Plaining complaining

[Colin]

Hi All
I'm new here to this board but from what I have read I think I will get the help I'm looking for, so here goes...
I have a dilemma with a 1990 Donzi 190 DL, Originally fitted with a 4 Cylinder Mercury I/O. This is my first boat and have thus far realy enjoyed using it. I purchased the boat used and thought that it would have been faster than it's turning out to be, tops out at about 35mph.
The original owner had a bigger motor installed. I would have thought that a boat when fitted with a bigger engine, in this case a Merc 4.3, would be faster and get on plane with greater ease. This was my assumption at the time. After driving the boat for a few month's I have noticed other boats (similar size and type) move through the water better. I have untold difficulty in controlling proposing that starts with the slightest provocation, waves or load shift. I find it hard to believe that is routine for a boat from Donzi without the modifications.
I have done some reading and have come to a rather amateurish conclusion that the added weight from the bigger engine is the at the root of the problem. Its added about 220lbs to the boat, obviously at the stearn.
So what I would like to know is:
-Can this boat be fitted with trim tabs?
I have noticed these on other boats and would think it difficult with the diving step on the transome, however you are the experts.
-Should I try a hydrofoil?
-Is this engine too big for the boat with regards to the weight of the engine?

-With the added weight of the engine does this effectively reduce the amount of people and/or weight the boat can now carry with respect to the original design capabilities?

Your help please, thanks in advance.

Colin

[Formula Jr]

Colin, I'll think about some of the stuff you've said. Need to look some other stuff up. All very good questions. While I'm looking for stuff, post what the numbers are stamped on the side of your prop hub and what your rpm reading is when at wide open throttle. Also write down what the CG load capacity plate says. You should have an Alpha 1 Drive. Does it porpose thru out all drive trim settings? I believe engine size isn't taken into consideration for certification load testing. In other words, the guy that bought a boat identical to yours with the bigger engine, has the same Capacity Plate even though you rightly point out there is a 200 lbs difference. Trim tabs can be added to anything. There are low profile configurations with the actuators laying horizontal. You should be getting 40 out of that 4.3. Also take a long straight edge and place it on your bottom near the transom. Check the running surfaces to see if they are flat, or if they bow out or in. Check closely along the chine. It was silly for Donzi in 1990 to put the 3.0 in some of these boats and there were some quality issues with the company that owned Donzi at the time. The cheapest solution is to buy two large lead bars and fasten them in the anchor locker at the bow. Its not pretty, but it works. I can'nt recommend tabs, or
the fin thingys till I know about the hull flatness. You can't correct a bad hook with tabs.

[Formula Jr]

In '92 Merc put out two of the 3 liter packages.
the 115 Hp standard and the 135 Hp LX
Both had Alpha drives with 1.98 to 1 ratios and most likely ran a 19 Inch Pitch
Prop.

I'm assuming you haven't changed the drive ratio which is now incorrect for
your V6 4.3 motor, which also came in two flavors: the 155 Hp and the LX 175 Hp version.
The 4.3 used a Alpha with a 1.84 to 1 ratio and is WOT rated at 4400 to 4800 RPM.

Using the Ideal Speed Formula:
((WOT RPM/DRIVE RATIO)XProp Pitch)/1056 = Ideal Speed in MPH.

We plug in your old drive ratio (1.98) and High WOT for the 4.3

((4800/1.98)x19)/1056 = 43.6 Given slip of 10% you're right there with 39 MPH.
But you're running at 35 so even with the odd drive there's something amiss.

Just for Giggles lets see what the top ideal speed would be for a normal 4.3 setup.

((4800/1.86)x21)/1056 = 51.3 Given slip, you're at 46, but you would need the LX four barrel
version to pull the RPMs needed.

Anyway, get back with some numbers so we can sort it out.

[RedDog]

It seems to me that porposing occurs when you have too much up-trim for the speed you are running. Try trimming back down until it stops, add throttle, and then slowly start trimming back up.

Hey, where is the spell check option now and how the hek do you spell porposing?

[Colin]

Hi Formula Jr et Al
My apologies for not getting back to you sooner with a reply to your request. I was on holiday and I was having fun on the water.

-looking at the hub of the prop I can only see 50440. This is a rebuilt prop from the marina where the boat was serviced.
Do props lose their cupping?
-At WOT (I had to double check) I can get about 38 MPH at 4700 RPM, about what your calculations show.
It should be noted that the serial numbers on the carb cover ,a two barrel, of the engine show those for the outdrive, an Alpha One, and they match so I would hazard a guess that the whole set up was installed from the previous boat.
-The "Mammal Swimming Action" issue. I would venture to say now after being more cautious with trim application, I might be inducing this, too much to soon. I have noticed that the trim indicator falls below 'down' when it's down 100%, and never reaches full up unless trailer mode is used. This makes it a bit difficult to tell exactly where the trim is in relation to the indicator. However I can tell that the drive is down. I can never get passed 50% trim at WOT without MSA. This past week-end I had tried additional sand bags, 200lbs++ worth in the bow and still the MSA sets in. The reason I must be missing the point of how to use trim effectivly and will have to be satisfied with using less. My gut feeling tells me ( ie:the boat) that things feel and sound better as I trim up. It planes better and I get more speed. I can adjust the throttle to cruise at lower speeds to suit but...I must be missing something.
As for the "hook" I'll have to wait untill the boat comes out of the water as I don't have a trailer.
I was looking at a Sea Ray Pachanga I noticed a very deffinitive curl in the hull as it met the stern, it was only in one section as well and even with the other side.
Is this normal or what you are refering to when you say hook?
Well I hope this helps, thanks again

Colin

[Formula Jr]

Colin, on the back hub of your prop, what are the numbers if you can read them. Should be 48-........something. Props never lose their cup. Its in the curve of the blades. But you have already put 200 lbs forward and still you are hopping. I'm lost on this one. There is no way to porpose (sp) with that much weight forward. I need a photo of the relationship of your drive to the transom with full down trim on the drive.