engine speed and props

[matt h]

my Mariner 2005 90 f/s is revving to 6750 and 41 kts, (i did back the throttle of sharpish) im running a 13" 19 pitch prop, standard Merc unit, what pitch prop would you recommend?? boats a humber 5.3,i keep getting stainless props
offered by dealers, i dont mind spending the money but it got to be right for the boat,
also there is a lot of movement on the engine via the rubber mounts,when under power. so much so its started hitting the plastic casing on the lower leg and damaging it,
do the mounts go soft??? they have been checked for tightness!!,any ideas??

[slimtim]

mounts don't sound too good.

props...speak to prop revs. Very helpful chap there called thomas. 6750 is obviously way too much, you sure thats right for a 90hp with 19" pitch prop??!! I reckon you need to knock off about 1250rpm! Thats means getting a 23inch prop but I'm pretty sure a 90hp won't swing that. I would say something is wrong...perhaps slipping prop bush/washer thingy if thats possible?

[Kernow Buoy]

Matt as a comparison I have a 5.35 Ribtec with a 90 hp Evinrude, I have a 19" stainless prop and max out at about 5300 rpm lightly loaded, and at about the 40kt mark. I'm happy with the prop size, 4 up I drop to about 5000rpm and 37/38 kts.Hope this helps

[fast fred]

for every + 1" of pich, your rpm should drop about 400rpm,
example- 19" of pich maxs out at 6500 rpm, change to a 20" pich, max rpm falls to about 6100 rpm.

[matt h]

thanks for that guys!! im going to very likely purchase a 13 inch 20 pitch stainless prop, anyboady got a nice one, or new one at decent dosh?? any dealerships??

[jyasaki]

What's the max rpm supposed to be? The 2006 100EFI max range is 5000 - 6000 rpm. Is the 2005 90 the same? If so, you're substantially underpropped.

I would guess a couple of inches of pitch increase are in order, or an inch pitch and about 3" diameter, if your lower unit will allow it. You do not want to run over the manufacturer's max rpm spec for any length of time.

If the spec is to 6500 rpm, you're closer to ballpark, but still a little high. In that case, I would think a larger diameter by a couple of inches, or an inch pitch would bring you in range nicely.

jky

[Andy_Rs600]

I can vouch for prop revs. Went down on thursday to get a new prop and took it out for an hour to try out before hand. Very good advice and service.

However, i agree with slim, those numbers sound like something's slipping unless you have the lighteset RIB in the world! Are you sure there isn't any ventilation/cavitation? Have you checked for any slip(look for sudden increase in RPM for small increase in throttle)

[matt h]

hello chaps!! thanks for all the help, i spoke to Prop Revs Today, (thomas) really help full, likely that the tachco is un calibrated and not actually revving that high,and the rev limiter should close in at 6400 rpm approx, anyway, am going to try some stainless props out shortly, so hopefully ,have a flying machine!!!

[alt]

I always thought that +1" brought the revs down about 200RPM?