Performance advice please/prop slip.

[Courageous]

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]

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]

That sounds about right to me.

[Courageous]

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]

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]

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]

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]

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]

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]

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?