Motor Mounting Height

I did some searching but couldn’t find anything. I remember someone posting about this years ago. I have a Alumacraft Dom sport with a 90 on it. Only getting about 35 MPH on the Gps and only about 5100 RPM WOT with a 17 pitch prop. I am reading on the internet I should be closer to 6000 RPM and 40 MPH with that setup. So I am thinking that my motor is too low. I have read that you run a line down the keel and the cav plate should be above this line. So my question is should I raise up? and if I do how high should I go? Do I put the line on the actual keel or the botom of the boat? this makes about an inch difference. Also do I measre the motor with it in the all the way down position or the trimmed position where I will be running?

Thanks for all the help in advance. This site is great for advice!

dont you mount it so the silver spinny thing gets wet when you’re in the water?????? just kidding!!!!!!

I experimented a few years back w/a Merc 125 on a Mr. Pike 17. I cannot recall which hole it was in, but moving it one hole down affected the performance beyond belief. It made it much, much worse so I put it back where I started from. I believe the cav plate was just above the bottom of the hull when I was done. We used an engine cherry picker w/a Merc lift ring. Hope this may help, Gregory

Here is a good article on where the correct height is. Personally, I don’t think you can tell if the motor is the right height without having the boat running across the water and looking at it. Of course, you can’t do it by yourself. You need somebody else to drive the boat while you look at the motor height. My motor was mounted on the top hole by the dealer, and I moved it up two holes, gained 300 RPM and a couple MPH.

I raised the motor by myself, using the trailer jack. Lower the jack all the way down. Tilt your motor down and put a block of wood under the skeg. Remove the two top bolts and loosen the lower bolts enough so the motor can slide. Slowly crank the jack up until you have the motor lined up with whichever holes you want to mount the motor on. It works best with two guys, but you can do it alone. Make sure your motor doesn’t catch on the transom cap as it will bend the crap out of it. Trust me on that one.

Here is the link.

Motor height

Read the attatchment and then printed it off. Going to check my setup out now and make some changes, hadn’t really gave it much thought before, but now? Great article very clear explanation.

I set the motors so the cavitation plate is just above the bottom of the boat. You can check this by trimming the motor down when it is on the trailer. After reading your post I would suspect the reason why you are not running higher rpms is the prop is too big. Double check your manual on the motor and look for operating rpms and confirm the specs. You will gain about 200 rpms for every degree of pitch. It is just a rule of thumb I go with. So if you drop down to a 15 degree pitch prop you should gain an extra 400 rpms. You really have to be careful when you raise the motor up on the transom. If you go too high with mounting position you will cause the motor to not get enough water up into the water pickup and overheat the engine.

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Only getting about 35 MPH on the Gps and only about 5100 RPM WOT with a 17 pitch prop.

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I gotta agree with Rigging Guy on this one. You should try another prop rather than moving the motor.

First, if you are getting 35 mph and like the overall performance – hole shot, hook up in turns etc, you may consider doing nothing. You can throw a lot of time and money at this one and only gain 1-2 mph in my opinion. The down side – you may lose other characteristics you like or end up with other problems like proposing.

I like Rigging Guys idea on the 15p prop. Definitely try a stainless here. Most boat dealers have a loaner prop program. Try a 3 and 4 blade prop and see what you like. Again, I would not expect a 5 mph gain here, 1-2 max!

-J.

Well did some looking and testing today. Went down to a 15 pitch prop and didn’t change anything else. Got up to about 5800 RPM and didn’t change the top end at all. Major hole shot though, I am on plane in under 2 seconds! Right now with the motor all the way down (not trimmed to cruzing speed) on the trailer, the cav plate is below the bottom of the boat. Watched on the water today. I can get the cav plate out of the water, but need to trim the motor all the way up. So now it is pushing the water more up in the air instead of straight out (ie roster tail) Does that sound normal? When I trim to where I think it should be the front of the cav plate is under that water and running water up the plate and out.

any advice? I think I am going to try rasing the motor one hole, But deathly scared of not getting enough water to the motor, especially on those 4 – 8 mile runs at WOT on the river!

