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Author Topic:   trimming up to gain more mph?
montaukman posted 05-05-2005 09:54 PM ET (US)   Profile for montaukman   Send Email to montaukman  
Hi Everyone,

I am trying to see if i can get my 170 to acheive the 40mph as it is speced out to do. I have the 90 merc 4 stroke.

Last trime i was out, i had the bimini opened and was alone and could only do 33mph as per the garmin GPS. The motor was fully in the down position.

Maximus, in a previous post, said that I need to trim the engine up to get the MPH's higher. I will try it but wouldn't that put more downward pressure on the hull and increase the drag?

I don't understand the principles of trimming vs not trimming. Is it realistic that this will help me gain some mph? I assume it must work but i just don't see it how.

Just curious if anyone does the same thing as suggested by maximus.

Thanks for the tip by the way maximus. Hope it works.

All the best,


rtk posted 05-05-2005 10:16 PM ET (US)     Profile for rtk  Send Email to rtk     
Trimming the motor up will bring the bow up.

Motor trimmed all the way down will help the boat plane easier. Trimming the motor up, while on plane, will get the bow up and typically increase speed.

Try it, you will like it!


elaelap posted 05-05-2005 10:41 PM ET (US)     Profile for elaelap  Send Email to elaelap     

Trim down/in if you need to to get up onto plane quickly, wait 'til your bow drops back down and the boat steadies on plane, then slowly trim up/out while increasing rpm. This will again raise your bow and shorten the wetted surface of your hull (the wetted surface will 'move aft' can see this by looking over the side at where your wake begins), thus increasing your speed. If the boat's steering starts feeling squirrely or the bow begins to oscillate up and down ('porpoising'), you've trimmed out too much and it's time to trim back in until the steering feels okay and/or the porpoising has stopped.

There are really no hard and fast rules about this stuff...'good' trim is affected by many factors and changes often during a boat trip--e.g., hull design, horsepower, how high up on the transom the motor is mounted, type of prop, how the boat is loaded and weight is distributed, windage (such as the use of a bimini or half a dozen rods and a salmon net sticking up out of rod holders, whether you and your shipmates are sitting or standing, etc), wind direction and strength, state of the water (calm, choppy, running into the stuff, across it, or downhill, etc), the speed your boat is travelling...and whether the moon is in Aquarius and Mercury is retrograde for all I know.

Have around with the trim's only a game. Once you get the feel of your boat you won't even need to look at the trim/tilt gauge to know when the motor is trimmed adequately. And you're not going to break anything or do any damage if you just trim back approximately one-fourth of the way according to your gauge and leave it there while you're getting used to other things on board...this should be adequate on most boats to get you onto plane without much trouble and then to allow for comfortable, if not optimum, cruising performance in most circumstances.


lin posted 05-06-2005 12:18 AM ET (US)     Profile for lin  Send Email to lin     
I recently repowered my Montauk with a 90HP 4 stroke-- I was finally able to get out on a really calm morning. I spent a good twenty minutes just trying different combinations of speed and trim-- It was calm enough that you could really feel the change of ride and see the difference in wake at different trims/speeds. I did get some porpoising at certain trims, but it trimmed right out. I really didn't have any method to my madness-- it was more just having fun and learning about how the new boat / motor combination performs.

If you're thru the break in period, go out and have fun learning how the boat runs-- see how quickly you can bring the boat on plane at different trims, see if you get RPM changes at different trims, etc. It all just seemed easier on a really calm day. It can even help if you have another person to watch the tach (sometimes I get fixated on looking for RPM changes and forget to watch the bay-- yikes)

WT posted 05-06-2005 12:53 AM ET (US)     Profile for WT  Send Email to WT     
I recommend getting a trim gauge for the novice boater. I had my dealer install mine for me. It sure beats turning you head around like the gal in the Exorcist while you are driving.


dauntlass 18 posted 05-06-2005 07:33 AM ET (US)     Profile for dauntlass 18  Send Email to dauntlass 18     
I call it the sweet spot .You stated the motor was fully down &the gps read 33 mph just apply a little up trim not much and wait a few seconds you most likley will see the speed will pick up and the motor sound will change.You can apply up trim again wait a few seconds and see if you get any more increase in performance.As you increase up trim performance will fall off and steering and ride get poor.The secret is to make little changes in trim adjust and wait for boat to react to the trim change.
Knot at Work posted 05-06-2005 08:18 AM ET (US)     Profile for Knot at Work  Send Email to Knot at Work     
You can actually feel a well trimmed boat. You go from a skip to a slice. The boat feels like it is on ice. You will notice an increase in speed with out an increase in rmp.

You have to find your boats sweet spot as mentioned.

Sea State dictates it as well but you can be on plane and slicing even in some foul sea.

montaukman posted 05-06-2005 08:30 AM ET (US)     Profile for montaukman  Send Email to montaukman     
Hi Everyone,

Well i must tell you i am so excited abut experimenting based on all your recomendations. I was a little bothered by the low top end and either it was ommitted in the manual or I missed it but i didn't see anything describing what you all are suggesting.

I guess it comes with boater's experience.

Anyway, thank you all for the help on this. I was going to go out tomnorrow and/or Sunday but now there is talk of a big storm. I may have to take a day off next week. Going through new boat withdrawal.

I will try a bunch of settings and will post back my results as soon as I can get out there?

Just out of curiosity, how many MPH can I expect to increase? Is 5 MPH realistic?

Thanks again for all the sage advice. what would I do without this forum?

All the best,


Knot at Work posted 05-06-2005 08:54 AM ET (US)     Profile for Knot at Work  Send Email to Knot at Work     

last recommendation. Ensure the Bimini is secured and the loose gear is stowed. Start in the full down position and increase throttle. Once on plane then bump the trim up as needed in increments. It won't take much!!!!

Good luck

bsmotril posted 05-06-2005 09:47 AM ET (US)     Profile for bsmotril  Send Email to bsmotril     
Also, be sure your GPS is set to read in MPH vs Knots. 33 Knots is about 37.5 mph. Been there, done that, myself.
Maximus posted 05-06-2005 06:15 PM ET (US)     Profile for Maximus  Send Email to Maximus     
I couldn't have put it better by the rest of the responses. Just get on plane with the motor fully down, and then slowly raise the motor, i.e. trimming. You will find that with too much trim you will cavitate the prop when turning.

I find that 33 mph is probably right on target with bimini up and motor down. Expect 40+ mph with the motor 'trimmed'.

Trimming is not only for speed gains, but can help provide a smoother ride depending on the water conditions. You may, for instance want the bow up to stay dry. There is probably a fuel efficiency gain with a properly trimmed motor as well.

For speed trials make sure your GPS is sampling as fast as possible. My handheld Garmin has an option for battery saver mode which reduces sampling rate to 3 second intervals.

dittybag54 posted 05-06-2005 06:55 PM ET (US)     Profile for dittybag54  Send Email to dittybag54     
I think Jeff put it well that you can actually feel a well trimmed boat go from a skip to a slice. It just lightens up and takes off.

Tony's explanation about it being a constantly changing game with no hard and fast rules was also a very good point. The one fast rule might be to not ever trim out too high.

I enjoyed his explanations but have noticed that when my Mercury is in retrograde it is usually because something has fouled my lower unit or we have picked up some bad fuel. We also prefer to boat when Jupiter lies with Mars if possible :0)
It's just common sense.


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