Montauk 17 with 90-HP

[JJ Donovan]

I have a 1990 Boston Whaler Montauk 17 with a 1990 Yamaha 90 HP 2 stroke. I just bought the boat used and would like to know the correct pitch and diameter of the prop, stainless or aluminum, and the best recommended brand for this application.


Thanks John Donovan

[Phil T]

Whaler owners and members have been tested and highly recommend the props listed below for the Montauk 17 and a Yamaha 90 2 stroke engine.

13" x 17" Yamaha Painted Stainless Steel
13-1/4" x 16" Yamaha Performance Series 3 Blade
13-1/4" x 15" Stiletto Advantage 4.25 (discontinued)
13-1/4" x 15" Turbo 1 4.25

If you select a different brand and/or model in a similar size it may not necessarily be appropriate. Blade design makes prop sizes unique. Size is not universal like a car tire.

The engine should be mounted 2 holes up. See this thread for mounting information: http://continuouswave.com/forum/viewtopic.php?f=7&t=739

[JJ Donovan]

Phil—thanks for the reply.

I forgot to ask what RPM at WOT is best for this engine and the expected top speed.

What is the best of the 4 props you recommended.

I am looking for the most fuel efficient of the four and am not looking for high speed .

John Dooonovan

[jimh]

...I am looking for the most fuel efficient [propeller] and am not looking for high speed...

In order to assess the fuel efficiency of a engine-boat-propeller combination, you will need some means to very accurately measure the instantaneous boat speed and instantaneous fuel flow rate. Without this sort of instrumentation, you will never know the fuel economy. There are many people who lack this sort of instrumentation, but freely give advice about the fuel economy of their boat, engine, and propeller combination, based on very imprecise measurements.

If the goal of your near-future use of the boat is to test for maximum fuel economy, you will first need to obtain the necessary test instrumentation, and then gain some experience using it with the present propeller. Once you get a good data set as a basis of the current fuel economy, then you can try a different propeller, run it long enough to collect an equivalent set of data, then compare the results with the first set.

I don't think you can get the "most fuel efficient" propeller for your particular boat and engine any other way.

When you conduct your fuel economy testing, be certain to maintain other influences constant. The other influences on fuel economy will be:

--air temperature
--air humidity
--wind speed and direction
--water current and wave condition
--water temperature and salinity

Also, you might give consideration to other aspects of a particular boat-engine-propeller combination, other than the highest possible fuel economy. In some instances, choosing solely on the basis of greatest fuel economy will not produce the best all-round operation. Other aspects of performance will be

--acceleration
--minimum planing speed
--freedom from ventilation or blow out in rough seas and turns.

If you test the four propellers mentioned by Phil, I would expect that the difference in fuel economy among them will not be very large. You might find a range from worst to best of perhaps 10-percent.

If fuel economy is really the ultimate goal, get a modern engine. It will improve fuel efficiency by more than 50-percent. This will be a much bigger improvement than a propeller.

[Phil T]

Ideal WOT for the 1990 Yamaha 90 motor is 52-5400 while the WOT rpm limit is 5500.

The desired WOT speed of this engine and model given a light load, single operator on a slight chop and no wind is 41-43 mph. 40 mph or more is a bullseye.

My 1987 Montauk with a 1987 Yamaha 90hp ran with the 13"x17" Yamaha Painted Stainless Steel.

Any of the 3 are good. Shop on price.

It is important to note that we talk about props, mounting heights, WOT rpm's etc and top speed. If the boat is setup accordingly the engine will perform properly at all speeds. Think of it as having a 12 speed bike. You want to adjust the chain derailer to travel across all the gears. If not done right it will limit you to only a few and they might not be the right ones to go slow, uphill etc.

[JJ Donovan]

Thanks for the reply from you both. I was working on the Yamaha today and cleaned up the prop. It doesn't look too bad, it had K 17 engraved on it and is stainless steel, it must be the original. The information you gave me will give me an idea of what to look for at WOT, so I will know that the prop is correct and also an idea on top speed. This engine has good compression, the lower end oil is clean and clear, installed a new impeller, so it should be good to go. New ETEC 90 installed is $10,300.00 plus tax, so I'm hoping my Yamaha will last for awhile.
Your willingness to take the time and share your knowledge is appreciated.

Thanks Again

John Donovan

[jimh]

The Yamaha two-cycle three-cylinder engines--a 70-HP and 90-HP--are very good engines. The power head--at least on the 70--was an exact copy of the OMC three-cylinder. These engines can run for years longer, as long as you properly feed them good-quality two-cycle lubricating oil. Don't skimp on the oil quality; try the recommended Yama-Lube. Avoid cheap oil from big-box discounters sold under house brands.

Because your engine is from 1990, the rubber components in the fuel system should be rated for use with ethanol-gasoline blended fuels. But watch out for using blended fuels with too much ethanol. Blended fuels with too much ethanol can cause deterioration of the rubber components in the fuel system and perhaps to other components like pumps and seals. Choose your fuel supplier carefully.

Keep the fuel supply hoses, primer bulb, and connectors in top shape. Any air leak into the fuel system can cause fuel starvation. In two-stroke-power-cycle engines the cylinder cooling depends on the fuel; with lean fuel the cylinder temperatures rise. Lubrication depends on fuel; with lean fuel there is lean lubrication. The combination of hotter cylinder and less lubrication leads to disastrous results in a cylinder. Keep an eye on all fuel-system hoses, connections, joints, etc.

I had some experience with a 1987 Yamaha three-cylinder 70-HP engine I can share:

--the c.1987 spark plug wire rubber boots were rather stiff and had a tendency to fall off the spark plugs; Yamaha may have improved the spark plug boots by c.1990. Check the spark plug wire boots to make sure they are firmly on the plug and not likely to come off. On my engine, I used a nylon cable tie to increase the tension of the boot onto the plug surface.

--check the gear case lubricant for signs of water intrusion. The rear propeller shaft seal could be worn and allowing water to enter the gear case. If you need a rear propeller shaft seal, you will have to unthread a circular ring nut. When I had the rear propeller shaft seals replaced on my engines, one of the ring nuts was corroded and could not be loosened. In order to remove the ring nut it had to be broken and taken out in pieces. The shop that did the work noted that the ring nut on the Yamaha was the same as on a Mercury engine, and they used a Mercury ring nut to replace the original (due to much easier availability and lower cost).

--cowling decals can be very expensive; protect the engine cowling decals with wax. Keep the cowling clean, washed, and waxed, and cover the cowling to protect against sunlight damage when not in use with some cloth engine cover that protects against ultra-violet radiation.

Re the propeller: a K17 is a good steel propeller, probably painted black. If the propeller is in perfect condition, I would not rush to get something different. But do not run the engine with a propeller that has bent blades or damaged blades. The load becomes unbalanced and causes shaft bearing and seal wear--more expensive to remedy than a new propeller will cost.

[JJ Donovan]

Thanks Jim, when I get everything finished and running I will send some pictures and a report of how things turned out.

John Donovan