Hello Brett62,
some very surprising fuel figures for that Chapperal.
Have a look at the following link, which is a Yanmar brochure for a 4LHA-STP Series , and study the power & fuel curves.
Is this motor comparable to what you plan?
http://www.yanmarmarine.eu/theme/yan...r-4LHA-STP.pdf
By my calculations;
RPM Prop HP Lt/hr gal/hr Hp/gal
3300 225 55 12.1 18.6
3000 180 38 8.4 21.4
2400 100 23 5.1 19.6
2200 80 18 4 20
2000 65 15 3.3 19.7
The figures for the Chapperal 25ft sportscruiser weighing in at 3356kg.
Cruise speed 25 Knots burning 18 lph = 80hp.
Cruise speed 22 knots burning 13 lph = 65hp.
Can a boat that weighs 3.3 tonnes really plane at 22 knots on 65 propshaft hp?????
The charts do show loads of torque at those revs, maybe that makes it possible but I do not think so.
Have look at this link, Bertram 25 230hp 4LHA + Mercruiser leg, which is broadly comparable to your Whittley.
http://www.nickpilgrim.com/newslette...newsletter.pdf
"The Bertram 25 ran at a brisk 20 knots and used just 80 litres of fuel over the 130km journey."
130 km = 68 nautical miles / 20knots = 3.5 hours
80 litres / 3.5 hours = 23litres per hour at 20 knots.
Which is 100 propshaft hp as per the Yanmar charts, which is a more believable result.
If my figures are incorrect, please let me know.
You may want to reconsider the fuel consumption side of the decision.