Boat speed?

[schmoedog]

How much speed should I expect from a Merc 115 an a 16 foot Wellcraft trihull?

 

[suzukidave]

Re: Boat speed?

Go to this website and click on the excel spreadsheet by Mike Guckin and fill in all the blanks about weight, rpm, etc... It will give you a pretty darn good estimate of the maximum speed for the boat and motor combo (you may have to discount the HP on your Merc by 10% if it's an older model with HP measured at the powerhead). <br /><br /> http://users.cybercity.dk/~CCC53569/prop_001.htmth <br /><br />Once you know your target speed, getting the right prop pitch to achieve it is trickier. Props are not perfectly efficient and tend to slip by 10-20% depending on design, materials and the boat itself. You can try multiplying your "effective pitch" on the spreadsheet by 1.15 as a starting point.

 

[Brento311]

Re: Boat speed?

Suzukidave, is that the correct link to the boat speed website? I clicked on it, and it took me nowhere. Thanks<br /><br />Brent

 

[JB]

Re: Boat speed?

Go find the thread on theoretical top speed in the engines forum.

 

[blifsey]

Re: Boat speed?

Take the "th" off the end of the link and it works. http://users.cybercity.dk/~CCC53569/prop_001.htm <br /><br />BL...

 

[Bob_VT]

Re: Boat speed?

Based on the motor and weight right around 38-40 mph.<br /><br />Bob

 

[Capt. Bob]

Re: Boat speed?

(Prop Pitch x RPM)<br />------------------- = Theoretical Speed in MPH<br />(Gear Ratio x 1056)<br /><br />Example:<br /><br />Prop size = 15" x 17"<br />Gear Ratio 1.68<br />WOT = 4,000 RPM<br /><br />(17 x 4,000)<br />------------- = 38.33 MPH<br />(1.68 x 1056)

 

[suzukidave]

Re: Boat speed?

Sorry about the typo onm the website<br /><br />The first formula based on weight and horsepower and max rated wot rpm gives you a target speed to strive for. <br /><br />Capt. Bob's formula helps you calculate what prop you would need to achieve that speed but you have to adjust it for slip, basically a 10-20% discount depending on hull, prop design and materials, trim etc...

 

[suzukidave]

Re: Boat speed?

By the way I would think you would get a lot better than 40mph with a 115 on a 16' boat unless its a darn heavy 16 footer.

 

[Thomas12]

Re: Boat speed?

i agree you should get better than 40 mph. i have 17" crestliner with 125 merc and it goes 58 mph.