The benefits of proper trim

Report the performance of your boat, supply as much information as possible (weight, HP, prop, speed, etc.).
Subject: Make the Glen-L design name the subject. If the design already has a posting, add your information to that post.

Moderator: Bill Edmundson

Post Reply
Denon Osterman
Posts: 556
Joined: Thu Jun 22, 2006 8:58 am
Location: toronto, CAN

The benefits of proper trim

Post by Denon Osterman »

After watching a video of someone driving their boat for the first time, with the trim fully "down", I decided to see just how much of a difference trim can make. I'm a strong advocate of always "trimming up" once on plane, and increasing the amount of "trim up" as the RPM and speed of the boat climb. The caveat, of course, is that one should always trim down when required as well...when slowing, cornering hard, etc.

Anyways, as a totally unscientific study, I took out my aluminium work boat (18' Thunderjet with an Evinrude 135 HO) and did a quick comparison. Here are the numbers:

- no gauge, but likely ~3/4 of the way "up", relative to the max the engine will let me while under speed (it will not let you start "trailering" the motor at speed).
-Exactly 5000 rpm - close to WOT, but not quite (it will do just over 5500 flat out, at which point I trim fully up as well)
-GPS measured 69 km/h (43 mph) fairly consistently

-without touching the throttle or steering wheel, I just held down the trim switch until the engine bottomed out.
-Just under 4500 RPM - at the same throttle!
-GPS measured 58 km/h (36 mph) fairly consistently.

Fuel consumption is a bit trickier as I don't have a guage to measure that, but bear with me - the most the fuel flow can change is the percentage rpm change, as the throttle was the same - the least it can change is not at all, since the throttle was the same. If it didn't change at all than fuel economy is directly related to speed changes - in this case 17%. If it is related to RPM, than the 10% decrease in RPM somewhat counters the 17% loss in speed, for an overall change of 7%

By trimming up, my engine gains a full 500 rpm - good for a 17% (!!!) increase in speed, and a 7-17% increase in fuel economy / range. YMMV, no pun intended :wink:


User avatar
Posts: 800
Joined: Thu Jul 25, 2013 1:07 pm
Location: Sacramento, CA

Re: The benefits of proper trim

Post by Jimbob »

You will feel the proper trim at any speed using this technique.
At speed, bring the trim "up" which will cause the bow of the boat to rise. Bring up the trim up slightly until the boat starts to porpoise, from there, move the trim back down slightly until the porpoise stops. You will feel the boat lifting free of the water when on plane, with less drag. You really don't need a trim guage, just start out with the trim down, trim up at speed, and then bring it back down as you slow down.

Jim Neeley
Building a Barrelback in Sacramento, CA

Denon Osterman
Posts: 556
Joined: Thu Jun 22, 2006 8:58 am
Location: toronto, CAN

Re: The benefits of proper trim

Post by Denon Osterman »

I agree fully. Between the sound of the engine (boy does it sound angry trimmed all the way down) and the feel of the boat and steering, you could be blindfolded and still trim properly in most cases.

User avatar
Bill Edmundson
Posts: 11898
Joined: Wed Nov 23, 2005 6:45 am
Location: Birmingham, AL, USA

Re: The benefits of proper trim

Post by Bill Edmundson »

I trim by sound!

Mini -Tug, KH Tahoe 19 & Bartender 24 - There can be no miracle recoveries without first screwing up.
Tahoe 19 Build

Posts: 128
Joined: Tue Aug 09, 2016 3:22 pm
Location: OR

Re: The benefits of proper trim

Post by TAB »

On my stoker ( mod vp tunnel)3/4" on the jack plate is good for 7 mph.( 84 to 91) It also makes the boat go from predictable too you really don't want too dump the air.

I always found 2hen striving for that last little bit of performance, having some one that can run next too you with a camera can really let you see when you have the least amount of wetter surface.

User avatar
Posts: 4799
Joined: Wed Nov 12, 2003 5:25 pm
Location: Coastal Georgia

Re: The benefits of proper trim

Post by kens »

here you go,,, here is proper trim,,,,,, the least wetted surface,,,,,,,,,
Oak is over rated, everything about it takes extra time; then it warps, splits or checks !!! :roll:

Posts: 2441
Joined: Thu Nov 10, 2011 8:48 am
Location: Lafayette, IN

Re: The benefits of proper trim

Post by hoodman »

Lanyard and life vest may have saved his life but what nearly killed him was idiotic driving.

Building a Geronimo......!

Post Reply

Return to “Speed and Performance”