Ventilation 65hp chrysler

[las]

Hi

I have just finished part of the restoration on my 13 feet selco and got my 1978 65hp Chrysler running but are having some issues with the handling.
Prop is a 10x17 aluminium i believe from the numbers stamped on it! It's hard to tell but I think it says 493 17

first test run with two adults showed that I could not trim the motor enough down to lower the bow, the bow lift was excessive and it was porpoising a lot.

I then tried to raise the motor up a bit, thinking the push would be more inline with the boats axis and therefor not raising the bow as much. I also fabricated a fiberglass wedge trimming the motor in to raise the stern a little.

second run with the wife and two kids was even worse, only now it was struggling to get on plane and still had massive bow lift. The added weight makes comparison hard, but I thought I had it nailed with the wedge.
At one point I had to go full throttle to get on plane and then back off, our highest speed with GPS was 27knots and there was more in it! but had to back off as it was pounding heavily and the family, well the wife didn't like it (neither did I).

I should mention that I do plan on adding trim tabs, hydraulic Bennett's, and are in the process of making the tabs... But I would like the boat to ride it's best without the need for tabs, thus requiring the least amount of trim when sailing and least drag.

Next test was a pressure plate, stole the design off the web! We went out again today and did see and improvement in bow lift and was porpoising less. But I was surprised to learn that it was now ventilating and I was not able to reach the same speed as earlier. It didn't feel like I had the same thrust from the motor.

I have a few theories but don't quite know where to start....

Being relatively flat bottomed the motor could sit to high when on plane! I mean, mount the ventilation plate 1" up on a deep V and you still have some hull in the water and hence the motor deeper, but a flat bottom would sit higher in/on the water?
After I mounted the pressure plate it might have lifted the stern up to the point of ventilation. The but in this theory is that it feels very heavy and generally sits rather low in the water...

Next I did hammer out a few dent in the prop between runs, but I'm usually not a bad handyman. Could this have lead to cavitation? I would think the dents in the prop before would more likely be the course of that.

My biggest worry is the prop slipping in the hub, prop's are hard to come by for such an old engine, but I ques this could also be the issue, but it's hard to tell. i did get it on plane and I would think it would take the most torque to achieve that, and maybe that's an indication that the hub is OK.

Wrong design of pressure plate? My theory is that allmost anything bolted to the ventilation plate would generate lift and perhaps aid in pressurizing the prop.
One thing I noticed was two "jets" of water coming of the motor at plane from what looked to be somewhere near the front of the pressure plate. I did make a slight curve on the inlet side on the pressure plates fins after the picture, like the ones you can buy have.

Propeller wrong for my type of boat?

One idea is to fabricate a pressure plate the extend all the way around the leg and the make a gentle up swept curve to trap the water and then lower the motor again.

That was a long story, sorry!
Any thoughts, ideas, whatever are most welcome.

regards
Lars

 

[Sea Rider]

Gee, too much issues. That's a very old motor and prop, it?s a non through hub one, seems already dead picking other prop pitches to play with, prop availability. Will need to intall an induction tach to check max wot revs as usually loaded on flat calm no wind water cond to see if inside min.max wot rpm range.

Happy Boating

 

[chrome dome]

don't know the boat but a 65 hp motor on a 13 ft boat is way too much, weight wise and power wise,
you will never totally get rid of the porpoising issues if you keep using it,
more suitable hp range would 15-30,

 

[las]

Sea Rider, I have a Faria tach I just need to hook up and figure out the poles on this beast. But that's not easy because it has an replacement ingintion/stator system, I'm working on it. Still, when not ventilating I wasn't able to keep the throttle wide open, at least in the conditions I encountered the other day.

chrome dome. I know. But I like the looks of it!
And besides that, it is actually relative low weight for 65hp (only 2 cyl.), has a very thin lower leg for less drag.

I could be wrong and probably is, but my plan wit h this motor/boat combo was to use some large trim tabs to act like a extension of the bottom and taking some of the added weight.
But like I said, now I have gotten the idea that I would like to make it handle better before the trim tabs.

What about the motor height! What does raising it do to the boat lift wise?

