Motor height on bracket

[JohnB]

I just got back from water testing my boat, and the only big issue is the motor is mounted too low. On a big merc efi, there is a fin/plate about 2.5" above the cavitation plate. When running at speed, 25-30 mph, the water is hitting this plate. I am pretty good at setting up transom mounted motors, but this is my first shot at a bracket mounted on. Where does the water hit realitve to the cavitation plate on everyones bracketed motor?
I raised the motor up 2.5", see what it does tomorrow.
Speed and effiency were off quite a bit with it mounted low.

[Bushwacker]

The normal formula for brackets is to raise the motor 1" (relative to a transom mount) for every foot of setback. Don Herman raised mine 2.5" for the 2.5' setback of the Hermco bracket. I was subsequently able to raise it another 2" and still have no problems with ventilation in hard turns unless I have motor trimmed pretty far out. Can run at 50-60% of max uptrim; it starts to porpoise at about 70%.

I'd say keep raising it until you run into ventilation or porpoising problems problems, but keep an eye on water pickup - need to make sure it's still below bottom of hull.

[JohnB]

bushwacker,
Thanks for the response.
When I initially set it up, I had it 2.5" higher than where it was when on the transom, setback is 30". It looked to me like it could easily go up 2" and still keep the cavitation plate wet. I raised it up 3 holes, and will test and see. Luckily, I started in the top hole, so I have a lot of room to move up if needed.

At rest, where is the waterline in reference to the top of the bracket? Mine is about 3" above the water line, about 1.5" between the bottom of the bracket and the waterline.
I mounted it that way because that was where Armstrong said too.

[strick]

On my 23 I believe I'm 4.5 inches above the keel with a 29 inch set back bracket.

strick

[JohnB]

strick,
I started a 2.5" with a 30" setback. This was according to armstrongs suggestion of 1" up for every foot. I have moved it up 2", to 4.5", and it seems to be close. I might be able to move it up another hole, but I need to get some time on it in the open ocean before doing that. I am close.
It's just that everything you read says 1" per foot, and that is definitely wrong for this application.
Thanks for the feedback

[1bayouboy]

I think part of the equation has to be the bottom of the bracket in relation to the bottom of the boat and it's overall shape. If the Armstrong bracket angles up from where it attaches to the boat transom to the back of the bracket, I'd think that would let you raise the motors higher than on a bracket lik ethe Seamark where the bottom of the bracket is parallel with the boat's bottom all the way back....... [img]/forum/images/graemlins/tongue.gif[/img] In the fisrt case the water can follow the angle up the bottom of the bracket. A lot more of the lower is hanging below the botto of the bracket. On a bracket lik ethe seamark I think you'd have to treat the bracket bottom like the bottom of the boat as far as motor height and anti-vent plate are concerned.......

What's the shape of your bracket bottom in relation to the keel??

[JohnB]

Mine is angled up quite a bit compared to hermco/etc.
One of the reasons I went this way instead of a hermco was I didn't want a hull extension, thus more water surface area and drag. Also, I think this puts the motor in cleaner water. At rest, it seems that the back of the boat is a little higher than it was when I had the motor on the transom. I think the floation chamber on the Armstrong excedes the weight of the bracket, and offsets the motor a little.

[Bushwacker]

At rest, where is the waterline in reference to the top of the bracket?

I took some measurements after taking above photo. Top of bracket is about 6" above bottom of bootstripe, which appears to be right at the waterline. Bottom of swim platform is about 4" above waterline.

The Seamark/Hermco bracket does NOT come all the way down to bottom of hull, as shown in following photo. The bottom of the flotation tub is about 4" above bottom of hull at center and it's clear of the water when on plane, although it probably acts somewhat like a big trim tab when you're coming up on plane.

[thehermit]

BTT

I am about 3 3/4 to cav plate and motor was acting to high but looked too low. Blowing out in turns. I am lowering it by about 1" to start the season. I should have adjustment back up from here.


Setback is 28"
Bracket keel is 4" above keel.

[hermco]

You might try removing the addon fin first. Just my opinion but I don't believe it will work with a bracket setup.