Raised Engine Off-Plane Performance - Moderated Discussion Areas

  ContinuousWave
  Whaler
  Moderated Discussion Areas
  ContinuousWave: Whaler Performance
  Raised Engine Off-Plane Performance

search | FAQ | profile | register | author help

Author Topic:   Raised Engine Off-Plane Performance

NJCoastFlyFish posted 06-02-2007 05:59 PM ET (US)
If an engine's anti-ventilation plate is raised 4-inches to 5-inched above the bottom of the boat, how is the boat's performance affected when it is not on plane? Say going 5-MPH?
I understand that jacking an engine up may help while on plane, but wouldn't it make it difficult for the engine while not on plane?
Binkie posted 06-03-2007 07:57 AM ET (US)
Jacking the motor up so the cavitation plate is 4-5" above the bottom is way too high, and the water intake will be clear of the water. About 1" is right. On my raceboat the cavitation plate is about 4" above the bottom and the nosecone is 1" below the bottom, but the water intake is on the bottom of the nosecone. Almost 1/2 of the prop is clear of the water when on plane. It idles along fine, but will cavitate when coming on plane, so I have to feather the throttle until the prop bites, and away I go. Not recommended for a pleasure boat.
Rich
Sal DiMercurio posted 06-03-2007 09:30 AM ET (US)
It has little to no effect at idle. The engine would have to be on at least a 5-inch setback jack plate, in order for it to function correctly, when mounted 4-inches to 5-inches above the bottom. The farther back, e.g., a 10-inch setback, the higher you can raise the engine, as the water coming off the bottom of the boat, raises up above the bottom the farther back you go.
Sal
jimh posted 06-03-2007 09:54 AM ET (US)
The performance at 5-MPH will probably not be optimized. On the other hand, if you cared to optimize the boat for best performance at 5-MPH you would probably have a much different set up.
NJCoastFlyFish posted 06-03-2007 11:27 AM ET (US)
Well I am in a fairly tricky situation with a lot of give and takes.
My suzuki will run better with a 6inch setback plate but the engine is already heavy on my 16ft hull and I imagine mounting it back another 6 inches will not help the static trim but it will help gas consumption and top and cruise speeds as per other reports on this site.
Also my boat is on a lagoon that is about a 20 minute ride to the bay, so for the first 20 minutes of each trip I am at 5 mph and having a raised engine hurting itself doing 5 mph is not good for 40minutes of each trip.
I know bigshot has his suzuki up about 5 inches if I read correctly swinging an 18inch prop and doing 39mph. This is where I would like to be but I think that more set back will hurt my boat more than help it and I might have to live with the less fuel economy and lowered speed.
Let me sleep on it before I buy the CMC unit and ask the dealer to re mount the engine-
Decisions decisions
Sal DiMercurio posted 06-03-2007 11:56 AM ET (US)
I don't see why a raised engine will hurt itself at idle for 20 minutes.
The engine dosen't know if it's raised, or sitting right down on the transom while at idle, all it knows is it's turning the prop at XXX rpms with very little effort.
The Suzuki is a 4 stroke engine & should be able to idle all day with no problems.
Even the 2 stroke DFI engines can idle 20 minutes or 20 miles [ as I do with my DFI almost everyday, while trolling for Salmon or Stripers ] with little to no adverse effect.
