How fast should my 20ft 454 be?

[urboyslick]

I just got my first boat! Its a 20 foot bowrider, 1979 Cordova by Fiber Cell. Its got a 330hp Carbd Mercruiser 454 with TRS drive, guessing it weighs 3800 wet. What kind of speed can i expect on this boat? I was doing 25mph at 3200 rpm (i thought it was pretty slow) Came with a 21p cleaver prop which i replaced with a mirage 21p since i felt i was digging the bow into the water so i wasnt geting a good speed reading to base my adjustments for propellers. i have been looking at pitch and prop slip calculators http://www.mercuryracing.com/prop-slip-calculator

Whats the max speed i should expect on this boat? I havent taken it out yet with the mirage 21 to check speed (local lakes have speed limits) im thinking i could definitely go more pitch on a small boat right?

 

[roffey]

Not sure what your boat will do but I can give you my stats, At 3200 RPM my boat is at 30 MPH, by the GPS. WOT is 4900 RPM with the outdrive trimmed up I can hit 54.2 on the GPS. My prop is a factory 23 inch and the motor is a 4.3 TKS, My boat is 2300lb and is a 20 foot BR (Stingray 198LX).

IMHO, your boat should be close to that.

What is your RPM at full throttle and are you using a GPS for speed?

My boat speedo stops working at 40 MPH and will start to go down, it did that since new.

 

[Oshkosh1]

With that I'd expect around 12mph per 1k of RPM. That prop/engine should be able to get you 50-52mph at WOT.

Now...how it's trimmed, load, loading, sea state, wind, current, temp...lots of variables can have an affect on your top speed on a day to day basis.

 

[Starcraft5834]

assuming your WOT recommended range is 4400-4800, have u gotten this boat up there yet with your current prop? you need to determine that first.... before you can assess anything... if you over rev the engine with a 21p, that would suggest you need more pitch.. if you cant hit 4400-4800 prm with a 21p, that would suggest you have to much prop- or pitch..

 

[jkust]

I like the look of that boat for a 1979. Is that a docking light down low on the bow? As for the prop, the Mirage is an aggressive prop, not as aggressive as the later version called the Mirage Plus, but will not be as fast on top end as some other props with different geometry. I have a Mirage Plus and it is a speed robber but quick out of the hole. I run the 17 inch Mirage Plus and get 53mph loaded up with 4, 60 gallons of gas and gear. Not fast on top but pulls nicely. Our dry weights are similar but that's probably the only similarity.

 

[Starcraft5834]

agreed...:rockon: that is a nice looking boat....

 

[urboyslick]

Thanks I love the boat too! Glad to be a part of iboats! Shes well taken care of, bought it from a park ranger he seemed really responsible, stringers replaced and new floor.

Not using GPS for speed
The 21p cleaver prop would spin really fast like 5-6k+ so i wouldn't full throttle it it seemed dangerous i didn't want to over rev that far
Yep those are docking lights on the hull

Is it okay to OVER rev these engines? Is it okay to run the 4600 rpm wot all day long? ive heard alot of mixed information on that..

 

[roffey]

Not sure if you understood the question, what is the RPM at WOT? I would think 5k would be the max and if you are going over that you need to change the prop for a higher pitch or keep an eye on the RPM. So just to answer your question, no it is not OK to over rev the motor.

 

[JASinIL2006]

You don't want to exceed the max RPM for your engine at WOT, so if you're getting above max RPM, you don't have the right right prop. I have a 350 Mag Alpha One Gen 2 in a 19' Larson (a fairly heavy boat for it's size) and I can hit 50 MPH easy with a 4-bladed prop, and even faster with three-bladed prop. Seems like a 20' boat with a 454 ought to be able to get up into that range at WOT.

I was trying to find out what the WOT RPM range for the 454 is, but I saw a number of values from 4400-5200. If you list your engine's serial number, someone can look up exactly what engine you have and figure out the WOT range.

Also, the guys who frequent the "Prop Questions" section of the iboats forums are really good at helping you figure out how your boat should be propped.

 

[urboyslick]

Not sure if you understood the question, what is the RPM at WOT? I would think 5k would be the max and if you are going over that you need to change the prop for a higher pitch or keep an eye on the RPM. So just to answer your question, no it is not OK to over rev the motor.

