Question for prop experts - 1989 Merc 100HP

[MartyTn]

I have a 1989 Merc 100HP (4 cyl) that cruises great with a 19" pitch (non-cupped) alum prop and it also does well pulling up a skier on two skiis, but it is very marginal pulling up an adult slalom skier (barely will).<br /><br />What difference would I see by using an 18" pitch stainless steel prop when pulling up the same slalom skier? (I'm not sure if the Merc 18" SS prop is cupped or not).<br /><br />I also have a 17" pitch (cupped) alum prop, and this will pop up a slalom skier quick, but the rpm's seem to be excessive and the boat uses a lot more fuel skiing with this prop. Also, this prop seems to ventilate or cavitate in sharp turns. I think I've seen in print that there should be about a 200 rpm difference in rpm with every inch of pitch reduction, but this 17" pitch cupped prop seems to me to be quite a bit more than 400 rpm higher than the 19" un-cupped alum prop at 30+ mph. <br /><br />Advice really appreciated. Thanks.

 

[moderator1]

Re: Question for prop experts - 1989 Merc 100HP

If the 17 is cupped, it should be almost the same rpm as the 19 un-cupped. I would say the 19 has been worked some if it spins that much less rpm...<br />SS props in most cases are always cupped.<br />We really need a base-line figure of both props to determine whats going on...<br />test each prop at wide open throttle and give us the rpm of each if you can.

 

[Coolmaker]

Re: Question for prop experts - 1989 Merc 100HP

And posting the RPM on each prop would be a really good start.... Are you guessing, or do you have a tach????

 

[MartyTn]

Re: Question for prop experts - 1989 Merc 100HP

Thanks for the responses.<br /><br />I did some further checking and I believe we're gonna need to leave the 17" pitch prop out of this picture because after inspecting it I found marks on the hub that I believe indicate this prop has been re-worked. This prop was on the boat when I bought it several years ago and I was told by the former owner that it was 17" pitch (13" diameter). The boat is equipped with speedometer, tach and tilt/trim, and at WOT 5500 RPM and best trim, the speedometer indicated only about 35-37 MPH with this prop. <br /><br />The (13" diameter)19" pitch prop, however, was new, and with this prop, at WOT, 5500 RPM, and best trim, the speedometer reads 43-44 mph. This prop has been a great fuel-efficient cruiser and skiier but now with kids getting to be adult size and wanting to slalom ski, this prop just won't quite handle it anymore.<br /><br />So, the goal is to hopefully get a little more pull-up power while still keeping some cruise/top-end ability to make things as fuel efficient as possible.<br /><br />I'm not sure how (or if) SS vs alum affects this situation and that's one of the reasons for this query. I've been checking prop ads for a year or so and that 13"x18" SS Merc prop seems like possibly a good bet, but that's a lot of $$ if it's not the right choice. <br /><br />All advice really appreciated. Many Thanks.