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Author Topic:   Predicted Speed- 1991 Guardian 19
Don SSDD posted 08-29-2014 06:21 PM ET (US)   Profile for Don SSDD  
Just curious, my boat has a 1994 140HP Evinrude 2 stroke, a 15-17 aluminum prop, and with me and half a tank of fuel, it topped out at about 5300 RPM and 29.8 knots on GPS.

Boat dry weight listed at 1750, outboard at 365 lbs, with fuel and me I figure 2500 lbs.

Is my speed in line with what I could/should be getting?



jimh posted 08-30-2014 12:10 PM ET (US)     Profile for jimh  Send Email to jimh     
Speed predictions generally depend on three parameters:


--hull form


In your case, we know:

Horsepower = 140

Hull form = well-known Boston Whaler classic hull

Speed obtained = 34.3-MPH (or 29.8-nautical miles per hour)

Using the algorithm developed by naval architect George Crouch for moderate planning hulls and implemented in my "Crouch's Calculator", we can use the algorithm to calculate the boat weight: 3,855-lbs.

To know if your boat is performing as expected, the weight of your boat has to be known. Do you think the calculated weight of 3,885-lbs is a good estimate of your boat's weight?


for my calculator that performs speed predictions.

Don SSDD posted 08-30-2014 03:32 PM ET (US)     Profile for Don SSDD    
Thanks Jim, the weight I am guessing was 2500 lbs, HP I assume with a 1994 140HP V4, I get less due to age, so I netted down to 130HP and using the calculator I get 41MPH, potentially.

The boat has a keel guard, that would likely cost about 1 or 2 MPH? The motor is mounted in the top hole of the 4 on the OMC, so I could move it up one hole and see what that does. Other than that, I can check out the motor a bit more. It surges a bit, sometimes idles rough, so I'll check the plugs and see if they all look the same.

Assuming the dry boat weight for an Outrage 18 is 1250 lbs, and they are rated for a maximum of 150 HP, it seems a 1750 lb dry weight Guardian 19, the same hull, should be rated for more than 150 HP. 40% more weight would equal 210HP.



jimh posted 08-31-2014 12:44 AM ET (US)     Profile for jimh  Send Email to jimh     
A rough estimate of total boat weight for an outboard boat like a Boston Whaler is double the hull weight. If the hull weight is 1,750-lbs, the boat probably weighs about 3,500-lbs fully loaded.
jimh posted 08-31-2014 12:46 AM ET (US)     Profile for jimh  Send Email to jimh     
Using these inputs

Weight = 3500
Power = 130
Hull Factor = 180

my speed prediction using Crouch's Calculator is 34.7-MPH. You report 34.3-MPH. That sounds like a very good estimation.

Don SSDD posted 08-31-2014 02:02 PM ET (US)     Profile for Don SSDD    
Thanks Jim.


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