HP vs Torque
 

OK, so now you know I don't know much, but can anyone tell me the difference between Torque and horsepower? My engine supposedly puts out 650 HP at 6200 RPM, but the torque drops off at 5800 RPM. I am getting ready to send my props to Bronson at Hill Propeller and the engine maker, HTM, says to get the RPM at WOT down to 5800 so I am at maximum torque. I guess all I have to do is write down the speed at different RPM for Hill, but am curious.

Thanks,
Tom

Torque gets you moving. HP keeps you there.

Torque is the physical measurable force, if you will, that any engine or lever arm producesand and is most commonly described in Ft-Lbs. Horsepower is a derived unit taking into account torque at some given rpm.

1 HP= 550 Ft-Lbs / sec which is translated into:

1HP = (Torque X RPM) / 5252

Torque provides acceleration, while horsepower nets speed.


I don't understand propping for your torque peek, though. You still have rpms left which will give you more horsepower. You should be as close to your HP peek as possible at WOT for max speed.

the angular acceleration of inertia equals torque

hp= rpm*tq/5252

actually on that extremely efficient hull I would think you should take advantage of all the hp your engine has to offer. that is something that is all part of dialing in a boat. maybe your engine builder does not want you to exceed a certain rpm for durability reasons??

Is it possible to net the same speed by using torque to spin a larger pitched prop which would lower the rpm.

But ... would that put more of a load on the engine than being able to use the HP to spin at a higher rpm.

Which is more cost effective(reliable) .... loading a BBC or RPM on a BBC?



:(

Power out = Power in X (1 - efficiency)

Props and drivetrains vary in efficiency(slips, friction) so it's always a tough call, testing is the best way, but your best speed will be achieved if your engine is operating at its dyno-proven WOT peek HP RPM.

Anything less and your not inputting all of your available HP.
Operating at the torque peek simply means that the engine is working harder and not able to achieve it max HP rpm. (Assuming torque peek is lower than HP peek, which 99% of the time it is).

Think about this....the larger the prop, the higher the dynamic loads WILL be on the powertrain....most of the time the drive will loose!

"(Assuming torque peek is lower than HP peek, which 99% of the time it is)" IT IS?...better change that.

My boat saturday went 98 on GPS at 6200 RPM with no rev limiter and I have not dialed it in at all yet. The bottom needs some work and the prop has nicks in it. HTM says it would turn 6500 at peak HP, but there is not enough torque to turn the props. They say to increase the pitch or cup the props until the RPM is more like 5800 or slightly less at WOT. I have Volvo Duoprop with E4 props which is the biggest they make. Bronson says he dialed in the RainX boat and is familiar with changing the E4 props.


Let me see if I understand torque and HP:

If I have 100 ft lbs of torque at 2000 rpm that is making 200000/5252 = 38 HP

If my torque drops off to 75 ft lbs at 4000 RPM, I have less ability to pull against resistance, but more HP - 57 HP

In a car going down the highway there is not so much resistance so it doesn't take so much torque unless I am trying to accelerate, but with a boat it is against resistance from the water, so torque is keeping me from going any faster even though I have more HP available due to higher RPM even if the torque is falling off.

My engine builder says it doesn't matter as long as I don't rev past 6500 RPM, but I don't have enough torque with the current set up to get there.

I have the theoretical calculations from Ocke at home, but I think I remember it should go around 110 and I would be very happy if I can get it over 100 all the time instead of only the perfect day.

If I am going in the wrong direction please let me know. I appreciate all the advice and education I can get.

Thanks,
Tom

Good point :cool:

I didn't consider the whole powertrain.

Not the number itself, but the rpm which they peek. Sorry.

Volvo duo on an HTM?

rpm and tq will always cross at 5252!

The actual value of the respective units will, but I was comparing rpms. The torque peak will typically occurr at a lower rpm than the HP peak rpm.

if your engine's peak tq is at 6500...and tq is < the amount your hull requires to obtain that speed??...then you engine builder should (or should have)....change cam timing and or cams...a cam good for 6500 rpm should not be in a application of 5800rpms....why have all that lift and increased ramp times if your not using it....it just takes from a durability prospective....

from your numers...it sounds like you NEED to go up in rpms!...your get's close to peak hp and are way past the 5800 mark....I WOULD TRY that first! try to dial in the props for 6500

Brad,
I have a batboat B24 and replaced the stock 350 with a HTM 565 engine. The boat runs much more stable with a duoprop according to those who tried the Bravo, but not as fast.

Audacity,
It sounds like you really know your stuff. My peak torque is at 5800 and I don't remember what the value is. The peak HP is 6500 RPM and is 650 HP according to HTM.

