1971 Nauset 90-HP: New propeller

Optimizing the performance of Boston Whaler boats
ShorLine
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Joined: Thu Jun 30, 2016 10:16 pm

1971 Nauset 90-HP: New propeller

Postby ShorLine » Sat Nov 05, 2016 12:03 pm

I just purchased a 1971 Nauset with a 1994 Mercury 90-HP two-stroke-power-cycle engine. The boat and engine are in excellent condition, and run fine. Twin Cities Marine is doing routine maintenance on the engine. I would like to replace (what looks to possibly be) the original propeller with a Mercury LASER II Stainless propeller. Is this a good choice? What is best size? Looks like a 20-pitch has been recommended in the past, but wanted to verify since this is a Special Order item for the dealer.
Thanks!

jimh
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Re: 1971 Nauset 90-HP: New propeller

Postby jimh » Sat Nov 05, 2016 3:54 pm

Since you mention Twin Cities Marine, I assume you are in Wisconsin. I think you are running out of boating season up there. I'd get the boat on the water with its current propeller, and gather some data. Determine the current propeller's pitch and model. Then you'll have a basis for a new propeller.

I suspect that a new Mercury LASER II propeller purchased at retail as a special order is going to be expensive. Are you buying it on the basis that you can return it if it's not the right pitch? That's the only way to go with buying a propeller. If you can't return it for credit on another propeller after a sea trial, it can get really expensive to test propellers.

Since your weather-window for on-the-water testing is going to be limited, you might want to postpone buying a new propeller until Spring.

ShorLine
Posts: 18
Joined: Thu Jun 30, 2016 10:16 pm

Re: 1971 Nauset 90-HP: New propeller

Postby ShorLine » Fri Dec 30, 2016 11:16 pm

Still trying to get the prop situation decided, and Twin Cities Marine recommended today that they put the boat in the water and do an RPM test to determine the best prop for the boat. [Give me your] thoughts on [putting the boat in the water at this time of year and testing].

I am surprised that there is not a standard stainless propeller recommended for the Mercury 90 HP motor on a Classic 16 or 17 when there have to be a lot of boats out there with this combo. [What are your] thoughts [about the lack of a standard propeller recommendation]?

jimh
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Re: 1971 Nauset 90-HP: New propeller

Postby jimh » Sun Jan 01, 2017 7:48 pm

Re your idea of testing on the water at this time of year: I don't recommend making a selection of a new propeller based on testing on the water in the Great Lakes in January. There are two concerns:

--in the cold air with low humidity, the engine will be able to develop more horsepower than it can in your typical boating in Summer, when air temperature will be much higher and so will humidity; and,

--in the cold water with temperatures much lower than your typical boating in Summer, the water density will increase and the propeller will tend to be more efficient.

A propeller selected by testing in cold air, low humidity, and very cold water will not deliver the same performance in hot air, high humidity, and warm water because the engine will be producing less power and the propeller won't work as efficiently. My boat always goes faster, gets on plane faster, and gets better fuel economy when I am running in colder weather, with lower humidity, and with cold water. But I do my propeller testing, for the most part, in mid-Summer, with normal air and water temperatures.

I don't have any thoughts on why there is not a "standard stainless steel propeller recommendation" for your boat. I am not sure who would establish that standard. Is there a particular organization you would turn to for publishing that standard?

If you want some advice from Mercury, they usually have a propeller selector website that will take data from you and crunch those data to provide a propeller prediction, but I wouldn't take that advice as gospel. I have used their selection quite a few times and it always suggested a propeller with too much pitch.

Nothing really replaces on-the-water testing, but you should do the testing under conditions that will be similar to your normal operation. Maybe I am missing the notion that you like to do most of your boat in January.