Tom

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.

Allan4

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

C_Spray

iTrader: (2)

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

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.

cobra marty

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.

mcollinstn

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.)

WETTE VETTE

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

h2owarrior

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.

Panther

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.

HyperBaja

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?

Panther

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?