Effect of Engine Mounting Height on Propeller Cavitation

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Author Topic:   Effect of Engine Mounting Height on Propeller Cavitation

prostock posted 05-22-2007 08:30 PM ET (US)
Have a (new!) 1979 Revenge Walk-Around with a Suzuki DT225 for power. When cornering to port, I am getting some cavitation. Motor is mounted about 2-inches above the transom.. Would lowering help eliminate the cavitation?
Tom W Clark posted 05-23-2007 09:33 AM ET (US)
No, what you are experiencing is VENTILATION. not CAVITATION.
Yes, lowering your motor *may* help but so may a better propeller, Let's see what you've got because I am confused.
There is no such thing as a 1979 Revenge Walk-Around. What boat do you have? Length? Model? In 1979 there was only the old banana hulled Revenge 19 and Revenge 21, neither of which would reasonably accommodate a Suzuki DT225.
Speaking of which, what year is the Suzuki? What is the shaft length? Exactly what propeller in on it now?
prostock posted 05-23-2007 05:54 PM ET (US)
Title says 79, but it must be a 1980 V-22 from looking at photo's on the site. 92 Suzuki Dt225 engine has 25" shaft and I will check the prop when I pull from water tonight.
Tom W Clark posted 05-23-2007 11:24 PM ET (US)
OK, you have a Revenge 22. That makes sense. A Suzuki DT225 would be a nice fit for that boat and the 25" shaft length is correct.
Normally the motor would be mounted one hole up on the transom (34"-7/8") Some propellers will support much higher mounting heights which is why I asked.
Part number for the prop would be best.
Tom W Clark posted 05-24-2007 12:13 AM ET (US)
...and to clarify: There *was* another Revenge model offered in 1979, the Revenge V-22 (aka Revenge 22). This model was introduced in 1979 as a 1979 model.
The 1979 Revenge 22 is NOT a Walk Around model. There has never been a classic 22 Walk Around as there has has been a Revenge 25 Walk Around.
prostock posted 05-24-2007 10:51 AM ET (US)
Ok...prop is a Suzuki 3 blade, 14 1/4 x18. Will be using as a commuter in Puget Sound so any suggestions are appreciated.
That makes sense on the year as what I can see on the plate looks like V-22. When I get it cleaned up the details will become clearer.
prostock posted 05-24-2007 10:56 AM ET (US)
Part No: 99105-00200-18P ... Stainless.
Tom W Clark posted 05-24-2007 02:55 PM ET (US)
Eric,
Do you know what engine speed (RPM) you see if you full throttle? How fast does your boat goes if you give it all its got?
prostock posted 05-25-2007 11:20 AM ET (US)
WOT is about 5500 and tops out at 40mph
jandrewg posted 05-26-2007 01:57 PM ET (US)
Just my two cents here, but yes, either lowering the engine or changing the prop could help here.
I don't know the WOT rating of your Sukuki, but the rule of thumb that I've found to be very close is this- raising the motor an inch will generally give an additional 2 mph and 200 rpm, with the reverse being true as well. You've raised it too much when the prop loses bite while turning. This is why the go-fast guys use a jack plate that will give the extra speed when running straight and the motor is raised, and can be lowered when cornering or in rough chop.
Also, adding an inch of pitch to the prop will lower rpm about 200 rpm or so. More pitch will often increase the tendancy to cavitate, less will have the opposite effect.
jandrewg posted 05-26-2007 02:01 PM ET (US)
You might also help your problem with a 4 blade prop. Expensive vs. lowering the motor, but they bite far better, and I believe are less prone to "ventilate" or "cavitate".
prm1177 posted 05-26-2007 02:36 PM ET (US)
My 2 cents here, as I spend a bit of time experimenting on my Outrage.
Lowering your engine will reduce the likelihood of ventilation, but not eliminate it in rough waters. It will also reduce your trim range and reduce your time to plane.
Moving to a 4 blade will give you better "bite" in rough water as well as better thrust and maneuverability at lower speeds. The disadvantage as was pointed out is a higher cost.
If your ventilation issue is only in rough water and the boat performs well in calm water and doesn't blow out in turns, I'd go with the 4 blade option if you can afford it. If you do have calm water ventilation issues, drop the engine one notch.

