# Ventilation VS Bow Dig



## stevenmurray1987 (Apr 25, 2018)

I feel kinda dumb chasing down a problem that might not even exist.....This outboard is power tilt only, correct?

And if so, is it supposed to be able to lift the motor up while underway (throttle down, moving)? 

Works fine on in idle or with a little throttle, but not strong enough to lift with much power applied.

At first I thought I had power tilt/trim issues, but after some research, looks like this is the way it was supposed to be.


1989 Yamaha 40 HP
2 Stroke, Oil Injected
3 cylinder, 3 carbs

















Of course I don't have a service manual.


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## eshaw (Apr 25, 2018)

Correct


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## stevenmurray1987 (Apr 25, 2018)

Okay thanks lol


Maybe this deserves a new thread, but here is my next issue.



Outboard tilted too far up- cavitation/ventilation during holeshot and turning. But on plane going straight, it performs well.

Outboard tilted too far down- Jumps on plane well, but bow digs pretty bad...water spray, horrible performance.


Basically my holeshot is slow and full of high RPM ventilation. I'm feathering the trim down button until I am up. But if I go too far down, then I'm bow digging. There is only a small sweet spot. And if I miss it, then I have to start over from the beginning. 

I can take some pictures of how the cavitaiton plate sits in relation to the bottom of hull. I will do that on my lunch break. 

I didn't think I mounted the engine too high, but it certainly seems to be the case.

I can think of two options here.

I will probably start with adding a whale tail/hydrofoil. Even though I think they look super dorky.

Other option would be setting the engine back a few inches with a fixed jackplate of some sort.

I just wish I could trim up while on plane and none of this would be an issue.


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## Bateman (Apr 25, 2018)

Have you played with the trim pin at all? Having tilt only may be making it hard for you to hit that sweet spot. Would also be interested in seeing a pic from the back of the boat with the engine lowered to see mounting height. 

Not an expert by any means, but I go out alone pretty often and play with the pin and weight/balance to see what works, what doesn't. Most of the time my optimal setting is good for holeshot and top end, but a good tight turn under power will cause it to blow out. 

I would question prop pitch, but looks like you have the factory 11.25" Yamaha wheel.


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## stevenmurray1987 (Apr 25, 2018)

I'll snap some pictures on my lunch break.

Not that this video is helpful to the conversation, but here is a quick video of me running it yesterday, just for the heck of it. 

https://streamable.com/dul5b


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## stevenmurray1987 (Apr 25, 2018)

Okay here is the motor trimmed (tilted) down, this position results in no ventilation, solid holeshot and quick to get on plane. Downside is the bow digs into the water and top end suffers horribly not to mention all the water spraying up in my face.







Here is the engine tilted up to the point where the bow digging is eliminated and top speed is achieved. Downside here is takes a while to get up on plane b/c it's ventilating the whole time. Also when I turn more than 5 or 10 degrees, it will ventilate then as well.







Couple more pictures for reference. Honestly it looks like it would be in the perfect spot to me, but apparently not. Little disappointing b/c I took extra care to measure everything 20 times when building the transom so it would be flush.


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## Bateman (Apr 25, 2018)

I think it could stand to come down a half inch. Mine is about .25" below flush. Every hull is different. Seems like most have it slightly below flush unless they are trying to go fast. 

Interested to see what some with more knowledge say.


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## DaleH (Apr 25, 2018)

stevenmurray1987 said:


> Here is the engine tilted up to the point where the bow digging is eliminated and top speed is achieved. Also *when I turn more than 5 or 10 degrees, it will ventilate then as well.*


Even on boats with power trim that can happen! FWIW I ran fuel computers on my OB powered tuna boats, with instant fuel performance readout in miles per gallon or MPG. When my trim was set for top speed and/or efficiency, yes ... it (single OB) or they (twin OB rig) can ventilate in a turn. What I would do in your case is:

(1) Try the trim pin as suggested

(2) Add a SE Sport 200 foil/fin. That brand specifically is the best rated in performance and not sacrificing top end, if at all. (Do not get the 'bolt on' plate feature). It will plane fast, eliminate bow rise, and keep you hooked up. Awesome on small boats without power trim. You also may be taxing that tilt system by trying to trim, although I'm sure there's a pressure bypass.