 

[Sea Rider]

Motor height is very important, if too low will experience drag issues on lower tail along water splashes over/out transom, if too high esxcessive aereation on tight ckose turns at speed and at choppy windy water cond, prop does not grip right, works with less water height around cav plate. With how many boaters you intend to go boating with. Assume it's a tiller driven OB, right ?

To dial OB height right will need driver and boater siting fully up front towards bow. Post a pic of your lower leg shot sideways to have a look. Will need to figure how to install a tach. Good hole shot is a must have to plane faster.

Happy Boating

 

[las]

No Title

I have been thinking all day long for what might have caused it to ventilate and I think it must be the high motor mounting combined with the pressure plate. I never tried to trim it out a bit when it ventilated, had it all the way in the whole trip, and that's frustrating. Because if I'm right that could have cured the ventilation at the expense of bow lift.
I think that although heavy the transom water level would still be low and now hitting the pressure plate at an angle causing it to take in air as well, maybe!

I moved the motor back to the initial "low" setting and took some pictures, didn't think of taking pictures before the move.... I know, sometimes you just forget halv of what your supposed to be doing.
There are two mounting holes spaced roughly 1.2 inches apart, so it was 1.2 inches higher before.

The plan is to have four people aboard, two adults and two 10-12 year kids.

I haven't tried it out yet, but plan to go tomorrow. And this time I will try and make one change at a time......

 

[Texasmark]

Why did you jack up the engine? Yes you are way too high. If you had a high performance prop and a good trim with a light load you could have fun with the engine jacket up that high. But from what I see/hear you don't. The prop is sucking in ambient air because it is too close to the surface and not built to bite when operated in that environment. Prop too high is usually a problem in higher speed turns and running headlong into a good chop.

Porpoising is caused by the thrust being at too high of an angle .....prop pushed away from the transom too far for a given speed; period.....unless you have a hook in your hull and that's the source. To make it stop you either "tuck it in"...move your prop closer to the transom with your PTT or tilt pin if no PTT, or speed up. If you can't speed up, your have to tuck it in. Moving weight forward can help to counteract it if you have that liberty....passengers are that liberty.

 

[las]

I jacked it up because I thought it would run better that way and like mentioned maybe the bow lift could be helped that way. I would say that current height is ok, wouldn't you?
I'm also in a learning curve and realize now that this old uncupped prop don't like beeing close to the surface.

The motor have been tucked in all the time, I even tried to build a wedge to tuck it further in.
But I do have a hull with a very uneven and rough surface that I plan to correct this winter, I know that can't be good and maybe I just have to fix that first... But it would be nice to know if I should go for a straight bottom or actually build in a little hook.

Passengers don't really have a lot of room to move in. so that's not an option either.

I will play some more with the pressure plate, maybe try to make a few other designs.

 

[randyray41]

If you take the pin out it will let motor go some bit lower. But if you put it in reverse without pin it will lift motor up. You can try that and see if it being any more "tucked in" helps with the ride.

 

[las]

Hi Randy, there's no pin holding it, the trim/tilt system does that and I have verified it's all the way down.

I planned to go sailing today but it's been to windy, hopefully tomorrow!

 

[randyray41]

Don't know why I thought it was manual trim.

 

[Texasmark]

If you have a "hook" in the hull, all bets are off!!!!!!!!! Fix that first then restart your engine performance question and drop your engine back down where it was. Actually if the AV plate has water running over it, it controls the attitude of the boat much better than when it's above like you now have it. The extra drag of more lower unit in the water slows you down somewhat, but you can't have your cake and eat it too. Fix the hook!

 

[jimmbo]

You say the hull is uneven. Do you mean it is not straight? If the hull has what is called a rocker. that is a deflection of the hull in the fore-aft direction that is best described as the shape of the runners of a rocking chair opposite of a Hook, it will make the boat unstable, and induce rocking-like movement called 'porpoising'.
If the hull does have a rocker, trimming the engine in just sets up a fight between the hull and a stern lifting thrust angle. You should chcck the last 5 or 6 ft of the hull with a straightedge. A 1/8" over 4ft is more than enough to cause problems. Adding a hook is not the solution, Truing up the hull is Once that is done, afterplanes(like trim tabs, but are not tilting down) will certain help.
The Lower Unit on your engine is actually a very design, Chrysler introduced it in 1977, it was huge improvement over the previous gearcases. It has the water inlets on the prop shaft centerline, and with the correct props could be run a very high transom heights. Don't go thinking that you need to be raising the engine higher, that was just an FYI.