How do you figure your hurting that engine?
Sal
NJCoastFlyFish posted 06-03-2007 02:41 PM ET (US)
Well the engine would probably have to turn more rpms to reach the same speed when raised on the transom while not on plane because it would be hard to find water. I guess it all depends on how far the set back is though and with my boat I can't set it back all that much
Binkie posted 06-03-2007 03:25 PM ET (US)
I don`t remember you mentioning your motor was on a setback bracket. There is a formula to tell you how much you can jack it up for each inch setback. I would just go with that. When not on plane or just idleing along, as long as the engine is getting cooling water its happy, If it ventilates coming on plane you need to look for another prop.
Rich
Sal DiMercurio posted 06-03-2007 05:49 PM ET (US)
I think your confused about the engine having to turn more rpms etc.
There's absolutly no reason why the engine would have to turn more rpms because of the jack plate.
The jack plate just gives you a bit of an edge when at speed.
It allows the prop [ if you don't change it ] to spin up more rpms & it allows the prop to run in cleaned, un-disturbed water farther in back of the transom.
The boat is settled down in the water at idle, why wouldn't it have plenty of water?
Even if the boat is on plane, the prop [ if the jack plate is set up right ] will have all the water it needs.
Sal
Teak Oil posted 06-03-2007 08:36 PM ET (US)
Sal is right, having the engine raised a couple inches will have no noticeable effect on your boat at idle whatsoever. You are worrying about nothing.
Tohsgib posted 06-04-2007 12:28 PM ET (US)
You have more to worry about than jack plates. You better find out why a 18" SS prop is only turning 4800rpm's.
crabby posted 06-04-2007 07:52 PM ET (US)
So long as you are pumping water through the motor it should have no ill effect. So what if you need to run a few more rev's to accomplish the same speed? Make certain you give a shot of reverse after you clear the lagoon before you put it on plane to make certain you have cleared any weeds or other blockages you may have picked up at low speed.
On a side note, my 90 hp etec can't hold a candle to my old 70 hp Evinrude when running slow with the motor tilted up in the shallows, either in forward or reverse. I can't explain it but the old 70 would idle through the muck (and go backwards through it) much nicer than the 90 does. Once the 90 is trimmed down onto the trim cylinders there is no issue, just when the motor is kicked up back in the flats with half the prop showing...
--pm
NJCoastFlyFish posted 06-07-2007 10:33 AM ET (US)
Tohsgib- I don't know how or why the 18in was only turning 4800 because yesterday I was out with two people on board and a 17in alum prop and turned 5500, a drop in 700rpm with 1in of pitch makes no sense. AND the mechanic who ran the boat and got those numbers was alone while on that test but it was in the motors first hour so maybe it needed to loosen up?
Well I guess in my mind a jacked up motor off plane seemed like it wouldn't work but I am mistaken. I am still debating whether I want to mess around with the trim of the boat at rest even more by placing the engine 6inched back.
I am trying to remember some college physics and torques law here to apply weight to the back of my boat in pounds to see what it would potentially look like if the engine was back 6 more inches than decide if it is worth the extra mph.

