Posts: 3507
Location: Elk River, Minnesota | Hiya,
I would say if you have gear and 2 people, you are sitting quite well with the numbers as stated. If you remove the 2nd person and their gear, I would bet you are sitting right at 6000 rpms if not a touch over that, which means you are quite close to optimum set up. Max rpm when you are lightly loaded is ideal. That way, when you do get that buddy that comes along with all the gear, you don't have to change props.
Now... depending on the prop and your motor mounting height, you can play a little as well. In most cases, motors are mounted in the lowest bolt hole or one bolt hole up to ensure that the prop (usually aluminum when sold) stays hooked up and gives decent overall performance, but it is usually not the most efficient for the rig. This is where steel props and motor mounting heights come in.
In your case, I'd be curious to know how well the bow trims with your current prop. Can you get the bow up and continue higher and create a porpoise, or does the prop blow out at some point?
The 681 series of boats have a forward mounted gas tank, which makes trimming the bow up a tougher task. That is why the previous poster would have trim issues as the raker series of props are bow lifting props, and perform best when the motor is running within a hundred or so rpms of max on a boat that does not have a huge amount of weight up front. So.. lower rpms overall, and the prop struggles to hold the bow up, thus the rise and fall of the bow. It also shows an affinity for a 4 blade prop to get the prop to hold while at trim, but is not a speed demon by any means. This is a very common characteristic of the older Ranger boats with a bow mounted gas tank. Ask Sled about his rig (He's got a 690 series) and loves his 4 blade!!
So...what possibilities do you have? After determining the exact prop and motor location, The first thing you can try is to move your motor up two more holes and test. What this will do is reduce the amount the bow rises on hole shot, but once on plane and trimmed to your best position, you should see an increase in RPM as you have less motor in the water and the potential that the entire boat is lifted further out of the water. The second thing you can look into are either having vent holes drilled in the prop hub, or go with a prop that is already vented. A vented prop will allow the motor to reach a higher rpm before the prop grabs, popping you up on plane quickly when it does...this comes with a price, though, and it will usually show up if you have to navigate in waters where your rpms are right at that point where exhaust is just about to blow by the holes rather than vent out through them. The boat will tend to surge and lose grip, regain it etc., which gets to be a real pain.
In a nutshell, I feel your speed gains will be minimal as you are close to where most rigs of that weight and size are with a 115 on them. Hole shot can be increased with a vented prop (I don't believe the sst is as it is just a steel prop made from the same basic casting as a stock aluminum). If you can move your motor up, you could potentially go up in pitch, although you will suffer hole shot, but a good vented prop might let you get away with it.
Props to consider: Michigan Rapture comes to mind first, and I wouldn't hesitate to try a raker 18 on it as well, given you can raise the motor, and potentially move some weight more toward the rear of the boat. My gut instinct says 4 blade, though which will really help keep that bow up... I don't recall right off hand, but I believe a mercury trophy plus would be an excellent option for you, given they have an exchangeable hub system in them so you can get the 13 spline hub you need. A 17 pitch, vented correctly would allow you to run your rig at, or just below the highest bolt hole position on the boat and still get good hook-up and bow lifting abilities.
Steve
Edited by VMS 2/10/2014 9:07 PM
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