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Old 09-27-2021, 08:29 AM
wh500special wh500special is offline
Join Date: Aug 2015
Location: Southern IL
Posts: 244

Originally Posted by Hapnen View Post


Do you know of anyway to determine if motor is to low besides raising it and trying?

Thank you for your time,
Sorry to ghost you on this, I've been away a bit.

Take everything I write below with skepticism and use your own good judgement and that of the more knowledgeable folks on this forum...

The anti-ventilation (AV) plate on the lower unit immediately above the propeller should be either coplanar with the bottom of the boat or maybe up to an inch above.

With the boat on the trailer and the motor trimmed down so that the AV plate is parallel to the bottom of the boat at the keel, stick a long straight edge against lowermost portion of the bottom of the boat (the keel) and let it protrude out the back to provide a reference line. The AV plate should be even with or higher than this line. If not, raise the motor assuming there is adjustment left in the motor mounting bolts.

From what I've read, each boat/motor/load/prop combination will have a magic depth for the motor that can only be determined experimentally. Having the motor mounted low will ensure that the prop never grabs any air (cavitates) but will reduce performance since there is more motor in the water than needed. This extra drag can cause porpoising and might not be fixable by just trimming.

I am about as far from being an expert on boat rigging as a person can be, so you'll probably want to solicit some additional opinions. In this case of too much motor depth I have the same issue on a smaller boat. In my case, the dealer hung my 40-horse Evinrude at the very bottom of its adjustment AND the motor has a gigantic lower unit analogous to the Mercury bigfoot. My AV plate is about an inch below the bottom of the boat.

Consequently, unless I have a ton of weight in the boat my boat softly porpoises as I run down the lake whether I have the thing trimmed in or out. I need to raise the motor and tinker with the setup, but I've not been very motivated to do so despite having it rigged this way for a long time.

If the motor was hung properly it's not a difficult thing to DIY.

There are (should be) two sets of bolts that hold your motor to your transom. The upper bolts go through holes drilled in the engine mounting bracket. The lower bolts go through slots.

With the boat on the trailer and unhooked from your vehicle crank the tongue jack down a little bit from level and trim the motor all the way down. Stick a block of wood under the skeg of the motor, then crank the tongue jack up until the skeg hits the block of wood.

Remove the upper motor mounting bolts from their holes. Loosen the lower slotted mounting bolts some. Crank the tongue jack up and observe the motor sliding up the transom. It's trial and error to get the bolts loose enough to permit the motor to slide but tight enough to keep it from flipping around aimlessly.

Pick a spot that puts the AV plate at or above the keel of the hull and reinstall the upper bolts then tighten. And water test.

I think dealers have gotten a lot better about rigging motors to boats properly through the years. I don't think it's out of the question that the original dealer for your boat just stuck the motor on the lowest spot since it was easy and guaranteed that the purchaser wasn't blowing out when trimmed way out or in turns and called it good.

So you may ask, "If this process is so easy - Mr. Keyboard Warrior - why haven't you done it?"

In my case, the dealer who hung the motor did not use the slotted mounting holes on the motor bracket. He used a second set of holes that was there that don't have slots. So mine requires redrilling. The SOB...

All of this assumes your motor is indeed mounted too low. If it's not I suppose it's possible you have a bent hull like someone noted to you. You'll probably see this when you get the straight edge out for the depth measurement.

Good luck,

Last edited by wh500special; 09-27-2021 at 08:33 AM. Reason: Made comment about replacing the motor; had two threads confused. Removed replacement comment.
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