50 hp prop size

[fishmonger.33]

i have a 50 hp mid 70s merc and i was wondering what size prop it would be safe to go with and what will give me the best performance? i believe it tacs out at about 5000 rpm

 

[Silvertip]

Re: 50 hp prop size

Without knowing what prop you have now, the type of boat, the speed at WOT, and only an "about 5000 rpm", we have no idea what to recommend. The very first post on this forum tells you what info is needed. What performance characteristics are you looking for? Better hole shot, better cruise, better top speed? There is no one-size fits all. You cannot have all three maximums with one prop -- only a comprise of all three.

 

[fishingdan]

Re: 50 hp prop size

What kind of boat? How long is it? What does it weigh on your typical outings (boat, engine, fuel, people, and any stuff you have in the boat)? What size prop today (look at numbers on the hub)? <br /><br />If you are hitting 5000 rpm at WOT today, you are pretty close. Just a little less pitch should get you into the right rpm range.

 

[fishmonger.33]

Re: 50 hp prop size

i have a very light weight 14 ft aluminium runabout rated for max of 60 hp and i hve a 15 pitch stainless steal prop on now and i get 35 miles. i would like good high speed and a good plain, possibly 40 miles and maybe a whale tail to help the plain, i have only myself in the boat 125 lbs. oh and what does WOT stand for? how cuold i get an acurate reading on the rpms without a gauge? sorry about all the questions but i am a beginner. thanks. fishmonger.33

 

[Silvertip]

Re: 50 hp prop size

If you don't have a tachometer, you have no idea (nor do we) as to what the engine is really turning now. WOT stands for wide open throttle. Here's the deal: You need to establish a benchmark when trying to figure out what prop to use. To do that you need to be sure the engine is running as it should. Then you need a tach and an accurate speedometer (GPS prefereably), and then run the boat at WOT and take speed and maximum RPM readings. If the engine is at maximum RPM (5500) increasing pitch will slow the engine down. Decreasing pitch will cause it to over rev. So you see you may already be at optimum performance for that combination. If the engine does not reach 5500, you need to decrease pitch. If the engine is over 5500 you need to increase pitch. Before you dash out and spend money on a prop, try raising the engine (mounting - not trim). The AV plate should be flush to about and inch or two above the bottom of the boat. When you get that setup properly, then take the speed/rpm readings and go from there.

 

[fishmonger.33]

Re: 50 hp prop size

thanks you have cleared things up a bit. thanks. fishmonger.33