My engine builder died in the HTM crash so not much I can do about wrong cam, etc. I already blew up one duoprop drive with a stock 502 and replaced it with the HP version of Duoprop along with the new engine. I am concerned with durability and drive very hard. (I guess that means I'm not that concerned;) ) I spend a lot of time in the air and sometimes miss with the throttles, but not that often. I love the acceleration from 75 - 90 and just want to break 100 all the time. It sounds like going for less pitch and higher RPM might actually get both the best speed and durability.

Thanks to all of you for the input.

Tom,
sorry to hear about the engine builder.

here are a few other key pints to keep in mind:...........

1. be sure you are not exceeding the drive specs with rpm...i would think you are currently in the hp department.

2. you have a dual prop so some rules and limitations have changed. you may be able to get away with adding diameter to one and take away from the other allowing only one to make contact at (your #) a given hi speed...Again, i have never done dial in work for a dual prop application...

3. you may want to investigate the limitations hydrodynamicly of your drive...you may be at its limits??...again, i have no idea. you may want to start a thread or call the manufacture to see what data is out there about others using this drive on applications above 100+ mph.

4. remember throttle input is required log before the prop(s) leave the water... it's not how fast you go,,,it's how well you go fast that wins everytime!!! you know someone said that in a magazine once.:rolleyes: :et: :D

Is that really you Joey ??? You sound so humble :D

:cool:

shhhhh,,,over on that side of the country!

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.:rolleyes: :rolleyes:

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)

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

G

I have heard the same great things about them.

TLee

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

That be da place :cool:

Notes from engine builder conversation
 

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

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.....

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

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.

This has turned out to be an interesting debate. Where is T2x and Dennis Moore?

Looks like there is the HP camp that says you should go for maximum HP by aiming for the highest RPM until you hit peak HP. If we measure the prop slip times pitch times RPM would we have maximum theoretical speed?

In the other corner we have Torque fans saying maximum speed is by raisning the pitch until you get down to the maximum torque. If we measure the prop slip times pitch times RPM would we have maximum theoretical speed? HTM otherwise known as "High Torque Marine" votes with this camp. Rumor has it they don't call their company "high effecient hulls" for good reason and it is the high torque alone that makes their boats so fast.

I don't know the numbers at all, but it would be interesting to see the theoretical difference numbers between the two ideas. Sounds like the HP fans would have better acceleration and durability in the drive train, is that true?

I talked to the mechanic I have been taking out in the batboat lately who owns the old "What's in Store" Warlock and he says it looks like my batboat is in the water a lot during WOT throttle passes and maybe there is a lot of drag compared to a boat that is running on the last foot of the boat. He mentioned the need for getting more lift from the props.

I am more confused than ever now, but having fun.:)

Cig1, generally speaking you run less risk of damaging a BBC by making it grunt than you do spinning it to the hill. When in doubt, I would go with less revs than more if given the choice, if you are talking motors. No reason to beat the motor to death for no gain, only way to truly know is by testing. Every combo is different, too many variables.

I had a very similar outcome as jdnca1 while prop testing. Key is you need to test and take notes, becasue it is hard to remember all those figures when you are out on the water!!!! I agree with jdnca1, Daze and Tomcat. Regardless, have fun

:D

As long as we're talking BatBoats and DuoProps, you won't see him here. :p

As far as gear ratio goes, lets back-calculate:

6200rpm/(assumed) 1.59 ratio = 3900 propshaft rpm

3900 rpm x 29.5 pitch (E4's) = 115,050 inches/miin

115,050 in/min / 12 ft/in = 9587.5 ft/min

9587.5 ft/min / 5280 ft/mile = 1.816 miles/minute = 108.9 mph (assuming zero slip)

(108.9 mph (theoretical speed)minus 98 mph (observed speed))/98 mph = 11% slip

This would seem to be a believable number for a llight boat with this much prop area, so it looks lik you have a 1.59:1 drive.

If you had a 1.47 drive it would give you a theoretical speed of 117.8 mph, and a slip factor of 20%, which doesn't seem right.

Going from E4's to E3's should give you another 330 rpm at the same speed (98 mph), so it looks like that won't really get you anywhere, although you'll have better acceleration...)

Sounds like you're pretty close to the right overall combiination, but something is holding you back. If you can keep everything equal and get to 6500, you should be running about 103 mph.

The DPX gearcase does create drag by running deep in the water, and because of this, begins to lose efficiency fairly dramatically at speeds over 90 mph. It has ran as fast as 108 in the 28' Rain-X Express boat with 720 hp, but they were using a lot more engine rpm and shorter pitch props. You said that there were some nicks and dings in your props - maybe all they need is a good S-class tuneup.

Which engine would you rather have?

Honda 2.2L turbo with 500TQ and 700HP or

BBC 540CID with 700 TQ and 500HP.

The power curves are so different. I'll take the one with less HP.

