Boat top speed question RPM vs MPH
 

I was wondering something, since new my boat hits top at 5700 RPM and max RPM for the motor is 6000 I can get 6000 at WOT but my speed dosn't increase ? Gets a little louder so more fuel just no speed ? I'm running a 4 blade prop it leaps onto plane is this a small trade off.

300 Rpm is/should be noticeable. Are u using the speedo or GPS for speed?

Prop pitch sounds like the root of it.
If it gets out of the hole fast and has great mid range, t tops off early, a different pitch will give you more top speed but y will lose some bottom end.
Cat

On the gps, I was thinking the same thing cat. Just never really thought about it.

Could be prop pitch or motor height. Lots of guys in the states use jack plates to adjust the height of the motor when on plane. Like Cat says, I would start with trying a higher pitch. What are you running now?


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If you can not max out the RPM with the pitch you have now, a higher pitch will run at a lower RPM than you are now. I doubt you will even get the speed you have now. Simple physics. If you are running regular gas instead of 91 or 93 octaine, that might make a difference.

I think he said he can get to 6,000 but with no ore speed than at 5,700.
300 RPM however is a bit too close.
wouldn't mess with the prop on second thought, but maybe a change pf fuel as Bar here suggested?
Yo ma just be chasing a ghost as well !:thinking-006:
Oh, aluminum prop?
I spent the extra bucks years ago on stainless props and it paid of dramatically, even with the same pitch we could tell the difference.:)
Cat

I'm getting the right RPM with the prop and just wondering why the last 300 didn't pick up any speed, I'm good actually extremely happy with my boats performance so I'll leave it alone. Cat its an aluminum prop, I might as well invest in a good stainless next season for sure. My outboard is on the bottom bolts, Lund rebel XL sport not sure if this matters. with three guys and all the gear it leaps on plain and cruzes at 33 mph at 5600 / 5675 RPM.
http://i394.photobucket.com/albums/p...psgaqcg7ia.jpg

You will notice a big difference with stainless, believe me!
I didn't think so either until I tried one out .
Cat

Not saying you should change, because it sounds like you already have a pretty good prop, but you will get more top end out of a 3 blade prop with more cup. But like others have said it will come at a cost on the lower end.

If you can try a new prop before you buy it I would. We got talked into an expensive SS prop that did not make a bit of difference compared to the aluminium one we originally had though the specs were the same.

Impossible question to answer without knowing all the variables ....

The most likely candidate is prop design ( pitch, #blades, material, etc...) - what are the 2 props in question?

You can also increase/decrease the top speed of any boat on plane by 1-2 mph just by a small adjustment to trimming angle (so was this angle consistent between the 2 tests?)

It also makes a difference if you get a small breeze of only a couple miles an hour different (which is hardly noticeable) or just an inch or two of wave.

On flat calm water, when I am at full throttle, I move the trim angle up or down, just a hair, and will notice the boat speeds up (or slows down) 1-2 mph. Pretty cool to know a tiny adjustment like this makes such a big impact.

Interesting. How does the prop material make a difference and what is it? What does a stainless prop give you that an aluminum doesn't?

I have no idea if it's true or not, but I heard that one advantage of aluminum is that if you hit something, your prop would take the damage itself rather than transferring it on to your drive system and create more expense. No idea if true.

For one, the material is heavier.
The other part of that is that the stainless dos not flex near so much when the throttle s applied so the transfer of power is greater.
When I ran the river with prop jobs, I spent a pile of money ( much more the long run) rebuilding aluminum props than stanless and once they are rebuilt they break even easier.
Stainless bends instead of breaking.
I would never go back to an aluminum prop if I were to g back to a prop job motor.:)
Cat

If you are happy with it I would not mess around. You can spend a lot on props for no real benefit.

On my I/O I just went from a highfive 23p stainless which gave me 45 MPH at 4,400 RPM to a 4 blade stainless 19P because they told me I would get more RPM and be more "efficient". It isn't, I am now at 47 MPH at 4,000 RPM.

I believe 4,800 RPM 4.3L mercruiser 0W322757

Sorry, I went from 23P to 21P (not 19). They said "It’s the cupping on the Rubex’s that make them so aggressive that the RPMs react with it"

Stain Steel gives you durability ie takes small hits and gravel better that Aluminium, as Cat said when he was running props in the river. But if your careful it won't matter that much.

