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I am returning here to update where we are with my Montauk 17 and its performance.
Since my last post I have done the following:
Raised the Mercury 90 HP two stroke so it’s now two holes up, mounted in the third hole.
New bottom paint. Now has a smooth painted bottom.
Installed a Mercury Laser II 13 1/4 20 Pitch SS prop as recommended (eBay find)
After raising the motor I ran it for a year with performance numbers a little better. WOT at 5200 RPM with same load as last test and produced top speed at 34 mph. Similar conditions. This was with my SS 22 pitch prop mentioned and pictured above.
Recently did new bottom paint and installed the new prop. Did some testing this week. Same load, water and conditions. Top speed is still in mid thirties, got it to36 mph but only seeing 5100 rpm at WOT. DID THIS SEVERAL TIMES. Rpm’s will not top 5100 rpm. Closer to 5000.
With two 2 degree reduction in pitch I would have expected an increase in RPM and a bit more speed.using stock tachometer and gaps speed both from my Garmin and my iPhone speed app.
I am beginning to suspect my motor. Could it be starting to tire? Last compression tests showed 110-112 in each of the three cylinders. Motor is 23 years old and I have no idea what hours it has on it. Starts and runs ok otherwise.
I would appreciate any opinions
Edited by alibertucci on 07/09/23 - 11:38 AM
2000 Montauk 17
Smooth Whalin’
Phil T wrote:
When keeping everything the same, raising an engine will result in an increase in rpm at the same throttle position of 125 rpm for each hole raised.
Are we measuring speed via GPS? Is the speed dlsplaying in Knts or mph?
Is the prop new in the box or used?
I am measuring speed by GPS using two devices, my Garmin, which is console-mounted, and an app on my iPhone. Both read the same. Speed was measured each time in MPH.
The prop is new, open box. Not a scratch on it still. Also purchased a new Quicksilver hub kit and installed it properly to 55 foot pounds.
Edited by alibertucci on 07/11/23 - 8:02 PM
2000 Montauk 17
Smooth Whalin’
When I raised the motor to two holes up I did not see any change in the rpm at WOT. Then after new bottom paint and the change in prop I again did not see an increase in rpm at WOT.
Did you strip all the paint before adding new coat?
That may have resulted in negating the rpm increases.
Given the compression, that WOT speed is not bad.
The bottom was sanded smooth professionally and given two new coats of paint. I had it done at a marina here, the only one local doing bottom painting.
You seem to have done everything recommend correctly, yet still the engine RPM/Propeller pitch/speed number do not calculate correctly. The engine should turn closer to 5500 RPM, and with a 20" prop, you should be getting 40-42mph
considering the boat is bottom painted. Paint DOES hurt top speed.
This leads me to believe one of two problems. Either the engine needs work, or the boat is heavier than it was originally. Do you know the history of the boat, such as has it suffered damage and been repaired? This can often add a lot of weight, as fiberglass and polyester resin repairs can be heavy. Bottom paint is often used to cover up major hull repairs. A trip to a weight station might be in order. To me, in your photos, the boat looks heavy. Maybe just my crazy observation! I have a Montauk 17, which I know to be all original and correct factory weight, and with a Mercury 115 does 50 MPH. A friend has one with a Mercury 90, and it does 43, both boats without bottom paint.
The engine also needs to be looked at, as many things could cause it to be low on power, such as a weak stator, missing on one cylinder, plugs, etc. A good mechanic will be able to determine the problem.
Do you know the history of the boat, such as has it suffered damage and been repaired? This can often add a lot of weight, as fiberglass and polyester resin repairs can be heavy. Bottom paint is often used to cover up major hull repairs.
I do not know the history of the boat but when the bottom paint was recently redone, it was completely sanded and there was no sign of repairs. Two new coats of bottom paint done in May, 2023.
No doubt that the boat is slightly heavy. I always have a full 22 gallons of fuel on board before I go out with the boat. Also have a SS steel bimini on board along with me and my wife and the usual gear and a some ice and drinks. On an older boat there is no doubt some water intrusion as well.
The engine also needs to be looked at, as many things could cause it to be low on power, such as a weak stator, missing on one cylinder, plugs, etc. A good mechanic will be able to determine the problem.
I will be looking at the motor through my mechanic. An experienced Mercury shop in my area.
Thank you Finnegan for your thoughtful reply. I really appreciate the advice and you sharing your knowledge.
