Prop advice.

[Pursuit2550]

I have a 91 Pursuit 2550 w/twin 93 Yamaha150's. I just took off the prop to grease them and I looked in the back and its a 13 3/4-19. There is no name, only thing is on the outside of the prop its stamped 19M. I am currentlly running 5200rpms@48mph. Should I just let it be since the engine max is 5500. I have only taken out the boat once since I just bought it but I think it performs pretty good. From idle, if you WOT it, it gets up and goes fast. At crusie speed if I remember correctlly it was 4-4200rpms@30-35. So should I just be happy or is there any gain to be had if I switch to diff. props (rpms, speed, fuel econ., etc.)

Thanks

 

[Pursuit2550]

Re: Prop advice.

Anyone have any advice.

 

[Silvertip]

Re: Prop advice.

Best performance is achieved when the engine can rev to the top of its recommended rpm band which is 5500 in your case. Before spending money on props, optimize the setup. In other words check the position of the anti ventilation plate in relation to the bottom of the boat. A good starting point for engine height is even to an inch above. You raise the engine on its mounting to raise it. Then retest. You may pick up a couple hundred rpm just by doing that. Since you've only had the boat out once, I suggest you do lots more testing to ensure you understand how to properly trim the boat for optimum performance. There is certainly nothing wrong with running the engines as they are currently set up. But I think there is more performance available without a prop change. If it comes down to needing a prop change, a 17P would pick up in the neighborhood of 300-400 RPM which would put you at the desired rpm. Prop design, aluminum or stainless, etc., are all things that affect the outcome of a prop change so work this issue carefully.

 

[MikDee]

Re: Prop advice.

Silvertip is right, but just to add, you have 2 drives in the water, a lot of drag, if you can move the engines up some so that at least 1/2 your cavitation plates are out of the water, considering the angle of the boat bottom on either side, I think you'll gain quite a bit of rpm, & speed. You might get away with just raising the motors up one hole in their mounting bracket, in fact that will probably do the trick!

 

[walleyehed]

Re: Prop advice.

The 19M is a Yamaha Brand prop...good design.
For every 5" of set back, you can raise the engine about 3/4" from the av-plate being centered level. 3/4" is 1 hole....test one hole at a time.

 

[Pursuit2550]

Re: Prop advice.

I checked the height and on the outside of the cavitation plate the hull is even with it on the inside of the plate its is 1" above the hull. I think if I go 1 hole up the hull will effect the water pick up. This is my 2nd boat and my last one was setup right so I have never had to deal with this. Whats my next step, take it out and see where the plate is in the water.

Thanks

 

[walleyehed]

Re: Prop advice.

you have room to go up one hole for sure....maybe 2. I would try one to start with. the water-line on the lower unit will be higher than the boat bottom due to the water seeking level after the hull displaces it.
With an engine 90 degs to the hull, can you take a measurement from the bottom of transom to the leading edge of the lower unit?

 

[Pursuit2550]

Re: Prop advice.

Thanks guys, I guess Ill be going up one hole and trying it out this weekend.

 

[Pursuit2550]

Re: Prop advice.

Well a month later and I moved up the motors one hole and took it out. My cavitation plate is good now. I took it out last friday and on my first post I was wrong about my numbers.
Before motors up= 45mph @ 5100
After(1 hole)motors up=46 @ 5200
So what now, is it worth spending the money on props. And if I go with a lower pitch will my fuel economy drop?

Thanks

 

[walleyehed]

Re: Prop advice.

If you're only in the second hole, go up one more....

 

[Pursuit2550]

Re: Prop advice.

Walleyehed, right now with the way it is, I can see the cavitation plate above the water. If I raise it one more hole wont that be to much. When I was testing it I could have gotten higher rpms but if I trimed to much I would gain rpms but lose speed. So I have to trim it to the perfect spot where I was getting the max rpms(5200) without losing mph. Also what about my other question about economy

 

[Dhadley]

Re: Prop advice.

OK, you followed Walleyehed's suggestion and picked up a free 100 rpm and 1 mph. That's from less drag. There's no way to look at the motors and tell it'll be too high one more hole up, testing is the only answer. You're actually doing pretty well for small diameter props. And you're getting away from the lugging situation but more rpm would be nice.

Are these 3 blade props? Is one gearcase a counter rotating?

After you raised the motors what did you set the toe-in at?

 

[Pursuit2550]

Re: Prop advice.

Yes they are 3 blade SS. One engine is counter rotation. The engines are perfectlly straight, no toe-in or toe-out.

 

[Dhadley]

Re: Prop advice.

Cool. The smaller diameter may stop you from going up but I'd try it as Walleyehed suggests. There are other solutions / things to try but you need to keep an open mind about engine height.

The 0 toe-in / out is fine on that boat. If anything you want to favor toe-in very, very slightly rather than any toe-out.