Prop Question

[Boilermaker]

Re: Prop Question

My stating that manifold pressure is the correct term is stricktly from an engineering standpoint>>>GAUGE PRESSURE(14.7PSIA) vs. ABSOLUTE PRESSURE>>> Absolute zero pressure = 29.92 inchs of mercury colume lift =, OR the ABSENTS of PRESSURE (NO ATMOSPHER).<br /> <br />As for compairing to a car running in overdrive>>><br />Would you run your car UP HILL at 3/4 to full throttle all day in overdrive, because that is the equivalent to what you are doing in your boat>>>Remember the medium the boat has to go thru (water) is much denser then what your car goes thru and creates a heck of a lot more drag. Also the cam of a marine engine is usually timed to place the torque peak a bit higher in the torque curve then your average family sedan.<br /> SO as for suggesting the higher part of the recommended RPM, as most here seem to think>>> I would have to agree. Treat your marine engine as if its going uphill all day, and gear accordingly.

 

[JasonJ]

Re: Prop Question

I actually believe that a marine engine has an easier life than an automotive engine. A marine engine does see a large load, but if set up properly, a marine engine is placed in a good powerband and stays there while in use. Everybody knows an engine is happier if it stays in a certain optimal powerband. That is why trains and large ships that used an engine to run a generator providing electricity for electric drive motors are more efficient and last longer. An automotive engine is left to idle for long periods which is bad, is often overreved or lugged due to poor driving technique, Stop and go driving and short trips never allow the engine to get up to proper running temps which promotes corrosion and oil contamination. Yeah, boat motors in saltwater envirnoments can take it hard, but cars drive in the rain exposing the drivetrain to moisture more than our boats see, due to boats not being used as much. Coastal living exposes cars to the corrosive salt environment as well, although not internally. <br /><br />Boilermaker, we all know true vacuum is a total absence, so it is just decreased pressure. It is referenced as vacuum though, and that is how most people look at it. My gauge says Vacuum, not Negative Pressure, and in most diagnostic books, it is referenced as vacuum. That is just how it is known, even if it is really less than sea-level atmospheric pressure. Heck, when I go to clean my carpet, I use a vacuum, not a less-than-sea-level-atmospheric-pressure carpet cleanamajigger .....

 

[Bondo]

Re: Prop Question

Yep,<br />Exactly What JasonJ Says......<br />And I'll add that boat engines see almost No Dust......<br /> "but if set up properly" <br />Are the Key Words.....<br /><br />And,<br />I'm Pretty Sure, We're in a Boating forum, talking about Boat Engines........Rather than a Laboratory......<br />Therefore, The Gauges on My Boat say Vacuum.... and the Gauge in My tubro-ed pick-up says Pressure.....<br />Sorry...........

 

[Boilermaker]

Re: Prop Question

<br />JASON, I was just making conversation, and pointing out that the term "MANIFOLD PRESSURE" is an exceptable and correct term. <br />BOATIST said "...My engine has no manifold pressure. I have vacume." >>>>Just having some fun with semantics.<br />I never intended to imply that the everyday use of the term VACUME is wrong.<br />On another note>>I LOVE the skanky beast pages. I have learned a few things on your pages THANKS for taking the time to post them!!!Great job on the carvings & TITANIC!!!! I also love older trucks and fine bicycles, although mine are road bikes & GMC 4x4. <br />Thanks <br />Jim

 

[Boatist]

Re: Prop Question

Pburchett Questions was can I use a 25 pitch prop.<br /><br />He now has a 21 pitch 3 blade ss prop and with full tank and 600 pounds of people/gear and WOT RPMS are 5400 on a engine with a recommended max of 4800 rpms.<br /><br />The answer is Yes a 25 pitch 3 blade ss of the same style he now has is the proper prop. This would give him a WOT RPMS of 4600 or right in the middle of the manufactures reccommended rpms.<br /><br />This would give him more speed, better fuel mileage, lower rpms to plane, Lower rpms for a decent crusing speed, longer engine life than being 600 rpms over red line.<br />It will take a little longer to get on plane or longer/slower hole shot.<br /><br />The other option worth looking into is to keep the same prop and check on price for a gear change to put him in the proper rpms.<br /><br />Vacuum is defined by webster's as "a degree of rarefaction below atmospheric pressure: negative pressure".<br /><br />My motor manual say to use a vacuum guage to check manifold vacuum and a compression guage to check cylinder head pressure.

 

[walleyehed]

Re: Prop Question

Boatist, I'm an aircraft tech by trade, and powerplants are my specialty...I know darn well what a 4-stroke engine does and is, and for your information, the "Highest" vacume in an intake, is when you let off the throttle....I'm sorry you don't understand this, and by the way, a "turbo-charger" is run directly off of the crank shaft, and a "turbo-normalizer", or a "Supercharger" is what is common in auto motive being run from exhaust gases...<br />Why do you think you plug-off the vacume line when setting base timing on an auto engine???<br />Because the HIGHEST "vacume" is at IDLE....

