Propeller options
 

First, I would like to thank you for allowing me to join your forum. I have recently aquired a 1976, 20' Seafari, the I/O model (boat is clean, always trailered, pretty sweet ride). The motor and outdrive combination currently installed, is a 140hp mercruiser (approx 200hrs), coupled to an Alpha 1 outdrive. Compression on motor checks out very well, and outdrive was completely gone through in the early part of the summer, prior to my taking posession. Both pieces (I/O and motor) came out of a 25' Blackfin with twins (guy took a plastic bag on the intake of other motor, and over heated it,... replaced both engines and outdrives last season). Here in-lies the problem. The propeller that is currently installed is a 13.75 x 21. I'm not getting the RPM's out of the motor. It is supposed to turn 42-4600 wfo, all I can get is about 3850 (I have been through the motor/fuel system, and I know I'm down to a torque issue from the propeller). I'm aware that a single inch of pitch should account for about 200rpm on the top end(this would translate to a 13.75 x 19), but before I go spend $$, I'm sure that somebody has a nearly identical set up they have proven and are happy with and has already been through this evolution. Any suggestions?

Re: Propeller options
 

Kahuna - Bojapete has the same setup as you have.

There is also an article in the literature section.
http://www.classicseacraft.com/broch...cle/index.html
That boat was turning a 15x21.

I spent (wasted) a great deal of time trying different props on my boat. I ended up with a Mercury Mirage 19pitch . . .

Two props I recommend are the Mercury Mirage plus or Highfive. Try the 19 pitch and find a Mercury Dealer that will let you try before you buy. Ebay is also a good option b/c you can find some very good deals ($200 - $250 for either prop).

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I've also heard that these Ron Hill props are excellent. He's an old school offshore racing guy on the west coast that been making props for 30+ years. The price is very good too!!

http://www.hillmarine.com/MercruiserAlpha.html

Re: Propeller options
 

the Mirage plus is a great prop, jsut takes some HP to turn it, the regular Mirage takes even more. That prop wass originally design for big block bravo set up's. Most out boards didn't have enough torque to spin them up, so they came out with the mirage plus, it has a little less surface area, and has ventilation holes in the hub for fine tuneing. If you boat will spin a Mirage, great! You'll have a very effecient set up, but due to the hp of your engine and boat size, I would probably start our with a same pitch regular old Mercruiser SS( Vengence) or Aluminum prop in the same pitch. Pitch change is not consistant when you change styles of props. Stay away from any props that have a lot of rake( swept back blades) they are for lighter faster boats requiring bow lift. Four blades, and even five blades generally don't have as much rake, and create a lot of stern lift, this helps to keep the boat on plane at a lower speed

Re: Propeller options
 

Hi Kahuna, my boat used a 19" pitch prop and I could reach about 4200-4300 rpm fully loaded, not the best but close enough. This gave me about 32 mph again fully loaded with the Bimini up so it's not a bad setup. Unfortunately the motor seized up this spring for unknown reasons. The mechanic told me these models tend to do that once in a while but I can't see why, especially when the oil and temperature were always OK.

By the way, are you in Hawaii? I'll be going to Oahu in Dec. to visit my son and grandkids, if by any chance you're close I'd like to see your rig if possible.

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Hey Botapeje, You are turning a 19 x 13.75. Ok, sounds good. I think I'll go with that. Since mine is a little older motor (still with low hours) I think I'll stay with the aluminum wheel vice the stainless. Just to give it a little more slip and try not to load the motor. I usually don't run the boat much over 20knots now, the sea-state does not generally allow for it. Besides, if I needed to get there faster, I should have left 15 minutes earlier. LOL. I usually run mine pretty light (full fuel, but light people and gear). It sounds like that 19 should work ok, unless they make an 18.

I'm actually located in eastern CT (New London)and use the boat on Long Island sound, but it has MS registration #'s (house on the cape, this is the CT boat, cheaper to register there). I'm actually the third owener of this boat. The original owner was in SC, and the boat was indoor rack stored up until about 4 years ago, teh second owner ran it for about 20hrs, shrink wrapped and put away until late winter this year. I use all the original canvas and cushions. I ripped out all the wiring and original power (raw water cooled motor). The boat has never been technically "restored", but it is in better shape than anything we could find that was under 5 years old. Besides, now I have got something that is worth something.... Happy to show it to you. I'll see if I can get a couple of pictures. Thanks for the help everybody!!

