Sea Ranger w/Honda BF150 Cavitation or Airation problem

[finfancy]

Running at 35 to 45 RPM gettting cavitation from engine.  Boat slows down when pushed to higher rpms.  Propeller pitch is 15.25 x 15 local Arima dealer suggested replace with 17 or 18x15.  Has anyone had a similar problem or experience and how was your problem solved?

[Threeweight]

With the motor trimmed all the way down, where does the capitation plate on the lower unit sit in relation to the center bottom of the hull?  Above or below it?  How old is the prop, and do you know if it ever hit anything?

With the throttle pegged all the way down, what RPM's does the engine reach?

Usually props are stamped with the diameter first, then pitch second.  You probably don't want to go up in diameter. 

[finfancy]

The plate is about half inch above the lowest part of the boat.  The prop is 2 yr old and has never hit anything. Has about 240 hrs on it.  Honestly have never pushed to limit.  Boat would start to slow down when it reached 3700 to 4000 due to the cavitating.  I had the numbers backwards the suggestion was to to 15x 17 with 4 blades currently have 3 blades.

[Threeweight]

Motor height sounds about right.  I asked about the age/hitting something in case it might have spun the rubber brake on the hub.

From what you describe, going up to a taller pitched prop could help.  If 15x15 now, 15x17 could work.  It would be helpful to know what it's maximum RPM's are with the current prop.  Ideally, you want a boat to be able to reach close to it's maximum recommended RPM's with a typical load (not that you have to run it there all the time, just that it is a good indication that the prop it has takes full advantage of the engine's potential power).  Each 1" in pitch you go up, will drop your RPM's by roughly 200.  If your motor is recommended to hit 6000 as it's maximum RPM's, I'd want a prop that could hit 5700-6000 wide open with my typical load.

Going to a 4 blade may help, but probably not as much as pitch.  The extra blade gives you more "bite" in the water, and more low speed thrust, at the cost of some fuel economy and top speed.  A 4 blade prop is typically 200 RPM less than the same pitch/diameter prop in a 3 blade.  I'm a fan of the cheap Solas Amita 4 blade aluminum props for Arima hulls.

[headduck]

Did it work fine and then all of a sudden did not?

[AP]

Are you sure it is trimmed down enough?  What hole is it currently set on?

I'd push the throttle all the way down just to be sure it isn't engine related.  if you have a cavitation/ventilation issue, you should VERY quickly hit the rev limiter for whatever that Honda is (6000rpm-ish?).  At least you'd know that the engine is OK.

[Markshoreline]

Threeweight I'm curious how increasing pitch would reduce cavitation???  What say ye?

[Threeweight]

Brook/Grizzle can explain the physics better than I can, but my understanding is that in a situation where the prop can't move enough water for the load it is under (torque being applied to the prop shaft vs. inertia or drag of the hull it is trying to move), it can start to cavitate by causing water vapor bubbles to form around the prop (like a giant high speed egg beater). 

Basically, a low pitched prop moves less water per revolution, but revs up higher and quicker for better acceleration... but it may cavitate if it can't move enough water for the load it's under.  A prop with a higher pitch moves more water, but won't rev as high or as quickly, and may lug the engine.

[Full-Pull]

The prop is 2 yr old and has never hit anything. Has about 240 hrs on it.  Honestly have never pushed to limit.  Boat would start to slow down when it reached 3700 to 4000 due to the cavitating.

Did it work fine and then all of a sudden did not?

I have the same question as headduck...did this issue appear suddenly, or has this been a chronic condition for an extended period?

If it is a NEW issue, I'd suspect a running problem in the engine. I've recently had a 'similar' issue, after the boat was stored for 2+ years.
I.e. engine was running fine (5800@ WOT); we idled down to drop crab pots, then it would not take additional throttle above 4000-4200 RPM. Stumbling and lean-popping.
Ran OK below that, but still felt a bit sluggish 'out of the hole'.
I strongly suspected a fuel-feed and/or fuel-contamination issue, so I cleaned/replaced all fuel filters, lines, VST tank, etc. No signs of water in fuel samples, and no faults indicated in ECU scan.
I haven't sea-trialed yet, but fingers crossed that it's been resolved. If not, it's time to pull & clean the injectors.

I think my boat is punishing me for the long layoff...no, wait...actually, it's the gasoline formulators doing the punishing!