Hop to:

Contact Us | RETURN to ContinuousWave Top Page

Powered by: Ultimate Bulletin Board, Freeware Version 2000
Purchase our Licensed Version- which adds many more features!
© Infopop Corporation (formerly Madrona Park, Inc.), 1998 - 2000.

Author	Topic: Raised Engine Off-Plane Performance
NJCoastFlyFish	posted 06-02-2007 05:59 PM ET (US) If an engine's anti-ventilation plate is raised 4-inches to 5-inched above the bottom of the boat, how is the boat's performance affected when it is not on plane? Say going 5-MPH? I understand that jacking an engine up may help while on plane, but wouldn't it make it difficult for the engine while not on plane?
Binkie	posted 06-03-2007 07:57 AM ET (US) Jacking the motor up so the cavitation plate is 4-5" above the bottom is way too high, and the water intake will be clear of the water. About 1" is right. On my raceboat the cavitation plate is about 4" above the bottom and the nosecone is 1" below the bottom, but the water intake is on the bottom of the nosecone. Almost 1/2 of the prop is clear of the water when on plane. It idles along fine, but will cavitate when coming on plane, so I have to feather the throttle until the prop bites, and away I go. Not recommended for a pleasure boat. Rich
Sal DiMercurio	posted 06-03-2007 09:30 AM ET (US) It has little to no effect at idle. The engine would have to be on at least a 5-inch setback jack plate, in order for it to function correctly, when mounted 4-inches to 5-inches above the bottom. The farther back, e.g., a 10-inch setback, the higher you can raise the engine, as the water coming off the bottom of the boat, raises up above the bottom the farther back you go. Sal
jimh	posted 06-03-2007 09:54 AM ET (US) The performance at 5-MPH will probably not be optimized. On the other hand, if you cared to optimize the boat for best performance at 5-MPH you would probably have a much different set up.
NJCoastFlyFish	posted 06-03-2007 11:27 AM ET (US) Well I am in a fairly tricky situation with a lot of give and takes. My suzuki will run better with a 6inch setback plate but the engine is already heavy on my 16ft hull and I imagine mounting it back another 6 inches will not help the static trim but it will help gas consumption and top and cruise speeds as per other reports on this site. Also my boat is on a lagoon that is about a 20 minute ride to the bay, so for the first 20 minutes of each trip I am at 5 mph and having a raised engine hurting itself doing 5 mph is not good for 40minutes of each trip. I know bigshot has his suzuki up about 5 inches if I read correctly swinging an 18inch prop and doing 39mph. This is where I would like to be but I think that more set back will hurt my boat more than help it and I might have to live with the less fuel economy and lowered speed. Let me sleep on it before I buy the CMC unit and ask the dealer to re mount the engine- Decisions decisions
Sal DiMercurio	posted 06-03-2007 11:56 AM ET (US) I don't see why a raised engine will hurt itself at idle for 20 minutes. The engine dosen't know if it's raised, or sitting right down on the transom while at idle, all it knows is it's turning the prop at XXX rpms with very little effort. The Suzuki is a 4 stroke engine & should be able to idle all day with no problems. Even the 2 stroke DFI engines can idle 20 minutes or 20 miles [ as I do with my DFI almost everyday, while trolling for Salmon or Stripers ] with little to no adverse effect. How do you figure your hurting that engine? Sal
NJCoastFlyFish	posted 06-03-2007 02:41 PM ET (US) Well the engine would probably have to turn more rpms to reach the same speed when raised on the transom while not on plane because it would be hard to find water. I guess it all depends on how far the set back is though and with my boat I can't set it back all that much
Binkie	posted 06-03-2007 03:25 PM ET (US) I don`t remember you mentioning your motor was on a setback bracket. There is a formula to tell you how much you can jack it up for each inch setback. I would just go with that. When not on plane or just idleing along, as long as the engine is getting cooling water its happy, If it ventilates coming on plane you need to look for another prop. Rich
Sal DiMercurio	posted 06-03-2007 05:49 PM ET (US) I think your confused about the engine having to turn more rpms etc. There's absolutly no reason why the engine would have to turn more rpms because of the jack plate. The jack plate just gives you a bit of an edge when at speed. It allows the prop [ if you don't change it ] to spin up more rpms & it allows the prop to run in cleaned, un-disturbed water farther in back of the transom. The boat is settled down in the water at idle, why wouldn't it have plenty of water? Even if the boat is on plane, the prop [ if the jack plate is set up right ] will have all the water it needs. Sal
Teak Oil	posted 06-03-2007 08:36 PM ET (US) Sal is right, having the engine raised a couple inches will have no noticeable effect on your boat at idle whatsoever. You are worrying about nothing.
Tohsgib	posted 06-04-2007 12:28 PM ET (US) You have more to worry about than jack plates. You better find out why a 18" SS prop is only turning 4800rpm's.
crabby	posted 06-04-2007 07:52 PM ET (US) So long as you are pumping water through the motor it should have no ill effect. So what if you need to run a few more rev's to accomplish the same speed? Make certain you give a shot of reverse after you clear the lagoon before you put it on plane to make certain you have cleared any weeds or other blockages you may have picked up at low speed. On a side note, my 90 hp etec can't hold a candle to my old 70 hp Evinrude when running slow with the motor tilted up in the shallows, either in forward or reverse. I can't explain it but the old 70 would idle through the muck (and go backwards through it) much nicer than the 90 does. Once the 90 is trimmed down onto the trim cylinders there is no issue, just when the motor is kicked up back in the flats with half the prop showing... --pm
NJCoastFlyFish	posted 06-07-2007 10:33 AM ET (US) Tohsgib- I don't know how or why the 18in was only turning 4800 because yesterday I was out with two people on board and a 17in alum prop and turned 5500, a drop in 700rpm with 1in of pitch makes no sense. AND the mechanic who ran the boat and got those numbers was alone while on that test but it was in the motors first hour so maybe it needed to loosen up? Well I guess in my mind a jacked up motor off plane seemed like it wouldn't work but I am mistaken. I am still debating whether I want to mess around with the trim of the boat at rest even more by placing the engine 6inched back. I am trying to remember some college physics and torques law here to apply weight to the back of my boat in pounds to see what it would potentially look like if the engine was back 6 more inches than decide if it is worth the extra mph.