---

When I asked a seller what speeds he achieved WOT, he replied :"per the operating instructions , the throttle is only advanced to 75% of max rpm. At that, it's about 20 mph." Does that sound right? And is he saying I can only get 75% or is it fixed to only go 75%? I would expect top speed of 40mph-ish +

OR let's say, what should I expect for speed on a 1989 Montauk w/original 1989 Mercury recently tuned at 3,750 RPMs?
Thanks.

---

Wow, I am shocked.

The 75% verbage sounds terrribly wrong to me.

Your engine has a wide open throttle (WOT) RPM limit of ~5500 rpms.

Classic Montauk's with a 2 stroke Mercury 90 hp report WOT speeds of 42-45 mph. Your boat's performance is horribly wrong. There are several intertwined factors that could be the cause:

Carburetors tuned- If your carburetors are not clean and synched your engine will not perform well.

Propeller - If your engine has the wrong size and pitch prop, it will not perform well. You want a prop that will reach 5500 rpm when running lightly loaded, full fuel and one person.

Mounting - If the engine is not bolted on the transom at the right height your engine many not perform well.

It is important to work on 1 variable at a time. First, check the prop. What is the make and size (part # stamped on the inside of the hub)

Are you on a lake or the bay/ocean?

---

How do RPMs translate to speeds. if 5500 RPMs achieves 42-45mph at WOT, what could one expect at 75% RPMs? or 4125rpms?
What would be a good pitch for a prop on this motor/boat combo?

---

The original engine on my boat was a 1989, 3 cylinder Mercury 2 stroke. It was mounted all the way down on the transom which was not unusual in that time frame. It came to me with a 21" pitch aluminum Quicksilver propeller (Mercury brand) and ran at about 41 mph on GPS with a light load.

I ended up using a Solas 19" pitch aluminum prop later on as it offered better all around performance with more than just me in the boat. I don't remember the RPM range but I could still get 39-40 MPH depending on the load.

Regardless of engine manufacturer, older engine or a new one, it's very important that you use a propeller that will allow the engine to get to the recommended wide open throttle RPM range.

---

A Mercury Laser II (also sold as a Quicksilver Lightspeed) prop in 20" pitch should be perfect for that engine on a Montauk.

But I would not buy that boat until you go for a sea trial, preferably with someone along who knows something about Mercs, Either the seller is a complete amatuer, and knows about engines in general, (that's a bunch of baloney he was giving you) or he is covering up an angine that may need serious work. The symptoms may sound like the high speed stator is out. If you buy the boat, expect to spend $1000-1500 getting the engine in shape, or even having to buy a different engine.

---

Consider as well that the engine is 28 years old, that's a long time especially if under salt water service. If it isn't running perfectly, negotiate your pricing based on the value of the hull as a candidate for being repowered.

When ours failed in 2012, I was already looking to repair some items; the pivot points the engine turned on were quite worn and sloppy. We couldn't find those replacement parts anywhere. Carb kits, water pump kits, fuel pump, etc. were no problem but anything more engine specific was starting to be a problem even 5 years ago.

---

Most boaters travel at the speed desired based on schedule, sea conditions and fuel economy.

75% of 42 is 31.5

---

Thanks. I Understand that math, but the speed does not increase in proportion to throttle position. It ncreases at a greater rate between 50-100%

---

Download an app to your cell phone that uses GPS to measure speed.

---

Where will it tell me what the current Prop is sin regards to specs?

---

Short answer: You need to sea trial the boat and make sure that it will run at in 5,200 to 5,500 rpm range with the throttle fully open. If the current owner won't allow that / the engine can't achieve it, it might be time to look at other boats.

Longer answer: Your quoted question above is not a question we can answer without a lot more information. However, note that we keep going back to what the engine RPM is at full throttle. It's not that we all go flying around at wide open throttle all the time, it's pretty rare that conditions allow that in a 17 foot open boat. It has to do with set up / running conditions the engine has experienced over its life, roughly 28 years in the case of the boat you're looking at.

Propeller pitch can be somewhat compared to gear ratios on a car; the higher the pitch, the further the theoretical movement through the water for each revolution. For example, with 0 slip, a 21 inch pitch prop would move the boat 21 inches forward per turn. There's no such thing as 0 slip by the way, as nothing is 100% efficient.

Too much pitch will "lug" the engine similar to driving a stick shift car up a steep hill in too high a gear. Too little pitch might cause the engine to rev too high, similar to driving a stick shift car at highway speeds in one of the lower gears.

