Originally Posted by clloyd
First post. Been researching williards. Saw your spreadsheet. Which prop did you decide on for best speed vs economy? Which would you choose with a generally lightly loaded boat but one that would occasionally have to punch into high winds and steep waves?
Saw 3.8 mph at 29kts posted elsewhere but your numbers dont back that up. Did you actually test all those props?
1. Yes, I've tested those 3 prop variations personally on my boat: 19 inch pitch, 19 inch pitch custom cupped, and 21 inch pitch. I wanted to try a 23 inch pitch with a smaller diameter but haven't yet (the 17 1/4 x 23...it's a stock size from Powertech)
2. So far, the 18 1/2 x 21 has given me the best economy. On a rough day here in southern California on my way out to the islands, 24-25 mph at 4 mpg is plenty for me. I couldn't go much faster anyways. I know of Hurricane 733's with twin 150 two strokes getting half that gas mileage. Yes, they have a top speed of 50 mph, but they can rarely use that in the open ocean. The hole shot isn't as good as the 19 but the mid range cruising numbers show ~2-3mph hour faster at 2200/2400/2600 rpm.
As for the "3.8 mph at 29kts" that you seen quoted by Warfare Rib, he has probably gotten close to those numbers. My hull has bottom paint. He has sold some very clean boats with the original gel and if not, he's redone the bottoms so they're shiny and glossy and ready to go fast. Let's do some math: 29 knots is 34.8 mph. He usually keeps the engines at 180 hp as the Navy specs them (he only bumps them to 210hp if the injectors or pump need a rebuild or a customer requests it). At 180hp, the engine is rated "Medium Continuous" which, according to Cummins, means "This power rating is intended for continuous use in variable load applications where full power is limited to six (6) hours out of every twelve (12) hours of operation." This means you can basically run at WOT for most of your cruising unless you're doing some long runs. WOT is 2500 rpm at that burns 10 gph @2500 rpm. With the 17 1/4 x 23 inch pitch prop and 8% slip (seems to be what I am getting), the calculation show 3.5 mpg. Not 3.8 mpg, but close when you're talking 0.1 increments.... But, he's also put 25 inch pitch props on before and he has smoother hulls. I don't know and can't vouch for him. Give him a call! He's probably redone more of these boats than anyone.