Torque "vs" Horse power

[MOP]

I like I am sure many have questions relating to HP "vs" Torque. I got caught up into T-V thing while propping the Beast, I ended up propping it down by 500 RPM and gained near 3 miles an hour. That got me intersted in trying to find some real knowledgeable answers which I think I finally have found.

The link below from Speed Wake has some of the best info I have found, well worth the full read.

http://www.speedwake.com/upload/showthread/t-24857.html

[ALLAN BROWN]

I'm not sure how it relates here, but when we developed new models, (49' Cary, 53' Magnum, 45' Magnum), the FIRST thing we did was to choose a propellor for the boat. These were inboard boats, which required an inclined shaft, a strut which controlled the diameter, etc. I read that whole Speedwake topic, and the one guy who made a lot of sense was 'cfm'. Unless you run WFO all the time, the rest of it is BS. If you want to outrun your friend, put on the prop that goes the fastest............

[MOP]

Allan it was a good post and I sort of agree with "cfm" on WOF, but where that lies prop wise is the question. I was curious if it better to prop for peak HP or torque thats what got me poking around. When I put the BravoX on the only prop I had was a 21 Mirage. After a few hours of break in I wound it up it turned up 62@5600, I tried a Merc 23 cleaver next it made the boat handle like crap but did 65.3@5250. Next I tried a 23 Quick Silver it was a sweet running prop and turned 65@5200, judging the results of the three props I figured that was the best it would do and I was pleased. All the talk about Solas got me thinking, I dropped a note to Big Griz he said the Solas had too much grab and to try 23 Turbo 4 blade as they seemed to be the best ptop on 22's. I Found a Turbo Ultima I expected it to drop the at least 200 rpm but only lost 100 and turned 65.5@5100. It had a very good hole shot and super sweet handling. I gained the most at 3500 my cruise it went from 43 to 45@3500, that was great for my usage. Griz felt 4800 top is a good target for the way my 383 was setup so now I want to try a 24 and possibly a 25 to see if it helps even more. GCarter got about the same results going up to a 25 gained a lot mid and top but only turned 4500. Anyway just my opinion and obsevation is spinning them high does not always give the best results.

Phil

[GEOO]

You want to prop the boat near max hp. If you prop the boat at peak torque you'll be over working and heating the engine and won't go as fast. Peak torque is where the engine has the best VE and should be close to your cruise rpm.

[DonCig]

Phil & George, I think that this is a great subject for discussion and I would love to see some of our engineer members pipe in. I would think that there is a point in the power curves where torque and HP balance each other out where no further speed improvement can occur due to the drag presented by the boat.
What this thought brings up is that the ultimate engine for a boat would be custom designed with a fixed ratio or relationship between the two power curves and the drag of the object in question.
In a heavy boat torque could have a higher value, in a lightweight boat, horsepower might be able to have a higher ratio.
If you look at the attached power chart for my new engine, it may make sense to give up some torque to gain some horsepower, since an 18' Donzi probably does not need 500 ft./lbs of torque to go 80 mph, it may be possible to go 90 mph by increasing the HP and decreasing the torque.
George is correct as it relates to heat produced at peak torque. I would think that you always want to prop above peak torque, but I can see how it is possible for a boat to reach its maxuimum speed below peak HP if the torque curve falls off to rapidly.
What are your guys thoughts?
Don

[MOP]

I am the first to admit I know little of how this stuff related, Don looking at your chart and the one Rootsy did for me they both look as if mid 5's is where they should be propped. But I am finding that propping mine down is giving better speed mid and top with my mild 383, Griz gave me a target RPM of 4800 for my setup I am still 300 over and will try for higher pitched try outs come spring.

Phil

[MOP]

You want to prop the boat near max hp. If you prop the boat at peak torque you'll be over working and heating the engine and won't go as fast. Peak torque is where the engine has the best VE and should be close to your cruise rpm.

George could you elaborate on the heat issue, looking at both Don's & my charts we run at peak torque from about 3000 up to just under 5000, I cruise at 3500 which is wher my torque is at its highest peak.

