Outboard Motor- Selecting Proper Hole for Installation

[Down Deep]

A while back I asked a question about saving fuel with a hydrofoil. Several of the replies included information about having the motor mounted on the proper hole. Can you shed some more light on selecting the proper hole --- How do you know the motor is mounted using the wrong hole --- What happens if you move it up --- What happens if you move it down???

Anything else we need to know....

Thanks

[marine_man]

Moving the motor up will help with your top end speed since there's less motor in the water for drag. The concern here is water pressure - if you're going to move your motor up to the third or fourth hole I'd seriously consider putting a water gauge on the motor, at least temporarily to verify you're getting adequate water pressure.

Moving the motor down will help with hole shot as you're getting more leverage from the motor and prop being lower in the water. Your top end will suffer, but you'll get better hole shot.

When I used to mount motors I would almost always mount them one hole up and experiment with it from there.

marine_man

[PerchJerker]

Another issue with having the motor too high is that it may cavitate or break loose in turns or at some cruising speeds

[VMS]

Hi,

Finding the correct hole placement for your motor is a test-and-run situation. I would agree that by moving the motor up you will get a more efficient running boat that should increase speed, but there is a limiting factor as well. Aluminum props will not be able to stay "hooked up" anywhere near as well as a stainless aftermarket prop.

No matter what you do, there is a compromise going on. Move the motor up, you gain speed, but there will be a potential loss of handling. Also, as was stated as well, you get too high and the prop will break loose on a slight turn even with the motor trimmed down (trimming down is usually good practice in any situation, but if it is trimmed all the way down and you turn slightly and the prop lets go, the motor is too high)

One thing I would disagree with, though, is that if you are using the mounting holes on the motor, you will NOT run the risk of losing water pressure to the point of damaging the engine. If that was a potential situation, the manufacturer would not have the different holes to begin with. Even if you go to the highest position, you'll have plenty of pressure.

I run a multispecies boat with a 90 yamaha 2 stroke. I added a 4" manual setback plate, which puts my motor's anti-ventilation plate up about 3-1/2 inches ABOVE the lowest point on the transom. I'm due for an impeller change, but even in this position, I am still get 18 pounds of pressure (I run a pressure gauge because I am above the mounting bolts on the motor itself.)

If you have a steel prop on, I'd say, move the motor up to the third hole, take it out and see how it performs. If the prop breaks loose while trimmed down and you make a moderate turn, it is probably one bolt too high. If it holds the water well, move it up to the top position and try again...

Many cases you can gain around 250 rpms, and potentially a couple mph increase. On hole shot, it make be a little longer, but the bow may not rise up anywhere near as much.

Lots of things to consider, but if you enjoy "tinkering" this is a fun project.

Steve

[boatfixer]

What boat and motor combo do you have? If this is an average set up, then as long as the anti-cavitation plate is even with the keel, that will generally give you the best all around performance. Now I'm sure there are instances were moving it a hole or 2 is sometimes necessary, but for the most part, you will give up performance somewhere to gain it elsewhere.

[Down Deep]

I have a 2007 17 Lund Explorer Sport with a 115 4s Merc. I run a 17 pitch aluminum prop. It is currently set on the 2nd hole from the top. I would rate the whole shot as fair and it tops out at 38 to 40 mph on the gps depending on the load. I've talked with Jay Soderboom the prop guy and he recommended I try a 17 SS prop which I am going to try this summer in order to tweak the performance.

When I took delivery on the unit it had a 19 inch prop and I couldn't get it on plane without taking a 2000 yd run. It was a pig. The dealer replaced it with the 17 which resulted in a big improvement. Last summer I bought a 15 for a spare and it jumped out of the water, but I could only get 32 MPH out of it.

[boatfixer]

 Originally Posted By: Down Deep

Last summer I bought a 15 for a spare and it jumped out of the water, but I could only get 32 MPH out of it.

Watch the tach.

[marine_man]

 Originally Posted By: Down Deep

Last summer I bought a 15 for a spare and it jumped out of the water, but I could only get 32 MPH out of it.

Be sure to keep tabs on your tach, like boat fixer said. If you're running at the upper end of your RPM range with your 17p prop (which I would suspect you are given your setup) a 15 may put you over...

marine_man

[VMS]

Couple of things I would check...

First, check the position of the trim bar(?) There will be a series of holes the bar goes into, and since your boat has trim and tilt, that bar should be in the lowest position to allow the Tuck under" as far as it can go. It should be there already, but if it is not, that would be part of issue with the slow hole shot.

Second, getting only 38 - 40 is a little on the low side of things. That boat should see somewhere around 42 - 43 when set up, and at minimum 40 with aluminum.

I think switching to steel would be a good thing too. Lots to try, and one will definitely out perform the rest. For that boat, I would see about trying a michigan ballistic. with that prop, you may actually see an INCREASE of RPM from your aluminum, and it will hook up well. I'd bet with any good steel prop, that motor can come up at least one hole, without any real loss in handling. Other props to try: Michigan Rapture, Mercury Lazer, tempest, Solas props,... Lots to choose from.

Sometimes, if you get in contact with a good prop shop, they will have demo props for you to try. Test a bunch of different ones and see which one works best first. Then, play with motor height from there

With the correct prop and motor height combination, I'd bet you could potentially see 44mph if no 45. Aluminum props on boat of 90 hp and up are just not going to cut it.

Steve

[Down Deep]

I'm headed to the garage. I will report back as we get rid of the ice. If you think of anything to chime in, please do so.

Thanks

Down Deep