Prop Question..........

[Theoutdoorsman]

I'm currently turning a 13 1/4 X 17 pitch alluminum prop with my '71 Mercury Inline Six Cylinder. The boat will run about 40 mph with a full load and I'm getting a really good hole shot (about 1 boats length to plane). Although I only run the motor at 5,000 RPM's for better fuel ecconomy, I suspect my maximum RPM range is a little high at 5600-5700. And, I can't go any bigger in pitch size. I am considering purchasing a steel wheel but I'm not sure if the advantages I might get will warrant the cost of the switch. Any thoughts as to what I could expect by making the change to steel? I suspect my RPM's will drop a bit, but I can probably stand to loose 200 or so anyway. Bare in mind, I'm currenty getting a good hole shot as well as acceptable top-end speed. In your honest opinion, will the switch be worth the cost? And what should I expect to gain? Thanks........Alan

 

[Dhadley]

Re: Prop Question..........

You may be a little shy on top rpm but its difficult to say since you are guessing. I would suspect that you could go with something like a 17" Stiletto and raise the motor (increase the X dimension) some. Thats one advantage of a ss prop over aluminum.<br /><br />Good luck!

 

[Spidybot]

Re: Prop Question..........

You say 'can't go any bigger in pitch'. Why is that?<br /><br />The rule of thumb regarding props is to determine your current rpm at WOT and work from that with whatever changes you are looking for, in mind.<br /><br />You should pay attention to the recommended max. rpm as this is important to your engine health.<br /><br />The benefits from SS tend to be minor if you're not aiming for a solution to a specific problem.<br />They do perform better overall too, but you might feel it's not really worth the price.

 

[Theoutdoorsman]

Re: Prop Question..........

I can't turn a 19 pitch prop. It's too sluggish. I can barely get on plane that's that's about as far as I can get. The figures I quoted in my original post on the RPM's aren't guesses. They are actual readings taken from the tac itself. Are you telling me that the RPM's I'm currently turning are shy of the max range? I know this sounds dumb, but what does "WOT" mean? Thanks.......Alan

 

[hammer250v8]

Re: Prop Question..........

WOT....Wide Open Throttle.....james

 

[muskyone]

Re: Prop Question..........

wot=wide open throttle

 

[Spidybot]

Re: Prop Question..........

OK. We aim for the max. rec. rpm at full throttle (WOT). Depending on your HP, your range could be stated as 5000-5500. The prop size that results in the upper end of the range gives you a better hole shot and a little less top end. If you want even more shot you can use a prop that will allow the engine to overrev at WOT and you will be responsible for keeping the rpm within the range by backing off on the lever.<br /><br />An inch up in pitch will reduce the rpm by 200 (as a rule). It's like gears in a car: Higher gear equals less accelleration but improves top end and cruising mpg (within reasonable limits).<br /><br />A SS-prop with a more sophisticated design would be right if you look for something extra, for example: <br />For a higher mount engine to acchieve less drag from the lower unit and more speed you'd need a prop capable of surfacing<br />To prevent ventilation/cavitation when cornering or trimming you'd need a prop with better grip.<br />To provide a more smooth pull you'd need a 4- or 5-blade prop.<br /><br />And so on.<br /><br />If you have other considerations like often hitting banks or rocks a SS will be stronger (and transmit more stress to your lower unit/gears etc.) A composite prop with exchangeable blades should be considered.<br /><br />On the more advanced side you could go for a shifting prop that will be ie. a 15" when aceelerating and a 21" once up and planing. Not sure if it's available for your engine but VERY shure it costs $$$$$s.<br /><br />A book about props and more is available for free download (8 mb) via this previous post