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Author Topic:   Speed Question
Digs1622 posted 05-23-2002 07:11 PM ET (US)   Profile for Digs1622   Send Email to Digs1622  
I have 25hp evinrude. I thought your speed semi-matched the actual horse power? Like 25hp should go maybe 30 or so. Ive been drivin alone (im 240lbs) and Im hitting speeds near 50 or greater and Im fairly confident that Im actually going close to that speed regardless of what the speedometer says (which says Im going that fast). Is this possible?


Digs1622 posted 05-23-2002 07:12 PM ET (US)     Profile for Digs1622  Send Email to Digs1622     
fyi--my boat is a 1991 13'ss
djahncke posted 05-23-2002 09:12 PM ET (US)     Profile for djahncke  Send Email to djahncke     
Is your speedometer calibrated in Kilometers/Hour? 50 sound too fast for that setup.

The perception of speed on the water is that you are going much faster than you really are. Personally I always take what the speedometer says with a grain of salt. They are by no means a highly accurate instrument.

Try checking your speed with a GPS.

JBCornwell posted 05-23-2002 10:19 PM ET (US)     Profile for JBCornwell  Send Email to JBCornwell     
Howdy, Digs.

A 13 with a 25 will top out at about 25 mph. A GPS will verify that.

Good luck.

Red sky at night. . .
JB :)

Clark Roberts posted 05-24-2002 07:50 AM ET (US)     Profile for Clark Roberts  Send Email to Clark Roberts     
JBC is right on the mark! SCN
where2 posted 05-24-2002 12:57 PM ET (US)     Profile for where2  Send Email to where2     
Speed has little bearing on horsepower. I could do 22mph in a Zodiac with a 9.5Hp Evinrude. However, on a neighbors 8' hydroplane, it would run considerably faster. If I put it on the back of my 15' Sport, it would run considerably slower.
Digs1622 posted 05-24-2002 07:59 PM ET (US)     Profile for Digs1622  Send Email to Digs1622     
Ok...Just curious to how many rpms at say 30mph? Or what is the formula for mph vs rpms??


JBCornwell posted 05-24-2002 09:50 PM ET (US)     Profile for JBCornwell  Send Email to JBCornwell     
Hi, Digs.

Calculating speed from rpm requires that you know the pitch of your prop and the % slip.

My figure above is from real life experience with 2 different 13s with different 25hp engines and optimized props. One even had T&T.

If I had lightened one of mine to get to 30mph it would have probably been turning 6500-7000rpm.

Believe DJahncke. Twenty five in a 13 feels like 50.

Red sky at night. . .
JB :)

jimp posted 05-24-2002 10:02 PM ET (US)     Profile for jimp  Send Email to jimp     
Digs -

My old 1963 13 with a Johnson 33 could top at 26.5 kts or just about 30 mph. But, boy was it fast! Dad's 38-ft Post Sportfisher with twin 300s (427 blocks) would walk away from it.


jimh posted 05-25-2002 12:17 AM ET (US)     Profile for jimh  Send Email to jimh     
[Questions about the performance of classic Whaler boats really belong in the message thread category called, of all things, "Classic Whaler: PERFORMANCE."--moderator]
crashq posted 06-04-2002 04:48 PM ET (US)     Profile for crashq  Send Email to crashq     
50 mph sounds a little too fast for a 25 hp on a BW13. My friend has a 1962 13 with a 40hp Merc 4-stroke with a 20 inch shaft( see cetacea page 57 - currently linked on front page). With two of us on board, it topped out at about 37mph (measured via GPS) on flat calm water. Despite the heavier weight of the 40hp 4-stroke, this was significantly faster than it was with the 1980's 25hp Johnson (15-inch shaft)that it had before this motor. Marine speedo's are notoriously inaccurate for various reasons. Rely on GPS (especially WAAS equipped GPS) for a more accurate reading over a long distance.

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