Boat speed 24' 454 king cobra 45 mph

[bigike]

Hey y'all, so i had my 95' Horizon 240 out on the water 2 months ago (in israel now) and we had my family of 4 and my buddies family of 4 on the boat. Boat weighs about 4,000 lbs. And has the 454 King Cobra drive package.

On GPS we were able to pull 45 MPH out of her with a 17px15" prop, aluminum. Does that sound about right? If i had to guess there was 1,100 to 1,200 lbs in people and gear, plus a big 3 person round tube tied to the sundeck...

I dinged the prop and have a untested 19p Balistic prop now.

I've ordered all the parts to repair the bellow, ujoint, gimbal and o-rings so i hope to have it back out soon

 

[chriscraft254]

Re: Boat speed 24' 454 king cobra 45 mph

Sounds ok to me with 8 people on board. Was she full of fuel also?

 

[HT32BSX115]

Re: Boat speed 24' 454 king cobra 45 mph

On GPS we were able to pull 45 MPH out of her with a 17px15" prop, aluminum. Does that sound about right? If i had to guess there was 1,100 to 1,200 lbs in people and gear, plus a big 3 person round tube tied to the sundeck...

It's really impossible to to say unless you also provide the RPM and drive ratio too.

A 24ft boat with that kind of HP should do close to 50 I would think, but it's hard to tell where you are without knowing if the engine is turning correct WOT RPM.

 

[sethjon]

Re: Boat speed 24' 454 king cobra 45 mph

45 sounds too good to be true with that size boat and a full load. I'm surprised you're using a 17 pitch prop too. Personally I'd sacrifice a little low end and save gas. I'd go with a 19 but to each his own.

 

[bruceb58]

Re: Boat speed 24' 454 king cobra 45 mph

I have a 24' boat with a 454 and a Volvo DuoProp. I can get up into the upper 40s with it. All depends on the weight of the boat, condition of the engine and the style of the hull. All 24' boats are not created equal.

 

[bruceb58]

Re: Boat speed 24' 454 king cobra 45 mph

I'm surprised you're using a 17 pitch prop too. Personally I'd sacrifice a little low end and save gas. I'd go with a 19 but to each his own.

Since you have no idea what his WOT RPM is with this prop, how can you even make this suggestion with the information given?

 

[sethjon]

Re: Boat speed 24' 454 king cobra 45 mph

Its easy. I'm on my 9th boat and have made more prop changes than I like to think about. First, I don't think you should go to WOT unless your looking to create problems down the road. Second, I didn't suggest that he change the prop for more RPMs which could be disastrous. I merely suggested that 45 MPH is plenty regardless of what the RPM is and to consider less revs with a little more top end. I respect what the engineers that do the design work for the boats and I respect guys like you too, but like I said I've had a lot of boats and have always done all repairs myself without ever having a serious problem. And I'm not even counting on my sons boats that I have had a hand in as well.

 

[bruceb58]

Re: Boat speed 24' 454 king cobra 45 mph

You figure out what prop pitch you need based on getting the WOT RPM to a recommended range. Has nothing to do with running the boat at WOT during normal operation. If you over prop a boat to not allow it to reach WOT RPM, you can cause serious harm to the engine. As far as suggesting a 17" prop for his boat/engine, do you have any experience with his drive/engine/boat combo? You do realize that different drives have different ratios and require different pitch props right?

 

[QC]

Re: Boat speed 24' 454 king cobra 45 mph

Yeah, got to side with bruce here seth. No way to discuss prop pitch without all of the numbers, regardless of experience.

 

[sam60]

Re: Boat speed 24' 454 king cobra 45 mph

Who is your parts source for my future needs, please?

 

[sethjon]

Re: Boat speed 24' 454 king cobra 45 mph

Sorry to disagree. If you never take it to WOT it doesn't matter which prop you have. If you take off slow and cruise at 25-30 then whats the difference? Like I said, I've made changes in props and engine specs for 30 years based on good sound logic and never had a problem. I don't think you can call it lucky after that much time.

 

[bruceb58]

Re: Boat speed 24' 454 king cobra 45 mph

If you take off slow and cruise at 25-30 then whats the difference?

If you do that, you couild easily be lugging the engine if you are over propped. Just like driving down the road at 35 MPH in overdrive all the time or starting from a dead stop in 2nd or 3rd gear instead of first. You are loading the crankshaft, pistons connecting rods and bearing surfaces way more than they should be in this condition.

