Revs & Screws

[david_e]

Perusing over this MBC forum since last October has got me wondering about the type of boat and engine(s) that you have and which boats you have them in. Now I see a speedboat going full pelt with an outboard so I presume he has one fwd gear, opened the throttle and is batting along at probably 5-6,000 rpm. So when I see a 40 footer like a Fairline or Sunseeker, as in the adds, batting along at similar speed - does they have diesels or petrols? I know petrols can be happy at high revs but thirsty, also I assume diesels are happier at much lower revs, so do they achieve the speed through a gearbox to increase the screw speed or is through the pitch of the prop or both or something else. Interested to find out if poss. Thanks.

 

[PGD]

B B Q time - me thinks !

here's one for flaming chaps - posting this sort of question is going get you a bit warm old chap - I asked about the S word for sorting out - or not - diesel bug - ow !

 

[tcm]

they'll be diesels if 40 footer. twin diesels to be precise, revving at max 2500 -3500, turbocharger kicking in at say 2,000. River-only boats should have 1 diesel (reliabilty no probe, just step off) and no turbo, cos high speed not required. The props contrarotate via handed gearboxes and handed props, and always go *that* way round.

a tiny number of 40foot USA boats will be petrols, and another v tinmy number will have 2-speed gearboxes, so these for speed crazed loonies. As with cars, sailboats, and powerboats, any craft faster than yours is ridiculous, and anything slower is quite unsatisfactory and of poorer design ;-)

 

[david_e]

Thanks for that Tcm.