prop sizing

[trevone]

I have re-engined my 23' sports day cruiser with two Yamaha 200hpdi outboards. It currently has 14X21"counter rotating props which seem too small. I was thinking 14X25" would be more suitable and economical for general cruising. Does anyone with a similar set up have any advise?

[pistnbroke]

when you have twin motors you usually go about 2 in pitch larger than the single engine set up ....so you need to check the wot revs and work from there ...there could be a rev limiter on these motors so check that wit hthe makers first

[trevone]

Thanks for that. My problem is I didn't get a chance to find WOT before the season ended here. I got it to about 60mph at 5000rpm when the boat was bouncing too much to control; OK I chickened out!
Max revs should be 5500rpm but from the feel of the thing I guess it would have gone to 6000 or more and probably 70mph.
The fuel tanks aren't big enough to do that for long (not at £5 a gallon); I am trying to get the thing more economical and more controllable at cruising speeds; say 25 mph.

[Submarine Tom]

Likely your peak economy is going to be around 30 knots.

Next time your wot and "bouncing around too much" (probably chine

hopping) try trimming down slightly (nose down on the engine trim).

That should stabilize your ride, perhaps at the cost of top speed.

14 X 23 would be my next prop choice/try. Don't let the other ones go

until you're sure your happy with the new props.

Sorry, no first hand experience with your set-up.

-Tom

[trevone]

Thanks Tom,
30 knots would be a nice cruising speed although the water I operate in could be a little choppy for that. I feared chine hopping might happen but hasn't yet. I had that on another boat and it can be scary. Last time I got to 60 mph the water became to rough to maintain the speed. The engines if anything were trimmed too low as the bows seemed very low; the boat was otherwise very steady until I ran out of flat water. The Atlantic can be a bit like that over here!
Trevone

[gonzo]

You chickened out with one motor and you are installing two?

[Joakim]

Originally Posted by trevone

The fuel tanks aren't big enough to do that for long (not at £5 a gallon); I am trying to get the thing more economical and more controllable at cruising speeds; say 25 mph.

The easiest way to achieve that is to remove and sell the other outboard. Changing propellers will not make a big difference at 25 mph and they are not free.