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I think you'll do OK since my motor's even older (1970!) and the aluminum prop works fine, especially here in South Florida where you have to run quite a bit in the shallow waters of Biscayne Bay until you can hit the pass and get out to deeper waters. Going through some shallow areas I've already banged up the prop in sand, small stones, coral and such. The aluminum absorbs the shock a lot better than SS and it's cheaper to repair or replace the prop than the lower unit! Good luck with yours! :rolleyes:

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Just got back from a ride, perfect day for it!!! 3850rpm, 30.2 knots on my Garmin plotter, full fuel, light load, trimmed out so it is just short of starting to porpious. Even at 3850, that motor doesn't sound like it should be spinning that fast. 2-stroke outboards sound better the faster you spin them. I have only run a Regulator with 225 4-strokes. This is my first experience with an I/O. I guess it is ok. At 3200, it is running 21-22knots. Plenty, still going to change the wheel.

Re: Propeller options
 

Drop down to a 19 pitch highfive and I'll bet you'll see close to 40mph @ 4500 on the top end. High five is a thin blade 19pitch prop that gives great sternlift and is super smooth. The hubs kits for these props are designed to break if you hit bottom and spares are about only $40.

http://www.mercurymarine.com/dealerlocator

and select "demo props" there are a list of dealers that will let you try props before you buy . . . You can always buy a aluminum 19P after trying some other props.

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Kahuna:
Thinking of moving up from 19" to 21" prop but your figures bother me. What is your drive ratio? If the 1.65 that translates to over 30% slip! Than again is your plotter reading from a speed transducer or the GPS. If GPS the speed through water is unknown and that's what the boat speed really is. Don't blow my head off for mentioning the obvious but are you reading MPH or Knots? (the plotters will read whatever you choose or has been set.)
If we go to a more reasonable 10% slip, than true hull speed - through the water - for a 21" wheel is 41.6 which allows for a 9 mph current, not unusual, moderate in fact in tidal channels but then do you have much tide or current issues in Hawaii? The Latitude suggests little tide but plenty opportunity for currents.
HBH

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140 mercruisers should have a 1.98-1 drive ratio, unless its be changed to high altitude gear set ( 2.20). the other ratios available are 1.84, 1.65, 1.50, 1.32. The later Gen II's had different ratios, but the are very close to these

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The 1.98 still gives a 21% slip, on the high side of reasonable.
Kahuna, next time you're out try a proper speed trial, run the reciprocal: first run at whatever bearing convenient, second at 180 degrees opposite, take the average of both runs.
Also, what does your wake look like?
Spares, is it common for the gear ratio to be changed by the user? How can the ratio be verified, short of counting teeth? How many reduction points are there on a sterndrive, more than one?
HBH

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PHNX,
The boat currently has a 13.75 x 21. Too much wheel. Need to drop down to get RPM's up. I'm going to have to go with the assumption that the gear is stock on the 1984 apha 1 drive. Agree, the number on the GPS is over the bottom, and not through the water. However in about 8' of water over a sandy bottom, with minimal tide effect, minimal wind, using a Tempo tachometer, the motor turns 3850 and the hull runs just short of 31knots trimmed properly just short of porpoising. Short of wind tunnel testing, or radar gun and putting a calibrated electronic tacometer directly on the motor, I feel the numbers I have got are close enough be able to make reasonable assumptions. I would like to go to a smaller wheel so that I'm turning 4500-4600 wfo with a light load. I spoke with a dealer adn found out that a 13.75 x 19 is not availabel. I don't have the available sizes for the alpha 1 in front of me here (think I left them at the office in Newport), but if I remember correctly, every time they drop two inches of pitch, they changed an inch in diameter. So the trick here I guess is going to be to try to guess right. The dealer did also say that I could take one, try it and as long as I didn't ding it up, I could bring it back, to ensure that I get the right one.

Re: Propeller options
 

I spoke with a dealer adn found out that a 13.75 x 19 is not available.

What's not available?

Re: Propeller options
 

i think you can get a 13.25 X 19, don't worry about the diamter too much, just use the pitch numbers to get you in the correct rpm( if you were running more hp at higher speeds it would be more of a concern), if you want to fine tune it you can change diameters, but I don't think you will gain enough to offset the price of propellers. Stay with the same brand of props while you're swapping them, I've seen measured pitches vary by manufactures by a wide margin

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Ok, that makes sense, stay with the same brand when swaping. I have worked through the fact that every inch of pitch is good for 200rpm's on a gas motor, but was not aware of how the change in diameter would work into the equation. I guess that i just won't get too excited about it, just monitor closely when I do the. At this point, I think it is going to have to happen next season.