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Prop size 30L

J

johnnynegaunee

Contributing Member
"I put a newer 3.0L replacemen

"I put a newer 3.0L replacement (for my tired 2.5L) into my '72 Starcraft Islander. Engine works great but R's now climb up to at least 5000 which is max on the tach. What prop do I need? How do you read the code numbers stamped on the prop?

I been searching the archive, I'm sure its there somewhere, but... Of course its easier sometimes to just ask.

Thanks,
John"
 
If its a Merc prop the last 2

If its a Merc prop the last 2 digits are usually the pitch.
Also it should be stamped in the side of the hub between 2 of the blades.

Rod
 
"Thanks Rod

I find two numb

"Thanks Rod

I find two numbers, 912K8, and 398G7. It looks like its about 14" in diam. Any way to measure the pitch of this thing here in the yard?

I'm guessing a 20 or 21 pitch is what I need from reading some of the prop Q's in the archive, that sound right?"
 
Here is a spreadsheet that I d

Here is a spreadsheet that I did a while back that will help you select the right prop.

Let me know if you have any trouble.

Rod

prop finder
Copy of Boat_Speed_Finder-79780.xls
 
"Let's try this again.

"Let's try this again.

Rod

<center><table border=1><tr><td>http://www.marineengine.com/discus/icons/mime_msexcel.gif""" align=left alt=""application/vnd.ms-excel"">Boat Speed Finder
[url=""][b]Boat Speed Finder.xls[/b][/url] (20.0 k)</td></tr></table></center>"
 
"Thanks Rod. I found that spr

"Thanks Rod. I found that spreadsheet in some other posts of yours while search the archives but could not open it as I don't have excel to run it. I assume you would need to input length, weight, engine hp etc. to get a prop recommendation."
 
"The 20 extra HP shouldn't

"The 20 extra HP shouldn't require a pitch change really, but you could take the prop to a prop shop and just ask for the next one up. 2" more pitch. 2" is all I could see the addition of 20HP requiring.
I would verify the tach readings first."
 
I went from a 140hp 3.0L to a

I went from a 140hp 3.0L to a 180hp 3.7L and I went up in pitch from a 19 to a 21 with a cupped prop and it was ok I think I may go up 2 more but I need more testing cause I am in fresh water half the time and salt water the other half and it reacts alot different in the 2 different waters
 
"John, what is the rpm max lim

"John, what is the rpm max limit on newer engine versus the older? (I do not have this data readily on hand). My memory fades, but I believe the older 2.5L had a limit of about 4300, and the 3.0L about 4800. If this is about right, this would suggest you could expect to attain higher rpm with the replacement engine running the same prop. Your tach unit may have been originally selected for an expected lower rpm engine.

I do remember the 2.5L I once had would run fine with a 19" pitch prop on a medium weight runabout, but a 21" pitch would not allow full rpm (actually reduced max rpm by 600) -- no power for skiing starts, but ideal for gas mileage just cruising (like overdrive). A 19" cupped was better performing than a 19" standard blade on that boat for keeping the engine within rpm limits and for power at the prop.

If you current prop is not a cupped design, try a cupped design of same pitch before going up in pitch as this will decrease higher rpm slightly and still give low end power for skiing."
 
"Thanks for the info, the 2.5

"Thanks for the info, the 2.5 would run the tach. to 44-4600 as I recall, while the newer, lesser houred 3.0L easily pegs the tach at 5K, with a little more throttle left. This is a aluminum 22 foot hull it's pushing. I believe it has a plain jane aluminum no frills prop. I have no problem running what I have , but was thinking a 20 or 21 might give better fuel economy, is that right?

I have an extra, unused 14 by 19 I was wondering also if people swap these.

Thanks,
John"
 
"Well, that is the rpm you are

"Well, that is the rpm you are getting, but the point was -- what are the engine's recommended rpm limits? My recollection, which could be faulty, is that the 3L engine has a higher allowed rpm limit than the 2.5L engine. The engine's limits are what you do not want to exceed or risk throwing bearings, rods, and pistons! What the printed dial's scale on the tach is not the limit you must be concerned with in equipping your boat with the proper propeller. Do not over rev!

You may be able to post a note at a prop shop or boat dealer that you have a prop for sale, or list on CraigsList. I doubt you can get a 'trade in', but you certainly can ask."
 
Prop repair shops frequently t

Prop repair shops frequently take used props on trade against other stock repaired props. Just a thought.

Rod
 
"John;
If you want to send me

"John;
If you want to send me the parameters on your boat I can run the prop sizing for you.
I need:
hp, maximum RPM
boat weight with gas and 1 person
Altitude above sea level if you are inland.
Drive ratio
Rod"
 
"Rod: the engine is a '72

"Rod: the engine is a '72 rated 140hp (I don't think this is prop shaft hp though) Weight 2800 lbs. (as you spec.'d) 600 ft up (Lake Superior) I have an R drive I don't know the ratio. As I stated above the old 120hp ran about 4400 on the tach. so I think it may be fairly accurate. The new engine turns the prop at 5000-5200 (the higher number is a guess, because 5K is the top end of this old tach. The Merc. manual says this engine should run 4200-4600 if I shoot for the 4600 WOT and 19 pitch gives me 5K, on paper a 21 should give the 4600, right? Thanks for all the help.

My ultimate goal is to run as gas stingy as I can, if the 19p and 3/4 throttle is it then I'm fine with that. If I get input to say a 20 or 21 will save $ then I'll hunt up someone to trade my new unused 19p with for a whatever-is-better.

Thanks,
John"
 
800 rpm gain for 20HP??? not p

800 rpm gain for 20HP??? not possible... I suspect your old 2.5 was really tired and maybe making 90HP and someone compensated with a really flat prop and so now you are way underpropped....
As far as what pitch gets you the best mpg...not predictable...the hull design and weight balance has too much to do with that... for about $150 you can get a lowrance fuel flow meter and watch your mpg on the dash
 
"True, it was a tired old girl

"True, it was a tired old girl, the 2.5L did need to be replaced, I don't really care if only made 90hp at this point I'm just interested in what prop to put on to run efficiently the motor I now have. The prop on my 21' Starcraft Islander looks like it has the same pitch as the new unused 19 that came with the boat. It seems a person should be able see pitch angle differences. Again if being a little underproped and running 3/4 throttle is the efficient way to run then no need to change. I just don't want to burn dollars I don't need to.

Thanks"
 
"Rod: the engine is a '72

"Rod: the engine is a '72 rated 140hp (I don't think this is prop shaft hp though) Weight 2800 lbs. (as you spec.'d) 600 ft up (Lake Superior) I have an R drive I don't know the ratio, maybe 1.65? As I stated above the old 120hp ran about 4400 on the tach. so I think it may be fairly accurate. The new engine turns the prop at 5000-5200 (the higher number is a guess, because 5K is the top end of this old tach. The Merc. manual says this engine should run 4200-4600 if I shoot for the 4600 WOT and 19 pitch gives me 5K, on paper a 21 should give the 4600, right? Thanks for all the help."
 
John;
The sheet predicts a to

John;
The sheet predicts a top speed of 43 mph @ 4600 RPM using a 22" pitch prop.
I think your drive ratio is likely 1.98.
So your estimate of a 21" pitch seems about right.

Rod
 
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