Motor Height and Prop Size

[danvillemarine]

I have a 16' Hydrasport with a 125 Mercury. The boat was switched over from Evinrude. The problem is it turns 5000 rpm at 30 mph. Am I correct that the cavitation plate should be at the bottom of the hull. It has a factory 19 pitch prop. Is it a matter of time testing props or is there a standard. Also what speed or target RPM should I be looking for. Thanks..

 

[rickdb1boat]

Re: Motor Height and Prop Size

AAA,<br /><br />Your plate should be even with the bottom of the hull for starters. You may want to try raising the motor up in 1/2 inch increments and see how it does before you change props. It's a matter of trial and error with the height and prop. You should be looking at least 5600-5800 RPM's wide open. A drop to a 17" prop will get you another 400 or so RPM's on the top end....15" will get you right where you need to be. But it would be best to know how you are set up before you make any prop changes right now...

 

[Texasmark]

Re: Motor Height and Prop Size

Something is terribly wrong. <br /><br />The Hydrasport is a high performance hull and as I recall padded. I remember fishing back in the '70's when I first saw them. One (Blue with Black lightening bolts molded into the side of the hull) had a 150 tower on it and a boat wake was coming at him on otherwise smooth water.<br /><br />He let the hammer down and that boat, and half the skeg shot out of the water as he crossed it.....I could see clear under the boat and could see the black skeg from 45 degrees off his starboard bow......and the prop just a shining and a chewing up the water.<br /><br />Your boat should do 50 mph or so with your tower. With my experience in that general area and knowing the boat and engine, if your throttle is maxed out at 5000 and you are only doing 30 with a 19" prop you have something wrong with the engine or the boat has the anchor dragging. Grin<br /><br />Granted, with a high performance SS prop (which is what you should have on that boat to realize how sweet it is), you could stand some jacking which would improve your performance even more than I show later. If you are just running a run of the mill Merc alum then you will probably do yourself more harm than good...cavitation, over revving.<br /><br />But let's get back to the numbers indicating you need to fix somethng in the engine first.<br /><br />The BAM prop slip calculator says that with your 2:1 gear box, which I recall they had....if it's less (like 1.87:1 you're worse off), says that with a 19" prop at 5000 rpm you should be doing a theoretical 45 with no slip. Assuming a reasonable 10% slip for that boat brings you down to 41. <br /><br />Long way from 30 baby!!!!!!!and it's not a linear equation (power vs speed.....takes a lot more power to run the higher speeds).<br /><br />That boat/engine combo ought to run a 21P easily at 5800 and get you 52 miles/hour with an estimated 10% slip.<br /><br />There are the numbers and no doubt in my mind that you can do it.....just need to get the dyno 125 hp out of that engine and the rest is academic.<br /><br />Mark