1995 Montauk 17 Porpoising with 90-HP

[ktval]

Hello everyone. I just installed Mercury 2001 90-HP outboard engine on my 1995 Montauk 17. [Later in the discussion upon direct solicitation we find out that the engine is a two-stroke-power-cycle engine with power trim and has a 20-inch shaft.]

I installed it on the second to last hole, almost all the way up. [This is an ambiguous description of the engine mounting height because we do not know how many mounting positions were available. There could be four sets of holes or perhaps five. Please describe the engine mounting height in terms of how many sets of holes or 0.75-inch increments the engine has been raised from the lowest position. As you have described the mounting height, the engine could be mounted two-holes up (if there are four sets of holes) or it could also be mounted three-holes up (if there are five sets of holes)--jimh]

Running a [LASER II] 20" propeller, the boat runs great, however it porpoises very badly at upper speeds. Any ideas?

[Oldslowandugly]

I would lower the motor one hole at a time until I got the best performance. In my case I had [another ambiguous description of engine mounting height. Please describe engine mounting height in terms of the number of holes or units of 0.75-inch the engine has been raised from the lowest position--jimh]

[Now appears to change the subject and begins to discuss the placement of the tilt/trim mechanical limit pin used with engines that do not have power trim--jimh]

The outer hole caused the boat to porpoise. The inner hole made the bow dig in. The middle hole was just right. Without power tilt you have to compromise between speed and handling by testing all the possible motor positions.

[jimh]

Fellas--please stop describing engine mounting height in these ambiguous manners. Please see the pinned article at the top of the PERFORMANCE forum for advice on how to describe engine mounting height:

The link is http://continuouswave.com/forum/viewtopic.php?f=7&t=739

Does the 2001 Mercury 90-HP engine have power trim? I would expect that it does.

What is the engine shaft length?

Is this engine a classic two-stroke-power-cycle engine? Or a FOURSTROKE? Both were available in 2001.

What is the engine weight?

[jimh]

Of all classic Boston Whaler boats, there is no model about which more performance information is available than the MONTAUK 17. The MONTAUK 17 has been re-powered by all sorts of engines, but among all those a 90-HP engine is by far the most common. I do not recall any prior reports in which a MONTAUK 17 with a 90-HP engine has been described as porpoising "very badly at upper speeds."

It is possible to induce a planing condition in just about any outboard-powered Boston Whaler boat which will result in an oscillation of the bow up and down. Such oscillations or porpoising occurs in certain combinations of waves, engine trim, and weight distribution. The usual remedy is to reduce engine trim to a lower trim position, bringing the bow down. It is quite normal that some hull, engine, and propeller combinations will create a very narrow range of trim that gives stable planing. Weight distribution also plays a role. Too much weight aft and little weight in the bow tend to make porpoising more likely.

With 90-HP on a classic MONTAUK 17, the top boat speed should only be in the 40 to 44-MPH range. This is not a particularly fast speed for the hull. If there were a natural tendency for porpoising, I would expect that we'd have many more reports about it. In comparison, there are reports of a 17-foot Whaler classic hull hitting 55-MPH (with 115-HP) "with no porpoising problems that couldn't be corrected by proper engine trimming." Compare at

http://continuouswave.com/ubb/Forum4/HTML/000464.html

Check the bottom surface of the hull near the stern for the possible presence of a hook, that is, a downward turn of the running surface as you approach the transom. It sometimes occurs in boats with loss of structural integrity in their hulls, perhaps from sitting on a trailer for a long time without proper transom support. The running surface should be flat as it approaches the transom.

[jimh]

Another problem with your MONTAUK 17, 90-HP engine, and LASER II 20-pitch propeller may be too much propeller pitch. The 90-HP engine probably lacks sufficient power to hold the bow of the boat up when planing because there is too much propeller pitch.

Give us a table of data showing engine speed and boat speed at various throttle settings. I suspect your choice of a 20-pitch propeller is probably holding down the engine speed and contributing to the problem of porpoising.

[ktval]

[T]he engine is a two-cycle, 20-inch-shaft and has power trim. The engine spins up to 5,400-RPM, which Mercury says is good.

I read [through] the many articles, and it [seems] to me that either mount the motor all-the-way-up or just one-hole down, which is what I did.

I will try playing with the trim some more.

[ktval]

[T]he engine has five mounting holes. It is mounted in the fourth one from the top, or second from the bottom.

[Phil T]

Please describe in detail the location and weight of the passengers, gear and fuel load in the boat. Detail weight of contents in console and bow locker.

What is the engine trim setting when the boat is porposing?
What are the wind conditions and sea state when the boat porposing?

[jimh]

[T]he engine has five mounting holes. It is mounted in the fourth one from the top, or second from the bottom.

Please read the pinned article at the top of this forum on how describe engine mounting position.

http://continuouswave.com/forum/viewtopic.php?f=7&t=739

We don't count holes from top or bottom. We describe the engine mounting height in terms of how many units of 0.75-inch or "holes" the engine has been raised from the lowest possible mounting position. Please adopt this method for describing your engine mounting position.

If you can't figure this out, please take a picture and we can figure it out for you.

[jimh]

I read [through] the many articles, and it [seems] to me that either mount the motor all-the-way-up...

