|09-27-2021 08:29 AM|
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.
|09-19-2021 12:11 PM|
|09-19-2021 10:02 AM|
How would creating a positive tilt which would bring the bow up help? I'm thinking it needs to be pushed down.
|09-19-2021 09:57 AM|
Unfortunately is has been like this for 15 years, I just thought it was caused by the width of the boat. I had read that this looks kike a common problem and the dealer would have to "bend" the boat to correct. I'm not sure if they would do even look at it because of the time.
Even moving all gear forward and live well full it will some times bounce.
Do you know of anyway to determine if motor is to low besides raising it and trying?
Thank you for your time,
|09-19-2021 09:37 AM|
It’s also possible that your motor is too low on the transom and the lower unit is creating extra drag. Might need to move it up a couple holes.
Is this a new problem or has it always been this way?
|09-19-2021 05:59 AM|
Try moving weight forward as previously suggested. If that doesn't work you can install a set of shims like these.
I have these on mine which is a similar to your rig and they work great.
|09-18-2021 10:57 PM|
Move weight forward and/or
4 blade SS prop
trim tabs if either of those do not do it
|09-18-2021 09:27 PM|
Alumacraft Navigator 165 60 HP
I have a 2005 Alumacraft Navigator 165 with a Yamaha 60hp tiller. The boat porpoises when planed. The motor is trimmed down and the propeller is a 11 5/8 - 11 ss. Any hints on what can be done?