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Old 12-11-2018, 10:48 AM   #1
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Performance formulas

Hi,

Is there a formula or calculator that can determine how much additional weight affects the speed? For planning boats.

Let's say, your 24000 lb cruiser will cruise at 24 knots. And you add 2000 lbs, how much less will the speed be, all other things being equal?

There's tons of charts and performance stuff like this for airplanes, would think there'd be a lot for boats, but a search turns up little.
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Old 12-11-2018, 11:20 AM   #2
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Boatdiesel has a prop calculator that will show speed effects of additional weight. You have to join though and it costs $25/yr for a basic membership. If you are a DIY diesel guy it is well worth it.



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Old 12-11-2018, 11:55 AM   #3
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Boatdiesel has a prop calculator that will show speed effects of additional weight. You have to join though and it costs $25/yr for a basic membership. If you are a DIY diesel guy it is well worth it.



David
David,

Yea, Im a member, will look for that....
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Old 12-11-2018, 12:54 PM   #4
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OK, you are a member. Go to the basic prop calculator. The advanced one takes a higher class of membership and is a PITA to use. Then enter the data for your boat and engine: weight, length, engine hp, etc. See what it gives you for wot speed. Then compare it to actual if you have that. Then adjust boat type or weight so it comes close to actual.


Then once you have it "calibrated" then you can add or subtract weight and see what happens. I have found that once calibrated to your specific boat, the calculator is reasonably accurate.



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Old 12-11-2018, 01:49 PM   #5
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The formulas you are looking for and more are in the propeller handbook by David Gerr.
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Old 12-11-2018, 02:11 PM   #6
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Hi,

Is there a formula or calculator that can determine how much additional weight affects the speed? For planning boats.

Let's say, your 24000 lb cruiser will cruise at 24 knots. And you add 2000 lbs, how much less will the speed be, all other things being equal?

There's tons of charts and performance stuff like this for airplanes, would think there'd be a lot for boats, but a search turns up little.
Years back I played with the various formulas and did not get too much satisfaction.
Our boat could easily vary from a full boat with many folks to an almost empty boat with just a few - maybe #5,000 difference.
From one extreme to the other our speeds would vary by less than 1/2 a knot at 17 knots or so. Items which would affect speed much more than the weight carried included: clean hull , clean running gear, balanced props, and fresh vs salt water.
Another practical note we towed various large RIBS over the years with one that weighed in at just over 1,200 #'s and another that weighed in at over #3,200 #'s.
Pulling these RIBS between 16 - 18 knots with a boat that was 48" overall and weighed around 33,000 #'s we saw these results....
- 19' RIB @ 1,200'#s , no noticeable difference in speed or fuel burn at 17 kts
- 24' RIB @ 3,200 #'s , about 1/2 knot or less at 17 knots
The best thing about the RIBS comparison is that often we would 'drop' the RIB off part way into the journey so we had a real time comparison between tow and non towed performance.
Hope this helps
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Old 12-11-2018, 06:45 PM   #7
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I have run a charter boat for the last 20 years that planes at 15 knots. The boat carries scuba divers in the extreme. At the maximum were talking 500 to 600 pounds per person with all their gear, time 6 people. Between running the boat empty and full, MPG can vary from 1.9 to 1.1. I've often tried to plot fuel increase relative to weight, and found it still varies beyond what I predict. Additionally, as your vessel gets more loaded, sea state and travel relative to the waves has a bigger impact. For my boat running with 2' seas burns more fuel than running into 2' seas, maybe has to do with less water touching the hull.

Imo, there are far to many variables for the simplistic computer program on the Boat Diesel site to have any real value. One only has to consider the length, beam, and angle of the wetted hull surface when on plane, to realize changing anyone of those could have a meaningful impact on fuel burn.

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Old 12-11-2018, 08:02 PM   #8
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No need to worry about the prop and all that. Google the words "Crouch formula" and run the math, it is very simple and surprisingly accurate. Start with something you know to check and "calibrate" the formula and then play with your weights otsee how it affects the speed.
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Old 12-11-2018, 08:19 PM   #9
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Imo, there are far to many variables for the simplistic computer program on the Boat Diesel site to have any real value. One only has to consider the length, beam, and angle of the wetted hull surface when on plane, to realize changing anyone of those could have a meaningful impact on fuel burn.
Ted

Have you used the boatdiesel calculator? It is much more sophisticated than you allege and more sophisticated than Crouch's formula. Once you dial in your boat, it is reasonably accurate at predicting changes in speed due to weight increases.


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Old 12-12-2018, 04:08 PM   #10
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The only real way to find out, is to run the measured mile, in both directions using a stop watch. Predicted Log competitors run the measured mile before every contest to ascertain what RPMs are needed to attain their predicted speed.
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Old 12-13-2018, 07:23 AM   #11
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If all you want to know is how a bigger noisemaker or a thousand pounds of batts will slow the boat an experiment is easy.

Get some garbage cans and fill them with fresh water till you have the added weight.

Then go for a ride with a GPS. Log any differences at various RPM.
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Old 12-13-2018, 08:50 AM   #12
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David,

Yea, Im a member, will look for that....

How did that work out?


David
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