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Author Topic:   Prop for Classic 18 Outrage
LUV2FISH posted 02-23-2001 11:51 AM ET (US)   Profile for LUV2FISH   Send Email to LUV2FISH  
I am trying to prop out my newly aquired 89 18ft outrage. It came with a 150 optimax and no prop. I found a post by lhg where he stated the weight for the boat is 1250lbs. I checked the mercurymarine website with there prop calculator. However the weight I put in will not give a prop size/pitch calculation. It says the power to weight ratio is too high and to call a professional. I keeped bumping up the weight until I hit 1700lbs where it calculated a theoretical pitch of 26.76 and a top speed of 66.99 miles per hour!!!! If I hit a big enough wake maybe I can jump one of the tankers coming into Charleston Harbor!! Actually, something probably isn't right here. I know the best thing is to stick one on and try it till I get the WOT to 5500 or so, but I was hoping for some input before hand from anyone with a similiar boat. Thanks for any help!
Peter posted 02-23-2001 06:00 PM ET (US)     Profile for Peter  Send Email to Peter     
I have an 1987 18 outrage with the original 1987 150 Johnson. The 150 turns a 14 1/2 by 21 OMC Raker 3 blade prop. Top speed under ideal flat conditions is just shy of 50. Assuming the gear ratio of the old Johnson is nearly the same as your Optimax, my guess is that you'll be looking for a prop with a pitch in the 19 to 23 inch range to get you in the WOT range.
Tom W Clark posted 02-23-2001 06:26 PM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
My 18' Outrage had a 150 hp Johnson and the boat had the usual amount of gear (kicker motor, dual batteries, ect) As I recall, I always used a 19" pitch stainless prop. I tried the 21" prop at one time and it was just a little too "tall" for anything other than one person, light on fuel kind of a load. Top speed under the right conditions was 48 m.p.h.
Peter posted 02-23-2001 07:46 PM ET (US)     Profile for Peter  Send Email to Peter     

My prop was spec'd for the motor mounted at the highest notch, which is the way it was rigged when I got it. Several years ago I brought it down a notch because it was too squirley in rough water, which is the typical conditions I find myself in.

Russ posted 02-23-2001 10:21 PM ET (US)     Profile for Russ  Send Email to Russ     
I have an '81 18.5 Outrage with a 150hp Mercury Black Max. It has a 141/2"x18" Merc aluminum 4 bladed prop. Performance varies, but the last time I checked for top speed, with two aboard and about 40 gallons of fuel, slight headwind, rippled water, it settled at 39mph as measured with a Furuno DGPS. I'm not sure it was perfectly trimmed out because I didn't want to risk having it ventilate at that rpm. (I really hate it when it does that. The engine might be mounted too high on the transom.) A known disadvantage of a 4 blade is lower top speed, but I like the advantages of lower planing speed, less steering torque and added quickness up to plane. The extra control is especially appreciated in a following sea. The reason for using aluminum instead of ss is that the boat is used in the western region of the Juan de Fuca Strait where there is a hazardous amount of floating wood; ss will save itself and sacrifice the gears if it hits something, in spite of the rubber shock absorber. The opposite is true with aluminum. Also, the weight of ss puts added stress on the gears when putting the engine in gear, and probably when running, too.
Peter posted 02-23-2001 11:34 PM ET (US)     Profile for Peter  Send Email to Peter     

The 1250lbs is the dry weight of the hull. You need to add the weight of the motor (~450lbs), a full fuel tank (~400lbs), two batteries (~ 80lbs), other miscellaneous gear (~???) and at least one person (~???) to get a more realistic operating weight. In all likelihood, the average operating weight is probably close to 2200lbs. When I used 18 feet, offshore fishing hull as the selecting factors, the Merc selector program recommended 19 inch pitch props. When I used 2200lbs and offshore fishing hull, the selector recommended 21 inch pitch props. So the range looks like 19 to 21 for the 150 Optimax. I would probably lean towards the 19 so you can get close to the WOT when fully loaded. Your actual top speed when lightly loaded should be close to 50 depending on mounting height and trim.

LUV2FISH posted 02-24-2001 12:06 AM ET (US)     Profile for LUV2FISH  Send Email to LUV2FISH     
Thanks for the quick responses guys!! Sounds like a four blade 19 should do it. I believe I can have it tweeked by my local prop shop if needed. I want to find a used one at a good price so once I buy it, it's mine. Ya'lls input will probably make the risk worthwhile and save me some money. Thanks so much!!
LUV2FISH posted 02-26-2001 02:36 PM ET (US)     Profile for LUV2FISH  Send Email to LUV2FISH     
To follow up, I got a merc offshore 13.75 X 19 4 blade prop from (excellent place for props) for $225 plus $20 shipping. This prop is one year old in excellent condition used as a spare, so they say...Anyway, the same one was close to $500 here locally new. It will be a while before I try it but I hope she tachs out at 5600 the first time. I'll keep my fingers crossed!
lhg posted 03-01-2001 06:16 PM ET (US)     Profile for lhg    
In checking my Merc prop charts, it looks like you made the right purchase. Top speed is shown as 51mph. With a performance prop like the 4 blade Offshore, the engine should be mounted in the 2nd or 3rd bolt hole, and you should get full rpm lightly loaded.

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