88 SPL WOT - Prop?

[trenth]

I've got a 17 foot Aquasport with a 1995 88 SPL that I am only able to get up to 5200 RPMs at WOT. RPMs verified on two tachs (TinyTach and dash tach). I'm pretty confident that the tachs are accurate. 5200 RPM puts my speed at 38mph (GPS). I'm running a 3 blade 15 3/4 x 15 prop. I've read that I should be up around 5800 or so so I've got some work to do but I can't figure out if it's engine set up or prop size.

About the engine setup:
130psi x 4 compression
Idle is at 650 in gear
Complete carb rebuild (yes I have an OEM manual)
Carb butterflies are perfectly horizontal WOT
Cavitation plate is exactly 1 inch above bottom of hull


Seems like everything is pointing to prop pitch but I know this boat is "supposed" to pull at least 5500 WOT with this prop (per the boat's manufacture specs). Anyone got any ideas? Is there something with engine setup that I've missed?

Another question: I've read that lugging this particular motor is no bueno but where I live it is ALWAYS windy. When I say "Windy" I mean really windy....like 20-30kts. It's a rare day that I can actually run my boat at WOT. Is cruising my boat at 4500 doing harm? Do people really run WOT all the time?

 

[crb478]

Re: 88 SPL WOT - Prop?

Your set up sounds about right, but you can still try to go up a hole if you have one left just make sure you still get water to the top end. Each hull is different and reacts differently so each set up is specific to that boat. How much does your boat weigh? Any chance that the foam is waterlogged? If it is not and it is running right then the next step after set up would be a prop.

As for your question it is fine to run at less than WOT and will not hurt the motor as long as it has the ability to reach WOT. The lugging comes when the parts are always straining to turn instead of spinning up. Kind of like going up a steep hill in overdrive in a small displacement car.

 

[trenth]

Re: 88 SPL WOT - Prop?

According to manufacturer spec the boat weighs 1800lbs. I'm fairly vertain the hull/foam is dry. The hull is a 1995 and is immaculate.

 

[crb478]

Re: 88 SPL WOT - Prop?

Are you sure that it is a 15 3/4 prop and not a 13 3/4 ? I would try to raise it if at all possible. You could pick up a couple hundred rpm there. Have you verified spark at high rpm? If you have a inductive timing light you could verify that everything is firing as it should, but I suspect that it is if you are getting 5200 rpm. You could try a different prop in the same pitch as there are differences in each prop, I am assuming that yours is not damaged.

 

[tashasdaddy]

Re: 88 SPL WOT - Prop?

moving to props.

 

[hwsiii]

Re: 88 SPL WOT - Prop?

Trenth, I believe you meant that the prop was a 13 3/4" x 15" prop. but if you are using a prop over 14.5" in diameter that is your problem. Your recommended WOT RPM is 4,500 RPM to 5,500 RPM and if you are at 5,200 RPM you are well within the RPM range.
I need your gear ratio, I show that is 2:1 and that is impossible with the speed and RPM you are talking about. And is the prop aluminum or stainless and what make and model of prop.


H

 

[trenth]

Re: 88 SPL WOT - Prop?

Yes, it is indeed 13 3/4. I mistyped. Good catch! According to my service manual the gear ratio is 13:26. The prop is aluminum although I'm not sure of the prop manufacturer. It does have what I would consider to be slight damage, although I don't know much about props so this "Slight" damage could have a more significant impact on performance than I imagined. I've attached a few pics. Could this damage cause the loss of a few hundred RPMs ?

Thanks

 

[hwsiii]

Re: 88 SPL WOT - Prop?

Trenth, it is very hard to tell by looking at prop pictures whether a prop has damage unless it is significant. But from what I see, I just see a few nicks and it looks fine.
Now to get back to the corret prop for your boat, my program shows a 15" pith prop is what you should have on it and it appears to be turning in the top 6% of recommended RPM.

But for you to be getting 38MPH out of a true 15" prop with a 2:1 gear ratio at 5,200 RPM is impossible. That is showing a negative slip of -3% and that is not possible.
There are 3 things that could be happening to show those numbers:
1. The engine is turning more than 5,200 RPM
2. The prop has a theoretical pitch of about 17" and thus the prop slip
is actually +9%
3. You forgot the speed it was going when you did the tests

To get the most speed out of your boat I would raise the motor up at least one hole and see what the RPM are at that time and see if the prop is ventilating too bad. You don't want to move it so high that it ventilates easily in turns, because when you get in rough water you will have to be giving it gas and letting off as the waves harm your speed by ventilation, by slowing it down or speeding it up depending on which way they are going. If that works move it up another hole and test it. That is my recommendation.


Prop Slip

H