Outboards need to run within an RPM range based on the manufacturers design; the 1989 Mercury on that boat needs to run at the speeds mentioned above at full throttle. Many years of operation with too much prop pitch will damage the crank and rod bearings and can cause them too fail. Not to open a separate can of worms, there are other mechanical issues that can keep an engine from running at full speed as others have mentioned above.

You don't want to buy someone else's problem, if you have your heart set on this boat it may be worthwhile to pay a Mercury tech to inspect the engine. Replacement engines are not inexpensive, trust me... Good luck!

Edit: I've seen a couple of your other posts where you mention price and a couple of pictures. Looks like a well kept boat and a pretty decent price for that boat here in the northeast. If it's in really good shape, the engine may have low hours. Maybe the current owner is a "collector" or a little quirky in the way he runs the boat to explain the 75% throttle statement. Again, best of luck.

---

Tested today on very choppy lake. Top RPM was 4,000 WOT. Propeller says: 48 77344 A40 17P. (Engraved on propeller) Top speed about 25MPH. Wind was strong.

---

I'm pretty sure that's a Mercury part number on the propeller you listed above and a 17 pitch. On paper, you should be underpropped and running above the rated RPM. As mentioned above, I have the twin of your boat and normally ran a 19 pitch. It would run on the high end of the RPM range with a light load.

Before you run the engine a lot more, I'd get it looked at by someone that's familiar with older Mercury engines. It could be something simple and possibly just cleaning and syncing the 3 carburetors. Pull the spark plugs and take a look at them as well. If one is worn or fouled that would affect performance quite a bit.

---

A Montauk will easily do 25 MPH with a 40 HP engine, so there is something seriously wrong with that 90 Merc that is only putting out 40 HP. Buyer beware, and don't say we didn't warn you.

An engine only putting out less that half of it's listed HP is not a cheap or simple fix. Start by getting a compression check to see if it has dropped a cylinder and needs a $3000 rebuild.

---

UPDATE: So I had the motor checked out. Compression good - spark good etc....
Found that one of the plastic connectors to the carbs was busted and only 2 of 3 cylinders were in use.

---

Glad to hear it was something simple. Keep an eye on your RPM the next time you attempt a full throttle run as the 17" pitch prop you're currently running is likely to over rev the engine as mentioned above. You should notice a big difference in performance with the engine running correctly.

---

What pitch prop would you recommend? 19?

---

Seems that enggass was/is in the same situation as myself, given his posts and the old Merc situation. I'm looking at a 1982 17 Montauk in Florida with an original 90HP Evinrude on it. The console and woodwork are in a very good condition. Brand new trailer. New steering, new 22 gallon Tank, new 8' Antenna and Radio, new Garmin 74DV and new Plexi dash panels. Hull seems to be in good condition from pictures (seeing it live tomorrow). Bimini is not great but does the job. The only "but" here is that the current owner says that, for the past year, he had no problem in keeping it a little over 30mph at 4500RPM. However, when he passes 4500RPM, in a matter of minutes, he gets an overheating alarm. According to him, he tried several solutions but nothing seems to work. This would be my first boat and I'm going with BWs for the durability, looks and easiness to keep and tow (I own a CUV). I like the low depreciation too. The owner wants $6,500 for it. I'm still not sure the price is OK, given the possible imminent need for repowering. What do you guys think about this motor situation?

---

See Larry's prop recommendation above. Post #5

---

vblm post18, sounds like a game changer on price. No way would I pay that for a boat and motor with mechanical issues. If compression is good, you could be looking at a large repair bill. Thermostats, water pump, stator, carbs, corrosion issues which could be a total loss. Its a gamble on the engine thats why its a game changer. You can always sell the engine on CL and reposer if it can't be fixed. those parts engines sell for under a thousand. So given that figure out what a hull and trailer cost based on the condition and go from there.

---

I had a similar overheat issue with a 1987 70 HP Mercury outboard. After doing all the easy things like water pump thermostats and poppet valve the problem persisted. I ended up pulling the power head off which was a chore since this was a salt water motor and many of the bolts were frozen. I also needed to remove the exhaust adapter plate, upper motor mounts, exhaust tube and quit a bit of other stuff to get to the gaskets and upper water tube seal. I replaced the large gaskets that sit below and above the exhaust adapter plate (took hours to clean the surfaces of the old gaskets off). I also replaced the upper water tube seal and the exhaust tube seal and any other rubber part while I was at it. I replaced all the old bolts with new ones. After getting it all back together success. The engine doesn't overheat anymore but boy was this a job. It took me many weekends and I lost a whole summer but learned quite a bit along the way.