Horsepower Question...

[morgro269]

This may be a dumb question, but, here it goes. People talk about adding horsepower to increase top speed. Does this really work? Isn't top speed going to be more a product of your RPM limit and/ or gear ratio ie- impeller pitch and speed of the impeller? It seems to me that if you are hitting your redline with the horsepower you have, adding horsepower is not going to increase your top speed. It will decrease the amount of time it takes to get to redline (acceleration). It will allow you to carry more weight to that redline but I don't see it increasing top speed without changing the other limiting factors. Can someone with more knowledge on the subject let me know if I'm having a brain fart and missing part of the equation?

 

[kthrash]

you are correct final drive ratio limits speed.. however how fast you get to speed is a function of horsepower..

Pulling a boarder up behind a boat with 100hp is way harder than pulling the same boarder up with 300hp..

 

[Bruce]

This is a perfect thread for @Speedling.

Our boats are limited by the size of the jets. At some point we need larger jets to take advantage of more horsepower.

 

[txav8r]

In a boat, hull design plays a roll in speed. And while you may eek out a few more mph's, with more power, the closer to the hull design speed you get, the more HP per mph it takes. You should also equate HP to $$$. Hills also have a limit for safe operation. Too much power or speed and the boat may be less stable.

 

[morgro269]

I can understand hull design playing a role in speed, but if you're hitting your redline, the hull is not limiting your top speed, the redline, final gear ratio is. I could see a different hull design giving you a few more M'sPH if you're not able to hit your redline with the given HP/ gear ratio. Brings me back to my initial conclusion- increasing HP alone will not increase top speed (assuming you are hitting redline).... when I need to go faster on my mountain bike, I change gears, unfortunately, I can't increase my horsepower....

 

[Speedling]

Oh man, i am bummed that i am out of town and have to answer on a phone!

I don't know if you are familiar with props at all, but noth pitch and size matter. It's a balance. You can increase the diameter on a prop easily and pitch easily if you have a change in horsepower.

A jet pump has a constant diameter. This means that pitch is really the only thing you can change to adjust for a horsepower increase. If go from 160 to 200hp for example, you can change from a 19 degree final pitch to a 22 degree pitch. The increase in pitch takes more torque to move the same rpms.

Rpms are important because you want your max torque at your max rpm otherwise you will run out of power when you are below your max rpm. This may be fine, you think, as you just won't go to a higher rpm. Torque and rpm equate to horsepower, which will give you your max speed. A lot can be said here about pump design but i am going to skip some for now.

The jet pump is just that, a pump. In order to pump more water, it takes more horsepower. This is an exponential function, of which i have been working on, but it is very complex due to different boat variables. But what this means is at a specific horsepower you will pump a specific amount of water, resulting in a specific speed. The problem is that with a specific pump size, you start hitting high amounts of resistance within the pump itself.

I really wish i had the computer at my hands to show you the graph, but basically at 160 hp, the 155mm pump is fairly efficient. 180 hp is starting to max it, and by 200 there really isn't a difference from the 180. The scarabs with 250 twins are going to use the extra 100hp to go about 2-4 mph more. Comparatively, the yamaha twins of 160 to 180 use a 40 hp difference to make the same or more difference in speed.

Honestly above the 200 mark they could use the aftermarket 160mm pump, but if people start modifying the 250hp engines, they really need to get an even bigger pump. 300Hp each should have a 170mm pump.

I do hope this helps! Sorry i had to be somewhat brief but i am out of town on work and on my phone!
Be sure to @ me for more questions!

 

[Speedling]

Now that people posted as well while i were typing, yes indeed hull design matters, but if we are talking about modifying engines for power and pumps to process more water, then i assume you aren't dropping this into a different hull, just making your boat faster.

I have found in messing with my equation that something to look at is the speed at which it comes on plane. Typically if it comes on plane quicker than other boats with similar horsepower, it will either be much lighter, or the top speed will be less due to usually having more surface area which is why that hull gets on plane so fast. A hull that is longer and narrower will have a longer time or higher speed to come onto plane, but will have a higher top speed.

Want speed? Here it is in importance imo:
Weight - get rid of it. Less hull in water and less for the engine to push!
Power - if you aren't at max rpms, this is what you want!
Impeller - if you are bouncing off the rev limiter, get more pitch.
Tower down! - somewhat obvious but keep the bimini down

Plenty of other things, like blueprint the pumps, true the hull etc but those are not going to get you any large gains.

Again, let me know if ya need more to chew on!

