Performance advice please/prop slip.

Courageous · 29 August 2011, 19:48

Hi chaps,

well, the saga continues. I have fitted the extension piece to my "L" leg and also the spare gearbox that I very fortunately had laying around. Just been for a sea trial and I am still limited to 28 knots WOT one up with 300L of fuel onboard.

For anyone not familiar: Valiant DR750, Mercury 150 now XL so the correct length. Planing plate removed. 19 pitch lazer SS prop fitted.

I am getting 5600ish WOT which is about 200 less than I was getting before with the ventilating arrangement.

5000 RPM gives me 24 knots which I calculate still very roughly to be 40% prop slip.

I would very much appreciate any thoughts or advice on this setup. In case someone suggests upping the prop pitch, we have a 17 and a 19 as she will be lugging 6 to 12 divers with full kit so a "tug" prop will probably be best with full load. I still don't understand the reason for this much slip with this prop though.

Ian

C-NUMB · 29 August 2011, 20:13

Is not Your boat slightly underpowered to expect high speeds? Pretty beamy boat and rough weight 700+200+300 fuel + driver, how much to expect from a 150 HP? Might be totally wrong but hands of, sounds that to much pitch on the prop, guess 15 or maybe max 17 would be closer?

willk · 29 August 2011, 20:14

That sounds about right to me.

Courageous · 29 August 2011, 20:18

Hmmm....... yes the 17 was on the boat when I bought her. With the L leg instead of the XL which placed the cav plate well above the skeg she performed better with the 19 prop but perhaps we should drop back to the 17 again. As it happens the high 5 I have bought from the UK is a 17 so that more or less makes the decision regarding what pitch we use from now on. Do you suspect that a lower pitch would reduce the slip? I believe the high 5 will make a significant difference anyway.

The 150 is the recommended engine size by Valiant by the way.

captnjack · 29 August 2011, 20:21

Are you expecting zero slip or something???

I am running a 13" prop which gives me 5900rpm WOT lightly loaded, about 30knots. Theoretically that's 6400ft/min (5900rpm x 13" /12) which would be ~63knots. Since I am actually doing about 30knots there's 47% slip.

I would drop to the 17" pitch or possibly less with that many divers.

For that length, a substantial deadrise(?) + that big of a load, I think you are quite underpowered with only 150hp.

captnjack · 29 August 2011, 20:24

Quote:

Originally Posted by Courageous

The 150 is the recommended engine size by Valiant by the way.

Maybe for passengers but for divers always use as much engine as the hull can handle. In your case 2x 150s would be much better for the weight of even 6 divers. 12 divers I don't see fitting onto a 7.5m RIB unless they are all petite ladies in 3mm wetsuits with 4kg weightbelts and tiny tanks.

Courageous · 29 August 2011, 20:25

Jack - do you not have a gearbox on your motor?

And no I'm not in search of the holy Grail, about 10 percent slip is reasonable IMHO.

Courageous · 29 August 2011, 20:26

Jack - the boat is certified for 22 pax (obviously not divers). The previous owner ran her on his dive school with 12 divers comfortably. She IS beamy.

captnjack · 29 August 2011, 20:26

Quote:

Originally Posted by Courageous

Jack - do you not have a gearbox on your motor?

sure 1.2 ratio I think. doesn't magically make the slip disappear though its still pretty substantial.

still think you are underpowered

C-NUMB · 29 August 2011, 20:27

If You have a gear ratio of 2.0, 13% slip and obtain 5800 rpm, your theoretical speed would be roundabout 31 knots with a 15 inch prop. Test the 17 but try to borrow a 15 as well, might be worth it?

Courageous · 29 August 2011, 20:30

15 - yes but as I say I bought the high 5 17" now and await collection. If Over propped is slip of this extent expected do you know?

doggypaddle · 29 August 2011, 20:30

Quote:

Originally Posted by captnjack

Are you expecting zero slip or something???

I am running a 13" prop which gives me 5900rpm WOT lightly loaded, about 30knots. Theoretically that's 6400ft/min (5900rpm x 13" /12) which would be ~63knots. Since I am actually doing about 30knots there's 47% slip.

