change from 19 pitch to 21?

[capn kurt]

I've finally been able to test my boat accurately. I'm getting 45.7mph according to gps, on my 13"dia X 19"pitch mercury aluminum prop while carrying 3 adults. This is on my '74 Switzer Craft 170 with a '74 Mercury 1150 115hp with 2:1 gearing.
I calibrated the tach and when I'm going 45.7, it's turning 5250rpm. My mercury manual says that the max operational range for my motor is 4800-5300. The boat is a 17', light, fast hull. I'm not sure about the weight, but it does say 900lbs on the placard. The motor is running perfectly and my holeshots are great, but I'd like to get more speed. If I jumped up to 21" would it be too much? Should I try a 20" pitch instead? If I can expect a 200rpm drop, that should still keep me in the operational range of the motor, correct? (5250 down to 5050). If I put on a 21, would I expect to see a 400 rpm drop? If so then I would still be within range, correct? Let me know what you guys think.

Thanks, capn

 

[SparkieBoat]

Re: change from 19 pitch to 21?

it would be in the operational range, but most outboards especially mercs like to be in the top end of that range. I would stay with what you have. You could test a 21P prop if you want but I would not buy one that I could not return. you will most likely want to go back to your 19, it looks like the perfect prop. I mean just how fast do you expect a 115 on a 17' boat to go??? 45.7 is pretty darn good. if you want to go faster get a 150, that will take you for a ride.

 

[jimmbo]

Re: change from 19 pitch to 21?

46 mph on a 17 ft boat using a crank rated 115 is doing pretty good. Going to a 20 will most likely give you 45.7mph @ approx 5050 rpm and a slower holeshot to boot. A 21 will be even worse You might gain a couple of mph using a GOOD performance Stainless prop but only if you can raise the engine up 1 1/2 to 2 inches higher on the transom. Not an easy task with your motor, new holes would need to be drilled. Get a post 1984 135hp, that will wake up your boat. Going from a crank rated 115 to a prop rated 135 should be good for 7 mph

 

[capn kurt]

Re: change from 19 pitch to 21?

Thanks for the info, fellas! Okay, well maybe I'll just try out a 20" prop if the opportunity arises. Otherwise I'll just have to try and be happy with what I've got. As far as "how fast do you want to go"...as fast as possible! So how much would a great condition '74 1150 115 hp outboard be worth? If I can offset the cost of a new motor by selling my current one that would be great. How much should I expect to pay for a similar condition '84+ 135 horse model?

 

[SparkieBoat]

Re: change from 19 pitch to 21?

the 74 will be worth about half of what a late 80s model 150 would cost. I would say around $800 for your 74, 115. probably find a good 87-91 150 for about $1500. (those XRs were really good motors)

 

[Titanium48]

Re: change from 19 pitch to 21?

A 1970s 150 hp "tower of power" might be a cheaper option, and with the same dimensions as your current motor it could be easier to rig.

 

[jimmbo]

Re: change from 19 pitch to 21?

A 1970s 150 hp "tower of power" might be a cheaper option, and with the same dimensions as your current motor it could be easier to rig.

It would eliminate the need to redrill the transom. However it will have a poorer holeshot than a V6, in fact the existing 115 will have a better holeshot. The 150 I6 runs rather unevenly between 2000 and 4000. Above 4000 thru 6000 it really packs a wallop and hauls the mail. The midrange roughness is most likely due to the radical port timing which is what gives it the high top end. I6 has a smaller gearcase, less drag but the bigger gearcase on the V6 allows for larger diameter props which can have more thrust.

 

[capn kurt]

Re: change from 19 pitch to 21?

Thanks for all of the ideas, fellas. I guess I should probably just stick with what I have, considering I only paid 1500.00 for a boat that runs well and a nice trailer. I don't want to spend 2500.00 minimum just to get an extra 7mph out of the thing, especially when most of my time is spent cruising at 30mph.

However, I could upgrade to a SS prop and gain something, correct? What difference could I expect between the ss and my stock aluminum, assuming I use the exact same size/pitch?

thanks,

capn

 

[Texasmark]

Re: change from 19 pitch to 21?

