Looking for more top end

[982100SR2]

I have a 1998 Maxum 2100SR2 (21ft) with a 5.0liter - 300HP Mercruiser engine powering a Alpha 1 Gen2 outdrive (1.47 ratio). I currently run a Mercury High-Five SS 5-blade prop 13.25" 19P. I typically get 50mph at about 5500rpm WOT. The boat weighs in at about 3300lbs.

I'm thinking of changing my prop to try and maximize the speed. I was told by one prop dealer I should go above 23P, but they didn't give me much reason other than losing the hole shot and getting up on plane slower. So I'm curious what the members here opinions are on what changes I can make without risking damage to the engine?

Thanks for your input.

 

[gm280]

:welcome: 982100SR2 to iboats...

WOW that is a huge jump in pitch from 19 to 23 in my opinion. Maybe there were talking about a three blade prop when changing that much. :noidea:

What is the upper limit to your engine's RPM range? Once you know that, you can figure a prop that will allow that RPM range to be met and that is all you can do with what you have. There are no magic props to give more then your engine/boat combination is capable of doing with the HP you presently have. After all that is a heavy weight setup. JMHO!

 

[Scott Danforth]

welcome aboard

if you have a 5.0, you have at most 260hp with an MPI motor assuming it was repowered recently. for '98, I believe the 5.0 was rated at 220hp carbed and 240hp with EFI and was available with the 1.62:1 ratio. the 5.7 was available in the magnum at 300hp.

with either motor, spinning at 5500 RPM will eat the motor eventually as both had a 4800 RPM limit

I would verify your tach with a good service/diagnostic tach and your speed via GPS.

 

[982100SR2]

Thanks for the responses.

Just to clarify I would be going from a 19P 5-blade to a 23P 3-blade. I'll check on the rpm range for my engine,

Scott Danforth, When I looked up the specs on my boat it stated 300HP for the 1998 5.0liter. From what I was able to find the 2100SR2 was some sort of special edition model that had increased horsepower from standard. My insurance co. also told me the same. seems high for a 5.0 liter carbureted, but I'm just going with what I was told.

(update)
I found this: http://boatspecs.iboats.com/Maxum__2100_SR2__1998/bp/66b55723

 

[jimmbo]

A 5.7 with FI may have reached 300, but not a 5.0. With a 1.47:1 gearing and a 19 inch prop, 5500 rpm works out to 60 mph, allowing for 10% slip

 

[982100SR2]

OK, so apparently the boat must have come with a 5.7liter option. My bad. Is there a formula you can share that I can use to figure out the theoretical mph based on my stated conditions?

 

[Starcraft5834]

what ever you run.. keep your engine rpm's within recommended specs or as Scott said, you will blow up your motor eventually..

 

[bajaunderground]

Post the engine and drive model numbers and someone that has access can tell you what exact motor you have? It sounds as if you're not 100% sure?

​On the prop I wouldn't just spend money going from a 19P to a 23P as that's a big jump. I would suggest trying different prop types and pitches before committing. I am skeptical on your numbers, I would have the tach verified (as mentioned) and use a GPS (one can be downloaded for free for your mobile phone) as a MPH gauge rather than the dream-o-meter that works off a pitot using water pressure/flows. ​According to your numbers, you have about 25% slip...I would expect 10-15%. Short of doing all that a 21P is where I'd start given all the other parameters?

Let us know the results?

 

[jkust]

OK, so apparently the boat must have come with a 5.7liter option. My bad. Is there a formula you can share that I can use to figure out the theoretical mph based on my stated conditions?

Just to expound on the 5.7 vs 5.0 a bit...it took the MPI version of the 5.7 which came out in model year 2002 to get 300HP as the previous MAG's didn't quite get there. The 300hp would have likely been the carbed big block. The big block however would have made more sense to pair with a Bravo 1 or 3 not an Alpha 1.
5500 rpms for any of these engines we are mentioning is just way to high. There's actually some great threads around on what constitutes a Mag and all of the various iterations.

Something just isn't adding up.