 

[Seadeals]

Increasing HP alone would help acceleration but not top speed assuming you are already making max rpm and make no changes to the impeller design. They are direct drive, so once the engines reach "redline" (which varies with engine type), the impeller is there too, so it too is maxed out. You can see variations in max attainable rpm at WOT though which could leave some speed on the table. With perfect stock impellers, cables rigged and WOT, my twin 110hp mr1s may have me a bit short (starboard) of total speed potential- see photo. Then again, the manual states maximum output of 110 HP at 8000RPM! There is also no "redline" drawn on the tachs and I know many of us with the MR1s make more rpm than 8000 at WOT. Does that mean we are all rigged wrong? Are tachs just approximate? I generally think the ECM is controlling the real max. Not to digress; but I agree with you. Start changing the impeller to something that needs more HP to spin up to top rpm and you should see more speed with increased HP.

 

[Bruce]

@Speedling, what I take from your post(s) is that since our impeller diameter is fixed the possible changes are in the pitch and the horsepower which is an expression of torque at a given rpm. Horsepower is calculated as (pound-feet of torque X RPM)/5,252.

If we are able to increase torque at the top end we can spin the impeller faster. But doing so may not produce additional thrust or do so efficiently depending on the pitch of the impeller.

If we want faster top speed we may be able to achieve that by reducing the pitch of our impellers allowing them to be spun faster and create more thrust at those higher RPMs.

If we want faster low end we can increase the pitch of our impellers creating more thrust at lower RPMs but this will reduce our top speed as our engines would not have enough torque to spin the higher pitched impeller up to the maximum rpm of the factory impeller.

I am uncertain if our engines are rev limited. MR1s are the engines that I am most familiar with. Here is a torque curve for two R1 motorcycle engines. I believe this is dyno torque levels which will be lower than the flywheel torque levels. The top lines are HP and the lower lines are torque.

Torque starts to drop just over 10,000 rpm which is exactly where HO 230s red line. I believe that this is the point where all available torque is used to spin the impeller. Spinning it faster would require additional torque at higher RPMs.

If we were able to increase torque between 10,000 and 11,000 RPM that would likely increase our top speed but would not necessarily increase our peak horsepower.

 

[Speedling]

Torque is the key because your rpm is set essentially.
@Bruce you are correct in your analysis.
I believe that we are actually about 72 ft lbs or better. We are 1052 cc engines where the bikes are 998cc which allows us to have a bit better torque.

Want more speed? You need hp and more pitch. This will be figured out by adding the power, then adding pitch until you just dip below your redline. Hard to do so call impros that will repitch for a small fee.

Want that hole shot? Reduce your pitch. This is essentially like putting lower gears in a car or leaving the car in first if you jave an automatic. You will redline quicker but the engine will spin faster to do the same work meaning less stress on the engine. I believe stock impellers vary when taken out and measured but i think if you do strictly tow sports you would get like 13/18's put on a mr1 ho engine boat, 14/19's on the 1.8's and i don't have a clue what the 998cc non ho mr1s have since they have 110 hp and different impellers

 

[Bruce]

Does Yamaha use different pitches for different models?

 

[txav8r]

Does Yamaha use different pitches for different models?

They certainly do, for different models and different engines. They also use ride plates on some and not on others...even with the exact same engine.

 

[Speedling]

They certainly do, for different models and different engines. They also use ride plates on some and not on others...even with the exact same engine.

The ride plates are only on the 240's i believe, and i believe that is because the hull was easier to basically keep the same but the engine placement required the pump be put further back. In order to protect/support everything correctly the ride plate was put on there. I remember some people having issues when those were not quite square and the boats steered a bit off.
The 21 footers had a bit more of a hull redesign and i don't think they required the ride plates. Can anyone confirm that?

The impellers i believe are even pitched differently from side to side!
For more information on why google pump loading

 

[scot71]

@Speedling My '04 has the 998cc non HO motors in it and they are listed as 140hp a piece and the impellers are 16.3° port/ 17.3° starboard. The engine RPM's max out at about 10200 rpm. I can't remember my max avg. speed but I know i've seen the speedo bounce around 50mph before in perfect conditions.

 

[Speedling]

@Speedling My '04 has the 998cc non HO motors in it and they are listed as 140hp a piece and the impellers are 16.3° port/ 17.3° starboard. The engine RPM's max out at about 10200 rpm. I can't remember my max avg. speed but I know i've seen the speedo bounce around 50mph before in perfect conditions.

I forgot that there is 3 mr1's! 110,140,160 hp versions! Sheesh, what a statistical nightmare!
Can't trust the speedo though. I have seen mine go to 60 mph, but only ever got 52 on gps.
Also, you would be surprised what the impellers actually read! Mine varied 6 degrees on the trailing edge! All three blades could read something totally different! I put on two 14/20 solas impellers and that waa the final kick to my boat to get me to 52 mph.

 

[scot71]

Yeah, I would say 45 to 47 mph is probably a more realistic number, and when I saw it bouncing off 50. I was by myself with absolutely calm water. I'll have to check the gps a few times this year just to see what I'm actually running.