The attach is my current boat at 17' with a '02 90 Merc 2.3 gearbox and a Ballistic 24XP SS which I just ported with 1/4" holes. It runs 48.5 GPS, 5600-5800 rpm (rated 5000-5500) with a 240# and 200#person on board plus gear, on a cool day. Alone with wind to my back I can just nudge over 50. The recent porting helps me to get a hole in the 3-4 second range.

I bought the boat used with a 17P, immediately went to a 21P and figured I could get more so bought the 24XP and did. So going to a higher pitch isn't all bad. If then engine's torque curve can handle it and the carbs can give you enough fuel to get to the higher rpm's, upping your prop may get more of your boat out of the water, reducing drag, reducing the load on the engine and allowing you to get both the speed and rpm's....porting rounds it out in assisting the hole shot.

Per your comment about alum and comparisons to SS you can't. Most alums are gp props (Turning Point Hustler an exception but still not as good as SS) whereas most SS are HP. Best bet is as you said to get a dealer to lend you props till you get it right.

My 2.3 to your 2:1 would make my 24P equivalent to a 21P on your engine. Your '74 is too early for prop rating so being power head rated will make your prop horses less by an estimated (by the experts) 10-15% making your equivalent (to mine) engine roughly 98 to 103 horsepower.

Soooooo I think you are going to do just fine with a 20-21 and the 21 will probably offer more suppliers and more performance.

My 2c

Mark

 

[capn kurt]

Re: change from 19 pitch to 21?

Thanks for the info, Mark. I got some new numbers over the weekend while on the river, headed downstream on some very calm water. I was traveling 46.3mph at 5500 rpm (I have been misreading the tach all along - forgetting that each notch is worth 200 rpm, not 100). The boat was full of gas (12 gal) and my buddy at 230 and me at 195 were on board. This was with a current of 1-3 feet per second, so that will obviously affect my speed accuracy. But, at that rpm I should AT LEAST be safe trying a 20" prop on there, right? (rpm max range at 5300 for my motor). I would like to go with a 21" just to see what effect it has on my hs and top speed and west marine has an aluminum mercury 'black diamond' 12 3/4 x 21 for $149.00 with the hub kit. I figured this would be a cheap way to try that pitch and then I would have a spare prop as well. What are your thoughts? Obviously, if the 21 proved to be a good prop, I'd really like to go with the SS for regular use.

 

[SparkieBoat]

Re: change from 19 pitch to 21?

have you tried jacking the motor up a hole or 2 yet?? still sounds like you have the perfect prop. I would only try another prop if you can take it back, or borrow one from a buddie. if you can jack the motor up a couple of holes without it venting then you could probably get a couple more hundred RPMs, you may even consider a manual jack plate. you should note that Texas boat is aluminum, that makes a big difference.

 

[Texasmark]

Re: change from 19 pitch to 21?

have you tried jacking the motor up a hole or 2 yet?? still sounds like you have the perfect prop. I would only try another prop if you can take it back, or borrow one from a buddie. if you can jack the motor up a couple of holes without it venting then you could probably get a couple more hundred RPMs, you may even consider a manual jack plate. you should note that Texas boat is aluminum, that makes a big difference.

Yes Yes Yes!

 

[jimmbo]

Re: change from 19 pitch to 21?

have you tried jacking the motor up a hole or 2 yet?? still sounds like you have the perfect prop. I would only try another prop if you can take it back, or borrow one from a buddie. if you can jack the motor up a couple of holes without it venting then you could probably get a couple more hundred RPMs, you may even consider a manual jack plate. you should note that Texas boat is aluminum, that makes a big difference.

Gents don't forget that this is a pre 1984 inline six. The only way to raise this engine on the transom is to drill at least six(I am assuming power trim) new holes to relocate the bolts. Or get a jack plate$$$$

 

[SparkieBoat]

Re: change from 19 pitch to 21?

$800-1500. manual jack plate is only about $100..not a huge investment, and could be used with any future motors..like a 1988 150 XR.