Getting the right engine

[wild garlic]

Ive just changed my old Perkins 4107 [35bhp] for a Nanni 38 [37.5].In the boat type application the chaps at Nanni reckon Ratio HP ton min is 6 bhp to every ton.However back in 1967 they quite happly fitted the perkins and the Francis Jones design used a Newage BMC if Nanni are correct as my boat is 6.5 Long tons I should have a 40 or 50 bhp unit

 

[Latestarter1]

Little unfair of your vendor to infer that you have purchased the wrong motor this late in the day..........

I do not like use of crude displacement rule, however it kind of works and I think I can kind of follow their logic, and yours for that matter.

Let me guess that your length W/L is around 30 ft giving a max hull speed of 7-7.4 knots. Minimum power required to drive at hull speed would be 30 Hp (22.37 kW). Remember old engines are rated at man sized Hp none of your pansy metric Hp stuff of modern motors so always better to look at power in kW when making apples for apples comparisons from spec sheets.

The Nanni 38 is 28 kW @ 3,000 rpm, your old 4.108 was 27 kW @ 3,000 rpm therefore your logic cannot be faulted when coming up with like for like choice.

However when your vessel was built only kids on the block were the BMC and Perkins, may not have been optimum for vessel but next jump up was a lot more iron.

Going back to the suggestion of larger 2 liter 40 Hp (29.4 kW) Vs 1.5 liter 28 kW motor looking at the logic of the Nanni guys, engine envelope size of larger Kubota is very similar to old 4.108, with rated speed at 2,800. If you want to cruise at close to hull speed the bigger motor may do the job in a slightly more relaxed manner. I see where they are coming from, however do not like their car sales pitch.

Whatever the choice both Kubota motors will work well and be far more socialble beasts than the Mr smelly that came out.

 

[wild garlic]

Thanks Latestarter, I didnt think I was to far wrong, price wise you jump quite abit to the 40bhp but I do understand that there where not many engines to fit back than even most of the tractors would of been petrol/paraffin