As gasoline engines go the Atomic 4 has a flat torque curve. There are various version of the A4 around, but the common 30 HP variant had about 35 ft/lbs at 1000 RPM, and "peaks" around 43 ft/lbs. From about 1800 RPM to 3500 RPM it stayed within about +/- 1.5 ft/lbs.
If you compare the Beta Marine 25 HP diesel ( BD902 ) it has a similar curve, maybe +/- 2.5 ft/lbs from 1600 to 3600. Since HP come from torque x RPM the HP curves are similar. On a 27 - 32 moderate displacement sailboat the Atomic 4 and a 1:1 transmission was an adequate match. A target would be to maintain 70 - 80% of hull speed at around 1800 RPM with some head wind and seas. On an A4 that would be around 13 HP. Fuel economy? I went by a gas station earlier today. Gas was $0.81 a litre, diesel was $1.15 a litre. Ouch! Michael Brown Windburn C&C 30-1 Date: Mon, 12 Jan 2015 18:18:32 -0600 From: "Dennis C." <capt...@gmail.com> To: Jake Brodersen <captain_j...@cox.net>, CnClist <cnc-list@cnc-list.com> Subject: Re: Stus-List C&C 37+ engine size Message-ID: <canir+ysq4pdcgevnyemk8274dt44cz2d07_ogpifgons0yq...@mail.gmail.com> Content-Type: text/plain; charset="utf-8" Josh pretty much restated my argument for repowering from an AT4 to a diesel. My main reasons were flatter torque curve and better fuel economy. The gasoline flammability issue had very little to do with my decision. Punching into a seaway with an AT4 sucks. To generate the horsepower, an AT4 needs rpm's. The 1:1 drive just results in making bubbles. A diesel with a flat torque curve, a reduction gear and a big pitched prop takes a bigger bite without cavitating. Blenders are great for frozen drinks but not for driving a boat through waves. The admiral and I spent a couple hours punching dead upwind in a narrow channel one day in 3-4 foot seas and mid-teens on the nose. Sucked. Sitting back in Mandeville was a brand new 25 hp diesel and all the parts to repower. We just didn't have time to complete the swap before that trip. As for the original thread, Yanmar, I think, recommends 1 hp per 500 lbs displacement. I think that's a bit light. I seem to recall Nigel Calder recommends 1.5 hp per 1000 lbs. Dennis C. Touche' 35-1 #83 Mandeville, LA
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