 

[gm280]

You asked about a formula to calculate speed for a given engine. There is such a formula. And here is how it works. You have to know the final output prop shaft RPM. Then you take the prop pitch (i.e. 23P), and for every revolution theoretically the boat moves 23 inches forward. So you multiple the output prop RPM time the prop pitch to get how far the boat theoretically travels in one minute. Then multiple that by 60 to get distance per hour. Then divide that number by 12 to get feet, and divide that number by 5280 to get MPH. I'm sure there are simpler methods but that does work. However, no boat travels the prop's pitch in distance per revolution. There is always slip. So factor in a 10% to 15% slip and you will get very close to the true speed.

 

[982100SR2]

Thanks for all the responses. I took the boat out yesterday and did some testing. First off, I was very wrong on the tach when I stated 5500rpm. The motor tops out at 4850rpm while traveling 48mph. The speed was verified with a GPS. So based on the previous comments, I am slightly high on my rpms, but increasing the pitch should help that out.

Sounds to me like I was sold a bill of goods on the engine HP. No matter what, the boat has plenty of power and is fast. I would just like to optimize it.

Thanks GM280 for the formula. Makes perfect sense to calculate it that way. I just didn't realize what the pitch number meant.

I have access to a 23P aluminum prop that I'm going to test tomorrow. I'll report the results when I get back off the water.

Thanks again for the responses!

 

[982100SR2]

I was able to borrow a 23p prop and had the opportunity the other day to test it out. Well....that was a bust. The boat could barely get out of it's own way trying to get on plane. Once it finally did get on plane I trimmed it out and was only able to achieve 43mph at just under 4400rpm at WOT. So, apparently my boat is already optimized with the 19p prop I am running. I'm not going to bother spending the money just to try a 21p. I'll just be happy with what I've got. Live and learn!

Thanks again for the responses.

 

[bajaunderground]

I was able to borrow a 23p prop and had the opportunity the other day to test it out. Well....that was a bust. The boat could barely get out of it's own way trying to get on plane. Once it finally did get on plane I trimmed it out and was only able to achieve 43mph at just under 4400rpm at WOT. So, apparently my boat is already optimized with the 19p prop I am running. I'm not going to bother spending the money just to try a 21p. I'll just be happy with what I've got. Live and learn!

Thanks again for the responses.

Cool, thanks for the update...don't hesitate to try a 4 blade 21P or a 20P, odds are you're right there at the 19P, but if the opportunity arises?

Cheers!

 

[gm280]

I was able to borrow a 23p prop and had the opportunity the other day to test it out. Well....that was a bust. The boat could barely get out of it's own way trying to get on plane. Once it finally did get on plane I trimmed it out and was only able to achieve 43mph at just under 4400rpm at WOT. So, apparently my boat is already optimized with the 19p prop I am running. I'm not going to bother spending the money just to try a 21p. I'll just be happy with what I've got. Live and learn!

Thanks again for the responses.

Yes, seems we all look for that absolute top end speed with our new boats. I know I did. But most of the time, we realize our boats come pretty much turned. And if you really think about it, 48MPH is pretty fast on the water, not lightening speeds but not too shabby either. Glad you got it all figured out. Happy and safe boating. :thumb:

 

[QBhoy]

Hi
As pointed out...your 5.0 (if it's the same age as boat) will be around 220hp, but more worryingly, as far as I know the Rev range at wot should be between 4400-4800rpm for that engine. I would change pretty quickly. Other point might be that with a 1:47:1 and a 19 pitch, you might be doing well to get 5500rpm. Check the tacho accuracy and check its not set to 3.0 or 4.3 mode on the back of it.
All the best.

 

[airshot]

Just some food for thought here, as a boater for more than 50 yrs I often hear of those looking for maximum speed from their watercraft. Just think for a moment....would you take your automobile out on the expressway and drop it into first/second gear then cruise the eway at 70 plus mph all day? Think about what your boat engine is doing....they have no transmission so by running at or near wot all day long is no different than driving at 70 mph while keeping your car in first/second gear. Your engine will be short lived...very short lived. Short bursts of wot are good but not for more than a few minutes at a time, if you read many of the posts here on this forum, typical cruising speed for engine longevity normally run around 3200-3400 rpm for a lengthy run. Just some food for thought !!