If this HAS been an ongoing, chronic problem (you mentioned 2 yrs/240 hrs of service), then yeah, I'd agree that you need to re-prop. :twocents:
Good Luck.
.

[finfancy]

Many are not understanding our situation.  Until this summer we have only been inshore fishing.  Never having to use higher rpsm.  Basically trolling close to shore all the time and in chop water so never going over 18 mph.  This summer started fishing for tuna in flat water and noticed all the boats passing us by.  We tried to catch up with them but couldn't because of this happening.  The engine runs good up to 3700 rpm or about 18 mph.  Then increasing rpm the boat slows down, so I never pushed the throttle down all the way.  The engine is running good.  It is well maintained; oil changes, rinsing every use, gas stabilizers, etc. Also the prop is in good condition.  No run into anything, no dents, chips, no overheating, and about 250 hrs on it. So as you mentioned the prop could be slipping (kelp wrapped around prop could have damaged gasket) or cavitation due to improper prop pitch.  The motor is mounted on the second hole down and the caitation plate is about 1/2" above the lowest point of the boat.  I read a suggestion on how to check if the prop is slipping.  What do you think?  Mark a straight line on the prop and hub, go full speed for a while.  Pull up the motor is the line is still straight it is not slipping if line is not aligned it is slipping.  Do you have any other suggestions on checking for slippage.  We have to travel 1 hr. to get to a body of water to put the boat in just to do any testing.

[Full-Pull]

Until this summer we have only been inshore fishing.  Never having to use higher rpsm.  Basically trolling close to shore all the time and in chop water so never going over 18 mph.

OK, got it.  I'd say definitely "under-propped", i.e. pitch is too low (think fine-thread screw).
Overall gearing is too low, prop might be breaking out...like a car in mud, spinning its tires in 1st gear...?

By combining known data (factory Arima SR21HT: 2150# dry, 48 gals fuel) with a few assumptions ("Inshore Hull" - "General-Purpose Use" - "1200# Persons & Gear"), the Honda Propeller Selector suggests the following 3-blade props:

* Solas Titan HR3: 15.25" x 17P
* Solas Lexor : 15.5" x 17P

Interesting to play with, here's the link:

http://marine.honda.com/parts/propeller-calculator

.

[headduck]

More questions than answers....

Wouldn't this also be evident in the ability to get on plane?

The pitch and rpms are related as well as top end, but it would seem that the "cavitation" may be more than prop pitch alone..?

Typically aren't we fine-tuning 1000 or less rpm with pitch? He is quite far from that degree of adjustment..? Some folks run 17 or 18 without problem and find problems when heavily loaded..?

Maybe the desire to push harder has uncovered a larger problem..?

[Full-Pull]

Agreed. While the current prop is not optimum, it's not waaay out of the ballpark.

IMHO, it shouldn't be falling on its face at 3,700 - 4,500.

Something else might be at play; maybe all the low-RPM operation is over-cooling the engine and has loaded up the plugs/valves/rings?

.

[Threeweight]

There may be something else causing the problem, but he says his boat and engine are very well maintained have no other issues.  If he had fouled plugs or carbon-fouled valves and rings, the motor would run like crap.

As far as whether the problem he's experiencing could be related to pitch, I sure think it could.  Folks who have problems with props and Arima's are typically propped too tall (someone puts a prop more suitable for a high-speed deep V on go-slow flat butted Sea Chaser or Ranger).  In that case, the problem they usually have is the motor takes a long time to get on plane (because the prop doesn't spin up freely), and they can go fast but don't reach the recommended RPM's for the motor.  We seldom see questions from someone who is propped too low.

I would think what he describes would be pretty typically of a slightly under-propped Arima.  The low pitched prop spins up freely and accelerates the boat just fine, but when the full horsepower of the motor starts to be applied the prop can't move enough water to transfer that power into forward thrust, and instead starts to waste energy exciting the water into water vapor (the egg beater analogy).  When cavitation starts, the more power you apply the worse it gets.

Remember there is no transmission on these motors, they are direct drive.... the position of the throttle and RPM's of the engine is directly related to how much torque is being applied to the prop.  He might have no problem between idle speeds and 18 mph because his engine is only applying 50% or less of it's power at those speeds.   When he tries to apply 75 or 100%, the problem emerges.

