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Old 07-31-2012, 02:39 PM   #81
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I just figured that the boat moves thru the water at a given speed per RPM regardless of what the water is doing. If the current is doing 2 knts with me across me or against me I am still moving thru the water at the same speed. Not Speed over ground.
Looking for that hull speed thru the water with enough load to keep the Cat happy
and the wallet if it matters.
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Old 07-31-2012, 03:49 PM   #82
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I just figured that the boat moves thru the water at a given speed per RPM regardless of what the water is doing.
That's what I've been trying to tell you.
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Old 07-31-2012, 03:58 PM   #83
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Originally Posted by skipperdude View Post
I just figured that the boat moves thru the water at a given speed per RPM regardless of what the water is doing. If the current is doing 2 knts with me across me or against me I am still moving thru the water at the same speed. Not Speed over ground.
Looking for that hull speed thru the water with enough load to keep the Cat happy
and the wallet if it matters.
SD
There are a few other factors such as momentum and chop but for the most past you have it now....just as a lot of people have suggested.

The real question for you is how fast? I hate the thought of going to Florida/Bahamas every year at 6-6.5 knots even though that's a great economy speed. Will going at 8+ knots (a 25% or so increase in speed) kill me if I spend another 50-100% in fuel? Not really as should another $3000 per year of boating when all of boating costs nearly $20,000 per year determine how fast I get someplace (assuming I steam 3200 miles per year)???

Just depends on whether I have those extra days to get where I want to be.
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Old 07-31-2012, 04:05 PM   #84
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Call me Homer Simpson.
doouh!!
I just wanted to know what thet RPM would be.

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Old 08-02-2012, 01:17 AM   #85
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Some real data for discussion

I thought I'd through some hard data into the conversation. What you'll see from the data is that there is no optimum speed. Going faster means less economy.

The forum doesn't support tables so it's going to probably look poorly formatted. MPH and MPG are nautical miles.

34' Displacement Hull
LWL 30'2"
1.4 * sqrt(LWL) = 7.7
1.1 * sqrt(LWL) = 6
Engine peak torque 1800 RPM

RPM MPH - GPH - MPG
1000 4.5 - 0.5 - 10
1200 5.1 - 0.7 - 7.8
1400 6.0 - 1.1 - 5.6
1600 6.8 - 1.6 - 4.4
1800 7.4 - 2.2 - 3.5
2000 7.8 - 3.3 - 2.4
2200 8.1 - 4.5 - 1.8
2400 8.6 - 6.1 - 1.4
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Old 08-02-2012, 09:06 AM   #86
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That is a great chart, what's it mean? I have a 34' mainship with the 200 (or 240 according to the engine manuals) Perkins and at 1400 I'm popping along at a solid 7+ kts.
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Old 08-02-2012, 09:14 AM   #87
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That is a great chart, what's it mean? I have a 34' mainship with the 200 (or 240 according to the engine manuals) Perkins and at 1400 I'm popping along at a solid 7+ kts.
it's an example of his boat's performance (I think)

it has nothing to do with your boat (I think)

it just shows that the basic trawler hull design (either full or semidisplacement) run at displacement speeds has a nice gradual speed/fuel consumption till you hit that number somewhere's
around the square root of the waterline length where fuel consumption starts to climb more rapidly than speed as power is applied. (I know)

sorta what some posters have been saying all along....
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Old 08-02-2012, 09:22 AM   #88
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The real question for you is how fast? I hate the thought of going to Florida/Bahamas every year at 6-6.5 knots even though that's a great economy speed.
Me too! My avatar picture is Moonstruck cruising in a "sweet spot" slightly above hull speed.
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Old 08-02-2012, 09:54 AM   #89
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it just shows that the basic trawler hull design (either full or semidisplacement) run at displacement speeds has a nice gradual speed/fuel consumption till you hit that number somewhere's
around the square root of the waterline length where fuel consumption starts to climb more rapidly than speed as power is applied. (I know)
Gotcha, I thought it was some kinda one size fits all this is what your boat should be doing with "x" parameters. And mine was no where near those numbers.

I've been boating my whole life and still can't understand why people like to run big boats with the bow pointed to the sky! On my particular boat I lose around .5 kt when I hit that speed where the stern drops and it takes roughly 100 more rpm (that requires quite a bit more throttle to achieve) to get the speed to begin to climb again. Having a full keel my boat never truly seems to "break free" where you can pull the throttle back and cruise on plane (my Shamrocks were the same way), too much wetted surface area. Our shrimping skiffs essentially the same hull as my Mainship without a keel and a foot more beam would break over and cruise even with large prop tunnels.
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Old 08-02-2012, 09:59 AM   #90
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Corrected numbers

I copied the wrong numbers for the speed. Here are the correct numbers with the average speed, rather than the speed of a run in one direction.

34' Displacement Hull
LWL 30'2"
1.4 * sqrt(LWL) = 7.7
1.1 * sqrt(LWL) = 6
Engine peak torque 1800 RPM

RPM MPH - GPH - MPG
1000 5.0 - 0.5 - 10
1200 5.5 - 0.7 - 7.8
1400 6.2 - 1.1 - 5.6
1600 7.1 - 1.6 - 4.4
1800 7.6 - 2.2 - 3.5
2000 8.0 - 3.3 - 2.4
2200 8.3 - 4.5 - 1.8
2400 8.7 - 6.1 - 1.4
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Old 08-02-2012, 02:13 PM   #91
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34' Displacement Hull

the hull pictured does not look like a displacement style hull.

Sure its cheap at low speeds , most boats are.

But the transom looks far to wide for a pure displacement boat.

Of course you get a nice fine useable aft deck , perhaps 2 -3 ft narrower would never be as comfortable.

The difference would only be 15% so at 1800 rpm ,, 15% of 2.2 gph would not save a big amount of bucks per year.
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Old 08-02-2012, 03:31 PM   #92
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Displacement shape

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Originally Posted by FF View Post
34' Displacement Hull

the hull pictured does not look like a displacement style hull.

Sure its cheap at low speeds , most boats are.

But the transom looks far to wide for a pure displacement boat.

Of course you get a nice fine useable aft deck , perhaps 2 -3 ft narrower would never be as comfortable.

The difference would only be 15% so at 1800 rpm ,, 15% of 2.2 gph would not save a big amount of bucks per year.
I guess I consider the DeFever to be a full displacement boat because of the deadrise. As you can see, there's not a lot of transom below the waterline, and nothing to provide lift.

My mileage numbers are generally a bit better than what I listed - these numbers were taken with a full load of fuel and mostly full water tanks.

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