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Quag

OK guys, I have been searching

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OK guys, I have been searching through the forums and I have come to find out that one of life's greatest mystery's is how to choose a prop for your boat....at least for a new boater.

I would like to pick up a new prop in the off season so I have a spare. I currently have a 15.25 inch prop 19P. It is on a Alpha 1 with a 3.0 liter Mercury. At WOT I did right around 40mph (perfect conditions) by speedo and I believe the RPM was 4800.

Couple things. I have looked all over for what the WOT RPM range should be and no luck . The owners manual that I got off line is pretty useless and could not find anything in the shop manual.

Quite honestly for a 4 cylinder it gave me me a decent hole shot and seemed to cruise fine loaded. No problem getting me up on skis or tubing etc. Just need to know what my top end speed should/could be, what I should be at WOT and if I have what seems like the right prop.

Any info and this great mystery would be great.

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4800 Is right where you should be. Top speed is a function of weight, wind, hull condition, etc, but when it's all said and done you need to protect the engine regardless of how fast you want to go or how good of a hole shot you desire. If it were mine I'd prop it a little south of 4,700 and use trim tabs for water sports, but I like to keep a stock engine toward the lower end of it's max RPM range.

Wingnut

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OK guys, I have been searching through the forums and I have come to find out that one of life's greatest mystery's is how to choose a prop for your boat....at least for a new boater.

I would like to pick up a new prop in the off season so I have a spare. I currently have a 15.25 inch prop 19P. It is on a Alpha 1 with a 3.0 liter Mercury. At WOT I did right around 40mph (perfect conditions) by speedo and I believe the RPM was 4800.

Couple things. I have looked all over for what the WOT RPM range should be and no luck . The owners manual that I got off line is pretty useless and could not find anything in the shop manual.

Quite honestly for a 4 cylinder it gave me me a decent hole shot and seemed to cruise fine loaded. No problem getting me up on skis or tubing etc. Just need to know what my top end speed should/could be, what I should be at WOT and if I have what seems like the right prop.

Any info and this great mystery would be great.

quag- if you do a search on bobman you will see how he has propped his 180 with a 4 banger or better yet PM him

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Quag, your right there...as Wingnut stated. Depending on tach accuracy, 4,700-4,800 RPM @WOT is right.

Can you tell me more about your prop, aluminum/SS, 3 or 4 blade?

My boat (180 SSi, 3.0L, 2.00:1 Alpha) came with a 3 blade aluminum 14.25 dia x 21P prop. I'm currently running a Vensura 4 SS 19P prop and my WOT RPM's are 4,750 with a top end of 43 MPH on the speedo. For my boat, another great option is an Alpha 4 14" x 20P (4 blade aluminum).

What does your boat weight? The dry weight on my 180 is listed as 2,100 lbs. I think yours might be heavier, thus explaining a slightly lower top speed (but probably rides nicer in rougher water).

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The Merc website shows the WOT range for current 3.0's as 4400 to 4800. I doubt it has changed much over the years. I like to prop for the high end of the range, so it sounds as if you are propped correctly.

You don't mention if the speed is GPS verified. 40 mph sounds just a tad low. Your speedo may be off a bit.

Overall, sounds like you are right where you should be.

brick

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Quag, your right there...as Wingnut stated. Depending on tach accuracy, 4,700-4,800 RPM @WOT is right.

Can you tell me more about your prop, aluminum/SS, 3 or 4 blade?

My boat (180 SSi, 3.0L, 2.00:1 Alpha) came with a 3 blade aluminum 14.25 dia x 21P prop. I'm currently running a Vensura 4 SS 19P prop and my WOT RPM's are 4,750 with a top end of 43 MPH on the speedo. For my boat, another great option is an Alpha 4 14" x 20P (4 blade aluminum).

What does your boat weight? The dry weight on my 180 is listed as 2,100 lbs. I think yours might be heavier, thus explaining a slightly lower top speed (but probably rides nicer in rougher water).

Bob and Brick, just an update. Dry weight is listed at 2450 but it has a 30 gallon tank, I think the newer models have a smaller tank and are certainly lighter. Prop is alum and 3 blade (think a MI Match) Sounds like I am close. All the sizes I have seen have been south of 15.25 which mine is. If I keep the same pitch, but get a smaller blade, how will that do for my top end speed?

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Dry weight is listed at 2450 but it has a 30 gallon tank, I think the newer models have a smaller tank and are certainly lighter.

