Prop selection: 25HP Merc on 17.5' alum boat

[matt61076]

I have a 17.5 foot tracker aluminum boat with a seriously undersized 25HP 2-stroke engine (my local fishing hole has a HP limit). The boat & motor are 1999 models. I currently get about 15mph WOT. I put a hydroplane on the engine and that seemed to really help get the bow down.

My tach is broken so I'm not sure what RPM I'm operating at. Anyone know the recommended operating RPM for this engine?

I'm wondering if changing the prop to a different pitch would help with performance? Or any other advice you might have to get more out of this limited HP setup.

 

[ACracing]

Re: Prop selection: 25HP Merc on 17.5' alum boat

I have a 17.5 foot tracker aluminum boat with a seriously undersized 25HP 2-stroke engine (my local fishing hole has a HP limit). The boat & motor are 1999 models. I currently get about 15mph WOT. I put a hydroplane on the engine and that seemed to really help get the bow down.

My tach is broken so I'm not sure what RPM I'm operating at. Anyone know the recommended operating RPM for this engine?

I'm wondering if changing the prop to a different pitch would help with performance? Or any other advice you might have to get more out of this limited HP setup.

Before changing props, I would recommend checking the current RPM's. You are possibly operating at min/max now? Changing prop (pitch, diameter) may help with speed, however, I think the main problem is the hp restriction at your local fishing spot. A 25hp motor on a 17.5 foot boat is the other obvious issue. Hope you can get it worked out! Good luck! Sorry that i could not give definite answers.

 

[steelespike]

Re: Prop selection: 25HP Merc on 17.5' alum boat

Welcome.First read the stickies at the top of the page also the helps at the prop store page.I believe a 25 is rated at about 32 on a light reasonably fast boat.
Depending on your load your boat could do about 20 to 25 lightly loaded and carefully setup.You need to get rid of the foil for now.
Motor height is a way to get more speed at no cost.The antivent(cavitation)plate just above the prop, even with the bottom is considered a starting point. Raise it to a point just before venting(cavitation) becomes a problem.
(be sure it pumps water).
Under ideal conditions you could gain 1 or 2 mph.Get us the prop size and your present max rpm and speed.If your motor is healthy and the load not too heavy a prop change may improve things.
Obviously you may need to trim as much weight as possible.
Your motor will use about 2.5 gph(gallons per hour) at wot(wide open throttle. If a 3 gal. plastic tank will work for you that will save some weight
cans instead of bottles A light danforth type anchor instead of a bucket of concrete.plastic dingy paddles instead of wood,move stuff at the stern forward.Don't haul extra weight you don't need,just the stuff you need for that trip.Carefully adjust motor trim if no power trim you may have to sacrifice
a little hole shot for top end.While your waiting for your test results,read up on the Turning point Rascal props.Stainless performance at Aluminum prices.