My 1979 C& L Trawler "Sugardoll" has two 90 hp ford Lehman's turning 22-18 three bladed props. The props are just about shot and the prop shop said they were to small for my boat and suggested 24-19 props instead. Does anyone out there run those 24-19 three bladed props and do they give you the performance and economy they said they would give. My vessel weight is 2600 lbs twin screws. Right now I'm running at 1800 RPM and getting 7.5 knots. Am I wasting hp and diesel with these old props? Thanks JRS

An awful lot of what you get for speed in a displacement boat has to do with the waterline length of your boat and added weights. Generally speaking, the shorter the boat, the less the theoretical hull speed using the formula 1.32 times the square root of the waterline length in feet to get hull speed in knots. I imagine you mistyped the boat weight as 2600 rather than the more likely 26,000 pounds. That probably means you are talking a boat with about 32 to 34 feet actual waterline length or a hull speed of 7.46 to 7.69 knots. The manufacturer probably engineered your boat to achieve hull speed when the properly propped engines are at maximum torque. Your engine's spec sheet shows max torque at around 1600 RPM. A friend of mine has a boat of about that size with twin naturally aspirated Lehman four cylinder engines (which he claims are 80 horsepower). His current props are 18X19 four-bladed, and he claims he cannot achieve max RPM on his engines, which suggests over-propping. Conversely, I have twin six-cylinder Lehman 120s in a boat with a 42-foot waterline (factory weight 34,000 but actual weight of 40,000 fully loaded) running 24X18 props, and I can achieve full design RPM and yet get hull speed of 8.6 knots at 1700-1800 RPM, probably because of forty-plus years of water soaked into the wooden hull and other added weights. I like to run at my engines' max torque which my curves, like yours, show as 1600 RPM and get just about 8 knots. I am guessing that 24-inch diameter props is too much prop for you. Get a second professional opinion. Rich Gano Calypso (GB-CL42 Hull 295) Panama City, FL > -----Original Message----- > From: Trawlers-and-Trawlering [mailto:trawlers- > bounces@lists.trawlering.com] On Behalf Of JOHN SWIERCZ via Trawlers-and- > Trawlering > Sent: Wednesday, February 25, 2015 9:16 PM > To: trawlers@lists.trawlering.com > Subject: T&T: Are my props to small > > My 1979 C& L Trawler "Sugardoll" has two 90 hp ford Lehman's turning 22-18 > three bladed props. The props are just about shot and the prop shop said > they were to small for my boat and suggested 24-19 props instead. Does > anyone out there run those 24-19 three bladed props and do they give you > the performance and economy they said they would give. My vessel weight is > 2600 lbs twin screws. Right now I'm running at 1800 RPM and getting 7.5 > knots. > Am I wasting hp and diesel with these old props? Thanks > > JRS > > _______________________________________________ > http://lists.trawlering.com/mailman/listinfo/trawlers_lists.trawlering.com > > To unsubscribe or modify your subscription options (get password, change > email address, etc) go to: > http://lists.trawlering.com/mailman/listinfo/trawlers_lists.trawlering.com > Trawlers & Trawlering and T&T are trademarks of Water World Productions. > Unauthorized use is prohibited.

John, Sizing props for an existing, running boat is pretty easy. We want props that allow your engines to just reach their max rated RPM at max throttle. If your present props approximate this, then they are a reasonable size/pitch. You didn't tell us your engines' rated top RPM's. The info you have provided seems to be at cruise speed, but prop sizing is normally matched to top speed. Now go out and, after a thorough warm-up, slowly advance the throttles to max. Tap the tachometers to be sure they aren't sticky. Note the RPM and your boat speed. Repeat while running the opposite direction if there is any appreciable current or wind. Then you'll have the data to determine the proper size and pitch. One more useful data point; how much clearance is there between your 22" props and the nearest part of the hull/ keel? Heavy vibration may result from getting this clearance too small. For a much more detailed understanding of prop sizing, you could see Dave Gerr's book, "Propeller Handbook". Warning: math is required. Mark Richter Mark's Mobile Marine Ortona, FL Sent from my iPhone >> -----Original Message----- >> From: Trawlers-and-Trawlering [mailto:trawlers- >> bounces@lists.trawlering.com] On Behalf Of JOHN SWIERCZ via Trawlers-and- >> Trawlering >> Sent: Wednesday, February 25, 2015 9:16 PM >> To: trawlers@lists.trawlering.com >> Subject: T&T: Are my props to small >> >> My 1979 C& L Trawler "Sugardoll" has two 90 hp ford Lehman's turning 22-18 >> three bladed props. The props are just about shot and the prop shop said >> they were to small for my boat and suggested 24-19 props instead. Does >> anyone out there run those 24-19 three bladed props and do they give you >> the performance and economy they said they would give. My vessel weight > is >> 2600 lbs twin screws. Right now I'm running at 1800 RPM and getting 7.5 >> knots. >> Am I wasting hp and diesel with these old props? Thanks >> >> JRS