Prop Question

[pburchett]

I have a 19 foot, 1986 Sea Ray Seville II Cuddy Cabin. 170HP inline 4 cylinder, alpha one outdrive with a 21 pitch, 3 blade SS prop. Great boat and it was used very little by the previous owner. I use it to cruise at mid range and to occasionally tow a tube. The boat has a dry weight of 2200lbs and I have one battery, 35 gallons of fuel, and 600 pounds of people/gear. It takes all of 2900 RPM to keep it on plane, and WOT is 5400RPM (more than the recommended 4800). <br /><br />The question: How big a pitch can I go to. I would like to increase my midrange speed. I think you can account for a 200 RPM drop for every inch in pitch you go up, but how much more MPH will one get? What size prop can I handle? I want a 25, but do not know if I can handle it. Would a 23 or 24 make any difference?<br />I thought at one time I saw a prop/slip/speed equation on here, but can not find it in the forum.<br /><br />Thanks

 

[walleyehed]

Re: Prop Question

23" is going to be about as good as it will get for ya, it looks like. Remember, your hole shot is going to get slower with every step up, and a 25" is going to put you below the RPM it should be. 23" would be the most I would recommend with the numbers you have given.

 

[crazy charlie]

Re: Prop Question

There will be others who can reco on size,pitch etc. I can tell you that you have a speed prop on now.I am suprised that you can tow a tube with it.You will need to get more of a working prop to keep you on plane at lower speeds and it will get you on plane at lower speeds.You can also try a doel fin if you like your prop and want to try the least expensive method first.Charlie

 

[Bondo]

Re: Prop Question

It's not quite as Simple as you'd like it to be.....<br />I think walleyehed is On the Right Track......<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)....... <br />

I can tell you that you have a speed prop on now

Charlie, What makes you Think This ????<br />Just because it's SSteel ????<br />I see nothing in the post to suggest this......

 

[pburchett]

Re: Prop Question

Charlie: I think (do not know for sure) but I think I have the lowest pitch prop that is reccomended for my boat as the recommended pitch goes from 21 to 25. Towing a tube is easy. Heck most of the time I trim for crusing and forget about it, even while towing or taking off. <br /><br />Bondo: Nothing I ever do is a simple as it is supposed to be. I was thinking the 23 would be about right, but was hoping to get a little extra speed and was dreaming of a 25 and more speed. Would you know approximately how many more MPH a 1 inch pitch provides. For example if I go from the 21 to a 23 pitch and I only increase in speed by 2 MPH then it will not be worth it for me. I would like to increase my speed by 5 MPH. Do not know if that is possible, but hoping. <br /><br />Oh ... on the RPM thing. 5400 RPM would definately sling major parts out of the old 170Hp 4 banger if held there for long. That is not to say that every once in a while I do take it to her limits for a little while. I mostly keep her down around 3500RPM as it is always a LONG walk back to the dock.

 

[Bondo]

Re: Prop Question

I Wish I had a link to explain it better,<br />But, Life Doesn't Work the way You Want it Too...!!! <br /><br />For your motor to Perform the way it's ment to, With out Destroying Itself, You Should Prop it to run at the Recommended WOT RPMs......<br />That way it's Not Over-rev'ed at WOT,<br />Nor is it Lugging, at Lower RPMs......<br /><br />If you go with the 25" prop, You'll be Lugging the $hit out of it...... It'll Detonate itself.....<br /><br />The Big Question about MPH, <br />MPH is the Product of a Properly Propped, Well Set-up, Perfectly Tuned, Marine Porpulsion(sp) System....<br />Your MPH , as you probably know, can go up or down by 4 or 5 mph, Just with the Trim Button.... <br /><br />My 1st Guess, for your boat, you want a 23", round-ear, Stainless Steel Prop.....<br /><br />OR, If you want the Absolute Best Prop in the World...........<br />Use the "Search" button,+ look up "Torque-Shift Prop"......<br />I've got 1, And have posted Lots of Info on them.... <br /><br />The Very Least you Have to do is,<br />Have your present prop Bent Up alittle bit... Any prop shop should be able to Help you there....

