prop time!

[mogfisher]

I have an 89 aquasport 17.5' center console boat. 2006 etec 90. Current prop is 3 blade aluminum 13.25 x17. Top speed at wot and trimmed out is 35 mph and max rpm of 4800. I would like to be operating in the optimal rpm range. These specs are a light load and the boat will often times be loaded heavy with gear. I've got a significant port lean while underway which can be corrected by shifting passengers. No idea if a prop will change this. I've plated with the torque tab but it doesn't help much. Maybe a little. Any thoughts? What prop?

 

[alldodge]

I have an 89 aquasport 17.5' center console boat. 2006 etec 90. Current prop is 3 blade aluminum 13.25 x17. Top speed at wot and trimmed out is 35 mph and max rpm of 4800. I would like to be operating in the optimal rpm range. These specs are a light load and the boat will often times be loaded heavy with gear. I've got a significant port lean while underway which can be corrected by shifting passengers. No idea if a prop will change this. I've plated with the torque tab but it doesn't help much. Maybe a little. Any thoughts? What prop?

You want the best of both worlds. WOT in range and good fuel economy when cruising. You have to give up one thing for another. If the boat comes out of the water OK when it has a heavy load I would leave it alone. If it does not, then I would suggest a 16 pitch ( if you can find one), or a 15 pitch. The issue is the prop will get you up quicker but you will turn more rpms at cruise. The other issue with less pitch is you can over rev at WOT.

 

[steelespike]

I think your tach may not be quite right.It may be off low a little. I do think you need a 15" prop.
Will raise the rpm and improve hole shot and will maintain rpm better with a heavy load.

 

[mogfisher]

Thanks for the input, guys. I guess I was under the assumption that it was not optimal for the motor to not be able to reach the the 5000-5500 mark. If thats not the case then I'll leave it. Im perfectly happy with the way the boat drives. Even with a huge load it planes reasonably. I have read that the boat should be in the 5000-5500 rpm range at WOT. Not necessary?

 

[mogfisher]

I think your tach may not be quite right.It may be off low a little. I do think you need a 15" prop.
Will raise the rpm and improve hole shot and will maintain rpm better with a heavy load.

Why do you think my tach is off? It's certainly possible. Been thinking about an Icommand gauge or a NMEA 2000 chartplotter. That should give me an accurate tach.

 

[steelespike]

Why do you think my tach is off? It's certainly possible. Been thinking about an Icommand gauge or a NMEA 2000 chartplotter. That should give me an accurate tach.

Your slip calculation is 9% seems a little low especially if the rpm is really 4800. Typical slip at wot on a boat like yours would be in the low double digits.
as rpm decreases slip usually goes up your rpm is kind of low yet your slip is 9%.
Thus I'm thinking rpm may actually a littler higher.But not a huge amount.Just to keep in mind.

 

[alldodge]

Why do you think my tach is off? It's certainly possible. Been thinking about an Icommand gauge or a NMEA 2000 chartplotter. That should give me an accurate tach.

What's your drive gear ratio ?

 

[steelespike]

I believe his gear ratio is 2.00.

 

[steelespike]

Thanks for the input, guys. I guess I was under the assumption that it was not optimal for the motor to not be able to reach the the 5000-5500 mark. If thats not the case then I'll leave it. Im perfectly happy with the way the boat drives. Even with a huge load it planes reasonably. I have read that the boat should be in the 5000-5500 rpm range at WOT. Not necessary?

The rpm range at the Etec site is 4500-5500 At 4800 light load I think is too low . As you start adding weight it could be dropping out of range.
Its a little worrisome that it seems the 17 is a ltitle too much prop holding you back a little. I think a 15 might get you to about 5200.
If you frequently run with a heavy load I think it will feel stronger with a 15.Not saying your in danger straining the motor It just will be easier for the motor to push.
Do you know the dry weight? Is it a deepV about 18-20 degrees at the transom?

 

[mogfisher]

Hey guys. Thanks for the info so far. I tried a 15 pitch and it still didn't get me there. I'm headed out tomorrow for more testing. I'm going to video the anti ventilation plate to see if my motor is at proper height. I'm also bringing a laptop with etec software to verify rpms. I'll report back my findings. It interesting that the 15 didn't make much top rpm difference but did improve hole shot. I've verified compression at 130 131 130 and fuel pressures look good.

 

[mogfisher]

Don't know the weight or angles but I can find out.

 

[steelespike]

I believe the 17 Osprey is 1150 dry weight actually both 17s I found were 1150.
I like the 15 over the 17. It should be easier to keep on plane at reduced throttle, faster hole shot.
If if a boat gets up quicker and can stay there at less throttle it could be a tiny fuel savings.
and rpm should hold up better as you add weight. You'll have to test to see which gives the better mileage.
If your really serious about mileage there are flow meters that give real time mpg.

 

[mogfisher]

Actually to be honest fuel burn isn't a problem. At least at this time. In 3 trips to my island house in the bay with a decent amount of fishing trips and trolling I only used 12 gallons. I'd rather burn it and keep it fresh in the tank rather than let it get nasty. Thanks for the info on the boat!

 

[steelespike]

The direct injection motors do get noticeably better mileage over a carbed motor.

 

[mogfisher]

Some things I confirmed tonight. Motor is definitely too low. Vent plate was at least 1-2" under water while on plane. Also. My tach is dead accurate as confirmed by brp software. So....since there is zero adjustment on this motor the way the transom is drilled (and cannot be correctly drilled) the only solution to my height problem is a jack plate. I was able to hit 4867 rpm before the boat began to porpoise like mad. I'm hoping the motor height is causing the porpoise. I can't get much trim out of it at all. If it's flat water it's OK but the slightest bump and it's a bouncy ride. Does the vent plate catch the water and cause a porpoise effect? That would make sense to me.

 

[funk6294]

Have you put a straight edge along the bottom of the bot to the cav plate to determine the distance between the two? Having the vent plate too low certainly will make the porpoising worse. I went through this on a Boston Whaler and by raising the motor up so the cav plate was about 1 inch above the bottom of the hull I had more usable trim, less bow rise while coming in a plane and better top end.

 

[mogfisher]

I did do that. The plate was about 1/2" above the keel. Leary it's not enough though as the plate should be above the water on plane and it's quite a bit submerged. I'm hoping that proper engine height will fix a few issues. I just wish I could do it without a jack plate but with this motor on this transom there is only one place to mount it. And it's wrong.

 

[steelespike]

Aren't there a choice of holes on the mount bracket you could raise it one hole at a time and test.
Shouldn't require a change in the transom holes.I've seen mounts with the bracket lip 2" above the transom.
Plate shouldn't be above water but barely visible under water.This may vary based on the boat design.

 

[mogfisher]

Yes there are holes but...Problem is that there is a bolt in the top hole of the bracket and at the bottom of the slot on the bottom of the bracket leaving zero adjustment as it sits. I'd love to just redrill the transom properly, but that has it's own challenges. The lower bolt as it sits is just barely below the floor in the bilge. Where the hole needs to be is exactly where the floor of the boat meets the transom so I cannot re drill this transom to allow adjustability of the motor.