icreasing top speed with merc 7.4 L

[JGIPSON]

I just got a 24'4" Vindicator, it has a 7.4 L mercruiser with a Bravo 1 outdrive. It has a SS 21 pitch Mirage prop. at WOT I am running 4600-4800 rpm, at about 50 MPH according to the gps. what kinda prop should I look into to to increase speed to the 57-62 range. I have no problem at all coming onto plane right now.

what about a 4 blade prop?

 

[Don S]

Re: icreasing top speed with merc 7.4 L

You need to post this in the Prop Forum

 

[JGIPSON]

Re: icreasing top speed with merc 7.4 L

i put it over there first...didnt seem to get any responses, so I thought I would try it here. I guess I could also add the question...anyother way beside prop change that I can increase RPM/speed range

 

[Don S]

Re: icreasing top speed with merc 7.4 L

It's very simple, to increase your speed 10mph, you need a LOT bigger engine. Just props ain't gonna do it.

 

[Bondo]

Re: icreasing top speed with merc 7.4 L

4600-4800 rpm, at about 50 MPH according to the gps. what kinda prop should I look into to to increase speed to the 57-62 range.

Ayuh,.......

Another 200hp,.... Might get you into that window with a new Prop...........

 

[Boomyal]

Re: icreasing top speed with merc 7.4 L

Lighten ship? Throw everything overboard.

 

[HT32BSX115]

Re: icreasing top speed with merc 7.4 L

Depending on the year, you are pretty much already at the correct WOT max RPM for that engine. you're not going to get much more speed out of it.

If you want more speed. you'll need to get a lot more engine. Even then, an additional 100 hp may only get you 5-10 more MPH at best.

A 4 blade will slow you down.


More speed will require several thousand more dollars!




Cheers,

Rick

 

[JGIPSON]

Re: icreasing top speed with merc 7.4 L

it is a 2000 model Vindicator, if that makes any difference. I was told by a mechanic that 5100 would be a good WOT max rpm. Is this too high?

 

[Don S]

Re: icreasing top speed with merc 7.4 L

The max rpm for a Mercruiser 7.4L MPI engine should be between 4200 and 4600 rpm according to Merc service manual. Not sure what your mechanic is using for information.

 

[JGIPSON]

Re: icreasing top speed with merc 7.4 L

Thank You, Im not sure what hes using either, that was just what he told me when i picked it up from the initial inspection I had them perform.

 

[SuperNova]

Re: icreasing top speed with merc 7.4 L

From personal experience I've found most people with I/O's don't know how to trim them properly for correct running attitude. I think if you were to trim yours correctly your top speed rpm would increase significantly. I honestly believe you should be able to run at least a 23" pitch prop with no difficulty and possibly a 26" pitch with a decrease in hole shot. I know others will disagree with me, but I spent a lot of time experimenting with props myself on different boats over the years and found there are a lot of "experts" spouting a lot of incorrect information. If you have the extra dough or a good relationship with your marina, it's definitely worthwhile to experiment.
--
Stan.

 

[mkast]

Re: icreasing top speed with merc 7.4 L

If you put a 26 pitch prop on that 24 footer, load the boat with 2 - 6 adults, complete with food and drinks, a full tank of fuel, that boat won't even get on plane unless everybody is on the bow.
If you want to go faster, get more (lots more) horsepower, or another boat.

 

[SuperNova]

Re: icreasing top speed with merc 7.4 L

I had a 21-footer with a 5.7 with a 26" fourblade stainless that still kicked *** even with 6 adults in it. It is worthwhile to experiment.

 

[Mischief Managed]

Re: icreasing top speed with merc 7.4 L

What's the max RPM for a 454 MAG? It displaces 7.4 liters as well and is made with a stouter reciprocating mass. I'd think the 7.4MPI would be boucning off the rev limiter at 4800 RPM (assuming it has one)

That said, assuming you are trimmed right, increasing top speed from 50 to 62will take almost exactly 50% more horsepower and a different prop. Changing the prop alone won't give you the speed you want. 57 MPH would require 29% more power.

 

[SuperNova]

Re: icreasing top speed with merc 7.4 L

My vote is to prop it can't you can't spin it properly--THEN go for more horsepower if you want. See what you can get out of your existing setup. Makes more sense than wildly guessing at stuff.
--
Stan

 

[mkast]

Re: icreasing top speed with merc 7.4 L

My vote is to prop it can't you can't spin it properly--THEN go for more horsepower if you want. See what you can get out of your existing setup. Makes more sense than wildly guessing at stuff.
--
Stan

You can change props on that boat until you're my age.
The boat weighs XXXX LBS.
The engine's output is XXX hp.
The boat will go at this speed until the horsepower or weight changes.
The engine is running at it's WOT rated RPM now.
Changing the prop will either raise the engine RPM at WOT and lower hull speed with a smaller pitch or lower the WOT RPM and lower the hull speed with a larger pitch prop.
That's not a wild guess, it's called PHYSICS.

 

[SuperNova]

Re: icreasing top speed with merc 7.4 L

There are also things called prop efficiency and hull efficiency, increase either of those and you can increase top speed without changing horsepower or weight. The theory you cite about horsepower required to push a given weight hull works better for non-planing type hulls(also known as displacement hulls) than it does for planing hulls, which is the type we are dealing with here.
--
Stan

 

[erikgreen]

Re: icreasing top speed with merc 7.4 L

Appreciate your "pen and paper theories" mkast, but how much real-world experience do you have? Physics also involves things like prop efficiency and the friction coefficient of the hull thru the water, things you don't seem to take into consideration.
--
Stan

Let's keep it friendly, shall we?

I would say that if switching props is something the owner wants to do, then it'll be shown pretty easily which of you is right, correct? No need to start personal attacks here.

One of the strengths of this board is that you can come here and get a variety of opinions on things, not a whole bunch of dogma about "the right thing to do". Let's keep it that way if we can.

Erik

 

[QC]

Re: icreasing top speed with merc 7.4 L

Well I was going to post some prop efficiency stuff as well and don't think Super Nova has been all that confrontational . . . The fact is that a pitch change of even the same model prop may increase top speed. If you check, almost all test data show that being propped toward the bottom of the WOT range is faster than being propped toward the top.

I didn't say propped this way is more fun, or more flexible across different loads, or would have equal hole shot. I said faster top speed, which was/is the topic . . .

 

[mkast]

Re: icreasing top speed with merc 7.4 L

what kinda prop should I look into to to increase speed to the 57-62 range.
what about a 4 blade prop?

Seven to twelve mph increase.
Now, tell me about prop efficiency!