 

[Mettaree]

Re: Prop Question

Bondo: Please e-mail me at w_jamesk@bellsouth.net<br />Would like to discuss torque-shift off forum.

 

[walleyehed]

Re: Prop Question

With a gee-wizz guess at prop slippage on your rig, the difference in speed calculates out to about 2.7MPH @4000RPM (gain from 21" to 23")<br />You may gain very slightly more with some serious tweaking with a SS prop, but it's costly, for the minimal gain.

 

[walleyehed]

Re: Prop Question

Here's the calculator....the unknown is slippage.<br /> http://www.boatpropellers.com/propcalc.html

 

[jam39vw995]

Re: Prop Question

<b>Bondo writes:<br />"That old 470 is going to Blow it's Cookies All over the Bilge at that RPM(5400)....... "</b><br /><br />Funniest thing I've read today. LOL

 

[crazy charlie]

Re: Prop Question

Bondo,I made the assumption of a speed prop from 2 things he said.Too many rpms at wot and struggling to stay on plane at 2900 rpms.Sounds like a speed prop to me or just plain wrong size. Charlie

 

[Bondo]

Re: Prop Question

Charlie, I agree he's got the Wrong prop.....<br />I hadn't thought about Blade Profile yet.....<br /><br />Jim, I Think I just sent you a letter... I'm changing 'puters,+ things are a little funny right now....

 

[Boatist]

Re: Prop Question

Pburchett<br />I have a 1980 21 foot Aluminum Runabout with a 140 HP 4 cyl I/O. I think the 25 inch prop is right for you or a gear change. As long as you get a simular style prop a 25 pitch would put you at 4600 RPM right in the center of the recommended range.<br /><br />Many here will tell you that you must be right at the max RPM or you are lugging it. I strongly disagree, 200 RPM below the MAX is not logging the engine.<br /><br />On my boat I have two props both 3 blade Stainless steel double cupped. <br /><br />With the 17 pitch my engine turns 4600 RPM with a top speed of 35 MPH and about 3.8 miles per gallon.<br /><br />With the 19 Pitch My engine turns 4200 RPMS with a top speed of 38 MPH and about 4.3 miles per gallon.<br /><br />I use the 17 pitch in the rivers and lakes where we tend to start and stop a lot more for other boats or marineas. Also crusies usually short, less than an hour.<br /><br />I use the 19 pitch mostly in the ocean where we do not start and stop much and sometimes run 2 hours or more. In Ocean Normally run 25 MPH or less due to Swells.<br /><br />In ocean a have tried both props many times to run out 26 miles to my favorite fishing spot. This trip I will log 70 miles plus or minus one. Some of these miles will be while drift fishing with out the engine running. My depth finder trip log, My loran trip log and my GPS trip log will all be very near 70 Miles when we get back to the fuel dock. With the 17 pitch I will burn 18 gallons, with the 19 pitch I will burn 15 gallons of fuel.<br /><br />From what they say here my engine should have blown back in 1981 when it was one year old but it is still running fine with no repairs except for a exhaust manifold and riser replacement.

 

[walleyehed]

Re: Prop Question

If the recommended RPM range is 4500-5000, and you are proped for 4500, the throttle plates are WIDE OPEN to get that RPM....High manifold pressure/low RPM...Very hard on an engine. If you are proped for 5000 RPM with a load, your throttle plates are still wide open, but the manifold pressure is less. When you pull back and cruise at say, 4500RPM, now your throttle plates are only about 75-80% open, hence lower manifold pressure for the given RPM, resulting in much longer engine life, better fuel economy, and much better boat performance.<br />Bottom line, if you want the best performance in all aspects, and best fuel economy, prop for Max RPM with a normal load....<br />Few people understand manifold pressure, and what it does to an engine in lower RPM situations, including engine runs hotter, oil gets dirty quicker (I wonder why), and it's darn hard on the fuel economy.

