jumping between props, advice??

[FlyingBoat]

This probably sounds like a hundred other posts on here, but I have not seen an identical scenario. I recently bought a 23' Larson DC230 SportCruiser, with 5.7 Merc, trim tabs, etc. It had a 14.5 x 19p aluminum prop. Performance was ok, cruised 33-35 at 3500 rpm, but only turned 4000 WOT. First answer, drop to a 14.5 x 17p, which I did. Should help the pickup for skiing and allow higher rpm, right?

Cruise speed about the same, maybe a little faster (35-37 @ 3500), still only turns 4200-4300 rpm WOT, but now the da@#d thing will not stay in the water, it porpoises something terrible!! Yes, I can crank the trim tabs down and stop it, but it feels like I am dragging a sea anchor. I cannot trim the drive up ANY at all or it gets worse! So, here I am driving around with my drive trimmed all the way down/in and trim tabs hanging out to keep the front end on the water.

Since the new prop did not increase the WOT to what I think it should be (4400-4600), I am wondering whether the 19p was the right prop, but something else is keeping the rpm down. Also wondering whether there is a technique (I am obviously newbie) to keep the porpoising down. As an unrelated issue (I think), it does seem to cavitate very easily with either prop. Either in a tight turn or while it is porpoising it can cavitate so I have to chop the power, allow it to regain, and then add again. Is there something wrong there? Would a large anti-cavitation plate help, or a "whale tail"? Would more weight in the boat help these issues?

Ideas welcome!!

Gene
1988 Larson DC 230 SportCruiser

 

[alldodge]

Re: jumping between props, advice??

This probably sounds like a hundred other posts on here, but I have not seen an identical scenario. I recently bought a 23' Larson DC230 SportCruiser, with 5.7 Merc, trim tabs, etc. It had a 14.5 x 19p aluminum prop. Performance was ok, cruised 33-35 at 3500 rpm, but only turned 4000 WOT. First answer, drop to a 14.5 x 17p, which I did. Should help the pickup for skiing and allow higher rpm, right?

Cruise speed about the same, maybe a little faster (35-37 @ 3500), still only turns 4200-4300 rpm WOT, but now the da@#d thing will not stay in the water, it porpoises something terrible!! Yes, I can crank the trim tabs down and stop it, but it feels like I am dragging a sea anchor. I cannot trim the drive up ANY at all or it gets worse! So, here I am driving around with my drive trimmed all the way down/in and trim tabs hanging out to keep the front end on the water.

Since the new prop did not increase the WOT to what I think it should be (4400-4600), I am wondering whether the 19p was the right prop, but something else is keeping the rpm down. Also wondering whether there is a technique (I am obviously newbie) to keep the porpoising down. As an unrelated issue (I think), it does seem to cavitate very easily with either prop. Either in a tight turn or while it is porpoising it can cavitate so I have to chop the power, allow it to regain, and then add again. Is there something wrong there? Would a large anti-cavitation plate help, or a "whale tail"? Would more weight in the boat help these issues?

Ideas welcome!!

Gene
1988 Larson DC 230 SportCruiser

Your WOT should be around 4400 to 4600. With the boat on the trailer the outdrive should be angled under the boat. Have you checked to see if your drive is going all the way down? How many blades are on your prop?

 

[hwsiii]

Re: jumping between props, advice??

Gene, the part that you didn't understand was that not all props are created the same.
The first prop you had the 19" pitch I presume had very little rake in the prop and thus was more of a stern lifter than a bow lifter, or it had a lot of cup in the trailing edge of the blades.

Cruise speed about the same, maybe a little faster (35-37 @ 3500), still only turns 4200-4300 rpm WOT, but now the da@#d thing will not stay in the water, it porpoises something terrible!!

The second prop 17" pitch evidently has a lot more rake but not enough to lift the bow of the boat and hold it, so you are experiencing porpoising, although this is not the only reason boats porpoise.
And since you said that it cruised at a higher speed at the same RPM, it evidently reduced your prop slip but started the porpoising.

I show your boat to have a 260 HP motor with a 1.32:1 gear ratio with a factory recommended max RPM to be 4,200 to 4.600 RPM, please verify this.

Prop Slip

And NO a whale tail will not cure your ventilation propblems.


I will be glad to help you but I need more information.

Iboats Boat and Motor Info

1. Year, make and model of boat
2. Length, width and base weight of boat, look for boat decal on back of boat
2a.What is the maximum recommended HP for your boat
3. Number of people and gallons of gas normally on boat
4. What do you use the boat for
5. Is it a Deep Vee and if so how many degrees of deadrise
6. Year, make. manufacturer and model of motor
7. HP and gear ratio of motor IMPORTANT
If you don?t know the ratio, you need to pull the plugs out and put a piece of tape across the prop and the lower unit and then cut it between the prop and the housing then do the same thing on the flywheel and turn the motor until the tape lines up with each other on the prop, it is easier if two people do this, so one can watch the prop while the other counts the revolutions of the motor
8. Manufacturer?s recommended Wide Open Throttle (WOT) range
9. Anti-ventilation Plate height above keel of boat if it is an outboard in inches, use a board under the keel and stiking out to the anti ventilation plate for a referene
10. Is it a bass boat or does it have a pad bottom
11. Does it have a hydrafoil, dolefin or trim tabs
12. Make, model, diameter, pitch, number of blades and whether SS or aluminum prop
13. WOT RPM and speed from your current prop and how much gas and how many people were in the boat for the test data and is the speed by GPS. Make sure you trim the prop up until it starts ventilating and then just trim in until it quits ventilating. If you do not have a tach you can buy a digital tachometer and hour meter here: http://cgi.ebay.com/ebaymotors/Tacho...Q5fAccessories
RPM _________ Speed ______ GPS ______ No. of people ______ Gal. Gas ________
14. Are you at sea level or a higher elevation, give us the elevation in feet
15. Has your motor been tuned up lately and have you checked that the carburetor butterfly is opening all the way, checked compression, and looked at the plugs and checked spark as well as timing and advance, all of the foregoing could be the reason your prop is not attaining full RPM
16. How long has this prop been on the boat and why, at this time, do you think it is the wrong prop
17. Does the prop show any damage that you can see
18. What problems are you trying to cure or what are you looking for the boat to do that it is not doing the way you think it should or to your expectations
19. If you are trying to attain a better cruising speed and fuel savings or trying to attain a faster speed I will want you to take your boat and run it with 1 or 2 people and give me the RPM and speed readings starting at 3,000 RPM in 500 RPM increments all the way to WOT.

REMEMBER, The numbers I give you will be NO better than the information you give me

The only thing I ask of you is to come back and give me a report of WOT RPM and speed for my database.



H