jdnca1

I have a 26 Cat with a natural 632 that makes 900HP @6600 and 820# tq at 5000.

I tried something last summer that might be useful to you guys.. before I changed everything by going to an Imco Xbox, etc.

Very hot sticky days...similar water conditions

I turned a 30 bravo (non lab) 6000rpm @100mph

I turned a 28 bravo (labbed) 6500 @98 mph

@6000 rpm this motor makes 860HP. I was convinced that if I propped the boat to turn the rpm for the addnl HP available I would see an increase in speed....NOT SO.

Now acceleration with the 28 was night and day over the 30, but the 30 was faster.

Addnl test with 30 only, a 175HP shot of NOS yielded 6700 rpm @109 mph. When I got off the NOS I could maintain 6300 @104.

My theory is the TQ the NOS provides gets the Hull through the point of max TQ before (NA) ...but once through (the wall) the HP of the motor can maintain a higher (implied) TQ value. Of course I am no engineer.

Cincinnati Race

Joey is 100% correct.

The boat you run is so efficient that a little TQ feels like a lot. Best thing to do is give your Prop shop all the info you have, and make sure that if you are not happy with them, that he will tweek them at no cost. Most offer this and stand behind their props 100%. If not, choose another prop shop.

TLee (Info via Reindl Powerboats)

cigarette1

Bronson at Hill Propeller will tweak until you are happy. I've used them.

G

Cincinnati Race

I have heard the same great things about them.

TLee

Tom

Bronson Propeller
Hill Propeller
Chad Hill Propeller
Bronson @Hill Propeller

Gaphrig
Gaphrig by Livorsi

Its all so confusing.

I think I got the right guy - Bronson @ Hill Propeller, but could somebody please give me the phone number for the right guy? I am hoping to send my props in after christmas.

Is it this place http://www.boatpropellers.com/index.html

Thanks,
Tom

cigarette1

That be da place

Tom

I really appreciate the advice you guys have been giving me and it has been an interesting lesson to boot. Thank you so much.

jdnca1 I don't really understand what you are pointing out, but it seems to go along with what the builder said. It is very interesting theory you have there about the torque getting it past a sticking point.

Here are the notes I took during my conversation with the engine builder. Maybe somebody can help make sense of them.

Roy at HTM says that 6000 is where the torque falls off. If i am hitting that i cannot go any faster unless I make the props higher pitch. As long as the moter turns at least 5500 I am ok. If I can't get to 5500 then the pitch is too much. If I get to 5800 rpm then I found the right pitch. 5800 is where the torque lets off.

carb - holly dominator 1050 CFM
brodix heads
brodix intake manifold
Gen 6 block
Callies Crankshaft
Eagle rods
holly distributor
EMI manifolds

Timing up to 34º 31-33 normal

C_Spray

iTrader: (2)

Here's my $.02 - just don't ask me to ride in that bad boy. (I might be willing to drive it, though....)

Before getting someone to hammer your E4's into something that they may not want to be, try to beg/borrow/steal a set of E3's. They will have about 1 1/2" less pitch, and will give you an idea as to whether or not you should try going higher in rpm. The added benefit of this approach will be a lower torque load on the drive. You're definitely pushing it at the numbers you're running, but at least you're boat's not too heavy, which helps.

Be very careful about any "trick tweaking" of props on DuoProp drives. the lower gear set is designed to split the load between the two props, and if you don't keep the prop loading balanced, you could have some problems. The idea with that drive is for only a very small outer diameter of the forward prop to do the work, so that the rear prop gets a cleaner flow of water. (If you measure carefully, you will find that there is just slighlty more pitch in this area of the front prop.) The rear prop is slightly smaller in diameter, has a more conventional shape, and carries about 1% more pitch than the fron prop to compensate. You can tweak the outer edge of the front prop, and cup the rear prop somewhat if you want pitch, but it is somewhat uncharted territory.

By the way, are you running a 1.59:1 ratio, or a 1.47? The 1.47 would definitely be a better deal for you in terms of both torque load and having to make customized props.

Still - try the E3's/MORE rpm first.....

Tom

Chuck,
Can you post a picture showing where to look on my DPX HP to find out the gear ratio? I've looked all over and can't tell. Any ideas where I might borrow an E3 pair?

thanks,
Tom

tomcat

Baja Daze and jdnca1 have hit the nail on the head. It's not just where the engine HP peak occurs that matters its where the maximum thrust is delivered to the water. So drive train losses and especially prop efficiency are important. Prop efficiency is defined as (thrust HP out)/(shaft HP in) X 100%.

Many people have tested with higher pitch and found more speed, even though their dyno sheet tells them that the engine doesn't make as much horsepower at the lower RPM. But drivetrain losses are lower and the higher prop may be operating closer to its maximum efficiency than the lower pitch prop was to its maximum efficiency.

If you don't have prop efficiency curves, about all you can do is test, test, test. But consider this, Mercury has a generic prop efficiency curve in their prop book, and it shows normal operating range for a prop extending from a low of 65% to a high of 80%. If that kind of variation can be found by changing props then you can pretty well forget about matching the engine's peak HP as your only goal.