sandcraft

Boat: 1993 32' Nordic OS straight deep v hull 8000# dry weight, had 100 gals of fuel during test.

Power: (2) 622ci chevy bowtie talldecks, na , approximately 700 hp 737 ftlbs tq@ 4986rpm ea.

Drives: b1 imcos with shorty lowers lwp 1.5:1

Current props: b1 4 blade 28p

Water conditions: 12-16" chop, freshwater lake

Results: 36 mph @ 3000 rpm
62 mph @ 4000 rpm
81 mph @ 4900 rpm All readings taken with GPS

I think the top end is very respectable for an older deep v and am satisfied with that. My cruise rpm is 3000 and I want to prop it to get as much cruise speed as I can without sacrificing too much of the awesome 40-80 mph punch this boat has. This was my first run with this boat after buying it and rebuilding the engines. I'm an experienced engine builder but a novice boater. Any help would be greatly appreciated.

throttleup

I would first verify your tachometers. Your slip seems to be too low. To get the speed you are seeing with your current set up I would expect you to turn about 5200-5400 rpm.

What is the goal of increasing your cruise speed? With the kind of torque your motors are putting out you will want to run as small a pitch prop as you can. That will reduce the load on the drives.

sandcraft

The motors seem to settle in and purr very nicely at 2900 rpm. since this is where it will spend most of its time, I would like it to move a little faster than 36 mph. If doing this will cause drive reliability issues, then Im up for suggestions.

bbladesprops

Cruising at 3000 and 36 mph is a little unusual for a boat and motor set up like yours. Most folks are looking to cruise a little faster. Otherwise they may be looking at different hull configurations for slower cruise speeds. anyway, thats up to you.

If your numbers are correct, the boat is running really well throughout the "last" 1/3 of throttle. The low end rpm numbers are as expected for your hull. The boat can't come alive till we get that big hull up and out of the water. If you want to get better cruise speeds in the 3000 rpm range, you will need to go to a 5 blade propeller. This will hurt the top speed since the 4 blade propellers are running so well there. Agreed, tons of low end torque and 5 blade hook up can hurt the drive in the long run. Or, just be real careful with your throttle input.

You need to determine the WOT rpm the engine wants to, or better yet, the WOT rpm you want them to run at. This will determine ultimate propeller selection.

Boat propping and road car gearing can be quite different.

sandcraft

WOT is holding at 4900-5000 rpm. I built the bottom end with an internally balanced rotating assembly to include Callies crank and Oliver steel billet rods. Even though it could easily take high rpm operation, I set the limiters at 6000 but dont want to exceed 5400 even on short bursts. The 62 mph at 4000 rpm feels great but the engines are considerably louder at that speed. I've been doing the testing with the engine hatch off, so after its reinstalled, A higher cruise rpm might be easier to bear. I am curious as to what the top speed could be with the weight and HP I have. The 40 to 80 punch is absolutely impressive and I would hate to mess that up.

throttleup

It's important to prop a boat motor at the proper WOT rpm. It doesn't matter where you want the motors to run at WOT. The motors need to run where they need to run. A good rule of thumb is to have the WOT around 1000-1400 rpm above the peak torque rpm.

Putting a prop on that is too high in pitch can cause all kinds of problems. A prop that's too big will cause excessive combustion chamber temperatures which can cause all kinds of engine problems. A prop that's too big can also cause a host of boat performance issues.

We work with high horsepower exotic powered boats everyday. If you have questions give us a call and we can better explain the in intricacies involved with getting a boat like yours in the proper prop.

throttleup

The 300XS is a nice motor.

It's very important to get the right prop on the boat so it runs in the manufacturers recommended rpm range at wide open throttle. This is especially true with outboard motors. In your case I would prop the boat at the higher end of the recommended range.

sandcraft

So I should prop my Nordic for 5900? And if so, which way do I go? Lower pitch? 3 blade? This is the part where I'm clueless.

bbladesprops

sandcraft,
Do you have dyno sheets for the engines. This will ultimately tell us where to prop the boat to make it live and give it the best performance. The answer to 5900rpm is, probably not. But, the HP curve will determine the answer. Propping to save the motor and picking a propeller model will self determine your cruise speeds.

marylandmark,
double check your operating range. Although I agree propping to the top is the best with most Merc OB's, I've stayed in touch with engineering on this one too and the HP is made in the lower ranges.

throttleup

You will want to reduce the pitch to raise the RPM. I would stay with the 4 blade props as they seem to run well on your boat.

Testing will ultimately give you the best set up for your boat. Is the max HP 700 or is it 700@4986? I would run with as small a pitch prop as you can. The smaller pitch props will ease the stress on your drives, which are being pushed to the limit with your power. If your motors make more horsepower at rpm above 5000 then you should pick up some top end speed with a smaller pitch prop.

We have props available for testing if you need them.