I would drop to the 17" pitch or possibly less with that many divers.

For that length, a substantial deadrise(?) + that big of a load, I think you are quite underpowered with only 150hp.

is that not 5900rpmX(13/26)x13/12=3196ft/min
the gearbox being 13teeth on the input gear 26 on the output gear????

Courageous · 29 August 2011, 20:31

No doggy, Jack doesn't have any teeth

Courageous · 29 August 2011, 20:35

Quote:

Originally Posted by captnjack

sure 1.2 ratio I think. doesn't magically make the slip disappear though its still pretty substantial.

still think you are underpowered

Yes Jack, it brings YOUR slip down to 9.5% which as mentioned above is pretty near expected.

doggypaddle · 29 August 2011, 20:35

Quote:

Originally Posted by Courageous

No doggy, Jack doesn't have any teeth

Surely that would be 100% gearbox slip

just found this on prop manufacturers site.

prop slip will range from 5% on a light cat to 25% on a heavy workboat. Most hi performance applications will be between 7 and 12 percent.

Courageous · 29 August 2011, 20:36

Yeah doggy I know but not as funny though eh?

Oscarguitar · 30 August 2011, 10:41

In your earlier thread about planeing technique you posted this in reply to one of my posts

"The little thing in front of the prop is the skeg! 25 inch transom and 20 inch leg doesn't quite work does it? You mentioned before that I am experiencing 46% slip or thereabouts. The boat is a standard DR750, the prop is a standard 3 bladed lazer in good nick - surely that only leaves leg length no?"

I see what you mean about the L v XL issue, but that doesn't seem to have solved the problem and it is a big boat so maybe it just isn't going to do what you want. My question is what do you mean by the skeg? Do you mean the gearbox housing of the outboard itself or a fin of some kind mounted to the hull? If this is on the hull and directly infront of the outboard leg it would create all the symptoms you describe, no bite, difficult to get on the plane and low top speed due to disrupting the flow of water over the prop. Even a small obstruction can cause big problems.

Any chance of a picture to show what you mean if it is not the leg?

Apologies if this sounds pedantic but I put your figures into a slip calculator the get the 46% and those slip levels are awful. It must be possible to improve. I don't think the High 5 17" will be the answer if top speed is important, good hole shot but not much top end.

I'm no expert, but Ken at the Propgods forum is probably the best in the business. See if he can give you any advice.

Courageous · 30 August 2011, 23:08

Hi Oscar and thanx for coming back in on this. By skeg I mean the base of the keel band. The very lowest strengthening strip along the hull. Having now extended the leg by 5 inches the cav plate is now level with this as it should be (I believe). So there is definitely no obstruction in front of the prop now. Still I get this horrible figure on the slip calc!

Regarding doing what I want - I have little expectation other than to bring the efficiency of the rig to within possibilities and that's somewhere close to the ten percent slip you quoted really. I suppose it is the wrong prop for the job and little more after all this?

If I had a 15 inch prop and ten percent slip I could expect performance in the mid thirties I suppose. I'm just amazed myself that with lowering the prop I haven't actually achieved anything really. I know there's more drag now but surely it should be better than it is now no?

I shall try to borrow a 15 inch prop and see how I get on........

Oscarguitar · 31 August 2011, 09:18

Best of luck and I hope it works out for you.

The reason I have some basic knowledge is that I get about 18% slip which to my mind is still too high so I have looked at ways to reduce it. I have raised my motor one hole (which helped me get another 200 rpm) and would like to go another but that would mean a jack plate or redrilling the transom which I dont want to do.

Have a look at this link regarding prop height.
http://www.veradoclub.com/smf/index.php?topic=137.0

My thoughts are that if your Anti Cav plate is level with the bottom of your hull it is too low but it does vary from hull to hull. The further back (set back) your prop is the higher it can go.

Any chance of a picture of this strengthening strip, I wonder if that is causing turbulence just in front of the prop.

And finally do have a look at the Prop Gods forum. It is USA based but the advice on prop selection type, diameter and pitch is the best around

Good luck again