puder

if my prop was slipping really badly (40-50%) would i be turning high rpms easily? (Such as if you make a hard low speed turn and gun it cuasing cavitation) How can i reduce prop slipage?

cobra marty

50% ?! Buy a different boat.

puder

acording to the prop calc it is about 37% but my memory since last season might be off on the RPM. I know I am running slower than I should be. SO iwoudl be willign to say even if i was off a little on eth RPMs I am still slipping moer than I should be

cobra marty

I'm at work. What is your excuse for being up at this ungoddly hour?

Gary Anderson

40%, holy s#@!, are you missing a blade on your prop?
Did this problem just start?
What boat, engine, drive prop are you running?
Gary

puder

ok

its 27' scarab with a single 330hp 454 and an alpha drive. (in 86 and 87 that is how they set it up) I am running a mirage + 3 blade 21p. My ratio is 1.32. She tops out at 47. And as I recall (its been a while since i ran her) it was at 4200 rpms.

when i ran the prop calc it saus my theoritiocal speed should be 63. My actual is 47, which leave my slip at 34%. My rpms might be a little lower than 4200.

Woudl goign down to a 19p 3 blade reduce slip? It woudl also bring eth RPMs up a little which would be ok becasue my engine is ok up to 4600. Would a smaller 4 blade help?

Does my lack of speed sounds like a prop issue or is it possible my engine isn't making decent HP? I woudl think it is relativley ok becasu ei am abel to turn a 21 up to 4200 and I have the steep gears (for "durability") Woudl changing to 1.50 gears help? I am considering converting to a bravo (hence the 1.50 question)

Also could the slipage be a result of a hook in the hull I know there is a hook in the last 2-3 feet. I am tryign to figure out if that was origoanlly in eth hullor its been created by being on a roller trailer for 15 years.

Any advice (other than get a new boat)?

Gary Anderson

Start with the simple stuff first. Is the prop in good condition (no burrs, dings, dents, missing blades, etc.), is it the correct pitch and diameter.
Is the boat trimming correctly? You dont have a load of concrete in the bow do you?
Make sure you haven't spun its hub or the coupler. They usually totally burn up at once.
Make sure there's no interference with water flow to the prop (water pickups, speedo pitot)
Is your thru hub exhaust bellows is in good shape and not bleeding exhaust gasses over the prop blades?
I'm assuming the drive is at the same height as it was originally.
Make sure your tach is correct. Make sure your speed reading is correct (gps not speedometer).
If I had to guess what the problem is (from 1000 miles away): You've got 1 or 2 bad spark plugs/wires. This will cause false high rpm readings on the tach, and the boat will be down on hp resulting in slower top speed.
Different pitch wont affect slippage that much. Larger diameter or more blades can help, especially if the X dimension is higher. But your slippage should be under 15%. So at 40%, you're WAY off and there's something more to this than being off by 1" pitch or 1/2" diameter.
Gary

Rambunctious

puder, my 211 liberator I had would turn a 21" up to 4800. perfect for the small block. the speed was 53 tops. the ratio of rpm/speed is 90.56. your ratio is 89.36 (4200/47). therfore your efficiency is 98.7% of what mine was. This seems reasonable since one prop is moving your 27 footer vs my 21 footer i had. so is 1.36 really the ratio for alphas? anybody, anybody?

a 4 blade will require (steal) more power to spin through the water, but the efficiency might be better. i would consider a lower pitch 4 blade to move the rpm up closer to 4600

the other solution is lab finishing. this reduces the parasitic lossed to spin the prop, freeing that power up to move the boat faster, thus moving the rpm into peak power range. Lot of recommendations on the board to talk to John York at York propellor. He will have lots of insight into your situation and give you suggestions. I spoke with him about labbing my props for my formula. He is very easy to talk to. 714-773-5444

I am at a similar situation with twin 330's in my Formula 272. I'm at 4400 on a good day. 4200 with 3 line monkeys aboard and racing to the pier in our local monthly group runs. I am saving my pennies to lab my two 25" props to break 64.99999mph. (bravo 1.5:1 ....btw) $125 each plus $25 shipping.

puder

liek i said when alphas came mater to BBC in 1986 and 1987 they use dteh 1.32 gears to make them last longer. The x dim is stock.

The prop is brand spankin new has maybe 10 hours on it and is in fine condition.

spark plug wires huh? I will have to try that. ANy way to test befor ei by a new set of wires?

As for exhaust i have thru hull and a block plate on the transom. The bellows is how it came from the factory. if it is in fact spark plug ires woulnd;t my engien not sound right? She sounds fine as far as i can tell.

any other ideas?

olysan

I just ran the prop calc based on your input and came up with 29% prop slip. This doesn't seem unreasonable considering the size and age of your boat, and your power package. Sorry but I don't think you've got much free speed hidden in there. I would try and prop it so that you hit that 4600 rpm mark.

Mike