Did you perform the same test on your old 4 horse that you did on the two 6 hp/4 strokes? It would be interesting to see the results. I too, have a Tohatsu 6hp, 2007 model, ultra long shaft on a North American Spirit 23, probably about 3400#s with all the s*** I've got on board. I get about 5.2 knots(GPS) in light air, and I can tell the unit is no where near top end..if I drop it down a bit, I can still get about 5, but again, light air, a headwind of any significance knocks it down a half knot or so, maybe a bit more. I have to agree with you on the economy.. on the first few "fillups".. I think the tip to the dockhand was more than the gas.. and I"m not the biggest tipper on the water. This is the only engine I have ever had on this vessel, so I have nothing to compare to; however, when I get it wet again next week, I intend to put an older 9.9 two stroke on for a comparison. Again, it would be interesting to see how those "bollard pull" results from your old 4hp/2stroke measure up to your newer 6. What would be even MORE interesting would be the same test between a NEW Tohatsu/Nissan, 4, 5, AND 6 hp.
I would be interested in how the 4 5 and 6 hp tohatsu/nissans compare against each other also.
I did do the tests on my old 4 hp johnson 2 stroke 20 inch shaft. It would generate 120 to 130 lbs of thrust consistently in a static / bollard test at full throttle. The tohatsu 6 hp produced an average 127 lbs of thrust with an 8 pitch prop in the same test. The tohatsu produced approx 160 lbs of thrust with the 6 pitch prop in the static test.
The 4hp 2 stroke would push my boat untrimmed at 5.65 to 5.75 knots over water consistently in good conditions. Trimmed It would push the boat at 6 to 6.2 knots over water. I have done 30 mile trips up the waterway under power and seen these speeds consistently over the entire trip with the old engine.
The 6hp 4 stroke would push my boat 5.35 to 5.4 knots and get to around 5.6 when the boat was trimmed with the 8 pitch prop. With the 6 pitch prop it would push the boat 5.45 knots and about 5.6 when trimmed. lowering the pitch did bring rpm up some on the water, but still left it much below the manufactures nominal 5500 rpm (6000 max rpm). The drop in pitch increased torque but because of the lower pitch did not increase top speed significantly. If the engine had been powerful enough to turn at the full 5500 rpm with these props then I would expect to have seen results consistent with what I expected of a 6hp engine. I ran the theoretical numbers through several online prop calculators and with my boat and these props running at their rated rpm I should see around 6.3 knots under average conditions and closer to 7 with the boat trimmed.
The online calculators did predict fairly accurately the performance that I actually see with the 4hp 2 stroke engine.
My observations lead me to think that the 6hp 4 stroke is putting out about the same energy as the lower powered 4hp 2 stroke and that in the high end of the rpm range that it is putting out very significantly less power. The torque curve for a 2 stroke has more power higher in the rpm range. I would expect in a normal straight up test between at 6hp 2 stroke and a 6hp 4 stroke that the 2 stroke would mildly outperform the 4 stroke in the top end. I would not expect a 4hp 2 stroke to outperform a 6hp 4 stroke though.