2016 FX21 & Confused

[JL]

So I have a 2016 FX21, 250 SHO and a 25T-1 that is blue printed and balanced by Lainier Propeller. Since I bought the boat I have been running right around 9 dots on the jackplate, the boat would run around 69mph-70mph at 5800 rpm's max. Last year I started playing with engine height. So I started by raising the jack plate higher figuring I need higher rpm close 6K, got to 10 dots and only gained 50 Rpm's, holeshot suffered and gained no top end. So I started to lowering the jackplate back down, all said and done I am at 7.5 dots, 5700 rpm's and my speed is 72mph.

Load/weight was the same.

Here's what I don't understand why lowering the jack plate and lowering rpm's made my speed increase? It doesn't make sense, does it? Also I think I need to be around 6000 rpm's which I would think should gain me more speed.

[utinator]

It does make sense if your prop was slipping at 10 dots. At the lower setting, your prop "grips" better. That will increase your speed and lower your rpms in that situation. Maximizing top end is all about getting the right balance of slip and grip.

You won't gain speed just because you increase rpms. It would depend on how you increase rpms. (Ex: reducing weight will increase rpms and speed. reducing drag by raising the engine will increase rpms and speed up to a certain point. lowering the prop pitch will increase rpms, but might not increase speed.)

Maybe try 8-8.5 dots...?

[Neilslure]

Wouldnt lowering the motor lift the hull out of the water more creating less drag?

[utinator]

Yes, it's possible, but only up to a certain point. When adjusting the prop shaft height, there is normally a small window where the boat runs the most efficiently. I call this small window, the "Sweet Spot". It is normally about .5"-.75" of vertical adjustment. If you set your motor higher than the sweet spot, you will gain rpms, but lose mph. If you set your motor below the sweet spot, you will lose rpms and mph. If you stay inside the sweet spot, you can potentially gain lift and reduce drag. This is where a hydro plate comes in handy.

[JL]

Thanks for the feedback! Yes I get what you are saying and I agree must be getting out the 'sweet spot' as I raise the motor. 8-8.5 dots saw no real improvement, just seemed odd as continued to lower engine height I continue to gain speed with a decrease in rpm. My other skeeters where not like this, for maximum speed, I always needed to be around 9-10 dots which equated to about 3"-2.75" below pad and at 6000 rpm's with the same prop (FX20 and ZX225). I haven't measured the actual prop to pad but it bet I'm closer to 4" below pad currently.

[bgibson]

If you carry a heavy load in the front of the boat you may need to lower the engine to gain leverage on the hull to lift the nose creating less drag.