It was going well past 5k so i didn't really try to find out. i don't think theres a rev limiter on my motor right? some guy said he was running 8k on his 454, is that possible?

Once i get my prop and RPMs dialed in (probably 4,600 RPM) , could i run WOT for long periods of time without worrying?

 

[R055]

It was going well past 5k so i didn't really try to find out. i don't think theres a rev limiter on my motor right? some guy said he was running 8k on his 454, is that possible?

Once i get my prop and RPMs dialed in (probably 4,600 RPM) , could i run WOT for long periods of time without worrying?

How long is "long"? Most will say it's okay for a couple minutes but you definitely don't want to be riding wot for extended periods of time.

 

[Baylinerchuck]

8k rpm on a big block would spell certain doom.

 

[Oshkosh1]

Yes...Assuming the engine is in good condition. Hell...I run that all day in my 4cyl 43 year old MGB. I'm sure a BBC or SBC will do the same.

Once i get my prop and RPMs dialed in (probably 4,600 RPM) , could i run WOT for long periods of time without worrying?

 

[roffey]

It was going well past 5k so i didn't really try to find out. i don't think theres a rev limiter on my motor right? some guy said he was running 8k on his 454, is that possible?

Once i get my prop and RPMs dialed in (probably 4,600 RPM) , could i run WOT for long periods of time without worrying?

IMHO, 5k would be the max so I think you are wise in not hammering it. Ideally it would be nice to try different props to see what gives you the best results. You are on the right path. It would be a guess on my part to tell you the right pitch, someone with more expertise will chime in. It sounds like you have a monster on your hands. My boat at 54 requires both hands on the wheel as it skates around a little.

 

[jkust]

The engine is designed to go WOT for long periods. It always feels to me uncomfortable going wot too long but generally long wot runs are just fine. It's me being illogical. Wot run at 53 with a low pitched/slow top end prop feels like going 30 in my current boat, no extra effort at the helm. What these engines aren't designed to be able to handle is overreving.

 

[Starcraft5834]

As JAS said, find out recommend rpm's for that engine....your hitting 5k + w room to spare..u need more prop..

 

[urboyslick]

Ok so i said screw it and went to the San Francisco Bay at the Berkeley Marina and managed to get some test runs in, just me and one passenger. I did like 45 MPH but that was where my prop was spinning at 5500 RPM, i believe it could reach 50 MPH if i held it longer and had maybe a better surface. This is with the Mirage 21, should i jump up to a 25 or 26 now? I believe I'm over reving about 900 RPMs, in the end id like to be at 4600RPM based on the specs. Its a 1979 454 330HP with a TRS 1.5 ratio.

Here she is at the Yacht Club





I've just ordered an Electronic Ignition upgrade made by Sierra. Also the Nauticus ST Smart Tabs to replace my current trim tabs, mine don't move because the switch is out. I hate constantly adjusting things... heard nothing but good about em, Good stuff right? If its not please let me know, I can send them back on Amazon..

 

[roffey]

My 4.3 is rated for 4800 RPM. WOT I am 4900 RPM at 54 MPH. I have a 23 inch prop. From what I read here trim tabs will slow you down but give you a better hole shot. I have to make sure the weight is distributed in my boat or I run lopsided, LOL. Trim tabs will fix that.

if you sharing pictures Ill show you mine, LOL.

 

[JASinIL2006]

SmartTabs on my boat didn't really cost me any top end speed... maybe 1 MPH, if that. Given that they help with the chine walking that can occur on some boats at higher speeds (like mine), it's a great trade-off.

Unless you know of a prop shop that will let you try and return props until you get the right one (many places don't allow returns), you're much better off starting a thread in the Prop Questions forum. Those guys are really good at steering you in the right direction when picking a prop.

 

[jkust]

Just want to impress upon you how bad it is to overrev by as much as you think you may be. It will most certainly not lead to a good outcome. Whatever you do, keep the throttle back a bit until you get wot resolved. It's interesting though, you mention that you were still able to possibly pick up some speed if you let it keep going...when you look at the HP curves on these marine engines, the HP drops off a cliff when you go past WOT meaning there's nothing to be gained. I would absolutely jump up several inches of pitch. The common sensical thinking is about 200rpms per inch of pitch.