Prop Cavitating (catching air?)

[SW_Boater]

Have a new 11 ft Saturn SD330 inflatable, and used, ~2000 Nissan/Tohatsu 8hp 2-stroke, that we took on the water for the first time. Have an issue I'm hoping someone can help us with.
Bear with my terminology as I have almost zero experience with outboards.

For some reason the prop seems to cavitate (catch air?!) when we try to get up on plane, like a clutch slipping on a car. If I give it full throttle the prop will cavitate, rev very high, than catch water for a moment, drop RPM's, then air again.

I've tried all 6-7 engine "trim" angle positions with similar results. Fwiw, the "top" part of the prop's outer turning radius is about 3" below the transom so I don't believe there's any issue with the Nissan's shaft length. The Saturn rear transom is about 14-15" deep. I believe the Nissan is a short shaft motor.

If we take the time to adjust the engine trim angle just so AND my wife positions herself perfectly fore-aft we can prevent the prop from cavitating and can apply full throttle; however, the bow is high and it won't quite get on a plane. Plus, the boat feels like it's dragging an anchor. Clearly something is wrong.

It cavitates riding solo in all engine trim positions. I only weigh approx. 140 lbs. My wife only weighs 110 lbs.

Just to be clear, the issue is not so much not getting on a plane, but the fact the prop appears to cavitate (and over rev) under med/high throttle conditions.

Do I have a bad prop? Current prop doesn't appear bent. There's some light nicks and wear on the leading edge, but that's about it.

Thanks in advance for any help!

 

[Sea Rider]

Try this : Trim OB to sit on transom perpendicular, that's at 90 deg, usually move trim pin to second/third hole out transom. With wife sitting up front and you sitting rear go for full throttle on flat, calm no wind water cond to plane out the Sib. Once on plane pull head out middle transom and check at which lower leg is wayer flow passing at speed. Can sit at rear tube or on deck next to transom.

Check if passing between middle or lower Cav Plate, report your findings, will go from there. Don't forget to fully inflate your Sib to al least 3.0 PSI on all air chambers including keel.

Happy Boating

 

[ondarvr]

Cavitation is something very different, you're experiencing ventilation.

Not all manufactures of motors and boats follow the same line of thought on motor height and transom height, plus inflatables tend to like the prop deeper in the water. Normally in your situation you can make it a little better with minor adjustments, you frequently need to lower the motor for best performance though.

 

[Fed]

You might have spun the prop hub, mark it & take it for a spin.
Google on 'how to check for a spun prop' to find out how to mark it.

 

[QBhoy]

Another thought might be that if your boat is of the full inflatable type (not a rib) and flat bottomed without an inflatable keel.....these types are well known for gulping big pockets of air under the hull. Often actually felt by your feet travelling under the hull and eventually to the prop, causing it to cavitate. If you have an inflatable keel...blow it up. If not, you might get a better result by experimenting with shifting your weight or load about the boat. Try and keep the weight of persons on the tubes.

 

[SW_Boater]

Try this : Trim OB to sit on transom perpendicular, that's at 90 deg, usually move trim pin to second/third hole out transom. With wife sitting up front and you sitting rear go for full throttle on flat, calm no wind water cond to plane out the Sib. Once on plane pull head out middle transom and check at which lower leg is wayer flow passing at speed. Can sit at rear tube or on deck next to transom.

Check if passing between middle or lower Cav Plate, report your findings, will go from there. Don't forget to fully inflate your Sib to al least 3.0 PSI on all air chambers including keel.

Happy Boating

Thanks for your reply, Sea Rider!

We tried what seemed every possible combination of trim while riding solo and with my wife on-board. It would only run steady at full throttle (without serious cavitating and jerking) at one specific trim position as long as my wife sat in a specific position. If she moved forward or back 6-8" from that position it would cavitate.

As you suggested, inflation pressure may be something we need to look at. We're using a 12vdc pump to inflate and it has a pressure gauge which could very well be incorrect. May try increasing the pressure, especially on the floor and keel tube, and see if that helps.

 

[SW_Boater]

Cavitation is something very different, you're experiencing ventilation.

Not all manufactures of motors and boats follow the same line of thought on motor height and transom height, plus inflatables tend to like the prop deeper in the water. Normally in your situation you can make it a little better with minor adjustments, you frequently need to lower the motor for best performance though.

Very possible we're experiencing ventilation and not cavitation.

Fwiw, our cavitation plate is about 2-3" lower than the bottom of the transom, so I'm hoping that's low enough.

 

[SW_Boater]

You might have spun the prop hub, mark it & take it for a spin.
Google on 'how to check for a spun prop' to find out how to mark it.

A spun-hub did enter my mind. The fact we were able to run full-throttle without any cavitation at a steady engine RPM a few times tends to let me believe it's not a spun-hub. However, if some of these simplier fix actions don't correct things, it probably wouldn't hurt to check just to rule it out. Thanks!

 

[SW_Boater]

Another thought might be that if your boat is of the full inflatable type (not a rib) and flat bottomed without an inflatable keel.....these types are well known for gulping big pockets of air under the hull. Often actually felt by your feet travelling under the hull and eventually to the prop, causing it to cavitate. If you have an inflatable keel...blow it up. If not, you might get a better result by experimenting with shifting your weight or load about the boat. Try and keep the weight of persons on the tubes.

Yes, possible we may have incorrectly inflated the boat causing these "big pockets of air" you mentioned. We may try increasing pressure, especially in the floor and keel, and see if that helps. Thanks!

 

[SW_Boater]

Again, appreciate everybody's replies!