Look, _some ventilation in power turns is to be expected_ with an OB set to the best running' trim angle. But there are steps you can take to help things. Hmmmm, can you add a trim gauge to that puppy? 

That's how I ran/run mine ... even adjusted the indicator angle displayed so that the best "running" attitude was firm on a line on the gauge ... I'd trim IN to turn and trim back OUT (up) to the line whilst running. Worked ... both on my big boats and on the 16' tin I run now.


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## Crazyboat (Apr 25, 2018)

You should be able to trim that engine while underway, it's the whole point of having tilt & trim. I have an F115 and you trim it down to jump on plane or to make a hard turn (tuck the prop under), you trim up while on plane to get best speed/RPM. Trim down some when it's a bit rough to cut through the chop (w V hull).

When trimmed out/up the wake directly behind the engine should flatten out, when trimmed down or in it should be a taller thinner type wake.

PS, when trimming up, trim up until your prop starts to lose grip, trim down just a tick and your golden.


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## Bateman (Apr 25, 2018)

Crazyboat said:


> You should be able to trim that engine while underway, it's the whole point of having tilt & trim. I have an F115 and you trim it down to jump on plane or to make a hard turn (tuck the prop under), you trim up while on plane to get best speed/RPM. Trim down some when it's a bit rough to cut through the chop (w V hull).
> 
> When trimmed out/up the wake directly behind the engine should flatten out, when trimmed down or in it should be a taller thinner type wake.
> 
> PS, when trimming up, trim up until your prop starts to lose grip, trim down just a tick and your golden.




Engine is tilt only. Not trim


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## stevenmurray1987 (Apr 25, 2018)

Crazyboat said:


> You should be able to trim that engine while underway, it's the whole point of having tilt & trim.




Yeah I thought I just had a weak trim setup, but turns out the unit is "tilt only" and was never intended to be able to raise the engine underway.

My local dealer didn't even know the answer, I actually called Yamaha customer service and they confirmed it in addition to a few posts I found online.
Was not easy to find a definitive answer though and apparently they still manufacture "tilt only" units.

My case is somewhat rare though b/c I modified my boat apparently to the point where it caused issues - that I am now trying to make up for. 

So my bad kinda and I'm not complaining or trying to lay blame. I'll get it figured out, sort of fun anyways. Trying to get my PHD in redneck hydrodynamics.


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## DaleH (Apr 25, 2018)

FYI - If you use the TRIM PIN and need a position in-between the 2 factory holes, add a bushing or sleeve over the rod to get it 1/2-way. Note where it would sit on the corresponding engine contact points, that'll help figure out what thickness you my need.


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## Crazyboat (Apr 25, 2018)

Sorry guy, going back to my days with having to use the pin, I'd say 2 things:

#1 just find a happy medium between optimum performance and quick to plane position and live with it.

#2, I've always thought if you mounted an egg shaped or oval steel rod with a handle welded to it, you could increase/decrease your trim while underway, getting the oval shaped thingamagiggy to stay at one setting is the hard part.


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## jethro (Apr 27, 2018)

You may think they look bad, but a hydrofoil is what you want. My 14' with 25hp manual tilt is basically useless without it. You'll holeshot better, you'll cavitate less and it will handle chop better. The only downfall is they certainly do rob a few MPH off the top end. Other than that small disadvantage it's a godsend for me.


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## DaleH (Apr 27, 2018)

jethro said:


> The only downfall is they certainly do rob a few MPH off the top end. Other than that small disadvantage it's a godsend for me.


Agreed. Note in testing .... the SE Sport foils/fins lost the least off the top end. I never run up there (WOT) much anyway (maybe 15-mins per trip on the way back in, _just to let her rip!_), so big whoop ....


