Step into your time machine!!! looking for your Prop input from the past

[ckraker]

Lets go back to 1973 (the Year of my 135 HP Evinrude) Or if you prefer we could go back to 1969 (the year of my 17 foot fiberglass sea ray.

Currently running a 13 1/2 x 15p propeller, Getting about 5000 RPM WOT and top speed of about 30. After reading a ton of posts it seems that my top speed is a little on the low side for a 17 foot boat with 135hp motor. I realize there are a lot of variables that come into play but, it's time for a new propeller and I'm thinking of dropping my pitch to a 13. I am certain that I'll be able to get my rpm's up to at least 5300 maybe a bit more, but what will I be losing? Will my top speed suffer or will I make up for the lost speed with the increase in RPM's. Does it seem odd to anyone that I would need to have that low of a pitch on that size boat to get my revs up? I feel like everyone else in this size boat has 19 or 17 pitch props and they are all hitting near 40 MPH. Again I know there are a lot of things that come into play here, but I think the old 115 that used to be on the boat pushed it close to 40 mph.

any input is really appreciated
Thanks.

 

[Maclin]

Re: Step into your time machine!!! looking for your Prop input from the past

The physics of this transcend time: Going down in pitch raises rpm and lowers top speed.

 

[Frank Acampora]

Re: Step into your time machine!!! looking for your Prop input from the past

30 MPH for any 135 HP engine on a 17 foot hull would be considered a little low! I would expect a 135 to turn a 17-19 pitch prop at about 5000RPM.

Before you go playing with props, find out why you need a 15 pitch prop to turn 5000 on a 17 foot boat. Is the boat heavy and waterlogged? Is weight distribution off? Is it a heavy hull OR are you carrying a heavy load? Is the engine set too low on the transom? Is compression low? Is the engine just old and tired? Is the ignition not up to snuff?

Just because the engine is a 1973, does not automatically mean it is a poor performer.

Just as a loose comparison, my 1979 Chrysler 140 on a 21 foot cuddy turns a 13 X 19 at around 5000 with a top speed of 33-35. I drop to a 17 pitch for skiing. And that 105 in my avatar top out at 45 on the 15 foot hull.

 

[steelespike]

Re: Step into your time machine!!! looking for your Prop input from the past

Your prop size seems reasonable to me. Your boat is probably about 1700 lbs dry weight no motor.
Your motor is rated at the power head So your actual hp is somewhat less compared to motors from 85 on.
Imo no more than 10% less. I feel you should be turning about 5500 with a 15" prop with a 2.00 gear ratio that puts you at about 35 mph. I do think you need to evaluate your setup.
Boats sometimes absorb water over time you may want to weigh the boat.Subtract motor, gear and trailer.
You could also check the foam under the floor for water.
Be sure the motor is in excellent tune,good compression.
Anti vent plate(just above the prop) about even with the bottom of the transom up to about 1" above. some pictures would help.
Boat bottom clean and smooth.

 

[ckraker]

Re: Step into your time machine!!! looking for your Prop input from the past

Whoa! I'm liking the responses. I'll get pictures on tonight. FYI I just got it out of the shop. Compression and timing are all good- it actually has very few hours on it. Trying to recall---- timing should be like 17 at WOT, right? What other engine variables affect top end RPM's?
About the water saturated foam.... I'm not sure there is any floatation foam in the boat at all. But it's my Dads boat and he did always say that it was a heavy boat. But Still-- 5000 RPM is not where I want to be-- Also I should mention Hole Shot is Super Fast, motor doesn't even sound like it's working.

The only ignition test I have done is a spark gap test--- Passed easily. what else could I test?

 

[steelespike]

Re: Step into your time machine!!! looking for your Prop input from the past

Super fast hole shot is sometimes an indication the boat has a hook.
The boat may seem like its glued to the water and it won't respond to up trim.
The boat may corner oddly with the bow seeming to affect it.
Instant hole shot and low rpm don't usually go together.

 

[Frank Acampora]

Re: Step into your time machine!!! looking for your Prop input from the past

If it is a loop charged engine, timing would be around 18 degrees BTDC. (I don't know if they made loopers in '73). If it is a crossflow engine, timing would be more like 32-34 Degrees BTDC. If your timing is retarded by 15-17 degrees, there is your problem with loss of speed. For more accurate timing info, post on the OMC forum. Sometimes timing info is posted on the engine flywheel shroud or the block.

 

[ckraker]

Re: Step into your time machine!!! looking for your Prop input from the past



Here are the pictures of the Lower unit with reference to the bottom of the boat. I think it's okay, but maybe some of you would know better. This winter I'm going to pull the floor out and check the floatation foam-- Still don't think that is the issue though.

About the timing---- the manual says 22 btdc at WOT. It is a cross flow--not sure why you think 32-34 is a better number???? Anyone have any input on that?

I did notice that the higher pitch propellers also have a smaller diameter. If I'm running 5000 RPM's WOT and getting 30 MPH with a 15 pitch 13 1/2 Diameter, what would I expect to see with a 17 pitch 13 diameter?

 

[Frank Acampora]

Re: Step into your time machine!!! looking for your Prop input from the past

I just threw it out as an example. The loop charged engines I have seen were timed at 18 degrees BTDC. I do have an OMC 65 that is timed at 34 by the flywheel sticker and many older crossflow engines are timed around 30-32. If your sticker says 22, then go with that.

If you are interested, the reason loop charged engines use much less timing advance than crossflow is because most loop charged engines use a flat topped piston with a hemispherical combustion chamber and "squish band". This allows a much more turbulent mixture and faster burning.
Since the function of timing advance is to have an equal burn time before and after top dead center, the faster burning charge of a loop charged engine does not need to be ignited as soon.

 

[82rude]

Re: Step into your time machine!!! looking for your Prop input from the past

all 135,s are wot at 5000 rpm!73-1976.btdc 22degrees,1973. 1974,, 75 76, at 20 degrees.