Estmated Correct Prop Size

[FreeBeeTony]

I recently bought a 24' Grady White Offshore with a 350 Magnum Mercruiser I/O. I believe it was rated at ~270HP at the crank. Boat never achieved WOT due to engine problems, it had a 15" prop on it. Went ~22mph at ~3200RPM's. Boat weights ~4000lbs w/ the engine.<br /><br />I converted the engine to a Vortec by replacing the heads and intake manifold. I believe this modification will push it to 310+ HP.<br /><br />What would/should the correct prop size be? If the 15" was correct for the 270HP, could I possibly go to a 17" and stay at the lower RPM?<br /><br />Not looking for a holeshot........just want to cruise at a decent speed at a relatively low RPM.

 

[walleyehed]

Re: Estmated Correct Prop Size

Keep in mind that a high cruise speed at a low RPM is going to cost you more fuel and run higher combustion temps.<br />I would test with the 15 with a normal load and see what you get at WOT..that 350 will handle 5500+ with no problem. Just run WOT long enough to trim for optimum performance and note the RPM.<br />That will give us the base-line we need to start with. The heads and intake may add some HP, but where??? will it be at Top-end or mid-range..we don't know. <br />What was the WOT range for the stock engine???

 

[FreeBeeTony]

Re: Estmated Correct Prop Size

I thought the WOT was 4400-4800 for the SBC's?

 

[Haut Medoc]

Re: Estmated Correct Prop Size

FBT, your WOT #'s are correct for marine applications....JK

 

[Haut Medoc]

Re: Estmated Correct Prop Size

I would try a 14"x19p....JK

 

[walleyehed]

Re: Estmated Correct Prop Size

Now that's brave suggesting a prop with NO base-line....<br />Let's start over...in your first post you said engine never achieved WOT due to engine problems...Let's not spend money for no reason...test with the 15 and see where it comes out, then, and ONLY then, can we make an educated recommendation.<br />p.s. That engine has been tested to 7500RPM and haven't had any internal issues with it...just so you know, 5500 is not going to hurt it...Granted you have different heads and intake, so keep it at or Below 5500 if it wants to go that high.....remember, this is just a test to establish a base-line..

 

[Dhadley]

Re: Estmated Correct Prop Size

In some cases with I/O's the rpm is kind of a range where the outdrive will live the longest. The SBC will take 5500-6000 with little problem but the outdrive wont last long. That combined with where the cam/heads/pistons make the most torque. <br /><br />Obviously the race outdrives will take a higher rpm. So will the Imco drives. But I'd bet thats not what you have.

 

[bdirect]

Re: Estmated Correct Prop Size

Don't know if it helps but i have a 24 cuddy weighes around 5,000 lbs. 350 mercrusiser 250hp.<br />On the mercrusier web site they list the rocommended prop for this engine and top rpm.<br />I run a mirage plus 19 on my boat

 

[FreeBeeTony]

Re: Estmated Correct Prop Size

thanks bdirect.........that's the kind of info i was looking for

 

[mbhenry]

Re: Estmated Correct Prop Size

If you go to Mercury Marine's web site, they have a prop selection guide that works as a base line. Just plug in the requested info and the site will give several props to start with.<br />MIke

 

[walleyehed]

Re: Estmated Correct Prop Size

Please also take this into consideration...very few times has that selector been accurate..in most cases, it will suggest a pitch of about 2" too high, I test props, I know...what they recommend never pans out in the end.<br />

If you go to Mercury Marine's web site, they have a prop selection guide that works as a base line.

This is not a base line...a base line is an actual test run with a known prop, on a SPECIFIC boat at current conditions.<br />

thanks bdirect.........that's the kind of info i was looking for

I apologize..I thought you wanted technical advice that would lead you to the prop that best suited YOUR needs.<br />As you will find out in the long run, there is more to set-up than the prop selector...I hope they take the prop back after you run it and find it's too low on RPM...

 

[FreeBeeTony]

Re: Estmated Correct Prop Size

geez walleyehed, what is your problem?<br /><br />lighten-up a little bit.<br /><br />I have a few different props I can try ranging from 15" to 19". <br /><br />I was just trying to get an idea on which one I should try first..........then I might not need to change it.

 

[walleyehed]

Re: Estmated Correct Prop Size

When I've answered the same question 400,000 times, and test props as well, I get a bit harsh when the person asking the question does not understand I just gave the best way to test to find the base-line. Simply put(and this is my point) there is no other way to find out, unless you have very deep pockets.<br />If you can get the correct prop the first try, you'll be the first on earth to do so.<br />You should also mention you had other props to try..brand, model and size.<br />Don't want ya going backwards on trying to find what you need is all.

 

[FreeBeeTony]

Re: Estmated Correct Prop Size

It's not my/our problem if you have answered the same question 400,000 times. <br /><br />If you're tired of answering questions then maybe you shouldn't repond.<br /><br />

I was just trying to get an idea on which one I should try first..........then I might not need to change it.<br />

Very simple.........end of post............<br /><br />Thanks.

 

[walleyehed]

Re: Estmated Correct Prop Size

It's not my/our problem you didn't get what I said the first time....try the 15 and get a base-line...again, very simple, and the results will point us TO the correct prop.<br />I'm here to help.<br />When I do respond, it's Fact-based. If I didn't respond, you may go buy 2 or 3 props and still not have the right one, especially if you follow the merc prop finder.<br />If you're afraid to hear the truth, don't ask the question. Just trying to help you, not hinder you.

 

[stevens]

Re: Estmated Correct Prop Size

Walleyehed - I've seen you state many times that the Merc prop selector tool gives too high pitch numbers.<br /><br />I don't doubt your word on this, but why do you think that this is so?<br /><br />I am thinking that while you may usually prop for top end speed and prefer the RPMs to be at (or even above) the top of the recommended RPM range, the tool takes a more conservative approach, landing the RPMs safely within the suggested range. In addition, you like to run with a high engine height, whereas the tool perhaps assumes a lower engine height to be more common?<br /><br />In my case, the tool suggests a 23", and this is perfect with a moderate prop like the Black Max or Vengeance. For a more aggressive design, like the Turbo or a Stiletto, I needed to drop to a 22". Dropping to 21" was too low with any of the props I've tried. By the way, all of this was exactly as you predicted. <br /><br />Anyway, my point is that the Merc prop selector seems pretty good to me, but that it does not necessarily suggest the perfect prop for every set-up. You still need to experiment some to find the exactly right one for your specific boat.

 

[QC]

Re: Estmated Correct Prop Size

FBT,<br /><br />You should listen to Walleyehed . . .<br /><br />Also the other boat's info is worthless without the drive info AND the gear ratio even then it is probably still worthless.

 

[Haut Medoc]

Re: Estmated Correct Prop Size

FBT, I'll lend you my 14x19 spare to test with......You gotta come and get it though.... ....JK

 

[FreeBeeTony]

Re: Estmated Correct Prop Size

Thanks for the offer JK..........<br /><br />I will probably start with the 17" and get a "baseline"............

 

[--GQ--]

Re: Estmated Correct Prop Size

freebeetony.... walleyehed was trying to help in your interest. the guy knows what he's talking about. if anything, you should thank him.