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Author Topic:   Procedure for trimming
Rocks posted 04-06-2004 01:05 PM ET (US)   Profile for Rocks   Send Email to Rocks  
Does anyone out there know if there is a way of determining the best cruise rpm/speed/trim for outboard pwertrimmed Whalers? Is it just trial and error, do you pick a speed then trim out and reduce power, maybe I missed something but I've never read anything on the best way to approach it.
Salmon Tub posted 04-06-2004 01:10 PM ET (US)     Profile for Salmon Tub  Send Email to Salmon Tub     
Rocks, it depends more on the particular situation than anything else. If you are running in a small static body of water such as most lakes, then the best approach is trial and error at different speeds, recording performance. In bodies of water that can vary in surface conditions, then you will need to use the whole range of trim depending on if you are heading into the seas, with the seas... In these conditions, normally you stay trimmed in when heading into seas, and trimmed out when going with the seas.
Moe posted 04-06-2004 01:37 PM ET (US)     Profile for Moe  Send Email to Moe     
It certainly varies from boat to boat, and even the same boat loaded differently, but best fuel mileage cruise speed is often 200-300 rpm over minimum planing speed.

As the boat gets on plane, I start throttling back and trimming out at the same time. Best trim angle for performance and economy will result in the highest engine rpm for a given throttle setting (i.e. the boat is putting the least load on the engine). You can watch the tach, but eventually get "an ear" for engine speed. Trim up until rpm no longer increases when you do. You may have to throttle back if that puts you higher in rpm than you wanted. If I go past optimum trim on mine, rpm begins to drop again. On some boats, you can't get that far because of porpoising or prop blow-out.

As mentioned, the state of seas sometimes overrides trim for best performance/economy.

Jerry Townsend posted 04-06-2004 01:55 PM ET (US)     Profile for Jerry Townsend  Send Email to Jerry Townsend     
As instructed by a very knowledgable dealer - adjust trim as necessary in coming up on plane with the rpm in a normal operating range. After coming up on plane, you will find that there is more resistance in turning to port or starboard (depending on the prop rotation). "Tweak" your trim to minimize this resistance - tending to equalize the resistance in turning to port or starboard.

I have also noted that once the trim is set - during a given day - leave it there when trolling or beaching or and the next time you are accelerating, your boat will snap right back onto plane post haste.---- Jerry/Idaho

Moe posted 04-06-2004 02:02 PM ET (US)     Profile for Moe  Send Email to Moe     
I sure can't do that with mine. If I take off from a stop with it trimmed out as I run, it goes bow-high and takes a lot longer to plane.


greyg8r posted 04-06-2004 07:04 PM ET (US)     Profile for greyg8r    

Thanks for asking this question! I was too embarassed to ask (even though I am a former community college instructor and should know better;-)


And thanks for the great responses.

Rocks posted 04-07-2004 02:12 AM ET (US)     Profile for Rocks  Send Email to Rocks     
Thanks guy's, and thanks for the email Moe, I've really learn't something there! While I'm having my week of asking silly questions: How do you know the point at which you're on the plane? Is there any indication or do you just get a feel for it? If I'm going to look dumb I might as well look really dumb! Moe have you thought about putting your gen on the reference section? I think there's probably a few guy's like me who would appreciate it!
Moe posted 04-07-2004 10:07 AM ET (US)     Profile for Moe  Send Email to Moe     
Take a look at this performance graph for a 170 Montauk:

Notice the dip in the curves between 2,000 and 3,500 rpm. Below that, the hull is in displacement mode of operation, easily pushing its way through the water. As speed increases beyond that, the boat begins climbing up on its bow wave.

This is the transition region between displacement and planing mode. The bow rises, the stern drops, and the motor is having to PUSH the boat through the water, as it is constantly climbing its own bow wave. This shows in the decreased fuel mileage. It also shows in the boat's wake, which is large.

As speed increases beyond this, the boat lifts up on the aft portion of its bottom, the bow falls, and the drag on the motor suddenly drops. The boat is now on plane. You can hear and feel the motor's "relief" of this burden as the boat gets on plane, and that's evidenced in the increase in fuel mileage. You'll also note a quicker rise in rpms, and the boat's wake decreases significantly.


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