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43 merc io prop change and running temp

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balaobill

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"have 4.3 v6 2 barrel in 91 tr

"have 4.3 v6 2 barrel in 91 trophy weights 4400 lbs was running a 14.5x17 prop that was giving me 20 knts at around 3600 to 3700rpms. At wot ran around 25 26 knts at around 4600 to4700rpm
replaced this prop after i spunn a hub with a 15x17 vortex prop. now i,m running 18 knts at around 3900 rpms. At wot running around 4900 to 5000 rpms at 25 to 26 knts. Does any think that the 1/2 inch increase in prop diameter would have that much of an influence on the rpms and speed. and do you think i should switch back to a 14.5 prop it seems less stressful on the engine to me.
one more question what would or should be the normal operating water temp. be.

thanks ,any help would be appreciated

bill"
 
On the water temperature. Is i

On the water temperature. Is it raw water cooled or an enclosed coolant system? Are you operating in the ocean or fresh water?
 
raw water cooled salt water o

raw water cooled salt water ocean and bay temps. in the high 60's to low 70's in summer 50 to 60 in fall early winter.


bill
 
"You need a 140 degree thermos

"You need a 140 degree thermostat. More than that may cause salt build up in the water jacket. If you're into engine longevity, a fresh water flush of the cooling system after each use would be an excellent maintainance practice."
 
"Going to the larger diameter

"Going to the larger diameter should have lowered your RPMs... you are over propped. There is a very fine balance between engine power and prop pitch/diameter and hull performance. You may have to play around with the prop size and pitch until you find the optimum for the type of boating you do. Sounds like your looking for economy at a decent cruising speed to get you where the fish are... 20-25 knts at a reasonable RPM, 3000-3500ish.

For as heavy as your boat is, I'd say your a little underpowered. The bigger prop isn't as efficient because the engine doesn't have enough umph to get the boat moving to take advantage of the added prop area, causing some cavitation. You're having to open the throttle more to get the same or even less speed... not good for the wallet.

I would recommend a smaller prop and maybe up the pitch... a little... one degree. If you jump to a 21 pitch prop, you'll never get on plane. 14x18 or 14x19 tops. You may end up right back where you started at 14x17, being the biggest prop you can turn with that engine.

Other things to consider are trim tabs and a hydrofoil. The trim tabs will make the hull effectively longer, therefore more efficient as it cuts through the water... same mass (therefore resistance to movement) divided over a larger area, kinda like drafting in NASCAR. And the hydrofoil minimizes cavitation by increasing the pressure the prop sees as it moves through the water.
Good luck."
 
thanks for the info troym.

thanks for the info troym.
going to try a 14.5x18 4 blade prop instead of the 3 blade and see what happens. I know what you mean by a fine line. And you are right boat is a little under powered.if i have to repower later on will switch up to a v-8 350. but for now it will take a little longer to get to fishing grounds but with gas the way it is thats ok. I proberly get another 25 hp if i went to 4 barrel carb. but lose fuel economy. will let you know how 4 blade prop works out. thanks
 
That's way too much prop!!

That's way too much prop!!!!!!!!
You'll be lucky to get on plane.
 
"That's a pretty neat calc

"That's a pretty neat calculator. If baloabill punches in the numbers from the previous two props he's used 14.5x17 and the 15x17, and solves for the prop slip, it'll prove how much more the bigger prop is doing nothing more than churning the water vice moving the boat."
 
"Agree Troy,

I'm using

"Agree Troy,

I'm using the link as a help guide to find out(and understand better) which is the best prop for the new power package installed on my boat.

Formerly had on a Maxum 2000SCL a 4.3LX (4 barrels carb)with Alpha 1 (1:1.84) propped with a Mirage 14.5 x 21". The repower is a 4.3 MPI with Alpha 1 (1:1.62). Due to the faster gear ratio and the fact that the engine only has runned for 6 odd hours, I'm still keeping the old prop, but as soon as I can experiment with WOT RPMs will test a 23" pitch one, given that the 4.3 MPI has (Mercury says)40 extra HP. I'm rather after the extra MPG rather than the extra MPH.

Will keep you posted on the results."
 
"Fred,
The change in gear rat

"Fred,
The change in gear ratio is going to drop the engine rpm for same given prop shaft rpm, but that is likely going to spend your added horsepower (the engine will be essentially working with less leverage to turn the shaft). For the same engine rpm, the prop shaft will be turning faster... that in itself will require more power. I'm guessing that the repower came out of a smaller/lighter boat?
I'm anxious to hear how it all works out. Please post WOT RPM and SOG with the props you test."
 
14.5 by 19 or 21. 19 is best

14.5 by 19 or 21. 19 is best all around prop but id use the 21p. the 4.3l has the power to pull it out if the hole and give you wicked fast top end. you should get 45 mph on the 21p at 4600 and thats flying. youll be pleasantly surprised!!!!
 
"As said I kept the Mirage 21p

"As said I kept the Mirage 21p that was on my former package (4.3 LX - 180 HP). The new one is 4.3 MPI - 220 HP.
With the old power package the boat did average 45 MPH (SOG)at 4600/4800 RPM as you correctly point out.