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Author	Topic: Effect of Engine Mounting Height on Propeller Cavitation
prostock	posted 05-22-2007 08:30 PM ET (US) Have a (new!) 1979 Revenge Walk-Around with a Suzuki DT225 for power. When cornering to port, I am getting some cavitation. Motor is mounted about 2-inches above the transom.. Would lowering help eliminate the cavitation?
Tom W Clark	posted 05-23-2007 09:33 AM ET (US) No, what you are experiencing is VENTILATION. not CAVITATION. Yes, lowering your motor may help but so may a better propeller, Let's see what you've got because I am confused. There is no such thing as a 1979 Revenge Walk-Around. What boat do you have? Length? Model? In 1979 there was only the old banana hulled Revenge 19 and Revenge 21, neither of which would reasonably accommodate a Suzuki DT225. Speaking of which, what year is the Suzuki? What is the shaft length? Exactly what propeller in on it now?
prostock	posted 05-23-2007 05:54 PM ET (US) Title says 79, but it must be a 1980 V-22 from looking at photo's on the site. 92 Suzuki Dt225 engine has 25" shaft and I will check the prop when I pull from water tonight.
Tom W Clark	posted 05-23-2007 11:24 PM ET (US) OK, you have a Revenge 22. That makes sense. A Suzuki DT225 would be a nice fit for that boat and the 25" shaft length is correct. Normally the motor would be mounted one hole up on the transom (34"-7/8") Some propellers will support much higher mounting heights which is why I asked. Part number for the prop would be best.
Tom W Clark	posted 05-24-2007 12:13 AM ET (US) ...and to clarify: There was another Revenge model offered in 1979, the Revenge V-22 (aka Revenge 22). This model was introduced in 1979 as a 1979 model. The 1979 Revenge 22 is NOT a Walk Around model. There has never been a classic 22 Walk Around as there has has been a Revenge 25 Walk Around.
prostock	posted 05-24-2007 10:51 AM ET (US) Ok...prop is a Suzuki 3 blade, 14 1/4 x18. Will be using as a commuter in Puget Sound so any suggestions are appreciated. That makes sense on the year as what I can see on the plate looks like V-22. When I get it cleaned up the details will become clearer.
prostock	posted 05-24-2007 10:56 AM ET (US) Part No: 99105-00200-18P ... Stainless.
Tom W Clark	posted 05-24-2007 02:55 PM ET (US) Eric, Do you know what engine speed (RPM) you see if you full throttle? How fast does your boat goes if you give it all its got?
prostock	posted 05-25-2007 11:20 AM ET (US) WOT is about 5500 and tops out at 40mph
jandrewg	posted 05-26-2007 01:57 PM ET (US) Just my two cents here, but yes, either lowering the engine or changing the prop could help here. I don't know the WOT rating of your Sukuki, but the rule of thumb that I've found to be very close is this- raising the motor an inch will generally give an additional 2 mph and 200 rpm, with the reverse being true as well. You've raised it too much when the prop loses bite while turning. This is why the go-fast guys use a jack plate that will give the extra speed when running straight and the motor is raised, and can be lowered when cornering or in rough chop. Also, adding an inch of pitch to the prop will lower rpm about 200 rpm or so. More pitch will often increase the tendancy to cavitate, less will have the opposite effect.
jandrewg	posted 05-26-2007 02:01 PM ET (US) You might also help your problem with a 4 blade prop. Expensive vs. lowering the motor, but they bite far better, and I believe are less prone to "ventilate" or "cavitate".
prm1177	posted 05-26-2007 02:36 PM ET (US) My 2 cents here, as I spend a bit of time experimenting on my Outrage. Lowering your engine will reduce the likelihood of ventilation, but not eliminate it in rough waters. It will also reduce your trim range and reduce your time to plane. Moving to a 4 blade will give you better "bite" in rough water as well as better thrust and maneuverability at lower speeds. The disadvantage as was pointed out is a higher cost. If your ventilation issue is only in rough water and the boat performs well in calm water and doesn't blow out in turns, I'd go with the 4 blade option if you can afford it. If you do have calm water ventilation issues, drop the engine one notch.