View Full Version : Optimum trim angle calculation
For all of you that might be interested.
Here is the mathematical way to figure out the optimum trim angle for any boat at any speed ( planning boat that is).
Please excuse any format and grammar mistakes. I didn't have much time to put this together.
I used a 18 foot donzi at 65 mph as my test boat. A few numbers I had to estimate and I am sure with a more accurate numbers I could get a better answer. Any how...
http://www.allardmarine.com/Trim%20Angle.htm
If your head hurts after looking at this it is ok...mine does too...
Chris
Tom Davis
10-30-2002, 12:16 PM
Chris, nice work! Try using math-cad next time, your head won't hurt as much.... I know the feeling. wink
Tom Davis
(Winni Bandits)
Ranman
10-30-2002, 12:35 PM
Well... It's obvious I'm not going to be a Naval Architect anytime soon! Guess I'd better get back to buying stuff.
Tom,
That is MATHcad :) :)
Chris
Woodsy
10-30-2002, 01:08 PM
Chris...
WHAT IS WRONG WITH YOU?? You are doing "recreational math"?? Nobody, and I mean nobody does quadratic equations for fun....
Good attempt at figuring it out in a mathematical sentence. Just think how complex the equation would have to be when you add in the variables of outdrive drag, chines, running pad surface, etc.... my head hurts just thinking about it.
Woodsy Von Donzi
Woodsy,
My head hurts working on it...all in due time...
Chris
Ok I admit I did math equations for fun...just don't call me normal :D
You are a man with toooo damn much time on your hands!! You can calculate all you want, but it will drive you nuts with too too many variables. No such thing a optimal! We need you for more important stuff, after all how many guys with Donzi's will have you really good formal knowledge. Keep up the good work at school!
mattyboy
10-30-2002, 02:53 PM
wow there's 10 minutes I'll never get back :p wink :D
now I know why I started drinking in High school eek!
then I said to my self, self you have a volvo 250
with trim
peg in up, middle or down
you do the math wink :D
only kidding Chris, go get em someday my grandkids might be studying the Allard theorum!!!
Matt
iI thought I would never need math again????
Hull #18-83
10-30-2002, 03:42 PM
Where is Ted Kazynski when you need him? That guy loves recreational math (and bomb building)!
Darrell
10-30-2002, 04:05 PM
I don't think I'll be using your formula, the only formula I want to see is the one I pass on the lake. However you do make me wish I work a little harder in college.
Darrell
Donzi Di
10-30-2002, 04:49 PM
Chris,
Okay so a scary thought - I was looking at the calculations that you were doing.
What other values did you use for the initial angles? Were they all whole numbers or did you break it out further?
Also, after you calculate the flat plate equivalent resistance, you use a value of 2 (I cannot replicate the symbol). Where did you get the 2 to start with?
Diana
BigGrizzly
10-30-2002, 05:43 PM
Does it work on both rocker and straight hulls and Arneson drives as well.
Di,
Good questions... I think the symbol you are referring to is the "lambda". We choose that value of 2 based on prior experience as a semi arbitrary guess. Since we are using the root solver function in that program we need a base “guess” for the program to start. Any number would work if it was reasonably close. If I used 3 there it wouldn’t change anything. If I used 43,000 it would mess stuff up a lot. The basis for the 2 guess comes from past experience. We generally know that lambda will be in the single digit range so something in that order of magnitude would work as our initial guess.
With regards to the angle accuracy yes it is very common to go a lot farther then whole numbers. I just wanted to keep it simple. For example my last HW assignment wound up to be 4.162. All the significant digits are great but I mean in reality who can trim a boat with that much accuracy…so there is no point.
Griz,
Good question on the rocker hulls. I think what you would need to do though would be to find the average angle of the rocker in the hull and add it to the number that you got from this data. This would be assuming that all of the wetted surface running in the water was part of the rocker and it wouldn’t take into account the “curve” of the rocker but I think it could give a good approximation.
With regards to the Arneson yes it would work. All it would change would be the variable in the beginning of the problem that takes into account the distance of the prop shaft below the vertical CG.
You must keep in mind though this formula system does not take into account the drag created, and therefore the lift created, by the drive ( regardless of what kind it is). This would significantly change the problem but as I said this was more of a learning thing for everyone as opposed to a definite way for you to calculate your optimum trim angle. In the future I will work up a sheet that does factor in the drive and running strakes.
Chris Allard
And in reality this didn’t take that much time. It was just a HW problem I adapted to a relevant craft. Hope you guys at least got something out of it.
harbormaster
10-30-2002, 06:39 PM
I just trim it up till it stops accelerating.
Doesn't that work too?
Chris,
I tried your way but had a hard time holding the steering wheel and working the calculator at the same time....
:D
roadtrip se
10-30-2002, 08:42 PM
Chris,
What happens if your trim gauge isn't accurate like ours?
Jill
Not drinking with you any more, because you might be killing valuable brain cells!
Guys guys guys...of course you aren't going to actually do this...in the voice from god :
Learning experience...not practical...learning experience....
Jill I bet your trim gauge works better then mine... wink Still trying to figure out what gauge goes to make room for one...
Madpoodle,
While I am still learning and at this point it is beyond me to mathematically prove this I can tell you from basic theory I would have to say that the drag caused by the drive could actually be MORE then the drag caused by the actual hull. that big anvil hanging off the back of the boat spinning that 15 inch water disrupter is a huge percentage of the drag in a Donzi style boat. So lets just assume that the drag from the drive is equal to the drag from the boat. That would put us at what 306 hp? Considering that this is actual delivered hp to the water after loses in the drive, transmission, prop et. al and remembering that a lot of this is calculated using estimations it sort of sounds reasonable to me.
The bravo isn't the problem but everytime I punch in the TRS numbers my calculator starts smoking and gives me error messages...
Chris
Need a sea drouge for that calc....
Donzi Jenn
10-31-2002, 10:53 AM
Chris,
You are a sick, sick boy! :p :p :p
Digger
10-31-2002, 11:19 AM
Hey Chris you get my email?
HyperDonzi
11-01-2002, 03:28 PM
If I had to do something like this, I would open it up, trim around to where speed is the best then drive it onto the trailer. then get out, get a protractor....
My eyes still hurt.
Good work
Tommy
clayman
11-01-2002, 03:36 PM
Chris, chris , chris, I only gave it a quick glance and quickly remembered how much I hated math classes. Good thing I am an artist, I'll design it, you make it work.
Good Job,
James
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