This is a prop question.

[MelLandry]

I'm doing a long block replacement in my 19.5ft 1978 open bow runabout. I'm going from the 27 year old marine 305 to a new crate 350 truck long block replacement. From what I understand, the cam is probably slightly different as well as the crank (as well as a few other components that will be exchanged ie. gaskets).

How am I supposed to determine what WOT should be, so that the engine can be properly propped? Should WOT fall at the peak HP or the peak torque range?

Thanks.

 

[Texasmark]

Re: What WOT in new automotive engine in a I/O?

Re: What WOT in new automotive engine in a I/O?

Think you might reconsider on your choice of things to do.

A Mercruiser, for one, engine is nothing more than a Ford or Chevy block crammed full of Merc parts. They had to do this to be able to run at WOT all day in the marine application. Your crate 350 is not designed for that.

You might contact a mechanic at your nearest Mercruiser marine dealership and verify what I said if you don't believe me.

Don't remember where I was made aware of that fact. I would like to quote a reliable source for you.

Sorry for the bad news, but better to know now than to find out 20 miles from shore and after investing all the time and money to do the conversion.



Mark

 

[MelLandry]

Re: What WOT in new automotive engine in a I/O?

Re: What WOT in new automotive engine in a I/O?

Think you might reconsider on your choice of things to do.

A Mercruiser, for one, engine is nothing more than a Ford or Chevy block crammed full of Merc parts. They had to do this to be able to run at WOT all day in the marine application. Your crate 350 is not designed for that.

You might contact a mechanic at your nearest Mercruiser marine dealership and verify what I said if you don't believe me.

Don't remember where I was made aware of that fact. I would like to quote a reliable source for you.

Sorry for the bad news, but better to know now than to find out 20 miles from shore and after investing all the time and money to do the conversion.



Mark

Thanks for the concern, but all the boat-critical components (carb, water pump, plugs, etc) will be marine parts. The only ones of concern were the crank and the cams, and the merc certified tech at a merc premier service center says it's fine.

But since he's a engine guy I figured I'd ask the prop nerds what they thought about propping. So if you have any thoughts about what the question I actually asked, please let me know.

 

[tommays]

Re: What WOT in new automotive engine in a I/O?

Re: What WOT in new automotive engine in a I/O?

The WOT on the motors is 4400 to 4800 RPM and its a lot easery on the motor if it can reach 4800 with a NORMAL load


Tommays

 

[walleyehed]

Re: What WOT in new automotive engine in a I/O?

Re: What WOT in new automotive engine in a I/O?

A "Marinized" engine has a balanced rotating mass.
The truck engine is NOT designed to run the same RPM, continuous, as the Marine version is.

"And", this needs to be in the I/O forum... moving there now......

 

[MelLandry]

Re: What WOT in new automotive engine in a I/O?

Re: What WOT in new automotive engine in a I/O?

A "Marinized" engine has a balanced rotating mass.
The truck engine is NOT designed to run the same RPM, continuous, as the Marine version is.

"And", this needs to be in the I/O forum... moving there now......

This is a prop question, not a I/O question. I'm looking for the prop on the new engine and what I should look for in deciding on a prop. I already have a discussion in the I/O thread about this. I cannot help that people respond about the engin in the prop thread when it should be about props. Please move this back to the prop thread.

 

[walleyehed]

Re: This is a prop question.

This was a question about WOT RPM. The problem is your engine is not a marinized version.
Anyone with any common sense will not recommend a prop for an auto engine placed directly in a boat, because we have no clue what the max continuous RPM should be.
The I/O guys will be able to tell you "IF" this will work, and then, what the RPM should be...
We have no reference to a prop anyway, because you haven't run it with any prop.

 

[MelLandry]

Re: This is a prop question.

So all "prop guys" can do is look at a chart? I could do that myself.

What if i put a weedeater engine in it? Where would the post go then?

Apparently moderators here are gods who grossly disdain anything they see as belonging to someone else. It would have been an atrocity to let the prop forum take a look at this.

 

[walleyehed]

Re: This is a prop question.

It's still in the prop forum.....
Do you just want a WAG at what prop it will use????
It's going to be about the same as that used on the 305 with a tad more RPM.
Another problem that happens in these cases, is the lower unit takes a beating because it wasn't designed for the torque of a bigger engine.
As for the "Chart", I have data of over 300 props on 5 different rigs, one simular to what you have in hull style.
If you have no Data to go by, how can you make an "educated" guess?
I hope you're not one of those that thinks there is a prop designed just for your set-up.....
Prop choice is a direct relation...DIRECT RELATION to Max RPM, that's how we do it......your first prop is going to be an experiment, from there you will have "Data" that will tell you if you need more pitch, more diameter, more rake, more/less blade area, blade design, etc.....there is no such thing as a fix-all prop. If ya want to do it right, then find out what the true Max RPM for "that" engine will be, and then we can make a somewhat educated wild guess at what prop you need.

 

[vipzach]

Re: This is a prop question.

There are lots of people that have swapped out truck motors for their boat. The guys are just trying to warn ya', settle down man! If the tech at merc says "do it", then why not?

What drive do you have? Your WOT should be what tommays said earlier. Your prop will probly be about the same, like walley said, "it will be an expirement" I would guess you will need somewhere around a 21-23p prop. That is just a guess though.

I would try out the prop you currently use and see how it runs on the water, what RPM's you get and how the hole shot and top speed are. Then post those details and people can help get you a good prop choice. Walley is one smart feller when it comes to props!!

 

[Tail_Gunner]

Re: This is a prop question.

So all "prop guys" can do is look at a chart? I could do that myself.