I'm assuming from everyone's responses the likelyhood of a bad prop is low?

Thanks!

 

[ondarvr]

Very possible we're experiencing ventilation and not cavitation.

Fwiw, our cavitation plate is about 2-3" lower than the bottom of the transom, so I'm hoping that's low enough.

Having the AV plate 2-3" below the transom is normally enough, but if air is being funneled under the hull and out the back right near the prop it can still ventilate easily.

 

[SW_Boater]

Well, finally made some progress. Increased the air pressure in the keel and floor area and the cavitation went down significantly. We also noticed the floor area near the transom and rear seat area was bulging upward when we applied a lot of throttle and tried to get on a plane. I put as much weight as possible in this area with my feet, when suddenly, the boat leveled out and got on plane. As long as I kept pushing down in this area with my feet we stayed on good plane. I'm guessing we were going at least 15 mph while on plane.

Assuming we have proper air pressure in all the cavities, I'm guessing we may have a "defective" boat. Assuming we don't (and our 12vdc pump's gauge is not accurate), then there's a possibility we may be running with too low of pressure in every cavity. Just ordered a K-pump air pressure gauge from Amazon so we can verify the air pressure in all the cavities. If they're low, I'm hoping increased air pressure will solve our problems.

Thanks again for everybody's help!

 

[Sea Rider]

Recapitulating

Did not payed much attention at the date that OB was manufactured, but that's a 17 year oldie. Don't know if bought new, second hand, with low, many hours use, anyway prop hubs don't last that long if OB was used, abused a lot.

Combo has too many issues to deal with. If wanting top water performance need the following :
-Inflate all tubes gradually to at least 3.0 PSI with pressure gauge.
-Inflate keel to 3.5-4.0 PSI.
-Inflate air deck to at least 12 PSI.
Re inflate all air chambers to specs once sib has rested for some minutes on water for pressure to stabilize.

Perform a prop hub slip test to rule out if already with prop issues.
-Remove prop.
-Mark 2 straight lines as in pic, let dry.



-Put prop back in, secure with split pin.
-Trim OB to sit perpendicular (90?) to water level to achieve best prop thrust.
-Sit inside middle deck you alone, with extended arm throttle fully, sib should plane faster than before.
-Go for at least 1 minute ride.
-Remove prop, check if marks have moved away from each other. If not hub is OK, if moved, hub is shot, kaputt!!

Which is the current prop pitch number stamped on prop body ?
-Would recommend going one less in pitch, combo will benefit with a faster hole shot and plane.

Air decks are not the best inflatables for speeding, if you find that inflating constantly air decks to perform right is a PITA, order a 330 alum deck from Saturn, will perform much better than any air deck. If going per that route, get a 9.8 HP carb to bump that 8 HP into an 9.8 HP, both models shares same CC powerheads, add a less pitch prop and now you're into something much better...

Report your findings,

Happy Boating

 

[SW_Boater]

Sea Rider, very much appreciate you taking the time to pass on all this useful info!

We had an opportunity to purchase an inflatable with hardfloors, but went air for the convenience of storing it in while traveling in an RV. Would have gone hard-floor from the git-go had I known these air floors were prone to deforming and causing all these issues. Live and learn. In any case, sounds like you're saying we can can purchase hard-floors for our Saturn SD330 after the fact? If so, good to know we may be able to improve things without having to purchase another boat. Still holding out hope our 12vdc pump is underinflating and we can improve things somewhat. Supposed to get our K-Pump air pressure gauge tomorrow.

"8.5" is stamped on the prop. I assume that's the pitch?

Yup, this Nissan's got a few years on it. Was going to purchase a new Tohatsu 9.8 4-stroke, but price and light weight (50 vs. 85 lbs) convinced us to take a chance on this used two-stroke. When we got the Saturn on a good plane yesterday (while pushing down on the bulging floor with my feet), it was scootin' along pretty good! If we can get this cavitation/ventilation issue rectified I think it'll work well for our needs.

 

[Sea Rider]

SWB, you're welcome,

Inflate and test as suggested, ideal would have been going straight to a 9.8 HP-OB. Contact Saturn and ask for the 330 alum floor set, should be available. Will take longer to assemble/disassemble, but will improve performance.

Don't forget the hub slip test, will go from there..

Happy Boating

 

[ondarvr]

The hub isn't slipping, once a hub starts to slip it gets worse quickly, after just a couple of minutes you can't go faster than an idle

 

[Fed]

I've had different results ondarvr, on again off again slipping and still sometimes even getting on the plane after a few failed attempts due to slipping.
It's a simple test to rule it out & often you can see the rubber fried a little with close inspection.

 

[ondarvr]

From my experience you might get 2-3 slip and grab chances, then it's all slip. In this case they tested it quite a bit with poor results, then made the correct changes in loading the boat and no more issues, doesn't sound like slip at all.

 

[Sea Rider]

Bear in mind that hubs don't go bad next day, oldie ones usually starts to have very slow slip issues, with use will eventually fully slip. OTOH OB is not delivering full wot rpm power. With portable OB's have had cases where combo didn't wanted to plane solo boating at all with no over revving whatsoever, hub slip test confirmed being kaputt. Installed one less pitch prop and made the boating day.

Oldie props are overlooked, same as with plugs. Have already the testing procedure, rule it out, work from there.

Happy Boating

 

[ondarvr]

Bear in mind that hubs don't go bad next day,

Happy Boating

They normally do fail quickly.

Once it starts to actually spin on the hub it won't last long, you aren't going to get multiple trips where it slips badly and then it starts working perfect again.