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I am considering changing my prop on my 1983 montauk it has a 90hp 1995 tohatsu
I am considering composite Is that good?
Question is what is best pitch size prop and 3 or 4 blade?
Generally to recommend a prop, one needs to know the engine's gear ratio. Except for OMC/BRP 90's, and pre-1987 Mercury L-6 90's, which have a 2.0 ratio, almost all 90's have a 2.33 gear ratio.
Assuming yours is 2.33, and runs at 5500 RPM max, and depending how powerful and fast the engine is, you would need either a 19" pitch prop, or a 21" pitch prop. For years, Boston Whaler has furnished either a Mercury SS Laser II performance prop in 20" pitch, or a 21" pitch Black Max aluminum on 17/170 Montauks with 90 HP Mercs.
I would stay away from a comp prop. I think they are worthless. Since you are obvioiusly looking to save cost, a good aluiminum prop is a much better investment, and I would recommend either a Mercury Black Max or Quicksilver Black Diamond (same props). I think they are the best perfroming aluminum prop you can buy, and the pricing is competitive. With Flo-torq II hub kit, these can be purchased for about $120 or less. If you are looking for an excellent 4-blade, Mercury has a brand new "Spitfire" prop out (or Quicksilver "Nemesis") that is just as fast a three blade aluminum with much better acceleration and holding. It can be purchased in 19 pitch or lower, but not 21".
This site has great pricing on Mercury/Quicksilve aluminum props.
Low pitch will definitely give you more load carrying capacity. As for speed, it's not so easy to determine, but pitch should not make much difference. This is determined by the engine RPM limit.
These Mercury Quicksilver aluminum props are so economical to buy @ $85, that you can hardly go wrong once you have an interchangeable hub kit. I recommend buying the lower 19" pitch, and see how the engine runs with the boat lightly loaded. If you are near the redline, you are all set. If the redline is exceeded, you need the 21" prop for another $85. Then the lower pitch can be used when you are boating with a heay load. You can't lose.
I do not recommend aluminum props for the 4-3/4" gearcase 150's and higher, but for the 4-1/4" gearcase 75-125's, they perform very well from my experiences if you are not planning much boating over 40 MPH, and don't need a lot of hull lift. I think the Mercury/Quicksiler branded aluminums are the best you can buy, and even run well at elevated transom heights. They are much better than the old "elephant ear" aluminums of yesteryear. Top end they will run within 1 or 2 MPH of an SS performance prop, but if you are not planning much W.O.T. running, you can get along fine with a good aluminum propeller.
Propeller performance is not universal across brands since blade designs are diffferent. An X by Y prop from Yamaha will not perform the same as the same X by Y by Mercury. The same is true for
aluminum versus stainless steel.
A 21 and 19 pitch prop from the same company and in the same material and design family will have a different top speed. The planing speed will be different. The low speed ride does not change.
I have a stainless Mercury Lightspeed in the 13-1/4" x 20" motor mounted with the top bolt in the 3rd hole down. On flat calm water i can hit 44mph at 5500 rpm
