Performance

Theoretical Performance Numbers

The following are just theoretical numbers I calculated before any actual sea trial just to see how close I could come.

Propeller  Efficiency: Kts = 0.0008234 * Pitch * RPM * Efficiency

72%

Calculated Max Cruise Speed (formula from Dick Tucker's Yanmarhelp Site)

Planning hull speed formula: SHP = shaft horsepower at the propeller, WT = total laden weight of the vessel in long tons (2240lb) (1016.04kg) and K = factor for different hull shapes and loaded waterline length, in feet.

Planning Hull Speed Formula: K * sqrt (SHP/WT) Value of K for Mainship 400 estimated 3.0

3.0 * sqrt (290 max cruise HP@3100rpm / 12.5 long tons) = 3.0 * sqrt 23.2 = 14.4

Fuel consumption @ max cruise approx.(3100 rpm) 17 gal/hr = .85 kn/gal.

(Speed formula use 2.5:1 reduction factor)

Kts @ 1800 rpm: .0008234 * 19.5 * 720 * 72% = 8.32 Kts.

Fuel consumption @ 1800 rpm: 4.5 gal/hr = 1.85 kn/gal.

While the above excercise was fun, sea trial indicates some error in estimating value of  K. The actual value of K works out to be about 2.3, which would be more typical of a semi-displacement hull.

Actual Speed and Estimated Fuel Consumption (Based on Yanmar Fuel Consumption Table)

*07/2004, GPS Speed, 1/2 fuel, full water, 4 persons on board, mid season bottom growth. **This is a much more conservative figure than the 10% normally used in these calculations. Weather - Calm sea, light variable wind, slack current, air temperature - low 70's

***Speed runs done 5/29/2005 240 gal. fuel, 70 gal. water, dinghy on davit, loaded for cruising, two persons onboard, air temp - low 60's.

Interesting observation: 200 rpm difference from 2200-2400 amounts to .4 kts., while a 200 rpm difference between 2800-3000 amounts to 1.2 kts. This would indicate that the hull is less efficient at transition speed (between hull speed and semi-planing speed) as most hulls are.) Increase in fuel burn remains pretty consistant with speed. Using trim tabs at lower speeds (hull speed through transition speed) will slow the boat slightly.

More about Speed of the 400.

Actual Fuel Consumption 2004 Season

I tallied up my fuel slips for this year and this is what I used:

314.2 gallons total.

80 hrs. on the generator. The book says .5 gal/Hr at 50% load. That accounts for 40 gallons fuel.

314.2 - 40  (for the generator) = 274.2 for the main engine.

95.5 hrs on the engine, 274 / 95.5 = 2.87 gal/Hr.

I ran the boat at about 8 knots average this summer of 2004. Mostly between 1800 - 2000 RPM. There were times we ran 3100 and times we ran 1100, (coming in and out of the marina), but we ran at mostly displacement trawler speed. The Yanmar specs call for about 5 gal/hr at 1900 rpm. This engine must be fairly lightly loaded at that speed because I did much better than that. 8kts / 2.87 = about 2.75 nm per gal. So, that was much better than the 1.5 nm/gal, I was estimating this spring before actual trials. I don't have a knot meter and log so I don't have the actual nautical miles through the water but this is pretty close. At this rate, and using a reserve of 100 gallons, the boat should have a range of about 550 miles with plenty to spare, almost twice as far as the estimated in the table above .

Actual Fuel Consumption 2005 Season

I tallied up my fuel slips for this year and this is what I used:

373.1 gallons total.

69.5 hrs. on the generator. The book says .5 gal/Hr at 50% load. That accounts for 35 gallons fuel.

373.1 - 35  (for the generator) = 338.1 for the main engine.

110.0 hrs on the engine, 338.1 / 110 = 3.07 gal/Hr. (average for the season)

This year I ran the boat a little harder at times, but things haven't changed much. My philosophy about running hasn't changed either. Run her at hull speed with tabs up or get her up to WOT, trim her for best speed, than back her down a couple of hundred RPM. I never run the boat at the 2600- 3000 RPM range. In that range the engine is working to hard for what you get for speed. Again this season, we did a lot more slow cruising (in the 7.5-8.0