 

[walleyehed]

Re: Prop Question

Pburchett, sorry for the confusion I've added to this...<br />To the original question, all I can say is Do as you wish, follow the info and make your decision accordingly, but I will place my money on a 23" being the most pitch you will be able to use efficiently.<br />If the 23" PROVES to be exceptable, please remember if you need to "Tweak" a prop, it is best to add bite or cup, rather than take it away. Simply put, you want to add efficiency, not remove it, and "increase" slip as a result.

 

[JasonJ]

Re: Prop Question

Actually Walleye, a supercharger is crank driven and a turbocharger is exhaust driven (in the automotive world), but I have the feeling that is what you meant to say (maybe aircraft term it differently).<br /><br />Boilermaker, I know you were making conversation , I am hoping that is what we all are doing. Got the boat pic, pretty sweet.<br /><br />Pburchett, I think the 25 will do you well for general cruising and a bit of increase in speed, but that motor is not the most torquey in the world. The 23 would probably be the best all around prop for skiing, tubing, and cruising. I think a 25 could be a bit much for skiing and tubing with that motor, but you could always swap props for a given situation. Run a 23 for playing, and toss the 25 on for cruising. Good luck either way....

 

[Bondo]

Re: Prop Question

Damn.... JasonJ Beat me Again......<br /><br />We Are Still talking about BOATS.......<br />Turbos are Gas Driven,+ Superchargers are Mechanically Driven....... They Both Increase manifold Pressure, as in Above the Local Atmospheric Pressure..... Hence they are Both known as Forced-Induction...... As opposed to Naturally Asperated(Vacuum, Caused by the Throttle Plates ! )<br /><br />As to the Original Question.....<br />I Still Think My 1st post Is the Right Answer....<br /> "The #s would lead you to think a 25" would work... Except, When you get up towards the Upper End of your RPMs, Increasing the Prop alittle will bring the rpms down Faster than you think, Because You're running Out of Available Horsepower......<br />I Also think a 23" will get you About where you Need to be...... If you're Off by an Inch, a Prop Shop can Beat it Up, or Down an Inch or so....<br />You Definetly are Way Under Propped Now.... That old 470 is going to Blow it's Cookies All over the Bilge at that RPM(5400)....... "

 

[walleyehed]

Re: Prop Question

JasonJ, you got me on the terminology...I was steaming somewhat as I typed, and the brain was working faster than my fingers, and yes, your terms are correct...<br />Bondo, the "blow cookies" thing is still killin' me...

 

[Boatist]

Re: Prop Question

Walleyehed<br />I think you need to read my post a little closer before you accuse me of not understanding. If you read my post I think you will find I said vacuum near zero at WOT and -18 to -22 at crusing speed.<br />I never said anything about Superchargers or turbo chargers in any post and it has nothing to do with his question. I plug off the vacuum line to the distributor to set base timing because I do not want any vacuum advance but I never said any thing about that either. At least we are talking vacuum now.<br /><br />I could agree with a 23 inch prop for pulling a skier. He would still be 200 rpms over max with full fuel tank and 600 lb of people and gear. With light fuel load and by himself even higher. I always suggest to run engine in the reccommended range and that would be a 25 inch prop, unless pulling a skier.<br /><br />His best bet is to find a dealer who will let him try a 25 inch prop with the agreement that he can exchange it for a 23 if it does not work out.

 

[walleyehed]

Re: Prop Question

Boatist, I need to apologize to you for my comments....Not a good excuse, but last night was a bad night for me to be allowed on line..<br />No harm intended.. <br />EDIT: I'll go along with trying a 25, and I hope it works, but I see the max HP available being used up before he gets that far...I may be wrong.

 

[waterinthefuel]

Re: Prop Question

Children, children can we stop the fighting so the fishing lies can recommence?

 

[muskyone]

Re: Prop Question

if your on good terms with you dealer see if he'll let you try both a 23 and 25 and decide from there most of the time the real world and the paper world are diff only a water test will let you know what prop you need

 

[Tracy Coleman]

Re: Prop Question

The first thing I would do is put a shop tach on it and a GPS and get some real figures.<br />Backfire

 

[pburchett]

Re: Prop Question

Good advice from all. Did not want everyone to argue, but I have picked up a lot of info.<br /><br />I am off to the prop shop (when weather is better, 17 F here this morning) to get a 23 and a 25. I wil try both and keep the one that performes best. <br /><br />Thanks for all the good information.

 

[Boatist]

Re: Prop Question

Purchett<br />Please come back and let us know how each works. 17F I do not think you will be towing any tuber for a while. Did you find a dealer willing to allow you to try both and buy one?

 

[AW]

Re: Prop Question

Originally posted by pburchett:<br /> I wil try both and keep the one that performes best. <br />

Always the best answer, if you can do it.