Difference in performance from 21p to 23p?

[TorchedGT]

I know this is a vague question, so just humor me haha.

The boat I just bought came with a dead engine (454, Gen VI - light mods, probably 370hp) and a 21p 14-3/4" Viper SS prop.

I yanked the engine and put in a 500hp 454 (bored 468, 10:1, dyno) and had the first run a couple weeks ago with the 21p - OBVIOUSLY this prop is way too small for the power.

This engine build/install and other initial purchase $$$$$ has set me back, so I'm not trying to spend $600 on a prop yet. My friend has a couple 23p Mercury Mirage Plus props sitting around and is happy to let me use one this coming weekend. It's a RH prop and mine is LH (don't ask why, it's a strange King Cobra / DONZI thing I guess) but since it's a cone clutch I/O the rotation doesn't matter.

I'm going to buy the hub kit for OMC and throw the prop on there this weekend - but honestly what do you think I'll see as far as performance gains?

The 21p put hardly any load on the engine, and it happily spun up to high RPM (I'm not going to hit rev limit since I'm still breaking it in; flat tappet cam) - I was usually seeing even RPM to MPH; 4000 rpm was ~42-43mph, etc - with tons of throttle travel remaining (like I said, there was just no load on the thing - I could peg the throttle and get pushed into the seat like I was driving a car but it would top out instantly).

My ideal prop is probably a 24P 4 blade but this should get me a little closer. The engine was run on a dyno for a while, so after this weekend I should be done with breaking it in and I can really see how quick it tachs out with a given prop.

Thanks,
-Alex

 

[Philster]

Re: Difference in performance from 21p to 23p?

What's the gear ratio?

Generally, a 21p on these boats is good for 52-55..a 23p is good for 61-63 and a 25p is good for 67-70. That is based on 3-blades.

@500 +/- HP, you should have a 68 MPH boat all day and night, and that translates perfectly into a 24p 4-blade.

Can you slap a Merc Bravo prop on her? If so, that is just about the best I/O prop going for boats of your size/type.

 

[TorchedGT]

Re: Difference in performance from 21p to 23p?

What's the gear ratio?

Generally, a 21p on these boats is good for 52-55..a 23p is good for 61-63 and a 25p is good for 67-70. That is based on 3-blades.

@500 +/- HP, you should have a 68 MPH boat all day and night, and that translates perfectly into a 24p 4-blade.

Can you slap a Merc Bravo prop on her? If so, that is just about the best I/O prop going for boats of your size/type.

Thanks for the info as always Philster! I didn't realize how large of a difference the 2" pitch would make, I'm becoming a little more excited about this weekend. Another negative side effect of the old prop was the fact that I'm just tossing gas $$ out the window - even if not ideal, the 23 should be a large improvement.

I believe the ratio is 1.43:1

This weekend I'm buying the Flo Torq II hub kit for the drive, so I should be able to run just about any Flo Torq Merc prop out there right? I'm not sure if that includes the bravos or if those are solid props.

I'm a little concerned about some literature for the hubs stating a 400hp limit, with a solid hub suggested for 400+ applications. I mean I suppose any hub that can grip for 400+hp isn't going to really protect the drive components in the event of an impact anyways lol! We shall see - not going WOT this weekend yet, but plan to the next time out.

 

[Philster]

Re: Difference in performance from 21p to 23p?

Okay, my numbers were based on 1.5 gear ratio, so the numbers I gave are still valid, +/- a mile per hour.

People exceed HP ratings for drives and drive components all the time. I know... Doesn't make it right! Generally, you'll hear that it's all-out WOT acceleration the places the tremendous load on the components. This begs to be debated.

Bravo is a 4B and comes with Flo-torq II solid hub kit to handle high horse power.

It's just a suggestion. You have a true V boat. A maximum bow lift prop will get the most hull out of the water (this does not mean it points the nose up). They are the key words in scoping a prop: maximum bow lifting.