With a very flat TQ curve you can prop for a few hundred rpms over peak b/c there is probably only 5-10TQ difference which is not noticable. Again a smaller pitch prop is more 'efficient' than a bigger pitch.

jdnca

Do you know whether your motor was dynoed with accessories attached and marine exhaust installed, or was it dynoed with bigtube dyno pipes?

I am in the HP camp, but I believe that there can be a significant difference in peak hp rpm when comparing a bare motor on dyno headers versus an installed marine engine with accessories and marine exhaust hooked to an outdrive with ujoints and 2 different 90 degree gearsets.

We all know that friction increases with rpm in any application. An outdrive will invariably have a different friction curve over rpm than the hp curve. A loaded (as in transferring 700 hp through a bunch of gears and a bunch of bearings loaded with side thrusts and prop thrust) outdrive will show a different friction curve than an unloaded one.

I don't know anybody who can dyno a propshaft on a Bravo at 700hp, but if you "could", and it didn't break the drive, then you could plot the torque and hp curves AT THE PROPSHAFT.

I believe the boat will show best speed when propped to the HP peak of the complete installed setup.

( I know a guy who owned a boat with sixes on it - they were "wetsump" sixes - he had them stolen off his boat and replaced them with drysump sixes. The boat picked up speed and 300 rpm with the swap. He swapped props to bring his rpm back to the previous rpm and he lost speed. Put the old ones back on and picked the speed up again. The drysump sixes gave him more hp at the propshaft cause the oil wasnt slinging all around the insides - It also changed the friction over rpm curve and it allowed him to better utilize the hp peak of the motors.)

I think a good rule of thumb on HP NA motors is to prop somewhere between peak TQ and HP. My motor hasn't been dynoed, but computer simulation programs show peak HP at 6500 RPM and peak TQ at 5500 RPM. I have tried many props that have let the motor sing 6500 and lug down to 5700. The fastest props are ones that let the motor run in the 6000 to 6200 RPM range. As for what mcollinstn mentioned about dyno numbers compared to actual boat numbers he is exactly right. Most dyno shops use automobile type full length headers and no accessories to get their numbers. The accessories shouldn't effect the where the peaks occur, only the actual number itself. Headers on the other do change the RPM levels where peak HP and TQ occur. True headers probably will make peak HP and TQ higher in the RPM range than the HP aluminum manifolds, which are pictured. In this case if the boat is propped from 5700 to 6200 it will probably be close. The only way to know is with a bunch of props and a radar gun. I was with JDNCA1 when he tested the 28 labbed prop at 6500 RPM and was surprised when it was slower than the 30. His dyno numbers showed peak HP at 6700 and I really thought more RPM was the ticket. His motor was dynoed without accessories and dyno headers I believe. He is running tube top CMI's which are 32" runners so he should be close to his dyno in terms of peak TQ and HP RPM's. Bottom line, try as many props as possible!
Good luck!!
Craig

Good discussion. I am a torque guy. I would rather run my motor at max torque that max HP. Remember HP is the rate at which and engine makes torque. Torque is the only thing that actually gets measured, HP is calculated. To get the most out of a set up you need to run at either max torque or max horsepower remembering that if you are running at Max HP, you have left some torque on the table. Torque is what is pushing the boat / turning the prop. High HP motors simply produce enough torque to turn the prop at a higher RPM.

Engines are typically more efficient at max torque as well. Most engines produce max torque at the same rpm range they develop their BMEP (brake mean effective pressure). This is when they are getting the most amount of air in the cylinder and can burn more of the available fuel.

I agree with the Torque guys here. I have always tried to prop the boat a little over max torque. Almost in the middle between max torque and max HP.

On my boat, the engine builder told me that I am making max hp at about 5800 rpms and max torque at about 5000rpms. When the boat was propped for max HP I was only able to get a max of 71mph. I ran the next size larger prop to 5500 rpm's and top speed was 74mph. Then I went to the next size 5300 rpms and the top end was 76 mph. I thought I was on a role so I went to the next size bigger prop turning 5000 rpms, and lost 2 mph.

But on the other hand, some boats like propeller speed. Some of the lighter and more efficient hulls use the propeller speed to give the boat lift. That did not work for my boat, so I went with what worked for my application. You have to test and tune to really find out. Do a little experimenting, but just don't blow the darn thing up testing it at WOT all the time.

Is there any "Rule of Thumb"??

Panther said- lighter more efficent boats like more prop speed
What holds true for bigger boats?
is it different with twins?

I think there is an infinite amount of variable's. Not all the lighter efficient boats like propeller speed. I have seen a lot of the smaller boats, especially boats with small blocks turning high RPM's.

I am going to stick to my guns with small props for rough water, and larger prop's for top speed. That's what has worked for me.

Any opinions?