You're correct again! :) I think the weight difference would easily account for the slightly lower speed. (2,100# vs 2,450# & 23 Gal vs 30 Gal) I wish I had the larger tank. ;)

Prop is alum and 3 blade (think a MI Match) Sounds like I am close. All the sizes I have seen have been south of 15.25 which mine is. If I keep the same pitch, but get a smaller blade, how will that do for my top end speed?

I think a smaller diameter would remove some prop loading and allow things to spin-up quicker. It might help the holeshot, but would lower your top end and maybe put you over your WOT RPM. You seem satisfied with the holeshot, so I'd leave things alone. I personally feel that the 3.0L's torque is unable to take advantage of a stainless prop. While I do have one, the added rigidity of the material is more beneficial to to powerplants that can really twist a prop. The 3.0L doesn't fall into that catagory. But I also feel that it's a good little motor in the right sized boat. ;)

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You're correct again! :) I think the weight difference would easily account for the slightly lower speed. (2,100# vs 2,450# & 23 Gal vs 30 Gal) I wish I had the larger tank. ;)

I think a smaller diameter would remove some prop loading and allow things to spin-up quicker. It might help the holeshot, but would lower your top end and maybe put you over your WOT RPM. You seem satisfied with the holeshot, so I'd leave things alone. I personally feel that the 3.0L's torque is unable to take advantage of a stainless prop. While I do have one, the added rigidity of the material is more beneficial to to powerplants that can really twist a prop. The 3.0L doesn't fall into that catagory. But I also feel that it's a good little motor in the right sized boat. ;)

Thanks for the info, I will get another 15.25 and 19P prop. Buying used and not knowing, just wanted to confirm things. BTW, I agree with you on the motor. Not going to be doing 60 anytime soon, but a friend of in has a big 8. I get by on two days on the water to his one with out filling up. For me, right now that is just right.

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I think the 15.25 is a little big for your boat. Can you try a 14.25 diamter? I think it will really help your hole-shot. Probably minimal or no difference in top end. You might have to go to a 21 inch pitch to keep RPM's at 4800. Less drag from smaller diameter equals higher top speed.

As far as SS vs Aluminum, I agree that not too much improvement, although I gave one of my friends my old used 14.25 X 19 SS and it incresed his top end by 2 mph. It was also a 3.0

And it looks good too!

brick

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Finally got wet yesterday and it was a great day, but that is for another post. Looking for some opionions on the prop again. I went ahead and got a 14.25 inch and 21 P prop from a 15.25 19P. I must say it made a pretty good difference. Better hole shot and I did have a better top end. By speedo I increased to about 45 (though I think it is off)

My concern is at WOT at one point it went up to 5000, however I ran it at WOT again and topped out at 45mph and 4800 RPM. Not sure of the accuracy of the tach, however I never saw it rev to 5K before.

I like the performance of the prop and quite honestly have no reason to rev the engine that high. Seems most everyone with my set up has a 14.25 and 21 P prop. Just looking for opinions on if I should change out since there is a possibility of reving to 5K. Go back to a bigger prop by 21 P?

Thanks for the help.

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Finally got wet yesterday and it was a great day, but that is for another post. Looking for some opionions on the prop again. I went ahead and got a 14.25 inch and 21 P prop from a 15.25 19P. I must say it made a pretty good difference. Better hole shot and I did have a better top end. By speedo I increased to about 45 (though I think it is off)

My concern is at WOT at one point it went up to 5000, however I ran it at WOT again and topped out at 45mph and 4800 RPM. Not sure of the accuracy of the tach, however I never saw it rev to 5K before.

I like the performance of the prop and quite honestly have no reason to rev the engine that high. Seems most everyone with my set up has a 14.25 and 21 P prop. Just looking for opinions on if I should change out since there is a possibility of reving to 5K. Go back to a bigger prop by 21 P?

Thanks for the help.

I'd keep what you have. Early in the season yet, and as summer heat and higher humidity come on, your engine will lose some RPM and hp. I think you're dialed right now for summer, especially if you were testing the boat with a light load.

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I'd keep what you have. Early in the season yet, and as summer heat and higher humidity come on, your engine will lose some RPM and hp. I think you're dialed right now for summer, especially if you were testing the boat with a light load.

Thanks and I did forget to add, I was the only one in the boat. I do think that RPM will come down with more weight.

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