Boat Speed?

[steveo32]

Hi, I found this link and I am wondering how accurate it is. This is the link
http://go-fast.com/boat_speed_predictions.htm

If you scroll down you can put in your total boat weight, horsepower at the shaft, and has a list to select the constant that would apply to you based on your boat. Then you click on CALCULATE button above speed and it will give you your boats top speed. You can also put in your top speed and leave horsepower blank and click CALCULATE above horsepower to see what your horsepower is. Just thought I would see what people thought.

I put my info in and it has a top speed of about 49mph. I have not taken my boat out yet, but this seems a bit high to me, I have a actual measured 15' Ebbtide Tri Hull that weighs 1500 lbs with full gear and full gas tanks, and motor with a 1975 70 hp 3 cyl Johnson outboard.

What do you think my top speed would be around?

 

[itstippy]

Re: Boat Speed?

There are way too many variables for a "plug in the stats" calculator like the link you show to give anything other than a rough estimate of potential top end speed. Set-up is the key to at least 25% of top end (or low end) performance. Weight distribution, prop pitch, hull speed, drag, etc. all factor in.
50MPH top end is a goal many shoot for; a true 50MPH by the GPS reading. To achieve that, or 49MPH as claimed, with the motor and boat you describe, would require optimum top end set-up. Realistically, plan on 40MPH top end, enough hole shot to easily pull up a skier, and enough cargo capacity to have passengers & gear without worrying about where they sit or where the stuff is stored. The motor will do that for you. It's the right motor for the boat.

 

[steveo32]

Re: Boat Speed?

Thanks for the reply, there was another site that had a chart and you would divide your horsepower by weight of boat and it would show you your top speed but I can't find it anymore. That other site showed my top speed around 40mph so it is obviously a little more accurate. Can't wait to get out on the water.

 

[Grits]

Re: Boat Speed?

Hello Everyone

Just for s and g I plugged in the data for my Key West 1900 CC with a 120 Evinrude. I solved speed and the calculation was about ten miles over. I then calculated the constant from known values. I also solved for speed and motor size. I changed the laden weight of boat to compensate for a larger motor. My boat weigh 2300 pounds and will run between 43 and 44 at 5,500 rpm. I know what boats like mine will do with larger motors. The calculator was all over the board even after I calculated the constant from known values It is fun but I do not feel it is very realistic. One thing for sure, I feel my boat will do over 53 with a 200; however, it is rated at 150 MHP.

Grits

 

[Chris1956]

Re: Boat Speed?

That calculator shows my 16' speedboat with the 135HP motor will go 84MPH. It is about 20-25MPH too high, at least. I do not think you should swear by these estimates...

 

[kenmyfam]

Re: Boat Speed?

25mph fast for me !!!!

 

[Silvertip]

Re: Boat Speed?

It was about 10% high (4 mph) in my case. When calculating the weight, are you including yourself and passengers. Weight is weight whether is an object or people.

 

[jay_merrill]

Re: Boat Speed?

I just used it too - I have a 17.5 trihull with an older 65hp Johnson on it. I used the heavy V-hull constant, estimated the loaded weight of my boat pretty closely and used a horsepower rating at the prop of only 50hp. The calculator says it should go 38 mph but my GPS says it only goes 28 mph at WOT.

I don't think this calculator is worth much.

 

[EricR]

Re: Boat Speed?

Oh that sounds like that rediculous "Desktop Dyno" program a freind used when I was putting together a small block Chevy some years back- it estimated I would have 323 hp and 363 ft/lbs of torque.

Subsequent drag strip passes (seven years and many passes worth) show I have about 240 crankshaft horsepower.

Those sim type programs are for speculation only!

 

[HighTrim]

Re: Boat Speed?

My numbers were actually fairly close.

You have to remember as Silvertip stated that you need to input an actual weight. Using a scale from the local dump works for me for weighing my boats, minus trailer. You also need to add your weight, plus weight of load onboard.

This is also based on prop hp, so any outboards manufactured before 1985 are not prop rated and need to be adjusted accordingly for the calculator.

 

[Grits]

Re: Boat Speed?

Hello Everyone,

I used the actual weight of my boat and then some. I have scales on my turf farm that are accurate - or + twenty pounds. I assumed a full tank of fuel, ice, two fat people, etc. I was most likely two hundred pounds over actual boat weight. That calculator was written by someone trying to sell motors.

Did anyone get an accurate speed with know values. I as I said above, I first calculated the constant, conservatively, and then went from there. When I used a constant for a heavy V, it was over.

Grits

 

[Melvin Hatcher]

Re: Boat Speed?

