Engine Change

[colvic]

If changing from an old to a new modern diesel engine of the same HP but lower revs, would I need to increase the size of the prop to compensate for the posible loss of power?


Cheers

Phil

 

[bedouin]

You also need to compare the gearboxes - what is important is the shaft revolutions rather than the engine revs, so divide the engine revs by the gearbox ratio to work out the shaft revs - and compare those.

That said, yes if you change engines you are likely to need to change props as well. That doesn't necessarily mean going for a larger prop, changing the prop pitch can also help.

 

[seahorse]

The "Westerly" site has a downloadable prog which is very useful.

 

[Johnjo]

If you search Propcalc Freeware you should be able to download a
free propellor sizing site, sorry cannot give web address but broke
my glasses and cannot read it!!!!! used it for my own engine,
well reccomend it, all the best with it.

mike

 

[Anchorite]

Can we have a few details of the boat: LWL, displacement, underwater shape
(keel, rudder position). I marinised a BMC 1500 from a Cambridge, must have been an early 70's model: it pushed my 7 ton, 28 ft wl at hull speed. I changed
it after 15 years 'cos it leaked oil at both ends and in the middle. Shouldn't
have! Prop was a 16x10 off a Hurth 2:1 box.
Suggestion: don't change your prop till you've seen how it goes with the new unit.
If you're out of the water now: make sure you can remove the prop (next time you're going to be swimming...). When you remove the cap on the end, then the
nut + washer, you should be able to read the diameter and pitch stamped on the
aft face. Replace the prop with a smear of marine grease.

 

[colvic]

Thanks for that. We have a 26ft. Colvic Watson motor sailer with the 1500 diesel which is full of sea water and has been since October. Thought I'd found a replacement but I'm not so sure but I'd have prefered to have stuck to the Thornycroft as she was performing ever so well...over 2000 miles last summer alone. At present its a 16 x 12 off a 1 1/4 shaft from a prm 2:1 gearbox.

Best modern equivalent is based on the Mitsubishi, is £4200 here and the same from the factory in Barcelona but then I'll need new shaft, stern tube , stern bearing and prop so best part of £5000 before sorting out anything else.

Thats boating though isn't.


Phil

 

[tr7v8]

assuming that it's the same horsepower developed at (engine) lower revs (normally because the engine is bigger) and not just by gearing then you'd expect to have an increase in pitch and/or diameter to absorb the same power. Various people can do sizing including Lancng Marine.

Jim
-------

 

[colvic]

Thanks everybody, you've been really helpful.

It seems the more time I spend with boats the more there is to learn.

Cheers

Phil