Powerboat Forums at SpeedWake banner
Cancel
Create thread New Forums

So what will happen if props too big?

Jump to Latest
1 - 20 of 31 Posts
S

Swamplizard

· Registered
Joined
·
1,886 Posts
Discussion starter · #1 ·
Having trouble getting straight answers from anrneson-savvy folks as to what props I should be running as I try to dial in the harley project boat. Some tell me I am overpropped since I can't pull 5000 rpm, others tell me that is normal for arnesons that expect props to be half out of the water. I cant do trial and error with $10k props.

current setup will not allow the motors to spool up over 4200 rpm (basically stock 502s at 425 hp carbed). I am not really that interested in top speed - more into cruising comfortably at 30-35 mph which it does at 3400 rpm.

assuming I leave the existing props on it and cruise the 502s mostly in the 3400-3800 rpm ranges - what will happen to the motors?
 
#3 ·
Doesn't seem like the type of drive should matter. If the engines are designed to turn at 5000 rpms at WOT, I would think they should be propped to turn at that rpm. Then reduce rpm's to your desired cruising speed.

I don't know what the stock 502 tq curve looks like. But I'll bet the mills are only turning a few hundred R's over peak tq at 4200. Like turbo said, seems like too much load on the engines.
 
Discussion starter · #6 ·
More info - little help there ThrottleUp:

I now have numbers to chare with you and am concernd the props might still be off...dont want to overwork motors and hurt them. Remember this has arneson surface drives that are typically over propped since the props are half out of the water at WOT.

Stock 502 have the following rpm/hp/torque
160/137/444
2000/175/456
2400/214/467
2800/254/475
3200/301/492
3600/335/489
4000/370/484
4200/374/468
4400/385/448
4800/369/403
5000/367/385

My 502s have 850 cfm holly carbs and have high torque crane cams and supposedly are 415-425 HP so the numbers are probably better than above.

I can cruise at 3100-3200 rpm at 28 knots but it is not a "high" plane (hull is not quite over hump and needs trim tabs).

At 3600-3800 she runs 35-knots and dead flat without tabs and high out of the water. Sounds better too.

I cant pull more than 4200 rpm WOT unless I trim the props so high that it is all rooster tail and feel like I am beating on it.

since the torque curve is flat from 2000-4300 am I running in the "meat" of the powerband? Am I overworking the motors or running in a safe zone?

Next flat weekend day on the ocean I will get better numbers speed (in knots since that's all my GPS gives me) versus every rpm range. Props are three blade 19/20 nibral round eared. 1:53 reduction in transmissions.
 
#7 ·
Julie has said that boats need to be propped so WOT RPM is 1200-1400rpms over torque peak. And engine is built for this and with HP peak at peak WOT RPM.

So, going by your graph it looks like 4400rpm is best for WOT.

All the rest should fall into place.
 
#11 ·
I agree cfm! The peak HP is at 4400 RPM....so that is were the speed is going to be found. Now if your goal is cruise speed then prop for a little less RPM.


Keep in mind as well that is some parts of the country the water temp, air temp and air density readings fluctuate greatly and this to will have an effect on your numbers. A good example of this is Florida. In the winter months we will see an increase in RPM of 100-200 RPM at WOT. So this should be kept in mind when you are testing in the winter.

Julie
 
1 - 20 of 31 Posts
About this Discussion
30 Replies
10 Participants
Last post:
Throttle Up
Powerboat Forums at SpeedWake
posts
921K
members
19K
Since
2002
A forum community dedicated to Powerboat owners and enthusiasts. Come join the discussion about performance, modifications, classifieds, troubleshooting, maintenance, and more!
Full Forum Listing
Explore Our Forums
General Boating Discussions Powerquest News Articles