As far as Aluminium being better for your drive train if the prop impacts something is no longer true. At least not on that new Merc of yours. They now have a sacrificial hub that will take the beating and spare the drive train. That being said you should pick up a spare sacrificial hub as they are usually quite cheap (and probably prop if you can afford it esp if you go on long holidays away from major centers).

It could be lots of things. It is hard to believe that your good threw 5700RPM and the next 300RPM you gain nothing? Serious Prop Slip on the last 300RPM? You can type in your info on a Prop slip calculator and maybe get a better understanding of what is happening.



As stated before, although it is fun too play around with props it can cost lots of money for sometimes little gains. It might not be worth it. I am guessing that based on the 33mph that you are running now, you probly have a 90hp mercury or something? They are not a High Performance engine. Just go fishing and enjoy it!!!

yup. I actually have a couple of hubs and washers, etc. in the glove box. Didn't originally know why. Now I do. Thanks. I have a prop wrench on board and also a spare prop. My 2008 Mercury Optimax 115 hp currently has a 4 blade prop on it, and I have a spare three blade prop in the rear storage compartment. I'm assuming the latter might be the original OEM. I have no idea what either prop pitch is.

Went out with another couple (4 adults in boat) on Sunday and was really displeased with how the boat first gets out of the hole, even at full throttle. Seemed to tilt up so much I couldn't see ahead and seemed to plow along and take forever to get on plane. Top end speed seemed OK at about 40 mph. I'll try again this weekend with 3 and see how it goes. I didn't notice the problems so much last time with 3.

The biggest difference you are going to see in hole shot (how quickly you get up on plane) versus top speed is going to be your prop pitch.

I run the lowest pitch I can, which allows me to jump right up on plane within - literally 3 seconds - but my top speed dropped by 15 mph compared to the high pitch prop that came with the boat (I think it was a 25 or 27). The high pitched prop pushed my Lund to around 60 mph. As fun as it is when the water is calm, there really is no need for me to go that fast normally.

The low pitch prop (I think it's a 17 or 19) on my 150 Honda (18foot Lund) gets me into the low 40's mph which is fast enough for me and I can still troll using the big motor when it's choppy and I need to (and stay well under 2.5 to 2.8 mph).

The reality is choosing the right prop for what you want.

If you want hole shot, low speed torque for tubing or skiing, better boat control, Better low speed trolling - get the lower pitch.

If you want screaming top speed, get the higher pitch and have some fun.

From that point choosing materials/different designs/number of blades/etc... can fine tune your boat for sure - but for immediate and dramatic difference - start with the pitch on a standard configuration of a prop.

The prop will have a number on it indicating the pitch.

My guess is your 4 blade is a speed prop and was replaced to give the boat top speed. Doing 40mph is pretty darn fast. Swap it with the factory one (which usually is one that is just a little higher in pitch versus the median range but probably lower than the aftermarket one) and you are likely to get better hole shot and get up on plane quicker. High speed props suck really bad when the boat is loaded up and heavy as they suffer with reduced torque which is needed to get you up and out of the hole.

99% of the people out there replace their props with a "faster prop" ..... I would bet dollars to doughnuts this is what you got.

Unfortunately, the same 99% of the guys don't realize the trade off they are making to the hole shot, bow rise and low speed torque when doing this.

Thanks! I'll check it out. I'm really not that interested in a high top speed. I'd rather have more bottom end power to get a skier up or simply get going and up on plane.

Are you increasing trim angle to go from 5700 to 6000 RPM at wide open throttle? If so, you are increasing prop slip and creating cavitation, which does in fact make your boat less efficient.

Aluminum props typically have less cup on the blade tip and suffer from blade flexing, both of those factors limit the amount of trim angle they can support before they lose efficiency. A stainless prop will give you some added bite at high RPM in comparison. That will carry the bow higher before it will blow out/cavitate and likely add some top end speed as you're reducing drag.

There are so many variables to selecting the correct prop - number of blades, pitch, diameter, rake, barrel length, all affect not only your speed and hole shot, but the attitude of the boat under power and how the boat handles. Motor height is also a significant factor in how a boat/prop combination performs.

In my opinion, you're very close to an optimum setup. Adding more pitch will slow your hole shot and possibly lug the boat when loaded up. You would likely see some small performance and handling gains by doing some testing and switching to a stainless steel prop, but on a 90HP motor, they won't be drastic.

