motor height

[lexkyboater]

Alright, I know this has been discussed to death, but I couldn't find any specifics on my setup. The most applicable post I found was http://forums.iboats.com/showthread.php?t=203309&highlight=motor+height, and while it mentioned starting with A/V plate 1 to 2 inches above the bottom of the boat, my 93 Yamaha 115 is more like 3 to 4 inches above the bottom of the "v" part of my boat. Haven't checked rpm yet, and haven't seen where the A/V plate is at plane speed. But, have confirmed the water pump seems to be pumping water okay at speed, and the prop cavitates in a mild turn.

When I ran it WOT yesterday, rpms didn't seem high, but speed gauge said 43 mph, which I'm sure is wrong. Fixed the tach this morning, so I can get exact rpm numbers tonight. Boat is a 1984 15 1/2 foot Cheetah bass boat. The mount holes and engine won't go any lower without a jack plate. It just seems that with this setup, the basic aluminum prop has got to be right at the surface, and I'm probably losing speed due to turbulent water. Anything I can do, or is this setup okay? TIA, Steve

 

[kenmyfam]

Re: motor height

Alright, I know this has been discussed to death, but I couldn't find any specifics on my setup. The most applicable post I found was http://forums.iboats.com/showthread.php?t=203309&highlight=motor+height, and while it mentioned starting with A/V plate 1 to 2 inches above the bottom of the boat, my 93 Yamaha 115 is more like 3 to 4 inches above the bottom of the "v" part of my boat. Haven't checked rpm yet, and haven't seen where the A/V plate is at plane speed. But, have confirmed the water pump seems to be pumping water okay at speed, and the prop cavitates in a mild turn.

When I ran it WOT yesterday, rpms didn't seem high, but speed gauge said 43 mph, which I'm sure is wrong. Fixed the tach this morning, so I can get exact rpm numbers tonight. Boat is a 1984 15 1/2 foot Cheetah bass boat. The mount holes and engine won't go any lower without a jack plate. It just seems that with this setup, the basic aluminum prop has got to be right at the surface, and I'm probably losing speed due to turbulent water. Anything I can do, or is this setup okay? TIA, Steve

Photo would help give a better opinion.

 

[lexkyboater]

Re: motor height

Will get some pictures tonight, but test this morning confirmed 5800 rpm at WOT. Speedo is saying 43 mph. Prop catches air real easy when turning left. Is 5800 rpm too high? Some newer Yamaha's say max rpm is 4500 - 5500, but other sites say 5800 is good.

 

[Bob_VT]

Re: motor height

When I went through my "testing phase" last year ...... I played with the motor height. When I was at max height I found that a tremendous amount of torque stress was on the steering and near impossible to make a left turn at WOT.

I have my av plate about 2" above. My set up is a 115 inline mariner on a 18' aluminum Fisher bass boat. http://forums.iboats.com/showthread.php?t=177235

 

[lexkyboater]

Re: motor height

Thaks for the helpful post reference Bob. That's interesting about the height and the left-handed turns. My boat also pulls to the right pretty bad, but the trim tab is adjusted at the farthest-most tick mark, I believe to the left. If you tilted the motor, and then looked at the trim tab, the top is at about the 11:00 position. Which means that when it's in the water, it's pushing the boat to the left right? Do I need to adjust it more to the left to keep from pulling to the right? See attached pic for reference.

 

[Bob_VT]

Re: motor height

Want more speed? Try this and it should have a noticeable effect. Take off that plastic wing! Give it a shot and you should actually gain speed. It also changes the steering a bit. Nothing to buy.... just remove.

 

[72SideWinderSS]

Re: motor height

If the boat pulls to the right, move the trim tab to the right to induce a left hand thrust.

 

[lexkyboater]

Re: motor height

Here's pics of the mount. I think it made a liar out of me because when I used a level it looks like the A/V plate is just about right at 2 inches above the bottom of the boat. The level is 2 1/16 inches wide and is level at the time of the pic. Can you guys see if that's right? Also, on that stabilizer, I had thought about taking it off but haven't tried it yet. I was thinking that if it's so far out of the water, it didn't matter, but in reality is probably dragging some. If I remove it, won't that leave 4 holes in the A/V plate?