It is just bad advice to give people...period!

 

[HT32BSX115]

Re: Boat speed 24' 454 king cobra 45 mph

EVERY engine manufacturer recommends "propping" an engine to produce "rated" (and upper point of the RPM range) AT WOT.

ANY gearing or prop pitch that will not produce "rated" RPM at WOT, (if lower) IS lugging the engine at ALL RPM.

It's pretty much a standard of the industry.


Here's what Mercury indicates in my OPERATION & MAINTENANCE MANUAL in bold print as in the manual.

PROPELLER SELECTION

IMPORTANT: Installed propeller must allow engine to run at the specified maximum wide-open-throttle revolutions per minute (RPM). Use an accurate service tachometer to verify engine operating RPM

Here's what Volvo Penta indicates in their Propeller Guide

W.O.T.
Wide open throttle. With the boat fully loaded the boat should run within
200RPM?s of the maximum rated RPM of the engine.

From Boatfix:

Check your Operator's Manual and note the RPM of the engine at its rated horsepower and the recommended operating range. Your objective is to check to see that the propeller allows the engine to run near the rated RPM, but under no circumstances outside the recommended operating range.​

From Evinrude-parts.com:

Important Facts:​

Choosing the correct pitch and number of blades will allow your engine to operate within the RPM range recommended by the manufacturer.

Tacoma Propeller suggests doing it the "standard" way, http://www.tacomapropeller.com/13296/Selecting-the-right-propeller.html

Selecting the Right Propeller for Your Boat and Engine
The best propeller size for your boat and engine combination is based on the recommended operating range at wide open throttle (w.o.t.) for your engine, which you will find in your operator's manual. This will be expressed in terms of a certain horsepower at a certain RPM (revolutions per minute).

The goal in prop selection is to determine what propeller style and size will maximize performance for your boat, while allowing your engine to operate in the recommended RPM range. The correct propeller will prevent the engine from going beyond its maximum rated RPM, yet allow it to reach the minimum rated RPM where maximum horsepower is produced.

Run the boat/motor at w.o.t. under normal operating load to determine the maximum RPM you are able to obtain. A tachometer is necessary for this test. Adjust the motor trim angle for the optimum performance. If during this test, you begin to exceed the maximum rated RPM of the engine, reduce throttle setting to a position where maximum RPM is not exceeded.

Sorry to bore everyone with this....


I just want to point out that the above techniques are pretty much the STANDARD of the industry for determining the correct prop for your boat. It doesn't matter which manufacturer, whether it's a inboard, outboard, I/O, 4-stroke, 2-stroke, etc.........

The procedure/technique is the same and produces the same results pretty much across the board.


I suppose this thread it a Prop thread now.........



Cheers,


Rick

 

[sethjon]

Re: Boat speed 24' 454 king cobra 45 mph

According to everything I've read you are absolutely correct, but after experimenting with so many combinations and never having a problem I don't agree. I'm not saying as an example, to take a boat and prop it so it screams at say 20 MPH which if you tried to go faster you'd blow the engine. I'm saying that you need enough prop to get up and go and one that will let you cruise to the speed you are comfortable with. Having said that, I have never had an engine failure or any kind of trouble from using the wrong prop.

 

[bigike]

Re: Boat speed 24' 454 king cobra 45 mph

Sorry, i was on a plane going half way around the world HAHA.

Ok, It's got a 19p prop now, but i haven't tested it and the OD is off now for repairs.

The ratio is 1.43 i believe, the HP is 330, unless its the HO then it's 370ish... Probably 330. Engine runs like a raped ape! and with the 17p (which the boat came with) would just literally JUMP out of the water! And it was full of fuel, 55 gallons!

The RPM wasn't verified, the tack is reading 6k, but thats just not right.

 

[QC]

Re: Boat speed 24' 454 king cobra 45 mph

Having said that, I have never had an engine failure or any kind of trouble from using the wrong prop.

Then you are a lucky man. Not arguing with your experience, but please understand that:

a) Many, many, many people have failed engines due to incorrect propeller selection. I/Os, OBs, 4 cycles and 2 cycles and
b) We must rebut as people use this information for making decisions. We play it pretty close to manufacturer recommendations and procedures here. Anything else would be irresponsible and erode our credibility.
c) What you say about fuel economy is technically and theoretically true, but a and b trump that.

