Nordicflame

iTrader: (4)

I know that with single engine applications that it takes 15-20 hp to net 1 mph. For example with a 100 hp increase on a single you should see a 5-7 mph increase.
Although this assumption/calculation seems to work out well for single motor applications, I don’t necessarily see the same holding true for a twin application.
For example, if you added a 100 hp per motor for a 200 hp total upgrade. I don’t see this adding 10-14 mph (or would it?)

Is there a formula or rule of thumb guideline for twins?
What have been your dyno proven results?

Thanks,
Dave

tomcat

Hi Dave: How's the new boat? Interesting question coming from a guy that just added another engine. The rule of thumb would be to add 15 HP/MPH per engine. Most speed predicting equations don't consider the number of engines/drives, just total HP vs. weight and hull type. If you post your current performance I will do a graph for you based on your planned HP.

Nordicflame

iTrader: (4)

Tom,
Here is the present performance at normal elevations…
79-80 GPS speed (71 to 72 GPS at 5000 feet elevation)
Twin 496HOs at 425 hp
Length and weight is 35’ and 8000 lbs

Projected hp increase to 525-550 hp

Also, I need to talk to you. The guy that bought my engine package is very interested in the R-Tech setup.

Thanks,
Dave

tomcat

Dave: I assumed 28 Bravo and 1.5 drive ratio, and gave you an additional 115 HP per engine. Based on your before numbers this would add 9 mph at normal elevation and you would need to increase pitch. Or keep the pitch you have and let the engine go to 5600 RPM at normal elevation, and maybe reach 5000 RPM at 5000 feet.

Tom
(416) 709-8714

Nordicflame

iTrader: (4)

So it appears by that calculation it takes a "total" of 25.5hp/mph on a twin setup. More than what a single would take, correct?
Yes those speeds were based on a set of 28s (5150rpm) at standard elevation and 26s (4900rpm) at 5000 feet.

I'll call you on the other deal today or tomorrow.
Dave

tomcat

Yes, it is the total HP that counts, and it is more HP/mph because it's a bigger, heavier boat, it doesn't have much to do with the number of engines.

You can see that at higher speeds the hull curve becomes steeper and it would take more HP/mph to produce the same increase. And if these engines were in a cat the whole curve would be lower and less HP/mph would be required for the same increase.

The amount of HP required for a 10 mph increase changes all the way up the curve. To get from 50 mph to 60 mph requires an additional 143 HP or 14.3 HP/mph. To get from 140 to 150 requires and additional 375 HP or 37.5 HP/mph.

I think the 15 HP/mph rule comes from the days when a fast boat went 60, and everybody naturally wanted to go 70.

35/25

I don't remember where I heard or read this, it was several years ago. But, a basic formula/rule for horsepower vs speed increases is that: the desired percent increase in speed requires twice that percent increase in horsepower. Or the inverse, an "X" percent increase in horsepower will yield an "X/2" percent increase in speed. I have used this simple formula many times out of curiosity when reading boat tests that compare different motors or even boat manufacturers speed claims with different engine options. It is pretty accurate and always in the ballpark.

Applying this to your example above: 100 horsepower increase (starting with 425 hp) is a 24% increase in power. This should therefore equate to half of that percent (12%) increase in speed. If you use the base line speed of 79.5 mph and add 12% you get an increase of 9.5 mph.

onesickpantera

iTrader: (1)

There's no set "rule" because it depends on the boat weight and how efficient the hull/drive combo is and like Tomcat said it's a sliding scale. If your boat does 50mph now and you want to go 55 it will take a lot less hp than going from 85 to 90.

Here's a fun calc to play with:

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

Plug in your current weight and hp and then play with the constant until you get your current mph. In your case a constant of 245 gives you 80 mph. Now plug in 1050 hp and you get 89 mph, just like Tomcat said.

Of course it's not exact but it seems to be very close. Unfortunately for me it was 2 mph off when I did my re-power.