Prop cupping question

[Bob Kimber]

Hi all,<br /><br />I have a 13 1/2" dia. X 15" pitch SS prop from my old 90hp inline six, which runs OK on my 3cyl 90 hp Merc but the WOT rpm is too high now motor is run in (6100 rpm). What rpm reduction could I expect to get if I could get more cupping on prop, I would have to ship it out of town to a suitable prop shop and would need at least 600 rpm reduction to make it worthwhile to cover the expense.<br /><br />Currently running stock Merc alum. 13 1/4 X 17" pitch prop which has WOT rpm from 5200 to 5400 depending on tidal current and wind, this is on f/glass half cabin estimated running weight 2600lb with 2 x 200lb adults, full fuel tanks, all fishing and safety gear etc and a full length vinyl canopy from cabin to transom. Current alum. prop gives good speed 30mph @ 4,400 and 39mph @ WOT (by GPS) plus good economy.<br /><br />Have tried 16" pitch SS prop but still get 5900 @ WOT, just wondering if I could get slightly more rpm than alum prop with modified SS prop but still be in recommended range of 5,000 to 5,500 rpm for this engine, boat is used for fishing only, so economy and engine life is the only consideration.<br /><br />Bob

 

[Boatist]

Re: Prop cupping question

Boatins Bob<br />It look like your aluminum 13 1/4 x 17 is correct pitch for your motor. The stainless steel prop has thinner blades and usually cupping and same diameter and pitch normally about the same rpms. With the cupping sometimes can raise motor a little for a little more speed and rpms. If it was me I would look for a 13 x 18 or a 13 x 19 stainless steel prop. The 19 pitch is going to put you at the bottom end of your motor rpm range about 5000 rpms. The 18 should be near 5200rpms. This is usuing your 17 aluminum as a reference.<br /><br />If use your 15 pitch @ 6100 stainless as a refference then a 17 should turn 5700, 18 @ 5500 and a 19 @ 5300 rpms. This is assuming props near the same diameter and same style. The 19 at 5300 rpms is going to give your less hole shot but higher speed and better fuel mileage. 1 inch more pitch equals about 200 rpms less, 2" = 400 less, 3" = 600 less, 4" = 800 less.<br /><br />If buying prop from a dealer give him your numbers and tell him what you are looking for. Then ask if the 19 pitch sounds right to him and if for some reason does not turn as expected if you can exchange for one that does.

 

[Bob Kimber]

Re: Prop cupping question

Hi Boatist,<br /><br />Thanks for your response, my problem is, we seem to have to rely on stock props and can't get the range of sizes and variations of pitch and diameter that you can in the US (at least where I live). The genuine Merc aluminium props available are standard sizes, the stainless props seem to be the same diameter but pitch is even number or 1" pitch less than the aluminium equivalent<br /># Aluminium prop <br /> 13 x 19 <br /> 13 1/4 x 17 <br /> 13 1/2 x 15 <br /> <br /># Equivalent SS prop<br /> 13 x 18<br /> 13 1/4 x 16 <br /> 13 1/2 x 14<br /> <br />I am quite happy with the overall performance of the alum prop but I would have liked to continue running the old SS prop if it could have been modified. I have tried the 16" pitch SS prop as I noted in the first post @ 5,900 rpm also tried 18" pitch SS prop (next size up) @ 5150 rpm but I felt the 18 was too close to the lower limit of recommended range.<br /><br />Thanks again for your input<br />Bob

 

[Boatist]

Re: Prop cupping question

Boating Bob<br />I guess I am a little confused. If you are asking if you can repitch a Stainless Steel prop the answer is yes but only a small amount and not cheap. The cupping of props I have seen has had more to do with how high you can run the motor. With a double cupped Stainless can usually raise motor about 1 inch for a little less drag and more speed. If you run offshore in rough water I am not sure running motor higher is a good idea.<br /><br />If the rpm you gave is from your boat with a heavy load then I would use that 18 pitch Stainless. The way I understand it is any prop that WOT RPMS is between 5000 and 5500 is ok. The 18 pitch would certainly give you your best speed and fuel mileage.<br /><br />My boat has a 140hp I/O and wot rpm range for motor is 4200 to 4600. I have three props a 17 aluminum that got bent but have repaired and keep in boat as a spare. A 13 3/8 x 17 Stainless that turns 4600 rpms at 35 mph and I use in river and lakes where we start and stop for other boats or marineas. also normally 30 minutes runs or less.<br />Second prop a 13 x 19 stainless that tunrs 4200 rpms at 38 mph. We use this prop mostly in ocean where fuel mileage is important as make many runs 2 hours or more one way. In ocean most days run under 25 mph due to Seas and Swells so not to pound the boat too much. This prop get better fuel mileage also. When it is ruff some day only run 10 to 12 mph at 2000 rpms. Anyway both props fall in my wot rpm range and we have been useing since about 1982 and never has any engine trouble. <br />Good luck hope you find what you want.

 

[Bob Kimber]

Re: Prop cupping question

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