 

[bigike]

Re: Boat speed 24' 454 king cobra 45 mph

Who is your parts source for my future needs, please?

Boats.net. I got the ujoint,exhaust bellow, gimbal bearing/seal, shift o-ring, drive shaft gimbal o-rings. I had to search for the water barb/gasket and found 1!!! LOL on the entire internet! and i figured i would change the hose too because they used a straight hose with a spring it in to replace the original. I found 1 on the internet too! haha Got some spicer u-joints from a drive shaft shop.

For gears you a screwed, unless you can find something on ebay...

Water pump.... YOUR ENGINE WAS MADE BY VOLVO! if you have a water pump go out and can't find one you can by the Volvo engine mount bracket and Volvo pump and run that!!!! Met a guy who did that, and ran a 600+ HP engine, King never gave him a issue.

Your not SOL if you have problems with the drive.... Many on Offshorerry LOVE these drives and boost the hell out of the engines behind them! LET HER EAT BABY! hehe

 

[HT32BSX115]

Re: Boat speed 24' 454 king cobra 45 mph

Water pump.... YOUR ENGINE WAS MADE BY VOLVO

Um...Correction, The engine was made by GM.



His pump is on the back of his 1992 model year DPEAMH OMC drive, for which parts are no longer available, so he MUST adapt something.

The raw waterpump and mount made by Volvo for later Volvo engines and drives will work, and I'll bet You could probably even adapt a Mercruiser (or any pump) by making up a bracket etc.

................................



bigike,



You can get some "BALLPARK" stuff by using a prop-slip calculator.

http://www.go-fast.com/Prop_Slip_Calculator.htm

I do not know what the max RPM for your 454 should be (It should be in your Owners Manual), but if it's anything like mine, you should have a 4600RPM max RPM.........(4200-4600 recommended)


So you should "prop" for 4600 with the "load" you'll usually run with (people, gear, fuel, # 6-packs etc)

Using 4600 as the goal, and 15% slip (very common for a single 3-blade prop in a planing hull..... )

With 1.43:1 ratio (are you sure it's NOT 1.41:1?) , 4600 RPM and 17p prop, I get 52mph theoretical (0 slip) speed........ and with 15% slip, about 44 MPH (actual or expected).



If you "prop" for 4200 RPM, it WILL be slower out of the hole.


Once you get your TACH verified (and be sure to use GPS for speed) you can make a better decision. The 19 will probably put you in the lower RPM and be a little slower out of the hole.


My previous 460 King Cobra, did about 57 with a SS 19 and 60 with the same prop cupped and re-pitched to 20p (all at 5000 RPM.....and I think my drive was 1.41:1.........Is that 4000lbs including the engine drive? Mine is 2900lbs EXCLUDING the engine and drive......


With the 17p prop it was considerably slower and would easily exceed 5000 RPM at WOT on plane.....but a LOT better out of the hole.......


With the Mercruiser 454 I now have, and the Thunderbolt V ignition, there's a REV-limiter that prevents operation above 4700 RPM.

I am not all that familiar with your engine and the Spitfire ignition (if you still have it?)....... If it also has rev-limiter capability, that'll prevent you from exceeding the max RPM for that engine.


All this prop-calc stuff is somewhat approximate. If your boat is heavier than normal, the bottom is fouled with marine growth, the engine is NOT producing normal power it will change everything!

Sorry, i was on a plane going half way around the world HAHA.

What? No excuse!!

The airline I work for has WIFI on all the airplanes!!!

Cheers,




Rick

 

[bigike]

Re: Boat speed 24' 454 king cobra 45 mph

I ride business class but never check the wifi.... Lufthansa kind of sucks.

 

[bigike]

Re: Boat speed 24' 454 king cobra 45 mph

yeah yeah yeah, it's a Volvo GL 454 package, the SX pump and bracket kit will bolt up and all you have to do is remove the impeller from the drive. I'm stocked up on impellers but when i use my last one i'll swap over to a engine one if i cant find a KC piece... I like where the pump is because it is just so easy to work on...

Just checked it's 1.42 to 1.

Think i'm gonna go buy some paint for it now

It feels fast, my buddy with a 22' 454 bravo III 24p said it was faster than his... That was a seat of the pants feel when we had the rear seating area out with the engine showing for a test drive after we fixed the floor.

I thought that 45 with all that weight and me not knowing how to trim a OD was pertty darn good