Motor height on bracket
 

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.

Re: Motor height on bracket
 

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.

Re: Motor height on bracket
 

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.

Re: Motor height on bracket
 

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

strick

Re: Motor height on bracket
 

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 :D

Re: Motor height on bracket
 

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??

Re: Motor height on bracket
 

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.
http://www.deep-blue-sea.org/seacraft/final4.jpg

Re: Motor height on bracket
 

http://i188.photobucket.com/albums/z...Prtbowview.jpg

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.
http://i188.photobucket.com/albums/z...mtrandbrkt.jpg

Re: Motor height on bracket
 

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.
http://i155.photobucket.com/albums/s...c/pic010-2.jpg

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

Re: Motor height on bracket
 

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

Re: Motor height on bracket
 

Almost a two year anniversery for this thread, lol. Long before me. Maybe I'll pull a measurement and post a pic.

Re: Motor height on bracket
 

Thanks Don....removed it to drop and service LU...I can put it aside :D Prop is a 15x15 OMC Alum.

Bruce...this thread was saved in my favorites ;)

Re: Motor height on bracket
 

I run an el-cheapo original Stingray fin on my bracket set-up. It works so well for me that I skipped putting trim tabs on and just run a 15x17 3 blade and might actually go up to a 19. It may not be the norm but it sure runs good on mine.

When I first got the boat wet my only complaint was popping it up on plane and that is a complete non-issue for me now along with having some other advantages. Seems to really help with blowout in hard turns in a sea which I love. Some people don't like that you lose some top end speed which I have seen on other boats. Not with mine, I lost nothing up top. My performance #'s are very good for an old 150 crossflow that sat neglected outside for six years and it surely isn't 150hp, it's not tired it's 22yo, with 40yo technology. I can get 44mph with half fuel and me and the wifey with normal gear, and I'm a few hundred rpm to much at full trim. Really a old OMC 140 V-4 looper would probably run faster and is far from a modern engine. I think the perfect motor for me would be a 25" 90's 60deg. V-6 looper 175 OMC. At only 350 lbs. I could probably lose the foil and the boat run just as good if not better and definitely faster.

I honestly always looked at fins/foils as crutch for various reasons and if it had any weird side affects I would have thrown it in the garbage and got a 4 blade and tabs. The fact that I have had such good results has me changing my thinking on foils/fins on some set-ups. I'll probably be running the foil on this motor til it dies, then we'll see for the next outboard.

Re: Motor height on bracket
 

That shoots my theory down! Good to know they work on brackets, I have used and installed on many boats without and like the results but never on a bracket boat. BruceB
do you know how high from keel you have the motor mounted?

Re: Motor height on bracket
 

Nah... I moved it up and down three times...I'll try to get a measurement and a pic today. I do remember thinking it looked to deep/low to me no matter what height I had it at...lol

Motor Height Measurement
 

I had a nice length of thin wall 1/2" aluminum that I duct taped to the keel and to the motor and checked it a few times. About 3" to the cav. plate and a setbeck of 30"

http://carphotos.cardomain.com/ride_...0136_large.jpg
http://carphotos.cardomain.com/ride_...133_medium.jpg
http://carphotos.cardomain.com/ride_...132_medium.jpg
http://carphotos.cardomain.com/ride_...134_medium.jpg

Re: Motor Height Measurement
 

I have a fin on my 20...when I bought the motor it already had one on it so I just left it on...the boat really jumps on plane and turns on a dime without any cavitation. My trim tabs were an unnecessary addition to this boat. Bushwacker has a fin on his Hermco bracket and he has said that he thinks it helps as well. I used to have a fin on a bracketed 23 Arena craft....cant remember exactly why I put it on but I think it was because of blowing out in turns and it fixed the problem. I think the main advantage of a fin is that it will get the bracketed boat up on plane faster. BTW I think fins look ugly..... but if they help

Re: Motor height on bracket
 

Don,

I raised my motor 2 holes from where you mounted it. Cav plate is now 3.25" above keel. It still handles fine; can put helm hard over at most any speed with no ventilation.
http://i188.photobucket.com/albums/z...thtclsup-1.jpg

I had a Doelfin on the old 115 and it seemed to help planning when I was heavily loaded on my Bahama trips; I could trim motor down and plane easily at 12-13 mph, allowing me to run comfortably SITTING DOWN in 3-4' head seas. When I added the 30" setback bracket and increased motor weight from 300 to 429 lbs, it wouldn't plane below about 18-20 mph! I added the Doelfin and that reduced min planing speed a couple mph, but it still wasn't as good as what I had before. I then tried a 4 blade prop and that got me back to where I was with the old motor. Denny

Re: Motor height on bracket
 

Thanks for the info guys. The fins seem to be a big plus. Now can you buy some more boats and let me install brackets for more testing? :D Just joking. The info you have provided will be quite useful to me and others with bracketed boats. ;)

Re: Motor height on bracket
 

After glassing/filling/painting/re drilling. I am @ 3". I paid the price for not drilling right the first time . Need to give it a try next month. One move up would still leave me a little lower than before. Going to seatrial without fin. Then fin can go back on if need be.