2018 Shoalwater 21 Cat, Yamaha F150, Bob's 6 inch hydraulic plate, Yamaha 3 blade 19 pitch Saltwater Series. bought it in march from a guy in Portland who ran it in Corpus Christi bay and i have been fighting it all summer.
with the plate above about halfway the prop blows out on the hole shot, revs, slips, then grabs and it takes forever to get on plane. plate all the way down it gets up fine but porpoises at 4200 with two of us in it, i have to trim down to stop it and then i am plowing. wide open with the plate at the top is 5400 and 44 on the GPS which seems low for a 150 on a 21.
so is this a prop problem, a plate height problem, or am i just running it wrong. i do not have a water pressure gauge, the boat came with the analog tach and that is it.
J
jack_plate_setup
SkipperCorpus Christi, TX19 posts, joined Sep 2025
#2
first thing, stop and put a water pressure gauge in before you touch anything else. 30 bucks for the mechanical one, ten minutes with a tee at the block. a tunnel feeds the prop aerated water on the hole shot and every setting on that plate is a guess until you can see pressure.
what you are describing, blowing out high and porpoising low, is a 3 blade with no cup on a cat hull. the 19p Saltwater Series is a good prop on a v hull. on a tunnel most of us end up on a 4 blade with a lot of cup. i run a PowerTech OFX4 17 on a 22 cat with a 150 and a 6 inch plate. plate about 2 inches up on the hole shot, then walk it up once i am on plane. it is not one setting, hole shot down, cruise up, and you learn it by the gauge not by feel.
other thing to check is where the motor sits on the plate. on a shoalwater the cav plate should end up around 3 inches above the tunnel roof with the jack plate at its lowest. if somebody before you bolted it a hole higher, the plate is already too high before you touch the switch.
Y
yamaha_f250_tech
Bay CaptainAransas Pass, TX41 posts, joined Sep 2025
#3
water pressure, same as he said. a 150 wants to see 10 psi and up at cruise and it will drop to nothing when the prop blows out. if you have been running the plate up with no gauge all summer you are one hot afternoon from a cooked head, i have done two of those on tunnel boats since june.
if you want to run it high on the flats, a low water pickup nose cone from Bob's or Hydro Tech makes a real difference on the 150. and since it is a 2018 with unknown history, put thermostats in it while you are in there, they stick, and pull the lower unit oil to see what color it is.
T
tunnel_hull_guyOP
DeckhandIngleside, TX9 posts, joined Sep 2025
#4
put the mechanical gauge in tuesday night, ran it wednesday morning before work. plate all the way down, 14 psi at 4000. plate up to the third light on the indicator and it dropped to 5 and the alarm never went off, so i guess that answers whether i was hurting it. going to try a 4 blade. anybody near Ingleside have an OFX4 or an OFS4 i can borrow for a weekend before i drop 600 bucks.
C
carla_runs_shallow
DeckhandPort Lavaca, TX8 posts, joined Sep 2025
#5
i have a 15 pitch OFX4 sitting in the garage from when i went from a 115 to a 150 on my 20 cat. it will be too much rpm for you at the top but the hole shot will tell you if the 4 blade fixes the blowout. i am in Port Lavaca, message me, you can have it for a weekend.
P
port_mansfield_flats
SkipperPort Mansfield, TX16 posts, joined Sep 2025
#6
one more thing nobody said. weight. two guys, a full livewell up front and 40 gallons of fuel changes where that hull wants to sit. my 22 cat porpoised until i moved both batteries out of the console and up to the back of the front deck. try that before you buy a prop, it costs nothing.
T
tunnel_hull_guyOP
DeckhandIngleside, TX9 posts, joined Sep 2025
#7
update. borrowed carla's 15 OFX4 last weekend. hole shot with the plate at 2 inches, no blowout, it jumped up like a different boat. 5950 at wide open which is the top of the range so the 15 is too small like she said. ordered a 17 pitch OFX4 from the dealer in Aransas Pass, 585 with the hub kit.
porpoising is gone with the plate down after i moved the batteries the way port_mansfield said, both of them were in the back of the console. gauge reads 12 to 13 at cruise now with the plate up about 2. and the motor was mounted a hole higher than the rigging guide says for a cat, i dropped it one hole while the prop was off. yamaha has the rigging guides on yamahaoutboards.com if anybody wants to check theirs. that was the other half of it.
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