If the motor is too low it will slow you down. I would try moving the motor up one hole. I would say as long as you dont get the cavitation plate more than 1/2 inch above the bottom you should not run into issues with overheating. Raising the motor up should give you a better hole shot and better top end. Just keep in mind that with raising the motor up you now will not beable to trim the motor up as high as before. This is ok, because you will get better performance with less triming. Sometimes the speed will not be a huge gain. If you raise the motor up too high you will notice the motor will tend to blow out(prop slip) on the corners when trimed up. I would say the average mounting height on most aluminum walleye boats will be in the second hole from the top. Most of the Lunds I have set up are this way. I have set some higher, but these guys are running stainless props. Stainless props will grab more water at higher mounting positions. I do agree with what John said too I really dont think you will gain alot, maybe two miles an hour by raising the engine. If it was my boat I would get the cavitation plate equal too or slightly higher than the bottom of the boat and stay with the 15 degree prop. If you decide to go too stainless you will most likely need to drop to 13 degree depending on the design and make of prop. Stainless are ussally 2 degree’s less than aluminums to maintain desired rpms. Dont forget to use silicon when you move the motor.

This a great site. The people on here have some great insight into a lot of areas. I checked my lower unit and it is at the right height as per the picture on the other site. I wasn’t having any issues, but just good to know. Thanks.

Just for reference, I run an e-tec 150 on an alumacraft Navigator 175 sport. Motor came from the dealer mounted on the top hole. It is now on the third hole from the top. I have the motor hooked up to my lowrance so I can monitor the engine temp. I have close to 150 hours on the motor since raising it, and have never seen the temp rise above the lower range of the recommended operating temps. I have made some 100 mile runs on the Winnebago system taking the girlfriend for rides without issues.

I run a 19 pitch stainless prop, get great hole shot, good handling, and 51 mph top speed at 5550 WOT RPM.

The major problem you will run into is the stock aluminum props performance becomes affected badly if you mount the engine higher then the second hole from the top.

Rigging Guy, That is interesting comment. Just got off the water. Put the motor up one hole last night and got all ready. I am now the third hole from the top on a 4 hole system. Put the 17 pitch on and hit it. Gained about 200 RPM, so now running 5400 just me in the boat. Topped out at 35 -36 depending on wind. Put the 15 pitch on and gained nothing it seemed like I almost lost a hair and running between 5700 and 5800RPM and nothing on the top end, still in that 35 – 36 MPH. Took a look at the back at WOT (I don’t reccomend doing that alone!!) and the cav plate is out of the water now. I think it is about right if not a tad high.

Weird thing I noticed. I was playing around with trim while running. This motor has a two stage trim, where the slow trim on the low end and it won’t let you go too high over a certain RPM. I can trim all the way up (to the point of a rooter tail) and I can’t get the prop to slip! I can even turn faily decent at high speed and trimed all the way up and still runs great! That is wierd to me since my last boat, I only had to trim just a hair to get up before the prop slipped.

I am not too worried about top speed at this point as 35 is faily good, however, I would like to be running closer to 6000 RPM for motor efficiency!

Any thoughts? Should I drop to a 13 or try a SS prop. Arrr I hate when things arn’t easy!

Thanks again for all the help.

fishhunter,
Thank you for posting this there is some very good info posted here. I assume that your Dom Sport is the 165 that is rated for a 90hp. I looked up the specs on your boat and the boat itself weights 1250lbs. I have a 1650 Lund Angler that is also rated for a 90hp and the boat by itself is weighs only 800lbs, I just put a 90hp 2 stroke Yamaha on her and like you I was only able to get 35MPH at WOT and my rig is 450lbs lighter than yours. I did not run it at WOT very long as it needs a new tac as my current tac is not compatible with the new motor. My rig is in the shop right now getting the new tac installed and a new axle on my trailer. (a story for a different post) I got two props with the motor but I am not sure what pitch they are, when I pick up the boat I will see if the dealer can tell me the pitch on both of them and go from there, from the info I am reading here sounds like I should run a 15 pitch prop. My old motor was a 2 stroke 40hp Evinrude and in perfect conditions I could hit 30MPH at WOT but most of the time would average 24MPH to 26MPH so I was very surprised when I was only able to get 35MPH out of the 90hp, I was expecting that I would be able to hit 40MPH at WOT with the 90hp. I will report back here on this post when I get the boat back with the new tac and find out what pitch prop I am running.