Like Walleye said,

If your RPM is increasing but your Speed isnt, they you could be cavitating the prop. i wouldnt worry much about the max WOT, and just keep it at 5700

[QUOTE=EZM;3249301]The biggest difference you are going to see in hole shot (how quickly you get up on plane) versus top speed is going to be your prop pitch.

I run the lowest pitch I can, which allows me to jump right up on plane within - literally 3 seconds - but my top speed dropped by 15 mph compared to the high pitch prop that came with the boat (I think it was a 25 or 27). The high pitched prop pushed my Lund to around 60 mph. As fun as it is when the water is calm, there really is no need for me to go that fast normally.

The low pitch prop (I think it's a 17 or 19) on my 150 Honda (18foot Lund) gets me into the low 40's mph which is fast enough for me and I can still troll using the big motor when it's choppy and I need to (and stay well under 2.5 to 2.8 mph).




Sorry EZM but......Not wanting to cause a fight but the only way your 18' Lund with 150hp honda is doing 60mph or remotely close to 60mph is behind your truck while you tow it to the lake. :) Sorry this is just bad information!!! It ain't happening. The 300hp mercury on lunds are touching 58mph to 60mph.....but this is a whole different story!

I would say you had all your weight to far to the stern. If you shift just one of those passengers to the front or have them sit on the edge of the casting deck between the windshields as you start off you will find it makes a world of a difference. Also shift as much weight (like anchors etc) up front. They can go back to their seat after getting up on plane.

Pretty sure your 4 blade prop is a power and not a speed prop by the way as most speed props are 3 blade.

Again weight distribution here and total weight in the boat is likely the answer here.

Good Luck

[QUOTE=thenaturalwoodsman;3249580]My 18.5' Lund with a 19P Black Max on a 150 4S Merc touches 48 mph on GPS with my family and all our gear in the boat on calm water. 5800 rpm. I can see getting low fifties with less gear, less people, and an IPS2 hull instead of an IPS. Add a higher pitch prop like a 23 and it's entirely possible to be mid-high fifties.

SS

To much trim can cause the slippage on the last bit of rpm. notice it on my boat if i trim out to far before getting up to speed. Once it's over 45mph the prop won't break lose, and the boat starts to fly.

Have a V-6 2-stroke 150 Merc spinning a 22 pitch prop @ 5800 rpm pushing a 17.5' boat with top end of 54mph.
Works out to 10% slippage from a therotical speed of 60mph.

Unfortunately just adding prop pitch isn't always possible and it doesn't necessarily equate to higher top end speeds.

It's more complicated than this, but simply speaking, RPM X Pitch = MPH. If you increase pitch but decrease RPM, there's no gain in top speed. Every outboard has a suggested WOT operating range, and you have to have the HP to turn the higher pitch prop at the recommended RPM to see a benefit.

Most outboards won't be able to maintain the correct RPM range if you add 4-6" of pitch beyond your optimum setup. Hole shot and cruising performance will be seriously reduced and your top end speed will actually be lower. There's also the potential for engine damage due to lugging the motor.

I don't want to fight either - I was just making a point.

The 300hp engine you are referring to, is not available on my series of Lund boats. 300hp options are reserved for much deeper, wider, larger and heavier boats in the Lund line up. I think, in 2013, the 150 was the max hp option ( I see it's 175 now). Max Hp is relative to the size of the boat and the ability of the boat to transom that weight and thrust.

Nevertheless - at this point - you are right - as I have no need or desire to get my boat to anywhere near 60 mph on the water. The boat originally had a really high pitched prop that was poor to plane but easily pushed my boat well into the 50's.

Keep in mind - The listed standard top speed on the website for Lund indicates 54 mph. No .... it's not quite 60mph ..... but I bet you could get there if you tinkered around a little with the right prop - I dunno - I never had a desire to try and play with props to get top speed out of the boat - maybe she won't get quite to 60mph - only experimentation can tell us that.

I guess my main point was - you can mess around and get really great top speed performance out of a set up - but the trade offs become pretty significant.

Is this with 3 people and a full load or without?
If the boat is running with a heavy load you are not going to be able to adjust the trim to take advantage of the extra rpm.
Try it with a light load and adjust the trim until you feel the boat start to get "loose" then trim back down just a touch.
I bet you see some more speed.