What if i put a weedeater engine in it? Where would the post go then?

Apparently moderators here are gods who grossly disdain anything they see as belonging to someone else. It would have been an atrocity to let the prop forum take a look at this.

Easy there my freind @ first glance it may seem the prop guy's are a bit digital, but i can tell you they are not. What they can tell you is very solid info if you give them a few details as to your current preformance. Ohh and they dont guess........ sometimes even get a bit crabby 8)

 

[ramster]

Re: This is a prop question.

Just try your current prop, I have feeling it will be a bit small with the new motor, we need to know what prop you ran, and what speed/rpm did it do?

 

[Crownie2]

Re: This is a prop question.

In answer to your question, most marine engines are propped close to their HP peak not their torque peak...

That said, I don't know how you are going to determine at what RPM that occurs.

 

[Reel-Fun]

Re: This is a prop question.

I would say have your engine dyno'd if you want to see data on it without taking it on the water. that can be pretty expensive, 1 question, dont they sell Marine inboards? why would you want to take a crate motor and toss it in your boat unless your driving an airboat i think your taking a risk. ive heard of people doing this and motors clunking out after 150 hours.

 

[Bondo]

Re: This is a prop question.

Wow,............
Quite a Few Misinformed people in This thread.......

Well,.... Mel,.....

What you're doing is Fine,...... A Truck Crate motor Cam is So Close to the Marine version,....
Don't Worry about it.......

Like I told you in your I/O thread,.....
The Crank is Fine,.....
The Cam is Fine,......
Actually,... So are the Gaskets,...... Most are Composites Anyway......

Just change out the Core Plugs to Brass,.... They maybe Already,... Check with a Magnet....

For your Prop,.......... I'd start with the 1 you had on the 305,.....
You're going to Need a Bigger 1,.. But you can get the Water Testing done with what you Have,........

I'm Guessing,..... But I'd Think you'll need 2 to 4 More "s of Prop.............

Good Luck..........

 

[walleyehed]

Re: This is a prop question.

Bondo, with all due respect, could you please confirm the internal components they showed us in school that were balance end to end and side to side, and the pistons and pins balanced to within 1/2 gram of each other, and the crank being balanced way closer than automotive standards was just a put-on, for show.
I talked with one of Mercs engineers today and he very highly recommended the engine be balanced at the least, by a competent machine/engine rebuild facility.
He also confirmed the crate truck engines do not contain the same innerds as the marinized version.
He told me even the ignition system is different in many ways.....Guess I haven't been around it long enough to try a truck engine.

 

[Bondo]

Re: This is a prop question.

Bondo, with all due respect, could you please confirm the internal components they showed us in school that were balance end to end and side to side, and the pistons and pins balanced to within 1/2 gram of each other, and the crank being balanced way closer than automotive standards was just a put-on, for show.
I talked with one of Mercs engineers today and he very highly recommended the engine be balanced at the least, by a competent machine/engine rebuild facility.
He also confirmed the crate truck engines do not contain the same innerds as the marinized version.
He told me even the ignition system is different in many ways.....Guess I haven't been around it long enough to try a truck engine.

Kenny,........
While the Merc. Tech wasn't Actually Lying to you,......
Most of what he was saying is Hype.........
For this General discussion,...We're talking about Standard Production Motors,......
Not the High-Preformance Motors.....That's where Merc. might, or might Not use Forged components.....

The "Internals" of a Truck SBC,+ a Marine SBC Are in fact the Same,.........The Cams are Very Close to the Same Grind,... And the Cranks Are the Same....
All SBCs are Balanced,.... As per the manufacturing standards,..... When I have an engine sent to the Machineshop,... I Always spent the extra to have the rotating assembley Balanced to a higher degree,.. Though it's probably Not really Necessary,.. It Does make for a Smoother running, Longer lasting motor.......

I'm out of Time,+ Gotta Go,....... I'll check back tonight.......

 

[Bondo]

Re: This is a prop question.

Bondo, with all due respect, could you please confirm the internal components they showed us in school that were balance end to end and side to side, and the pistons and pins balanced to within 1/2 gram of each other, and the crank being balanced way closer than automotive standards was just a put-on, for show.
I talked with one of Mercs engineers today and he very highly recommended the engine be balanced at the least, by a competent machine/engine rebuild facility.
He also confirmed the crate truck engines do not contain the same innerds as the marinized version.
He told me even the ignition system is different in many ways.....Guess I haven't been around it long enough to try a truck engine.

Kenny,........
While the Merc. Tech wasn't Actually Lying to you,......
Most of what he was saying is Hype.........
For this General discussion,...We're talking about Standard Production Motors,......
Not the High-Preformance Motors.....That's where Merc. might, or might Not use Forged components.....

The "Internals" of a Truck SBC,+ a Marine SBC Are in fact the Same,.........The Cams are Very Close to the Same Grind,... And the Cranks Are the Same....
All SBCs are Balanced,.... As per the manufacturing standards,..... When I have an engine sent to the Machineshop,... I Always spent the extra to have the rotating assembley Balanced to a higher degree,.. Though it's probably Not really Necessary,.. It Does make for a Smoother running, Longer lasting motor.......

And,..... Of Course the Ignition is Different,....... This discussion is about Long Block Motors,..... Not what's bolted Too Them......

I'm out of Time,+ Gotta Go,....... I'll check back tonight.......

 

[Bondo]

Re: This is a prop question.

Oooops,..... Wrong Button,.......

I just wanted to add the line about the Ignition.......

 

[horsefly38425]

Re: This is a prop question.

Y tell'em bondo By the way marine gas comes from the same truck as auto gas. I see it drop a load 2-3 times a week