Propping out Yamaha F115

[Tmj2121]

Hi I'm looking for some help on finding the right prop for my boat/motor.
2003 Alumacraft Tournament Sport 175
2003 Yamaha F115
Current props I've ran are - stock aluminum 13x19p 3 blade yamaha
stainless steel 13.25x17p 3 blade cabelas
the aluminum 19p runs better than the stainless one and I'm at 34.5mph GPS @ 5800 rpm, I'm looking for more speed but I have some weird stuff happening with my performance.
Read carefully: I started with my motor mounted at the LOWEST setting resting on the transom. Anti ventilation plate dead even with the bottom of the hull. Had very poor handling with excessive blowout and a lot of trim to plane out good, which would result in more blowout, no tach at the time but was at 37mph with the aluminum 19p
I RAISED my motor, now 3/4" above transom and AV plate 3/4" above the hull bottom. Great handling, no blowout up to 5000rpm and 30mph in turns and short trim angle to feel planed out, minimal ventilation. Top speed DECREASED to 34.5mph and with new tach installed I'm at 5800rpm. I cruise at 5000rpm and 30mph

Can someone help me? I feel like I should be able to hit 40 mph based on other very similar boat/ motor combos.....thanks

 

[WesNewell]

Assuming you have the 20" transom model, I'd say you still have the engine mounted too low. And if you have a jack plate on it, it's probably way too low. Best way to tell is to keep av plate in line with hull bottom. Open it up wot and view position of av plate in water. It should be just above and parallel with the water line with no water going over the top of the plate.If that's not possible, the an photo from the rear of the boat with the av plate in line with hull bottom will help decide what you need to do.

 

[porscheguy]

What happens when you put on the stainless prop with the engine raised one hole? That prop may run better with the motor higher.

the reason you lost speed is because aluminum props just don’t grip as well, and the higher they are, the worse they get. See what the stainless does.

 

[Texasmark]

What happens when you put on the stainless prop with the engine raised one hole? That prop may run better with the motor higher.

the reason you lost speed is because aluminum props just don’t grip as well, and the higher they are, the worse they get. See what the stainless does.

Some reasons: SS has ports maybe, for better hole shot without sacrificing WOT speed, high rake helps in gripping and especially desired in high mounted engines, cupped blade tips that aid in preventing blowout and better gripping along with the rake, thin blades that cut the water easier increasing speed with everything else being equal.

I'd say comparing one to the other is like comparing apples to oranges.

The higher you can put your lower unit without blowing out the better your WOT performance.
The less boat you have in the water at WOT the better the performance.
The better the prop grips the better the performance especially in rough water at speed and in tight high speed turns.
The higher you can trim your engine out without blowing out the better your WOT performance.

The 115 on that hull, lightly loaded should put you in at around 45 or so if setup right....which includes prop selection.

How about some pictures.
What is the max recommended RPM of that engine...5500?
Know the gear ratio...2:1?

 

[QBhoy]

Hi. Some good advice above there. Generally an ally normal prop isn’t much good at doing much out of the ordinary.
Even after you have raised the engine. I think you are still too low. A quick boat should ideally be set up with the plate 1”-3” above the line of the hull generally. The lower end of “quick” like yours perhaps would be good at 1” above.
Regardless. You really need a good stainless for this. It’s half your battle for sure.
Without seeming like a smart arse...it won’t be blow out you are experiencing. That’s a whole different and much scarier thing altogether. Normally reserved for boats running over 70/80 mph.
I really think you need a good prop capable of creating lift which will in turn allow more trim without loosing grip. And almost certainly at least a 19” going by your numbers.

 

[Texasmark]

"I really think you need a good prop capable of creating lift which will in turn allow more trim without loosing grip."

Well there't your "Rake" designed for bow lift, getting the hull up and out of the water to reduce drag and increase speed that comes with most SS props with any degree of pitch....pitched for fast boats, not barges.

 

[Faztbullet]

What brand SS prop did you run/try? Sounds like it needs a lil tweaking.........