 

[las]

No Title

Thanks for the replies so far.
Yes the outside hull is in very bad condition and I plan on fixing it this winter.
The bottom is not straight as far as I can see and with a piece of wood held to it. I think it has both rocker and hook at various points, some areas are fixed with heavy pieces of fiberglass cloth, giving a huge rocker in the right transom area.
When sailed two years ago with another engine I was not able to get it on plane but it rode level! Of course it was waterlogged and no way the engine at the time could lift the bow....

In hindsight I can clearly see that I must address those issues before the hull will perform well, but I still plan to go ahead and try to make it work for the remainder of the season.

The benefit is you get a ton of knowledge along t he way....

 

[Texasmark]

I used the term "hook" figuratively meaning any deformation of the hull from it's original design. Usually the problem comes with old boats with wooden stringers and such that sit for years on a poorly supporting trailer, out in the weather, partially filled with rain water, bow down. It forces the hull to be concave in that area which makes for what appears to be bulges in other areas. The point we are making here is that it sounds like your hull is your problem.

 

[las]

I think you are right and I can't wait to get it fixed.

Went out today and with the lower motor mount it sailed a lot better, no ventilation and just what I would call normal bow lift. Although we where only one adult and two kids it sailed a lot better and at WOT it was the best it can be at the moment I think, we even hit 29 knots on the GPS.

When at full speed I had to steer a fair amount left to go straight, could that be the pressure plate or is it just normal?

 

[Sea Rider]

When at full speed I had to steer a fair amount left to go straight, could that be the pressure plate or is it just normal?

Is the pressure plate the horizontal fin seen on top antiucav plate ? If so, personally would remove both side vertical plates and leave only the doel fin. Does the engine have a trim tab under anti cav plate ? If not could adjust steering load, there should be a bolt at middle swivel bracket for that purpose, adjust CW enough for boat to travel on a straight line when tiller is released. Is engine sitting right on middle transom, check.

Happy Boating

 

[las]

There's only an exhaust snout, no trim tab. But I can live with it, no big deal.

I would think the vertical part of the pressure plate would actually keep the direction better, but I could be wrong. perhaps I'll try and make a different design fin and see what it does.
Engine sits in the middle.

Come think about it, I just installed a speed log before going out and it sits in the right side! Could that cause enough drag to actually steer the boat?

No matter what, several possibilities comes to mind for adjusting the straight line tracking. I could build a fin mounted on the pressure plate to correct the problem, or trim tabs once installed.

 

[Texasmark]

"There's only an exhaust snout, no trim tab. But I can live with it, no big deal."

"When at full speed I had to steer a fair amount left to go straight, could that be the pressure plate or is it just normal?"

Look again. I can clearly see in your provided picture that shows the exhaust snout 2 very distinguishing features:
1. There is a finite space between the snauze and the anti vent plate looking at the rear portion of the snauze. if it were part of the AV plate casting it would roll into the plate as both pieces would have been in the same mold. Not the case....The snauze is a separate part.

2. Looking closely at the front of the snauze, I think I see a bolt head. That means to me that the thing is removable (which theory #1 supports) and possibly turnable, meaning that you do in fact have a trim "tab". It makes no sense to get by without one when everybody else building steerable propulsion systems has to have one to stabilize the steering on larger drives and offset the propeller's torque. You have a propeller, it has torque. That "moment" transfers to the engine and its "opposite and opposing by the same amount" (tossing in a little Physics here....forget the Theorem number).

 

[las]

I see what you mean but although not part of the AV plate it's not adjustable, the pictures may cheat a little because there's no room to move it either way, it's recessed into the AV plate.

Don't know how these old Chrysler's were meant to be trimmed, but I'll figure something out.