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## turbotodd (Apr 29, 2018)

In the dozen or so tin's (and one 'glass) boat, I've found one use for a hydrofoil. On the glass boat, the transom was junk and I wasn't spending the money to fix it, so I attached some thick aluminum to it and mounted the motor. Just used the Aluminum as a "stiffener". The weight of the aluminum + 3 batteries + all my junk in the storage boxes in the aft of the hull put the motor DEEP at low speeds, such that a holeshot was next to impossible. It would stand the bow up so high that I could not see over it, period, and if the GF was with me, it wouldn't even break over onto plane. Once I had her run forward and once it broke over, it was good to go...48 mph...so I put a hydrofoil on it. The problem SOLVED, but it also lost 3 mph. Raised engine one more hole on the transom and got my MPH back but it also affected the holeshot every so slightly.

If you're blowing out, the prop's getting air (or exhaust) to it. That can be caused from a lot of things, USUALLY from a hull that's causing a lot of turbulence, a bad prop, perhaps a spun hub? Is there any sort of a transducer mounted within 12" of the prop? Especially directly in front of the prop. How about a livewell drain? I have seen a pair of drains cause some turbulence too...get on plane, worked fine for a little while, and as soon as they was drained, it was pulling air into the water stream causing some ventilation. They were directly in front of the prop, exiting the transom just below the 2 intakes (one for each well). Have you tried a different prop yet? If the hull's got a bunch of imperfections near the edge, it'll not help matters any. Ideally you want a smooth transition between bottom and transom, with few (or no) dings, big dents, etc. in the last 3 feet of hull. I had one that was hooked (big dent from the bottom up) about 3", and roughly a foot in front of the transom and it would also blow out easily. Sometimes a LITTLE hook is beneficial but you have to put a LITTLE in at a time. Rocker (bent from the top downward) will cause issues too but not usually ventilation/blowout. 

The pin is adjustable. Just like a "small" outboard. That's what it's there for. If you find that one hole is too far "in" (plowing water with the bow), and the next hole is too far "out" (blowout or porpoising), you can get yourself a metal sleeve that fits over the pin; which is what I do. That can get you "in between"...yes mine blows out in turns too...completely normal. I also do not have power trim (or tilt for that matter).

A 4 blade prop is less likely to "blow out"-and is more tolerant to turbulent water, higher mounting heights, they run smoother and get out of the hole better....BUT...lose a mph or two (usually). A bent or dinged prop is MORE likely to blow out. Have run into that with a 15HP yamaha, guy who owned it said he'd tried everything he knew of, and I suspected prop right off the bat...gave him a spare I had laying around and said here try this. Called me about an hour later and said his prop looked fine but he tried it anyway, and picked up 6 MPH, came out of the hole better, etc. He was right, his old one "looked" perfect visibly. But it wasn't obviously. 

I "Think" that a two stage PT&T unit from a later model 50hp will fit your motor; and I think that'll give you trim. But you'll have to research which years will work as I do not know. I replaced one on a 50 earlier this year and it had trim...looked identical to yours. A lot of those motors (especially the C40's) didn't have trim...Tilt only. You may also find a bolt-on PT&T from a different 40hp...maybe a P40? It'd be worth researching. Some of these PT&T units show up on ebay and not real expensive, either, in some cases. Wish I could get one to fit my 25 (but won't...clamp bracket is completely different and to my knowledge, the long shaft bracket won't work on a short shaft motor).


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## DarrellS (Dec 17, 2018)

stevenmurray1987 : Did you get the prop blow out under control ? What did you have to do to make it better for you ? Thanks .


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## Stumpalump (Dec 17, 2018)

DarrellS said:


> stevenmurray1987 : Did you get the prop blow out under control ? What did you have to do to make it better for you ? Thanks .



I don’t know what he did but if he did not drop the mounting height he still has the problem.


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## DarrellS (Dec 18, 2018)

turbotodd : I kind of have a similar problem with my results being like his . I will start a new thread instead of side tracking this one .


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