According to my theoretical calculations it should be now doing 49.7 MPH at 4600 RPM (assuming a 12% slippage)with the new sterndrive because the gear ratio is 1.62 instead of 1.84.

Maybe it looks like I'm geting to ambitious when thinking in a 23p.

I'm not after more speed, agree that 45 MPH is more than enough, just trying not to waste fuel at crusing speed.
Tests at WOT under load should tell me if its possible."
 
"Troy the engine has 15 hours

"Troy the engine has 15 hours already and the manual says it can (and should) be spinned to high revs for short runs (less than 5 mins. at a time)

I did so yesterday (for less than a minute) and with the old mirage 21" pitch prop, reached 4800 RPM effortless and the throttle lever still had some way to go. At this point the speed meter(SOG - GPS) read 50 MPH. Didn't dare to push it further but will do so after the first service (in 5 more hours) with a 23" pitch. Will keep you posted"
 
"Man, that thing is a rocket!

"Man, that thing is a rocket!

Seeing what gas costs, I'd add at least another two incches of pitch. It may plane a bit slower, but gas mileage and engine life will be improved.

Jeff"
 
"Yes Jeff I'm very happy w

"Yes Jeff I'm very happy with what I did (mean repowering) because now I'm convinced the boat is worth it.

I have the same hope, that it will handle a 23" pitch without trouble. Will let you know as soon as the break in period is over."
 
"Balaobill,
I have a 1991 Tro

"Balaobill,
I have a 1991 Trophy 2352 WA w/t hardtop that has a 4.3L engine in it and I am running a 14.5x17 Mercury Offshore 4 blade SS prop. I get 35 mph at 4800 rpm. My engine was a 2bbl but when I bought the boat I had engine problems that needed a lot of the engine components changed due to salt water corrosion. I choose to put on an Edelbrock 4bbl intake and a 1409 4bbl carb to get the engine up to 205hp. I don't really know what the 2bbl would have done but I changed the prop from a Solas 14.4x17 4 blade aluminum to the SS prop. So one of the previous owners must have decided that the 14.5x17 was the right prop for the boat."
 
"Balaobill, to help me underst

"Balaobill, to help me understand the effect of the different variables involved I investigated performance tests of other boats published in the net, and put together some formulas and data on an active calculation sheet. If you are familiar with Excell the attached file might help you to. Please let mew know if you find any trouble in operating it.

<center><table border=1><tr><td>http://www.marineengine.com/discus/icons/attachment_icon.gif""" align=left alt=""gas,mileage calc.xls,application/octet-stream"">Excell file
[url=""][b]Prop[/b][/url] (29.2 k)</td></tr></table></center>"
 
federico install the 23 prop m

federico install the 23 prop most definately and let us know how you make out.
 
Agree Skinnyowen. I'm anxi

Agree Skinnyowen. I'm anxiously waiting for the break in period to end so I can test it after the first service.
 
"Well I´m back after a while.

"Well I´m back after a while.
Been out on vacations on a spot where there is no access to internet (thanks God)
After compleeting the break-in period (20 hours) have installed a Smart Craft SC1000 monitor with the intention of getting accurate readings.

It turned out that what I took for 3000 RPM (as measured by the original gauge) was actually 2670 and at WOT the engine did 4400 RPM instead of 5000 plus the analog meter read.

To make a long story short, I decided to stick to my 14.5 x 21"P SS Mirage.

Attached you can find a performance sheet I have put together with actual readings. The boat was with the gas tank full, about 120 pounds of load and 2 X 150 pounds persons.
<center><table border=1><tr><td>http://www.marineengine.com/discus/icons/attachment_icon.gif""" align=left alt=""application/octet-stream"">Performance Calculator
[url=""][b]Performance calculator.xls[/b][/url] (24.1 k)</td></tr></table></center>"
 
"What format is that attachmen

"What format is that attachment in?

I can't read it on my browser. I'm guessing it's excel from the "xls" file extension, which should be okay to post as an attachement. Can anyone else read it?"
 
"The file attachent is Excell

"The file attachent is Excell on Windows XP.
to see if I had made any mistake while attaching I tried and succesfully opened it from the board, but of course its me who sent it so I'm opening with the same version.
You shouldn't have any problem though unless perhaps you have a very old version. Let me know if I can help you any further."
 
"Federico,

Thanks. I have W

"Federico,

Thanks. I have Windows XP and a newish version of excel, Microsoft Office 2003.

Can anyone else open it?

Usually attachments work pretty well here..."
 
"Just in case I'm attachi

"Just in case I'm attaching it once again... Good luck.

<center><table border=1><tr><td>http://www.marineengine.com/discus/icons/mime_msexcel.gif""" align=left alt=""application/vnd.ms-excel"">Performance
[url=""][b]Performance calculator.xls[/b][/url] (24.1 k)</td></tr></table></center>"
 
That one opens fine!

Wow...

That one opens fine!

Wow... very thorough analysis.... thanks! A good example for others to use.
 
Thanks Andrew!

The calc. sh

Thanks Andrew!

The calc. sheet is active so if you change parameters you can adapt it to other boats. E.g. if you change the prop pitch you get different results.

Enjoy!!!
 
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