 

[Boatist]

Re: Prop Question

The reason they have a recommended WOT RPM is so you can stay in a range that will not harm your engine. His max RPM range is 4800. Running at 4600 WOT RPM is well with in the reccomended range and will increase the life of his engine over running at 5400 rpms. I know you never use over drive in your car and probably stay in second or third because you think it improves gas mileage but I have news your gas mileage will go down and your engine will not last as long.<br /><br />My boat has no manifold pressure at all in fact I have manifold vaccum. At full throttle it is very near zero. At cursing speeds it is in the -18 to -22 lb range. With in the reccommended rpm range a higher pitch will increse speed and fuel mileage, hole shot will take longer.

 

[Jdeagro]

Re: Prop Question

pburchett;<br /><br />Lots of good advice on props from all of the above but would like to add something to the pile.<br /><br />What I hear is that you would like more speed per RPM and maybe more top end (at less RPM). You also do not want to loose the low end performance (hole shot and lower pspeed).<br /><br />Increase the pitch of the prop untill you are under the recommended WOT 4800 RPMs. This will give you more speed per REVotution of the engine (increased fuel econ also). You will likely low the low end performance since now you are starting off in "2nd gear". You will need to use trim tabs to regain ( and much more) what you lost with the prop change. Your bois ideal for Smart Tabs (ST1290-60). check this site for info from those who have used Smart Tabs and check the Mall for pricing, or other retailers.

 

[walleyehed]

Re: Prop Question

Boatist, no insult intended, but you DO have manifold pressure even in a normally aspirated powerplant, and no, I don't use overdrive unless I'm going downhill (East, and south) or no-wind situations.<br />The "Vacume" you speak of is the pressure decrease when the flow, or movement of fuel/air is in motion because the piston is on it's way up...this is not the time period in which manifold pressure is taken-it's between those pulses. The rush of fuel/air in itself causes pressure at the peak of the pulse, and the rush of fuel/air is MUCH greater at lower RPM with the throttle plates wide open.<br />It is fact that more fuel is used when the plates are wide open to obtain the same speed at a lower RPM, whether design limited or within the manufacturers range, it's directly proportional.<br />If your RPM range is 4200-4800, 4800 RPM will be the most efficient over 4200...Keep in mind, the plates are fully open in both cases...hope this clears it up some...

 

[Boatist]

Re: Prop Question

Walleyehead<br />We are talking about a 4 cycle engine. My engine has no manifold pressure. I have vaccum. This is created by the piston sucking in air on the intake stroke and the throttle plate restricking the air flow. The venturi creates a less than atmapheric presure or vaccum which allow fuel to inter the air stream and enter the cylinder on the intake stroke. My car gets better gas mileage in the higher gears and overdrive and so does my boat. I check both on every fill up. <br /><br />We will just have to agree to disagree.

 

[Boilermaker]

Re: Prop Question

Well, not to throw gas on the fire, BUT, technically, there is NO SUCH THING as vacume>>only absence of pressure>>>In a normally aspirated engine, 2 or 4 stroke, we have a NEGITIVE change in pressure below the throttle plates & venturi,compared to atmospheric. But its still considered manifold PRESSURE.

 

[JasonJ]

Re: Prop Question

Yeah, those 470s have a 4800 peak rpm recommendation. I have a blown up one in my garage that belongs to a buddy who did not have it set up right.<br />As far as vacuum or pressure, a normally aspirated engine is always considered to be under vacuum, and a turbocharged/supercharged engine is considered to be under pressure. No way around that. Also, it is recommended by preety much anyone that when driving a car you run it in the highest gear you can at a given speed as long as the engine is within its torque range and not being lugged. Running too low a gear just wastes gas. The only exception is towing a heavy load or driving a performance oriented vehicle at its limit, like on a race track or whatever. Daily driver, just leave the OD on..