On the trim tab, it's turned to the left pretty far as you can see in the pics, so when you say move to the right, the front is to the right, which seems that it would correct a right pull to the right? So the question is (without researching this), do I move the front farther to the right or move it back more towards center?

 

[BillP]

Re: motor height

Its hard to tell from the pics but if the wing and cavitation is above water level at speed and slight cavitation happens when turning you are at best height for speed. Take a straight edge and run it along the boat bottom and back to the fin...if the fin is higher you are ok. Fins mounted this way will let you run faster, not slower. If the fin is hitting solid water (not propwash) at high speed you are too low and scrubbing speed...which is the typical reason people blame fins for not working. If propped for max rpm without the fin, expect to over-rev with the fin when trimmed up for max speed.

bp

 

[lexkyboater]

Re: motor height

Are you saying to properly check height of A/V plate, the boat should be in the water, and run that level/straight edge along the boat bottom back to the motor? It seems that if it's 2 inches above the boat bottom with it level, the A/V plate and wing would be out of the water, but I need to check that at speed. I guess I should remove the fin and just see if I get more speed. More speed means it was scrubbing water as you put it?

Not sure what you mean it will over-rev with the fin when trimmed up? It seems that the fin would drag water worse when you trim out and cause it to lose rpms? I did trim out quite a bit yesterday when testing and it got dicey and started to porpoise when I pulled it up too much, so backed it back down. Speed and rpm stayed about the same if I remember right so I saw no advantage in trimming up and in fact lost some control. Maybe that means the wing was grabbing water?

 

[steelespike]

Re: motor height

Sounds like the height is about right.If you get a prop with a little cup in it it may help the ventilation.It may lower the rpm as well.You may want to try a couple of props at the dealer.

 

[72SideWinderSS]

Re: motor height

On the trim tab, it's turned to the left pretty far as you can see in the pics, so when you say move to the right, the front is to the right, which seems that it would correct a right pull to the right? So the question is (without researching this), do I move the front farther to the right or move it back more towards center?

If your motor is pulling to the right, then adjust the aft most part of the trim tab to the right.

OBTW: if your A/V Plate is running above the water, then the trim tab and the bat wing have no effect.

 

[BillP]

Re: motor height

Are you saying to properly check height of A/V plate, the boat should be in the water, and run that level/straight edge along the boat bottom back to the motor? It seems that if it's 2 inches above the boat bottom with it level, the A/V plate and wing would be out of the water, but I need to check that at speed. I guess I should remove the fin and just see if I get more speed. More speed means it was scrubbing water as you put it?

Not sure what you mean it will over-rev with the fin when trimmed up? It seems that the fin would drag water worse when you trim out and cause it to lose rpms? I did trim out quite a bit yesterday when testing and it got dicey and started to porpoise when I pulled it up too much, so backed it back down. Speed and rpm stayed about the same if I remember right so I saw no advantage in trimming up and in fact lost some control. Maybe that means the wing was grabbing water?

Fins only need to be in the water at low speeds to help lift the stern...however, at high speed when trimmed up they keep water from defecting away from the prop and let it bite at a higher trim angle. At a point the prop gets too high and ventilates...which increases rpms too much and over revs the engine. A tach tells you rpms but a speedo or gps tells when trimmed too high because the speed drops. Last time I checked my boat with and without the fin it gained 6 mph (32 to 38 mph) with the fin. I'm under propped with the fin at trimmed high top speed and have to pull the throttle back slightly to keep from bouncing off the rev limiter.

 

[lexkyboater]

Re: motor height

Okay, finally got around to adjusting the trim tab and you're absolutely right - moving the aft most part to the right did get the steering to stabilize in the middle. Thanks. The previous owner had the aft most part all the way to the left, and now it's almost all the way to the right. Really odd he didn't fix that. Now with the trim in at speed it tends to go the right and with the trim out it tends to go to the left. I think I've got it mostly right after about 3 adjustments.

