happy at either hull speed or planing speeds. If you're cruising in between you'll have a huge wake behind you and the bow will be high up. Very poor fuel economy.
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Old 07-03-2019, 05:49 PM   #12
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Okay, well thats important. Planing hulls (like ours) are most happy at either hull speed or planing speeds. If you're cruising in between you'll have a huge wake behind you and the bow will be high up. Very poor fuel economy.
Yep, we call that no manís land.
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Old 07-20-2019, 05:09 AM   #13
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Sea Ray

Thx I will try to find Hull SPEED
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Old 07-20-2019, 05:21 AM   #14
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Hull speed has both a theoretical and a practical definition. The practical one is when you are going just fast enough to keep up with your bow wave. Any faster requires you to climb over it burning lots of fuel and hp. That is what happens to you with two engines at 1,200 rpm.


Theoretical hull speed is 1.34*sqrt(LWL) or in your case about 8.5 kts. You will probably hit it with two engines running 1,000 rpm. BTW that rpm is about as low as I would want to run a diesel and still keep it up to operating temps to limit sooting.


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Old 07-20-2019, 05:28 AM   #15
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Thx DJ

What ďLWLĒ stand for?

Paul
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Old 07-20-2019, 06:56 AM   #16
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LWL is the length of the waterline. Another term youíll hear is LOA, which is length overall (tip to tip).
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