Hello Everyone

Just for s and g I plugged in the data for my Key West 1900 CC with a 120 Evinrude. I solved speed and the calculation was about ten miles over. I then calculated the constant from known values. I also solved for speed and motor size. I changed the laden weight of boat to compensate for a larger motor. My boat weigh 2300 pounds and will run between 43 and 44 at 5,500 rpm. I know what boats like mine will do with larger motors. The calculator was all over the board even after I calculated the constant from known values It is fun but I do not feel it is very realistic. One thing for sure, I feel my boat will do over 53 with a 200; however, it is rated at 150 MHP.

Grits

How are you measuring your speed, speedometer or GPS and what pitch prop are you running on your Evinrude?
I am only getting 43-44 mph with my 1900 with three people and full tank of gas. I am running a 19 pitch SST prop.

Regards,

 

[Grits]

Re: Boat Speed?

Marvin,

My speed was measured with a GPS. I am running a 13x19 factory SS prop.

That is with two people, two ice chest, fishing tackle and a full tank of fuel.

What engine are you running and how much does it weigh. My 120 only weighs about three hundred fifty pounds.

What are you turning at you top speed?

Good Luck

Grits

 

[Melvin Hatcher]

Re: Boat Speed?

Marvin,

My speed was measured with a GPS. I am running a 13x19 factory SS prop.

That is with two people, two ice chest, fishing tackle and a full tank of fuel.

What engine are you running and how much does it weigh. My 120 only weighs about three hundred fifty pounds.

What are you turning at you top speed?

Good Luck

Grits

My speed is measured with a Garmin 545-S GPS, the engine is a 1985 XP150 Evinrude Model # E 150 STL CO H, Serial # J 0578813 weighing 381 lbs. and turning a 14 1/2X19 SST OMC prop which pushes my 2003 1900 to 43-44 mph @ 5100-5200 rpm.

Regards,

 

[Dhadley]

Re: Boat Speed?

I plugged in the numbers for one of the old race boats (we know the exact speed and weight) and the numbers were actually very close.

 

[Grits]

Re: Boat Speed?

DHadley

That is interesting. Maybe it works because your hull is built for speed. In other words, it fits the formula.

I am pretty close on my boat weight. We have scales on the farm we use to weigh our trucks that are accurate +-20 lbs. I think the weight I used on the formula was +- 100 lbs.. I played with the boat weight, within a range that I would have weighed, and it was never close.

Melvin, I do not know much about props but are you sure you have it propped right. Your motor is only a few pounds heavier than mine, maybe fifty pounds.

What is the shaft length of you motor?

Grits

 

[Melvin Hatcher]

Re: Boat Speed?

DHadley

Melvin, I do not know much about props but are you sure you have it propped right. Your motor is only a few pounds heavier than mine, maybe fifty pounds.

What is the shaft length of you motor?

Grits

Grits,

My motor has a twenty-inch shaft and is mounted via the second hole from the bottom. I have considered moving the motor all the way up using the bottom hole. However, I don't want it too high causing the the prop to surface in high seas.

I tried a 17" pitch and slowed down with a slight increase in rpms and I tried a 21" pitch prop and picked up a couple of mph but, dropped to 4800-4900 rpms.

Therefore, I finally settled on the 19" pitch SST.
I have two friends with late model 115 Evinrudes, one a 1995 115 on a 17' cc Seaswirl and one with a 2002 115 on a 1900 cc Key West.

Nether can keep up with me or come close when we race out to our favorite crabbing spots. I am always there first dropping my traps when they finally arrive.

Regards,

 

[Grits]

Re: Boat Speed?

I guess mine just runs good. I am the last person you want to take with about props. I may be able to help you with sails but props are out of the question.

FYI-Your motor is mounted exactly like mine. Is yours a V6 or V4? It sounds like were have the same frame etc. Is your motor a 2L?

Grits

 

[Melvin Hatcher]

Re: Boat Speed?

Is yours a V6 or V4? It sounds like were have the same frame etc. Is your motor a 2L?

Grits

The XP150 is a V6 and is 149.4 Cubic Inches.

Regards,

 

[guy74]

Re: Boat Speed?

I used a constant of 200 in the equation, and it came out pretty close to the performance I got out of the 2 boats (3 different boat motor combos) that I've tested. One was my V156 Glastron with a 60hp Johnson on the back (1750lbs), it would go 36mph(gps), caculated at 37mph. Next, same boat, 100hp Evinrude (1800lbs), I got 48mph(gps), calculated 47mph. Third one GX 150 Glastron with 90hp Evinrude(aprox 80shaft hp)(1600lbs), 47mph(gps), calculated 45mph. These are appox weights( I've weighed the boats and these weights are close with me and my brother in the boat, as they were tested). just some FYI