As far as that fin, I took it off and the front end pitches (porpoises?) up and down pretty bad at lower speed. I didn't even try it at higher speed since it was dark when I went out and frankly it kind of made me nervous to go faster. The front end was oscillating about 2 1/2 feet or so at about 15 mph, so I gave up and put the fin back on. I honestly don't care how fast it could go if the behavior is like that at lower speeds. I tried it without the fin and with the trim all the way in and almost halfway up, and it still was pitching, so I thought it wasn't satisfactory without the fin. I'm not sure I understand why it would do that though? I would assume at higher plane speeds that fin will be just skimming or out of the water since my motor's so high, but am not real interested in trying if I can't get rid of that pitching at lower speeds.

BTW, my rpms are still running around 5800 per the tach at WOT, and trimming out and in makes no discernable difference in rpm, only you can feel the drag decrease when trimming out. The speed goes up a little, but it gets dicey when I pull the trim out past the half way point, so I usually don't exceed half way out on the trim. It sounds like you guys may be saying with the way it's catching air and the high rpms, I should probably get a different prop? Are you saying aluminum generally doesn't cut it because it doesn't have enough cup?

Thanks for all the feedback, Steve

 

[steelespike]

Re: motor height

As I understand it you can get aluminum props with cup. I don't know how effective they are.Stainless props are really good but expensive and if you hit something they are harder on the gearcase.I wouldn't be surprised if a stainless in the same size would reduce your venting problems and reduce your rpm while maintaining speed.
If you want better info go over to the prop forum with as much info as possible. rpm, speed,type of boat,type of use prop size etc.

 

[lexkyboater]

Re: motor height

Okay, some more questions on this. I've got a built-in 8 inch transom offset, but no jackplate. See attached picture. How does this affect motor height? I was talking to a local dealer about my prop (trying to find a prop shop to identify the pitch) and he said new bass boats with a built in offset and a jack plate run the bullet even with the bottom of the hull, which really surprised me.

I didn't really think about this before, but seems that this built in offset would change things so that I could let the motor run higher as opposed to a flat transomed boat? My prop isn't great, also attached, as it has no cupping, and is a rather large 13 1/2 inch diameter. It ventilates badly when turning, especially to the left.

I'm heading down to Proper Props in Bronston Kentucky this weekend (they're at 606-561-6005 if that helps anyone). They're a prop shop and have hundreds of used props so I can also try some out and get some recommendations on that too.

 

[lexkyboater]

Re: motor height

Can anyone comment on this? Since the motor is a standard length, C115TLRR, and it's already at the bottom-most mounting hole, that seems wrong to me. It seems that I've got the motor at the lowest possible position that the manufacturers intended, which means I'm dragging. Do I just need to get someone to drive and check it to see if the A/V plate is under water?

 

[bigtwin]

Re: motor height

My brother has a 17' Crestliner with a Yamaha F115, when the dealer rigged the boat they had the motor mounted in the lowest position (A/V plate about an inch above the bottom of the hull) the stock aluminum prop would ventilate when trimmed and in turns, after switching to a Turbo II stainless steel prop. we were able to raise the motor 2-3 more inches and gained 4-5 mph with no ventilation.
I think you need to find the correct stainless steel prop first and then adjust the motor height, a good stainless prop. (like the Turbo II) will bite alot beter at higher mounting heights then an aluminum prop.
Good luck!

 

[Silvertip]

Re: motor height

You are porpoising 2 - 2-1/2 feet at 15 MPH???? You are barely on plane at 15 mph. Are you trimmed out or in at that speed.

 

[lexkyboater]

Re: motor height

I agree on the prop. I ended up going to the prop shop and getting then to put some cup in the prop, and a little extra rake/roll, and put some ventilation holes in it. The prop doesn't ventilate anymore when taking off, but still does on turns. They said 4 blade will help that too. Rpms dropped from 6800 to 6700 and speed is about the same. Probably should have invested instead on a new stainless prop.

Hey Slivertip, porpoising at about 15 mph is right and tried trim all the way in up to 1/3 out, still pitches up and down badly. Seems odd.