Steve, was your gages on your boat for an Evinrude specificaly? IF they were the stock Lund gages all you had to do was pull the tach and change the setting on the back of the gage to 6p setting and it will be right setting for Yamaha. On the back of the gage their is a little dial selector that allows you to change the setting of analog readings for the different brands of motors. You can also double check your tach with a hand held rpm gage that hooks up to the spark plug wire.
Every hull can be alittle different even from the same brand and make. So go with what works best. Sometimes alittle tinkering really pays off. If you dont have any problems blowing out in the turns I would leave it their. Now that your motor height sounds right, you could see if some your buddies have any stainless props that would work on your boat. A 13-15 stainless should work fine. The 15 might lower your rpm a little, but remember the real target rpm you should shoot for is peak horsepower. Your manual should list were the motor makes peak power at. It is generally 400-500 rpm less than highest rpm of the total rpm range.

Also the power tilt and trim does have two stages. The trim mode is tilted down all the way and will come up slowly until it hits the tilting stage which will pick up speed and raise up to the end of its range, on most motors. The trim stage will hold the motor in place when running at any speed, where the tilt will not hold it. If the motor is in the tilt stage when running, the motor is too low in the water, so the mounting hieght needs to be changed up higher. I dont know how to explain this any better.
Depending on the hull will dictact how high the motor can go. Typically on the Lunds, especially with the IPS hull, the second hole seems to fit the bill with a aluminum prop. This is why I mentioned about mounting higher then the second hole. If it holds good and does not affect performance, by all means go with it. The motor is just getting more efficent.

Ok. So just checked on alumacrafts web site. They are running a 90 4 stroke on my same rig with a 19 pitch prop. they get 39 MPH top end at 5900 RPM. Intersting that the 4 stoke seems to have more power to turn a 19 and get the RPMs up? I was always told that the 2 strokes have more low end torque to turn higher pitch props than 4 stokes?????

Steve,

I have a buddy that is running a 1700 angler (very close to the same weight as mine) tiller with a 70 2 stoke yamaha on it. with a 19 pitch prop, he runs 40 – 42 GPS no problems with 2 guys in the boat. Another reason I am confused as to why a 70 can push a boat faster than a 90 (even though I have more wind drag with the windshield).

Thoughts anyone?

Some times the factory tests can be confusing. I am not sure about Alumicraft, but some test the boats with little gas and no equipment on board. So the boat is not fully rigged and weighs a lot lighter. I know when I tested my 18 pro-v se with a 150 HPDI I was getting 56 miles an hour with no equipment. Now when I added the t-8 and bowmount with batteries and all the other stuff a walleye guy needs I could only manage 53 at its best, 52 on the average. I also had to drop from a 23 to a 21 proseries to get the best rpm and speed. Like I said before some hulls just act different. Wieght distribution is another big killer for speed. Wieght on the back of the boat really slows the boat. So in Steve’s instance he has an extra 110 pounds of T-8 hanging off the back if I remember right. Too much wieght up front slows you down fast too. It seemed to me that if I could find a way to keep the weight of extra items towards the middle of the boat my speed would not alter alot. This was just a characteristic of my boat.

Rigging Guy,

One more question for you. Talked to the Dealer a bit today and they said I am likely over the power curve where I am at. He said that since I didn’t gain anything (speed or RPM’s) when going up one hole with the 15 pitch prop that I am just spinning more and burning more gas for nothing. Any truth to this. I have settled on the 15 pitch prop, but and debating if I should drop it back down one hole (2nd form top) if I really am over the power curve.

Thanks!

pm sent