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Old 10-10-2011, 03:15 PM   #1
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Default 2005 1800MX Prop selection

Hi guys,

Reasonably new owner of a 2005 1800MX 3.0L 135hp Mercruiser and looking for a bit of advice regarding props.

Usage is pretty much wakeboarding all over so not too fussed with regards to top end.

Current prop (came with it) is a Laser II 19p....Current performance is around 35mph top speed (sorry dont know rpm - new to this!!) (which seems low) and seemingly an age to plane.

What Im looking for a low speed planing and good hole shot. Prop selector on Mercury web says recommends high 5 19p - whats all your lot opinion and is the 35mph speed normal or a symptom of issues?

I can get a SH high 5 21p but not sure if thats too much pitch?

When giving it full throttle I get a fair bit of vibration as well which seems a bit odd (like the prop is ventilating).

Thanks all in advance
Ben
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Old 10-10-2011, 03:47 PM   #2
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Ben
welcome to the zoo...
well..there's a few things we need to verify before you go plopping down tons of money on a new prop....like where are you located so you can buy us all beer...........(might wanna introduce yersef some)...
2nd is ..what is your wot rpm??..ie when your out on smooth water..hit the throttle to wide open throttle and see where the top speed/rpm is...
how much gear and people do you have on board the boat....fuel....food..beer....etc.....this will tell you which way to move the prop ....
to answer your other question about cavitating....it may well be doing that or it may be spinning the prop hub....so...the thing you need to find out is 1)wot rpm...you should be between 4200-4800rpm....2) how heavy the boat is with gear..(take everything off you don't need)....and what the prop condition is...ie nicks, dents, out of true...etc....take it off and to a prop shop to have it tuned back up...

now to answer your question about props....if you go up in prop pitch..ie 19 to 21...you will loose about 400 rpm off the top end and will probably gain some mph .....now if you go down in pitch...ie 19 to 17..you will gain 400 rpm..ie 4200 rpm to 42400 rpm or roughly 4300 rpm(acct for gauge error).......
going down will decrease your hole shot time or time to plane....going up will increase it.....make sense???

SP
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Old 10-21-2011, 01:52 PM   #3
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If its any help, i have a 1900SR2 witha 4.3L, i use an 18P 3 balde when i pull skiiers or am on a small lake... she drives like a vette but only has a top in around 35-40 MPH. When we go to larger lakes an I know I'm going to be running around alot (poker runs, bar runs, etc.), i'll put on a 21P 3 balde aluminum and my top end moves to 45-50 MPH but a lot less responsive out of the hole. I also use Smart Tabs to help get her on plane more quickly in all scenarios (love them)

Some will recommend stainless steel props but if you boat in areas like i do in the midwest where there is often sumbergred timber and other obstacles in the water, i recommend Aluminum props for the one reason a lot of folks hate them, they break easy. I'd much rather replace a $100-110 prop than the gears in my outdrive . Not sure where you live but check the web for a place called "midwest prop", they have great prices and service.
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Old 10-21-2011, 05:04 PM   #4
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All great points. The only thing i can add is, I doubt the prop will dictate how slowly you can run while still staying on plane. Tabs and trim will really come into play here. I don't suggest a hydrofoil. While that will give the stern lift and allow you to run on plane at slower speeds, the lift is being placed on either the drive and transom assembly, or the outboard and the transom mount, depending on whether it is an I/O or OB. You're better off suing tabs and trim which obtains lift from the hull. The smart should have manual adjustments which are statically set. Possibly try setting at a more aggressive setting. Otherwise, adjustable tabs such as bennet (hydraulic) or Lenco (Electric) might allow you more control and adjustability over a smart tab.

One more thing to keep in mind. If you're goal is not top end, but rather hole shots and slower speeds for towing sports, make sure not to go too low in the pitch. While lowering pitch will lower top end speed and decrease the hole shot times, you want to make sure that the Max. RPM is not exceeding the Max. RPM rated for the motor. In testing, you should still test WOT and make sure you remain within the range of the motor.
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Old 10-22-2011, 02:51 PM   #5
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Would agree with Shrew on use of tabs for planeing, however I think on smaller craft (<20'), on-the-fly adjustable tabs can be dangerous, so i highly recommend the SmartTabs. I isntalled the SX model with 60# pistons and am extremely pleased with the results... planes faster, no more chine walking, and bites into turns. When looking for solutions i also considered hydorfoils but after researchinn further, decided I didn't want to drill holes in or put the stress on the outdrive... not that drilling 16 holes in the transom was any less nerve-racking
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Old 11-10-2011, 10:26 PM   #6
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all good advice. As long as your motor is in good working condition, those advices are true for a straight forward answer.

If you have 500 bucks to spend, I would buy a redline weber carb to replace the stock one. I have a 2008 maxum 1800mx with the 3.0 and the weber. I run a 21 pitch prop and with 9 people (I know, too many people), I can pull up a heavy tuber. If I have just me, my 0-30mph is 3 seconds. Hope that helps
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