Logo
FULL SITE MENU

Canbt get DT8516b CC into the powerband

B

bowhuntrrl

Contributing Member
"I bought a 16 SportCraft CC a

"I bought a 16 SportCraft CC a few months ago and have been ironing out the bugs. Now that I have it running well, on to the next problem. The boat came with a 19P prop and sounded like the engine was really lugging. It would only do about 25-28 mph. I installed a tach and bought a 13 1/4x17 pitch prop and took it out today. I'm just barely getting 5000 rpm so the engine is still not getting into the power band which is 5500-6000 @ WOT, at least that is what I was told. I'm really having a hard time believing that I need to drop down to a 15 pitch prop !!!! The motor starts great, idles fine, and the compression on all the cylinders are between 125-132. The carbs have been rebuilt, the fuel pump diaphragms have been replaced, and the plugs are new. The fuel supply is from an external tank and is 89 octane.

Anyone have any thoughts ?????"
 
"Here's some numbers for y

"Here's some numbers for you. The boat has a dry weight of about 1100 pounds, motor has a dry weight of about 270 pounds, add gas, battery, standard safety gear and any other "rigging" and that would put displacement at what, maybe 1600-1700 pounds.

Now throw in your normal passenger load, say 400 pounds for two guys with their junk for the outing and you are pushing a ton (literally) through the water.

So here's what the prop calculator says about your set-up:

a 12D 15P is the correct prop for that set-up and even less than 15 pitch would be called for if your added weight, on a regular basis, exceeded the 400 I used just to come up with some numbers, and if you were only cracking 5K with a 17 pitch, you are probably a touch heavier than I input...

The 12D 15P should allow the motor to run at 5600 rpms and get you a top speed, on a calm, cool day, in the high 30's"
 
"Actually, I will correct one

"Actually, I will correct one comment I made above. I stated that you were probably a touch heavier than I input.

In fact, re-reading your post, the 13" diameter would account for the slight drop in rpms, and since a quick look indicates that props for this model are pretty much in the 13" diameter (give or a take), although the prop calc gives an "ideal recommendation", a 13D 15P would probably be the best you could use, with perhaps a top rpm of about 5500...."
 
"Thanks Graham, I've prett

"Thanks Graham, I've pretty much resigned myself to the fact that I will need the 13 3/4 x 15 pitch prop that is the next one down, in fact it looks like the lowest that I can go with a Michigan Wheel Vortex prop. It just hurts to buy a prop and use it for 10 minutes. I guess I can use it for a spare although I still have the original 19 pitch Suzuki prop. Someone on another forum had a Suzuki brochure for 86-87 motors and he said that the brochure states 5000-5600 for the WOT rpm range. I was told 5500-6000 on that same forum. Unfortunately, my Clymer manual doesn't have that spec at all. What does your information yield for that spec ????"
 
"Graham, one other question. D

"Graham, one other question. Doesn't the gear ratio of the lower unit have an effect on what pitch the prop will be ??? It seems to me that gear ratio has to be figured in, along with slippage."
 
"Yes, gear ratio is a factor,

"Yes, gear ratio is a factor, definately, and as such, my prop calculator requires the specs specific to your motor, which were input.

The factors that are considered for calculation of pitch are, hull type, length at the waterline, total displacement, engine type and configuration, propshaft horsepower (which is converted to brake horsepower by the software), rpm required to develop maximum horsepower, gear ratio and maximum prop diameter for the particular motor.

From all that, the software calculates and accounts for prop slip, determines a maximum speed, and pitch ratio which is then correlated to the optimum diameter and pitch of a standard, "uncupped", 3 blade propeller.

That's the math and science of the "recommendation". Obviously, if the motor has been altered from factory spec with regards to gear ratio or horsepower, or is not perfectly tuned, inappropriately mounted and if the general weather conditions are not exactly in line with the ISO standard definition and the motor is not operated at sea level, then the calculation can be out by as much as 14%.

It is for that reason that still the best way to prop a boat is in the spring on a day with low humidity and a tempurature of about 70 degrees, carrying a "usual" load on board and confirmed by a tach.

And then also obtaining a second prop, usually 2" less pitch, which should be swapped "in" when either, you carry a heavier load, or as the summer progresses and the motor labours to breathe under the heat and humidity.

The lower pitch at that point would allow your motor to get back to it's optimal rpm range...

I'm not trying to be a smart *&^() here with a whole lump of information, however, this has been an excellent opportunity to kinda spew all this information out. Hopefully others with similar questions or concerns will read this post.

Prop'ing a boat is perhaps one of the least understood area's with regards to boating in general and ultimately can have the most profound effect on performance and handling of the boat and the useful life of the engine. Far too many select their prop because "this guy" at the marina has a 16 footer and he's running a 21 pitch, or nearly as bad, simply want to know what the "OEM" prop was.

An OEM prop is middle of the road at best. If your boat is rated for 200 horses, you are only running 100, you will need a significantly lower pitch than the guy running that 100 on hull only rated for that. In fact, at least in the case of the big Evinrudes, they don't come with an OEM prop anymore - it is selected to "fit your rig".

Running a lower pitch doesn't necessarily lead to "less speed".

If your motor can only turn a 19 pitch at 2100 rpms (driveshaft rpms divided by gear ratio) and slips like crazy, the boat is going to travel slower than it would with a 15 pitch at 2500 rpm that is easily "biting" the water..."
 
"Sorry, didn't answer your

"Sorry, didn't answer your first question, spec on the motor is 5000-5600 WOT range according to my info..."
 
"Thanks for the info. I unders

"Thanks for the info. I understand the principles of propping a boat, I just found it hard to believe that when the boat was rigged new that they were that far off in the setup. According to the previous owner (who bought the boat new), it has never gone over 25 mph. When I took it out after the work I did to "bring it back" after a 4 year sleep, I could tell that it wasn't revving up very well by the sound since I've been working on engines for 45 years or so. That's why I installed a tach to see where I really was in relation to the power band. It still takes a bit to get on plane with the 17P, but I'm sure the 15 is what it wants. It has been my experience that these SportCrafts are heavy hulls for their size and my latest, this 16' CC Fisherman is no exception. I think that the 13 3/4x15 P plus raising the engine bracket up 1 hole will optimize my setup. I'll check back with the results when I accomplish the changes. Thanks for the plethora of informative information."
 
"Here's an update: I got t

"Here's an update: I got the 13 3/4 x 15P prop. I had the chance this evening to take it to the lake. There was just a slight breeze putting some light ripples on the water, conditions were very nice. From the moment that I mashed the throttle down, I knew we were in business. I had forgotten to trim the motor down and the boat stood straight up !!!! I trimmed it and it popped right on to plane. I stopped it, trimmed it down and tried the hole shot again. It jumped right on to plane with no fuss at all. The top speed was still 34 mph, only this time @ 5500 rpm. I can't believe that dropping only 1 pitch raised the revs over 500 rpm, but it did. I tried to find a good cruising speed and came up with 26 mph @4200 rpm. That should be a good compromise speed to save gas. Overall, I'm pretty happy with this Michigan Wheel Vortex prop. The only upgrade that I might do in the future would be to a 15P stainless which might give a few more mph and a few hundred less rpm.

Our next experiment is to raise the engine up 1 hole since the cav plate is about 1" below the bottom of the hull. After that, I will install the Bob's machine torque plate to see if that changes anything."
 
Back
Top