---

Hello Al,

I have a 1990 Montauk 17 with the original 1990 Mercury 90 HP (2-stroke) engine and a 13” x 19” stainless Mercury propeller, a setup much like yours, although you have a fancier polished propeller with vent holes. I have used my boat in lakes, bays, and the near offshore Gulf of Mexico for over 30 years. In preparation for the installation of a hydraulic jack plate, I recently did some performance testing on my boat, and I thought you might like to review the results:

Location: Lewis Smith Lake, AL (MSL 500 feet). Wind S 5 mph, temperature 78F.
One person aboard, 12 gallons fuel in port Mirax tank, one battery at stern (starboard).
All values average of upwind and downwind measurements.
- Time to plane (from idle speed until bow begins to descend at full throttle): 2.0 seconds with trim fully down/in.; 4.0 seconds with trim in cruise position.
- Speed @4000 rpm (cruise): 28.5 mph (statute) (I usually get 25-26 mph with three people, 24 gallons of fuel, two stern batteries, and a full 72 quart ice chest ahead of the center console).
-WOT 5,400 rpm, with speed 39 mph (cruise trim); mild chine walking encountered.

As you can see, my performance is better than yours despite my prop having 3” less pitch and a more basic propeller, although I admit that your boat was more heavily loaded. It is indeed likely that your engine is mounted too low. However, before you pull off your motor (if you haven’t already), I would like to suggest the following:

1. Make sure your motor is tuned up, your tachometer is accurate, your trim tab is set appropriately, your bottom is smooth, and your propeller is in good shape, otherwise the advice below and in the other replies will be of no value.

2. Check the position of your motor’s ventilation plate in relation to your keel. (If you are unsure how to do this, I suggest you read this article:
https://www.boats.com/how-to/the-outboard-expert-boost-speed-with-outboard-engine-height-adjustments/

You may encounter suggestions on this and other discussion boards to mount your engine “2 or 3 holes up”, but this advice assumes that you have an engine of standard length, a standard transom height, and bolt holes in the transom that are at the standard height (1 and 7/8 to 2 inches from the top bolt hole centerline to the top of the transom). Your bolts look like they may be a little higher up than that, and your dealer may have done that on purpose to ensure that the lower bolts clear the bottom of the splashwell. (If your bolt holes are closer than 1 and 3/4 inches from the top of the transom, I suggest you consider installing a transom reinforcing plate for the top bolts). With respect to the position of the motor, what really matters is how deep the lower unit is in the water while the boat is underway. If the lower unit is deep, the increased drag will slow down the boat and make steering a bit harder while making the propeller less likely to ventilate in sharp turns or in rough water. If the motor is high, you’ll go faster and the unit will be easier to turn, but you will encounter more prop ventilation issues. If it’s really high, you could even reduce the amount of water entering the intake holes below the ventilation plate. This is why some high-performance boats have water pressure gauges and low water-intake holes.

My Mercury owner’s manual says that the ventilation plate should be positioned from one inch below to even with the keel, preferably even, and many boats are sold this way. However, my ventilation plate is 1.25” ABOVE the keel; I think the dealer mounted it that way because one of the splashwell drain holes was installed too close to the midline, and this prevented the dealer from installing the motor any lower. My engine is mounted “2 holes up”, and I’m guessing by the position of your bracket, your ventilation plate is about even with the keel. You will find that most people on this and other discussion boards recommend the ventilation plate be 1.5” above the keel, but given my own experience and the possibility that you might encounter rough water in Currituck Sound, I think if your ventilation plate is 1.25” or even 1” above the keel, your performance should be satisfactory. Fine tuning the height isn’t really an option with your setup, as moving the engine up or down by one bolt hole changes the height by 0.75”, and pulling the engine to change the height is a huge hassle, which is one reason why people install jack plates. I’m satisfied with my boat’s performance (although I occasionally experience ventilation at the top of a large swell and in sharp turns), and I don’t expect much improvement with a jack plate, but I do look forward to lowering the motor a bit when I’m in rough seas and raising it to the max when I’m idling in shallow water as I approach Timbalier Island.

3. Once your motor/ventilation plate is where you want it, you should repeat your performance runs under conditions like those you had before your modification. I suggest you trim and check your boat speed at your “cruise rpm”, the motor speed which gives the maximum fuel efficiency. If you have a fuel flow meter and a GPS unit, you can calculate this rpm (and that is the best method), or perhaps you can find the info for your boat on the Internet; otherwise, you’ll have to default to the engine manufacturer’s number (which is about 4,000-4,200 rpm for my motor). Once you have trimmed your motor to get the maximum boat speed at that setting, I suggest you leave the trim set that way to get your time to plane and WOT speed. (Strictly speaking, the trim should be changed whenever you encounter a different boat speed, boat weight or weight distribution, or water condition, but I find this a hassle and rarely do it. When I leave the dock, I trim the motor all the way down/in, get on plane, set the trim at cruise rpm, and leave it there for the rest of the day.) You should also determine your motor's wide-open throttle (WOT) rpm with the appropriate trim. In general, after your boat is optimally set up, your WOT rpm should be close to the upper limit of the range specified by the manufacturer, maybe within 100-200 rpm. My motor's recommended WOT range is 5,000 to 5,500 rpm, and I think you will find that most people set up their boats to get a WOT just at or below the top of the recommended range. If you are still not there after repositioning your motor, only then should you consider swapping out your prop to achieve your performance goals.

Michael