It may be that he also has a problem the rake of the prop blades, or their shape, but it sure sounds like he is slightly under propped. 

[First Cabin]

You guys have it pretty well covered, but I will add one more possibility...

Are there any transducers installed near the motor that may be interferring with water flow to the prop at higher speeds?

[AP]

I was thinking the same thing.  Is there ANYTHING that could be introducing air before water even gets to the prop?

I assume you have a tachometer since you know the rpms.  You say the boat "slows down" when you get to 3700 rpms but what does the tach do at that moment.  If it is truly prop cavitation, you should see the rpms spike in a hurry since the prop broke free.  If that doesn't happen, I think you have a motor problem.  What happens if you punch it full throttle from a dead stop?  Does it bog down.  Jump out of the hole?  Immediately cavitate?

The idea you described got testing slippage sounds like a good one.

[Markshoreline]

OK, finfancy said second hole down... aren't there four holes?  I raised my motor up one hole from the third hole down and now have cavitation problems so will move the motor down one notch.
Recommend finfancy do so as well. 

[finfancy]

Thank you everyone for your thoughts and suggestions.  Since my Sea Ranger 21 was purchased and set up (new set up including motor mounting) by Arima factory in Bremerton, WA.  I called Terry;technical support team at Arima yesterday.  Terry and Three weight have the same opinion. He also said that usually Sea Ranger 21 with 150hp will be equipped with 15x17 propeller and he went on to say that some Oregon State tuna fishing Arima Sea Ranger are equipped with a 4 blade prop.  But he was unsure of the pitch.  Terry promised to do more research and will call back after checking our factory specs and this blog and give a more   conclusive answer  For best performance which is best? I will let you know what his answer is when he calls.

[Threeweight]

If you go 4 blade, look for a design that is a stern lifting prop (not a high-speed bow lifting prop).  The shape and rake of the blades are different between them.  A lot of prop dealers assume that if you ask for a prop for a 21' fiberglass walkaround with a 150, you need the same prop they would put on a 21' Striper or Trophy.

I am a fan of the Solas Amita 4 blade aluminum props, but I have no idea what options they offer for your motor.  I would think a 15x17 4 blade would work well (it would just generate about 200 less RPM's than a 3 blade of the same pitch).

[beancounter]

I have the same boat and motor same hole mount. I just bought a solas amita 4 blade 15 pitch. Should be trying it out next week hopefully and will report back.

[finfancy]

Since I haven't heard from Terry at Arima; Question to Beancounter,  I would be anxious to hear the results of your testing.  Question to Threeweight, I am very concerned about gas economy; having a smaller gas tank I have been wanting to go for Tuna fishing I would like to get 100 miles out of my 45 gals in the tank.  I have heard from other owners they get 3-4miles/gal.  I have no clue since we have this cavitation problem.  By the way out latest catch out of San Diego.  Oops ?? can't post pic.

[Threeweight]

3-4 mpg seems quite doable with your motor/boat combo (if everything is working properly).  I typically fish Tuna! out of a 22' Stabicraft aluminum pilot house with twin Honda 90's... she averages 2.5-3 mpg.  Your Arima should be a bit more efficient (a single 150 is more economical than twin 90's, and the hull is lighter and more efficient than the deep-V of the Stabi.)

When I say a 4 blade is less fuel efficient than a 3 blade, that is mainly at higher RPM's.  At the mid-range cruising speeds (4000-4500 RPM) they are pretty comparable.

[Chief of the Boat]

Finfancy,
I will get you some data tomorrow, I am back to three days a week.
Terry

[Chief of the Boat]

Finfancy,
I reached out to our Arima Dealers that rig Hondas their recommendation is a Power Tech 17 pitch 3 bladed stainless, with the motor mounted on the second hold.   As a member of the spent over $800 bucks propping a boat, I wished I would of called Power Tech and got their  :twocents:.  The upside Chasin Baitman got a good deal on a Solas S/S.

[finfancy]

I want to thank everyone for their input of knowledge and experience in helping me to solve my problem.  I wanted to go for the 4 blade Amita as was suggested by Threeweight and I had read elsewhere that it has good stern lift.  I went with the Solas Titan HR3 prop 15.25 x 17 because it was recommended by Honda outboard calculator and it is OEM and oh yes, the price was $250.  As soon as I receive it I will take it for a test run.  Thank you again everyone.