Why so much slip?

i'm running a four winns liberator 261 with twin 351 clevor engines, windsor block with cleveland heads. The power output is 600+hp total. volvo penta 290sp drives are mounted with 1,61 gearratio. The propellers have 14 inch diameter and 24 inch incline.

the boat tops out at 45-47 knots, doesn't mather if you have half or full throttle. The engines easily revs 5500 rpm, trimmed out about 6 deegrees.

slip ratio incline rpm speed [knots]
0,31 1,61 24 5500 46,55
0,15 1,61 24 5500 57,35

as the figures show we have a huge slip, about 31%.
I wonder if someone have experimented with nosecones to these drives? One thing we have considered is that the engines weighs less than ordinary marine engines and that the boat stand to much on the nose!
Any suggestions for a solution are welcome

 

If your getting 5500 rpms thats good, but if thats with a full load its under proped and I would go with a larger pitch that would give about 4800 rpms with load and 5500 rpms with light load. I say play with your props. Also consider to get you dialed in go on boat trader and look up the boat just like yours and find a add that claims the boat does 60 give em a call and ask what props and drives they are running and what rpm. But I say you need more prop.
Peace,
CaptRon

 

So the more throttle you apply above half throttle the more the nose pushes over and the prop slips without an increase in speed? So you are looking for ways to decrease lift at the stern and/or increase lift at the bow?

 

My believe is that the main reason for the boat not to go faster is the drives, they are hydrodynamicly useless above 40 knots it's like hitting a wall.

The nose doesn't "dive" according to throttle position only with increase in speed

The boat runs very flat at 47 knots, can't say how much this affects speed, but if we increase speed by putting nosecones on the drives this effect will increase and the boat might get dangerous to drive.

So what i'm looking for is first to improve the penta 290SP drives with nosecones to reduce drag and give the propellers a better environment to work in, can this make the drive trim more efficient when it comes to lifting the bow? Second a way to decrease lift at the stern and/or increase lift at the bow as "woodboat" suggested.

If anyone has experience in putting nosecones on penta drives, or any other, that can give me an idea of what to do i would appreciate it.

Woodboat let me know what you're thinking regarding the increase/decrease in lift.

The 5500 rpm's is with light load, the propellers we are using is the biggest available from penta.

 

Four Winns/VP 290/Slip

navy boi,

I don't believe that the hydrodynamics of the drive is the, main, limiting factor here. Although efficiency drops as speed increases with the 290 drive and a nose cone will probarbly help I have seen many boats with 280 and 290 drives go 50+ knots with decent efficiency.

If the boat drops the bow at higher speeds you drag more surface and loose speed and effiency.

Were are the drives installed (vertically) ? Measure from the hull bottom to the bottom of the cavitation plate.

What prop, exactly, are you running ? SS ? 3 or 4 blade ? Brand ?

Not that familiar with this particular boat - Is it designed to run 50+ knots ?

Karl

 

I've seen boat ads with boats claiming to do 55 knots with 590 hp and 60+ knots with around 660hp, both with heavier engines, none with penta drives though.

it's a standard 3-blade VP prop the biggest they have for the drive unfortunately not steel, originaly 23 inch incline but now 24. Will get the exact data, and do the measurements

 

Navy boi; it is either your engines or your props.

When going from half throttle to full throttle; rpm increases ~linearly with throttle with no speed increase then your props are most likley cavitating and you're just heating up water.

When going from half throttle to full throttle; rpm and speed remains nearly constant with throttle increase then your engines are up against their torque limit.

While running bow down may increas drag, my money says that if you hit a speed wall at half throttle, then your props are not correct for the situation. As you point out, they are standard VP props and I would bet that with the speed you want (60 knt) and the wheels you have (14") that the blade shape/skew is incorrect for the cavitation region. I would start looking for some props especially designed to operate in the cavitation region that you are in.

 

This looks interesting http://www.miwheel.com/MIWheel/products/outboard/ballistic.htm#ball

 

This is what happens just boiling water. And as mensioned before we have a slip greater than 30%

Thanks for the site woodboat

 

Navy boi,

The 14x23 (24) alu prop is not the prop for this application. The "biggest" motor Volvo used this wheel for was the "AQ311" - a 350 Chevy with 300 hp @ 5200. At 5200 boats with this prop/engine combo ran 46 - 50 knots.

To start off with you need to go SS. Alu will start to flex/bend/twist at higher speeds and torque input. Also - From my previous post - Have you been able to check were the drive is located ?(bottom of boat to bottom of cavitation plate)

"600+ hp" - At what rpm ?

Karl

 

The cavitation plate is in line with the bottom of the boat. How do you decide the optimum location? would a lower placed drive effect bow up?

the engines puts out 305 hp each at about 4800 rpm. The engines are special made and I don't have exact numbers.

 

Navyboi,

Drives are fine where they are. Raising or lowering the drives will not change running attitude of the boat.

One more item to check but not critical to performance: Messure the distance between the dipsticks on the upper gear and the distance between the bolt that holds the trim fin in the cavitation plate. Messure with the drives at 0 degree trim and pointing straight ahead. Take out the slack by pressing the drives apart. Compare the two meassurements. The distance between the dipsticks should be 10-12 mm greater than the distance between the bolts for the trim fin.

Apart from that I'm submitting that you are dealing with two issues:

1. Change to SS props.

2. Get the running attitude at high speed correct. The only way to do this (without changing the bottom of the boat) is to shift weight from forward of cgl to aft of cgl.

Good Luck
Karl