Transom Bracket - twin outboards

lobo1 · #1 07-18-2010, 11:19 AM

no luck on the dropping of the motors to the lowest hole - may even be worse. had 25 mph winds so it was too choppy to do too much of a trial but it I was out long enough to figure out this is not the correct setup. I also put my 300 pound son along with my wife and daughter in the bow and this is not helping.
My trim tabs appear to be 12x12 and working fine. There is nothing dragging under my boat - the transducer shoots thru hull - I am pretty sure the motors are dragging tho.
I have added a bunch of pictures to my photobucket that I took yesterday the links are below.
I appreciate any feedback.
4 photos are of the boat at rest in the water with motors all the way down- I have on board 90 pounds of lead in the bow, 100 gals of fuel, 3 batteries, and a cooler with drinks.
I have some others pics of the props themselves, and how the motors are mounted etc with them all the way down and on the trailer.

http://i962.photobucket.com/albums/a...s/IMG_0436.jpg
http://i962.photobucket.com/albums/a...s/IMG_0437.jpg
http://i962.photobucket.com/albums/a...s/IMG_0438.jpg
http://i962.photobucket.com/albums/a...s/IMG_0439.jpg
http://i962.photobucket.com/albums/a...s/IMG_0435.jpg
http://i962.photobucket.com/albums/a...s/IMG_0428.jpg
http://i962.photobucket.com/albums/a...s/IMG_0427.jpg
http://i962.photobucket.com/albums/a...s/IMG_0426.jpg
http://i962.photobucket.com/albums/a...s/IMG_0425.jpg
http://i962.photobucket.com/albums/a...s/IMG_0424.jpg
http://i962.photobucket.com/albums/a...s/IMG_0429.jpg
http://i962.photobucket.com/albums/a...s/IMG_0431.jpg
http://i962.photobucket.com/albums/a...s/IMG_0432.jpg
http://i962.photobucket.com/albums/a...s/IMG_0433.jpg
http://i962.photobucket.com/albums/a...s/IMG_0434.jpg

thanks in advance!
Steve

Fr. Frank · #2 07-18-2010, 03:26 PM

Put a straight 2x4 along the hull bottom directly in front of the lower unit to see where the cavitation plate sits in relation to the hull bottom.

It's hard to tell from your photos, but if it is as it appears, I'd say you're mounted with your cavitation plates almost dead even with the hull bottom, and that's about 3 to 4 inches too low. Your cavitation problem in turns would then be (in part) a result of having your cavitation plate causing excessive turbulence from running too deep.

Use the straight 2x4 as a straight edge along the hull back to the lower unit, and, with your engines at neutral trim, put the engines where the cavitation plate is about 3" to 3 1/2" above the straight edge. That's usually a pretty good starting place.

Hopefully Peter B will chime in about where his were mounted.

JohnB · #3 07-18-2010, 05:43 PM

IMHO....
In this picture

it looks like the motors are reverse trimed. If you take a look at the boot strap, and put a ruler on the picture, it looks like the motors are angled in, underneath the boat. I have been on a couple of boats with yammie 4 strokes that had similar problems, and they required "wedges" to correct the condition. The energy of the prop is pushing up, and pushing the back of the boat up. This is causing cavitation, as the props push the stern up, when you come off a wave, it cavitates, drops back down, and then catches water again.

I think yammie did this on purpose to overcome the additional weight of the 4 strokes, to get the boats on plane faster, so they reverse angle it. Once you get on plane, try trimming up some and see if it helps keep it in the water. The bow should get lighter, and the stern should stay in the water better. I bet this rig is a handfull in a following sea set up like that.

It is really hard to set the height from a picture, but I am pretty sure your motors are too low, not too high. You might want to talk to armstrong, they were helpful when I was setting mine up. BTW, the water comes up about a little over an inch for every 12"'s of setback on the motor.

Put a straight edge on the bottom of the hull extending back, and I would mount the cavitation plates 2.5"'s above that line.

Old'sCool · #4 07-18-2010, 06:30 PM

I agree, they look too low. I would try raising them, remove the foils,and work with someone on props to dial it in. The 4 blade has helped mine.

lobo1 · #5 07-18-2010, 07:02 PM

will take a straight board and a level with me tomorrow and then have them raise them up. Will take one hole up just to get it back to where it was - so I will either go two or all 3 holes up this time.

it is a bear in a following sea. i am not comfortable with it at all - makes me a bit nervous actually.....

one of my favorite spots to fish this time of year is about 19 miles offshore in Lake Michigan - I cant go out there with this boat like it is.

JohnB · #6 07-18-2010, 08:04 PM

Quote:

it is a bear in a following sea. i am not comfortable with it at all - makes me a bit nervous actually.....

one of my favorite spots to fish this time of year is about 19 miles offshore in Lake Michigan - I cant go out there with this boat like it is.

IMHO, the motors are lifting the stern up and driving the nose into the water coming off from following seas. I was on a contender that used to do this, he put wedges in, raised the motor up, and it made a world of difference. The way you have them also will suck a lot of gas.

Get those motors at the right height, and than use your trim to get the motors into a neutral, vs negative trim state. It would really help to have someone drive the boat so you can look at the cavitation plates. If you can, shoot some pictures looking down at the plate while it is running.

lobo1 · #7 07-19-2010, 11:26 AM

I will have the motors raised up a few holes - I can trim the motors to neutral. For 70 bucks I'll certainly add the wedges but cant seem to make my brain understand the additional benefit I will gain - above and beyond trimming the motors to a neutral position.

thanks again,

steve

Old'sCool · #8 07-19-2010, 02:01 PM

It's my undertanding the wedges were designed (unless mounted upside down) to increase negative trim, which you appear to have plenty.

McGillicuddy · #9 07-20-2010, 12:11 AM

I think John is dead on about the negative trim - you have excess and that need a way to govern that, wedges or a 2x4 - whatever keeps your prop shaft and cavitation plate mostly parallel to your keel. Like Dana Carvey mimicking Uncle George Sr., "Don't want to be \ nor / wanna be somewhere in between, sure..."

Fr. Frank's suggestion on measuring engine height is very important. Straight edge on bottom of hull in line with each motor skeg and cavitation plate parallel to hull bottom. Seacrafts run best with cav plate about an 1-1/2" above the hull bottom, additionally, Armstrong asks for 1" of rise for every foot of bracket setback...you do the math.

Finally, I'm with reelescape - ditch the foils. Use the the trim tabs for efficient planing and hull control.

Different props may help but I think the other three issues should be dialed in first. Good luck, and keep us posted.

Fr. Frank · #10 07-20-2010, 08:33 AM

Gillie said:

Quote:

Seacrafts run best with cav plate about an 1-1/2" above the hull bottom, additionally, Armstrong asks for 1" of rise for every foot of bracket setback...you do the math.

Finally, I'm with reelescape - ditch the foils. Use the the trim tabs for efficient planing and hull control.

Different props may help but I think the other three issues should be dialed in first. Good luck, and keep us posted.

That 1.5" inches where Seacrafts do so well is for transom mounted motors. For every foot of bracket, add 1 inch to the 1.5" above the hull starting point. So for a 30" setback bracket, your starting point would be having the bottom of the cavitation plate 3.5" above the hull.