Could you please give a pointer or URL to those articles which you read on CONTINUOUSWAVE that told you to mount an engine on a MONTAUK 17 in the highest possible mounting position? I don't think there is any advice given like that, but, if there is, I want to know about it and remove it, before it confuses anyone else as it appears to have confused you.

... or one-hole down...

There is no engine mounting position that is "one-hole down." The engine starts at all the way down, and then is moved up.

Please read the pinned article at the top of this forum on how describe engine mounting position.

http://continuouswave.com/forum/viewtopic.php?f=7&t=739

[jimh]

A classic Mercury 90 CLASSIC two-stroke-power-cycle carburetor engine of c.2001 has the following specifications:

GEAR RATIO = 2.30:1
WEIGHT = 303-lbs
POWER TRIM = standard
RECOMMENDED RPM RANGE = 5,000 to 5,500
Taken from a Mercury catalogue in my collection

This engine is perfect for a classic Boston Whaler MONTAUK. The propeller calculator figures a boat speed of 40-MPH with SLIP=10 with 5,400-RPM engine speed and a propeller pitch of 20. That sounds about right for a MONTAUK with 90-HP. Compare at

http://continuouswave.com/whaler/refere ... power.html

There should not be any problems with a continual oscillation of the bow up-and-down while on plane at 35 to 40-MPH if the engine trim is properly adjusted.

I will try playing with the trim some more.

This is likely to be where you will find a remedy to the bow oscillation on plane. If nothing obtains from that experiment, try adjusting the engine mounting height.

The most likely starting place will be to mount the engine one-hole or 0.75 inch above the lowest possible mounting position. I am not really sure where you engine is presently mounted, but, if I had to guess from your various descriptions based on various reference points, it sounds like the engine is mounted three-holes-up from lowest, That position is quite unusual and perhaps too high for best all-around performance. It could account for what sounds like an inability of the engine and propeller to hold the bow up in a raised trim position.

[ktval]

OK I will try and lower the motor, it is set at 2.25" right now and see how that works. thank you for your help!!

[Jefecinco]

This may seem superfluous but I have to say it. On all my boats I've selected propellers (often with a lot of help) that can achieve the manufacture's maximum recommended RPM or slightly above or below that number at WOT. I.f I want to proceed at top speed I open the throttle. Sometimes depending upon water conditions, the load in the boat, weather, etc. the boat will porpoise at full throttle. My tendency is to get on plane at WOT with the engine trimmed full in and as plane is achieved to throttle back a bit and to trim the engine out until it porpoises a little. I then trim the engine in until the porpoising stops.

Usually I like to burn as little fuel as possible so I slow down to a speed slightly higher than required to maintain plane. Throttling back sometimes makes it difficult to maintain plane, crossing a wake can cause the boat to fall off plane into a plowing speed. The solution to this is usually a slight RPM increase and a slight adjustment in of the trim.

I don't think there is a perfect propeller or engine mounting height for our boats. Our conditions are almost constantly changing. Having said all this I believe your engine is mounted low. I would go up two holes and test.

[jimh]

...the motor...is set at 2.25" right now...

I guess no amount of asking you to please use the "holes-up" method of description is going to make any effect. Let's see if we can do the math:

2.25-inches x (1-hole/0.75-inch) = 3-holes

Your engine is mounted three-holes up.

[jimh]

... I believe your engine is mounted low. I would go up two holes and test.

JEFE--I think the engine under discussion is already mounted three-holes up. It is not possible to raise the engine two more holes.

[Jefecinco]

Jim--thank you, clearly I misunderstood what I thought I read. I like that you have presented the standard method for describing engine height mounting and asked us to adopt it. It's simple and easy to picture in my mind.

Have we asked the depth of the water over the anti-ventilation plate while the boat is on plane?

[jimh]

As JEFE suggests, the position of the outboard engine anti-ventilation plate relative to the water line when the boat is running on-plane and the engine trim is optimized can be a good indicator of engine mounting height. In a prior discussion with many illustrations, you can see various engine installations and the position of the anti-ventilation (A-V) plate relative to the water line. This discussion from the OLD FORUM is found at

A-V Plate Pictures
http://continuouswave.com/ubb/Forum4/HTML/006491.html

[ktval]

I moved the motor down one hole, and that seemed to help things out a bit. Thank you everyone for your help in this matter.

[jimh]

Good outcome.

Do you have any data on engine speed and boat speed?

Did you find any articles which you referenced earlier that told you to always mount the engine as high as possible?

Re how to describe engine mounting height:

Please read the pinned article at the top of this forum on how describe engine mounting position.

viewtopic.php?f=7&t=739

We don't count holes from top or bottom. We describe the engine mounting height in terms of how many units of 0.75-inch or "holes" the engine has been raised from the lowest possible mounting position. Please adopt this method for describing your engine mounting position.

[ktval]

The engine turned 5500-RPM and the GPS receiver showed 43.5-nautical-miles-per-hour. It was a nice light chop on the water, trimmed up well, about 6-gallons of fuel, and just me at 230-lbs.

[jimh]

A boat speed of 43.5-nautical-miles-per-hour is 50-MPH. Are you certain that your GNSS receiver has been adjusted to show SPEED OVER GROUND in units of nautical-miles-per-hour? The reason for asking is your reported boat speed seems to be much faster than any other reports of a MONTAUK 17 with only 90-HP. You may have the fastest MONTAUK 17 with a 90-HP engine in history.