Phil

[DonCig]

Not to complicate things.

[Rootsy]

first off when you run your motor at peak torque (or where your VE is greatest) you are laboring the motor if it is under full load... just think about you wanting to run but you have a big rock tied around your waist... your legs can move faster but that big load is holding you back and you begin to sweat and tire pretty quickly...

there is a balance in propping a boat... with about 5 or 6 variables... or more... you could turn this into a big ole discussion if you wanted to get really intimate... i've pondered this for too long over the past years...

[turbo2256]

Our shop runs a 427 SBC. We run 3.70 gears, 31" slicks and shift around 6 grand. Pulls around 140ish in the quarter. Hows that for low end torq?

[RickSE]

To me "laboring the motor" has always meant that you were continuously running under load, below peak torque, with the throttle wide or significantly open. Peak torque is just the lower end of the sweet spot on a motor. The sweet spot begins at peak torque and runs all the way up to peak HP. I agree that propping to peak torque is a mistake. Take advantage of the sweet spot and prop to peak HP. Cruise close to peak torque at part throttle and save WOT for peak HP.

[blackhawk]

Way too many variables for a cut and dry answer, but I would never prop for peak torque. Lots of reasons but the biggest is you would lose too much acceleration! That being said, propping for peak hp isn't always the best either as the boat may run faster a few hundred rpm more or less. Every boat is different, every driver is different. Prop the boat for the combo of speed, cruise and handling that YOU like. Test, test, test! :D

[cigarette30]

One interesting point that may apply to many of the newer 22's with the 496 HO. I had a another boat (26' Sonic) where the boat was only slightly over propped and lost 6 mph, due to not allowing the motor to "turn" in the upper range of the factory specified recommendations. The boat was running 4600, and + - 6 mph under the expected top end. The factory and Merc. Racing said change the prop just enough to get the motor in the range it was designed, and the motor would pick up another 200-300 rpm. I was very skeptical to say the least. I simply had the 25" prop "cleaned up" (i.e.:edges sharpened, blades thinned, and a minor amount of cup reduced, and darn if they weren't right, 5100 and 5 mph GPS I was completely amazed! The Sonic factory and Merc. were not surprised at all.

Moral in this case, from the owners manuals, to factory reps, turn them right to top of the recommended range, in the case of the 496 HO 5050/5150.

[ALLAN BROWN]

I think it is time to bore you with this story again. In 1964, we built the first 16' Donzis. Standard power was the 4-banger 110 Volvo. Without testing the boat, we guaranteed 40mph with this set-up. We were slightly (hah!) dismayed when it ran 36mph. Jim Wynne and I got the job of making it go. Jim brought all of his racing props (Anal S.O.B. had them in Crown Royal bags) and all the prop manufacturers sent their 'best guess' props. If Jim had one outstanding character trait, it was discipline. I could take a set of props with similar characteristics, and after a couple of runs, pretty much predict the performance of the rest of the props. Not Jim. After a couple of days, we were up to 38 mph, and stuck. We had tried 30 some props! Sitting on the dock at sunset, drinking a beer, he said "We have tried every prop but one...".

On the dock was a 3 bladed, left hand (Jim Wynne, you know) aluminum round ear 15" x 15" prop that was so thick that it looked like a forging that one might machine a prop out of. It was fully 1/8" thick, and square all around the perimiter of the prop. "You've got to be kidding". Well, you can guess the next line. It ran 42 mph with both of us aboard! We never once looked at the torque, HP, BMEP, etc.

[MOP]

Allan you are never ever boring, I ead your posts with great interest! Thanks again.

Phil

[Lenny]

I LOVE your recollections AB. Never a dull moment. :) What about a "book".

[MOP]

Great idea Lenny, Allan needs a nice quiet place in the sun a few:beer: and a:smileybo: to take care of the literary end of it.

MOP

[turbo2256]

Book book book book

[DonziDave]

I gotta' go with Brownie...Sometimes you have to sit at the feet of the master and listen.....